F Saxer

Bjurström Mf

Conaghan Pg
email: p.conaghan@leeds.ac.uk

Prof Francis Berenbaum

Pain-Phenotyping in Osteoarthritis: Current Concepts, Evidence, and Considerations towards a Comprehensive Framework for Assessment and Treatment

Keywords: phenotypes, osteoarthritis, osteoarthritis pain, drug development, patient reported outcomes

Objectives: Pain as central symptom of osteoarthritis (OA) needs to be addressed as part of successful treatment. The assessment of pain as feature of disease or outcome in clinical practice and drug development remains a challenge due to its multidimensionality and the plethora of confounders. This article aims at providing insights into our understanding of OA pain-phenotypes and suggests a framework for systematic and comprehensive assessments.

Methods: This narrative review is based on a search of current literature for various combinations of the search terms "pain-phenotype" and "knee OA" and summarizes current knowledge on OA pain-phenotypes, putting OA pain and its assessment into perspective of current research efforts.

Results: Pain is a complex phenomenon, not necessarily associated with tissue damage.

Various pain-phenotypes have been described in knee OA. Among those a phenotype with high pain levels not necessarily matching structural changes and a phenotype with low pain levels and impact are relatively consistent. Further subgroups can be differentiated based on patient reported outcome measures, assessments of comorbidities, anxiety and depression, sleep, activity and objective measures such as quantitative sensory testing.

Conclusions: The complexity of both OA as disease and pain in OA prompt the definition of a set of variables that facilitate assessments comparable across studies to maximize our understanding of pain, as central concern for the patient.

Introduction

Osteoarthritis (OA) is a complex multifactorial disease and global health care challenge affecting more than 500 million people. [START_REF]Collaborative-Global Burden of Disease Network[END_REF] Not only is OA a major cause of reduction in quality of life and activities of daily living, with substantial socio-economic impact, [START_REF] Hunter | The individual and socioeconomic impact of osteoarthritis[END_REF][START_REF] Araujo | Quality of Life and Functional Independence in Patients with Osteoarthritis of the Knee[END_REF] but has also been associated with increased mortality. [START_REF] Nuesch | All cause and disease specific mortality in patients with knee or hip osteoarthritis: population based cohort study[END_REF] Total joint replacement is typically the 'last resort' but approximately 20% of patients remain symptomatic after the procedure. [START_REF] Beswick | What proportion of patients report long-term pain after total hip or knee replacement for osteoarthritis? A systematic review of prospective studies in unselected patients[END_REF] In the absence of treatments that can halt or reverse the OA process, and despite much research over decades, there remains a huge unmet medical need.

For a "treatment of OA" claim for a medication that targets the underlying pathophysiology, regulatory authorities require benefits on how patients feel, function or (their joints) survive. [START_REF] Kim | Concept Endpoints Informing Design Considerations for Confirmatory Clinical Trials in Osteoarthritis[END_REF] While structural changes are objectively quantifiable, validly assessing non-structural outcomes (i.e., pain or function) remains complex.

Previous research has established the concept of OA-phenotypes. [START_REF] Mobasheri | The future of deep phenotyping in osteoarthritis: How can high throughput omics technologies advance our understanding of the cellular and molecular taxonomy of the disease?[END_REF][START_REF] Mobasheri | Recent advances in understanding the phenotypes of osteoarthritis[END_REF] , i.e., the existence of observable patient characteristics that systematically differ between groups of patients affected by OA. Phenotyping thereby allows a stratification of a heterogeneous patient population and may be reflective of different underlying pathologic mechanisms defining different endotypes. [START_REF] Angelini | Osteoarthritis endotype discovery via clustering of biochemical marker data[END_REF][START_REF] Mobasheri | Molecular taxonomy of osteoarthritis for patient stratification, disease management and drug development: biochemical markers associated with emerging clinical phenotypes and molecular endotypes[END_REF] The existence of different OA pain-phenotypes [START_REF] Carlesso | Identifying pain susceptibility phenotypes in knee osteoarthritis[END_REF] adds an additional layer of complexity. This narrative review aims at summarizing key concepts of pain-phenotyping, presenting current evidence. Pain is the most important symptom of OA and its treatment central to patients' well-being. The manuscript tries to capture the complexity of OA-pain that underlines the need for personalized and targeted management approaches based on a better understanding of pain-phenotypes and underlying mechanisms. We argue that a better understanding of these aspects is crucial for designing meaningful future trials and measuring treatment success. The ultimate goal is to establish a framework for systematic and comparable pain assessments in OA patients, with the intention of developing and allocating targeted treatments that meet patients' and societies' expectations.

Pathophysiology of pain in OA

Pain is defined as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage". [START_REF] Raja | The revised International Association for the Study of Pain definition of pain: concepts, challenges, and compromises[END_REF] This definition underlines the complex interaction of pain triggers with biological, psychological and social factors (see Supplementary Table 1). [START_REF] Cohen | Chronic pain: an update on burden, best practices, and new advances[END_REF] This definition also makes abundantly clear that the absence of a structural correlate does not disqualify a sensation as pain, and that pain can persist despite the normalization of structure. It remains unclear why or which patients transition from acute to persistent or chronic pain. [START_REF] Nijs | Nociplastic Pain Criteria or Recognition of Central Sensitization? Pain Phenotyping in the Past, Present and Future[END_REF] In principle, chronicity should be assumed in most OA patients with a typical pain duration of >6 months; indeed a "chronic pain" phenotype is consistently reported. [START_REF] Dell'isola | Identification of clinical phenotypes in knee osteoarthritis: a systematic review of the literature[END_REF][START_REF] Arendt-Nielsen | Pain sensitisation in osteoarthritis[END_REF] Pain perception, processing and transition to chronic pain are the result of experiencedriven neuro-structural changes [START_REF] Kuner | Cellular Circuits in the Brain and Their Modulation in Acute and Chronic Pain[END_REF] , neuro-immunologic crosstalk 18,[START_REF] Hiraga | Neuroplasticity related to chronic pain and its modulation by microglia[END_REF] and (epi)genetic mechanisms. [START_REF] Olesen | Association Between Genetic Polymorphisms and Pain Sensitivity in Patients with Hip Osteoarthritis[END_REF][START_REF] Barowsky | Cross-Disorder Genomics Data Analysis Elucidates a Shared Genetic Basis Between Major Depression and Osteoarthritis Pain[END_REF] In principle, pain perception occurs in several "morphologic layers".

Peripheral joint nociceptors are activated by mechanical, thermal or chemical stimuli such as cytokines or chemokines released as part of inflammatory processes and cartilage degradation in OA. This can also trigger vascularization and ingrowth of additional nociceptors perpetuating the stimulus. [START_REF] Fu | Osteoarthritis: the genesis of pain[END_REF] Continuous or repetitive stimulation of nociceptors can reduce activation thresholds leading to peripheral sensitization with primary hyperalgesia (an abnormally increased sensitivity to pain at the site of tissue damage) or allodynia (pain from otherwise non-noxious stimuli such as light touch), which may be present in OA. [START_REF] Arendt-Nielsen | Pain sensitisation in osteoarthritis[END_REF] Nociceptor activity is transmitted via C-fibers (slow, burning pain) or A-delta fibers (fast, sharp pain) to the cell body situated in the dorsal root ganglion of the spinal cord. The activity is further transmitted to higher systems, whereas inhibitory and excitatory influences from the local cellular environment as well as thalamic centers, brainstem and cerebral cortex modulate the pain perception [START_REF] Kuner | Cellular Circuits in the Brain and Their Modulation in Acute and Chronic Pain[END_REF][START_REF] Kuner | Structural plasticity and reorganisation in chronic pain[END_REF] , explaining the interrelation between pain and affect [START_REF] Kuner | Cellular Circuits in the Brain and Their Modulation in Acute and Chronic Pain[END_REF][START_REF] Garland | Pain processing in the human nervous system: a selective review of nociceptive and biobehavioral pathways[END_REF] , but also the impact from expectation, observed in placebo and nocebo phenomena [START_REF] Turk | Assessment of Psychosocial and Functional Impact of Chronic Pain[END_REF][START_REF] Frisaldi | Understanding the mechanisms of placebo and nocebo effects[END_REF] .

Based on the above mechanisms, primarily three types of pain have been discerned (with some overlap) in OA: I) Nociceptive pain is triggered by tissue damage and often responsive to NSAIDs. [START_REF] Da Costa | Effectiveness and safety of non-steroidal anti-inflammatory drugs and opioid treatment for knee and hip osteoarthritis: network meta-analysis[END_REF] Pain in OA was thought to be purely nociceptive [START_REF] Hunter | The symptoms of osteoarthritis and the genesis of pain[END_REF] with inflammation as potential pathophysiologic trigger and driver of pain. [START_REF] Berenbaum | Osteoarthritis as an inflammatory disease (osteoarthritis is not osteoarthrosis!)[END_REF][START_REF] Vincent | IL-1 in osteoarthritis: time for a critical review of the literature[END_REF] The innate immune system, [START_REF] Miller | The innate immune response as a mediator of osteoarthritis pain[END_REF] and especially macrophages play crucial roles in knee OA-pain through induction of inflammatory mediators, [START_REF] Miller | Osteoarthritis joint pain: the cytokine connection[END_REF] growth factors [START_REF] Malfait | Basic Mechanisms of Pain in Osteoarthritis: Experimental Observations and New Perspectives[END_REF] and proteinases, [START_REF] Chen | Macrophages in osteoarthritis: pathophysiology and therapeutics[END_REF] and are reciprocally stimulated via nociceptor-secreted neuropeptides. [START_REF] Geraghty | Neuroimmune interactions and osteoarthritis pain: focus on macrophages[END_REF] They also impact pain processing at the level of dorsal root ganglia and literature supports their role in pain sensitization and neuropathic pain. [START_REF] Ioan-Facsinay | Initiating pain in osteoarthritis (OA): is it the mast cell?[END_REF][START_REF] Miller | CCR2 chemokine receptor signaling mediates pain in experimental osteoarthritis[END_REF] Preclinical animal models evaluating anti-ADAMTS5 (a disintegrin and metalloproteinase with thrombospondin motifs 5), [START_REF] Miller | Therapeutic effects of an anti-ADAMTS-5 antibody on joint damage and mechanical allodynia in a murine model of osteoarthritis[END_REF] or antibodies targeting Toll-like Receptors [START_REF] Miller | An emerging role for Toll-like receptors at the neuroimmune interface in osteoarthritis[END_REF][START_REF] Sharma | Inflammation and joint destruction may be linked to the generation of cartilage metabolites of ADAMTS-5 through activation of toll-like receptors[END_REF] in knee OA support the idea of neuroinflammatory mechanisms in OA-pain. Similarly, the neurotrophin NGF (nerve growth factor) has been implicated in OA-pain and inflammation [START_REF] Malfait | Basic Mechanisms of Pain in Osteoarthritis: Experimental Observations and New Perspectives[END_REF][START_REF] Malfait | Targeting neurotrophic factors: Novel approaches to musculoskeletal pain[END_REF] . NGF is increased in OA joints and promising clinical results for pain relief have been reported in humans and animals [START_REF] Miller | Nerve growth factor blockade for the management of osteoarthritis pain: what can we learn from clinical trials and preclinical models?[END_REF][START_REF] Fda | FDA Approves Novel Treatment to Control Pain in Cats with Osteoarthritis, First Monoclonal Antibody Drug for Use in Any Animal Species[END_REF][START_REF] Fda | FDA Approves First Monoclonal Antibody for Dogs with Osteoarthritis Pain[END_REF] .

NGF is released in response to mechanical stress and inflammation [START_REF] Pecchi | Induction of nerve growth factor expression and release by mechanical and inflammatory stimuli in chondrocytes: possible involvement in osteoarthritis pain[END_REF] , its role in the context of inflammation however is not fully understood yet [START_REF] Minnone | NGF and Its Receptors in the Regulation of Inflammatory Response[END_REF] , which may explain the safety concerns that finally led to a negative benefit risk evaluation for an anti-NGF antibody by the FDA (food and drug administration) [START_REF]Tanezumab Monoclonal Antibody Against Nerve Growth Factor[END_REF] . In addition, histamine receptors have been implicated in nociception and chronic pain. Subtypes are expressed in the peripheral and central nervous system and play a role in the modulation of nociceptive transmission. [START_REF] Obara | Histamine, histamine receptors, and neuropathic pain relief[END_REF] II) Nociplastic pain is a result of central dysregulation and sensitization, and refers to "pain that arises from altered nociception despite no clear evidence of actual or threatened tissue damage causing the activation of peripheral nociceptors or evidence for disease or lesion of the somatosensory system causing the pain" (IASP (International Association for the Study of Pain) definition). [START_REF] Raja | The revised International Association for the Study of Pain definition of pain: concepts, challenges, and compromises[END_REF][START_REF] Kosek | Do we need a third mechanistic descriptor for chronic pain states?[END_REF][START_REF] Soni | Central Sensitization in Knee Osteoarthritis: Relating Presurgical Brainstem Neuroimaging and PainDETECT-Based Patient Stratification to Arthroplasty Outcome[END_REF] Yet, links between disease duration and measures of central sensitization seem weak [START_REF] Neogi | Sensitivity and sensitisation in relation to pain severity in knee osteoarthritis: trait or state?[END_REF] and most patients improve markedly after joint replacement, suggesting a peripheral driver of the pain experience. [START_REF] Pinedo-Villanueva | Identifying individuals with chronic pain after knee replacement: a populationcohort, cluster-analysis of Oxford knee scores in 128,145 patients from the English National Health Service[END_REF] Nociplastic pain is decoupled from the pathology at the joint level though also associated with neuroimmunologic changes. In view of the impact of central pain modulation, treatments such as patient education, sleep hygiene, and psychological treatment [START_REF] Yong | Pain Medicine : An Essential Review[END_REF] or, centrally acting substances such as NMDA (N-methyl-D-aspartate) antagonists [START_REF] Yong | Pain Medicine : An Essential Review[END_REF] , cannabis-based medicines [START_REF] Bennici | Safety of Medical Cannabis in Neuropathic Chronic Pain Management[END_REF] , tricyclic antidepressants, 5-hydroxytryptaminenoradrenaline reuptake inhibitors and gabapentinoids 53 55 may be beneficial as adjuncts in improving this type of pain. Similarly, sympatholytics may be beneficial in nociplastic and possibly neuropathic pain. [START_REF] Yong | Pain Medicine : An Essential Review[END_REF] III) Neuropathic pain is typically associated with structural nerve damage [START_REF] Costigan | Neuropathic pain: a maladaptive response of the nervous system to damage[END_REF] , the morphologic correlate of which currently remains elusive in OA and may be related to comorbidities rather than OA (e.g., diabetes, lumbar radiculopathy, etc.). A recent matched pair approach in a cohort of knee OA patients suggested a potential neuropathic pain component in 8.2% (based on PainDETECT). These patients differed from their likely non-neuropathic counterparts (matched for pain intensity) in having a higher degree of functional impairment and more painful joints but generally less pronounced radiographic joint changes. [START_REF] Van Helvoort | Neuropathic pain in the IMI-APPROACH knee osteoarthritis cohort: prevalence and phenotyping[END_REF]

Methods

This narrative review is based on a non-systematic search of current literature in Ovid MEDLINE® using the search terms "pain-phenotype" and "knee osteoarthritis" in various combinations to identify articles covering the area of interest. To evaluate potential surrogate measures for pain-phenotypes PubMed ® was searched for biomarkers evaluated in the context of OA. The search was then expanded to crossreferenced biomarkers and interventions.

Studies examining knee OA pain-phenotypes

The relevance of the different mechanisms for pain perception in OA underlines the importance of distinguishing the predominant pain type or mechanism for a successful treatment allocation especially in relation to nociceptive vs non-nociceptive pain. This distinction can be achieved via pain-phenotyping, i.e., the differentiation of patient clusters based on observable traits associated with differences in pain experience.

Table 1 may be placed here

Various studies have used phenotyping approaches to characterize pain-phenotypes in OA as summarized in Table 1. Murphy et al. [START_REF] Murphy | Subgroups of older adults with osteoarthritis based upon differing comorbid symptom presentations and potential underlying pain mechanisms[END_REF] cross-sectionally evaluated the cooccurrence of centrally mediated symptoms in older adults with hip or knee OA and identified three pain-phenotypes. Those with the highest pain levels also showed high levels of depression and fatigue, low sleep quality and a high burden of comorbidities potentially indicating a higher overall impact from central mechanisms of pain perception. Patients in this cluster had the highest disease impact on health-related quality of life. The second cluster had intermediate levels of depression and fatigue, low levels of pain and good sleep, possibly indicative of a mixed peripheral and central painphenotype. The third cluster had overall low levels of pain, fatigue or depression, but a poor sleep quality. This could be patients with a predominantly nociceptive pain type. [START_REF] Murphy | Subgroups of older adults with osteoarthritis based upon differing comorbid symptom presentations and potential underlying pain mechanisms[END_REF] However, because this evaluation was cross-sectional, directionality and mechanisms cannot be discerned.

Finan et al. [START_REF] Finan | Discordance between pain and radiographic severity in knee osteoarthritis: findings from quantitative sensory testing of central sensitization[END_REF] also evaluated patient reported outcome (PRO) information on anxiety/depression symptoms, sleep and pain catastrophizing but included the congruence between pain and structural changes versus quantitative sensory testing (QST). They dichotomized pain (cut-off 4.22 out of 20 on WOMAC (Western Ontario and

McMaster Universities Osteoarthritis Index) pain subscale score) and radiographic grade (Kellgren-Lawrence 1 and 2 vs. 3 and 4) resulting in four combinations. The highpain groups trended towards higher impact in psychosocial function, which was significant for patients with high-pain and low radiographic grade. The most notable finding was that the high-pain and low Kellgren-Lawrence group exhibited hypersensitivity on several QST modalities at unaffected anatomic sites, suggesting a propensity towards central pain sensitization. In contrast, the other discordant group with low-pain and high-Kellgren-Lawrence were the least pain-sensitive. [START_REF] Finan | Discordance between pain and radiographic severity in knee osteoarthritis: findings from quantitative sensory testing of central sensitization[END_REF] Similarly, Egsgaard et al. [START_REF] Egsgaard | Identifying specific profiles in patients with different degrees of painful knee osteoarthritis based on serological biochemical and mechanistic pain biomarkers: a diagnostic approach based on cluster analysis[END_REF] aimed at identifying pain profiles in patients with OA based on psychological measures, QST, Kellgren-Lawrence grade and biomarkers. Compared to controls, the four resultant clusters had higher disease impact on physical functioning, quality of life and pain response. In the order of pain impact (low to high), the cluster of patients with overall low pain sensitivity and higher CPM (conditioned pain modulation) than controls had the lowest pain. The next lowest pain cluster showed increased temporal summation at the arm only (TS) and CPM and pressure pain thresholds (PPT) comparable to controls, potentially indicative of an early stage of chronification. Two clusters showed reduced PPTs, enhanced TS and reduced CPM. In addition, one of those clusters was characterized by greater hyperalgesia, lower general health and pain catastrophizing. While both of these clusters showed alterations in pain thresholds quantifiable with QST, the one additionally affected by lower general health and pain catastrophizing reported the highest values on the three WOMAC subscales, suggesting an additive effect on pain experience. [START_REF] Egsgaard | Identifying specific profiles in patients with different degrees of painful knee osteoarthritis based on serological biochemical and mechanistic pain biomarkers: a diagnostic approach based on cluster analysis[END_REF] In addition to psychological measures, radiographic OA grade and patient characteristics, Kittelson et al. included extensor strength in their approach to painphenotyping of the OAI (osteoarthritis initiative) database [START_REF] Kittelson | Determination of Pain Phenotypes in Knee Osteoarthritis: A Latent Class Analysis Using Data From the Osteoarthritis Initiative[END_REF] , as well as a community sample that comprised participants with symptomatic OA and healthy older adults as controls. [START_REF] Kittelson | Determination of Pain Phenotypes in Knee Osteoarthritis Using Latent Profile Analysis[END_REF] In both samples they identified four pain-phenotypes, one primarily characterized by a high burden of comorbidities, one by a high level of psychological distress and pain, and one with high extensor strength and a low overall burden of disease. Participants from the community sample in this latter group often had a history of knee trauma or surgery. [START_REF] Kittelson | Determination of Pain Phenotypes in Knee Osteoarthritis Using Latent Profile Analysis[END_REF] A fourth pain-phenotype was identified in both analyses; in the OAI, this fourth phenotype was characterized by a high proportion of joint line and pes anserine tenderness [START_REF] Kittelson | Determination of Pain Phenotypes in Knee Osteoarthritis: A Latent Class Analysis Using Data From the Osteoarthritis Initiative[END_REF] . In the community sample, the fourth phenotype was differentiated by low target knee PPTs. [START_REF] Kittelson | Determination of Pain Phenotypes in Knee Osteoarthritis Using Latent Profile Analysis[END_REF] Reducing heterogeneity due to differences in OA severity, Frey-Law et al. [START_REF] Frey-Law | Pain sensitivity profiles in patients with advanced knee osteoarthritis[END_REF] analysed pain-phenotypes in patients scheduled for knee arthroplasty and identified five phenotypes based on psychological assessments, patient characteristics, QST, pain characteristics, function and quality of life. One pain-phenotype exhibited low pain sensitivity but high PPTs at the target knee. Another exhibited average pain sensitivity to all tested stimuli. In contrast, three clusters showed high sensitivity to pain. These three clusters differed in their sensitivity to TS, heat and pressure pain, and punctate pain, respectively. There was no relevant impact from the other evaluated characteristics except a predominance of males in the low pain group. Interestingly, in the high pain sensitivity group, high punctate and high heat and pressure pain sensitivity translated into higher clinical pain levels, while TS did not [START_REF] Frey-Law | Pain sensitivity profiles in patients with advanced knee osteoarthritis[END_REF] .

Evaluating thermal measures of QST as potential indicators of central sensitization and neuropathic pain and their correlation with pain levels, pain characteristics and function, Wright et al. [START_REF] Wright | Cold Pain Threshold Identifies a Subgroup of Individuals With Knee Osteoarthritis That Present With Multimodality Hyperalgesia and Elevated Pain Levels[END_REF] compared a community sample of patients with painful knee OA to pain-free volunteers. Patients with OA displayed lower PPTs than pain-free volunteers at the index knee but not at other sites. In addition, patients with OA showed cold pressure pain on average at higher temperatures than pain-free controls at the index and contralateral knee, as well as a distant site. This cold hyperalgesia was pronounced in a subgroup of 44% of patients. These patients also had a tendency towards reduced thresholds for pressure and thermal pain at sites other than the target knee, higher pain levels, higher functional impact and higher PainDETECT scores. Despite the differences in QST between the groups, there were no differences in psychological impact. [START_REF] Wright | Cold Pain Threshold Identifies a Subgroup of Individuals With Knee Osteoarthritis That Present With Multimodality Hyperalgesia and Elevated Pain Levels[END_REF] In the only longitudinal study to date to assess pain susceptibility by Carlesso et al. [START_REF] Carlesso | Pain Susceptibility Phenotypes in Those Free of Knee Pain With or at Risk of Knee Osteoarthritis: The Multicenter Osteoarthritis Study[END_REF] , four distinct phenotypes were identified among people with or at risk of knee OA who were free of persistent knee pain at baseline. Interestingly, the group that was the most sensitized based upon PPT measures had a 2-fold higher risk of developing persistent knee pain compared with the group that had the least sensitization based upon PPT and TS. Further, the group that exhibited TS was not at increased risk for developing persistent knee pain. [START_REF] Carlesso | Pain Susceptibility Phenotypes in Those Free of Knee Pain With or at Risk of Knee Osteoarthritis: The Multicenter Osteoarthritis Study[END_REF] The other factors that were examined (i.e., widespread pain, pain catastrophizing, depressive symptoms, poor sleep) did not differentiate between the groups, and thus did not contribute to risk of developing persistent knee pain.

Heat and cold hyperalgesia have recently further been evaluated by Carlesso et al. [START_REF] Carlesso | Use of IMMPACT Recommendations to Explore Pain Phenotypes in People with Knee Osteoarthritis[END_REF] in an analysis of pain-phenotypes in patients presenting with knee OA. The analysis was based on the IMMPACT recommendations for pain-phenotyping, i.e., "pain variability, intensity and qualities, somatization, anxio-depressive symptoms, sleep, fatigue, pain catastrophizing, neuropathic pain, and quantitative sensory tests". [START_REF] Edwards | Patient phenotyping in clinical trials of chronic pain treatments: IMMPACT recommendations[END_REF] The three pain classes separated based on PRO information (consistent high, intermediate or low disease impact). The results for QST were less clear. Temperature sensitivity and PPTs separated the least affected from the two other classes. Only TS was significantly different for all the classes. [START_REF] Carlesso | Use of IMMPACT Recommendations to Explore Pain Phenotypes in People with Knee Osteoarthritis[END_REF] TS has also been demonstrated to separate clusters in other cohorts [START_REF] Finan | Discordance between pain and radiographic severity in knee osteoarthritis: findings from quantitative sensory testing of central sensitization[END_REF][START_REF] Egsgaard | Identifying specific profiles in patients with different degrees of painful knee osteoarthritis based on serological biochemical and mechanistic pain biomarkers: a diagnostic approach based on cluster analysis[END_REF][START_REF] Frey-Law | Pain sensitivity profiles in patients with advanced knee osteoarthritis[END_REF][START_REF] Carlesso | Pain Susceptibility Phenotypes in Those Free of Knee Pain With or at Risk of Knee Osteoarthritis: The Multicenter Osteoarthritis Study[END_REF] , and to potentially predict acute postoperative pain intensity and chronic postsurgical pain. [START_REF] Sangesland | Are preoperative experimental pain assessments correlated with clinical pain outcomes after surgery? A systematic review[END_REF][START_REF] Van Helmond | Is Preoperative Quantitative Sensory Testing Related to Persistent Postsurgical Pain? A Systematic Literature Review[END_REF] Two studies evaluated clinical pain-phenotyping and included imaging. In a community sample of older adults, Pan et al. identified three subgroups of patients with knee pain. [START_REF] Pan | Differentiating knee pain phenotypes in older adults: a prospective cohort study[END_REF] A predominantly female class including patients with high local pain, a high burden of emotional problems and limited structural changes was identified, while another class was dominated by males with low disease impact but definite structural changes. The third class was healthy overall with limited signs of structural OA and low levels of knee pain, assumed by the authors to comprise participants with early OA. Pain levels between the high and low pain groups consistently differed over 10.7 years and were not necessarily correlated with the presence of radiographic signs of OA.

In another cohort study of community dwelling adults, Burston et al. [START_REF] Burston | The impact of anxiety on chronic musculoskeletal pain and the role of astrocyte activation[END_REF] evaluated the impact of anxiety and depression on incident knee pain. They report an odds ratio (OR) of 1.71 for incident knee pain at twelve months in individuals with baseline anxiety (adjusted for depression), and a 1.66 OR in patients with baseline depression (adjusted for anxiety). These insights complement a preclinical OA model that demonstrated astrocyte activation as potential correlate of altered pain perception in animals with elevated baseline anxiety-like behaviour reversible after introducing a centrally acting anxiolytic. [START_REF] Burston | The impact of anxiety on chronic musculoskeletal pain and the role of astrocyte activation[END_REF] In summary, the above-described studies clearly demonstrate the existence of several OA pain-phenotypes, which seem differentiable based on objective measures and PRO information. Many approaches suggest a low pain-phenotype as well as a phenotype with high pain perception and impact. Interestingly, few articles on OA and OA painphenotypes specifically report pes anserine tenderness [START_REF] Kittelson | Determination of Pain Phenotypes in Knee Osteoarthritis: A Latent Class Analysis Using Data From the Osteoarthritis Initiative[END_REF][START_REF] Demanse | Unsupervised machine-learning algorithms for the identification of clinical phenotypes in the Osteoarthritis Initiative database[END_REF][START_REF] Mcgonagle | Differentiation between osteoarthritis and psoriatic arthritis: implications for pathogenesis and treatment in the biologic therapy era[END_REF] , which may confound OApain perception and OA pain-phenotyping. Furthermore, the observed differences and similarities in previous OA-phenotyping analyses underline the importance of the choice of input variables for the allocation of clusters in phenotyping. [START_REF] Demanse | Unsupervised machine-learning algorithms for the identification of clinical phenotypes in the Osteoarthritis Initiative database[END_REF] The observed differences in pain perception and painphenotypes do not necessarily correlate with the extent of radiographic changes. There seems to be a certain overlap between structural OA and OA pain-phenotypes if imaging is included as an input variable. [START_REF] Kittelson | Determination of Pain Phenotypes in Knee Osteoarthritis: A Latent Class Analysis Using Data From the Osteoarthritis Initiative[END_REF][START_REF] Kittelson | Determination of Pain Phenotypes in Knee Osteoarthritis Using Latent Profile Analysis[END_REF][START_REF] Pan | Differentiating knee pain phenotypes in older adults: a prospective cohort study[END_REF][START_REF] Demanse | Unsupervised machine-learning algorithms for the identification of clinical phenotypes in the Osteoarthritis Initiative database[END_REF] Whether imaging information dominates differences between phenotypes, or if pain-phenotypes are associated with structural changes assessed on imaging merits further investigation.

To further differentiate pain-phenotypes, the degree of altered neurobiological signalling appears to be particularly relevant; specific questionnaires and QST measures, especially TS and PPTs or thermal sensitivity appear to be important.

Limitations of existing tools to identify OA pain-phenotypes

Pain measurement in OA studies primarily focuses on questionnaires that inquire about the intensity, pain on movement and a limited range of pain characteristics to capture the pain experience (Supplementary Table 1). However, most of these questionnaires do not differentiate the underlying pain mechanism(s) at play in any given individual.

Further highlighting the complexity of OA, numerous biomarkers (as potential indicators of pathophysiologic mechanisms in OA) and interventions have been evaluated in the context of structural and symptom (pain) OA outcomes (Supplemental Table 2). Patients with different pain-pheno-and endotypes may report similar pain intensity and dimensions. These pain measures therefore may not be suitable to categorize patients but should be used as outcome measures to explore treatment effects. To identify different pain-pheno-and -endotypes, assessments should include clinical/biological information as well as medical history (e.g., burden of comorbidities, signs of dysfunctional pain experience or pain quality, sleep, anxiety and depression, physical activity and assessment of somatosensory function by QST, see Supplemental Table 3). Given the above-described convergence of structural OA and OA pain-phenotypes if imaging or performance measures are added to the clustering, the selection of input variables has to be carefully considered.

Comorbidity impacts pain [START_REF] Dominick | Unpacking the burden: understanding the relationships between chronic pain and comorbidity in the general population[END_REF] and various measures are used to estimate the burden of comorbidity (comprehensively summarized by Stirland et al. [START_REF] Stirland | Measuring multimorbidity beyond counting diseases: systematic review of community and population studies and guide to index choice[END_REF] ). It is however vital to consider a score's "original purpose and the outcomes for which it is validated". [START_REF] Stirland | Measuring multimorbidity beyond counting diseases: systematic review of community and population studies and guide to index choice[END_REF] Scores developed to predict mortality (e.g., Charlson Comorbidity Index) may be unsuitable to reflect the burden of comorbidity and its impact on physical functioning.

Affective states such as anxiety, depression or pain catastrophizing influence pain modulation and perception of pain. While there are diagnostic criteria and tools to identify and grade anxiety and depression, a consensus regarding how to measure catastrophizing has not yet been reached. [START_REF] Petrini | Understanding Pain Catastrophizing: Putting Pieces Together[END_REF] Measures of emotional dysregulation or positive and negative affect can also be useful. [START_REF] Watson | Development and validation of brief measures of positive and negative affect: the PANAS scales[END_REF][START_REF] Koechlin | The role of emotion regulation in chronic pain: A systematic literature review[END_REF] Kinesiophobia has been reported as predictor of disability impacting quality of life in various pain conditions; it has been associated with chronic pain and thus may also present a useful addition. [START_REF] Boersma | How does persistent pain develop? An analysis of the relationship between psychological variables, pain and function across stages of chronicity[END_REF][START_REF] Alshahrani | Association between Kinesiophobia and Knee Pain Intensity, Joint Position Sense, and Functional Performance in Individuals with Bilateral Knee Osteoarthritis[END_REF] Exercise can positively influence pain [START_REF] Naugle | Physical activity behavior predicts endogenous pain modulation in older adults[END_REF] ; pain and activity may have a reciprocal relationship in some individuals; it may therefore be misleading to assess one without the other. [START_REF] Trudeau | Assessment of pain and activity using an electronic pain diary and actigraphy device in a randomized, placebo-controlled crossover trial of celecoxib in osteoarthritis of the knee[END_REF][START_REF] Lo | Validation of a new symptom outcome for knee osteoarthritis: the Ambulation Adjusted Score for Knee pain[END_REF] This results in methodologic challenges. Objective performance tests are subject to day-to-day variability and reflect what patients are able to do under observation rather than what they habitually do in their free-living environment. The domain of activity, in the future, may best be captured using digital devices that allow the measurement of indicators in the free-living environment like step count, activities at a certain heart rate or radius of mobility. Similarly, objective assessment of sleep structure may be obtained using wearable technology. [START_REF] Smith | Experimental sleep disruption attenuates morphine analgesia: findings from a randomized trial and implications for the opioid abuse epidemic[END_REF][START_REF] Fabbri | Measuring Subjective Sleep Quality: A Review[END_REF] Measuring elements of sleep is increasingly recognized as an important aspect to understanding the pain experience since sleep and pain are also closely inter-related; pain may disrupt sleep, and sleep disturbance negatively impacts descending pain inhibitory pathways, heightens pain sensitivity and attenuates opioid analgesia. [START_REF] Smith | Experimental sleep disruption attenuates morphine analgesia: findings from a randomized trial and implications for the opioid abuse epidemic[END_REF][START_REF] Smith | The effects of sleep deprivation on pain inhibition and spontaneous pain in women[END_REF][START_REF] Finan | The association of sleep and pain: an update and a path forward[END_REF][START_REF] Irwin | Sleep Disruption and Activation of Cellular Inflammation Mediate Heightened Pain Sensitivity: A Randomized Clinical Trial[END_REF] These examples underline the importance of systematically assessing pain and potential confounders in an integrative approach.

Considerations for a broader collection of pain measures

This summary highlights the complexity of the pain experience as multidimensional physical and psychological phenomenon, as well as of the plethora of assessment tools.

It also suggests the existence of different patterns of observable traits, OA painphenotypes, which likely reflect different underlying mechanisms contributing to the overall pain experience. Striving for the development of a personalized and targeted management of OA, pain is a critical factor, and central to patients' well-being. OA-pain is associated with multiple pathophysiological mechanisms reflected in distinct phenoand endotypes. This implies the need to systematically define those pain-pheno-and -endotypes independent of the underlying OA pheno-and -endotype.

We therefore suggest the systematic collection of additional pain-related data, such as pain quality, including potential signs of sensitization and other altered neurobiological mechanisms, burden of comorbidity, presence of anxio-depressive psychopathology, sleep quality and physical activity as a minimal set of assessments. Other aspects such as pain catastrophizing, kinesiophobia, dysregulation of affect, etc. may play an important role. At the moment there is however less consensus about their independent relevance and optimal tools for the assessment of these concepts. Similarly, the potential application of this additional pain-related data necessitates further evaluation. The individual use of the PRO information could lead to unnecessary fragmentation of the patient population. The use of patient response or patient characteristics patterns in form of phenotypes for subgroup analyses or treatment allocation though could support drug development. Pain-phenotyping could be specifically valuable to discriminate treatments without any effect on pain, from those that target specific pain processes.

QST allows valuable additional insights into pain processing. Necessary expertise, equipment and time for valid assessments may be challenging, thereby impacting the implementability of comprehensive QST protocols in large multicenter trials.

Nevertheless, future research may guide the construction of targeted somatosensory assessment-batteries based on their discriminative value e.g., in combination with PROs, which would allow a broad implementation and add relevant scientific value to OA trials.

One challenge has been the comparability of various PROs that focus on slightly different clinical domains. Georgopoulous et al. have recently demonstrated, that harmonized results of the 4 most widely used PROs for pain assessment produce similar patient acceptable symptom states and are thus comparable. [START_REF] Georgopoulos | Harmonising knee pain patient-reported outcomes: a systematic literature review and meta-analysis of Patient Acceptable Symptom State (PASS) and individual participant data (IPD)[END_REF] To increase our knowledge about pain-phenotypes from published and future studies, a similar concept to generally interpret and compare PRO results could be applied, leveraging established cut off values [START_REF] Burston | The impact of anxiety on chronic musculoskeletal pain and the role of astrocyte activation[END_REF] . Alternatively cut off values such as tertiles or quartiles of the original score range could be used. [START_REF] Carlesso | Pain Susceptibility Phenotypes in Those Free of Knee Pain With or at Risk of Knee Osteoarthritis: The Multicenter Osteoarthritis Study[END_REF] The latter approach is based on the assumption, that for a score e.g., ranging from 0-100 with 100 denoting high impact from a given pathology, people who score between 0-25 or 0-33 are less likely to be impacted, compared to those scoring between 66-100 or 75-100. While on a granular level, the different scores may convey different nuances of patient experience (and thus allow focus in a specific project), a separation in tertiles or quartiles in principle allows the clear identification of highly vs marginally affected individuals for comparison with other studies. This could also facilitate the implementation of systematic PRO-based assessments in clinical practice to allow individualized treatment approaches.

The legacy of numerous failed trials, the increasing cost pressure on healthcare systems, and the public and individual health burden of OA are concerning. Given the increase in mechanistic understanding, the field is under a certain pressure to develop medicines that address patients' symptoms and halt or reverse OA. One prerequisite for the development of worthwhile treatments is the establishment of clinical endpoints that provide a meaningful reflection of disease modification and long-term patient benefit. This can only be accomplished if we better understand and measure pain in OA which could also give further insights in the pain structure relationship. However, to achieve real progress, data need to be comparable. Systematic generation of data that allow OA pain-phenotyping may be one piece of the puzzle towards a "treatment of OA".

Table 1 : Summary of key OA pain-phenotyping studies

 1

	Version 1, 15.04.2023	1/18

Table

  Click here to access/download;Table;Table 1 rev.docx

	Reference Population Murphy et al. 58 2011 older adults (≥65y) with hip or knee OA and signs of primary fatigue Reference Population Finan et al. 59 2013 Baseline of study to evaluate psychological treatments in OA patients with/without insomnia Reference Population Egsgaard 60 et al. 2015 full spectrum from no clinical OA to clinical OA, randomly selected from pre-existing database 40-80y controls with no OA and little or no pain Reference Population Kittelson et al. 61 2016 OAI from the incident and progression cohort Kittelson et al. 62 2021 Recruitment from community (healthy elderly) and orthopaedic clinics (OA patients) 50-85y Reference Population Frey-Law et al. 63 2017 Baseline of TANK (TENS After New Knee) study NCT01364870 ≥30y scheduled for primary total knee joint replacement Reference Population Wright et al. 64 2017 adults with painful knee OA Reference Population Recruitment from community pain-free volunteers (≥50y) Pan et (healthy elderly) and al. 70 2019 orthopedic clinics (OA patients) 50-85y	Sample Size (for comparative studies n of OA patients : n of controls) 129 (69% knee OA) Sample Size (for comparative studies n of OA patients : n of controls) 113 Sample Size (for comparative studies n of OA patients : n of controls) 280 (216:64) Sample Size (for comparative studies n of OA patients : n of controls) 3494 183 (152:31) Sample Size (for Sample Size comparative studies n of OA patients : n of controls) (for Sample Size (for 218 comparative studies n of OA patients : n of controls) 120 (80:40) comparative studies n of OA patients : n of controls) Tasmanian Older Adult Cohort Study	Phenomenon relationship among pain, fatigue, and physical activity Phenomenon Association between self-reported levels of pain with measures of central sensitization in the absence of moderate-to-severe radiographic evidence of pathologic changes of knee OA Phenomenon Comparator Comparator Approach na Hierarchical agglomerative cluster analysis Cross sectional Community sample Comparator Approach na. cross-sectional multivariate general linear modeling Approach identification of knee pain profiles identification of marker patterns correlating to pain profiles non-OA knees largely independent of pain Principal Components Analysis (PCA) with clustering using Ward's method with squared Euclidean distance Phenomenon Comparator Approach Knee OA pain-phenotypes based on 1) knee OA pathology 2) psychological distress 3) altered pain neurophysiology 4) relation to patient characteristics na Latent Class Analysis cross sectional cluster analysis (4-year follow-up visit) with some longitudinal information Knee OA pain-phenotypes based on 1) multimorbidity 2) psychological distress 3) pain sensitivity healthy community dwelling elders Latent Profile Analysis Cross sectional Community sample 4) knee impairment or pathology Phenomenon Comparator Approach QST pain sensitivity profiles in advanced knee OA na Principal Components Analysis (PCA) and Principal Axis Factoring (PAF) with clustering using Ward's method with squared Euclidean distance Phenomenon Comparator Approach widespread cold, pressure, and heat hyperalgesia pain free control OA patients Standard statistics Phenomenon Comparator Approach 963 knee pain-Latent Class phenotypes in Analysis an older in OA patients differences in QST measures, levels of pain, with and without wide-spread cold hyperalgesia Cross sectional Community sample population Cross sectional Community sample pain characteristics, and perceived function in patients with wide-spread cold hyperalgesia	Evaluation Patient Characteristics Brief Fatigue Inventory WOMAC -5 times daily NRS pain assessment -Illness burden (41 somatic symptoms) -Timed up-and-go test -Activity measured via Actiwatch -Pittsburgh Sleep Quality Index (PSQI) -Center for Epidemiologic Studies Depression Scale (CES-D) Evaluation Patient Characteristics  STAI  CES-D  PCS  PSQI  Radiographic disease severity (Kellgren/Lawrence)  QST  PPT  CPT  Mechanical phasic pain  Thermal phasic pain  Sensitivity to tonic pain  CPM Evaluation Patient Characteristics  OA grade  Comorbidities  Number of painful joints  Pain duration  Pain localization  WOMAC  Lequesne functional index  EQ-5D  Pain catastrophizing  QST  PPT  TS  CPM  Biomarkers  VICM  CIM  CRP  CRPM  CIIIM Evaluation Patient Characteristics  Numeric Pain Rating Scale (NPRS)  WOMAC  Radiographic severity of knee OA  MVIC  Tenderness of the knee joint  Modified Charlson Comorbidity Index  Number of pain sites (as surrogate for central sensitization)  CES-D  Modified version of the coping strategies questionnaire-catastrophizing subscale  20-meter timed walking test at self-selected walking speed  Health seeking behavior (unstructured question) Patient Characteristics  Visual analog scale (VAS)  WOMAC pain  ICOAP  Normalized knee extensor strength at maximum voluntary isometric contraction (MVIC)  Functional Comorbidity Index (FCI)  Pain Catastrophizing Scale (PCS)  Walking speed  Health seeking behaviour (unstructured question)  Symptom duration  QST  PPT (target knee) Tampa Scale for Kinesiophobia, modified Charlson Population Characteristics 61% female Age: 72.2 (+/-9.8), range 65 to 90 y BMI: 30.5 +/-5.9 kg/m 2 , range 21.5 to 49.9 Self-reported duration of pain (months) 132.1 (146.5) range 0 to 708 BFI total 4.5 (2.0) range 0.25 to 8.75 WOMAC pain 7.9 (3.4) range 2 to 20 WOMAC stiffness 3.3 (1.7) range 0 to 8 WOMAC disability 20.9 (10.3) range 3 to 42 Population Characteristics 66.7% female Age: 61.05 +/-8.93 y BMI: 30.94 +/-5.85 kg/m 2 Population Characteristics 64% female Age: 61.7 +/-10.0 y BMI: 33.9 +/-7.0 kg/m 2 Population Characteristics OA 59.2% female Age: 64.9 +/-9.0 y BMI: 28.9 +/-5.0 kg/m 2 similar symptom duration and health seeking behavior OA 64.5% female, control 64.5% female Age: OA 65.2 +/-8.5 y, control 64.9 +/-9.0 y BMI: OA 30.2 +/-6.0 kg/m 2 , control 26.7 +/-4.6 kg/m 2 similar symptom duration and health seeking behavior Comorbidity Index, PPT (regional/distant), Radiographic Severity of Knee Osteoarthritis, CES-D evaluated but excluded based on weaker correlation with pain intensity (Spearman correlations) Evaluation Population Characteristics Patient Characteristics 54.6% female (50% in control group) Age: not reported BMI: not reported Evaluation Population Characteristics Patient Characteristics OA 55% female, control 60% female Evaluation Population Characteristics Patient Characteristics 50% female (sampling strategy) Age: OA 64, range 50 to 86 y; control 64, range Age: 62.8 +/-7.4 y BMI: 27.7 +/-4.6 kg/m 2 51 to 86 y OA 38% obese,  HPT  WDT  CPT  CDT  PPT  QST  Pain intensity (rest and movement) via 21-point NRS  Pain duration  Analgesic medication  State-Trait Anxiety Inventory (STAI) anxiety subscale  Geriatric Depression Scale (GDS), 5-item version  PCS  KOOS  SF-36  QST  PPT  HPT and HPTol  Punctate Pain Intensity via VAS  TS via tonic heat stimulus control 10% obese Brief Fatigue Inventory BFI total 4.5 (2.0) range 0.25 to 8.75 OA WOMAC pain, 18.5/50 WOMAC OA WOMAC (PQAS) assessment scale  Pain quality  PainDETECT function, 53.4/250 Short-Form Health Survey (SF-36) 43.75% (n=35) cut off based on 12.25°C cold hyperalgesic	Phenotypes/Groups no significant differences in patient characteristics Cluster I: 36% highest scores on all measures -high stiffness, high disability, TUG 13.5 +/-8.9 Cluster II: 30% subclinical depression, moderate fatigue, moderate illness burden, overall low pain, low sleep disturbance -stiffness moderate, disability low, TUG 10.5 +/-2.1 s Cluster III: 34% relevant sleep disturbance, mild pain, low fatigue and depression scores, low illness burden -low stiffness, moderate disability, TUG 10.2 +/-2.3s Phenotypes/Groups Low pain/low knee OA grade (21.2%): overall lowest BMI High pain/high knee OA grade (28.3%): reduced distant (and local) PPT vs low pain groups, high rate of depression, anxiety, sleep disturbance and pain catastrophizing, overall highest BMI Low pain/high knee OA grade (23.90%): overall oldest group High pain/low knee OA grade (26.5%): significantly increased pain response to distant mechanical phasic stimuli and thermal phasic pain compared to high knee OA groups, reduced distant PPT vs low pain groups, high rate of depression, anxiety, sleep disturbance and pain catastrophizing, overall youngest group no differences in CPM or QST measures locally, education and income as significant covariates Phenotypes/Groups Principal components: PC1: physical health questionnaires PC2: peripheral, central, and spreading sensitization, PC3: biochemical markers, PC4: pain catastrophizing, PC5: temporal summation. Profile A (12.5%): moderate impact on WOMAC/ Lequesne, low to moderate catastrophizing, near normal TS, high CPM and PPT as potential sign of resilience, still reduced QoL Profile B (27.3%): moderate impact on WOMAC/ Lequesne, low to moderate catastrophizing, near normal TS, moderate CPM but reduced PPT, reduced QoL Profile C (39.4%): moderate impact on WOMAC/ Lequesne, low to moderate catastrophizing, increased TS, reduced CPM and PPT, reduced QoL, CRP near normal Profile D (18.9%): higher impact on WOMAC and especially Lequesne, increased catastrophizing, increased TS, reduced CPM and PPT, reduced QoL Profile E (1.9%): outlier cluster, not reported in detail controls low impact on WOMAC/Lequesne, moderate CPM and PPT, low TS Phenotypes/Groups Class 1: on average older than all other classes, higher proportion of females, slowest walking speed, high level of comorbidities Class 2: on average older than class 3/4, high levels of knee joint tenderness, weak extensor strength and high proportion of pes anserine tenderness Class 3: highest pain level, psychological distress, highest number of painful sites and more severe radiographic OA Class 4: mild radiographic OA, low levels of pain and comorbidity, highest average extensor strength Group 1 (9% of pt with knee pain): characterized by high FCI scores (upper gastrointestinal, osteoporosis, heart disease, asthma), slower walking speed than group 2/4 ("weakness and heightened pain sensitivity pain levels also in KOOS, hyperalgesia 47.50% hyperalgesia, correlation with multimorbidity") Group 2 (63% of pt with knee pain): low PCS and FCI (vs group 1 and 3), higher target knee PPT and lower extensor strength than healthy elderly or group 4 ("weakness and heightened pain sensitivity") Group 3 (11% of pt with knee pain): characterized by pain catastrophizing, higher pain ratings than group 2/4 ("weakness and heightened pain sensitivity with pain associated distress") Group 4 (17% of pt with knee pain): characterized by high PPT vs all other groups, otherwise similar to healthy elderly, highest proportion of pt with previous knee surgery or trauma ("normal strength, low pain sensitivity") at rest, gait and range of index knee, 37.50% between cold hyperalgesia Phenotypes/Groups Low Pain Sensitivity Profile (18.3%): low QST based standardized pain sensitivity before and after adjustment for age and sex Average Pain Sensitivity Profile (38.5%): average movement no relevant impact from other assessments apart from sex. Men were allocated predominant in low pain sensitivity cluster. After adjustment higher pain sensitivity for non-white and/or hispanic individuals Phenotypes/Groups no significant differences in patient characteristics contralat. knee, 43.75% distant site) Cold hyperalgesic vs and PainDETECT scores and surface and paradoxical subscores in Phenotypes/Groups pain quality assessment scale Class 1 (25%): highest non-hyperalgesic OA proportion of females, on patients: average more emotional sign. lower cold detection and cold pain threshold at all sites cold-hyperalgesic vs non-cold hyperalgesic problems, higher burden of comorbidity, more severe knee pain and more painful sites, lower OA patients, no difference knee structural damage, QST based standardized between non-hyperalgesic lower education pain sensitivity, after adjustment for age and sex more pronounced difference in PPT and HPT vs low pain sensitivity cluster High Pain Sensitivity Profile temporal summation (20.6%): isolated high TS with low values for other qualities, effect pronounced after adjustment High Pain Sensitivity Profile high heat and pressure pain (17.9%): before adjustment, after adjustment similar to average pain sensitivity cluster with TS as main discriminator, higher pain levels than pure TS cluster also in KOOS, at rest, gait High Pain Sensitivity Profile high punctate pain (4.5%): average for all qualities especially after adjustment except punctate pain with highest sign. higher overall WDT 17.50% distant site; heat patients with cold 17.50% contralat. knee, pain and disability in 11.25% index knee, patients, higher WOMAC HPT (heat hypoesthesia hyperalgesia in OA in OA, no differences in based on cold no differences in SF36 and range of movement OA patients vs pain-free Class 2 (20%): more controls males, higher level of sign. lower warmth education, fewer painful detection threshold at sites or structural knee index knee and distant site abnormalities, lower levels (cold hyperalgesic patients of pain OA vs pain free: sign. higher index knee PPT in OA (pressure hyperalgesia: 22.50% index knee, 16.25% contralat. knee, 3.75% vs all others), sign. lower Class 3 (50%): overall warmth detection lowest prevalence of knee threshold at contralateral pain, comorbidities, knee (cold hyperalgesic radiographic OA, structural patients vs pain free damage and low BMI controls, but not vs other OA patients), lower heat distant site) sign. higher CDT at index and contralat. knee (cold hypoesthesia 11.25% index knee, 17.50% contralat. knee, 17.50% distant site) distant site contralat. knee, 43.75% no sign. difference at index knee, 37.50% and contralat knee PPT, hyperalgesia 47.50% sign. higher index knee distant site; cold and controls. cold hyperalgesic patients no difference between pain threshold at all sites consistently WOMAC (cold hyperalgesic patients vs other OA patients), but and	Commented [MB2]: Maybe rephrase to avoid misunderstanding. As I understand it, men are predominant in the low pain sens. cluster. Commented [MB1]: Especially?
	Pain in knee OA Pain in knee OA Pain in knee OA Pain in knee OA Pain in knee OA Pain in knee OA Pain in knee OA Pain in knee OA Pain in knee OA Pain in knee OA Pain in knee OA		Version 1, 15.04.2023 Version 1, 15.04.2023 Version 1, 15.04.2023 Version 1, 15.04.2023 Version 1, 15.04.2023 Version 1, 15.04.2023 Version 1, 15.04.2023 Version 1, 15.04.2023 Version 1, 15.04.2023 Version 1, 15.04.2023 Version 1, 15.04.2023 Version 1, 15.04.2023	2/18 3/18 4/18 5/18 6/18 7/18 8/18 9/18 10/18 11/18 12/18 13/18

painful sites Class 1 > Class 2 > Class 3 over average 10.7 y

			Sample Size Sample Size Sample Size
			(for (for (for
	Reference Population Reference Population Reference Population	comparative studies n of OA comparative studies n of comparative studies n of OA	Phenomenon Phenomenon Phenomenon	Comparator Comparator Comparator	Approach Approach Approach	Evaluation Evaluation Evaluation	Population Characteristics Population Characteristics Population Characteristics	Phenotypes/Groups Phenotypes/Groups Phenotypes/Groups
			patients : n of OA patients : patients : n of
			controls) n of controls) controls)
	Burston Carlesso Carlesso	participants MOST orthopaedic	230 (130:100) 852 343	associations pain Pain-phenotype	Non-OA na na	Spearman Latent Class Latent Class	Patient Characteristics Patient Characteristics Patient Characteristics	OA 61.9% female, 55% female 63% female	Impact of anxiety (25% Pain susceptibility Class 1 (49%): overall low
	et al. 71 2019 et al. 65 2019 et al. 66 2022	from a community-based cohort study ≥40y population 50-79y having/at risk of developing knee OA without persistent knee pain specialist confirmed diagnosis of OA ≥40y	3274 for impact of anxiety (351 anxiety at baseline) on incident knee pain at 12 months 3767 for impact of knee pain (1020 with baseline knee pain) on incident anxiety at 12 months	between knee pain, pain spread, anxiety, and depression susceptibility phenotype (PSP) based on development of persistent pain at 2 years identification based on IMMPACT criteria	patients	correlation and linear regression Analysis observational longitudinal Analysis observational longitudinal	 WOMAC pain  Number of painful sites  MRI characteristics (cartilage defects, bone marrow lesions, effusion-synovitis)  Radiographic presence of knee OA  Education level  Single mental health item from the short form-8  4-item comorbidity questionnaire (heart attack, diabetes, hypertension, rheumatoid arthritis)  HADS  Intermittent and Constant Osteoarthritis Pain scale (ICOAP)  Numeric Rating Scale (NRS)  Widespread pain index (WPI)  QST  PPT  TS  Coping Strategies Questionnaire (single item for pain catastrophizing)  OA severity (Kellgren- -CES-D Lawrence)  QST  PPT	control 58.2% female Age: OA 60.27 +/-9.61 y; control 63.06, +/-8.88 y BMI: OA 27.1 +/-4.56 kg/m 2 , control 30.09 +/-6.62 kg/m 2 Age: 67y BMI: 29.5 kg/m 2 Age: 64y BMI: 32kg/m 2	of population) anxiety sign. associated with all pain measures and PPTs after adj. for depression odds ratio (OR) for incident knee pain at 12 months in patients with anxiety 1.71 (adj. for depression) OR for incident anxiety at 12 months in patients with knee pain 1.18 (after adj. for depression) OR for incident anxiety at 12 months in patients with depression 3.20 Impact of depression (10% of population) OR for incident knee pain for anxiety) analyzed with depression 1.66 (adj. in other aspects at 12 months in patients phenotypes (PSP) PSP 1 (34%): pressure scores in all assessed measures (i.e. low severity) or marginal signs pain sensitivity (~16-of central sensitization 26%), facilitated TS (33-according to QST 35%) Class 2 (40%): overall PSP 2 (31%): pressure moderate scores in pain sensitivity (0-6%), assessed measures, but facilitated TS (2-10%), high pain variability, mixed 22% non-caucasian QST values PSP 3 (23%): pressure pain sensitivity (75-89%), facilitated TS (53-58%), 74% female, higher risk of developing incident knee pain PSP 4 (12%): pressure pain sensitivity (0-4%), facilitated TS (82-90%), 26% female, 23% non-Class 3 (11%): overall highest scores in assessed measures (except pain variability), QST values for PPT patella, TS, cold pain and CPM heat pain as indicator of relevant central sensitization caucasian, mean age 70% decreasing function no relevant differences from
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class 1 to class 3 considering walk fast and climb stairs, no significant difference for sit stand

  Body Mass Index; BFI: Brief Fatigue Inventory; CES-D: Center for Epidemiologic Studies Depression Scale; FCI: Functional Comorbidity Index; GDS: Geriatric Depression Scale HADS: Hospital Anxiety and Depression Scale; ICOAP: Intermittent and Constant O steoarthritis Pain scale KOOS: Knee Injury and Osteoarthritis Outcomes Score; MVIC: Normalized knee extensor strength at maximum voluntary isometric contraction; NRS: Numeric Rating Scale; NPRS: Numeric Pain R ating Scale PCS: Pain Catastrophizing Scale; PRIME-MD: Primary Care Evaluation of Mental Disorders; PQAS: Pain quality assessment scale; PSQI: Pittsburgh Sleep Quality Index; S F-36: Short-Form Health Survey; STAI: State-Trait Anxiety Inventory; WOMAC: Western Ontario and McMaster Universities Osteoarth ritis Index; QST: Quantitative Sensory Testing; CDT: Cold Detection Threshold; CPT: Cold Pain Threshold; CPM: Conditioned pain modulation; HPT: Heat Pain Threshold; HPTol: Heat Pain Tolerance; PPT: Pressure Pain Threshold; TS: Temporal Summation; WDT: Warmth Detection Threshold CIM: Collagen I Metabolite; CIIIM: Sollagen III Metabolite; CRP: C-Reactive Protein; CRPM: C-Reactive Protein Metabolite; VICM: Citrullinated Vimentin Fragment;

		 PPT
	Abbreviations: pt: patients BMI:	 TS  CPT  HPT  CPM (Conditioned
		pain modulation)
		 Self-report Charlson
		comorbidity index
		 Life Orientation Test-
		Revised scale
		(dispositional
		optimism)
		 Chronic Pain Self
		Efficacy Scale
		 Kellgren-Lawrence
		grade
		 Knee Injury and
		Osteoarthritis
		 Modified Pain Detect Outcomes Score
		Questionnaire (KOOS) activities of
		 Hospital Anxiety and daily living subscale
		Depression Scale  Core measures of
		(HADS) functional performance
		(1) transition from sit to  Patient Health stand, 2) walk fast and Questionnaire-15 (self-administered version of 3) climb stairs
		the Primary Care  Healthcare Utilization
		Evaluation of Mental (via provincial
		Disorders (PRIME-MD) insurance system in
		diagnostic instrument) one vicinity)
		 Pain Catastrophizing
		Scale
		 Multidimensional
		Fatigue Inventory
		 Pain variability (NRS 3
		times via text for a week)  Average pain intensity (NRS, recall 1 week)  Pittsburgh Sleep Quality Index (PSQI)	increasing health care utilization of 44% and 240% for class 2 and 3 respectively compared to class 1
		 Short form McGill Pain
		Questionnaire 2
		 QST
	Pain in knee OA Pain in knee OA	Version 1, 15.04.2023 Version 1, 15.04.2023	17/18 18/18

Acknowledgements

We thank Shafaq S Shaikh for her help in compiling the various patient reported outcome measures and the insightful discussions including also Christel Naujocks and Daniel Kuessner. We also thank all colleagues, labs and patients who by their work and trial participation helped to generate these insights.

Franziska Saxer is employee and shareholder of Novartis, she is affiliated to the University Basel and member of the European Union Medical Devices -Expert Panel section Orthopaedics, traumatology, rehabilitation, rheumatology Alexa Hollinger is a medical fellow at Novartis, she is affiliated with the University Hospital Basel and the University of Basel. Martin Flores Bjurström has no competing interests to declare Philip G Conaghan reports consultancies or speakers bureaus for AbbVie, AstraZeneca, EliLilly, GlaxoSmithKline, Grunenthal, Janssen, Levicept, Merck, Novartis, Pfizer, Stryker and UCB. Philip G Conaghan is supported in part through the NIHR Leeds Biomedical Research Centre. The views expressed are those of the author and not necessarily those of the NHS, the NIHR or the Department of Health. Tuhina Neogi reports consultancies for Pfizer-Eli Lilly, Novartis Matthias Schieker is employee and shareholder of Novartis and owner LivImplant GmbH Francis Berenbaum reports consultancies from AstraZeneca, Grunenthal, GSK, Eli Lilly, Nordic Bioscience, Novartis, Pfizer, Servier, Peptinov, 4P Pharma, 4Moving Biotech. Honoraria for lectures from Pfizer, Viatris. Stock owner of 4Moving Biotech

Abbreviations

ADAMTS5

Author contributions

AH and FS have collected the information for the tables and performed literature research. All authors have been involved in the analysis and interpretation of the data and contributed to the final manuscript.

Role of the funding source

The manuscript has been developed as part of a medical fellowship by AH founded by Novartis Biomedical Research. Also, FS and MS have received salaries from Novartis during the work on this manuscript.

The funder had no influence on the study design, data interpretation or publication strategy.

Competing interests


Sarah Tournayre

Olivier Mathieu

Maxime Villiet

Noemie Besnard

Vincent Brunot

Delphine Daubin

Laura Platon

Philippe Corne

Kada Klouche

Romaric Larcher
email: .romaric.larcher@chu-nimes.fr

Factors associated with meropenem pharmacokinetic/pharmacodynamic target attainment in septic critically ill patients treated with extended intermittent infusion or continuous infusion

Keywords: Meropenem Continuous infusion Extended intermittent infusion Pharmacokinetic ICU AKI, acute kidney injury, BMI, body mass index, CI, continuous infusion, Cmin, trough concentration, Css, steady-state concentration, CVVHD, continuous veno-venous haemodialysis, CVVHDF, continuous venovenous haemodiafiltration, eGFR, estimated glomerular filtration rate, EII, extended intermittent infusion, EUCAST, European Committee on Antimicrobial Susceptibility Testing, ICU, intensive care unit, IHD, intermittent haemodialysis, IQR, interquartile range, KDIGO, Kidney Disease: Improving Global Outcomes, MIC, minimum inhibitory concentration, PD, pharmacodynamic, PK, pharmacokinetic, SAPS, Simplified Acute Physiology Score, SLED, sustained low-efficiency dialysis, SOFA, Sequential Organ Failure

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Antimicrobial resistance and carbapenem consumption are in-creasing worldwide [START_REF] Klein | Global increase and geographic convergence in antibiotic consumption between 20 0 0 and 2015[END_REF] . Among the carbapenems, meropenem is one of the most commonly used in the treatment of severe healthcare-associated infections and, to a lesser extent, community-acquired infections [2] . Meropenem mainly displays timedependent killing [START_REF] Williams | Pharmacokinetics/Pharmacodynamics of β-Lactams and Therapeutic Drug Monitoring: From Theory to Practical Issues in the Intensive Care Unit[END_REF] ; therefore, in intensive care units (ICUs), either extended intermittent infusion (EII) or continuous infusion (CI) of meropenem are recommended to optimise the time during which the plasma concentration of meropenem is maintained above the minimum inhibitory concentration (MIC) of the treated microorganism(s) [START_REF] Alffenaar | Antimicrobial therapeutic drug monitoring in critically ill adult patients: a Position Paper#[END_REF] .

Optimal management of patients with sepsis and septic shock requires the earliest initiation of broad-spectrum antibiotic therapy in a regimen that achieves effective therapeutic concentrations [ 4 , 5 ]. In this setting, the choice of the best administration modality may influence patient outcomes [ 4 , 5 ]. Population PK analysis with Monte Carlo simulations suggested that CI outperforms EII in achieving the pharmacokinetic/pharmacodynamic (PK/PD) target in ICU patients [ 6 , 7 ]; however, data comparing both regimens are scarce in routine practice.

This study aimed to determine the meropenem plasma concentrations achieved in critically ill septic patients treated with CI and EII of meropenem, and to assess the factors independently associated with attainment of the PK/PD target and toxicity threshold.

Material and Methods

Study Design and Settings

A single-centre, retrospective, observational study was con-ducted in the 20-bed medical ICU of the Montpellier University Hospital, France. All consecutive patients admitted to the ICU be-tween 1 January 2019 and 31 March 2020 and who had at least one meropenem therapeutic drug monitoring (TDM) were screened using the Pharmacology and Toxicology Department database; each patient's medical chart was then reviewed (by ST and RL). The study included adult patients treated with meropenem for severe infections [START_REF] Evans | Surviving sepsis campaign: international guidelines for management of sepsis and septic shock 2021[END_REF] , and when a patient had more than one meropenem plasma concentration measurement, only the first meropenem concentration was considered. The study excluded patients aged < 18 years, who were pregnant or who did not fulfil the criteria of the TDM sampling protocol (see below).

Meropenem Dosage Regimens

The meropenem (Meropenem Arrow®, Arrow Laboratories, Lyon, France) dosage regimen was selected by the treating physician. During the study period, meropenem was administered by either: (i) EII, 2 g over 5 h, three times/day, or (ii) CI, 1g over 4 h, six times/day. Patients treated with CI received an initial loading dose of 2 g over 30 min. All patients were treated with one of these two dosage regimens for at least 24 h, then the treating physician could make dosage adjustments based on the estimated glomerular filtration rate (eGFR) as recommended [START_REF] Thalhammer | Pharmacokinetics of Meropenem in Patients with Renal Failure and Patients Receiving Renal Replacement Therapy[END_REF] . Glomerular filtration rates were estimated by creatinine clearance measured by 24-h UV/P creatinine at the time of TDM [START_REF] Silva | Recommended Antibiotic Dosage Regimens in Critically Ill Patients with Augmented Renal Clearance: A Systematic Review[END_REF] . Augmented renal clearance was defined as an eGFR > 130 mL/min/1.73 m 2 with a normal serum creatinine [START_REF] Silva | Recommended Antibiotic Dosage Regimens in Critically Ill Patients with Augmented Renal Clearance: A Systematic Review[END_REF] .

Meropenem Therapeutic Drug Monitoring Sampling Protocol

Blood samples were taken 15-30 min before the start of the next infusion (i.e. at trough or Cmin) in patients treated with EII, and at least 24 h after the start of CI (i.e. at steady-state or Css, actually reached in 5 h) in patients treated with CI. Sampling was performed within 2 days of treatment initiation or a change in dosage regimen. The Pharmacology and Toxicology Department measured meropenem plasma concentrations using an ultra-high performance liquid chromatography method adapted from Legrand et al. [START_REF] Legrand | Simultaneous Determination of Eight β-Lactam Antibiotics, Amoxicillin, Cefazolin, Cefepime, Cefotaxime, Ceftazidime, Cloxacillin, Oxacillin, and Piperacillin, in Human Plasma by Using Ultra-High-Performance Liquid Chromatography with Ultraviolet Detection[END_REF] . Total plasma concentrations of meropenem were considered equivalent to free concentrations [ 4 , 11 ].

Meropenem Pharmacokinetic/Pharmacodynamic Target

The recommended PK/PD target of meropenem plasma concentration in critically ill patients is 50-100% of the dosing interval with a free drug concentration above the MIC of the treated microorganism (50% ƒT > MIC to 100% ƒT > MIC ) [START_REF] Alffenaar | Antimicrobial therapeutic drug monitoring in critically ill adult patients: a Position Paper#[END_REF] . However, most evidence suggests an optimal PK/PD target at a free concentration above four-fold or even five-fold the MIC throughout the whole dosing interval (100% ƒT > 4xMIC to 100% ƒT > 5xMIC ) [START_REF] Alffenaar | Antimicrobial therapeutic drug monitoring in critically ill adult patients: a Position Paper#[END_REF] .

As no exact MIC values were available, the critical MIC break-point for susceptibility of Enterobacterales and Pseudomonas aeruginosa to meropenem of 2 mg/L was used to calculate PK/PD target according to the European Committee on Antimicrobial Susceptibility Testing (EUCAST) [START_REF]Breakpoint tables for interpretation of MICs and zone diameters[END_REF]. Hence, PK/PD target attainment was defined as Cmin or Css ≥10 mg/L corresponding to 100% ƒT > 5xMIC. As recommended, this study determined a toxicity threshold as Cmin or Css > 50 mg/L [START_REF] Alffenaar | Antimicrobial therapeutic drug monitoring in critically ill adult patients: a Position Paper#[END_REF] .

Data Collection

Demographic, clinical, pharmacological and laboratory data were collected from patient's medical charts. Patient conditions were evaluated using the Charlson Comorbidity Index and disease severity by calculation of the Simplified Acute Physiology Score II (SAPS II) and Sequential Organ Failure Assessment (SOFA) score, 24 hours after ICU admission. Acute kidney injury (AKI) was defined according to the 2012 Kidney Disease: Improving Global Outcomes (KDIGO) guidelines.

Statistical Analysis

Quantitative data were described as median and interquartile range (IQR) and qualitative data as number and percentage. The study population was divided according to the attainment (Cmin or Css ≥10 mg/L) or nonattainment (Cmin or Css < 10 mg/L) of meropenem PK/PD target concentrations. Categorical variables were compared using χ2 tests and continuous variables were com-pared using the Wilcoxon test or the Fisher's exact test, as appropriate.

Factors independently associated with the attainment of meropenem PK/PD target concentrations were assessed using a logistic regression model. A conditional stepwise regression was per-formed to select the most informative variables (with P ≤0.2 in the univariate analysis for entry into the model). Finally, given the small number of events, the three most clinically relevant and statistically significant variables in the univariate analysis were selected for inclusion in the multivariate analysis. Interactions and correlations between explanatory variables were carefully checked for. The same analyses were performed to determine the factors associated with the attainment of the toxicity threshold (Cmin or Css > 50 mg/L).

All tests were two-sided and P-values < 0.05 were considered statistically significant. Analyses were performed using R software version 4.2.0 (The R Foundation for Statistical Computing, Vienna, Austria).

Results

Patients

Among 1125 patients admitted during the study period, 110 pa-tients had at least one meropenem TDM representing 238 blood samples. Of these, 70 patients were included in the analysis (Supplementary Figure ). The characteristics of the study population are presented in Table 1.

Most of the patients (69%) included in the study were men and the median age and body mass index of the population was 69 years (IQR 60-74) and 24 kg/m 2 (IQR 22-29), respectively. At ICU admission, the median Charlson index, SOFA score and SAPS II were 4 (IQR 3-7), 6 (IQR 5-10) and 54 (IQR 41-67), respectively.

Sepsis and septic shock ( n = 65; 93%) were the main reasons for ICU admission, and pneumonia ( n = 37; 53%) and urinary tract infection ( n = 13; 19%) were the main sources of infection. Thirty-five of the patients (50%) had AKI, including 13 who required renal replacement therapy, whereas 13 (19%) had augmented renal clearance.

Characteristics of patients treated with EII and CI were balanced, except for eGFR: 30 mL/min/m 2 (IQR 30-84) vs. 79 mL/ min/m 2 (IQR 30-124), and renal replacement therapy requirement: 11 patients (33%) vs. 2 (5%), respectively.

Pharmacokinetic/Pharmacodynamic Target Attainment

Characteristics of meropenem TDM are summarised in Table 2 . Meropenem was administered by CI at a median dose of 6 g over 24 h (IQR 6-6) in 37 patients, and by EII at a median daily dose of 3 g (IQR 2-4), divided into three doses/day (IQR 2-3) administered over 5 hours (IQR 5-5) in 33 patients. Among the patients treated with EII, 21 (64%) achieved the target concentration, whereas 31 (97%) of those treated with CI achieved it (P < 0.001). The median plasma concentration of meropenem was Cmin = 16 mg/L (IQR 8, 23) in the EII group and Css = 34 mg/L (IQR 27, 49) in the CI group (P < 0.001).

In detail, among the 13 patients (19%) who were below the meropenem concentration target of 10 mg/L, 12 received EII ( P < 0.001) and 10 had a daily dose of meropenem < 40 mg/kg, including eight patients who had an eGFR < 50 mL/min. Of the remaining five patients who did not reach the meropenem concentration target, all had an eGFR > 120 mL/min, including one patient treated with CI of meropenem.

On the contrary, nine of 11 patients with meropenem con-centration above the toxicity threshold were in the CI group ( P = 0.036). However, three patients were reported with neurological adverse effects attributable to meropenem: two in the EII group and one in the CI group ( P = 0.6) with meropenem concentrations at 9 mg/L, 21 mg/L and 53 mg/L, respectively. Four patients treated with EII and 10 treated with CI died ( P = 0.12), bringing the ICU mortality rate to 20.0% (14 patients).

As illustrated in Figure 1, those treated with CI were at greater risk of overdose, whereas those treated with EII were at risk of meropenem underdosing. The meropenem plasma concentrations were also modified by the eGFR and the dose/kg of total body weight. Finally, 27 patients treated with CI (73%) and 19 treated with EII (58%) were within the therapeutic range (10-50 mg/L) of meropenem plasma concentrations (P = 0.2).

3.3.

Factors Associated With Attainment of Meropenem Pharmacokinetic/Pharmacodynamic Target Concentrations and Attainment of Meropenem Concentrations Above the Toxicity Threshold

The results of univariate and multivariable analyses are presented in Supplementary Tables 1 and2. A meropenem dose > 40 mg/kg of total body weight (OR 12.23, 95% CI 1.76-197.0; P = 0.03) and administration by continuous infusion (OR 16.28, 95% CI 2.05-407.5) were independent factors associated with attainment of meropenem PK/PD target concentrations, whereas higher eGFR was associated with non-attainment of 10 mg/L (OR 0.98, 95% CI 0.97-0.99; P = 0.02). The only factor independently associated with the attainment of meropenem concentrations above the toxicity threshold was daily dose of meropenem > 70 mg/kg of total body weight (OR 35.5, 95% CI 5.61-410.3; P < 0.001).

Discussion

This study reported the results of a single-centre retrospective cohort study including 70 septic ICU patients. It found that meropenem CI more frequently achieved the therapeutic target than EII. Furthermore, it found that meropenem CI was independently associated with achievement of the PK/PD target. Other factors associated with achieving the PK/PD target were lower eGFR and daily meropenem dose ≥40 mg/kg, whereas daily dose > 70 mg/kg was independently associated with plasma concentrations above the toxicity threshold. Some have pointed out that ca. one-third of the ICU patients treated with intermittent bolus infusion of meropenem failed to achieve the lowest PK/PD targets 100% ƒT > 2 mg/L [START_REF] Scharf | Therapeutic Drug Monitoring of Meropenem and Piperacillin in Critical Illness-Experience and Recommendations from One Year in Routine Clinical Practice[END_REF]. Thus, CI and EII have been proposed to limit the risk of underdosing [ 4 , 6 , 7 , 13 ]. The current study found that CI outperforms EII in real-world situations, as it has been suggested in population PK analysis with Monte Carlo simulations [ 6 , 7 ], confirming that CI is the best dosing strategy to increase the likelihood of achieving the most optimised PK/PD targets, especially in ICU patients [START_REF] Scharf | Therapeutic Drug Monitoring of Meropenem and Piperacillin in Critical Illness-Experience and Recommendations from One Year in Routine Clinical Practice[END_REF] . The cur-rent study also reported plasma meropenem concentrations ranging 8-100 mg/L after CI, whereas concentrations ranged from < 2 mg/L to > 100 mg/L after EII. This finding is consistent with data [START_REF] Silva | Recommended Antibiotic Dosage Regimens in Critically Ill Patients with Augmented Renal Clearance: A Systematic Review[END_REF] showing that CI provides more predictable antibiotic PK profiles by limiting the impact of changes in volume of distribution and/or renal clearance.

Another interesting result was the high median meropenem concentration at 34 mg/L (IQR 27, 49) achieved in CI-treated patients that enabled optimising the treatment of bacteria classified 'susceptible, increased exposure' to meropenem or even treating bacteria classified 'resistant' according to the EUCAST breakpoint [START_REF]Breakpoint tables for interpretation of MICs and zone diameters[END_REF] . Using CI, a PK/PD target of 100% ƒT > 4xMIC [START_REF] Alffenaar | Antimicrobial therapeutic drug monitoring in critically ill adult patients: a Position Paper#[END_REF] was achievable for the treatment of bacteria with MICs up to 8 mg/L or even MICs up to 16 or 32 mg/L [START_REF] Cojutti | Population Pharmacokinetics of High-Dose Continuous-Infusion Meropenem and Considerations for Use in the Treatment of Infections Due to KPC-Producing Klebsiella pneumoniae[END_REF] for PK/PD targets ranging from 50% ƒT > MIC to 100% ƒT > MIC [START_REF] Alffenaar | Antimicrobial therapeutic drug monitoring in critically ill adult patients: a Position Paper#[END_REF] . Clinicians might then reconsider the use of meropenem to treat infections due to Enterococcus faecalis (MIC range 2-16 mg/L) and anaerobic bacteria [START_REF]Breakpoint tables for interpretation of MICs and zone diameters[END_REF], whereas imipenem was considered superior in these indications [2]. Similarly, meropenem CI may be an attractive dosing strategy for treating carbapenemase-producing Enterobacterales [START_REF] Cojutti | Population Pharmacokinetics of High-Dose Continuous-Infusion Meropenem and Considerations for Use in the Treatment of Infections Due to KPC-Producing Klebsiella pneumoniae[END_REF].

As with the mode of administration, the total daily dose matters and the current results suggest that a meropenem dose range of 40-70 mg/kg/day (maximum 6 g/day) may limit the risk of underdosing and overdosing. Clinicians should be careful with obese patients [START_REF] Alobaid | Effect of Obesity on the Population Pharmacokinetics of Meropenem in Critically Ill Patients[END_REF] ; however, a lower antibiotic daily dose may be appropriate with CI [START_REF] Dulhunty | Continuous betalactam infusion in critically ill patients: the clinical evidence[END_REF] or in those with lower eGFR [ 13 , 16 ]. In agreement with these results, Scharf et al. [START_REF] Scharf | Therapeutic Drug Monitoring of Meropenem and Piperacillin in Critical Illness-Experience and Recommendations from One Year in Routine Clinical Practice[END_REF] reported that patients with eGFR < 70 mL/min did not benefit most from CI administration of meropenem and did not require a dose > 3 g/day.

The use of meropenem CI has some drawbacks. Some authors have reported that meropenem also exhibited a concentration-dependent killing and suggested that optimisation of both T > MIC and peak concentration/MIC ratio should be considered [2]. The risk of physicochemical incompatibility is increased with CI, which therefore requires a dedicated intravenous line [START_REF] Dulhunty | Continuous betalactam infusion in critically ill patients: the clinical evidence[END_REF]. Meropenem is not stable upon prolonged exposure to room temperature [START_REF] Dulhunty | Continuous betalactam infusion in critically ill patients: the clinical evidence[END_REF]. Based on previous studies [START_REF] Berthoin | Stability of meropenem and doripenem solutions for administration by continuous infusion[END_REF][START_REF] Curti | Stability Studies of Antipyocyanic Beta-Lactam Antibiotics Used in Continuous Infusion[END_REF], the CI of meropenem over 24 h was performed by dividing the total daily dose in four to six syringes of 60 mL infused over 4-6 h, four to six times/day. Finally, regarding the risk of meropenem-related toxicity, some authors [START_REF] Imani | Too much of a good thing: a retrospective study of β-lactam concentration-toxicity relationships[END_REF][START_REF] Beumier | Elevated β-lactam concentrations associated with neurological deterioration in ICU septic patients[END_REF] have reported that neurotoxicity may be more related to high peak concentrations than to high trough concentrations. These results are in agreement with the current results and those of other authors [START_REF] Roberts | The Effect of Renal Replacement Therapy and Antibiotic Dose on Antibiotic Concentrations in Critically Ill Patients: Data From the Multinational Sampling Antibiotics in Renal Replacement Therapy Study[END_REF] who have shown that meropenem CI was safe, although it increased the risk of overdose.

This study had several limitations. First, it was limited by its retrospective, single-centre design, small cohort size, and heterogeneity of the study population that included different meropenem dosing regimens and patient renal function due to its observational nature, which may have limited the generalisability of the results. However, it reported real-life data, observed in a case-mix of septic ICU patients, consistent with PK studies using Monte-Carlo simulations [START_REF] Dhaese | Population pharmacokinetics and evaluation of the predictive performance of pharmacokinetic models in critically ill patients receiving continuous infusion meropenem: a comparison of eight pharmacokinetic models[END_REF][START_REF] Cojutti | Population Pharmacokinetics of High-Dose Continuous-Infusion Meropenem and Considerations for Use in the Treatment of Infections Due to KPC-Producing Klebsiella pneumoniae[END_REF]. In addition, to limit bias in data collection, both electronic and paper charts were reviewed to include data collected prospectively at the bedside by nurses. Second, the study was not randomised and the choice between EII and CI was left to the discretion of the physicians. Most of them were trained in PK/PD and had chosen EII in patients with AKI, which may have limited comparability of the groups. Third, the study did not re-port any association between CI and microbiological or clinical out-comes. Nonetheless, several studies have reported that EII or CI of β-lactams improved outcomes [START_REF] Alffenaar | Antimicrobial therapeutic drug monitoring in critically ill adult patients: a Position Paper#[END_REF]. Finally, due to confounding actors the estimation of meropenem neurotoxicity may have been skewed and it did not investigate meropenem-associated renal toxicity [ 4 , 19 ].

Conclusions

In this cohort of meropenem-treated septic ICU patients, those treated with CI achieved higher median plasma meropenem con-centration (Cmin or Css) than those treated with EII. Continuous administration of meropenem was a better dosing strategy to achieve the optimal PK/PD target (100% ƒT ≥10 mg/L), especially in patients with the higher eGFR. These results also highlighted the importance of a daily meropenem dose between 40-70 mg/kg to achieve an optimal PK/PD target. Further studies are needed to confirm these results and to assess the impact of continuous ad-ministration on ICU patient outcomes, particularly in case of infections caused by difficult-to-treat resistance bacteria such as non-fermenters or carbapenemase-producing Enterobacterales.

Declarations

Funding:

This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.

Competing Interests:

The authors declare that the research was conducted in the absence of any commercial or financial relation-ships that could be construed as a potential conflict of interest.

Ethic Approval:

This study was conducted according to the guide-lines of the Declaration of Helsinki. The Institutional Review Board of Montpellier University Hospital approved the study (No. 2019_IRB-MTP_03-01) and waived the need for signed consent to participate.

Sequence Information:

Not applicable.

Author contributions:

RL concepted and designed the study; ST, NB, VB, DD, LP and PC collected the data; ST, KK and RL analysed and interpreted the data; ST and RL wrote the first draft of the article; OM, MV, KK and RL revised it critically for important intellectual content; all authors approved the final version of the sub-mitted manuscript.