Samuel Salvaggio

Nicolas Masson

Alexandre Zénon

Michael Andres
email: michael.andres@uclouvain.be

The predictive role of eye movements in mental arithmetic

Keywords: mental arithmetic, visuospatial attention, attention shift, eye tracking

Behavioural studies have suggested that number manipulation involves shifting attention along a left-to-right oriented continuum. However, these studies provide little evidence about the time course of attention shifts during number processing. We used an eye-tracker with high spatiotemporal resolution to measure eye movements during the mental solving of addition (e.g., 43+4) and subtraction problems (e.g., 53-6), as a proxy for the rightward and leftward attention shifts that accompany these operations. A first difference in eye position was observed as soon as the operator was heard: the hearing of "plus" shifted the eye rightward compared to "minus".

A second difference was observed later between problem offset and response onset: addition shifted the eye rightward and upward compared to subtraction, suggesting that the space used to represent the problem is bidimensional. Further analyses confirmed the fast deployment of spatial attention and evidenced its relationship with the carrying and borrowing procedures triggered by the problem presentation. The predictive role of horizontal eye movements, in particular, is essential to understand how attention contributes to narrow down the range of plausible answers. We propose that attention illuminates significant portions of the numerical continuum anticipatively to guide the search of the answer and facilitate the implementation of solving procedures in verbal working memory.

functional process or an epiphenomenon (for a discussion of this issue, see Andres, Salvaggio, Lefèvre, Pesenti, & Masson, 2020). The measures obtained with chronometric methods do not allow us separating these options because they typically index any phenomenon that takes place between the stimulus and the response. As an alternative to chronometric methods, two studies recorded eye movements to track attention while participants were solving arithmetic problems. Hartmann and colleagues (2015) evidenced attention shifts concomitant to the encoding of the arithmetic problem but only on the vertical axis, suggesting that addition directed attention upward relative to subtraction. Masson and colleagues (2018) found attention shifts along the horizontalbut not the verticalaxis while participants were computing the answer. These conflicting results raise a third issue related to the dimensions of the mental space used for number manipulation. The vertical axis is often used to teach or represent numbers in daily life and the introspective reports of number synaesthetes witness the importance of this dimension [START_REF] Seron | Images of numbers, or "when 98 is upper left and 6 sky blue[END_REF], Piazza 2006). Finally, an important aspect of calculation concerns the procedures used to solve arithmetic problems. Large problems put a different load on working memory, especially when they imply carrying or borrowing procedures [START_REF] Lefevre | Selection of procedures in mental addition: Reassessing the problem size effect in adults[END_REF][START_REF] Logie | Counting on working memory in arithmetic problem solving[END_REF], potentially increasing the need to rely on a visuospatial medium.

We made the assumption that eye movements contribute to narrow down the range of plausible answers by guiding the search of the answer on a left-to-right oriented continuum as information about the operator (addition/subtraction) and the calculation (carry/borrow) is processed. If the eye position reflects a dynamic search process, the gaze should drift leftward or rightward as soon as the direction of the operation is known. We used eyetracking as a proxy to investigate how the cognitive system progressively integrated the information carried by the numbers, the operator or their association, to predict the location of the answer on the mental numerical continuum. Furthermore, we examined the role of cognitive load by comparing the eye movements elicited by arithmetic operation in carry and non-carry problems. Finally, we also explored vertical eye movements as previous studies suggested they might play a role in arithmetic problem solving [START_REF] Hartmann | Spatial biases during mental arithmetic: Evidence from eye movements on a blank screen[END_REF]. To do so, we took advantage of the high temporal resolution of the eye-tracker enhanced by the fragmented presentation of the different elements of two-digit addition and subtraction problems. The task was coupled with the passive viewing of landscape pictures, and their mirror version, both carefully counterbalanced over the different conditions of the experiment to exclude perceptual biases. The rationale was to provide participants with a rich visual context that stimulates eye movements and thereby increases sensitivity to number-related attention shifts. The use of a background image is common to other studies and proved superior to the use of a blank screen, as evidenced by the number of saccades elicited during a trial and the effective detection of number-related attention shifts (Salvaggio, Masson & Andres, 2019;[START_REF] Sahan | Eye-movements reveal the serial position of the attended item in verbal working memory[END_REF].

Method

Participants

Thirty-two French-speaking University students with normal or corrected-to-normal vision took part in this experiment in exchange for course credits (twelve males; three lefthanded; mean age and standard deviation, M=22.8, SD=4.2). Sample size was estimated using G*Power 3.1 [START_REF] Faul | G* Power 3: A flexible statistical power analysis program for the social, behavioral, and biomedical sciences[END_REF]. A sample size of 28 participants achieves a 95% power to detect a true difference between addition and subtraction in horizontal eye position (Cohen's d = 0.6; [START_REF] Masson | Time course of overt attentional shifts in mental arithmetic: Evidence from gaze metrics[END_REF] with α = 0.05 using a onetailed paired t-test. The experimental procedure was approved by the ethical committee of the Psychological Sciences Research Institute at UCLouvain (B403201629166).

Apparatus

The experiment was run on a PC equipped with a 22-inch LCD screen (1920x1080 pixels; refresh rate: 60Hz). The participant wore Sennheiser PC8 USB headphones equipped with a microphone (Sennheiser, Wedemark, Germany). The software E-Prime 2 (Schneider, Eschman, & Zuccolotto, 2002) controlled stimulus presentation and recording of the verbal response. An Eyelink 1000 tower-mounted camera was used to track eye movements (SR Resarch, Mississauga, Canada; sampling rate: 1000Hz; average accuracy range: 0.25° angle to 0.5° angle; gaze tracking range of 32° angle horizontally and 25° angle vertically). Participants were placed at 60 cm from the screen. Prior to each experimental block, the eye tracker was calibrated to the screen using a built-in 9-point protocol.

Stimuli materials

The arithmetic problems consisted of 144 unique 2-digit ± 1-digit problems that participants heard through headphones. The first operand (O1) ranged between 11 and 79. The unit of O1 could be 1, 2, 3, 7, 8 or 9 and the decade of O1 ranged from 10 to 60 for the addition problems, and from 20 to 70 for the subtraction problems. The second operand (O2) was either 4 or 6. Therefore, carrying procedures were required to solve addition problems with O1 units 7-9 and borrowing procedures were required to solve subtraction problems with O1 units 1-3.

Overall, addition and subtraction problems were matched for the magnitude of their results (mean and SD of addition results: 45±18; mean and SD of subtraction results: 45±18). The numbers and operators were recorded in Belgian French, and the audio file duration was adjusted to 750 ms for O1 and 500 ms for the operator and O2. The visual stimuli consisted in 18 high-definition coloured pictures of urban and natural landscapes and their horizontal flip (1920x1080 pixels). The same set of pictures were used for addition and subtraction problems, with and without a carry, which means that each picture was presented once in all four conditions. Within each condition, the different pictures were randomly associated with arithmetic problems to exclude any systematic association with a given problem. We presented both the original pictures and their horizontal flip to be sure that if something in the original picture grabbed the participant's attention on a particular side, it would be mirrored in the same condition later.

Task and Procedure

During the task, the gaze was free, but participants were instructed to move their eyes back on the central fixation cross (size: 0.5° x 0.5°) displayed at the beginning of each trial, in black on a grey background, during 500 ms. After the fixation cross disappeared, the first operand (O1) was presented for 750 ms, followed by a 625-ms blank interval, and then the operator (+/-) for 500 ms; after another 625-ms blank interval, the second operand (O2) was presented for 500 ms. The total duration of the problem presentation was thus 3000 ms.

Participants were asked to respond to the arithmetic problem verbally, with a deadline of 6000 ms from problem onset. The picture was displayed at the time of O1 onset and remained on screen for all the duration of the trial. The next trial started immediately after the offset of the picture.

Data analysis

Erroneous trials (18%) were excluded from the rest of the analyses (NADDITION = 8 %,; NSUBTRACTION = 10 %). Response latencies (RL), measured from problem offset, were entered into a repeated measure analysis of variance (ANOVA) with OPERATION (subtraction vs. addition) and CARRY (carry vs. no carry) as within-subject factors. The spatial and temporal parameters of eye movements were extracted using Eyelink® Data Viewer (SR Research Ltd., Mississauga, Ontario, Canada). Eyetracking data were considered as outliers if they corresponded to locations outside the screen. The pupil size derivative was also systematically inspected to refine the search for outliers due to signal loss. The signal loss was associated with a departure from linearity in the derivative data (typically > 50 SD), which clearly denotes an abnormality given the relatively low frequency of pupil size variations. This method is preferable to the visual inspection of eye position because the signal loss is immediately reflected in the pupil velocity profile, whose better signal-to-noise ratio leaves no place for ambiguity, and the decision to remove the data is independent of the analysis of lateralized eye movements. We interpolated the missing data only if the signal loss was less than 500 ms, which corresponds to the time interval of an eye blink (Schiffman, 1990). Moreover, if signal losses cumulated beyond the limit of 2000 ms within a single trial, we removed the trial from the dataset. The percentage of excluded trials resulting from outlier detection (4%) added up to the percentage of error trials (18%), for a total of 22% of excluded trials across the full dataset. In the remaining data (78%), the vertical and horizontal coordinates of eye position were aggregated in bins of 10 ms on a time period of 6000 ms running from the onset of the number word. The difference between subtraction and addition problems was tested in every bin using a paired t-test and clusters were constituted by grouping all bins that showed a significant difference (p<.05) and a maximal gap of 100 ms (10 bins) between them. Effect size was estimated at the cluster level by taking the t statistic of the maximum difference across all the bins contained in the cluster and dividing it by the square of the number of participants. The multiple comparison problem was addressed by means of a nonparametric random permutation procedure (for details, see Salvaggio, Masson & Andres, 2019;[START_REF] Maris | Nonparametric statistical testing of EEG-and MEG-data[END_REF].

The analyses were made with MATLAB 2018a (MathWorks Inc., 2018) and SPSS 24.0 (IBM

Corp., 2016).

Results

Response latency

There was no effect of OPERATION, F(1, 31) = 3.47, MSE = 9637, p = .072, ηp 2 = .101 (MADDITION = 948 ms from problem offset, SDADDITION = 268; MSUBTRACTION = 974 ms from problem offset, SDSUBTRACTION = 235), but a main effect of CARRY, F(1, 31) = 134.8, MSE = 28797, p < .001, ηp 2 = .813, indicating a higher RL when arithmetic problems involved shifting to the superior or inferior decade (M= 1132 ms, SD= 288) compared to when they did not (M= 787 ms , SD= 236). A significant interaction was found between these two factors, F(1, 31) = 14.3, MSE = 7440, p = .001, ηp 2 = .316, indicating that the CARRY effect was larger for subtraction (MCARRT = 1167 ms, SDCARRY = 291; MNO CARRY = 771 ms, SDNO CARRY = 226; t(31)=11.54, p <.001) than for addition (MCARRT = 1078 ms, SDCARRY = 309; MNO CARRY = 796 ms, SDNO CARRY = 266; t(31)=9.07, p <.001; see Figure 1).

Horizontal eye drift

The statistical comparison of horizontal eye position evidenced a rightward deviation in addition trials relative to subtraction trials (Figure 2A and 2B), on two different time periods, just after the operator onset, from 1770 to 2210 ms (p<.001, Cohen's d=.48, maximal difference = 13 pixels or 0.32° angle), and during the response production stages, from 3900 to 4040 ms after problem onset (p=.001, d=.39, maximal difference = 16 pixels or 0.39° angle). Figure 2 shows an initial deflection toward the left side of space, irrespective of the arithmetic operation, before addition re-directed attention rightward to the screen center, creating a difference with subtraction.

In the second analysis, we separated carry and no-carry problems to test whether the early move of attention along the horizontal axis anticipated the need to implement carrying/borrowing procedures. Of note, in the present experiment, the structure of the problems made it possible to anticipate such carrying or borrowing procedures from operator onset: if the first operand ended with 7-8-9, addition required a carry in any case (+4 or +6); if the first operand ended with 2-3-4, subtraction required a borrowing in any case (-4 or -6). In the carry condition, addition showed a rightward deviation relative to subtraction just after the operator onset (+ or -), between 1720 to 1920 ms (p<.001, d=.48, maximal difference = 22 pixels or 0.52° angle), and during the response production stages, between 3670 to 4160 ms (p<.001, d=.42, maximal difference = 19 pixels or 0.47° angle). In the no-carry condition, however, no difference was observed between the two arithmetic operations (Figure 3). The response time analysis showed that the execution of carrying/borrowing procedures induce an extra time cost. In order to control for the possible influence of response latency on the difference in eye position between the two operations, we conducted an additional analysis over the 1000 ms preceding each oral response (i.e. the time interval between the problem offset and the response). This response-locked analysis replicated the continuous rightward deviation of the gaze in addition trials relative to subtraction trials for the carry problems only (p<.001, d=.52, from 1000 ms to 230 ms before the response, maximal difference = 22 pixels or 0.52° angle).

In the third analysis, we exploited another particularity of the stimulus set to further investigate the predictive role of attention shifts and disentangle the influence of number magnitude and arithmetic operation on their direction. In order to match the two arithmetic operations in response magnitude, we created a subset of addition problems with a first operand between 11 and 19 (plus 4 or 6) and a subset of subtraction problems with a first operand between 71 and 79 (minus 4 or 6). Because these problems were exclusively linked to addition or subtraction, it was possible for participants to perform attention shifts anticipatively, before the operator onset (+ or -), from the decade of the first operand (11-19 or 71-79). We tested whether these problems could have influenced the time course of horizontal attention shifts.

First, we replicated the main analysis after excluding these problems, thus limiting the eyetracking data to problems that were associated both with addition and subtraction (i.e. problems with a first operand between 20 and 60). This control analysis revealed the same pattern of results as described above: a difference in horizontal eye position between subtraction and addition was observed between 1780 ms and 1990 ms (p<.001, d=.57 maximal difference = 17 pixels or 0.41° angle), right after hearing the operator, and between 3880ms and 4050ms (p<.001, d=.54, maximal difference = 18 pixels or 0.44° angle), during the response production.

Second, we specifically compared the two subsets of problems where it was possible to guess the operation from the decade of the first operand (11-19 or 71-79). In these problems, a significant difference in horizontal eye position was observed very early, before the onset of the operator, from 1390 ms to 1550 ms (p<.001, d=.47, maximal difference = 38 pixels or 0.93° angle; see Figure 4). These findings corroborate the idea that participants noticed the regularities in the stimulus set and moved their attention anticipatively when the side of the numerical continuum where the answer lies could be inferred from the first operand. Another important implication of this analysis is that it clearly shows it is the operation and not the number magnitude that is responsible for the direction of attention shifts. The mental number line is usually considered as a long term memory representation where numbers occupy a fixed position. If the position assigned to numbers on this mental number line played a role in arithmetic problem solving, then the subset of addition problems with numbers 11-19 should shift attention leftward compared to the subset of subtraction problems with numbers 71-79.

We found the opposite, meaning that attention shifted in the direction implied by the arithmetic operation, i.e. rightward for addition and leftward for subtraction, irrespective to the position of the operands on the two ends of the numerical continuum.

Vertical eye drift

A significant difference in vertical eye position was observed during the response production stages, from 3500 ms to 4200 ms after problem onset, indicating a late deviation downward in subtraction relative to addition trials (p<.001, d=.43, maximal difference = 9 pixels or 0.21° angle). This late contribution contrasts with the predictive nature of horizontal attention shifts (see Figure 2C and2D).

Discussion

We used a high-temporal resolution eye-tracker to measure eye movements during the solving of addition and subtraction problems, as a proxy for the spatial attention shifts associated with numerical cognition. Pictures were presented on screen to stimulate eye movements and the arithmetic problems were presented in a fragmented manner to isolate attention shifts over time. The main analysis showed that arithmetic operation shifted eye position at two distinct time windows. A first difference was observed just after hearing the operator: the plus sign shifted the eye rightward compared to the minus sign. A second difference was observed later between problem offset and response onset: addition shifted the eye rightward and upward compared to subtraction. Supplementary analyses confirmed the early onset of horizontal attention shifts and showed they resulted from the comparison of problems that required carrying and borrowing procedures. These results allow three important conclusions regarding the cognitive mechanisms that govern number and space interactions during arithmetic processing.

First, horizontal attention shifts occur predictively, i.e. before exact computation has started. The results showed that addition deviated the gaze rightward, compared to subtraction, as soon as the operator was heard. In problems where the first operand implied an addition or a subtraction due to systematic associations in the stimulus set, attention moved in the direction of the expected operation even before the operator was presented. These data demonstrate the fast deployment of spatial attention during arithmetic processing. This predictive role is essential to understand how attention potentially contributes to narrow down the range of plausible answers. We propose that attention illuminates significant portions of the numerical continuum anticipatively to facilitate the incremental process during exact computation. A similar proposal is that attentional shifts could contribute to give an approximative idea of plausible answers that would then be compared to the solution obtained by an exact procedure, preventing absurd errors (Marghetis, Nunez, & Bergen, 2014). Our results are compatible with this proposal but refines it by highlighting the dynamic nature of the approximation, which actually takes place as soon as it is possible to predict the direction of the operation. The predictive role of spatial attention in mental arithmetic has been noticed in former chronometric studies showing for example that the display of a first operand and an arithmetic sign is sufficient to prime the detection of lateralized targets (Liu et al., 2016;[START_REF] Masson | Spatial biases in mental arithmetic are independent of reading/writing habits: Evidence from French and Arabic speakers[END_REF][START_REF] Yáñez | Learning to run the number line: the development of attentional shifts during single-digit arithmetic[END_REF]. Our study brings direct evidence for such predictive shifts by combining the interleaved presentation of operands with eye-tracking.

Second, horizontal attention shifts are driven by the execution of arithmetic procedures in working memory. Two results force this conclusion. When the analysis is restricted where number magnitude and arithmetic operation predict shifts in opposite directions, eye movements clearly reflect the direction implied by the operation. In other words, addition deviated the gaze rightward relative to subtraction even when addition problems were made of numbers located on the leftmost end of the continuum and subtraction problems were made of numbers located on the rightmost end of the continuum. Close examination of the data shows that each trial started with a leftward ocular drift that cannot be explained by arithmetic operation or number magnitude. This initial deflection toward the left side of space has been observed in several other studies using auditory verbal stimuli (Myachykov, Cangelosi, Ellis & Fischer, 2015;[START_REF] Myachykov | Ocular drift along the mental number line[END_REF]Salvaggio, Masson & Andres, 2018;[START_REF] Sahan | Eye-movements reveal the serial position of the attended item in verbal working memory[END_REF]. It is reminiscent of our natural tendency to scan visual scenes from left to right, possibly due to the dominant role of the right hemisphere in the deployment of spatial attention [START_REF] Foulsham | Leftward biases in picture scanning and line bisection: A gaze-contingent window study[END_REF]. Because of the generic deployment of attention from left to right, we assume that the first operand was positioned on the left side of the mental space, irrespective of its magnitude, and served as an anchor for searching the answer of the calculation. Then, the analyses showed that horizontal attention shifts were mainly driven by those problems that implied a carrying or borrowing procedure. The rightward deviation induced by addition problems, relative to subtraction problems, was actually observed only when the computation of the answer required crossing decades. Response time data showed that carrying and borrowing procedures are costly, which suggests that spatial attention is involved to alleviate the working memory load. Of note, in the present experiment, the structure of the problems made it possible to anticipate such carrying or borrowing procedures from operator onset: if the first operand ended with 7-8-9, addition required a carry in any case (+4 or +6); if the first operand ended with 2-3-4, subtraction required a borrowing in any case (-4 or -6).

Third, vertical attention shifts were detected, suggesting that the space used to represent arithmetic problems is bidimensional. The analysis of vertical attention shifts is admittedly exploratory and requires a cautious interpretation for two reasons. One of the limitations is that the pictures used in our experiment were not flipped vertically. Hence, the particular bottomto-top orientation of the visual scenes, with close objects in the inferior part and far objects in the superior part, could have primed an association between addition (i.e. incrementing) and upward movement (i.e. moving away) and the reverse for subtraction. After all, a top-to-bottom orientation was equally plausible to represent the incremental process if one thinks of the progression of a text on a page or the progression of rows in a table of figures. Moreover, contrary to the horizontal gaze deviation, the difference in vertical eye position was observed in the late stages of calculation, from 3500 ms to 4200 ms after problem onset. We did not investigate further the origin of vertical attention shifts because the absence of evidence for an early contribution in the main analysis did not justify a study of their predictive role in followup analyses. The conclusion is that arithmetic problem solving entertains movements of attention along the vertical axis, as well, but their late onset forces us to adopt a different perspective on their role. We propose that horizontal attention shifts help to narrow down the range of plausible answers predictively and keep them active in working memory especially when cognitive load is high. In contrast, vertical attention shifts might contribute to refine the spatial frame in later stages of calculation, for example by representing small increments on the portion of the numerical continuum where the answer has been identified in the former stages.

This interpretation is compatible with some introspective reports of number forms, which refer to a horizontal continuum regularly interleaved with volutes describing the organization of digits within a decade [START_REF] Galton | Visualised numerals[END_REF][START_REF] Hubbard | Interactions between number and space in parietal cortex[END_REF], and with eye-tracking data suggesting a contribution of both dimensions to number comparison and arithmetic [START_REF] Hartmann | Spatial biases during mental arithmetic: Evidence from eye movements on a blank screen[END_REF]2016;[START_REF] Masson | Shifts of spatial attention underlie numerical comparison and mental arithmetic: Evidence from a patient with right unilateral neglect[END_REF][START_REF] Salvaggio | Eye position reflects the spatial coding of numbers during magnitude comparison[END_REF]. However, the present experiment does not allow us to exclude that vertical attention shifts are epiphenomenal to the resolution of the problem, given their late time course, and confirmatory research is needed to establish their role in mental arithmetic. Another interesting idea that remains to be investigated is that the bidimensional representation of numbers can be flexibly used, as a function of one's personal experience or of the task at hand, and could eventually extend to a third dimension defining close and far space [START_REF] Aleotti | Numbers around Descartes: A preregistered study on the three-dimensional SNARC effect[END_REF]. Classical cognitive architectures of numerical cognition posit that small arithmetic facts are solved through verbal memory retrieval [START_REF] Dehaene | Two mental calculation systems: A case study of severe acalculia with preserved approximation[END_REF], whereas large arithmetic problems require the manipulation of magnitude representations (Mc Closkey, Caramazza & Basili, 1985;[START_REF] Dehaene | Two mental calculation systems: A case study of severe acalculia with preserved approximation[END_REF], in particular when the problem implies carry or decomposition [START_REF] Campbell | Cognitive arithmetic across cultures[END_REF][START_REF] Lefevre | Selection of procedures in mental subtraction[END_REF][START_REF] Lefevre | Selection of procedures in mental addition: Reassessing the problem size effect in adults[END_REF][START_REF] Pesenti | Mental calculation in a prodigy is sustained by right prefrontal and medial temporal areas[END_REF]. However, these classical models do not specify the mechanisms involved in the manipulation of number magnitude during arithmetic problem solving. Recent proposals acknowledge the role of spatial attention in arithmetic problem solving but they were mostly developed on theoretical or neurophysiological grounds [START_REF] Chen | Spatial intuition in elementary arithmetic: A neurocomputational account[END_REF][START_REF] Dehaene | Two mental calculation systems: A case study of severe acalculia with preserved approximation[END_REF][START_REF] Hubbard | Interactions between number and space in parietal cortex[END_REF][START_REF] Knops | Recruitment of an area involved in eye movements during mental arithmetic[END_REF], providing little insight into the cognitive mechanisms that elicit the transient mapping of numbers on a spatial continuum and cause directional attention shifts. Our eye-tracking study highlights the predictive role of attention in converting the arithmetic problem into a spatial frame that represents the plausible answers, thereby reducing the cognitive load and paving the way for the implementation of efficient solving strategies. Supporting evidence for this view comes from studies of patients with unilateral spatial neglect, a brain disorder affecting the ability to orient attention to the contralesional hemispace [START_REF] Heilman | Mechanisms underlying hemispatial neglect[END_REF]. It has been shown that subtraction problems give rise to more errors than addition problems in right-brain-damaged patients exhibiting left neglect [START_REF] Dormal | Causal role of spatial attention in arithmetic problem solving: Evidence from left unilateral neglect[END_REF].

In contrast, a single case with right neglect, following left brain damage, was reported to have more difficulties with addition than subtraction problems [START_REF] Masson | Shifts of spatial attention underlie numerical comparison and mental arithmetic: Evidence from a patient with right unilateral neglect[END_REF]. This double dissociation cannot be explained by current numerical cognition models unless one considers that the ancillary role of spatial attention in arithmetic problem solving. The impairment of attention orientation along the vertical axis has received less attention in the literature, though a few cases of vertical neglect have been reported (e.g., Rapcsack, Cimino [START_REF] Rapcsak | Altitudinal neglect[END_REF][START_REF] Seron | Images of numbers, or "when 98 is upper left and 6 sky blue[END_REF][START_REF] Andres | Increased cognitive load reveals unilateral neglect and altitudinal extinction in chronic stroke[END_REF], and the question of whether vertical neglect might impact on mental arithmetic remains to be further tested.

Our time course analysis thus shows that the cognitive system optimally integrates the terms of the problem to direct spatial attention toward the probable location of the answer on the mental numerical continuum. It is clear from the predictive nature of these attention shifts, which typically occur before the presentation of the second operand, that their primary role is not to identify the exact answer but to reduce uncertainty about its probable location. Moreover, the solving of arithmetic problems cannot be uniquely spatial because this would imply that the location of the answer is known before an attention shift is triggered, which is not compatible with the early attention orienting response revealed by our eye-tracking data. Our results favour a probabilistic model where attention shifts are embedded in working memory processes: the number word sequence provides positional information (landmark) to anchor the spatial frame of the problem, whereas (directional) attention shifts contribute to adjust the frame as calculation progresses thereby reducing the part of the number sequence maintained in verbal working memory. The role of spatial attention would thus be ancillary to verbal working memory in the incrementation process, just as the movement of a flashlight guides our steps into darkness. Beyond the arithmetic domain, our results converge with others to support a working memory model attributing a role to spatial attention in the serial processing of verbal information (Abrahamse, van Dijck, Majerus, & Fias, 2014;[START_REF] Sahan | Eye-movements reveal the serial position of the attended item in verbal working memory[END_REF]van Dijck, Abrahamse, Majerus & Fias, 2013). Grounding the resolution of arithmetic problems in the interaction between spatial attention and verbal working memory is a promising avenue for further research.

To sum up, our results demonstrate that the recruitment of spatial attention is linked to the arithmetic procedures triggered by the problem presentation. The operator triggers the setting of a spatial frame oriented leftward or rightward, relative to the first operand, to represent the numerical relationships relevant to the arithmetic task. We tentatively assume that horizontal and vertical attention shifts observed later, around response production, reflect a recalibration of the spatial frame as computation progresses. In this view, attention shifts are not the signature of movements along a mental number line where numbers occupy a fixed position but they rather reflect working memory processes operating on a spatial representation created to face the actual demands of arithmetic processing, for instance the implementation of carrying or borrowing procedures in working memory.

FiguresFigure

  Figures

Figure 2 .

 2 Figure 1. Response latencies as a function of arithmetic operation and carrying procedure.

Figure 3 .

 3 Figure 3. Horizontal eye drift for no-carry (left) and carry (right) problems. The graphs

Figure 4 .

 4 Figure 4. Horizontal eye drift for problems where it was possible to infer the arithmetic

Acknowledgements M.A. is a research associate at the Fonds National de la Recherche Scientifique (FRS-FNRS, Belgium) and N.M. is a post-doc researcher supported by grant PDR-T.0047.18 from the Fonds National de la Recherche Scientifique (FRS-FNRS, Belgium) and grant FNR-INTER/FNRS/17/1178524 from the National Research Fund of Luxembourg (FNR, Luxembourg). This research was supported by grant PDR-FNRS T.0245.16 from the Fonds National de la Recherche Scientifique (FRS-FNRS, Belgium). AZ is a research associate at the Centre National de la Recherche Scientifique, funded by IdEx Junior Chair and ANR JCJC (ANR-18-CE37-0009-01) grants.

Funding M.A. is a research associate at the Fonds National de la Recherche Scientifique (FRS-FNRS, Belgium) and N.M. is a post-doc researcher supported by grant PDR-T.0047.18 from the Fonds National de la Recherche Scientifique (FRS-FNRS, Belgium) and grant FNR-INTER/FNRS/17/1178524 from the National Research Fund of Luxembourg (FNR, Luxembourg). This research was supported by grant PDR-FNRS T.0245.16 from the Fonds National de la Recherche Scientifique (FRS-FNRS, Belgium). AZ is a research associate at the Centre National de la Recherche Scientifique, funded by IdEx Junior Chair and ANR JCJC (ANR-18-CE37-0009-01) grants.

Availability of data and material

The custom codes written for the current study and the datasets generated during and/or analysed during the current study are available on OSF following this link.

Conflict of interest statement

On behalf of all authors, the corresponding author states that there is no conflict of interests.


Ilaria Motta

Martin Boeree

Dumitru Chesov

Keertan Dheda

Gunar Günther

Charles Robert Horsburgh

Yousra Kherabi

Christoph Lange

Christian Lienhardt

Helen M Mcilleron

Nicholas I Paton

Helen R Stagg

Guy Thwaites

Zarir Udwadia

Reinout Van Crevel

Gustavo E Velásquez

Robert J Wilkinson

Lorenzo Guglielmetti
email: lorenzo.guglielmetti@aphp.fr

Recent advances in the treatment of tuberculosis

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.

Background

Tuberculosis is a global health challenge and one of the leading causes of death worldwide. In the last decade, the tuberculosis treatment landscape has dramatically changed. After long years of stagnation, new compounds entered the market (bedaquiline, delamanid and pretomanid) and phase III clinical trials have shown promising results towards shortening duration of treatment for both drug-susceptible (Study 31/A5349, TRUNCATE-TB, SHINE) and drug-resistant tuberculosis (STREAM, NiX-TB, ZeNix, TB-PRACTECAL). Dose optimization of rifamycins and repurposed drugs have also brought hopes of further development of safe and effective regimens. Consequently, international and World Health Organization clinical guidelines have been updated multiple times in the last years to keep pace with these advances.

Objectives

This narrative review aims to summarize the state-of-the-art on treatment of drug-susceptible and drug-resistant tuberculosis, as well as recent trials results and an overview of ongoing clinical trials.

Sources

A non-systematic literature review was conducted in PubMed and MEDLINE, focusing on the treatment of tuberculosis. Ongoing clinical trials were listed according to the authors' knowledge, and completed consulting clinicaltrials.gov and other publicly available websites (www.resisttb.org/clinical-trials-progress-report, www.newtbdrugs.org/pipeline/trials).

Content

This review summarizes the recent, major changes in the landscape for drug-susceptible and drug-resistant treatment, with a specific focus on their potential impact on patient outcomes and programmatic TB management. Moreover, insights in host-directed therapies, and advances in pharmacokinetic and pharmacogenomics are discussed. A thorough outline of ongoing therapeutic clinical trials is presented, highlighting different approaches and goals in current TB clinical research.

Implications

Future research should be directed to individualize regimens and protect these recent breakthroughs by preventing and identifying the selection of drug resistance and providing widespread, affordable, patient-centered access to new treatment options for all people affected 106 by tuberculosis. 107

Main article

Introduction

Tuberculosis (TB) remains a global health challenge, with an estimated incidence of 10.6 million new cases occurring in 2021, according to the 2022 World Health Organization (WHO) Global TB Report. [START_REF]Global tuberculosis report[END_REF] The incidence of multidrug-resistant/rifampicin-resistant TB (MDR/RR-TB) is increasing, with an estimated 450,000 new cases in 2021.

Existing treatments for drug-susceptible (DS) and drug-resistant (DR) TB for adult and children have saved millions of lives. However, TB is still a leading infectious cause of death with 1.6 million deaths (including 187,000 people living with HIV) occurring in 2021 and in the near future could replace again COVID-19 as the leading cause of death by a single infectious agent. [START_REF]Global tuberculosis report[END_REF] Recent therapeutic advances have dramatically renewed the landscape for DS-and DR-TB treatment. This review aims to highlight these major changes and their potential impact on patient outcomes and programmatic TB management.

Treatment of drug-susceptible TB

For much of the last 50 years, DS-TB has been treated with the so-called 'short-course chemotherapy' regimen. Administered over six months, the regimen was the result of a series of clinical trials conducted between 1946 and 1986 by the British Medical Research Council. [START_REF] Fox | Studies on the treatment of tuberculosis undertaken by the British Medical Research Council Tuberculosis Units, 1946-1986, with relevant subsequent publications[END_REF] These trials demonstrated that an 'intensive' phase of two months of rifampicin, isoniazid, and pyrazinamide, followed by a 'continuation' phase of four months of rifampicin and isoniazid, could cure most patients. Pyrazinamide was added to the regimen in the intensive phase to permit treatment shortening from nine to six months. [START_REF] Fox | Studies on the treatment of tuberculosis undertaken by the British Medical Research Council Tuberculosis Units, 1946-1986, with relevant subsequent publications[END_REF] This 'one-size-fits-all' regimen subsequently became the global standard, recommended for treating all forms of DS-TB. Short-course chemotherapy has saved millions of lives, but six months of pill-taking challenges patients and TB treatment programmes alike. There has, therefore, been much interest in shortening the regimen. There were early signals that the addition of fluoroquinolones might shorten treatment, with trials suggesting that these drugs reduced the time-to-sterilisation of sputum when added to the standard therapy. [START_REF] Rustomjee | A Phase II study of the sterilising activities of ofloxacin, gatifloxacin and moxifloxacin in pulmonary tuberculosis[END_REF][START_REF] Conde | Moxifloxacin versus ethambutol in the initial treatment of tuberculosis: a double-blind, randomised, controlled phase II trial[END_REF] However, three independent phase III trials published in 2014 showed that four-month regimens containing moxifloxacin or gatifloxacin did not meet the pre-defined non-inferiority margins when compared to the standard six-month regimen. [START_REF] Merle | A Four-Month Gatifloxacin-Containing Regimen for Treating Tuberculosis[END_REF][START_REF] Jindani | High-Dose Rifapentine with Moxifloxacin for Pulmonary Tuberculosis[END_REF][START_REF] Gillespie | Four-Month Moxifloxacin-Based Regimens for Drug-Sensitive Tuberculosis[END_REF] Nonetheless, subsequent analyses of the results of these trials have shown that specific subgroups of patients may benefit from less than six months of treatment. [START_REF] Imperial | A patient-level pooled analysis of treatmentshortening regimens for drug-susceptible pulmonary tuberculosis[END_REF][START_REF] Imperial | Precision-enhancing Risk Stratification Tools for Selecting Optimal Treatment Durations in Tuberculosis Clinical Trials[END_REF] Investigators turned to shorten treatment by optimising the pharmacokinetics of the drugs used, especially the rifamycins (rifampicin, rifabutin, rifapentine). Clinical studies have confirmed that rifampicin doses up to 40mg/kg/day were well-tolerated and increased early bactericidal activity, [START_REF] Brake | Increased bactericidal activity but dose-limiting intolerability at 50 mg•kg-1 rifampicin[END_REF] but whether high rifampicin doses can safely shorten therapy, or improve outcomes from TB meningitis, remains the subject of ongoing trials (Table 1). Recent data from a large phase III trial (Study 31/A5349) showed that rifapentinea rifamycin with a longer half-lifeused in combination with isoniazid, pyrazinamide and moxifloxacin, can shorten therapy to four months. [START_REF] Dorman | Four-Month Rifapentine Regimens with or without Moxifloxacin for Tuberculosis[END_REF] In May 2022, the WHO conditionally recommended that eligible persons aged ≥12 years with pulmonary DS-TB may receive this four-month regimen. [START_REF]WHO consolidated guidelines on tuberculosis. Module 4: treatment -drug-susceptible tuberculosis treatment[END_REF] A reinvigorated anti-TB drug pipeline has enabled new approaches to treatment (Table 1). A recent phase II trial compared pretomanid -a new nitroimidazole -with either rifampicin or rifabutin, in combination with isoniazid and pyrazinamide, against the standard 6-month regimen. [START_REF] Dooley | Assessing Pretomanid for Tuberculosis (APT) Study Team. Assessing Pretomanid for Tuberculosis (APT), a Randomized Phase 2 Trial of Pretomanid-Containing Regimens for Drug-Sensitive Tuberculosis: 12-Week Results[END_REF] The pretomanid-rifabutin regimen induced faster bacterial killing in sputum than the other regimens, but with more frequent hepatic adverse events, probably due to the pretomanid/pyrazinamide combination, which may temper the use of this combination for future DS-TB treatment. [START_REF] Eristavi | SimpliciTB Results and Hepatic Safety of Pretomanid Regimens +/1 Pyrazinamide[END_REF] It has long been recognised that there is a subset of patients, often with less severe TB, that may be cured with less than six months of therapy. [START_REF]Sputum-smear-negative pulmonary tuberculosis: controlled trial of 3-month and 2-month regimens of chemotherapy[END_REF] The SHINE trial showed that sixteen weeks was non-inferior to six months of treatment in children with DS, non-severe, smear-negative TB. [START_REF] Turkova | Shorter Treatment for Nonsevere Tuberculosis in African and Indian Children[END_REF] The TRUNCATE-TB trial investigated a strategy of giving eight-week treatment regimens to adults with mild or moderately severe pulmonary TB, with the possibility to extend treatment in those with poor response and retreatment for relapses. [START_REF] Paton | Treatment Strategy for Rifampin-Susceptible Tuberculosis[END_REF] One strategy arm using an initial eight-week combination of bedaquiline, linezolid, isoniazid, pyrazinamide, and ethambutol met the 12% non-inferiority margin, with marked reduction in total time on treatment, and without major safety concerns. Overall, 3 (2%) participants out of 189 in the successful strategy arm extended regimen and 24 (13%) started a second treatment course. Two participants in the bedaquiline-, linezolid-containing arm relapsed with confirmed acquired phenotypic drug resistance to bedaquiline (and clofazimine). Implementation research is needed to evaluate the outcome of such strategy in diverse populations. Of note, the definition of TB severity is heterogeneous among the aforementioned studies and would greatly benefit from consensus on validated criteria. [START_REF] Imperial | A patient-level pooled analysis of treatmentshortening regimens for drug-susceptible pulmonary tuberculosis[END_REF][START_REF] Imperial | Precision-enhancing Risk Stratification Tools for Selecting Optimal Treatment Durations in Tuberculosis Clinical Trials[END_REF] Treatment of isoniazid-resistant, rifampicin-susceptible TB Isoniazid resistance without concurrent rifampicin resistance is the most common type of M. tuberculosis resistance worldwide, present among an estimated 10.6% of all TB cases in 2019. [START_REF]Global tuberculosis report[END_REF] In 2018, WHO recommended a regimen consisting of six months of rifampicin, ethambutol, pyrazinamide, and levofloxacin to treat isoniazid-resistant TB, following an individual patient data (IPD) meta-analysis containing 3,923 patients with isoniazid-resistant, rifampicin-susceptible TB which indicated that the addition of a fluoroquinolone, compared to six months of standard treatment with or without isoniazid, increased the likelihood of treatment success (adjusted odds ratio: 2,8 [95% confidence interval: 1,1-7,3]). [START_REF]WHO consolidated guidelines on tuberculosis. Module 4: treatment -drug-susceptible tuberculosis treatment[END_REF][START_REF] Fregonese | Comparison of different treatments for isoniazid-resistant tuberculosis: an individual patient data meta-analysis[END_REF] This recommendation was, however, conditional, based on a very low certainty of evidence. In instances of noncavitary disease, low bacillary burden, or pyrazinamide toxicity, European-American guidelines have suggested that pyrazinamide may be given only during the first two months of treatment, provided the fluoroquinolone used is later-generation. [START_REF] Nahid | Treatment of Drug-Resistant Tuberculosis. An Official ATS/CDC/ERS/IDSA Clinical Practice Guideline[END_REF] Patients with fluoroquinolone resistance or contraindications are generally recommended to be treated with rifampicin, ethambutol and pyrazinamide only for six months. However, these two latter recommendations lack clinical trial evidence and are based on expert opinion. When additional drug resistance is detected or highly likely, individualised regimens are needed.

At the time of the WHO guidelines, there was no clear evidence if using high-dose isoniazid within such regimens was beneficial. This is likely to be influenced by the resistance mutation(s) present (for example, mutations in inhA and its promoter are usually associated with lower-level resistance than katG mutations) and the patient's acetylator status. [START_REF] Dooley | Early Bactericidal Activity of Different Isoniazid Doses for Drug Resistant TB (INHindsight): A Randomized Open-label Clinical Trial[END_REF] There is recognition that isoniazid may be included in regimens simply due to the use of fixed-dose combination pills. As the majority of the evidence for the treatment of isoniazid-resistant TB derives from secondary observational data, bespoke clinical trials (potentially drawing from emulated target trials) would be needed to strengthen the evidence base. [START_REF] Stagg | Isoniazid-resistant tuberculosis: a cause for concern?[END_REF]

Treatment of MDR/RR-TB

In 2018, the results of an IPD analysis with more than 12,000 patients with MDR/RR-TB, and an observational cohort about the impact of bedaquiline on TB mortality in South Africa, led to substantial changes in the recommendations for management of patients with MDR/RR-TB. [START_REF] Nahid | Treatment of Drug-Resistant Tuberculosis. An Official ATS/CDC/ERS/IDSA Clinical Practice Guideline[END_REF][START_REF] Dooley | Early Bactericidal Activity of Different Isoniazid Doses for Drug Resistant TB (INHindsight): A Randomized Open-label Clinical Trial[END_REF] The recommendation to change from 18-20 months of treatment to an all-oral, shorter, 9-12 month regimen, as well as the recommendation against the use of injectables (namely capreomycin and kanamycin), marked a drastic shift in the management of patients with MDR/RR-TB. [START_REF] Organization | WHO consolidated guidelines on tuberculosis. Module 4: treatmentdrug-resistant tuberculosis treatment[END_REF][START_REF]Rapid communication: key changes to treatment of drug-resistant tuberculosis[END_REF] STREAM Stage 2 trial was a phase III trial that compared a nine-month injectable containing regimen (four months of kanamycin, isoniazid, prothionamide, and nine months of moxifloxacin, clofazimine, ethambutol, pyrazinamide) with a nine-month all-oral regimen where bedaquiline replaced kanamycin. The primary endpoint, favourable treatment outcome, was reached with the injectable-containing regimen in 71% of participants and with the all-oral regimen in 83%. [START_REF] Goodall | Evaluation of two short standardised regimens for the treatment of rifampicin-resistant tuberculosis (STREAM stage 2): an open-label, multicentre, randomised, non-inferiority trial[END_REF] Most importantly, grade 3/4 hearing loss was documented in only 2% of participants receiving the all-oral regimen vs. 9% in the injectable-containing regimen.

The WHO recommended the nine-to twelve-month regimen with bedaquiline (and the option of replacing ethionamide with linezolid given for two months) in 2022 for the treatment of MDR/RR-TB without fluoroquinolone resistance as second option. [START_REF]WHO consolidated guidelines on tuberculosis. Module 4: treatment -drug-resistant tuberculosis treatment, 2022 update[END_REF] The TB-PRACTECAL trial consolidated the evidence that MDR/RR-TB can be treated successfully with a six-month regimen. [START_REF] Nyang'wa B-T | A 24-Week, All-Oral Regimen for Rifampin-Resistant Tuberculosis[END_REF] A regimen with bedaquiline, linezolid, pretomanid and moxifloxacin (BPaLM) was documented in the modified intention to treat analysis to be superior to the standard of care (89% favourable outcomes in BPaLM group versus 51% in standard of care). At least as important as the efficacy of the regimen were the safety results: only 25% patients on BPaLM, compared to 60% on standard of care suffered a grade 3/4 adverse event within 108 weeks after randomization. [START_REF] Nyang'wa | TB-PRACTECAL final efficacy and safety results[END_REF] The trial was stopped early due to the superiority of the BPaLM regimen and the WHO recommended it (very low certainty of evidence) as the preferred treatment option for fluoroquinolone-susceptible MDR/RR-TB in 2022 guidelines, even if the trial included also participants with fluoroquinolone-resistant TB. [START_REF]WHO consolidated guidelines on tuberculosis. Module 4: treatment -drug-resistant tuberculosis treatment, 2022 update[END_REF] The NExT trial supports the potential of bedaquiline, linezolid (600 mg daily) and fluoroquinolones to shorten MDR/RR-TB treatment to six months. [START_REF] Esmail | An All-Oral 6-Month Regimen for Multidrug-Resistant Tuberculosis: A Multicenter, Randomized Controlled Clinical Trial (the NExT Study)[END_REF] An interim analysis of the BEAT-Tuberculosis trial with bedaquiline, linezolid and delamanid for 6 months showed also high efficacy with 87% obtaining a favourable outcome. [START_REF] Conradie | High rate of successful outcomes treating RR-TB with a delamanid-bedaquiline regimen in BEAT Tuberculosis: an interim analysis[END_REF] The MDR-END trial, using a non-bedaquiline based regimen with delamanid, linezolid, levofloxacin and pyrazinamide for 9-12 months, showed 75% success and the regimen was non-inferior to a 20-24 month regimen based on WHO 2014 MDR-TB guidelines. [START_REF] Mok | 9 months of delamanid, linezolid, levofloxacin, and pyrazinamide versus conventional therapy for treatment of fluoroquinolone-sensitive multidrug-resistant tuberculosis (MDR-END): a multicentre, randomised, open-label phase 2/3 non-inferiority trial in South Korea[END_REF][START_REF]Companion handbook to the WHO guidelines for the programmatic management of drug-resistant tuberculosis[END_REF] Table 2 shows completed and ongoing trials not yet published concerning treatment of MDR/RR-TB.

Unfortunately, despite all progress with new regimens, the scarcity of drug resistance testing against bedaquiline, linezolid, pretomanid, delamanid, and other key drugs is a substantial threat to all the progress made in the treatment of MDR/RR-TB. [START_REF] Günther | Availability and costs of medicines for the treatment of tuberculosis in Europe[END_REF] The lack of user-friendly, standardized phenotypic drug susceptibility testing limits not only the scale-up of diagnostics, but also undermines the trust of treating physicians in their implementation in clinical practice. It is crucial to implement widespread routine surveillance systems for drug resistance. [START_REF] Saluzzo | Mind the gap. Rolling out new drug resistant tuberculosis regimens with limited diagnostic tools[END_REF] Moreover, drugs included in these regimens are not accessible everywhere and their availability is jeopardized by unacceptably high costs in many countries. [START_REF]DR-TB Drugs Under the Microscope[END_REF] Treatment of MDR/RR and fluoroquinolone-resistant TB Challenges that clinicians face when managing patients with pre extensively drug-resistant (pre-XDR) TB, defined as RR/MDR-TB with additional fluoroquinolone resistance, include limited efficacy with current regimens, a high adverse event profile, unaffordable costs for most settings, and the potential to amplify drug resistance given the limited availability of registered novel drugs. [START_REF] Dheda | Global control of tuberculosis: from extensively drug-resistant to untreatable tuberculosis[END_REF] The only ongoing trial is reported in Table 3. The BEAT-India trial specifically recruited persons with pre-XDR-TB and used a six to nine-month four-drug regimen (bedaquiline, linezolid at 600 mg daily, clofazimine, and delamanid), 139/153 participants (91%) had a favourable outcome, though linezolid-associated toxicity was considerable. [START_REF] Padmapriyadarsini | Bedaquiline, Delamanid, Linezolid, and Clofazimine for Treatment of Pre-extensively Drug-Resistant Tuberculosis[END_REF] Over half of the participants developed myelosuppression (85, 52%) or neurotoxicity (69, 42%) of any grade, although 34 patients were able to take a lower (300 mg) dose of linezolid. NiX-TB and ZeNix trials used a three-drug 6 months BPaL regimen in pre-XDR-TB or MDR-TB with previous failure (linezolid was dosed 1200 mg daily for 6 months in NiX-TB and 600 mg or 1200 mg daily for 2 or 6 months in ZeNix). Neither study had a control arm and the number of participants included was relatively small. Nix-TB showed ~90% favourable outcome rate in 109 participants, with 81% experiencing peripheral neuropathy and 48% myelosuppression. ZeNix confirmed the efficacy results (favourable outcome ranged between 84% and 93% across different linezolid doses groups) and the risk-benefit ratio seemed in favour of the group that received linezolid at 600mg daily for six months. Nine participants had baseline phenotypic bedaquiline resistance, of whom six had a favourable outcome. [START_REF] Conradie | Bedaquiline-Pretomanid-Linezolid Regimens for Drug-Resistant Tuberculosis[END_REF][START_REF] Conradie | Treatment of Highly Drug-Resistant Pulmonary Tuberculosis[END_REF] The BPaL regimen can be prescribed in case of proven fluoroquinolone resistance, according to WHO recommendations (very low certainty of evidence). [START_REF] Who | Rapid communication: key changes to the treatment of drug-resistant tuberculosis[END_REF] The optimal linezolid dosing posology remains to be established, as current WHO recommended dosing (600 mg daily throughout the treatment) is based on very low certainty of evidence; ongoing efforts may inform policies on reduced/intermittent linezolid administration [START_REF] Guglielmetti | Evaluating newly approved drugs for multidrugresistant tuberculosis (endTB): an adaptive, multi-country randomized controlled trial[END_REF][START_REF]Evaluating newly approved drugs in combination regimens for multidrug-resistant TB with fluoroquinolone resistance (endtb-Q)[END_REF]. In patients with more extensive disease and with unfavourable linezolid pharmacokinetics (sub-optimal linezolid levels relative to minimal inhibitory concentration (MIC)), [START_REF] Wasserman | Linezolid Pharmacokinetics in South African Patients with Drug-Resistant Tuberculosis and a High Prevalence of HIV Coinfection[END_REF] there are concerns about the amplification of resistance, even if evidence is still lacking. Moreover, monitoring linezolid side effects outside clinical trial settings in high-endemic, low-resource areas may be challenging.

In summary, the available findings seem to indicate that a six to nine-month, three-to four-drug regimen to treat fluoroquinolone-resistant MDR/rifampicin-resistant disease is feasible.

Although there is no solid evidence base, where appropriate (multiple poor prognostic features), it would be reasonable for clinicians to opt for a four-drug regimen (i.e. bedaquiline-linezoliddelamanid-clofazimine as in the BEAT-India regimen) or to extend the duration of the regimen in case of culture positivity at the four-month time point when using a six-month regimen.

Overall, it is imperative that capacity for drug-susceptibility testing of Group A drugs (fluoroquinolones, bedaquiline and linezolid), and pretomanid, is urgently developed and rolled out. Concerningly, emerging bedaquiline resistance acquisition has been reported in programmatic setting in South Africa, Moldova and other countries. [START_REF] Ismail | Bedaquiline resistance among patients not responding to a drug resistant treatment regimen in South Africa[END_REF][START_REF] Derendinger | High frequency of bedaquiline resistance in programmatically treated drug-resistant TB patients with sustained culture-positivity in Cape Town, South Africa[END_REF][START_REF] Chesov | Emergence of bedaquiline resistance in a high tuberculosis burden country[END_REF][START_REF] Kaniga | Bedaquiline Drug Resistance Emergence Assessment in Multidrug-Resistant Tuberculosis (MDR-TB): a 5-Year Prospective In Vitro Surveillance Study of Bedaquiline and Other Second-Line Drug Susceptibility Testing in MDR-TB Isolates[END_REF] Host-directed therapy Host-directed therapy (HDT) for TB may either boost host defence ('antimicrobial') or control an exuberant inflammatory phenotype ('anti-inflammatory'). Determining the correct timing of HDT is a challenge. It is equally complicated to identify underlying TB endotypes, defined as distinct immune, epigenetic, metabolic, molecular and transcriptional profiles. In addition, recognised immune risk factors include Mendelian susceptibility to mycobacteria, untreated HIV-1 infection, or TNF inhibitors use. In HIV-1 infection, provision of antiretroviral therapy reduces individual risk for developing TB by 60-80% and reduces mortality, and is thus the most effective HDT widely in use. Conversely, excessive dysregulated immune responses may contribute to tissue damage and even death, such as in tuberculous meningitis, or HIVtuberculosis immune reconstitution inflammatory syndrome (TB-IRIS).

Whilst there has been considerable activity recently on preclinical evaluation of HDT, clinical trial evidence is lacking. Interferon-gamma modestly increased bacterial clearance and resolution of fever in patients with cavitary TB in a single randomized-controlled trial, [START_REF] Dawson | Immunomodulation with recombinant interferon-gamma1b in pulmonary tuberculosis[END_REF] and TNF-and interleukin-1 antagonists have shown to be effective in steroid-refractory paradoxical reactions. [START_REF] Armange | The use of TNF-α antagonists in tuberculosis to control severe paradoxical reaction or immune reconstitution inflammatory syndrome: a case series and literature review[END_REF][START_REF] Van Arkel | Interleukin-1 receptor antagonist anakinra as treatment for paradoxical responses in HIVnegative tuberculosis patients: A case series[END_REF] Vitamin D3 potentially has both antimicrobial and anti-inflammatory actions through promotion of autophagy and the induction of antimicrobial cathelicidin: [START_REF] Chung | Vitamin D-Cathelicidin Axis: at the Crossroads between Protective Immunity and Pathological Inflammation during Infection[END_REF] however, clinical trial evidence of the benefit of systematic addition of vitamin D3 has been modest or non-existent. Metformin therapy of diabetes mellitus associates epidemiologically with benefit, but did not lead to earlier sputum conversion in a recent trial. [START_REF] Padmapriydarsini | Randomized Trial of Metformin With Anti-Tuberculosis Drugs for Early Sputum Conversion in Adults With Pulmonary Tuberculosis[END_REF] A type 4 phosphodiesterase inhibitor and everolimus, a mTOR inhibitor, both modestly enhanced recovery of lung function at end of therapy in a recent trial in South Africa. [START_REF] Wallis | Adjunctive host-directed therapies for pulmonary tuberculosis: a prospective, open-label, phase 2, randomised controlled trial[END_REF] The clearest evidence of anti-inflammatory benefit exists for corticosteroids, which are associated with 30% lower mortality of HIV-1 uninfected TB meningitis, [START_REF] Thwaites | Dexamethasone for the treatment of tuberculous meningitis in adolescents and adults[END_REF] and reduce constriction and hospitalization in TB pericarditis, [START_REF] Mayosi | Prednisolone and Mycobacterium indicus pranii in tuberculous pericarditis[END_REF] and both prevent and improve outcome of TB-IRIS. [START_REF] Meintjes | Prednisone for the Prevention of Paradoxical Tuberculosis-Associated IRIS[END_REF][START_REF] Meintjes | Randomized placebo-controlled trial of prednisone for paradoxical tuberculosisassociated immune reconstitution inflammatory syndrome[END_REF] However, this benefit may vary according to different patient genotypes (i.e. leukotriene A(4) hydrolase) and pro-inflammatory cytokine concentrations (i.e. in cerebrospinal fluid of TB meningitis patients). [START_REF] Tobin | Host genotype-specific therapies can optimize the inflammatory response to mycobacterial infections[END_REF][START_REF] Whitworth | Elevated cerebrospinal fluid cytokine levels in tuberculous meningitis predict survival in response to dexamethasone[END_REF]

Pharmacokinetics and pharmacogenomics

Advances in pharmacokinetics have accelerated the pace of TB drug development. In a salient example, pharmacometric analyses of two clinical trials optimized rifapentine dosing from an initial posology of 10 mg/kg daily to a fixed 1200 mg daily dose of rifapentine as part of the newly-approved four-month regimen. [START_REF] Dorman | Four-Month Rifapentine Regimens with or without Moxifloxacin for Tuberculosis[END_REF][START_REF]WHO consolidated guidelines on tuberculosis. Module 4: treatment -drug-susceptible tuberculosis treatment[END_REF][START_REF] Savic | Defining the optimal dose of rifapentine for pulmonary tuberculosis: Exposure-response relations from two phase II clinical trials[END_REF] While pharmacokinetic studies have demonstrated that rifampicin exposure increases at least dose-proportionally, [START_REF] Boeree | A Dose-Ranging Trial to Optimize the Dose of Rifampin in the Treatment of Tuberculosis[END_REF][START_REF] Peloquin | Pharmacokinetic Evidence from the HIRIF Trial To Support Increased Doses of Rifampin for Tuberculosis[END_REF] and that higher rifampicin doses exhibit dose-and exposure-response relationships, [START_REF] Boeree | High-dose rifampicin, moxifloxacin, and SQ109 for treating tuberculosis: a multi-arm, multi-stage randomised controlled trial[END_REF][START_REF] Velásquez | Efficacy and Safety of High-Dose Rifampin in Pulmonary Tuberculosis. A Randomized Controlled Trial[END_REF] clinical trials have yet to confirm whether treatment shortening is possible with high-dose rifampicin (Table 1). [START_REF] Turkova | Shorter Treatment for Nonsevere Tuberculosis in African and Indian Children[END_REF] The pharmacokinetic analysis of SHINE and results from a separate cohort study indicated substantially reduced drug exposures in children in lower weight bands and in those who transition onto adult doses (≥25 kg), underlining the need for doses to account for the higher mg/kg requirement of smaller individuals. [START_REF] Chabala | Pharmacokinetics of First-Line Drugs in Children With Tuberculosis, Using World Health Organization-Recommended Weight Band Doses and Formulations[END_REF][START_REF] Denti | Optimizing Dosing and Fixed-Dose Combinations of Rifampicin, Isoniazid, and Pyrazinamide in Pediatric Patients With Tuberculosis: A Prospective Population Pharmacokinetic Study[END_REF] Despite significant gains in treatment shortening for adults, adolescents, and children with DS-TB, pharmacokinetic studies suggest that treatment approaches tailored to patient characteristics may be achievable.

Pharmacogenetic evaluations

have not yet gained guideline endorsements in the treatment for TB. The best described pharmacogenetic signal to date, for isoniazid acetylator status, has been considered-but not recommended-by the WHO to inform the use of high-dose isoniazid for the treatment of DR-TB for rapid acetylators. [START_REF](PK/PD) of medicines used in the treatment of drug-resistant tuberculosis[END_REF][START_REF] Gausi | A Semimechanistic Model of the Bactericidal Activity of High-Dose Isoniazid against Multidrug-Resistant Tuberculosis: Results from a Randomized Clinical Trial[END_REF] The recent development of a cartridgebased multiplex quantitative PCR assay on the GeneXpert platform that differentiates NAT2 acetylator genotype signals raises hope that the personalization of treatment, based on host genetic polymorphisms, may be within grasp. [

71]

Future research priorities

There remains a pressing need to find well-tolerated, safe, short regimens for both DS and DR-TB, [START_REF] Dorman | Four-Month Rifapentine Regimens with or without Moxifloxacin for Tuberculosis[END_REF][START_REF] Nyang'wa B-T | A 24-Week, All-Oral Regimen for Rifampin-Resistant Tuberculosis[END_REF] including in particular a better-tolerated alternative for linezolid. Cure of most nonseverely affected patients with a two-month duration of treatment may be achievable. Longacting injectable drugs also have transformative potential, for both prevention and treatment of TB. In parallel with the quest for new regimens, it is important to evaluate strategic, more individualised, treatment approaches, or individual risk-based strategy such as those tested in the TRUNCATE-TB trial. [START_REF] Paton | Treatment Strategy for Rifampin-Susceptible Tuberculosis[END_REF] Efficient testing of multiple new regimens requires identification of a biomarker that is a reliable surrogate for relapse-free cure. [START_REF] Heyckendorf | Prediction of anti-tuberculosis treatment duration based on a 22-gene transcriptomic model[END_REF] This would accelerate the identification and advancement of promising regimens to testing in definitive trials, as well as guide physicians decisions to individualise treatments. [START_REF] Heyckendorf | Tuberculosis Treatment Monitoring and Outcome Measures: New Interest and New Strategies[END_REF] Testing adjunctive host-directed therapies, with the goals of enhancing bacterial clearance and minimising post-tuberculosis lung damage, is an important but neglected research direction. Understanding TB endotypes may enable a stratified approach to use such host-directed agents. [START_REF] Dinardo | Gene expression signatures identify biologically and clinically distinct tuberculosis endotypes[END_REF][START_REF] Dinardo | Tuberculosis endotypes to guide stratified host-directed therapy[END_REF] Implementation research remains critically important to evaluate and optimise outcomes in programmatic settings. There is a need to improve treatment outcome definitions based on long-term outcome benchmarks. [START_REF] Maier | Long-term treatment outcomes in multidrug-resistant tuberculosis[END_REF] Research on the optimal approaches to roll out molecular diagnostic drug susceptibility tests accompanying the availability of new regimens is important to ensure that affected patients receive appropriate therapy. [START_REF] Günther | Availability and costs of medicines for the treatment of tuberculosis in Europe[END_REF] Barriers accessing new drugs (including rifapentine) and regimens are significant, [START_REF] Guglielmetti | Rifapentine access in Europe: growing concerns over key tuberculosis treatment component[END_REF][START_REF] Lange | Regimens for Drug-Resistant Tuberculosis[END_REF] and there is a need for research into how these can be overcome to ensure rapid translation of new findings into practice. This particularly applies to special populations, such as children of all ages and pregnant women: inclusion of these groups in future clinical trials should be prioritized. Table 4 summarizes research priorities.

Conclusions

The last two decades have seen major changes in the management of TB. The availability of new compounds, coupled with renewed interest in TB regimen development, has led to impressive achievements which will have to be sustained in the coming years. While the focus in recent years has been in treatment shortening with new drug combinations, future aims may include improving current regimens by increasing the quality of supporting evidence (including operational and programmatic data), reducing toxicity and optimizing efficacy, for instance by enhancing pharmacokinetic properties, identifying optimal HDT, and further individualizing regimens. In parallel, future efforts should be directed to protect these recent advances by preventing and identifying the selection of drug resistance and providing widespread, affordable, patient-centred access to new treatment options for all people affected by TB. Table 1:

Tables

  Registered, unpublished clinical trials for the treatment of drug-susceptible tuberculosis (as of July 6 th , 2023). , H = isoniazid, Lzd = linezolid, TB = tuberculosis, Z = pyrazinamide, Lfx = levofloxacin, Mfx = moxifloxacin, Bdq = bedaquiline, Pa = pretomanid, Dlm = delamanid, Rbt = rifabutin, Cfz = clofazimine.

Table 3 .

 3 DS-TB = drug-susceptible tuberculosis, Lfx = levofloxacin, Bdq = bedaquiline, Pa = pretomanid, Z = pyrazinamide, Mfx = moxifloxacin, HRZE = isoniazid + rifampicin + pyrazinamide + ethambutol, MDR-TB = multidrug-resistant tuberculosis, Dlm = delamanid, Lzd = linezolid, Cfz = clofazimine, Cs = cycloserine, Pto = prothionamide, NS = not specified, NA = not applicable, TIW = three times weekly, WGS = whole genome Recently completed and ongoing, unpublished trials on rifampicin-resistant, fluoroquinolone-TB (FQ-resistant TB) in ≥15 year-old with pulmonary tuberculosis according to a validated rapid molecular test , Lzd = linezolid, Dlm = delamanid, Cfz = clofazimine, Pre-XDR TB = pre-extensively drug resistant tuberculosis, FQ = fluoroquinolone. Shorter, well-tolerated and safer regimens for drug-susceptible and drug-resistant tuberculosis Sustained early development pipeline of new anti-TB compounds, including long-acting injectable drugs Tailored treatment approach  Treatment strategies based on more individualised treatment, including the identification of criteria to define TB severity  Surrogate biomarker of relapse-free cure Host-directed therapies  Improved understanding of TB endotypes  Host-directed therapies to accelerate bacterial clearance or reduce post-TB morbidity Implementation research Identify barriers to access to new drugs (including special populations, for instance children) Optimise rollout of drug-susceptibility testing for new drugs (including rapid molecular tests and evidence on relationship between phenotypic-genotypic resistance profiles) by Wellcome (203135). For the purposes of open access, the author has applied a CC-BY public copyright licence to any author-accepted manuscript arising from this submission.

Table 2 . Recently completed and ongoing, unpublished trials on rifampicin-resistant tuberculosis treatment (excluding 376 fluoroquinolone-resistant tuberculosis) (as of July 6th, 2023). 377

 2

	Trial	Phase	Control	Country	Experimental treatment regimen(s)	Treatment	Notes	Clinicaltrials.g
			Arm			duration		ov identifier
						(months)
					Recently completed trials
							750-1000 mg Lfx qd
				Peru, South	Lfx 11, 14, 17 or 20 mg/Kg plus
	OptiQ	II	No			6	achieved target	NCT01918397
				Africa	background regimen
							AUC/MIC
			Yes (only				Not non-inferior to
	SimpliciTB	II	for DS-	8 countries	Bdq, Pa, Z, Mfx	4	HRZE; no comparator	NCT03338621
			TB)				for MDR-TB arm
				South Africa,
	SUDOCU	II	No		Sutezolid, Bdq, Dlm, Mfx	3	regimen well tolerated	NCT03959566
				Tanzania
	TREAT-TB						91% favourable	CTRI/2019/01/0
		III	No	India	Bdq, Dlm, Lzd and Cfz	6-9 months
	(India)						outcomes

Acknowledgements: IM, LG and RvC are part of the Study Group on Mycobacteria (ESGMYC) of the European Society of Clinical Microbiology and Infectious Diseases and would like to acknowledge the support from the Study Group.

Funding: GT and CLi are supported by the Wellcome Trust. HRS is supported by the Medical Research Council (MRC), UK [MR/R008345/1]. GEV is supported the National Institute of Allergy and Infectious Diseases at the U.S. National Institutes of Health grant number K08 AI141740. RJW is funded by the Francis Crick Institute which receives funding from Wellcome (CC2112), Cancer Research UK (CC2112) and the Medical Research Council (CC2112). He is also funded

Ongoing trials

Author contributions: IM and LG conceptualized and supervised the review process, wrote part of the initial draft, revised the full draft, and approved the final manuscript; all other authors wrote part of the initial draft, revised the full draft, and approved the final manuscript.

Transparency declaration:

Conflict of interest disclosure: IM, MB, DC, KD, GG, CRH, YK, CLa, CLi, HM, GT, ZU, RvC, GEV, RJW, and LG have nothing to declare HRS reports honoraria in 2018 for speaking at and attending an event organised by the Latvian Society Against Tuberculosis, which was sponsored by Otsuka and Johnson and Johnson; NIP has received grant funding paid to institution from Janssen, drug donation for trials from Pfizer and Sanofi, and speaker fees from Janssen.