Zahraa Hammoud

Riham Gharib

Sophie Fourmentin

Abdelhamid Elaissari

Hélène Greige-Gerges

New findings on the incorporation of essential oil components into liposomes composed of lipoid S100 and cholesterol

Keywords: aqueous solubility, encapsulation, essential oil components, henry constant, liposomes, release

The encapsulation of essential oil components into liposomes was demonstrated to improve their solubility and chemical stability. The aim of the present study is to investigate the effect of chemical structure, henry constant (H " ) value and aqueous solubility of essential oil components on their liposomal encapsulation. Based on ethanol injection method, the essential oil components (estragole, eucalyptol, isoeugenol, pulegone, terpineol, and thymol) were encapsulated in lipoid S100-liposomes. Static headspace gas chromatography method showed that H # of estragole, isoeugenol and eucalyptol was approximately 10 times greater than that of pulegone, terpineol and thymol. Besides, dynamic light scattering and diffraction technique revealed that all liposomal vesicles were of micrometric in size and the presence of phenols, isoeugenol and thymol, promoted enlargement of vesicles. Incorporation in liposomes was better (encapsulation efficiency > 90 %) for the essential oil components exhibiting low aqueous solubility, estragole, isoeugenol, and pulegone. Besides, efficient entrapment in liposomes (loading rate > 18 %) was obtained for isoeugenol, terpineol and thymol. This suggests that the presence of hydroxyl group in the structure and exhibiting lower H " ameliorated the entrapment of essential oil components into liposomes. Furthermore, release rate from liposomes varied among the investigated essential oil components and it was controlled by loading rate of essential oil components into liposomes, size of liposomal batches, location of essential oil components within lipid bilayer, and cholesterol incorporation rate of liposomes. Finally, liposomal encapsulation of isoeugenol, pulegone, terpineol, and thymol was suitable to protect them as a considerable concentration was retained in liposomes after 10 months with respect to initial concentration.

Introduction

Essential oils are complex mixtures composed mainly of monoterpenes and phenylpropenes and biosynthesized in nature by aromatic plants for their protection against various pathogens [START_REF] Bakkali | Biological Effects of Essential Oils--a Review[END_REF]. Also, essential oils received remarkable attention in food, cosmetics, and pharmaceutical industries. For instance, they are widely used as flavouring agents, food preservatives, and in manufacturing several dosage forms including suppositories, capsules, soap, perfumes, and others [START_REF] Asbahani | Essential Oils: From Extraction to Encapsulation[END_REF]. In the present study, the phenyl propenes; estragole and isoeugenol, and the monoterpenes; eucalyptol, pulegone, terpineol, and thymol were chosen as models of essential oil components. These components exhibit considerable biological effects including antimicrobial, antifungal, antinociceptive, anti-inflammatory [START_REF] Dogan | Chemical Composition and Biological Activities of Leaf and Fruit Essential Oils from Eucalyptus Camaldulensis[END_REF][START_REF] Kfoury | Solubility, Photostability and Antifungal Activity of Phenylpropanoids Encapsulated in Cyclodextrins[END_REF][START_REF] Sousa | Pharmacological Activity of (R)-(+)-Pulegone, a Chemical Constituent of Essential Oils[END_REF][START_REF] Júnior | Acute and Neuropathic Orofacial Antinociceptive Effect of Eucalyptol[END_REF][START_REF] Detoni | Evaluation of Thermal-Oxidative Stability and Antiglioma Activity of Zanthoxylum Tingoassuiba Essential Oil Entrapped into Multi-and Unilamellar Liposomes[END_REF][START_REF] Wattanasatcha | Thymol Nanospheres as an Effective Anti-Bacterial Agent[END_REF][START_REF] Riella | Anti-Inflammatory and Cicatrizing Activities of Thymol, a Monoterpene of the Essential Oil from Lippia Gracilis, in Rodents[END_REF] antioxidant [START_REF] Kfoury | Effect of Cyclodextrin Complexation on Phenylpropanoids' Solubility and Antioxidant Activity[END_REF], and anesthetic [START_REF] Reiner | Effects of Propofol and Other GABAergic Phenols on Membrane Molecular Organization[END_REF] activities.

The selected essential oil components differ by their octanol/water partition coefficient (Log P) value, their aqueous solubility, and their henry constant (H " ) value. According to literature, log P values varied between 2.6 (Pubchem) and 3.4 (Pubchem) for isoeugenol and estragole respectively (Table 1). Also, their aqueous solubility ranged between 178 for estragole [START_REF] Yalkowsky | Aquasol Database of Aqueous Solubility[END_REF] and 7100 mg/L for terpineol [START_REF] Li | Preprints of Papers Presented at the 209th ACS National Meeting[END_REF] (Table 1). Besides, H " values of the chosen essential oil components were determined in literature as estimated from the vapour pressure and aqueous solubility at 25 °C using the vapour pressure/aqueous solubility method [START_REF] Us Epa | Estimation Program Interface (EPI) Suite. Ver. 4.1[END_REF]. The values ranged between 2.23 × 10 &' atm ( ) /mol for terpineol and 4.62 × 10 &* atm ( ) /mol for estragole (Table 2). However, to our knowledge there are no experimental H " values determined for the essential oil components we are studying.

The use of essential oils components is hindered by their volatility, poor water solubility and instability in presence of heat, light and oxygen [START_REF] Turek | Impact of Different Storage Conditions on the Quality of Selected Essential Oils[END_REF]. Hence, their encapsulation in different carrier systems including liposomes, cyclodextrins, solid lipid nanoparticles, micelles, and others, was demonstrated to overcome these drawbacks and preserve their activities.

Liposomes are spherical microscopic vesicles comprising a central aqueous compartment enclosed by a membrane constituted mainly of phospholipids and may contain cholesterol. They are biocompatible, biodegradable, non-immunogenic and non-toxic [START_REF] Gharib | Liposomes Incorporating Cyclodextrin-Drug Inclusion Complexes: Current State of Knowledge[END_REF]. Thus, liposomes are suitable for delivery of bioactive compounds in which hydrophobic substances can be entrapped within the lipid bilayer, hydrophilic molecules within the aqueous internal cavity and amphiphilic ones at the waterbilayer interface [START_REF] Laouini | Preparation, Characterization and Applications of Liposomes: State of the Art[END_REF]. It had been reported that liposomal incorporation represents an efficient opportunity for improving solubility and chemical stability of essential oils components [START_REF] Detoni | Evaluation of Thermal-Oxidative Stability and Antiglioma Activity of Zanthoxylum Tingoassuiba Essential Oil Entrapped into Multi-and Unilamellar Liposomes[END_REF].

The interaction of the selected essential oil components with dipalmitoylphosphatidylcholine (DPPC) membrane has been studied by Raman spectroscopy, differential scanning calorimetry and fluorescence anisotropy [START_REF] Gharib | Interaction of Selected Phenylpropenes with Dipalmitoylphosphatidylcholine Membrane and Their Relevance to Antibacterial Activity[END_REF][START_REF] Gharib | Effect of a Series of Essential Oil Molecules on DPPC Membrane Fluidity: A Biophysical Study[END_REF]. Results demonstrated that the essential oil components exhibited a membrane fluidizing effect. Besides, their incorporation, except eucalyptol, decreased the transition enthalpy (∆, -) value and increased the gauche conformers leading to an increase in the disorder of lipid bilayer.

Several studies prepared liposomes containing various essential oil components and characterized them in terms of their size, drug encapsulation efficiency (EE) and loading rate (LR), release kinetics, storage stability, and biological activities [START_REF] Sebaaly | Preparation and Characterization of Clove Essential Oil-Loaded Liposomes[END_REF][START_REF] Gharib | Interaction of Selected Phenylpropenes with Dipalmitoylphosphatidylcholine Membrane and Their Relevance to Antibacterial Activity[END_REF]other references).

Furthermore, few studies discussed the effect of drug properties on liposomes characteristics. Zhigaltsev et al. [START_REF] Zhigaltsev | Liposome-Encapsulated Vincristine, Vinblastine and Vinorelbine: A Comparative Study of Drug Loading and Retention[END_REF]) compared the characteristics of egg sphingomyelin/cholesterol liposomes containing three structurally closely related vinca alkaloids; vincristine, vinorelbine and vinblastine and demonstrated that lipophilicity of drug, determined by its log P value, influences its loading and release kinetics from liposomes. Nevertheless, our study is the first to deal with a series of components and estimate the factors that modulate their liposomal encapsulation and release.

In the present study, liposomal formulations containing estragole, eucalyptol, isoeugenol, pulegone, terpineol and thymol were prepared by ethanol injection method using lipoid S100 and cholesterol as liposomes constituents. The liposomal batches were characterized for their size, drug encapsulation efficiency (EE), loading rate (LR), and drug release from liposomes. Phospholipids and cholesterol incorporation rates were also determined. Finally, the stability of liposomal suspensions was evaluated after storage for 10 months at 4 ⁰C. The effect of bearing a hydroxyl group in essential oil components structure, the physiochemical properties of essential oil components including Henri constant value and aqueous solubility, and the final liposomal composition experimentally determined (phospholipid: cholesterol: drug molar ratio) on liposome characteristics is discussed.

Materials and methods

Materials

Non-hydrogenated soybean phosphatidylcholine lipoid S100 (94% soybean phosphatidylcholine, 3% lysophosphatidylcholine, 0.5% N-acyl-phosphatidylethanolamine, 0.1% phosphatidylethanolamine, 0.1% phosphatidylinositol, 2% water, 0.2% ethanol) was supplied by lipoid GmBh, Germany. 4-Amino-3hydroxy-1-naphthalenesulfonic acid was purchased from Fluka, India and hydrogen peroxide from Fisher Scientific, UK. Cholesterol (94%) was purchased from Sigma-Aldrich, Japan; ammonium molybdate, eugenol, isoeugenol, potassium dihydrogenophosphate and thymol from Sigma-Aldrich, Germany;

pulegone and terpineol from Sigma-Aldrich, Switzerland; estragole from Sigma-Aldrich, China; eucalyptol and triton X-100 from Sigma-Aldrich, USA; and methanol HPLC grade from Sigma-Aldrich, France.

Absolute ethanol and sulfuric acid were purchased from VWR Pro-labo chemicals, France. Cholesterol assay kit was purchased from Spin react Company, Spain.

Preparation of liposomes

Ethanol injection method was applied to prepare the liposomal formulations. The required amounts of lipoid S100 (10 mg/ml) and cholesterol (5 mg/ml) were dissolved in absolute ethanol. 10 ml of the obtained organic phase was later injected using a syringe pump (Fortuna optima, GmbH-Allemagne) into 20 ml aqueous phase at a flow rate of 1 ml/min, under magnetic stirring at 400 rpm, and at room temperature. As soon as the organic phase was in contact with the aqueous phase, spontaneous liposomes formation occurred. The liposomal suspensions were then kept for 15 min under stirring (400 rpm) at room temperature. Finally, ethanol was removed by rotary evaporation (Heidolph instruments GmbH and co., Germany) at 41 ⁰C and 60 rpm under reduced pressure. Both blank liposomes and drug loaded liposomes were formed in which the essential oil component (estragole, eucalyptol, isoeugenol, pulegone, terpineol or thymol) was added to the organic phase at a concentration of 2.5 mg/mL. Each of the batches was prepared in triplicate. The liposomal formulations were stored at 4 ⁰C prior to analysis.

2.3.

Characterization of liposomal formulations 2.3.1. Measurement of liposome particle size Particle size of liposomes was determined by a Laser granulometer (Partica Laser scattering, particle size distribution analyser LA-950V2; HORIBA, Japan) designed for measuring particle size between 0.01 and 3000 μm. Liposomal particle size and percentage of each population in the suspension were measured and data were expressed as mean ± standard deviation.

Determination of phospholipids: cholesterol: drug molar ratio in the liposome structures

The un-retained components in preparations were separated from the retained ones by submitting liposomal suspension to centrifugation at 15,000 rpm and 4 ⁰C for 1 hour using a vivaspin 500 centrifugal concentrator (Sartorius Stedim Biotech, Germany, MW cut off=10,000 Da). Aliquots were removed from liposomal suspensions then sonicated for 10 minutes in ice to determine the total concentration of liposomal constituents (phospholipids, cholesterol, and drugs). Also, aliquots were taken from filtrate to determine the free concentration of liposomal constituents. The phospholipids: cholesterol: drug molar ratio for each formulation was later calculated.

Assay of phospholipids

The phospholipids concentration in the liposomal suspension and liposomal filtrate was quantified through Bartlett's method. The organic phosphates in the samples (500 µL from filtrate, total liposomal suspension, and standard solution of phosphorus) were digested by the addition of 400 µL sulfuric acid (5M) at 200 °C for 1h. Then, the organic phosphates were oxidized to inorganic phosphates through incubating the samples in the presence of 100 µL H . O . (30 %) for 30 min at 200 °C. Through interaction with ammonium molybdite (4.6 ml), the phosphomolybdic complex was formed. The latter was reduced to a blue product upon interaction with 4-amino-3-hydroxy-1-naphthalenesulfonic acid (200 µL) at 100 °C for 15 min and the absorbance of this blue compound was later measured at 815 nm using a UV-vis spectrophotometer (U-2900, Hitachi High-Tech Science Corporation, Japan is the initial mass of phospholipids added to the organic phase during liposomes preparation.

Dosage of cholesterol

For quantifying the total and free concentrations of cholesterol in suspension, enzymatic colorimetric method was utilized. 1 ml of the cholesterol assay kit containing cholesterol esterase, cholesterol oxidase, peroxidase, and 4-aminophenazone, was added to samples (10 μl of cholesterol standards, filtrate and total liposomal suspension). Cholesterol in the samples was oxidized by cholesterol oxidase into 4cholestenona and hydrogen peroxide. The latter reacts with 4-aminophenazone in the presence of peroxidase to form a coloured complex, quinonimine, of a colour intensity proportional to the cholesterol concentration in the sample. The absorbance of the compound was measured at 505 nm using a UV-vis spectrophotometer (U-2900, Hitachi High-Tech Science Corporation, Japan). Stock solution of cholesterol was prepared in triton X-100 at a concentration 2 mg/ml and diluted to final concentrations ranging between 0.05 and 1.5 mg/ml. The incorporation rate IR (%) was determined based on the following equation:

UV WXY (%) = Z [\] ^ & Z [\] _ Z [\] `abcdef ghcij × 100 (Equation 2)
where M #k3 : is the cholesterol mass in the liposomal suspension, M #k3 < is the mass of cholesterol in filtrate and M #k3 =>?@ABC DE@FG is the initial mass of cholesterol added to the organic phase during liposomal preparation.

Determination of drug encapsulation efficiency and loading rate

The EE (%) and LR (%) values of the selected essential oil components were calculated after HPLC analysis of the samples. The analysis was performed using an analytic column (C 18: 15 cm× 4.6 mm (Agilent)) at a wavelength of 204 nm. The mobile phase was made up of methanol and water (70:30) for all samples except for pulegone (65:35). The flow rate was set at 1 ml/min. The injection volume was 20 µl.

Stock standard solutions of the selected essential oil components were prepared in methanol at a concentration of 1 mg/ml then diluted to desired concentrations. Linearity was proved between 1 to 250 μg/mL for estragole; 50 to 1000 μg/mL for eucalyptol; 0.2 to 100 μg/mL for isoeugenol and thymol; 2.5 to 100 μg/mL for pulegone; and 1 to 100 μg/mL for terpineol. Besides, stock standard solutions (1 mg/ml) of the internal standards were prepared in methanol and diluted to desired concentrations. Thymol (100 μg/ml) was used as an internal standard for the dosage of estragole and isoeugenol. Eugenol was used at a concentration of 1 μg/ml for eucalyptol dosage and 2 μg/ml for pulegone, terpineol and thymol dosages.

100 μl of each solution (standard solution, liposomal suspension, or liposomal filtrate) was added to a solution of internal standard (100 µl) and methanol (200 µl). The EE (%) value of each formulation was calculated as follows:

EE (%) = [mLnM] p=p@q & [mLnM] r>GG [mLnM] p=p@q
× 100 (Equation 3)

where [Drug] wKwNx and [Drug] yLTT correspond to the total and free drug concentrations in liposomal suspension respectively.

Besides, the LR (%) values were calculated according to the following equation:

LR (%) = { qBD=F=|@q F}FDGAFB=A & { rBqp>@pG
{ =>?@ABC DE@FG × 100 (Equation 4)

Where m xPQKSK{Nx SnSQTOSPKO and m yPxwLNwT correspond to the mass of drug in liposomal suspension (total)

and liposomal filtrate (free) respectively. ( ÄÅÇÉÑÖÜ áàÉâä is the initial mass of drug added to the organic phase during liposome preparation.

In vitro drug release Kinetics

The dynamic release of drugs from liposomes was studied by multiple headspace extraction (MHE) coupled to gas chromatography. 1 ml of each drug loaded liposome sample was added to 9 ml of water in 22 ml sealed vials; the corresponding free drug (estragole and thymol at 1 ppm; isoeugenol at 200 ppm; terpineol at 5 ppm; pulegone at 2 ppm and eucalyptol at 0.5 ppm) was also introduced in 22 ml sealed vials. After equilibrium, vials were submitted to 30 successive gas extractions at constant time interval (8 min), oven temperature of 60°C, transfer line temperature of 250°C and using nitrogen as a carrier vector.

1 ml of the vapour present in the gaseous phase was then transferred to gas chromatography (GC) for analysis. All measurements were conducted using an Agilent G1888 headspace sampler coupled to a Perkin Elmer Autosystem XL gas chromatography equipped with a flame ionization detector and a DB624 column. GC conditions were set as follows: GC cycle of 8 min and column temperature of 160°C for estragole, eucalyptol, terpineol, pulegone and thymol and of 180°C for isoeugenol. The percentage of essential oil component remaining at time t was calculated as follows:

ãåçéåèêëíå ìî çå(ëïèïèí åññåèêïëó ìïó éì(òìèåèê = ö õ ö ú × 100 (Equation 5)

where, At and A1 corresponds to the area of the chromatographic peak of studied essential oil component at time t and at the first extraction, respectively.

Determination of loading capacity

The amount of essential oil component in liposome was quantified using MHE. The concentration of the analyte in the gas phase of the vial decreases at each extraction steps. Thus, the sum of the analyte amounts removed in the individual extractions will be equal to the total amount of analyte present in the original sample [START_REF] Kolb | Static Headspace-Gas Chromatography: Theory and Practice[END_REF].

A linear regression was constructed for the standard and the samples using the following equation:

lnAt =-q(t-1) + lnA1
where A1 is the area of the chromatographic peak of essential oil component at the first extraction; At is the peak area of essential oil component at the extraction time t and -q is the slope of the regression curve that illustrates ln At as a function of (t -1).

The sum of peak areas corresponding to the total amount of essential oil component present in the sample or in the standard are obtained as follows:

∑ û ü = ö ú †&°
Loading capacity (LC) is expressed as µg of essential oil component / mg of liposomes.

LC = £LnM G §D ( ¶M)

{NSS Ky xPQKSK{TS ({M) (Equation 6)

Study of liposome storage stability

After 10 months of storage at 4 °C, the hydrodynamic particle size of the liposomal formulations was measured by dynamic light scattering and diffraction technique. Besides, the free and encapsulated concentration of each essential oil component in liposomal suspension was assessed after 10 months via through HPLC analysis and the deduced values were compared to those obtained immediately during preparation.

Determination of Henry constant

Henry's constant law (HC) is the vapor-liquid partition coefficient value of the various essential oil components which was determined by static headspace-gas chromatography (SH-GC). HC is expressed as follow [START_REF] Kolb | Static Headspace-Gas Chromatography: Theory and Practice[END_REF]: 7)

H # = # ® # © (Equation
where CG is the concentration of drug in the vapor phase and CL, the concentration of drug in the aqueous phase For the determination of HC in water, several headspace vials were prepared containing different amounts of water (0.5; 0.6; 1, 2, 3 and 5mL for estragole, pulegone, eucalyptol and 0.1; 0.2; 0.3; 0.4; 0.5 and 0.6 mL for thymol, isoeugenol and terpineol) to which the same amount of drug was added (10 µL from a standard solution of 10000 ppm). The headspace settings were as follows: an oven temperature of 30 or 60 °C and an equilibrium time of 30 min. The transfer line temperature was set at 250 °C. The GC settings were a described above.

Using the phase ratio variation method described by [START_REF] Kolb | Static Headspace-Gas Chromatography: Theory and Practice[END_REF], the values of HC were determined by the relationship between the reciprocal chromatographic peak areas and the vapourliquid volumetric ratio (VG/VL, with VG and VL being the vapour volume and the liquid volume, respectively):

† ö™ ´= † ¨™≠ ™ ´+ † ¨X[
(Equation 8)

where A is the peak area, α is a constant incorporating several parameters and HC is the vapour-liquid partition coefficient, directly calculated from the ratio of the slope and intercept of the fit of the experimental data obtained after allowing a vapour-liquid equilibrium time of 30 min in the oven.

Results and discussion

Henry constants of the studied essential oil components

Applying the static head space-gas chromatography method, H # of the selected essential oil components The total and free phospholipids concentrations in liposomal suspension were assessed through Bartlet's method and the incorporation rates (IR) values were calculated according to equation 1 and results are summarized in table 3. Standard curve was constructed by plotting the corresponding absorbance against the concentration of phosphorus in mM. The linear relationships were evaluated by regression analysis with the least squares method and the correlation coefficient was found to be 0.999.

For blank liposome, phospholipids IR value was high (94.2 ± 6.45 %). Thus, most of the initial mass of phospholipids used for liposome preparation was embedded in liposomes. The effect of essential oil components addition on phospholipids retention in liposomes differed among the tested components.

Compared to initial phospholipids: cholesterol: drug molar ratio used for liposome preparation and to blank liposomes, it is obvious that the addition of estragole and eucalyptol did not widely affect the loading of phospholipids in liposomes. However, the presence of isoeugenol, pulegone, terpineol and thymol significantly reduced phospholipids retention in liposomes and their effect decrease in order of pulegone > thymol > isoeugenol > terpineol. Hence, isoeugenol, pulegone, terpineol and thymol replaced phospholipids molecules in lipid bilayer.

Upon interaction with phospholipids bilayer, certain molecules cause the variation of both transition temperature Tm and enthalpy difference (∆H), thus act as "substitution impurities" of membrane taking place of lipid molecules. Instead, other molecules cause Tm, but not ∆H, variation, thus act as "interstitial impurities" by insertion among the flexible acyl chains of lipids [START_REF] Cristani | Interaction of Four Monoterpenes Contained in Essential Oils with Model Membranes: Implications for Their Antibacterial Activity[END_REF]). In particular, Gharib et al. [START_REF] Gharib | Interaction of Selected Phenylpropenes with Dipalmitoylphosphatidylcholine Membrane and Their Relevance to Antibacterial Activity[END_REF][START_REF] Gharib | Effect of a Series of Essential Oil Molecules on DPPC Membrane Fluidity: A Biophysical Study[END_REF] studied respectively the interaction of several phenylpropenes and monoterpenes, including the ones selected in this study, with DPPC liposomes. As a result, it had been investigated that the essential oil components, except eucalyptol, act as substitution impurities, taking place of lipid molecules. On the other hand, eucalyptol acts as interstitial impurity and intercalate in the bilayer thus does not affect phospholipids loading in membrane. Moreover, it had been proven that the presence of estragole induces less decrease in Tm and ∆, -in comparison to different phenylpropenes studied and ∆, -decreased significantly in presence of estragole from molar ratio of 100:5 in a concentration-dependent manner. Thus, the phospholipid: cholesterol: estragole molar ratio (113:110:2) determined in our study could provide a logical reasoning for the lower effect of estragole on phospholipids incorporation rate compared to other components studied.

Cholesterol

Using the enzymatic colorimetric method, cholesterol concentration in liposomal suspension and filtrate was determined and cholesterol IR values were calculated according to equation 2. Results are shown in table 3. For blank liposomes, IR of cholesterol was 77.6 ± 1.18 %. Compared to empty vesicles, a significant increase in the cholesterol IR was only obtained upon the addition of estragole and terpineol to liposomes. [START_REF] Gharib | Interaction of Selected Phenylpropenes with Dipalmitoylphosphatidylcholine Membrane and Their Relevance to Antibacterial Activity[END_REF][START_REF] Gharib | Effect of a Series of Essential Oil Molecules on DPPC Membrane Fluidity: A Biophysical Study[END_REF] demonstrated that the selected essential oil components are able to fluidize the membrane by decreasing the phase transition temperature of DPPC vesicles. Hence, we may propose an increase in cholesterol incorporation into estragole and terpineol loaded liposomes with increasing membrane fluidity induced by the presence of estragole and terpineol respectively. On the other hand, loading of eucalyptol, isoeugenol, pulegone, and thymol into liposomes reduced cholesterol incorporation in the formulations. Hence, eucalyptol, isoeugenol, pulegone, and thymol replaced cholesterol in lipid bilayer. This agree with Frang et al study [START_REF] Fang | Effect of Liposomes and Niosomes on Skin Permeation of Enoxacin[END_REF]) which reported that certain lipid soluble drugs compete with cholesterol molecules for the hydrophobic space in liposomal membrane, thus decease cholesterol incorporation in liposomes.

Drugs

The retention times on HPLC were 8.4 ± 0.04, 6.9 ± 0.09, 3.5 ± 0.00, 7.8 ± 0.06, 6.3 ± 0.01, 5.6 ± 0.01 for estragole, eucalyptol, isoeugenol, pulegone, terpineol and thymol respectively. Besides, the retention time of eugenol was 3.4 ± 0.00 with all drugs, except with pulegone it was 4.3 ± 0.005. These results are due to the different methanol: water ratio (65:35) used for HPLC analysis of pulegone compared to that used for analysis of the other molecules (70:30). The calibration curves were constructed by plotting AUC £LnM AUC POwTLONx SwNO£NL£ ⁄ against the drug concentrations in μg/ml. The linear relationships were evaluated by regression analysis with the least squares method and the correlation coefficient (V . ) ranged between 0.989 and 0.999. After HPLC analysis, the EE and LR values of estragole, eucalyptol, isoeugenol, pulegone, terpineol and thymol into lipoid S100-liposomes were calculated according to equations 3 and 4 respectively.

The entrapment of drugs in liposomes, determined by their LR values, varied among the studied essential oil components. LR values of essential oil components bearing a hydroxyl group, isoeugenol, terpineol and thymol were high (> 18 %) and decrease in order of ≥V üà¥-ĵ > ≥V ÖâÄä∂ÇäÑĵ > ≥V üäÅáÖÑäĵ . However, the non-hydroxylic essential oil components, estragole, eucalyptol, and pulegone represented lower LR values (< 5 %) and ≥V ä∂Üɵ¥áüĵ > ≥V á∂µäÇÄÑä > ≥V äâüÅÉÇĵä . Therefore, the presence of hydroxyl group in essential oil component structure enhanced its incorporation in liposomes. This implies that isoeugenol, terpineol, and thymol would be incorporated within the lipid bilayer with their hydroxyl group located toward the polar groups of phospholipids and hydrophobic parts are buried inside the lipid bilayer. Our findings were in good agreement with other studies in literature reported by Reiner et al. [START_REF] Reiner | Effects of Propofol and Other GABAergic Phenols on Membrane Molecular Organization[END_REF] where phenols, mainly thymol, interact and insert in egg phosphatidylcholine vesicles due to the presence of free hydroxyl group in their structure and their incorporation decrease viscosity of monolayers. Besides, it had been reported that the presence of hydroxyl group in monoterpenes structure enhances its membrane fluidizing effect [START_REF] Gharib | Effect of a Series of Essential Oil Molecules on DPPC Membrane Fluidity: A Biophysical Study[END_REF].

Therefore, increasing fluidity of membranes promotes the incorporation of essential oil components.

On the other hand, the lower LR values of estragole, eucalyptol and pulegone may be attributed to the greater loss of these components due to their higher H " compared to other selected components.

Concerning ) between EE of essential oil components into liposomes and aqueous solubility of the components was calculated and its value was 0.85, with the exception of terpineol. Besides, hydrophobicity is another factor that may influence drug encapsulation in liposomes [START_REF] Zhigaltsev | Liposome-Encapsulated Vincristine, Vinblastine and Vinorelbine: A Comparative Study of Drug Loading and Retention[END_REF]. Indeed, hydrophobicity of essential oil components determined by their log P values increased in order of isoeugenol (2.376, 2.6) < eucalyptol (2.74) < terpineol (2.98) < pulegone (3.08) < estragole (3.13, 3.4) ~ thymol (3.34).Thus, our results demonstrated that hydrophobicity of essential oil components did not influence their encapsulation into liposomes.

From the above results, bearing a hydroxyl group in the chemical structure of essential oil components, exhibiting low aqueous solubility and low H # value are the main parameters that improve essential oil components encapsulation in liposomes, as evidenced by the high EE and LR values.

Loading capacity of drugs

Through MHE method, amount of drugs in formulations was quantified for calculating loading capacity (LC) of essential oil components. Table 3 represents the LC value, expressed as µg of essential oil components / mg of liposome. It is important to note that loading capacity represents the total mass of drug in suspension rather than the mass encapsulated in liposomes so it would not correlate with LR and EE values determined before.

Based on LC values, we could demonstrate that isoeugenol represents the highest retention level in liposomal suspension (LC was 4.9 µg/ mg) with respect to other components; LC of isoeugenol was about 10 times greater than that of other components studied. Pulegone, terpineol and thymol showed moderate levels of encapsulation in suspension with LC being in order of pulegone (0.1 µg/mg) < terpineol (0.6 µg/mg) ~ thymol (0.6 µg/mg) respectively. Estragole and eucalyptol were less retained in suspension (LC < 0.02 µg/ mg). The difference in LC could be related to variation in total mass of liposomes affected by final liposomal composition (phospholipid: cholesterol: drug molar ratio) and amount of water accumulated in vesicles. Also it may be related to the difference in H # of the components used in which components with less H # value were better retained in suspension. For Pho: Cho: drug ratio normal font is based on initial moles of liposomal component used for liposome preparation, bold is based on liposomal components embedded in liposomes.

Liposomal particle size measurement

Table 4: Percentage distribution and mean particle size of populations for blank liposomes and the drug loaded liposomes at day of preparation and after 10 months of storage at 4 °C.

Italic: after 10 months of storage at 4 °C Through laser granulometry, the particle size distribution of the liposomal batches was measured and compared between the various batches. Table 3 represents the number, percentage distribution and mean particle size of populations obtained in each formulation. For blank liposomes, a single micrometric sized (6.16 ± 0.51 μm) population appeared. Thus, larger vesicles were obtained in comparison to literature. This difference can be ascribed to the mechanism by which vesicles are formed during the selected method [START_REF] Lasic | The Mechanism of Vesicle Formation[END_REF]. During ethanol diffusion into water phase, phospholipids dissolved in the organic phase precipitate at the water/organic boundary phase forming bi-layered phospholipids fragments (BPFs). BPFs seal off to form vesicles and the short distance between them enhances the possibility of forming larger vesicles by facilitating their coalescence probability [START_REF] Yang | Fast High-Throughput Screening of Temoporfin-Loaded Liposomal Formulations Prepared by Ethanol Injection Method[END_REF]. Moreover, this difference (in what? In particle size?) could be related to the different methods used for size characterization. Indeed, some are determined using dynamic light scattering (DLS) apparatus (Zetasizer Nano ZS; MalvernInstruments Ltd, France) allow measurement of only submicron size range populations [START_REF] Storti | Particle Size Distributions by Laser Diffraction: Sensitivity of Granular Matter Strength to Analytical Operating Procedures[END_REF]. However, laser granulometry technique used in this study is designed for measuring particle size between 0.01 and 3000 µm.

Regarding the number of populations obtained, eucalyptol and pulegone loaded liposomes were the most similar to blank liposomes in which one micrometric sized population appeared. However, the presence of other essential oil components induced an increase in number of populations obtained. For isoeugenol and thymol loaded liposome two micrometric sized populations appeared. For estragole and terpineol loaded liposomes three populations arose; one in nanometric and two in micrometric sizes (table 4).

The effect of adding the selected essential oil components on particle size varied among the selected essential oil components (table 4). In comparison to blank formulation, the addition of estragole and terpineol promoted the production of nanometric batches. On the other hand, the incorporation of estragole, isoeugenol, terpineol and thymol boosts the formation of larger vesicles (and accordingly reduced the percentage of smaller micrometric vesicles) compared to blank liposomes. It might be that the accumulation of lipophilic drugs in the hydrophobic part of the membrane affects the interactions between the acyl chains of phospholipids and induce swelling of the membrane, leading to formation of larger vesicles [START_REF] Sikkema | Mechanisms of Membrane Toxicity of Hydrocarbons[END_REF]. Moreover, large vesicles were reported to be clusters or aggregates of smaller particles [START_REF] Domazou | SIZE DISTRIBUTION OF SPONTANEOUSLY FORMED LIPOSOMES BY THE ALCOHOL INJECTION METHOD[END_REF].

Besides, the mean particle size of estragole, eucalyptol, pulegone and terpineol loaded liposomes did not significantly differ from that of blank batches; the size of the major population obtained was close to size of blank liposome. However, the encapsulation of the phenolic components, isoeugenol and thymol, in liposomes increased liposome particle size in which the size of the major population (70.76 ± 5.65 and 27.7 ± 5.89 µm for isoeugenol and thymol respectively) was significantly greater than that of blank liposome (6.16 ± 0.51 µm). It had been demonstrated that the essential oil "Eucalyptus camaldulensis" [START_REF] Moghimipour | Preparation and Characterization of Liposomes Containing Essential Oil of Eucalyptus Camaldulensis Leaf[END_REF]) and the essential oil component, eugenol, induce a considerable increase in vesicles size [START_REF] Sebaaly | Preparation and Characterization of Clove Essential Oil-Loaded Liposomes[END_REF]. Moreover, Reiner et al. [START_REF] Reiner | Effects of Propofol and Other GABAergic Phenols on Membrane Molecular Organization[END_REF] had demonstrated that five phenol compound, including thymol, insert in egg phosphatidylcholine-liposomes in regions between the polar groups (choline molecule), the glycerol backbone and the first atoms of the acyl chains. This arrangement induces a reduction of the phospholipids head group repulsive force and decreases the mobility of hydrocarbon chains, thus induce an increase in particle size. Therefore, the presence of phenolic hydroxyl group in thymol and isoeugenol structure promotes an increase in liposome vesicle size. The experiment of drug release was conducted at 60 °C using MHE method. Figure 1 depicts, at each extraction step, the remaining percentage of essential oil components as free molecules or when encapsulated in liposomes.

We can notice based on figure 1 that liposomal encapsulation of estragole, isoeugeol, and thymol reduced their volatility and permitted their sustained release. At each extraction step, the remaining percentage in liposomal suspension was considerably greater than that in the free form. However, liposomal encapsulation of eucalyptol, pulegone, and terpineol did not influence their release.

In their free form, the studied essential oil components exhibited different release pattern and the remaining percentage at the last extraction differed among the various essential oil components (figure 1). The latter increased in order of estragole (74.8 %) < eucalyptol (85.6 %) < isoeugenol (85.9 %) < thymol (91.1 %) < pulegone (94.1 %) < terpineol (96.8 %). This order correlates with that of H " determined at 60 °C, except for isoeugenol (# $ were 0.53 and 0.89 with and without isoeugenol respectively).

Moreover, when loaded in liposomes, essential oil components represented different release profiles.

Isoeugenol and thymol manifested biphasic release patterns in which they remain unreleased for a period of time (first phase), followed by a slow release over the second phase. The other essential oil components exhibited one phase release pattern with different release rates. Terpineol and pulegone showed slow release rate while estragole and eucalyptol showed rapid release rate. In addition, the remaining percentage at the last extraction increased in order of estragole (78.9 %) < eucalyptol (85.6 %) < pulegone (94.9 %) < terpineol (95.6 %) < isoeugenol (96.5 %) < thymol (96.4 %).

Based on the above results, we could notice that EE of essential oil components into liposomes, chemical structure of components, their aqueous solubility values, and their log P values did not seem to influence their release rate from liposomes. Moreover, release of components was not affected by their H & so liposomes efficiently retained them and consequently reduced their volatility.

Indeed, LR of essential oil components into liposomes controlled their release rate. Linear relationship was found between the percentage of essential oil components determined at the last extraction step and LR of essential oil components into liposomes, except for pulegone; # $ was 0.54 with pulegone while 0.95 without pulegone.

Besides, results of release rate from liposomes may be attributed to the size of formulations and the location of essential oil components within the lipid bilayer [START_REF] Juliano | PHARMACOKINETICS OF LIPOSOME-ENCAPSULATED ANTITUMOR DRUGS AND IMPLICATIONS FOR THERAPY[END_REF].

Based on results of particle size measurement (batches have size greater than 0.5 µm) and method of liposomes preparation, it can be stated that the formulations are multi-lamellar vesicles [START_REF] Rongen | Liposomes and Immunoassays[END_REF][START_REF] Jaafar-Maalej | Ethanol Injection Method for Hydrophilic and Lipophilic Drug-Loaded Liposome Preparation[END_REF]. Also, size of isoeugenol and thymol loaded liposomes was significantly greater than that of other formulations. Therefore, isoeugenol and thymol may be encapsulated within the internal lamellas of liposomes and must pass several barriers before being released to the outer environment. This explains the biphasic release pattern of isoeugenol and thymol when loaded in liposome. Moreover, regarding the location of essential oil components within lipid bilayer, [START_REF] Gharib | Interaction of Selected Phenylpropenes with Dipalmitoylphosphatidylcholine Membrane and Their Relevance to Antibacterial Activity[END_REF][START_REF] Gharib | Effect of a Series of Essential Oil Molecules on DPPC Membrane Fluidity: A Biophysical Study[END_REF] studied the interaction of the studied essential oil components with DPPC liposomes and specified their location within the bilayer. As a result, it had been proven that position of double bond in propenyl group of phenypropenes control its incorporation in liposomes where isoeugenol, possessing the double bond at position C7-C8 in the propenyl side chain, inserted deeply in the bilayer in comparison to estragole. Also, the presence of hydroxyl group allows a deep bilayer incorporation of terpineol and thymol in comparison to pulegone and eucalyptol.

Pulegone exhibited low LR value (2.91± 0.32 %) and previous study reported that it did not deeply incorporate in DPPC bilayer [START_REF] Gharib | Effect of a Series of Essential Oil Molecules on DPPC Membrane Fluidity: A Biophysical Study[END_REF]). Nevertheless, it exhibited a slow release rate (94.9 % remained at the final extraction step). This could be ascribed to the lower cholesterol incorporation rate (39.6 ± 5.09%) for pulegone loaded liposomes in comparison to the other prepared vesicles. It had been confirmed that liposomal retention of floxuridine was enhanced by decreasing cholesterol content in the formulation (Tardi et al. 2007) and idarubicin is retained better in cholesterol free vesicles in comparison to cholesterol containing ones (Dos [START_REF] Santos | Improved Retention of Idarubicin after Intravenous Injection Obtained for Cholesterol-Free Liposomes[END_REF]. Cholesterol helps to decrease the membrane fluidity and allows at the phase transition temperature (Tm) of phospholipids the switching from fluid phase of phospholipids bilayer to gel phase [START_REF] Liu | Kinetic Stability and Membrane Structure of Liposomes during in Vitro Infant Intestinal Digestion: Effect of Cholesterol and Lactoferrin[END_REF]. Besides, it was demonstrated that cholesterol may induce a reduction in permeability of liposomal membrane [START_REF] Sezer | In Vitro Evaluation of Enrofloxacin-Loaded MLV Liposomes[END_REF]. Hence, the presence of lower cholesterol content in the formulation may delay the release of some essential oil components from liposomes.

Hence, size of liposomal batches, IR of cholesterol into liposomes, LR of essential oil components, and the location of components within lipid bilayer are the main factors that affect essential oil components release rate from lipoid S-100 liposomes.

Storage stability

After 10 months of storage at 4 °C, the stability of the formulations was examined. All of the prepared batches, except thymol loaded liposomes, showed a significant increase in liposome particle size (table 3). These results were similar to those obtained by Sebaaly et al [START_REF] Sebaaly | Preparation and Characterization of Clove Essential Oil-Loaded Liposomes[END_REF] in which eugenol loaded lipoid-S100 liposomes particle size increased after storage for 2 months at 4 °C.

For each formulation, the total and free concentrations of essential oil components in liposomal suspensions were determined at day of preparation and after 10 months. Figure 2 summarizes the obtained results. In general, a considerable amount of essential oil components remained in liposomal suspension (> 50 %), except for estragole loaded liposomes, in comparison to initial amount. Besides, with respect to initial concentration of essential oils incorporated in liposomes; a noticeable concentration of isoeugenol, pulegone, terpineol, and thymol was retained in liposomes after 10 months. Those findings correlate with that of release study in which isoeugenol, pulegone, terpineol, and thymol exhibiting a delayed release were satisfactory incorporated in liposomes after 10 months.

Thus, the encapsulation of isoeugenol, pulegone, terpineol and thymol in lipoid S-100 liposomes is efficient for protecting them.

4-Conclusion

Drug loaded lipoid S-100 liposomes encapsulating diverse essential oil components were prepared by ethanol injection method and characterized via several techniques. The parameters that affect liposomal particle size, EE and LR, release kinetics and liposome storage stability had been discussed.

The addition of the phenolic essential oil components, isoeugenol and thymol, induces enlargement of vesicles in comparison to blank liposomes. Besides, EE values into liposomes were greater for essential oil components exhibiting lower aqueous solubility. Regarding the LR values, results were different.

Hydroxylated essential oil components were better entrapped in liposomes than non hydroxylated ones and incorporation was better for essential oil components with low henry constants.

Furthermore, the release profiles were variable among the various formulations. Liposome particle size, LR of essential oil components into liposomes, location of components within liposome bilayer, and IR of cholesterol into liposomes were the key parameters that affect release of essential oil components from liposomes. Finally, liposomes loading isoeugenol, pulegone, terpineol, and thymol were stable after long term storage at 4°C. Hence, liposomes could be useful for the encapsulation of essential oil components extending their various applications. However, several factors control their incorporation into liposomes must be considered in future studies.

Highlights to review

• At 30 °C, H " of estragole, eucalyptol and isoeugenol were approximately 10 times greater than that pulegone, terpineol and thymol

• Isoeugenol, pulegone, terpineol and thymol replaced phospholipids molecules while eucalyptol, isoeugenol, pulegone, and thymol replaced cholesterol in lipid bilayer.

• The presence of hydroxyl group in essential oil component structure enhanced its incorporation in liposomes

•

Figure 1 :

 1 Figure 1: The remaining percentage of various essential oil components, in their free and liposome encapsulated form after different extractions. (A) estragole; (B) eucalyptol; (C) isoeugenol; (D) pulegone; (E) terpineol; (F) thymol .

Figure 2 :

 2 Figure 2: Storage stability of the various prepared liposomal batches: Remaining concentrations of free and encapsulated drugs in liposome suspension

••

  Exhibiting high H & could be a factor that causes reduced incorporation of essential oil components in liposomes Incorporation in liposomes was better for the essential oil components exhibiting low aqueous solubility The incorporation of estragole, isoeugenol, terpineol and thymol boosts the formation of larger vesicles • The presence of phenolic hydroxyl group in thymol and isoeugenol structure promotes an increase in liposome vesicle size

Table 1 : Structure, log P, and aqueous solubility of the studied essential oil components

 1

	Essential oil	Structure	Log P	Aqueous solubility
	component			(mg/L)
	Estragole		3.4	178
			(Pubchem)	(Yalkowsky and
				Dannenfelser
				1992)
	Eucalyptol		2.74	3500
			(Griffin, Wyllie,	(Yalkowsky, He,
			and Markham	and Jain 2010)
			1999)
	Isoeugenol		2.6	665; 810
			(Pubchem)	(Kfoury et al.
				2014; HERA 2012)
	Pulegone		3.08	276
			(Griffin, Wyllie,	(US EPA. 2012)
			and Markham
			1999)
	Terpineol		2.98	7100
			(Li and Perdue	(Li and Perdue
			1995)	1995)
	Thymol		3.3	900
			(Pubchem)	(Yalkowsky and
				Dannenfelser
				1992)

Table 2 :

 2 Estimated and experimental Henry constants of the selected essential oil components at 30 and 60 °C

	Drug	Henry constant				Henry constant (30 °C)	Henry constant (60 °C)
		(atm ( ) /mol at 25 °C)
	Estragole	4.62 × 10 &*	a		0.03	0.07
	Eucalyptol	1.1 ×10 &*	a	0.01	0.05
	Isoeugenol	3.6 × 10 &'	b		0.03	0.1
	Pulegone	5.87 × 10 &Ø	a		0.005	0.009
	Terpineol	2.23 × 10 &'	c		0.001	0.003

was determined and results are presented in table 2. First, H # values increased upon increasing temperature from 30 to 60 °C. Thus, increasing temperature induced essential oil component to reside more in the gaseous phase. This is in agreement with Copolovici et al study

[START_REF] Copolovici | Temperature Dependencies of Henry's Law Constants and Octanol/Water Partition Coefficients for Key Plant Volatile Monoterpenoids[END_REF]

which demonstrated strong influence of temperature on H # value of each of the ten plant monoterpenes, including terpineol, at temperature range 25-50°C. Moreover, H # of the selected essential oil components followed the same order at 30 and 60 °C except isoeugenol; which was more affected by temperature increase in comparison to other essential oil components. Furthermore, H " of estragole, eucalyptol and isoeugenol were approximately 10 times greater than that pulegone, terpineol and thymol at 30 °C. Finally, we could notice that the experimental H # values, determined in our study at 30 °C, were consistent with the predicted H # values present in literature (V . > 0.99) except for isoeugenol. It was demonstrated that temperature increase greatly affect H # of isoeugenol so it may represent great variation between 25 °C (temperature used for estimating H # in literature) and 30 °C (temperature used to determine H # values in our study) in comparison to the other components.

Table 3 :

 3 Phospholipids and cholesterol incorporation rates (IR), drug encapsulation efficiency (EE), drug loading rate (LR), phospholipids: cholesterol: drug molar ratio and loading capacity for the various essential oil components loaded liposomes

	Liposomal components	Pho: CHO:	Pho: CHO:	Loading
		drug molar	drug molar	capacity
		ratio	percentage in	(µg/mg)
			liposome
			structure

• Size of liposomal batches, cholesterol IR of liposomes, LR of essential oil components into liposomes, and location of essential oil components within lipid bilayer are the main factors that affect essential oil components release rate from liposomes.