Enhancement of docetaxel solubility using binary and ternary solid dispersion systems.

Abstract Context: Poor biopharmaceutical properties and toxicities associated with the intravenous formulation of docetaxel (DTX) necessitate the exploration of an alternate oral route of delivery. Objective: This study aims at enhancing the solubility of poorly soluble drug, DTX with the help of solid dispersion (SD) technique. Method: DTX SDs were formulated with selected solubilizers, including Kollidon 12PF, Lutrol F68, Soluplus and Hydroxypropyl-β-cyclodextrin in different weight ratios. Freeze-drying method was used to prepare the binary and ternary SDs. Kinetic solubility of the SDs was evaluated in order to select best DTX-solubilizer combination. Best performing combination was then characterized using differential scanning calorimeter (DSC), powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Results and Discussion: Among all SDs tested, Soluplus outperformed all the excipients at equivalent weight ratio. Binary SD of DTX and Soluplus (1:10) resulted in the highest improvement in solubility (362.93 ± 11.01 µg/mL). This is approximately a 93-fold increment as compared to the solubility of crystalline DTX (3.9 ± 0.2 µg/mL). This exceptional performance can be attributed to solid-state transformation as well as micellization. Conclusion: Among all the excipients tested, Soluplus dispersion is the most promising candidate for oral formulation development.


Introduction
The rate and extent of drug absorption from solid oral dosage form mainly depend on the solubility and dissolution rate of the active pharmaceutical ingredient. In case of poorly soluble drugs, the solubility and dissolution rate often act as a rate-limiting step in absorption. Enhancements of these properties are a major challenge for pharmaceutical industry 1 . Several techniques have been explored to enhance the aqueous solubility including, complexation, micronization, micro-emulsions, particle size reduction, self-emulsifying micro and nano disperse systems, preparation of SDs, salt and prodrugs.
Docetaxel (DTX) belongs to family of taxane and is widely prescribed chemotherapy drug for the treatment of breast, nonsmall cell lung, prostate, gastric, head and neck cancer. It acts by halting the cell cycle at G2/M phase via binding and subsequent stabilization of tubulin, leading to cell death 2 . DTX is classified under Class IV drug 3 of the Biopharmaceutics Classification System (BCS) having poor solubility and permeability. The low bioavailability of DTX (510%) 4 is mainly due to poor solubility in water and its affinity to the multi-drug efflux pump P-glycoprotein (P-gp) and extensive metabolism by CYP 3A4 in gut wall and liver [5][6][7] . The poor bioavailability of DTX has made solubility an essential aspect to be augmented during product development.
Currently, DTX is available in single-dose vials with polysorbate 80 for the intravenous (IV) infusion. However, oral dosage form will offer obvious inherent benefits. Oral dosage form is generally cost effective, easier to use, and most importantly reduces the risk of severe hypersensitivity reaction that are related to IV administration of polysorbate 80 8 . Certainly, an oral dosage formulation will increase patient compliance as well as improve the therapeutic outcome 3 .
Among the various solubility enhancement techniques, SD preparation is considered simple, economical and amenable to scale up 9 . SD is defined as the dispersion of one or more API in an inert carrier in the solid form. This technique has been effective in increasing the drug solubility, dissolution rate and bioavailability compared to pure drug and physical mixtures. Studies on drug SDs that have been shown to improve solubility include Carbendazin, Cilostazol, Danazol, Ibuprofen, Carbamazepin, Nimodipine, Ritonavir and Tacrolimus 3 . It is understood that the solubility enhancement mechanisms could be attributed to the ability of SD in reducing the particle size of the drug at molecular level and increase in the saturation solubility and/or transforming the drug from crystalline state to amorphous state 10 . Common methods for preparation of SDs include solvent evaporation, freeze-drying (FD), hot melt extrusion, fusion and spray drying. Spray drying and hot melt extrusion are industrially scalable 11 but are of little value for small-scale discovery settings 12 . Methods involving heating are limited due to drug stability. FD is one of the widely used techniques for milligram quantities and has good efficiency in terms of yield 12 .
SDs increase the apparent solubility of the compound by achieving supersaturation. The phenomenon of supersaturation can be explained by ''spring and parachute'' concept proposed for the dissolution of drug 13,14 . When thermodynamically stable form of a drug is dissolved into the medium, the concentration of drug increases until it reaches an equilibrium solubility (path A, Figure 1). When a high-energy form (''Spring'') comes in contact with a medium, the solubility reaches a highest point, S max corresponding to a time T precipitation , after which the solubility start to decline and reaches a point where the solution of a drug is in equilibrium with its thermodynamically most stable solid form (path B, Figure 1); the solubility corresponding to this point is the S equilibrium (Figure 1). When the thermodynamically unstable, high-energy form (''Spring'', e.g. an amorphous dispersion) of the same drug is subjected to the dissolution, it follows path C. Wherein, it achieves higher maximum solubility value by supersaturation (S ss ), and maintain it for a certain time before entering into decline phase. The time corresponding to this maintenance phase is the hang time provided by the excipients which act as a ''parachute''. However, the high-energy form of drug molecules club together and eventually precipitate as the most stable form over the period to give a solubility value equivalent to S equilibrium . As compared to dissolution experiments, in solubility studies non-sink conditions are maintained. But, the ''spring and parachute'' concept can be extended to kinetic solubility measurements for rank ordering different SDs given that the initial amount used for the solubility measurements is kept constant.
The present study was undertaken to improve the therapeutic performance of DTX through solubility enhancement using SD formulated using Lutrol F68 (L), Kollidon 12PF (P), Hydroxypropyl-beta cyclodextrin (HP-b-CD) and Soluplus(S).
Lutrol F68 is a non-ionic poly (ethylene oxide) (PEO)-poly (propylene oxide) (PPO) co-polymer that is commonly used as solubilizing agent, surface active, emulsifying and dispersing agent. Kollidon 12PF is a member of the low-molecular polyvinyl pyrrolidones (PVP) and is used as solubilizing agent, dispersant or crystallization inhibitor. HP-b-CD is a cyclic oligosaccharide, derived from starch. HP-b-CD improves the aqueous solubility by providing the hydrophobic cavity for the lipophilic guest drug molecules. Soluplus is a graft co-polymer of poly-vinyl acetate and poly-vinyl caprolactam on a polyethylene glycol backbone. It is known to solubilize the poorly soluble drugs and serve as the matrix polymer for the solid solutions. It exhibits an excellent solubilizing property for the drugs from the BCS Class II and IV 15,16 .
In this study, SDs were prepared to compare the performance of excipients either alone or in combination. Comparison of Kollidon 12PF and Lutrol F68 combination would be of special importance as chemically this combination represents the copolymer composition of Soluplus (i.e. vinyl derivative and polyethylene glycol). Excipients were selected on the basis of solubility parameter (SP) approach. SD samples were prepared using FD, characterized and evaluated for various physic-chemical parameters.

Materials and methods
DTX anhydrous was purchased from Shanghai Jinhe Bio-Technology Co., Ltd, Shanghai, China. HP-b-CD and Lutrol F68 were purchased from Sigma-Aldrich, Petaling Jaya, Selangor, Malaysia. Kollidon 12PF and Soluplus were gifted from BASF Australia Ltd (Melbourne, Australia). All other materials and reagents were of analytical grade.

Solubility parameter analysis
SP was used as a selection criterion for the polymeric excipients. SP is a measure of cohesive energy density of a substance. Cohesive energy is related to the molar heat of vaporization and can be calculated from the heat of vaporization for low-molecular weight substances. In studying polymers, indirect methods are preferred which include comparative swelling and solubility in a solvent of known cohesive energy. One of the mathematical approaches for determining the SP is the Hansen solubility parameter (HSP) calculation. It defines a compound on the basis of dispersion (d d ), polarity (d p ) and hydrogen bonding (d h ) properties 17 . Each of these parameters accounts for the atomic, molecular and electronic level interactions. SP approach has been successfully used as a measure of miscibility of the drugs in the polymeric excipients for the preparation of SDs 18,19 . Greater the miscibility, lesser is the chance of recrystallization, hence literature value of DTX was compared with the polymer to select the polymeric carriers.

Preparation of the binary and ternary solid dispersions
All the SDs were prepared by FD method for two reasons: (1) DTX is an expensive drug and (2) FD allows small quantities to be processed with 100% yield.
DTX stock solution (10 mg/mL) was prepared in absolute ethanol. HP-b-CD, Soluplus and Kollidon 12PF were dissolved in purified water at 10 mg/mL concentration. One milliliter of DTX solution was mixed with an appropriate amount of polymer solution to make different ratios of binary and ternary dispersions ( Table 1). The final volume was adjusted with water so that ethanol did not exceed 10% v/v in the solution used for the FD. For 1:1 HP-b-CD inclusion complex preparation 20 , DTX solution was added to the HP-b-CD solution and stirred at room temperature for 2 h. Solution was filtered through 0.45-mm syringe filter (PVDF). Before FD, solutions were frozen at À80 C for at least 6 h and then subjected to lyophillization in Novalyphe-NL 500 (Savant Instruments Corp., Holbrook, NY) lyophilizer for at least 24 h at À45 C and 7 Â 10 À2 mbar pressure. The SDs were then stored in the desiccator for further analysis. Best performing binary ratios were further combined to make ternary dispersions. Table 1 shows the different combinations and ratios used for preparing dispersions. Amorphous DTX was prepared using solvent evaporation method 21 using Buchi Rotavap II instrument. 50 mg of DTX was dissolved in 5-7 mL of acetone, then the mixture was dried at 55-60 C under vacuum (500-600 mbar). The product was collected and stored in desiccator before analysis.

Kinetic solubility test
The dispersion equivalent to about 1 mg of DTX was weighed and kept for solubility study in 2.5 mL of 0.1 M, pH 6.8 phosphate buffer at ambient temperature. The samples were continuously rotated using a mechanical shaker (Axyos Technologies, Brisbane, Australia) throughout the test. Solubility samples were collected at the predetermined time interval and filtered through 0.45-mm PVDF syringe filter. Subsequently, the filtrates were diluted using acetonitrile. The analysis of the samples was performed on an HPLC (Shimadzu, Kyoto, Japan) system equipped with a UV-VIS detector [SPD-20 A], DGU-20A3 online degasser, CBM-20 A system controller, SIL-20AHT autosampler and a LC solution Chromopac data processor. Zorbax Eclipse XDB-C 18 (4.6 Â 150 mm, 3.5 mm) analytical column was used. The mobile phase used was acetonitrile and ammonium acetate pH 5 (0.02 M) at the ratio of 57:43. The injection volume was 20 mL with a flow rate of 1 mL/min, and detection was performed at 230 nm 22,23 .

Scanning electron microscopy (SEM)
Samples were platinum coated using Quorumtech K757X sputter coater to get 10-nm thick deposits. SEM CamScan MX2500 (Ottawa, Canada) was used to study the morphology of DTX and the SD samples. It was operated in secondary electron mode at an accelerating voltage of 15 kV.

Fourier transform infrared spectroscopy
FTIR patterns were recorded between 800 and 4000 cm À1 at 2 cm À1 scan rate using Shimadzu Fourier Transform Infra-Red Spectrophotometer 8400S (Tokyo, Japan). Conventional, potassium bromide (KBr) pallet method was used for the sample preparation.

Differential scanning calorimetry
Thermal analysis was performed using differential scanning calorimeter (DSC) (TA Q3 Model 2920, New Castle, DE). Hermetically sealed samples were heated at 10 C/min from ambient to 220 C under nitrogen purge (50 mL/min).
Powder X-ray diffractometry X-ray diffraction patterns were obtained from a Rigaku MiniFlex 600 bench top XRD (Rigaku Corporation, Tokyo, Japan). Radiations were generated from Cu source (Ka, ¼ 1.54 Å ) operating at 30 kV and 20 mA. Samples were scanned at a step size of 0.02 between 3-40 2y.

Statistical analysis
All experiments were performed in triplicate and the values are reported as mean ± standard deviation. The analysis of variance (ANOVA) and Tukey HSD post-hoc test was performed using the IBM SPSS software (Armonk, NY). A probability level of p50.05 was considered statistically significant.

Solubility parameter analysis
It has been observed that less than 7 MPa 1/2 difference in the SP leads to favorable interactions between drug and polymer, resulting in mutual miscibility 18,24 . Although this value has been proposed on the basis of specific drug-polymer combination, it can serve as a criterion for the excipient selection. SP value for DTX is 26.6 MPa 1/2 . It was compared with the polymeric excipients as shown in Table 2 25,26 . Using above value as a guideline, it can be predicted that the excipients having value between 19.6 and 33.6 MPa 1/2 should be miscible with DTX, whereas an excipient with SP value less than 19.6 or greater than

Kinetic solubility test
Prepared SDs were subjected to kinetic solubility analysis. Figure 2 shows the 24 h solubility values for DTX, binary and ternary dispersions. From here on, the solubility at 24 h will be referred as equilibrium solubility. Crystalline and amorphous forms of DTX show nearly the same equilibrium solubility of about $3.9 mg/mL, which is in close agreement with the previously published data 2,29 . Though both forms show 5 min of T precipitation value, but the amorphous forms show slightly higher S max value, 14.23 mg/mL as compared to crystalline form (Table 3).
For all the combinations of SDs, the equilibrium solubility is greater than any of the DTX forms. Equimolar complex of DTX with HP-b-CD showed an equilibrium solubility of $5 mg/mL and S ss of about $6 mg/mL at 10 min. S equilibrium was 1.3-folds higher than the crystalline form.
All the binary combinations of DTX and Kollidon 12PF show polymer concentration dependent improvement in the S equilibrium value ranging from 5.4 to 10 mg/mL. Improvements were also seen in the hang time, as shown in Table 3. Binary dispersions of Soluplus at all the ratios showed significant improvement in the solubility value ranging from 53.4 to 363 mg/mL. These are 24 h solubility numbers, as no T precipitation was observed.
Ternary dispersions were prepared from the best combination of the binary dispersion. In the case of Kollidon 12PF, 1:10 ratio was found to give best performance considering the S max and T precipitation (12.7 mg/mL at 30 min). This combination was considered as one part when combining with the third component, i.e. Lutrol F68 to get 1:2, 1:5 and 1:10 weight ratios. As compared to binary combinations, S max values were found to improve but there was a negative correlation as far as the hang time was concerned.
Molecular complexes of DTX:HP-b-CD (1:1) were combined with Soluplus in 1:2, 1:5 and 1:10 weight ratios. Increase in apparent solubility as well as T precipitation was observed when compared to inclusion complex alone (Table 3).

Statistical analysis
ANOVA was performed at T 20 and T 45 min solubility data for the best-performing formulations ( Table 4) and it was found that there was significant difference at both time points. The kinetic solubility profiles and the details of the statistical tests can be found under Supplementary material (S1 and S2). The results of post-hoc Tukey test showed that at 20 min time point, formulations DH (p ¼ 0.98), DP 1:10 (p ¼ 0.91) and (DP)L 1:10 (p ¼ 0.60), did not differ significantly from the crystalline DTX. But the formulations containing Soluplus, i.e. DS 1:10 (p ¼ 0.00) and (DH)S 1:10 (p ¼ 0.00) were significantly different from the crystalline DTX. Similar observation was made at 45 min. The formulations DH (p ¼ 0.91), DP 1:10 (p ¼ 1.00) and (DP)L 1:10 (p ¼ 0.44) were statistically not different as compared to crystalline DTX, while Soluplus containing formulations differed significantly from the crystalline DTX. When the Soluplus containing formulations were compared at T 20 and T 45 , it was found that at T 20 there was significant difference in the solubility of DS 1:10 and (DH)S 1:10, but at T 45 min there was no statistical difference between the two (p ¼ 0.43). Combination of Lutrol F68 with DP 1:10 SD did not improve the solubility significantly at T 20 and T 40 (p ¼ 0.99 and p ¼ 0.72, respectively). Addition of Soluplus to HP-b-CD complex significantly improved the solubility at both time points.

Scanning electron microscopy (SEM)
SEM analysis was performed to study the morphological feature of best combinations from each SD group. DTX shows needleshaped acicular crystals ( Figure 3A). Dispersion of DTX along with the Soluplus, HP-b-CD and Lutrol F68 combinations  exhibited a typical sponge-like morphology ( Figure 3B, C and D). No traces of crystalline DTX were observed.

Fourier transform infrared spectroscopy
Fourier transform infrared spectroscopy (FTIR) was performed on the best performing binary combination, i.e. DTX and Soluplus. The IR spectra are shown in Figure 4. Spectrum of DTX showed peaks corresponding to N-H and O-H stretching around 3460 cm À1 . Aliphatic and aromatic C-H stretches were observed between 2820 and 3060 cm À1 and C ¼ O stretching corresponding to the 22 peak around 1720 cm À1 . Soluplus showed peaks around 3450 cm À1 corresponding to O-H stretching, and carbonyl stretching at 1635 and 1740 cm À1 . Peaks corresponding to C-H stretching 16 were observed around 2930 cm À1 . There were no peak shifts/appearance or disappearance of characteristic peaks in the SD of DTX and Soluplus (1:1) as compared to pure DTX and Soluplus. It indicated that there was no chemical incompatibility and the DTX is chemically intact in the Soluplus matrix.

Differential scanning calorimetry
DSC was performed on Soluplus dispersion to determine the solid-state changes in DTX dispersion preparation. In the thermogram of freeze-dried Soluplus, no thermal event was recorded ( Figure 5A), indicating the amorphous nature of the polymeric carrier. Amorphous DTX do not show an event corresponding to the melting peak shown by the crystalline DTX at 169.13 C ( Figure 5C and D). In the case of DTX Soluplus dispersion (1:1) no endothermic event corresponding to melting of DTX was observed, indicating the amorphization of the DTX in the SD ( Figure 5B).

Powder X-ray diffractometry
Powder X-ray diffractometry (PXRD) was performed to confirm the solid-state characteristics of the Soluplus dispersion. Figure 6 shows the PXRD pattern for DTX Soluplus (1:1) dispersion, freeze-dried Soluplus, amorphous and crystalline DTX. As expected, crystalline form of DTX showed sharp characteristic diffraction peaks confirming the crystallinity of the initial form used for the preparation of SDs. In freeze dried Soluplus and amorphous DTX samples broad hallows were observed indicating the amorphous nature. The PXRD pattern of DTX Soluplus (1:1) dispersion did not exhibit crystallinity which corroborates the DSC results, where no thermal events were recorded corresponding to the melting of DTX.

Discussion
Amorphous form of DTX was prepared successfully using the solvent evaporation method. Amorphization was confirmed using DSC and XRD analyses. Amorphous DTX showed kinetic advantage as compared to the crystalline form (S max Crystalline DTX5S max amorphous DTX). However, equilibrium solubility in both cases is nearly same. This suggests the conversion of amorphous to a crystalline form by the end of the solubility experiment 3 . In this study, anhydrous crystalline DTX was used. It is known that the DTX trihydrate is the thermodynamically most stable form 30 , hence both amorphous and the anhydrous crystalline form could have converted to a trihydrate form by the end of the study. Tatini et al. 31 have reported the conversion of methnolates and ethanolates to trihydrate form in presence of 95% relative humidity. Hence, it is important to point here that, no attempt was made to analyze the solid form after the solubility analysis as it is imperative that all the DTX after solubility analysis would eventually convert to trihydrate form. SP was used as a selection criterion for preparing the binary and ternary SDs of DTX. As predicted, Lutrol F68 yielded amorphous product. In the case of Soluplus and Kollidon 12PF, SDs were prepared successfully despite the moderate miscibility predicted by SP. It is worthwhile to highlight here that, SP takes into consideration the van der Waals-type interactions but specific directional interactions like hydrogen bonding are outside the limits of this concept 32 . Similar deviation has been observed for other drug-polymer combination 33 , and this appears to be the case  Kinetic solubility was used as a performance criterion for SDs. All the binary and ternary SDs prepared showed higher S ss (except DH and DP 1:2) as compared to the S max of crystalline DTX. In the case of binary dispersion of HP-b-CD and Kollidon 12PF at 1:2 ratio though S ss was lower, S equillibrium was higher than crystalline DTX. Besides, the hang time of 5 min was achieved (as compared to crystalline DTX), which means that the higher amount of solubilizer used in the above two cases acted in sustaining the solubilization for longer but at the same time shielded the DTX instant solubilization. From here on, results for each excipient with all combinations (binary/ternary) will be discussed one by one.
In the case of binary HP-b-CD dispersion, improvement in S 24 h and T precipitation was observed with a decrease in the S ss value. Previously, HP-b-CD has been shown to improve the   solubility of DTX to 7.43 mg/mL at 40% w/w concentration 27 . It appears that a higher amount of HP-b-CD is required for a meaningful increment in the solubility value. Ternary dispersion of HP-b-CD with Soluplus clearly showed an increment in the S ss , T precipitation and S 24 h . This appears to be primarily an effect of Soluplus, and it will be discussed at greater length in the later part of the discussion. Kollidon 12PF binary SDs exhibited higher equilibrium solubility and longer T precipitation at all the ratios, with up to 2.6fold increment. T precipitation of DP 1:10 reach up to 30 min. Viscosity increment has been considered as one of the factors for delayed precipitation 34 when PVP K30 (MW $40 K) is used 3 . In this study, Kollidon 12PF was used which has lower molecular weight (MW 2-3 K) as compared to PVP K30. The viscosity of the medium does increase due to Kollidon 12PF but it is for sure, going to be lesser than the PVP K30 at the equivalent concentration. And also, there is a high potential of hydrogenbonding interaction between PVP hydrogen bond acceptors 35 and DTX hydrogen bond donors. So, it is not difficult to surmise that the overall increment could be a combined effect of viscosity 34 , hydrogen-bonding interactions 36 and shielding of DTX by PVP 3 . To further enhance the performance Kollidon 12PF, it was combined with Lutrol F68. When the binary dispersion of Kollidon 12PF is compared with a ternary dispersion containing Lutrol F68, there is a clear advantage in terms of S ss and S 24 h due to surface activity of Lutrol F68. These ternary dispersions showed lesser T precipitation , which means the rate of DTX release from the dispersion is higher as compared to binary dispersion with Kollidon 12PF alone. This infers that DTX dissociates faster from Kollidon 12PF in the presence of Lutrol F68.
Soluplus binary dispersion showed the best performance when equilibrium solubility is compared with all the other dispersions studied. Solid-state characterization, i.e. DSC and PXRD, clearly showed the successful amorphization of the DTX. No T precipitation was observed within the duration of experiment. This suggests that Soluplus acts as a precipitation inhibitor and keeps the supersaturated condition for more than 24 h. Shamma et al. 16 have shown that an increase in the concentration of Soluplus in the SD can increase the drug solubility. This is mainly because of the micellar solubilization. The critical micelle concentration (CMC) of Soluplus was reported to be 0.0007% w/v at 37 C 16 . Soluplus forms micelles by reorganizing the hydrophilic polyethylene glycol moiety into a micellar wall and the vinyl acetate and vinyl caprolactam side chains are buried inside the micelles. Again, there is a potential of hydrogen bonding interactions between the hydrogen acceptor groups in the vinyl acetate and vinyl caprolactam group 10 with DTX. However, in solid state these interactions were not evident in the infrared spectroscopy,  possibly due to overlapping of the peaks. So it can be concluded that the kinetics of solubilization are positively influenced by both, formation of amorphous form (''Spring'') as well as micelle in the solution (''Parachute''). Ternary dispersion of Soluplus and HP-b-CD showed a decrease in equilibrium solubility and hang time when compared with the Soluplus binary dispersion, although it is an improvement when compared with a binary dispersion with HP-b-CD alone. It appears that the kinetics of solubilization is relatively suppressed in the case of ternary dispersion. In general, it has been observed that the presence of cyclodextrins increases the critical micellar concentration of the surfactant by binding with the hydrophobic part 37,38 . This could explain the suppressed solubility and hang time observed when Soluplus is combined with HP-b-CD. From post-hoc analysis, binary Soluplus (1:10) and ternary dispersion with HP-b-CD (1:10) showed no significant difference at 45 min time point, but when overall profile was considered in terms of S ss , T precipitation /hang time and S equilibrium , binary dispersion of Soluplus at 1:10 outperformed not only the ternary dispersion with HP-b-CD but all the other SDs studied.
It is worthwhile comparing the results of Kollidon 12PF/Lutrol F68 combinations with Soluplus alone due to similarity of the chemical composition. It was observed that the ternary complex of Kollidon 12PF and Lutrol F68 exhibited maximum T precipitation of 10 min [(DP)L 1:10] and S max of about 22.32 mg/mL [(DP)L 1:2] as opposed to the S max of about 362.93 mg/mL (DS 1:10) without precipitation. These results can be explained from two perspectives, i.e. T precipitation and S max on the basis of molecular weight/dissociation and mechanism of solubility enhancement. Kollidon 12PF is a low-molecular weight polymer as compared to Soluplus (11.8 K); in addition, the presence of Lutrol F68 could have led to faster DTX dissociation. As for the solubility enhancement, Kollidon 12PF acts by amorphization alone as opposed to Soluplus whose action was supplemented by micellization.
As per the statistical analysis, Soluplus dispersions show higher solubility at T 20 and T 45 min which essentially points to the fact that DTX was converted to a metastable amorphous form (''Spring''), and the polymers used helped to maintain supersaturation for longer duration (''Parachute'') relative to crystalline DTX. Though metastable forms present attractive avenue for increasing apparent solubility, form conversion during storage could lead to formulation instability 39,40 . Hence, in solid dispersion (SD) approach a rigid polymer help in avoiding this conversion, and storing formulation below the glass transition temperature 41 could also add to the shelf life of the product.
Recently, a clinical study has reported encouraging results from a ternary SD of DTX with PVP K30 and SLS in 1:9:1 weight ratio 3 . When combined with ritonavir, a CYP 3A4 inhibitor, no significant difference was observed in the DTX premix solution and the SD. In addition, inter-individual exposure variability was decreased in the SD group. This study reports a dissolution test which was performed in water for injection at 37 C. The best performing combination (vide supra) afforded the S max of $210 mg/mL, T precipitation of $9 min and equilibrium solubility of $20 mg/mL. If these results are compared with the kinetic solubility study performed in this study, it strongly suggests that the DTX and Soluplus combination could yield potentially higher bioavailability when combined with ritonavir.

Conclusion
SP approach can be a good guide for the polymer selection provided that no specific interactions are involved. SDs prepared with solubilizers using the FD method is a promising technique to enhance solubility of drugs. The results obtained shows that Soluplus, Kollidon 12PF, Lutrol F68 and HP-b-CD were effective in increasing solubility of the poorly soluble drug. The type of solubilizer and DTX-solubilizer ratio are critical in the selection of appropriate SD combination. Among all the SDs studied, Soluplus SD in 1:10 ratio exhibited the best performance in terms of equilibrium solubility and delayed precipitation. This article also demonstrates that the typical excipient combinations, i.e. polymer and a surfactant explored for SD preparations could be replaced with a graft copolymer Soluplus with greater efficiency at equivalent weight ratio. Thus, Soluplus is the best option for DTX SD preparation. However, further investigations dealing with stability and in-vivo behavior of this SD formulation is necessary to ensure therapeutic usefulness.