Improving Solubility and Pharmacokinetics of Meloxicam via Multiple

May 29, 2012 - (5) Therefore, much effort has been devoted to the improvement of drug solubility in pharmaceutical development, with a special emphasi...
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Improving Solubility and Pharmacokinetics of Meloxicam via Multiple-Component Crystal Formation David R. Weyna,† Miranda L. Cheney,† Ning Shan,*,† Mazen Hanna,† Michael J. Zaworotko,‡ Vasyl Sava,§ Shijie Song,§ and Juan R. Sanchez-Ramos§ †

Thar Pharmaceuticals Inc, 3802 Spectrum Boulevard, Suite 120, Tampa, Florida 33612, United States Department of Chemistry, University of South Florida, 4202 East Fowler Avenue, CHE205, Tampa, Florida 33620, United States § James A. Haley Veterans Affairs Medical Center, Tampa, Florida 33612, United States ‡

S Supporting Information *

ABSTRACT: Meloxicam is a nonsteroidal anti-inflammatory drug prescribed for rheumatoid arthritis, osteoarthritis, postoperative pain and fever. Meloxicam exhibits low solubility in acidic aqueous media and a slow onset of action in biological subjects. An oral dosage form of meloxicam with enhanced aqueous solubility is desired to enable a faster onset of action and its use for mild-to-medium-level acute pain relief. With this in mind, we examine the solubility and pharmacokinetics of 12 meloxicam cocrystals with carboxylic acids. Dissolution studies of meloxicam and its cocrystals were performed in pH 6.5 phosphate buffer solutions at 37 °C. In addition, pharmacokinetic profiles over four hours were acquired after oral administration of a 10 mg/kg (meloxicam equivalent) solid suspension in rats. The majority of meloxicam cocrystals were found to achieve higher meloxicam concentrations in dissolution media and enhanced oral absorption compared to that of pure meloxicam. All meloxicam cocrystals were converted to meloxicam form I when the slurry reached equilibrium. To better understand how cocrystallization impacts the absorption of meloxicam after oral administration, correlations between the in vitro and in vivo data were explored. The results suggest that the meloxicam cocrystals with a faster dissolution rate would exhibit increased oral absorption and an earlier onset of action. KEYWORDS: pharmaceutical cocrystal, multiple-component crystal, solubility, dissolution, pharmacokinetics, meloxicam

1. INTRODUCTION The absorption of an orally delivered medication is a critical physiological process that transports the active pharmaceutical ingredient (API) into the bloodstream and enables the distribution, metabolism and excretion of the API in the body. The majority of orally administered drugs are presented to the body as solid or semisolid dosage forms,1 where absorption occurs as a two-step process. Initially, the oral dosage formulation is released and dissolved into the biological medium (e.g., gastric or intestinal fluid) secreted in the gastrointestinal (GI) tract. This is followed by permeation of the API through the GI membranes, driven by either a concentration gradient or active transport process.2 Thus, solubility and permeability are the two key physical properties for oral drug absorption. The US Food and Drug Administration (FDA) also classifies orally administered APIs based upon solubility and permeability in the Biopharmaceutics Classification System (BCS).3 Interestingly, the majority of APIs fall into the BCS class II (low solubility, high permeability) category.4 In addition, approximately two-thirds of APIs under development to date exhibit aqueous solubilities of less than 0.1 mg/mL, which can limit their clinical performance.5 Therefore, much effort has been devoted to © 2012 American Chemical Society

the improvement of drug solubility in pharmaceutical development, with a special emphasis on APIs exhibiting poor dissolution profiles.6 Various formulation techniques, such as micronization, nanonization and dispersion in polymer matrices, are commonly applied to enhance drug solubility with limited success achieved.7,8 Alternatively, the modification of crystal form is also used to improve the solubility of API molecules as it can influence both solvation and lattice energies.9,10 The rearrangement of molecular packing in the crystal lattice can dramatically change the solid-state thermodynamics and therefore result in a (sometimes significant) modification of relevant physicochemical properties, such as solubility and dissolution rate. In many cases, the molecular packing rearrangement is facilitated by the introduction of a second molecule into the crystal lattice thereby forming a multiple-component crystal. For example, it has been reported that the sodium salt of naproxen achieved an improved solubility over the free base and therefore exhibited a Received: Revised: Accepted: Published: 2094

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Table 1. In House Solubility and Smax Data in 25 mM Sodium Phosphate Buffer at pH 6.5 and 37 °C for Meloxicam Coformers and Cocrystals

faster Cmax (i.e., maximum plasma concentration) after oral drug administration.11 To date, drug solubility enhancement by developing novel salts12 and inclusion compounds13 is well established in the pharmaceutical industry. More recently, pharmaceutical cocrystallization14−16 has been studied as an attractive alternative for solubility enhancement of BCS class II drugs.17−22 A pharmaceutical cocrystal is defined as a multiple-component crystal comprising two or more components that are solids under ambient conditions, are present in a stoichiometric ratio, and interact by hydrogen bonding, where at least one component is neutral and at least one component is an active moiety. It has been demonstrated that pharmaceutical cocrystals can profoundly change the physicochemical properties, including the solubility, of drug molecules.23,24 The strategy of using pharmaceutical cocrystals to improve aqueous solubility has been particularly successful for BCS class II drugs that are polymorphic and/or difficult to ionize.25−27 Numerous poorly soluble drugs have been studied in this context to identify novel crystal forms and improve their aqueous solubility and subsequent oral absorption. For example, carbamazepine, which exhibits a poor solubility and a low potential for salt formation, has been successfully cocrystallized with various cocrystal formers. The carbamaze-

pine:saccharin cocrystal has been of particular interest because of its improved stability and pharmacokinetics, thus making it a viable alternative to the marketed product.19 In contrast to the abundant crystallographic and solubility data on pharmaceutical cocrystals, available pharmacokinetic (PK) data for cocrystals in the literature are limited to date. Not surprisingly, the correlation between the in vitro dissolution/ solubility and in vivo PK data for pharmaceutical cocrystals has been explored even less. To the best of our knowledge, in the context of cocrystals, correlations between in vitro dissolution and in vivo PK data have only been exemplified in the analysis of AMG 517 cocrystals.21 It was observed that the increase of dissolution rate by cocrystallization resulted in an improved in vivo absorption of AMG 517. However, a strong linear correlation between the in vitro and in vivo data of AMG 517 cocrystals was not evident. Conceivably, a robust correlation between the in vitro and in vivo data would be a useful tool for predicting the PK behavior of novel cocrystals. In addition, such a correlation could also help to define the solubility acceptance criteria and possibly be used as a surrogate for further bioequivalence studies.28,29 With this in mind, we have continued our pharmaceutical cocrystal development for meloxicam, with a special focus on the improvement of 2095

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2.3.2. Differential Scanning Calorimetry (DSC). Differential scanning calorimetry was performed on a Perkin-Elmer Diamond DSC with a typical scan range of 25−280 °C, scan rate of 10 °C/min, and nitrogen purge of ca. 30 psi. 2.3.3. Fourier Transform Infrared Spectroscopy (FT-IR). FT-IR analysis was performed on a Perkin-Elmer Spectrum 100 FT-IR spectrometer equipped with a solid-state ATR accessory. 2.4. Dissolution Study. Powder dissolution experiments were conducted in 25 mM sodium phosphate buffer solutions at pH 6.5 and 37 °C to simulate intestinal physiological conditions. For each dissolution experiment, 50 mg of the crystalline solid and 100 mL of buffer solution were used. The amount of solid introduced into the slurry was sufficient to maintain a supersaturated solution for the duration of the study. Prior to the study, the particle size of meloxicam form I and its cocrystal forms was controlled to be between 53 and 75 μm by sieving. For a typical dissolution test, the solid powder was introduced into the buffer solution at time 0, and the slurry was stirred at ca. 120 rpm by a magnetic stir bar. Approximately 3 mL of sample solution was taken from each slurry by a polypropylene syringe at 15, 30, 45 min, 1, 2, 4, and 24 h. The sample solution was then filtered through a 0.45 μm nylon filter and diluted appropriately for further analysis. After each sampling, the volume of the liquid removed from the slurry was not compensated. If the dissolution profile was not equilibrated by 24 h, the slurry was continued until equilibration was observed. A dissolution profile was considered equilibrated when the meloxicam concentration in the solution remained stable for at least two adjacent collection time points. Specifically, slurries of 8 and 11 were equilibrated within 24 h; slurries of 2, 4, 9, 10, and 12 were equilibrated between 24 and 48 h and thus additional slurry samples in these experiments were collected at 48 h; slurries of 1, 3, 5, 6, and 7 were equilibrated between 48 and 72 h and thus additional slurry samples in these experiments were collected at 48 and 72 h. After the last sample collection, the remaining solid material in the slurry was filtered, dried, and characterized by PXRD and DSC analyses. Solution samples from the dissolution study were analyzed under ambient conditions with a Shimadzu Prominence high performance liquid chromatography−ultraviolet spectroscopy (HPLC−UV) system composed of the following units: an SIL 20AHT autosampler; a SPD 20A UV/vis detector; a CBM 20A communications bus module; LC20 AT liquid chromatograph; DGU 20A5 degasser. A Thermo Scientific Hypersil ODS C-18 (200 mm, 4.6 mm, 5 μm) column with a flow rate of 1 mL/min was used. The mobile phase contained 50 mM sodium phosphate buffer (pH 6.5) and acetonitrile in the volume ratio of 75:25. The samples were diluted with mobile phase, and the absorbance was measured at 366 nm. The dissolution/ solubility testing for each crystal form was repeated in triplicate. The mean concentration of each time point (n = 3) was used in the evaluation of the dissolution profiles. All concentration values in the dissolution profiles of meloxicam and its cocrystals are presented as meloxicam equivalent concentrations. 2.5. Pharmacokinetic Study. 2.5.1. In Life Study. Eightweek male Sprague−Dawley rats with jugular vein catheters implanted were purchased from Charles River Laboratories International, Inc. (Wilmington, MA) and housed in a temperature-controlled room for at least 48 h before the PK study. The rats were fasted overnight and weighed immediately before dosing. Pure meloxicam was dosed to 10 rats, and each meloxicam cocrystal was dosed to 5 rats. Prior to the study, the

solubility and pharmacokinetics, as well as the relationship between these two properties. Meloxicam (4-hydroxy-2-methyl-N-(5-methyl-2-thiazolyl)2H-1,2-benzothiazine-3-carboxamide-1,1-dioxide) is a nonsteroidal anti-inflammatory drug (NSAID) originally developed by Boehringer Ingleheim and prescribed for rheumatoid arthritis and osteoarthritis,30 postoperative pain31,32 and fever.33 The pure drug substance exists as a yellow solid that is practically insoluble in water34 and is considered a BCS class II drug. Meloxicam has pH dependent solubility related to its multiple ionization states.35,36 Oral dosage forms of meloxicam have a Tmax (i.e., time to reach maximum concentration) in the human body of about four to six hours, and it can take more than two hours for the drug to reach its therapeutic serum concentration.37 The slow onset of meloxicam restricts it from potential application for the relief of mild-to-moderate-level acute pain. While an accurate prediction of the time required to reach the therapeutic concentration for acute pain relief is difficult for meloxicam in the absence of clinical data, by estimation, attainment of the therapeutic serum concentration within approximately 30 min after dosing would be required. This estimation was determined upon evaluation of other NSAIDs indicated for acute pain relief.38,39 Previous attempts to alter the PK profile of meloxicam have resulted in various inclusion complexes,40 solvates,34 and salts.35,41,42 Preparation of polymorphic crystal forms of meloxicam36 have also been attempted, although unsuccessfully, to improve its dissolution profile.43,44 Recently, we have prepared a set of meloxicam cocrystals with carboxylic acids via the supramolecular synthon approach.18 In this study, we investigate the dissolution and PK profiles of meloxicam and its cocrystals, and further evaluate the correlation between the in vitro and in vivo data.

2. MATERIALS AND METHODS 2.1. Materials. Meloxicam was purchased from Jai Radhe Sales, India, with a purity of 99.64% and was used without further purification. The original drug substance employed in the dissolution and pharmacokinetic studies described herein was identified to be meloxicam form I exclusively.36 All other chemicals were supplied by Sigma-Aldrich and used as received. 2.2. Cocrystal Synthesis. Meloxicam·1-hydroxy-2-naphthoic acid (1), meloxicam·salicylic acid form III (2), meloxicam·succinic acid (3), meloxicam·4-hydroxybenzoic acid (4), meloxicam·glutaric acid (5), meloxicam·maleic acid (6), meloxicam·L-malic acid (7), meloxicam·benzoic acid (8), meloxicam·DL-malic acid (9), meloxicam·hydrocinnamic acid (10), meloxicam·glycolic acid (11), and meloxicam·fumaric acid (12) cocrystals were synthesized as previously reported45 and used in the dissolution and PK studies. Details of cocrystal synthesis are reported in the Supporting Information. The molecular structures of all cocrystal formers (coformers) are presented in Table 1. 2.3. Solid-State Characterization. The powdered solids of meloxicam and its cocrystals used in this study were characterized using the following solid-state characterization techniques. 2.3.1. Powder X-ray Diffraction (PXRD). Solids were characterized using a D-8 Bruker X-ray powder diffractometer using Cu Kα radiation (λ = 1.54178 Å), 40 kV, 40 mA. Data were collected over an angular range of 3° to 40° 2θ values in continuous scan mode using a step size of 0.05° 2θ and a scan rate of ca. 5°/min. 2096

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Figure 1. First four-hour dissolution profiles of meloxicam form I and 1−12 (pH 6.5 and 37 °C).

min. The injection volume was 20 μL, and the absorbance was measured at 360 nm. All serum concentration values in the PK profiles of meloxicam and its cocrystals are presented as meloxicam equivalent concentrations. Microsoft Excel 2003 (Microsoft Corp., Redmond, WA) was used to evaluate the PK data using a noncompartmental approach. The mean meloxicam concentration of each time point (n = 5 or 10) was used in the PK evaluations.

particle size of each test article (meloxicam or its cocrystal) was controlled by sieving to be between 53 and 75 μm. For each animal, the dosing formulation was prepared by suspending 10 mg/kg of meloxicam or its cocrystals (meloxicam equivalent) in 1 mL of a solution mixture of 5% polyethylene glycol 400 (PEG 400) and 95% methylcellulose solution (w/w %). The methylcellulose solution was prepared by dissolving 0.5% methylcellulose in water (w %). The suspension was administered as a single dose to each animal via oral gavage. Serial blood samples (0.2 mL) were obtained from the catheter at predose (0), 0.25, 0.5, 0.75, 1, 2, and 4 h after oral administration. The blood sampling of each animal was discontinued after the 4 h time point, as this study was focused on the identification of meloxicam cocrystals exhibiting a faster rate of oral absorption which could possibly lead to an early onset of action. Blood samples were centrifuged with an Eppendorf centrifuge at 3000 rpm and 4 °C for 10 min in order to obtain serum samples. All serum samples were stored at −80 °C for subsequent HPLC bioanalysis. The study design was approved by the Institutional Animal Care and Use Committee (IACUC) at the University of South Florida (Tampa, FL) prior to the study. 2.5.2. Bioanalysis. A 12.5 μg/mL piroxicam methanol solution was used as the internal standard (IS). In a typical sample preparation, 50 μL of rat serum sample and 200 μL of IS were added into an individual Eppendorf microcentrifuge tube. The tube was shaken by hand and then allowed to sit for 20 min. The process was repeated, and then the sample was transferred to a 0.2 μm Nylon-66 Microfilterfuge tube (Rainin, Oakland, CA) and spun at 10,000 rpm for 4 min. A clear methanol sample solution (200 μL or less) was separated from serum proteins, and 160 μL of clear methanol solution was transferred into an HPLC vial. HPLC bioanalysis was carried out on Perkin-Elmer Instruments LLC equipment composed of the following units: series 200 gradient pump; 785A UV/vis detector; series 200 autosampler; NCI 900 network chromatography interface and 600 series link. The machine was operated by Total Chrome Workstation (Perkin-Elmer Instruments LLC). Sample holder temperature was set to 4 °C, and a 250 mm × 4.6 mm × 1/4 in. Microsorb-MV 300-5 C-18 column was used. The analytes were eluted with a mixture of phosphate buffer (pH 3.0) and methanol (1/1, v/v). The temperature of the column was 40 °C with a flow rate of 1 mL/

3. RESULTS AND DISCUSSION 3.1. In Vitro Dissolution. As meloxicam exhibits limited solubility under acidic conditions, it is believed that meloxicam and cocrystals thereof would be more likely to dissolve and absorb under physiological conditions exhibiting higher pH values. Therefore in this study, powder dissolution results of meloxicam and its cocrystals were obtained from 25 mM sodium phosphate buffer solutions at pH 6.5 and 37 °C. The time−concentration dissolution profiles of meloxicam form I and its 12 cocrystals for the first 4 h are presented in Figure 1. In the dissolution experiment of meloxicam form I, rapid dissolution of the drug was observed in the early phase, achieving a concentration of ca. 128 μg/mL after 4 h. From 4 to 24 h, the concentration of meloxicam in solution increased a mere 19% and reached an equilibrium solubility of ca.153 μg/ mL. At the end of the dissolution experiment, the remaining solid in the slurry was collected, dried and characterized. It was confirmed that the remaining solid was meloxicam form I. The dissolution profiles of meloxicam cocrystals were also determined. Compared to pure meloxicam, all cocrystals exhibited faster dissolution rates in the early phase. In particular, 1, 2, 3 and 6 exhibited at least a 10-fold higher meloxicam concentration compared to that of the parent drug within the first two hours, followed by a decrease in meloxicam concentration over time, as shown in Figure 2. Such dissolution profiles, described as “spring-and-parachute” curves, have been reported previously.46 Among those cocrystals, 1 exhibited the highest peak concentration (Smax) of 343 μg/mL after 2 h of stirring in the buffer solution. After 48 h, the meloxicam concentration in the slurry of 1 decreased 47% to a value of 182 μg/mL, which is similar to the solubility of pure meloxicam form I. While the majority of cocrystals exhibited similar or higher equilibrium concentrations compared to that of meloxicam 2097

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between meloxicam cocrystals and corresponding coformers, the apparent solubility of each coformer used in this study was also determined in the dissolution medium at 37 °C (Table 1). Interestingly, it was found that no correlation was evident between the Smax values of meloxicam cocrystals and solubilities of the coformers. Among the coformers discussed in this study, 1-hydroxy-2-naphthoic acid, salicylic acid, succinic acid, and maleic acid exhibited the lowest aqueous solubilities; while in contrast, the corresponding cocrystals (1, 2, 3, and 6) showed the highest Smax values. On the other hand, L-malic acid exhibited the highest apparent solubility among all cocrystal formers while the associated cocrystal (7) exhibited one of the lowest Smax values. The result indicated that a more soluble coformer did not necessarily result in a cocrystal with a higher Smax in this study. The correlation between Smax and melting point data for meloxicam cocrystals was also explored. It has been demonstrated that the melting point and the solubility of multiple-component crystal forms could be well correlated, as rationalized by the van’t Hoff equation and Walden’s rule,48,49 while more recent case studies on pharmaceutical cocrystals offered a different suggestion.27,50 To enable an analogous evaluation in this study, the Smax of the meloxicam cocrystals in molar fraction was plotted against the corresponding melting points. A poor linear correlation between the Smax and melting point data of the 12 cocrystals was observed. The deviation could be potentially due to the neglect of solute−solvent interactions in the ideal solution which is adopted in the van’t Hoff equation. Moreover, the simplified estimation for entropy of fusion by Walden’s rule could also contribute to the poor correlation. Since the molecular arrangement in the crystal lattice is associated with the thermodynamics of cocrystal dissolution, a correlation of cocrystal solubility at equilibrium and supramolecular arrangement was also attempted. In a previous study based on a data set of two meloxicam cocrystals,51 it was believed that the solubility enhancement of meloxicam cocrystals was associated with the absence of NH···OS interactions in the crystal lattice. To further validate this correlation, six meloxicam cocrystals (i.e., 1, 2, 3, 5, 7, and 12) with known crystal structures were included in the analysis. The

Figure 2. Dissolution profiles of meloxicam form I and selected cocrystals from 0 to 72 h (pH 6.5 and 37 °C).

form I, further analysis revealed that other meloxicam cocrystals (i.e., 5, 6, and 7) presented concentrations upon equilibrium below that of pure meloxicam form I (Figure 2). Among those, 5 achieved one of the lowest equilibrium concentrations, even though 5 showed a more rapid dissolution rate than that of meloxicam form I in the early phase, resulting in a meloxicam concentration of 106 μg/mL at 45 min. The meloxicam concentration in the buffer solution continued to increase slowly until an equilibrium concentration of 132 μg/mL was reached at the 48 h time point. After the last sample collections, all solid materials remaining in the slurries were characterized via PXRD and DSC analyses. In all cases, the cocrystal was converted to meloxicam form I when the slurry reached equilibrium. To better understand how cocrystallization impacted the solubility of meloxicam, the solubility data were explored to reveal potential correlations between coformers and the corresponding cocrystals. It has been demonstrated by a set of pharmaceutical cocrystals of a hexamethylenebisacetamide pyridine derivative that the solubility of the coformer can affect the solubility of the corresponding cocrystal in a linear fashion.47 In order to validate the solubility correlations

Figure 3. PK profiles over 4 h following 10 mg/kg (meloxicam equivalent) single-dose oral administration of meloxicam form I and 1−12 in rats. 2098

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solubility data of those six cocrystals were compared with the supramolecular arrangements in the corresponding crystal structures. The result showed that the absence of NH···OS interactions in the meloxicam cocrystals does not necessarily correspond to the increase in meloxicam concentration at equilibrium. All six cocrystals showed the absence of NH···O S interactions between meloxicam molecules. Consistent with the conclusion in the aforementioned study, 1, 2, and 3 exhibited an increased solubility at equilibrium compared to that of meloxicam. However, the other cocrystals showed lower solubilities than that of meloxicam. Particularly in the case of 5, the cocrystal showed an equilibrium solubility value 14% lower than that of meloxicam form I. 3.2. In Vivo Pharmacokinetics. As part of the further development, the early PK profiles of meloxicam form I and its cocrystals after single-dose oral gavage administration were determined in rats. The mean serum concentration at each time point was used for the PK evaluation. After dosing pure meloxicam form I, the mean serum concentration of meloxicam increased gradually from 0 to 24 μg/mL within the first hour (Figure 3). A slower rate of increase in serum concentration was found in the following three hours, where a meloxicam concentration of 39.4 μg/mL was eventually obtained. The PK profiles of meloxicam cocrystals were also determined. Within the first hour, almost all cocrystals exhibited higher meloxicam serum concentrations compared to pure meloxicam, indicating a faster oral absorption after administration (Figure 3). The serum concentrations after 15 min were ca. 2-fold higher for 6 out of the 12 cocrystals when compared to pure meloxicam form I. Among these cocrystals, 2 exhibited the highest serum concentration of 42.1 μg/mL at ca. 15 min after oral gavage. The meloxicam serum concentration was subsequently maintained with minor fluctuation until four hours after dosing. Based on the available PK data of meloxicam and its cocrystals, the onset of action for meloxicam cocrystals was estimated. It was previously reported that the onset of action for pure meloxicam is approximately 2−3 h after oral dosing and the time of onset is associated with the serum concentration level of meloxicam.52 To facilitate the evaluation of potential onset improvement for all meloxicam cocrystals, the mean meloxicam serum concentration at 2 h after oral dosing of pure meloxicam in rats (i.e., 30.2 μg/mL) was used as a reference to quantify when the cocrystal reached the hypothetical therapeutic concentration after receiving an equivalent oral dose. By estimation, the concentration of 30.2 μg/mL was achieved approximately 11 min after oral administration of 2, indicating a potential for rapid onset of action. Similar potential for onset enhancement was observed in the PK profiles of 1 and 7, which reached the hypothetical therapeutic concentration in 21 and 24 min, respectively. The time required to reach the serum concentration of 30.2 μg/mL after oral cocrystal dosing as well as the AUC0−4h (i.e., area under the serum concentration curve from 0 to 4 h) of each meloxicam crystal form is presented in Table 2. To better understand how the PK behavior was affected by the solubility, the correlation between in vitro dissolution and in vivo PK data for meloxicam cocrystals was evaluated. First, the evaluation of the in vitro and in vivo data was performed on an individual crystal form basis. For each cocrystal, the in vitro mean dissolution data were plotted against the in vivo mean serum concentration data on a time-point-by-time-point basis over four hours and processed via linear regression analysis. Almost all meloxicam cocrystals exhibited relatively strong

Table 2. AUC0‑4h and the Time Required To Reach the Hypothetical Therapeutic Concentration (30.2 μg/mL) after Oral Administration of Meloxicam or Its Cocrystals crystal form

AUC0−4h (meloxicam equiv-μg·h/mL)

timea (min)

meloxicam form I 1 2 3 4 5 6 7 8 9 10 11 12

6622.74 10572.70 9948.09 9030.40 8588.14 8163.22 8070.02 7810.02 7744.98 7571.28 6316.05 5648.32 4495.38

120 21 11 42 38 80 42 24 88 106 150 208 240

a Time required to reach a serum concentration of 30.2 μg/mL after oral administration.

linear correlations between the dissolution and PK data, with R2 values ranging between 0.7347 and 0.9743. Overall, approximately 50% of the cocrystals resulted in R2 values greater than 0.9. These results indicated that, for cocrystals of BCS class II drugs such as meloxicam, the in vivo PK data are likely to be predicted based on a point-to-point correlation with the appropriate in vitro dissolution data. The correlations between in vitro and in vivo data of pure meloxicam and selected cocrystals are presented in Figure 4.

Figure 4. Correlation of in vitro mean dissolution and in vivo mean serum concentration data (time-point-by-time-point) on an individual crystal form basis, exemplified by meloxicam form I, 1 and 2.

Regression analysis based on the dissolution and absorption rates of all cocrystals was also performed. To facilitate the analysis, the dissolution rates for meloxicam form I and its cocrystals (except 2, 7 and 11) were obtained as the slope of linear regression of the in vitro dissolution concentration mean values within the first 45 min, where the dissolution profile remained in a relatively linear range. Meloxicam concentrations at 0 and 15 min in the slurries were used in order to obtain a more accurate representation of the dissolution rates for 2, 7 and 11. The rates of absorption for meloxicam and its cocrystal were estimated based on initial in vivo PK mean serum concentrations, assuming that meloxicam metabolism, distribu2099

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in vitro dissolution rate for a meloxicam cocrystal could be indicative of the onset of action after its oral administration.

tion and excretion initially play a limited role. Thus, the rate of absorption for each cocrystal (except 2 and 7) was calculated as the slope of the linear regression for the in vivo PK mean serum concentrations within the first 45 min. The rates of absorption for 2 and 7 were calculated using the first 15 and 30 min in vivo mean serum concentrations, respectively, in order to obtain a more accurate representation of the absorption rates. A relatively clear trend was observed when the dissolution and oral absorption rates of meloxicam and the 12 cocrystals were plotted together, as shown in Figure 5. A linear regression

4. CONCLUSION The dissolution profiles of meloxicam form I and 12 corresponding cocrystals with carboxylic acids have been determined in pH 6.5 phosphate buffer solutions at 37 °C. Among all cocrystals studied, 9 out of 12 exhibited a greater maximum meloxicam concentration (Smax) compared to that of meloxicam form I. At the end of the dissolution experiments, all cocrystals in the slurry were transformed to meloxicam form I. To better understand how cocrystallization influenced the dissolution/solubility behavior of meloxicam, linear regression analysis between the physicochemical properties of the cocrystals and coformers was performed. A poor correlation between the solubility of the coformers and the maximum solubility of the corresponding cocrystals was observed. In addition, analysis of the melting points and Smax data for the cocrystals also failed to show a strong correlation. It is noted that, for meloxicam cocrystals, the prediction of cocrystal dissolution behavior based on the physicochemical properties of the coformers remains unreliable. Further analysis showed that the correlation between the solubility enhancement and supramolecular arrangement in the cocrystal lattice was also proven to be inconsistent. Clearly, the intermolecular interactions between adjacent meloxicam molecules in the crystal lattice only played a limited role in the thermodynamics of cocrystal solubilization. The PK profiles of meloxicam and its cocrystals were acquired after single-dose oral administration in rats. The collection of PK data was restricted to within four hours after dosing, as the in vivo study was focused on meloxicam cocrystals with an early onset of action. The majority of meloxicam cocrystals exhibited an increase in the oral absorption rate, in comparison to pure meloxicam form I. In particular, 1 and 2 exhibited the hypothetical therapeutic serum concentration of 30.2 μg/mL within 21 and 11 min, respectively, suggesting their greater potential in the development of meloxicam for mild-to-medium-level acute pain relief. As an additional step of investigation, the PK data of the cocrystals were compared to the relevant dissolution data. When the analysis was performed on an individual basis, the meloxicam concentrations in the dissolution media were well correlated with the meloxicam serum concentrations at the same time point. Moreover, the correlation analysis based on all meloxicam cocrystals indicated that those cocrystals with faster dissolution rates could also exhibit higher rates of oral absorption and possibly a more rapid onset of action. As part of the future work, meloxicam and selected meloxicam cocrystals from this study (particularly 1 and 2) will be further evaluated in a more sophisticated animal PK study to obtain a complete set of relevant PK parameters. In addition, the early onset of selected cocrystal candidates will be corroborated in an animal efficacy study.

Figure 5. Correlation of in vitro dissolution rate and in vivo absorption rate of meloxicam form I and 1 −12.

model of the data produced an R2 value of 0.7067. 1 and 2 are among the cocrystals that exhibited the highest rates of dissolution while these two cocrystals also showed the most rapid absorption after oral administration. The correlation between the rate of cocrystal dissolution and the hypothetical time of onset was evaluated as well. The rates of dissolution and the reciprocal of the hypothetical time of onset values for meloxicam form I and 12 cocrystals were plotted (Figure 6). A relatively good correlation between cocrystal dissolution rate and the reciprocal of the time to reach the therapeutic concentration was found, as evidenced by a linear regression R2 value of 0.8474. The result suggests that the

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ASSOCIATED CONTENT

S Supporting Information *

The details of meloxicam cocrystal synthesis, solid-state characterization results of the solids obtained from the dissolution studies, dissolution and PK profiles of meloxicam form I and 1−12 over 72 h, and correlation coefficients (R2) generated from time-point-by-time-point linear regression analyses on an individual crystal form basis are provided.

Figure 6. In vitro dissolution rate plotted against the reciprocal of T. T denotes the time required to reach the hypothetical therapeutic concentration (i.e., 30.2 μg/mL) after oral administration of meloxicam form I or 1−12. 2100

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AUTHOR INFORMATION

Corresponding Author

*Thar Pharmaceuticals Inc, 3802 Spectrum Boulevard, Suite 120, Tampa, FL 33612. Phone: 1-813-978-3980. Fax: 1-813903-0649. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mr. Raymond K. Houck for stimulating discussions and valuable input to the project. REFERENCES

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