Engineering cocrystals of paliperidone with enhanced solubility and dissolution characteristics

In the present study, co-crystals (CCs) of Paliperidone (PPD) with coformers like benzoic acid (BA) and P-amino benzoic acid (PABA) were synthesized and characterized to improve the physicochemical properties and dissolution rate. CCs were prepared by the solvent evaporation (SE) technique and were compared with the products formed by neat grinding (NG) and liquid assisted grinding (LAG) in their enhancement of solubility. The formation of CCs was confirmed by the IR spectroscopy, powder X-ray diffraction and thermal analysis methods. The saturation solubility studies indicate that the aqueous solubility of PPD-BA and PPD-PABA CCs was significantly improved to 1.343±0.162mg/ml and 1.964±0.452mg/ml, respectively, in comparison with the PPD solubility of 0.473mg/ml. This increase in solubility is 2.83and 3.09fold, respectively. PPD exhibited a poor dissolution of 37.8% in 60min, while the dissolution of the CCs improved tremendously to 96.07% and 89.65% in 60min. CCs of PPD with BA and PABA present a novel approach to overcome the solubility challenges of poorly water-soluble drug PPD.


INTRODUCTION
One of the significant challenges the pharmaceutical industry faces today is the development of drugs with required aqueous solubility (1). As poorly aqueous soluble drugs exhibit low dissolution in biological fluids, they are insufficiently absorbed into the human body (2). With the advent of crystal engineering technology, pharmaceutical cocrystals that exhibit improved physicochemical and mechanical properties have emerged as a potential alternative (3,4). In recent years, pharmaceutical cocrystals have been promising techniques in modulating the physicochemical properties of drugs and thus aiding in enhancing the solubility of Biopharmaceutical Classification System (BCS) class II and IV drugs (5). A pharmaceutical cocrystal is a multicomponent system which is structurally homogeneous, comprising two or more components in stoichiometric ratios, including an ionic or a molecular active pharmaceutical ingredient (6,7). The possible association occurring between the drug and the coformers is through weak interactions like hydrogen bonds (8). The functional groups commonly responsible for hydrogen bond formation are carboxylic acid, amide and alcohol groups (9). A supramolecular homosynthon is an association of identical functional groups, while heterosynthon is made of two different functional groups (10). Cocrystals could offer potential advantages of improved solubility, dissolution, bioavailability and synergistic/ additive effects of the components (11).
In the present investigation, cocrystals of paliperidone were developed using benzoic acid (BA) and 4-amino benzoic acid (PABA) as coformers by the solvent evaporation technique. Structural characterization of the cocrystals was performed by Fourier Transform Infrared spectroscopy (FTIR), Differential Scanning Calorimetry (DSC), powder X-Ray diffraction studies (PXRD) and Scanning Electron Microscopy (SEM). The CCs were also evaluated through saturation solubility studies, micromeritic studies and in vitro drug dissolution (20).

Materials
Paliperidone was obtained as a gift sample from MSN laboratories, Hyderabad. Para amino benzoic acid and benzoic acid were procured from Finar chemicals Limited, Ahmedabad. Dichloromethane and hydrochloric acid were purchased from Thermo Fisher Scientific India Pvt Ltd.

Design of cocrystals of PLP
The formation of cocrystals occurs due to possible intermolecular interactions between the drug and the coformers through hydrogen bonding. Paliperidone is a benzisoxazole derivative possessing 1 hydrogen bond donor group (-OH) and 7 hydrogen bond acceptor groups (4 N atoms, 2 O atoms and 1F atom). As for the coformers used, BA possesses 1 hydrogen bond donor group and 2 hydrogen bond acceptor groups and PABA has 2 hydrogen bond donor groups and 3 hydrogen bond acceptor groups. These groups may be responsible for the probable formation of hydrogen bonds (21).

Synthesis of physical mixtures by neat grinding (NG)
PPD (1mmol) and coformers PABA (1mmol) and BA (1mmol) were ground for 30min in a mortar and pestle using the neat grinding (NG) technique. The product obtained was stored in a desiccator until further use (22).

Synthesis by liquid assisted grinding (LAG)
PPD (1mmol) with PABA (1mmol) and BA (1mmol) were ground for 30min using DCM as a solvent. The obtained powder was vacuum dried at 40℃ for 24h and stored at room temperature until further use (23).

Synthesis by solvent evaporation (SE)
The drug and coformer in equimolar ratios (1mmol each) were dissolved in 10ml of DCM using sonication for 30min. The obtained product was sealed with aluminium foil, which was perforated to allow the solvent to evaporate slowly under room temperature, in order to obtain the cocrystals (24).

Saturation solubility studies
Saturation solubility studies were performed for the pure drug and products obtained by NG, LAG and SE by adding them in excess quantity into vials comprising 10ml distilled water and 0.1N HCl separately. The capped vials were placed in a water bath shaker at room temperature for 24h to attain equilibrium. The supernatant was filtered through Whatman filter paper. The filtrate was diluted by considering 1ml of filtrate and making it up to 10ml with the respective solvents. The solutions were analysed spectrophotometrically at 275nm to determine the concentration of the drug. All the studies were conducted in triplicate.

FTIR studies
FTIR study of all the samples were performed on the FTIR Agilent Cary 630 series. Around 1-2mg of sample was directly placed on the diamond ATR and scanned from 4000-400 cm -1 . The peaks obtained were interpreted to study the formation of CCs (25).

Differential Scanning Calorimetry (DSC)
Thermal analyses of the samples were carried out by placing 1-3mg of sample in crimped sealed aluminium pans and scanned from 50-350 ℃ at a heating rate of 10 ℃/min under continuously purged nitrogen flow rate of 50ml/min (26,27).

Powder X-Ray Diffraction studies (PXRD)
For analysing the samples through PXRD studies, all the samples were irradiated with Cu Kα, 40kV, 40mA. Data were collected at room temperature over an angular range of 2 ˚ to 40˚ 2Ɵ value in continuous scan mode at a scan rate of 5 ˚/min (28).

Scanning Electron Microscopy (SEM)
The surface topography of the formed CCs was characterized using SEM. The samples were mounted on an aluminium stub and coated with a thin gold layer. The analysis was performed using JSM 6360, Joel, which was operated at 10kV acceleration voltage (29).

Evaluation of micromeritic properties
The obtained CCs were characterized for various micromeritic properties like bulk density, tapped density, Hausner's ratio, Carr's compressibility index and angle of repose because these properties are influenced by particle size, shape and surface texture (30,31). The following equations were used in the calculation of these properties:

Determination of percentage yield
The percentage yield of the product was calculated considering the weight of the formed CCs and the initial weight of the drug and coformer taken (32).

Estimation of drug content
The prepared CCs by SE were dissolved in 100ml of 0.1N HCl in weights equivalent to 10mg of drug. The solutions were filtered through a Whatman filter paper. 10ml of the filtrate was taken and diluted to 100ml. The absorbance was measured at 275nm and the concentration was calculated (33).

In vitro dissolution studies
In vitro drug release was studied using a USP type II dissolution apparatus. Drug and CCs prepared by SE were considered equivalent to a 6mg dose and placed into 900ml of 0.1N HCl. The entire study was carried out at a temperature of 37 ± 0.5 ℃, paddle rotation speed of 50rpm for a time period of 60min. 5ml aliquots were withdrawn at 5, 10, 15, 30, 45 and 60min time intervals, filtered, diluted 10times and analysed using a UV-Vis spectrophotometer. The quantity of the sample removed was replaced with an equal quantity of fresh dissolution medium. All the experiments were conducted in triplicate (34).

Saturation solubility studies
Solubility analysis was conducted both in distilled water and 0.1N HCl. The solubility of pure drug was 0.473mg/ml and 0.788±0.022mg/ml respectively in distilled water and 0.1N HCl. The products obtained by NG and LAG exhibited solubilities that were slightly higher, while those prepared using the SE technique exhibited significantly increased solubilities compared with the pure drug. The aqueous solubilities of PPD-PABA CCs and PPD-BA CCs obtained by SE were enhanced by 3.09-fold (1.964±0.452mg/ml) and 2.83-fold (1.343±0.162mg/ml) respectively. CCs of PABA prepared using the SE technique exhibited an increase in 3.88-fold (3.064±0.864mg/ml) and that of BA CCs was an increase in 2.87-fold (2.565±0.013mg/ml) in 0.1N HCl. This tremendous increase in solubility may be due to the formation of CCs from the SE technique. (35).

Scanning Electron Microscopy
SEM images of PPD and CCs formed from SE technique were presented in Fig.6. SEM analysis revealed a difference in the surface topology of the CCs when compared to that of the pure drug. It was observed that pure PPD exhibited irregularly shaped particles in various sizes. The morphology of PPD-PABA CCs appeared to be slender, elongated needle like crystals. While that of PPD-BA exhibited short, broad, plate like particles. SEM images of both CCs were found to be different from those of the pure drug indicating a change in the morphology of the CCs (42, 43).

Evaluation of micromeritic properties
The micromeritic properties of the CCs as shown in Table I were found to be lowered in comparison with that of the pure drug, indicating an improved flow.

Determination of percentage yield and estimation of drug content
The percentage yield of the CCs formed between PPD-BA and PPD-PABA was 93.89±0.56% and 95.1±1.23% respectively. The drug content of the PPD-BA and PPD-PABA CCs was 98.04±1.84% and 98.72±0.41% respectively.

In vitro drug dissolution studies
The dissolution profiles of the drug and the CCs were shown in Fig. 7. They clearly indicated that drug had poor dissolution, and an increase in dissolution of the drug was observed in formulations prepared using NG, LAG as well as in the CCs 44 . PPD exhibited a dissolution of 37.80±0.56% in 60min, while the CCs of PABA and BA showed 96.07±0.67% and 89.65±1.95% respectively in 60min. This enhancement in dissolution may be due to CCs formation between the drug and coformers that alter the physicochemical properties. The FTIR spectrum of the CCs showed a difference from that of the drug and coformers used. Shifts in the wave number indicate that the new molecular interactions influence the positions of the functional groups, as they are related to the wave numbers of the functional groups that participate in the formation of hydrogen bonds (46). SEM studies showed a change in the surface morphology of the CCs when compared to the drug, resulting in needle like and plate like CCs. From the DSC thermograms, the melting point of the CCs was found to be reduced when compared to the drug, which might be due to weaker interactions in the CCs. All these confirm the formation of CCs between the drug and the coformers used.
The angle of repose of the pure drug was 31.07±0.21˚, while that of the CCs prepared using BA and PABA as coformers was 26.05±0.96˚ and 27.64±0.99˚ respectively. This indicates an improved flow of CCs when compared to pure PPD. Hausner's ratio of CCs of BA and PABA was obtained as 1.15±0.83 and 1.11±0.82 respectively when compared to 1.25±0.17 in the case of pure drug. This indicates excellent flow characteristics. Carr's index values were 13.20±0.12 and 10.52±0.41 in case of CCs, when compared to that of 20.40±0.09 as in case of PPD. This indicates good flowability of the CCs in comparison with the pure drug. The CCs exhibited an enhanced solubility and dissolution rate, which is assumed to be due to hydrogen bonded CCs, which dissociate quickly due to the weaker heteromolecular interactions in the CCs, leading to an enhanced dissolution rate in comparison with the drug (47). This might also be due to an increased affinity for the solvent and lowered interfacial barrier energy that generally occurs due to the formation of CCs. The coformer incorporation modifies the physical properties of drug. The aqueous solubilities of PPD-PABA CCs and PPD-BA CCs obtained by SE were enhanced 3.09-fold and 2.83-fold respectively. Literature review showed a solubility of PPD at 0.115mg/ml in water and a 1.48-fold increase in aqueous solubility of PPD CCs when prepared using boric acid as a conformer (19). The dissolution of the CCs is significantly enhanced when compared to the drug due to increased wettability and weaker associations (48,49).

CONCLUSION
Pharmaceutical CCs of PPD were prepared with BA and PABA using the SE technique. The formation of CCs was confirmed by FTIR, DSC and XRD studies and supported by the SEM analysis. The prepared CCs exhibited higher solubility and dissolution compared with the pure drug. They also exhibited improved flow properties. Hence, cocrystals have proven to be an alternative strategy to overcome the solubility and dissolution problems of the poorly soluble PPD.