Pharmacokinetics and comparative bioavailability of allopurinol formulations in healthy subjects

Mechanosynthesis of Stable Salt Hydrates of Allopurinol with Enhanced Dissolution, Diffusion, and Pharmacokinetics

Allopurinol (ALO) is a medication that treats gout and kidney stones by lowering uric acid synthesis in the blood. The biopharmaceutics classification system (BCS) IV drug exhibits poor aqueous solubility, permeability, and bioavailability. To overcome the bottlenecks of ALO, salts with maleic acid (MLE) and oxalic acid (OXA) were synthesized using the solvent-assisted grinding method. The novel multicomponent solids were characterized by PXRD, DSC, TGA, FT-IR, and SEM images. The crystal structures of these salts with variable stoichiometry were obtained using Rietveld refinement from the high-resolution PXRD data. The proton from the dicarboxylic acid is transferred to the most basic pyrimidine "N" of ALO. The N-H···N hydrogen-bonded ALO homodimer is replaced by the N+-H···O- ionic interactions in ALO-OXA (2:1:0.4) and ALO-MLE (1:1:1) salt hydrates. The organic salts improved solubility and dissolution up to 5-fold and the diffusion permeability up to 12 times compared to the native drug in a luminal pH 6.8 phosphate buffer medium. The salt hydrates were exceptionally stable during storage at 30 ± 5 °C and 75 ± 5% relative humidity. Superior dissolution and diffusion permeability of the ALO-MLE salt resulted in improved pharmacokinetics (peak plasma concentration) that offers a promising solid dosage form with enhanced bioavailability and lower dosage formulation.

Sample preparation and quantification of polar drug, allopurinol, in human plasma using LCMSMS

A fast, selective and reproducible LC-MS/MS method with simple sample preparation was developed and validated for a polar compound, allopurinol in human plasma, using acyclovir as internal standard (IS). Chromatographic separation was achieved using Agilent Poroshell 120 EC-C₁₈ (100 × 2.1 mmID, 2.7 µm) analytical column. The mobile phase was comprised of 0.1%v/v formic acid-methanol (95:05; v/v), at a flow rate of 0.45 mL/min. The effect of different protein precipitation agents used in sample preparation such as methanol, acetonitrile, a mixture of acetonitrile-methanol and a mixture of acetonitrile-acetone were evaluated to optimize the extraction efficiency of allopurinol and IS. The use of acetone-acetonitrile (50:50, v/v) as protein precipitating agent shortened the sample preparation time and improved the recovery of allopurinol to above 93%. The IS-normalised matrix factors at two concentration levels were 1.0, with CV of 5.1% and 4.2%. Allopurinol in plasma was stable at benchtop for 24 h, in autosampler tray for 48 h, in instrumentation room for 48 h, in freezer after 7 freeze-thaw cycles and in freezer for 140 days. Allopurinol stock standard solutions were stable for 140 days at room temperature and in the chiller. The short sample run time of the validated bioanalytical method allowed high throughput analysis of plasma samples in pharmacokinetic study of an allopurinol formulation. The robustness and reproducibility of the bioanalytical method was reaffirmed through incurred sample reanalysis (ISR).