Biorelevant Media Slows the Solution-Mediated Phase Transformation of Amorphous Spironolactone

Effect of Structurally Related Compounds on Desupersaturation Kinetics of Indomethacin

PURPOSE

The pharmaceutical literature contains examples wherein desupersaturation from high concentrations does not proceed to equilibrium concentration of the thermodynamically most stable form but remains above equilibrium. The purpose of the current research was to investigate the effect of structurally related compounds on desupersaturation kinetics as a possible explanation for a higher than equilibrium solubility after crystal growth of γ-indomethacin (γ-IMC).

METHODS

Three structurally related compounds (SRC) - cis-sulindac (c-SUL), trans-sulindac (t-SUL) and indomethacin-related compound-A (IMC-A) -were investigated. Desupersaturation kinetics to the most stable γ-IMC, in the presence of c-SUL, t-SUL or IMC-A, was measured at pH 2.0.

RESULTS

The SRCs c-SUL and t-SUL were effective crystallization inhibitors of IMC, while IMC-A was not a potent crystallization inhibitor of IMC. Among the sulindac isomers, t-SUL was a stronger crystallization inhibitor. The apparent solubility of γ-IMC crystals grown from supersaturated solutions in the presence of SRCs matched the equilibrium solubility of γ-IMC. During crystallization of IMC in the presence of IMC-A, the concentration of IMC-A declined initially but rebounded as supersaturation and crystallization rate of IMC declined, suggesting that IMC-A itself became incorporated in the IMC crystal lattice at higher degrees of IMC supersaturation.

CONCLUSIONS

The results suggest that high apparent solubility after crystallization of IMC reported by several authors is not related to the presence of IMC-A impurity. The greater IMC crystal growth rate inhibition by t-SUL than by c-SUL was consistent with the proposed orientation of SUL molecules adsorbed on the IMC crystal, providing a mechanistic understanding of the inhibition.

Solution-Mediated Phase Transformation of Aripiprazole: Negating the Effect of Crystalline Forms on Dissolution and Oral Pharmacokinetics

We aimed to evaluate the effect of crystalline forms of aripiprazole, an antipsychotic drug for schizophrenia, on the dissolution rates and oral pharmacokinetics. Solubility, intrinsic dissolution rates, and tablet dissolution rates of the monohydrate (MA) and the anhydrous form (AA) were measured in various aqueous media while monitoring the phase transformation by ATR-FTIR. And their oral pharmacokinetics in dogs were compared. The intrinsic dissolution rate of MA was lower compared to AA, confirming its thermodynamic stability relative to AA in water. Phase transformations during the solubility measurement were media-dependent: In simulated gastric fluid, both AA and MA changed to HCl salt form, whereas AA and HCl salt form transformed to MA in simulated intestinal fluid. In vitro dissolution rates and dog oral pharmacokinetics of AA and MA tablets were similar. The results suggest that the solution-mediated transformation to HCl salt or MA negates the effect of different crystalline forms on dissolution rates in vivo and, consequently, on oral pharmacokinetics. We emphasize the importance of the dissolution tests employing various bio-relevant media for better prediction of in vivo performance and the selection of a solid form for development.