Effect of Counterions on Dissolution of Amorphous Solid Dispersions Studied by Surface Area Normalized Dissolution

Solubility enhancement has become a common requirement for formulation development to deliver poorly water soluble drugs. Amorphous solid dispersions (ASDs) and salt formation have been two successful strategies, yet there are opportunities for further development. For ASDs, drug-polymer phase separation may occur at high drug loadings during dissolution, limiting the increase of drug loadings in ASD formulations. For salt formation, a salt form with high crystallinity and sufficient solid-state stability is required for solid dosage form development. This work studied the effect of counterions on the dissolution performance of ASDs. Surface area normalized dissolution or intrinsic dissolution methodology was employed to eliminate the effect of particle size and provide a quantitative comparison of the counterion effect on the intrinsic dissolution rate. Using indomethacin (IMC)-poly(vinylpyrrolidone-co-vinyl acetate) ASD as a model system, the effect of different bases incorporated into the ASD during preparation, the molar ratios between the base and IMC, and the drug loadings in the ASD were systematically studied. Strong bases capable of ionizing IMC significantly enhanced drug dissolution, while a weak base did not. A physical mixture of a strong base and the ASD also enhanced the dissolution rate, but the effect was less pronounced. At different base to IMC molar ratios, dissolution enhancement increased with the base to IMC ratio. At different drug loadings, without a base, the IMC dissolution rate decreased with the increase of drug loading. After incorporating a strong base, it increased with the increase of drug loading. The observations from this study were thought to be related to both the ionization of IMC in ASDs and the increase of microenvironment pH by the incorporated bases. With the significant enhancement of the drug dissolution rate, our work provides a promising approach of overcoming the dissolution limitation of ASD formulations at high drug loadings.