Amino Acid Conjugation: An Approach to Enhance Aqueous Solubility and Permeability of Poorly Water Soluble Drug Ritonavir

Objective: The objective of present work is to improve physicochemical and pharmacokinetic profile of poorly water soluble HIV protease inhibitor, ritonavir (RT) by preparing its amino acid conjugates. Methods: Ester conjugates of ritonavir with various amino acids were synthesized by a simple esterification process using dicyclohexyl carbodiimide (DCC) as a coupling agent. The synthesized compounds were characterized by thin layer chromatography (TLC), fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) and mass spectroscopy. All conjugates were evaluated by saturation solubility and hydrolytic stability studies. Cytotoxicity and permeability studies were conducted using Caco-2 cell line. Results: All amino acid conjugates showed a significantly higher aqueous solubility compared to the pure RT. With respect to hydrolysis, alkaline hydrolysis of conjugates was rapid relative to acidic hydrolysis. No cytotoxicity was shown by conjugates for concentration as high as 100μM, which indicates promising therapeutic potential. Transport studies across Caco-2 cells showed that prepared amino acid conjugates improved the permeation of RT compared to pure RT. Conclusion: In vitro studies demonstrated that amino acid conjugation of RT may be an effective strategy to improve its aqueous solubility as well as permeability and can be used to improve oral absorption and thereby oral bioavailability of protease inhibitors. Keywords: Ritonavir, amino acids, conjugates, solubility, HIV, hydrolysis