Solubilization of thiazolobenzimidazole using a combination of pH adjustment and complexation with...

The thiazolobenzimidazole 1-(2,6-difluorophenyl)-1H,3H-thiazolo[3,4-a] benzimidazole, TBI, is an experimental drug for the treatment of AIDS which exhibits a low water solubility (11 micrograms/mL) and is therefore difficult to administer in an injectable solution dosage form at a target solution concentration of 10 mg/mL. The compound has a single ionizable functional group and exhibits an increase in solubility with decreasing pH consistent with a pKa of 3.55, but the maximum solubility attainable by pH adjustment has been shown to be only 0.4 mg/mL (at pH 2). TBI has been found to form inclusion complexes in either its neutral or its protonated form with 2-hydroxypropyl-beta-cyclodextrin (HPCD). The equilibrium constants for 1:1 complex formation were found to be 81 and 1033 M-1 for the protonated and neutral species, respectively. Although the formation of protonated complex is less favored in comparison to the neutral complex, the contribution of this species to the overall solubility of TBI predominates at low pH. Thus, using a combined approach of pH adjustment and complexation with HPCD, a solubility enhancement of 3 orders of magnitude is possible. NMR proton spectroscopy and molecular modeling studies, conducted to understand the orientation of TBI in the complex and the effect of protonation, are described.