Tannic acid as a co-former in co-amorphous systems: Enhancing their physical stability, solubility and dissolution behavior

Co-amorphous systems have been increasingly investigated to improve the solubility and dissolution rate of poorly soluble drugs. Considering the ability of tannic acid (TA), a polyphenolic compound, to form hydrogen bonds with compounds that contain carbonyl groups, we hypothesized that tannic acid will also be effective in stabilizing amorphous form of drugs in co-amorphous systems. Co-amorphization by TA of two poorly soluble model drugs, carbamazepine (CBZ) and indomethacin (IND) was investigated. Tannic acid facilitated the amorphization of studied drugs and successful co-amorphous systems were obtained as proved by powder X-Ray diffraction (PXRD). Differential scanning calorimetry (DSC) confirmed the homogeneous structure as indicated by the existence of a single Tg for each co-amorphous product. The expected molecular interactions between phenolic groups in TA and carbonyl groups in the studied drugs (CBZ and IND) were confirmed by analyzing their infrared spectra. Drug-TA co-amorphous formulations showed an enhanced equilibrium solubility over the individual drugs. Powder dissolution test under sink conditions showed improved dissolution profiles of drug-TA co-amorphous formulations compared to the corresponding crystalline drugs and physical mixtures. Tannic acid also showed a superior stabilizing effect. CBZ-TA co-amorphous system was physically stable at dry conditions (up to 6 months at 40 °C), under 60% relative humidity (up to one month at 20 °C), and in solution (after 48 h of solubility measurements), as revealed by PXRD examination of the remaining solid after solubility measurement. However, IND-TA co-amorphous formulation remained stable at dry conditions up to 6 months at 4 °C and up to one month at 60% relative humidity at 20 °C. These findings demonstrate the potential of tannic acid as a promising co-former in co-amorphous systems of poorly soluble drugs.