Liposomal encapsulation of the tuberculocidal drug, pyrazinamide, for increasing safety and potency

Mycobacterium tuberculosis infects approximately one-third of the world's population and causes about three-million deaths each year. Pyrazinamide is a key drug in the short- course chemotherapy of pulmonary tuberculosis and is included in several drug regimens recommended by the IUAT and WHO. Liposomes are taken up avidly by macrophages and are targetted naturally to the reticuloendothelial system. These facts propelled our interest in formulating pyrazinamide in liposomal form and studying various aspects on this formulation preceeding future work on treatment of Mycobacterium tuberculosis. Pyrazinamide liposomes were formulated using L-alpha-lecithin and also L-alpha- dipalmitoyl phosphatidyl choline [DPPC], as the major phospholipids. Encapsulation efficiencies of the prepared liposomes were estimated by spectrophotometric measurement of free pyrazinamide at 268.4 nm. Charged liposomes effected higher pyrazinamide entrapment compared to neutral liposomes, with the exception of 7:2 neutral pyrazinamide liposomes which exhibited the highest trapping efficacy. Pyrazinamide liposomes were characterized by transmission electron microscopy and differential scanning calorimetry [DSC]. The DSC-thermograms obtained revealed interaction between DPPC and the other phospholipids included in the preparations indicating liposome formation. In-vitro release kinetics study was conducted to anticipate the liposomes' behavior under the hydrodynamic stress in-vivo. The drug release pattern was biphasic and followed first order kinetics. The negative liposomes exhibited higher drug release compared to the neutral liposomes. The formulation 7:2 neutral pyrazinamide liposomes showed maximum drug retention. Lung disposition of pyrazinamide liposomes was conducted in an experiment employing 7:2 neutral pyrazinamide liposomes and free pyrazinamide. The results indicated that liposomal pyrazinamide was detected, in mice lungs, in higher amounts lasting for a period of 14-days compared to free pyrazinamide. These studies are a predictor for further studies involving the efficacy of pyrazinamide liposomes for treatment of Mycobacterium tuberculosis