In vitro protein binding and tissue distribution of D(+) usnic acid

In vitro protein binding of D(+) usnic acid in rabbit plasma and purified bovine serum albumin was investigated by equilibrium dialysis. The drug was highly protein bound, approximately 99.2%, and the extent of protein binding remained constant at usnic acid concentrations in the range of 1-5 micrograms/ml. The extent of binding, however, tended to decrease at low albumin concentrations and higher drug concentrations; Scatchard plot analysis indicated the existence of two classes of binding sites with association constants of 34.3 x 10(-6) and 1.43 x 10(-6) M respectively. Tissue distribution studies of usnic acid were undertaken in rats following i.p. administration. Usnic acid was well distributed into well perfused organs. The tissue/plasma ratio in lungs was high, which could be advantageous in a therapeutic agent for pulmonary tuberculosis.