Solubilization of liposomes by weak electrolyte drugs II. Cefotaxime

The solubilization of dimyristoylphosphatidylcholine (DMPC) liposomes by the weak electrolyte drug, cefotaxime (CFX), has been studied as a function of pH, DMPC, temperature, presence of cholesterol (CHOL), and method of liposome preparation. At 7.5mMCFX the lag time for solubilization increased, the rate of solubilization decreased, and the minimum turbidity reached increased as a function of DMPC at pH 1.0 and 40C. Solubilization was most pronounced at pHs below the pKa but inhibited at least one pH unit above the pKa. The critical mole ratio of unionized CFX:DMPC, Rec, for solubilization was estimated to be 0.12. Reducing the temperature slowed the rate of solubilization as did the addition of CHOL. Encapsulation of CFX in liposomes did not significantly reduce CFX degradation, k1=0.048h-1 at 40C and a complex of DMPC and a degradation product of CFX precipitated as rectangular crystals. As a result, an increase in the turbidity of solubilized systems was observed from about 20h to 48h depending on the conditions. Liposomes in the gel state or with at least 20% CHOL did not undergo an apparent reversal of solubilization. It is concluded that the inclusion of weak electrolyte drugs existing predominantly as the unionized species in liquid crystalline state liposomes may undergo a slow solubilization process not necessarily recognized during characterization.