CHEMICAL AND ENZYMATIC DEGRADATION OF GANCICLOVIR PRODRUGS - ENHANCED STABILITY OF THE DIADAMANTOATE PRODRUG UNDER ACID CONDITIONS

We report the chemical and enzymatic hydrolysis of two hydrophobic prodrugs of ganciclovir (3 = dipropionate ester; 4 = diadamantoate ester). Both prodrugs undergo hydrolysis showing a pH dependence of k(obs) = kH + aH+ + k(o) + kHO-aHO- and a pH of maximum stability near pH 5. Only 4 exhibited a shelf life (t90) greater than 2 years. Compound 4 reacts significantly slower than ganciclovir in acidic media, even though the adamantyl esters provide additional reaction sites (which would be expected to increase the rate of degradation) that are distally removed from the guanine ring system, offering negligible steric or electronic substituent effects. Both 3 and 4 hydrolyzed in tissue homogenate, where k(obs) followed liver > intestine much greater than skin. Based on these findings of chemical stability, lipophilicity, rand acceptable rate of enzymatic cleavage by skin esterases, 4 meets several of the criteria required for the topical sustained delivery of ganciclovir.