Beta-glucuronidase-cleavable prodrugs of O6-benzylguanine and O6-benzyl-2'-deoxyguanosine.

Glucuronic acid linked prodrugs of O(6)-benzylguanine and O(6)-benzyl-2'-deoxyguanosine were synthesized. The prodrugs were found to be quite stable at physiological pH and were more than 200-fold less active as inactivators of O(6)-alkylguanine-DNA alkyltransferase (alkyltransferase) than either O(6)-benzylguanine or O(6)-benzyl-2'-deoxyguanosine. Beta-glucuronidase from both Escherichia coli and bovine liver cleaved the prodrugs efficiently to release O(6)-benzylguanine and O(6)-benzyl-2'-deoxyguanosine, respectively. In combination with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), the prodrugs were not effective adjuvants for HT29 cell killing. However, as expected, incubation of these prodrugs with beta-glucuronidase in the culture medium led to much more efficient cell killing by BCNU as a result of the liberation of the more potent inactivators, O(6)-benzylguanine and O(6)-benzyl-2'-deoxyguanosine. These prodrugs may be useful for prodrug monotherapy of necrotic tumors that liberate beta-glucuronidase or for antibody-directed enzyme prodrug therapy with antibodies that can deliver beta-glucuronidase to target tumor cells.