Inactivation of O⁶-alkylguanine-DNA alkyltransferase by folate esters of O⁶-benzyl-2′-deoxyguanosine and of O ⁶-[4-(hydroxymethyl)benzyl]guanine

O⁶-Alkylguanine-DNA alkyltransferase (alkyltransferase) provides an important source of resistance to some cancer chemotherapeutic alkylating agents. Folate ester derivatives of O⁶-benzyl-2′-deoxyguanosine and of O⁶-[4-(hydroxymethyl)benzyl]guanine were synthesized and tested for their ability to inactivate human alkyltransferase. Inactivation of alkyltransferase by the γ-folate ester of O⁶- [4-(hydroxymethyl)benzyl] guanine was similar to that of the parent base. The γ-folate esters of O⁶-benzyl-2′-deoxyguanosine were more potent alkyltransferase inactivators than the parent nucleoside. The 3′-ester was considerably more potent than the 5′-ester and was more than an order of magnitude more active than O⁶-benzylguanine, which is currently in clinical trials to enhance therapy with alkylating agents. They were also able to sensitize human tumor cells to killing by 1,3-bis(2- chloroethyl)-1-nitrosourea, with O⁶-benzyl-3′-O-(γ-folyl) -2′-deoxyguanosine being most active. These compounds provide a new class of highly water-soluble alkyltransferase inactivators and form the basis to construct more tumor-specific and potent compounds targeting this DNA repair protein.