Substitution of aminomethyl at the meta-position enhances the inactivation of O⁶-alkylguanine-DNA alkyltransferase by O⁶- benzylguanine

O⁶-Benzylguanine is an irreversible inactivator of O ⁶-alkylguanine-DNA alkyltransferase currently in clinical trials to overcome alkyltransferase-mediated resistance to certain cancer chemotherapeutic alkylating agents. In order to produce more soluble alkyltransferase inhibitors, we have synthesized three aminomethyl-substituted O ⁶-benzylguanines and the three methyl analogs and found that the substitution of aminomethyl at the meta-position greatly enhances inactivation of alkyltransferase, whereas para-substitution has little effect and ortho-substitution virtually eliminates activity. Molecular modeling of their interactions with alkyltransferase provided a molecular explanation for these results. The square of the correlation coefficient (R²) obtained between E-model scores (obtained from GLIDE XP/QPLD docking calculations) vs log(ED₅₀) values via a linear regression analysis was 0.96. The models indicate that the ortho-substitution causes a steric clash interfering with binding, whereas the meta-aminomethyl substitution allows an interaction of the amino group to generate an additional hydrogen bond with the protein.