Resistance-modifying agents. 8. Inhibition of O-6-alkylguanine-DNA alkyltransferase by O-6-alkenyl-, O-6-cycloalkenyl-, and O-6-(2-oxoalkyl)guanines and potentiation of temozolomide cytotoxicity in vitro by O-6-(1-cyclopentenylmethyl)guanine

A series of O-6-allyl- and O-6-(2-oxoalkyl)guanines were synthesized and ev aluated, in comp ari son with the corresponding O-6-alkylguanines, as poten tial inhibitors of the DNA-repair protein O-6-alkylguanine-DNA alkyltransfe rase (AGT). Simple O-6-alkyl- and O-6-cycloalkylguanines were weak AGT inac tivators compared with O-6-allylguanine (IC50 = 8.5 +/- 0.6 muM) With IC50 values ranging from 100 to 1000 muM. The introduction of substituents at C- 2 of the allyl group of O-6-allylguanine reduced activity compared with the parent compound, while analogous compounds in the O-6-(2-oxoalkyl)guanine series exhibited very poor activity (150-1000 muM) O-6-Cycloalkenylguanines proved to be excellent AGT inactivators, with 1-cyclobutenylmethylguanine (IC50 = 0.55 +/- 0.02 muM) and 1-cyclopentenylmethylguanine (IC50 = 0.39 +/ - 0.04 muM) exhibiting potency approaching that of the benchmark AGT inhibi tor O-6-benzylguanine (IC50 = 0.18 +/- 0.02 muM). 1-Cyclopentenylmethylguan ine also inactivated AGT in intact HT29 human colorectal carcinoma cells (I C50 = 0.20 +/- 0.07 muM) and potentiated the cytotoxicity of the monomethyl ating antitumor agent Temozolomide by approximately 3- and 10-fold, respect ively, in the HT29 and Colo205 tumor cell lines. The observation that four mutant AGT enzymes resistant to O-6-benzylguanine also proved strongly cros s-resistant to 1-cyclopentenylmethylguanine indicates that the O-6-substitu ent of each compound makes similar binding interactions within the active s ite of AGT.