Synthesis of 131I-labeled glucose-conjugated inhibitors of O6-methylguanine-DNA methyltransferase (MGMT) and comparison with nonconjugated inhibitors as potential tools for in vivo MGMT imaging

O(6)-Substituted guanine derivatives are powerful agents used for tumor cell sensitization by inhibition of the DNA repair enzyme O(6)-methylguanine-DNA methyltransferase (MGMT). To provide targeted accumulation of MGMT inhibitors in tumor tissue as well as tools for in vivo imaging, we synthesized iodinated C(8)-alkyl-linked glucose conjugates of 2-amino-6-(5-iodothenyl)-9H-purine (O(6)-(5-iodothenyl) guanine, ITG) and 2-amino-6-(3-iodobenzyloxy)-9H-purine (O(6)-(5-iodobenzyl) guanine, IBG). These compounds have MGMT inhibitor constants (IC(50) values) of 0.8 and 0.45 microM for ITGG and IBGG, respectively, as determined in HeLa S3 cells after 2-h incubation with inhibitor. To substantiate that the (131)I-(hetero)arylmethylene group at the O(6)-position of guanine is transferred to MGMT, both the glucose conjugated inhibitors ITGG and IBGG and the corresponding nonglucose conjugated compounds ITG and IBG were labeled with iodine-131. The radioiodinations of all compounds with [(131)I]I(-) were performed with radiochemical yields of >70% for the destannylation of the corresponding tri-n-butylstannylated precursors. The binding ability of [(131)I]ITGG, [(131)]IBGG, [(131)I]ITG, and [(131)I]IBG to purified MGMT was tested. All radioactive compounds were substrates for MGMT, as demonstrated using a competitive repair assay. The newly synthesized radioactive inhibitors were utilized to study ex vivo biodistribution in mice, and the tumor-to-blood ratio of tissue uptake of [(131)I]IBG and [(131)I]IBGG was determined to be 0.24 and 0.76 after 0.5 h, respectively.