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

O⁶-Substituted guanine derivatives are powerful agents used for tumor cell sensitization by inhibition of the DNA repair enzyme O⁶-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 C₈-alkyl-linked glucose conjugates of 2-amino-6-(5-iodothenyl)-9H-purine (O⁶-(5-iodothenyl) guanine, ITG) and 2-amino-6-(3-iodobenzyloxy)-9H-purine (O⁶-(5-iodobenzyl) guanine, IBG). These compounds have MGMT inhibitor constants (IC₅₀ values) of 0.8 and 0.45 μM for ITGG and IBGG, respectively, as determined in HeLa S3 cells after 2-h incubation with inhibitor. To substantiate that the ¹³¹I−(hetero)arylmethylene group at the O⁶-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 [¹³¹I]I^- were performed with radiochemical yields of >70% for the destannylation of the corresponding tri-n-butylstannylated precursors. The binding ability of [¹³¹I]ITGG, [¹³¹]IBGG, [¹³¹I]ITG, and [¹³¹I]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 [¹³¹I]IBG and [¹³¹I]IBGG was determined to be 0.24 and 0.76 after 0.5 h, respectively.