Differential degradation rates of inactivated alkyltransferase in blood mononuclear cells and tumors of patients after treatment with O-6-benzylguanine

O-6-Alkylguanine-DNA alkyltransferase (AGT) repairs O-6-alkylating DNA addu cts generated by alkylating therapeutic agents. Therefore, AGT activity may be an important marker of tumor and normal tissue sensitivity to chemother apeutic agents and a predictor for the success of chemotherapeutic regimens . It is rapidly inactivated by O-6-benzylguanine (BG) that mimics its subst rates, O-6-methylguanine and O-6-chloroethylguanine DNA adducts. In a Phase I clinical trial, BG was given in increasing doses (from 10 to 120 mg/m(2) ) by 1-h infusion. We previously reported depletion of AGT activity, and in this report, we demonstrate the relationship between degradation of BG-ina ctivated AGT protein and the depletion of AGT activity in peripheral blood mononuclear cells (PBMCs) and tumor samples obtained by computed tomography -guided cutting needle biopsy from patients prior to BG and either 2 or 18 h after BG. In PBMCs, BG inactivated AGT activity by over 95-100% at the en d of a 1-h infusion, and depletion was maintained for 18 h. In contrast, AG T protein remained almost unchanged for up to 18 h after BG, suggesting tha t inactivated AGT proteins remain immunoreactive and are not rapidly degrad ed in PBMCs. In patient tumor biopsies, AGT activity was depleted similar t o 90% 2 h after BG. Tumor AGT protein levels were reduced to similar to 40% of pretreatment values when detected by either Western blot or immunohisto chemistry staining. In tumor samples obtained 18 h after BG, >95% inactivat ion of tumor AGT activity was observed at BG doses of 36-80 mg/m(2), and co mplete depletion of tumor AGT activity occurred at 120 mg/m(2) BG. However, residual AGT protein (5-10% of baseline) was detectable in all tumor sampl es. Therefore, the degradation of BG-inactivated AGT protein appeared to be much more rapid in tumors than that in PBMCs, which may impact on AGT rege neration rates as well. Because degradation of BG-inactivated AGT takes pla ce slowly, antibody-based measurements of AGT protein correlate poorly with depletion of AGT activity immediately after BG. Thus, biochemical activity measurements remain the appropriate monitor of AGT during therapeutic modu lation. These data provide the first and conclusive evidence of differentia l degradation rates of inactivated AGT in PBMCs and tumors of patients afte r treatment with BG and suggest that immunoreactive AGT measurements in PBM Cs are a poor surrogate for AGT activity in tumor tissue.