Temozolomide: The effect of once- and twice-a-day dosing on tumor tissue levels of the DNA repair protein O-6-alkylguanine-DNA-alkyltransferase

Temozolomide (TMZ) is a methylating agent of the imidotetrazine class, whos e cytotoxic product is O-6-methyl-guanine DNA adducts, which initiate a fut ile recycling of the mismatch repair pathway causing DNA strand breaks and apoptotic cell death in mismatch repair proficient cells. The DNA repair pr otein O-6-alkylguanine DNA alkyltransferase (AGT) repairs these adducts in a suicide manner and reduces the cytotoxic action of TMZ. An antitumor thre shold is reached when sufficient adducts are formed by TMZ to inactivate AG T. In this study, we evaluated the relation between TMZ dosing and AGT depl etion in patients with deep visceral tumors and in peripheral blood mononuc lear cells (PBMCs) to determine whether the dose of TMZ was sufficient to i nactivate AGT and lead to therapeutic efficacy. To do so, we compared singl e dose therapy with a novel twice daily regimen in a laboratory correlate-d riven Phase I dose escalation study. p.o. bolus dose TMZ 200 mg/m(2) daily times five was compared with the same bolus on day I followed by nine doses at 12-h intervals of 50, 75, 90, or 100 mg/m(2). Dose-limiting toxicity in the bid regimen (grade IV thrombocytopenia and neutropenia) was seen at 10 0 mg/m(2), cumulative dose 1100 mg/m(2), and the maximum tolerated dose was 1010 mg/m(2). The degree of tumor tissue AGT activity depletion measured i n biopsies before and on day 5 of therapy varied widely, between 0 (in 3 pa tients) and 99% (in 1), with the majority of patients (10 of 15) having 52- 84% tumor AGT depletion. In contrast, AGT activity in PBMCs fell rapidly du ring TMZ administration to undetectable levels in all dosage groups on day 5 but did not correlate with tumor AGT depletion. TMZ pharmacokinetics were dose proportional; no accumulation occurred >5-day period in the bid regim en. Two partial responses were seen, lasting 3 and 4 months. Five additiona l patients achieved prolonged stabilization of disease for 4-6 monthly cycl es. This is the first study to document that at maximum tolerated doses, TM Z depletes PBMC AGT but only partially and variably depletes visceral tumor AGT in most patients, even during twice daily dosing. Drug combinations or schedules designed to maximally deplete tumor AGT might improve TMZ effica cy.