Marked inactivation of O6-alkylguanine-DNA alkyltransferase activity with protracted temozolomide schedules

Abstract

Temozolomide, an oral DNA methylator that inactivates the DNA repair enzyme O 6 -alkylguanine-DNA alkyltransferase (AGAT), has demonstrated anticancer activity on protracted schedules. Protracted schedules may lead to an 'autoenhancement' of temozolomide's inherent cytotoxic potential by cumulative reduction of the cell's capacity for AGAT-mediated DNA repair and resistance. This study was undertaken to characterise AGAT inactivation and regeneration in the peripheral blood mononuclear cells (PBMCs) of patients treated on two protracted temozolomide schedules. O 6 -alkyl guanine-DNA alkyltransferase activity was measured in the PBMCs of patients treated on two phase I protracted temozolomide studies. Patients were treated daily for either 7 days every 2 weeks (Schedule A) or 21 days every 4 weeks (Schedule B). The effects of various temozolomide doses (75 -175 mg m À2 ), treatment duration (7 -21 days), and temozolomide plasma levels on AGAT inactivation and regeneration, as well as the relation between AGAT inactivation and toxicity, were studied. O 6 -alkyl guanine-DNA alkyltransferase activity in PBMCs was measured serially in 52 patients. Marked inactivation of AGAT occurred following 7 days of temozolomide treatment, with mean AGAT activity decreasing by 72% (Po0.0001). Similarly, mean AGAT activity decreased by 63 and 73% after 14 and 21 days of treatment, respectively (Po0.001 for both comparisons). O 6 -alkyl guanine-DNA alkyltransferase inactivation was greater after 7 days of treatment with higher doses of temozolomide than lower doses and remained markedly reduced 7 days post-treatment. However, AGAT inactivation following temozolomide treatment for 14 and 21 days was similar at all doses. On the continuous 21-day schedule, AGAT inactivation was significantly greater in patients who experienced severe thrombocytopenia than those who did not (90.375.5 vs 72.5716.1%, Po0.045). In Conclusion, protracted administration of temozolomide, even at relatively low daily doses, leads to significant and prolonged depletion of AGAT activity, which may enhance the antitumour activity of the agent.

Rechallenge with temozolomide in patients with recurrent gliomas

“…This regimen may yield similar results with respect to efficacy, but a higher rate of toxicity, specifically lymphopenia and infection, have been reported [8,9]. The latter two regimens aim at a dose intensification, which could enhance the direct cytotoxic effect of TMZ and which might deplete the DNA repair enzyme O 6 -methylguanine DNA Hau, 5 methyltransferase (MGMT) [10]. This enzyme modulates resistance to alkylating chemotherapeutic agents [11].…”

Dose finding and O6-alkylguanine-DNA alkyltransferase study of cisplatin combined with temozolomide in paediatric solid malignancies

“…This was somewhat higher than previous values (Middleton et al, 2000b: range 2.2 -25.6, mean 12.6) but similar to other ongoing studies (range 2.3 -51.6, mean 15.5: GPM unpublished results). Another study of adult patients has reported a range of B0.6 to B19 Fm mg À1 DNA and a much lower mean of 6.974.7 Fm mg À1 DNA (Tolcher et al, 2003). Some of these differences may be a consequence of the age or disease status of the study groups.…”

“…Previous studies reporting MGMT in adult PBMCs have also shown a range of extents of depletion by temozolomide. A variety of dose regimen and schedules have been examined but complete depletion of MGMT has been observed in very few patients and very wide variations in depletion are always observed (Lee et al, 1994;Gander et al, 1999;Middleton et al, 2000b;Tolcher et al, 2003). The reasons for this have yet to be established.…”

“…Although our evaluations are in a limited number of samples and need to be confirmed and extended by further studies, they are supported by previous studies. Thus, lower MGMT activities in PBMCs have been associated with increased haematological toxicity in a phase II study of temozolomide in adult melanoma patients (Middleton et al, 2000b) and inactivation of MGMT was correlated with haematological toxicity in a phase I study of temozolomide (Tolcher et al, 2003). Nevertheless, to address the questions of MGMT inactivation by cisplatin in relation to tumour responses and toxicities, it would be necessary to undertake larger studies, ideally in direct comparison with equivalent numbers of patients receiving temozolomide alone.…”

Tailored temozolomide therapy according to MGMT methylation status for elderly patients with acute myeloid leukemia

“…Protracted administration of temozolomide may lead to a marked and sustained inactivation of MGMT leading to an ''autoenhancement" of temozolomide's inherent cytotoxic potential by cumulative reduction of the leukemic blasts' capacity for MGMT-mediated DNA repair and resistance [20]. MGMT activity can be reduced by approximately 80% in patients treated with this dosing schedule [20]. Thus, we designed an exploratory clinical study to test two hypotheses: (1) Can temozolomide therapy be tailored according the MGMT methylation status in patients with AML?…”