Enhanced therapeutic effect of methotrexate in experimental rat leukemia after inactivation of cobalamin (vitamin B12) by nitrous oxide

Abstract

Exposure to nitrous oxide interferes selectively with the coenzyme function of vitamin B12 and causes inactivation of methionine synthetase, with subsequent impairment of folate metabolism and reduction of cellular proliferation. In a rat leukemia model (BNML) we investigated the combined administration of nitrous oxide, inactivating vitamin B12, and methotrexate (MTX), a folate antagonist inhibiting the enzyme dihydrofolate reductase. Through different mechanisms, both agents decrease the availability of tetrahydrofolate, and subsequently of other reduced folates, with increased impairment of folate-dependent synthesis of thymidylate. Effects on leukemic growth and on hematological values in rats demonstrated enhancement of the therapeutic effect of MTX by exposure to nitrous oxide. With several treatment schedules, the results of combined treatment were seen to be better than additive when compared with the effects of single agents. In particular, pretreatment of leukemic rats with nitrous oxide for 3 days before administration of MTX appeared effective. With higher doses of MTX, concomitant exposure to nitrous oxide even resulted in toxic effects. These findings were in accordance with the results of some metabolic studies performed in leukemic rats. De novo synthesis of thymidylate in leukemic cells, when studied by means of the deoxyuridine suppression test, showed the most severe disturbance with combined treatment consisting in MTX (0.5 mg/kg) and nitrous oxide pretreatment for 3 days. Intracellular levels of folate and dTTP were lowest with 2 and 3 days' pretreatment before MTX, respectively. It is concluded that this interaction of nitrous oxide and MTX can result in enhanced metabolic and therapeutic effects of low doses of MTX. Inactivation of vitamin B12 appears to be a potentially useful addition in cancer chemotherapy. © 1986 Springer-Verlag.