Anti-leukemic potential of methyl-cobalamin inactivation by nitrous oxide. 
Myelo-cytotoxicity of extended nitrous oxide (N2O) inhalation was described almost forty years ago and then incidentally applied already with temporary success for suppressing leukemia.  In 1948 the accompanying megaloblastic maturation arrest was explained by inactivation of the methylcobalamin coenzyme and subsequent folate deficiency.  We studied the anti-leukemic effect of N2O on a transplantable acute leukemia in B(rown) N(orway) rats.  Progression of this B,N,M(yelocytic)L(eukemia) was measured as spleen and liver weights, and leukemic blood cell counts.  The deoxyuridine (dU)-suppression test provided in vitro indication of the functional folate activity of leukemic cells.  Breathing of N2O-oxygen considerably reduced but did not eradicate, BNML-proliferation.  Addition of anti-metabolites, interfering with some enzyme in the folate metabolism beyond the methylcobalamin co-enzyme dependent methionine synthase step, acted at least synergistically.  The anti-leukemic effect of cycloleucine, which reduces S-adenosyl-methionine synthesis by inactivation of methionine adenosyltransferase, was moderate but became much stronger with N2O inhalation.  Methotrexate, a potent anti-leukemic agent by inhibiting tetrahydrofolate (THF) generation through inactivation of di-HF reductase, became highly anti-BNML, even in low dosage when combined with or preceded by N2O. 5-Fluorouracil, which inhibits methylene-THF dependent thymidilate synthase, itself was surprisingly anti-BNML, but also became much more potent with previous or concomitant N2O exposure.  Preliminary dU-suppression test results with human acute leukemia cells, exposed to N2O and/or folate antagonists in vitro, correlated well with the in vivo BNML-experiments.  Combining the anticobalamin activity of N2O with an anti-folate therefore seems to be a promising chemotherapeutic approach.