BACKGROUND: Nitrous oxide inactivates cobalamin which is important in the folate-dependent synthesis of thymidylate. Also, methotrexate has the anti-cancer activity. The aim of this work was to determine the optimal pressure and exposure time of nitrous oxide that maximize the suppression of cancer growth, and the adequate level of methotrexate that maximize the anti-cancer activity of nitrous oxide.

METHODS: Acute lymphoblastic leukemic cells and normal lymphocytes were cultured in hyperbaric chamber at 1, 2 and 3 atm of 74% nitrous oxide in 24, 48 and 72 hours at 0, 0.3, 0.5 and 0.7 µM of methotrexate, respectively. The results were expressed in the ratio of cell number in hyperbaric chamber to that in the incubator.

RESULTS: Compared to control, the growth rates of cancer cells and lymphocytes were 0.767, 0.990 at 1 atm, 0.592, 0.880 at 2 atm and 0.718, 0.864 at 3 atm of nitrous oxide in 24 hours. The survival fraction of cancer cells and lymphocytes were 0.767, 0.894 in 24 hours, 0.800, 0.630 in 48 hours, and 0.571, 0.597 in 72 hours, at 1 atm of nitrous oxide. The growth rates of cancer cells and lymphocytes were 1.012, 0.745 at 0 µM, 0.912, 0.696 at 0.3 µM, 0.77, 0.647 at 0.5 µM and 1.133, 0.506 at 0.7 µM of methotrexate.

CONCLUSIONS: The pressure increase of nitrous oxide significantly suppressed the growth of lymphocyte but not cancer cells. However, growth of cancer cells and lymphocytes were significantly reduced at high concentration of methotrexate and larger exposure time.