DISPARATE AFFINITIES OF ANTIFOLATES FOR FOLYLPOLYGLUTAMATE SYNTHETASEFROM HUMAN LEUKEMIA-CELLS

Previous work showed that acute myelocytic leukemia blasts accumulate less long chain polyglutamates of methotrexate (MTX) than acute lympho cytic leukemia blasts when incubated with this radiolabeled antifolate . This difference likely explains the increased sensitivity of lymphoi d leukemias to short-term exposure of MTX as compared with myeloid leu kemias. In this study, we examined the basis for differences between l ong chain MTX polyglutamate accumulation between different leukemia ce ll types using both leukemia cell lines and blasts freshly isolated fr om blood of leukemic patients, The major difference found between leuk emia cells that accumulate long chain polyglutamates and those that do not were differences in K-m values for the enzyme folylpolyglutamate synthetase. K-m values did not change with partial purification of thi s enzyme, indicating that interfering substances in crude lysates were not responsible for this difference, We postulate that there may be d ifferences in the properties of this enzyme related to tissue specific expression, In contrast to MTX, both Tomudex (Zeneca Pharmaceuticals, Wilmington, DE) and 1843U89, potent inhibitors of thymidylate synthet ase, have low K(m)s for folylpolyglutamate synthetase, and polyglutama te forms of these inhibitors are accumulated to the same degree in bot h myeloid and lymphoid acute leukemia cells, paralleling the equivalen t cytotoxicity found between myeloid and lymphoid leukemia cell lines. Based on these results, we believe a clinical trial of Tomudex in pat ients with acute myeloid leukemia is warranted. (C) 1997 by The Americ an Society of Hematology.