The intracellular synthesis of polyglutamyl derivatives of both
methotrexate (4-amino-N-10-methylpteroylglutamic acid) and
7-hydroxymethotrexate, the primary plasma metabolite of
methotrexate in humans, was evaluated in a
methotrexate-sensitive,
acute lymphoblastic leukemia cell line, MOLT 4. These studies were performed using a highly specific ion-pairing high-pressure liquid chromatography method which permits the simultaneous determination of
methotrexate,
7-hydroxymethotrexate, and their corresponding polyglutamyl derivatives. When MOLT 4 cells were exposed to 1 microM
methotrexate, the monoglutamate attained a steady state after 30 min, and polyglutamyl derivatives having from one to 4 additional glutamyl residues were observed over 4 hr. Four additional metabolites were also detected upon incubation with 1 microM
7-hydroxymethotrexate. On the basis of the retention times for these compounds relative to
methotrexate polyglutamyl standards and since these metabolites reverted to
7-hydroxymethotrexate upon treatment with a preparation of hog kidney
conjugase, they were identified as polyglutamyl derivatives of
7-hydroxymethotrexate. The identification of
7-hydroxymethotrexate polyglutamyl derivatives in vitro raises the possibility of an important new dimension in the pharmacological action of
methotrexate. We investigated the effect of extracellular
7-hydroxymethotrexate on net
methotrexate uptake and metabolism when cells were exposed simultaneously to 1 microM [3H]-
methotrexate and unlabeled
7-hydroxymethotrexate. A decrease in the levels of both intracellular
methotrexate and the corresponding polyglutamyl derivatives was noted for cells treated with 1 or 10 microM
7-hydroxymethotrexate. However, no appreciable effect of
7-hydroxymethotrexate on the amount of polyglutamyl derivatives formed relative to the total intracellular
antifolate was noted. These studies show that in MOLT 4 cells (a) both
methotrexate and
7-hydroxymethotrexate are rapidly converted to polyglutamyl derivatives, and (b)
7-hydroxymethotrexate interferes with net
methotrexate accumulation and metabolism when present simultaneously in the extracellular medium. These results, moreover, suggest a potential role for
7-hydroxymethotrexate in modulating the biochemical effects of
methotrexate in vivo.