GAMMA-GLUTAMYL HYDROLASE AND FOLYLPOLYGLUTAMATE SYNTHETASE ACTIVITIESPREDICT POLYGLUTAMYLATION OF METHOTREXATE IN ACUTE LEUKEMIAS

Decreased methotrexate (MTX) long-chain polyglutamate formation is ass ociated with MTX resistance whereas high levels of MTX polyglutamate a ccumulation are found in the blasts of leukemia patients who respond t o therapy and have improved outcome. The steady-state level of long-ch ain MTX polyglutamates depends on the balance of activities of two enz ymes: folylpolyglutamate synthetase (FPGS), which adds glutamates to M TX in a gamma-carboxyl linkage, and gamma-glutamyl hydrolase (GGH) or conjugase, which sequentially removes the terminal glutamate residue o f MTX polyglutamates. FPGS and GGH activities as well as the formation of total and long-chain MTX polyglutamates were measured after incuba tion with [H-3]MTX in 15 blast samples from patients with acute leukem ias (myeloid and lymphoid). The ratio between GGH and FPGS activities was better at predicting the amount of polyglutamate accumulated in th e 24-h [H-3]MTX assay compared to the determination of either activity alone. The linear regression curve relating the relative levels of lo ng-chain polyglutamates/total polyglutamates with the ratio of GGH/FPG S showed an r value of 0.81 (P < 0.001). These data suggest that the e valuation of both these enzymes at diagnosis may be used as a predicto r of MTX polyglutamylation and therefore for response to MTX therapy a nd outcome.