Amplification of the dihydrofolate reductase gene is a mechanism of acquired resistance to methotrexate in patients with acute lymphoblastic leukemia and is correlated with p53 gene mutations.

Although dihydrofolate reductase (DHFR) gene amplification is a common mechanism of resistance to methotrexate (MTX) in tumor cell lines, with the exception of a few case reports, the incidence of this phenomenon as a mechanism of MTX resistance in the clinic has not been reported. We studied 38 untreated patients and 29 patients in relapse with acute lymphoblastic leukemia (ALL) for gene amplification and p53 gene mutations. Three patients were studied both at diagnosis and at each of two relapses after treatment with MTX. Nine of 29 relapsed patients (31%) had low-level DHFR gene amplification (two to four gene copies) associated with increased levels of DHFR mRNA and enzyme activity. Of significance was a correlation of gene amplification with p53 mutations in seven of nine relapsed patients (P < .001). Low-level DHFR gene amplification may be an important cause of MTX resistance in ALL and strengthens the concept that mutations in the p53 gene may lead to gene amplification as a consequence of defective cell cycle control.

Selective treatment of SCID mice bearing methotrexate-transport-resistant human acute lymphoblastic leukemia tumors with trimetrexate and leucovorin protection.

Impaired transport of methotrexate (MTX) is a common resistance mechanism of tumor cells to this drug. Trimetrexate (TMTX), a second-generation folate antagonist, is still active against MTX-transport-resistant cells because it enters cells by passive diffusion and does not use the reduced folate transport system for cell entry. Therefore, although leucovorin (LV) protects MTX-sensitive cells from TMTX toxicity, MTX-transport defective cells are poorly rescued by LV. Severe combined immunodeficiency mice bearing MTX-transport-resistant CCRF-CEM acute lymphoblastic leukemia tumors were treated with TMTX alone or with the combination of TMTX and LV, with tumor regressions in both groups (P < .001) and without significant toxicity. These results indicate that TMTX with LV protection may be a useful therapeutic regimen for patients with MTX-transport-defective acute lymphoblastic leukemia. Furthermore, resistance to TMTX plus LV may result in reversion to MTX sensitivity.

Acute monocytic leukemia: a myeloid leukemia subset that may be sensitive to methotrexate.

Impaired polyglutamylation of methotrexate (MTX) and thus poor retention is believed to be the basis of intrinsic resistance in blasts from patients with acute myeloid leukemia (AML) to MTX. We studied additional samples from patients with this disease, and confirmed that polyglutamylation of MTX was poor in ANLL blast cells. However, in one subset of ANLL, acute monocytic leukemia, (M5) leukemia blasts were found to be capable of accumulating and forming long-chain MTX polyglutamates. An acute monocytic leukemia cell line, THP-1 also was found to accumulate high levels of MTX polyglutamates and was relatively sensitive to MTX, strengthening the concept that M5 blasts may be sensitive to this drug. MTX may be an overlooked drug for the treatment of acute monocytic leukemia.

A phase II study of continuous infusion of trimetrexate in patients with refractory acute leukemia.

Trimetrexate, a second-generation folate antagonist, is a potent inhibitor of dihydrofolate reductase with a broader spectrum of activity and different mechanism of entry and intracellular accumulation than methotrexate. Six patients with refractory or relapsed acute leukemia were treated with a 5-day continuous infusion of trimetrexate of 8 mg/m2/day after an initial loading dose of 4 mg/m2 to achieve a target plasma concentration of 0.2-0.5 microM. In 4 patients with peripheral blasts at study entry, transient decrease or disappearance of blasts was observed, although no decrease of bone marrow blasts occurred. Mucositis was dose-limiting and severe in 4 patients. Neutrophil and platelet nadirs occurred on day 5-12 postinfusion. Because of dose-limiting mucositis, this dose schedule of trimetrexate is not recommended for further studies in refractory acute leukemia. However, other dose schedules (24- to 72-hr infusions) and its use as a modulating agent with thiopurines or leucovorin in patients that are resistant to methotrexate should be explored.

Pharmacologic-guided trial of sequential methotrexate and thioguanine in children with advanced malignancies.

PURPOSE: Based on in vitro studies that have shown synergistic effects of sequential administration of methotrexate (MTX) and thioguanine (6-TG), we conducted a pharmacologically guided trial of sequential MTX and 6-TG to determine the following: (1) the maximum-tolerated dose (MTD) of 6-TG; (2) the nature of the dose-limiting toxicity; and (3) the modulation effect of MTX on 6-TG given by this sequence and schedule. PATIENTS AND METHODS: Thirty-one children with advanced malignancies (acute leukemia, n = 10; lymphoma n = 10; and solid tumors, n = 11) were treated weekly for 3 weeks with a 2-week rest; treatment consisted of a fixed dose of MTX (30 mg/m2 over 24 hours) followed by a 2-hour infusion of 6-TG in escalating doses. RESULTS: Measurement of plasma MTX, 6-TG, and mononuclear 5-phosphoribosyl-1-pyrophosphate (PRPP) levels indicates that the desired biochemical modulation and serum levels were achieved. Nonhematologic toxicities were mild and the dose-limiting toxicity was bone marrow depression. A 300-mg/m2 dose of 6-TG with MTX is considered the MTD. Responses were noted in patients with lymphoma. CONCLUSION: Encouraging antitumor effects were produced with this regimen in heavily pretreated patients with lymphoma, particularly Hodgkin's disease (HD). The durations of responses were 17, 13+, 12, 9, and 7+ months. A phase II trial of the MTX/6-TG combination is warranted for the treatment of relapsed lymphoma.