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Methotrexate (MTX) transport was examined in 27 patients with untreated acute lymphocytic leukemia (ALL) and 31 patients with relapsed ALL using a previously described fluorescent MTX analog (PT430) displacement assay (Blood 80:1158, 1992). Only 13% of untreated patients were considered to have impaired MTX transport, whereas more than 70% of relapsed patients had evidence of impaired MTX transport. To further characterize the basis for this defect, Northern analyses for the reduced folate carrier (RFC) were performed on the RNA available from the leukemic blasts of 24 patients in whom MTX transport had been measured. Six of nine samples with impaired MTX transport had decreased RFC expression (one had no detectable RFC expression), while three had no decrease in RFC expression. None of 15 samples with normal MTX transport had decreased RFC expression. A reverse-transcriptase polymerase chain reaction (RT-PCR) assay was developed to quantitate RFC mRNA expression more accurately. Decreased RFC expression was demonstrated in six of the nine samples with impaired MTX transport, confirming the results obtained by Northern blot. These data indicate decreased RFC expression associated with impaired MTX transport is observed in relapsed ALL following treatment with MTX-containing therapy.

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.

A rapid and sensitive procedure for the separation of methotrexate (MTX) polyglutamates2 using capillary electrophoresis (CE) is described as it applies to the in vitro assay of the enzyme gamma-glutamyl hydrolase (GGH, EC 3.4.22.12). Distinct separation of MTX and polyglutamylated forms (up to glu4) is achieved within 10 min using a 75 microns I.D. capillary (50 cm, +25 kV), and enables quantitation of both reactant and enzyme products. As activity can be reliably determined using less than 5 x 10(5) eukaryotic cells, this new technique can be used to measure GGH in patient tumor samples and investigate the relationship between GGH levels and clinical MTX resistance.

Methotrexate, a folic acid antagonist, is used extensively not only for the treatment of cancer but also for the treatment of rheumatoid arthritis, psoriasis, and autoimmune disease and for the prevention of graft-versus-host disease after transplantation. The drug is also used as an abortifacient.1,2 Other folate antagonists are used to treat bacterial infections (trimethoprim), malaria (pyrimethamine), and Pneumocystis carinii infection (trimetrexate with leucovorin).3,4 As with other drugs used to treat infectious diseases or cancers, the development of resistance limits the effectiveness of these folate antagonists. An understanding of the mechanisms of resistance to this class of drugs is . . .

PURPOSE Trimetrexate (TMTX) is a dihydrofolate reductase inhibitor, which, like methotrexate (MTX), has been shown to potentiate fluorouracil (FU) cytotoxicity by increasing phosphoribosylpyrophosphate (PRPP) levels. We investigated the safety and efficacy of a sequential TMTX/FU/leucovorin (LV) combination. PATIENTS AND METHODS Forty-one patients with advanced gastrointestinal carcinoma (mostly colorectal) received variable doses of TMTX followed 24 hours later by FU/LV (500 mg/m2 of each drug). Almost all patients had received previous chemotherapy. The initial 19 patients were treated on a 3-week-on/1-week-off schedule without any significant toxicity; the remaining patients were treated for 6 consecutive weeks followed by a 2-week rest period. TMTX was escalated in 30-mg/m2 increments from 20 to 110 mg/m2 in separate patient cohorts. When the 110-mg/m2 dose of TMTX was reached, the FU dose was escalated from 500 mg/m2 to 600 mg/m2. RESULTS The partial response (PR) rate in assessable patients with colorectal cancer (all previously treated) was 20% (seven of 35; 95% confidence interval, 7% to 33%), and with other gastrointestinal cancers was one of four patients. Median survival has not been reached with a median follow-up of 13.5 months. The maximum-tolerated dose (MTD) was 110 mg/m2 for TMTX, 500 mg/m2 for FU, and 500 mg/m2 for LV on a 6-weeks-on/2-weeks-off cycle. The principal toxicities were grade 3 or 4 diarrhea, which occurred in 17% of patients, and hypersensitivity reactions, which occurred in 26% of patients. CONCLUSION TMTX can be administered at maximal doses in combination with FU and LV without increasing toxicity. The PR rate of 20% in advanced colorectal carcinoma patients previously treated with chemotherapy is encouraging and merits further study.

The detection of a monoclonal lymphoid population in a clinical specimen is highly suggestive for a neoplastic lymphoproliferative disease. There are only few disorders where mono- or oligoclonality is of benign origin and is not associated with malignancy. The detection of a clonal lymphocytic population is thus of great diagnostic interest. Traditionally, the molecular detection of clonal populations is performed by Southern blot analysis of rearranged T-cell receptor or immunoglobulin genes. However, Southern blot analysis cannot be performed on small biopsies of tissue, because of the requirement for sufficient high quality DNA. T-cell receptor (TCR) gamma genes rearrange in the majority of T-lineage acute lymphocytic leukemias (ALL). Southern blot analysis show rearrangements in about 60% of pre-B lineage ALL, but not in mature B-ALL [1–3]. Immunoglobulin heavy chain IgH genes rearrange in Band pre-B ALL but not in T-ALL [4]. Considerable diversity is created in the junctional region of rearranging variable (V) and joint (J) genes by imprecise recombination and by the insertion of random nucleotide sequences (N regions) [5–7]. Immunoglobulin heavy chain genes additionally recombine various diversity (D) genes [8]. The highest variation is found in the third complementarity determining region (CDR3) [9]. The unique junctional sequence of rearranging genes is a clonespecific “molecular fingerprint” of an individual lymphocytic clone. Rearranging immunoglobulin genes undergo secondary rearrangements probably predominantly by an exchange of the VH gene leading to a subclone formation in about one third of all cases with ALL [10, 11].

Methotrexate (MTX) is a clinically important antifolate that has been used in combination with other chemotherapeutic agents in the treatment of malignancies including acute lymphocytic leukemia, osteosarcoma, carcinomas of the breast, head and neck, choriocarcinoma and non-Hodgkin's lymphoma. The primary target of MTX is the enzyme dihydrofolate reductase (DHFR) which catalyzes the reduction of folate and 7,8-dihydrofolate to 5,6,7,8-tetrahydrofolate. Understanding of MTX action has revealed how cells acquire resistance to this drug. The four known mechanisms of MTX resistance are a decrease in the uptake of the drug, a decrease in the retention of the drug due to defective polyglutamylation or an increase in polyglutamate breakdown, an increase in the enzyme activity and a decrease in the binding of MTX to DHFR. The molecular basis for some of these mechanisms has been elucidated in MTX resistant cell lines; in particular the occurrence of gene amplification resulting in increased DHFR and point mutations resulting in altered DHFR with reduced affinity for MTX. Cloning of the human folylpolyglutamate synthase gene and the reduced folate transport gene have been reported recently and should facilitate the identification of the molecular basis of these resistant phenotypes. DHFR protein has been shown to regulate its synthesis by exerting an inhibitory influence on its own translation. Addition of MTX relieves this inhibition thus providing a possible molecular explanation for the rapid rise in DHFR activity noted in some cells after MTX administration. Alterations in genes involved in regulating the cell cycle such as cyclin D1 and the retinoblastoma (Rb) gene have also been shown to influence cellular response to MTX. Overexpression of cyclin D1 in HT1080, a human fibrosarcoma cell line, results in decreased MTX sensitivity. The molecular basis of this observation is under investigation. Abnormalities in the Rb gene may also have profound effects on MTX sensitivity. Rb interacts with the family of transcription factors called E2F reducing transcription of genes that contain E2F binding sites in the promoter regions e.g. DHFR. When Rb is deleted or rendered nonfunctional levels of "free" or unbound E2F are high resulting in enhanced transcription of genes such as DHFR. This results in increased DHFR protein and may lead to MTX resistance. As the knowledge regarding mechanisms of resistance increases newer approaches to circumvent such resistance or to target resistant cells can be undertaken.