A phase I study of trimetrexate, an analog of methotrexate, administered monthly in the form of nine consecutive daily bolus injections

Trimetrexate glucuronate (TMTX) is a methotrexate (MTX) analog that is active against transport-deficient MTX-resistant tumor cells. We performed a phase I study of TMTX administered by daily bolus for 9 consecutive days since this schedule is one of the most active in experimental murine tumor models. The drug was administered in this fashion every 4 weeks for at least two cycles. Fifteen patients with refractory metastatic cancers were studied and all had received prior chemotherapy. The dose-limiting toxicity was a rapidly reversible thrombocytopenia first seen at a daily dose of 4.0 mg/m² which occurred 7 days after the end of TMTX administration. There was great inter-and intrapatient variability in the platelet nadirs observed in the six patients treated at 4.0 mg/m². One patient died of massive hemoptysis during a platelet nadir at that dose level. Granulocyte counts never dropped below 1500/mm³. Only one patient had significant non-hematological toxicity: a radiation recall skin toxicity along with a self-limited maculopapular rash. One patient with melanoma and lung metastases treated at 4.0 mg/m² had a partial response. TMTX plasma levels were measured by HPLC every 3 days prior to daily dosing in patients receiving 4 mg/m² to determine whether drug accumulation occurred during this prolonged administration schedule. Nadir drug levels varied from less than 0.02 to 0.35 μM and did not seem to increase during the 9-day schedule in individual patients. By comparison with other phase I trials, the hematologic toxicity of TMTX seems to be schedule-dependent, with less drug being tolerated and more severe thrombocytopenia observed with more protracted treatment protocols. A firm phase II starting dose for daily bolus x 9 schedules is difficult to recommend in view of the variable toxicity observed in the patients treated at 4.0 mg/m² daily, who, in addition, had all been extensively pretreated. A reasonable starting dose might be 3.0 mg/m² daily with built-in dosage increases or decreases.