Pharmacokinetics of leucovorin rescue using a new methotrexate-independent biochemical assay for leucovorin and N5-methyltetrahydrofolate.

A rapid biochemical assay for leucovorin (LV) and N5-methyltetrahydrofolate (mTHFA) has been developed, using a cell-free extract of Escherichia coli. The reaction used was the formylation of [14C]methionyl-tRNA fMet E. coli, in which product formation is dependent on added folate derivatives. The actual formyl donor, N10-formyltetrahydrofolate, is generated from LV in the presence of ATP or from mTHFA in the presence of NAD+ or NADP+. The assay is sensitive to approximately 5 X 10(-8) M l-LV and 5 X 10(-7) M l-mTHFA in serum. There is no cross-reactivity between LV and mTHFA. The presence of methotrexate (MTX) has no effect on assay results. This assay has been used to determine LV and mTHFA levels in patients receiving LV rescue after high-dose MTX infusions. Patients received 96 mg/m2 of LV as a single iv dose at the conclusion of the MTX infusion. Serum levels of LV and mTHFA were followed for the next 6 hrs. The initial (5-min) level and LV was about 10(-5) M, and its concentration in the serum decreased rapidly. The alpha-phase half-life of LV was about 15 mins, but LV was readily detectable by the assay for 3-4 hrs. There was rapid apparent conversion to mTHFA, as this compound was also detectable at the initial time point. The level of mTHFA increased for at least 60 mins, being equimolar with LV by 30 mins, and then decreased slowly with an apparent half-life of 2-3 hrs.