Accumulation of tetrahydrofolates in human plasma after leucovorin administration

Summary

Reduced folates in plasma after i.v. and oral leucovorin administration were estimated by a ternary complex assay based on the incorporation of CH₂FH₄ into a stable complex with Lactobacillus casei thymidylate synthase and [³H]FdUMP. Each of the reduced folates, CH₂FH₄, FH₄, and 5CH₃FH₄, could be quantitatively recovered from plasma by this approach even in the presence of high concentrations of the parent compound leucovorin. Examination of the accumulation kinetics of these reduced folates showed that after i.v. administration of 20 mg d,l-leucovorin to a healthy volunteer, FH₄ and, to a lesser extent, CH₂FH₄ accumulated to maximal levels very early (<15 min), with a subsequent depletion that had a half-life of approximately 30 min. Accumulation of FH₄ reached a peak level that was 12% of the maximal level of 5CH₃FH₄ achieved and more than 3 times greater than the pretreatment level of this common, circulating reduced folate form. Similar accumulation patterns were observed in a female patient with metastatic colonic cancer who was undergoing methotrexate (MTX)/fluorouracil therapy followed by i.v. leucovorin (15 mg). FH₄ also accumulated, but to a lesser extent and over a longer period of time, when the same dose of leucovorin given orally. When several similar doses of leucovorin were given prior to the experimental dose, greater accumulation and duration of the FH₄ response was observed. We propose that accumulation of FH₄ and CH₂FH₄ could provide a circulating source of the reduced folate thought to be the active form for both high-dose MTX with leucovorin rescue and enhancement of fluorouracil activity.