The natural and unnatural diastereomers of leucovorin: aspects of their cellular pharmacology.

Abstract

A mixture of the natural ((6S)-LV) and unnatural ((6R)-LV) diastereomers of leucovorin (LV, (6RS)-LV) is administered clinically in combination with 5-fluorouracil (FUra) to overcome drug resistance due to insufficient intracellular folate concentrations (1). Initially described as a growth factor for Pediococcus cerevesiae, LV has a S chirality at the 6 carbon of the tetrahydropterine ring (2). Following intravenous administration of a mixture of the (6S) and (6R) isomers of leucovorin to patients, plasmatic (6S)-LV disappears rapidly with a T0.5 of 30 min. while (6R)-LV has a slower clearance (To.5 of 7.5 hours) (1,3). The natural isomer is readily transported into cells where it has no known folate cofactor activity. Greenberg was the first to show that LV was converted to another folate involved in purine biosynthesis (4). The first enzymatic step involved was later found to be methenyltetrahy-drofolate synthetase (5-formyltetrahy-drofolate cyclodehydrase, EC 6.3.3.2) which catalyzes the irreversible and stereospecific ATP and Mg2+-dependent transformation of (6S)-LV to N5–10-methenyltetrahy-drofolate. Sheep liver enzyme activity was first partially purified (5) and the enzyme was more recently highly purified and characterized from Lactobacillus casei (6) and rabbit liver (7). We have recently obtained and characterized highly purified human liver methenyltetrahydrofolate synthetase (8) and will describe in the following paper its purification and characteristics. We next took advantage of the enzyme characteristics to prepare stereochemically pure (6R)-LV (9) and will present its transport characteristics and biological activity in human leukemic CCRF-CEM cells.