Folic acid (pteroylglutamic acid), an essential water-soluble vitamin, consists of a pteridine ring joined to PABA (para-aminobenzoic acid) and glutamic acid.10 Folic acid is the most common of the many folate congeners that exist in nature and perform essential cellular metabolic functions. Folic acid is often called vitamin B9. After absorption, folic acid is reduced by dihydrofolic acid reductase (DHFR) to dihydrofolic acid and then tetrahydrofolic acid (THF), which accepts one carbon groups. Tetrahydrofolic acid serves as the precursor for several biologically active forms of folic acid, including 5-formyltetrahydrofolic acid (5-formyl THF), which is best known as folinic acid, leucovorin, and citrovorum factor. These biologically active forms of folate are enzymatically interconvertible and function as cofactors, providing the one carbon groups necessary for many intracellular metabolic reactions, including the synthesis of thymidylate and purine nucleotides, which are essential precursors of DNA.29,32,36,38,43 The minimum daily requirement of folic acid is normally 50 µg, but pregnant women and nutritionally deprived, acutely ill patients may require 100 to 200 µg.10,12
Leucovorin is a mixture of the active and inactive diastereoisomers of 5-formyl THF of which the levo form is active and available as levoleucovorin.6 Both are available as the calcium salt, with the same chemical formula, C20H21CaN7O7, and a molecular weight (MW) of 511 Da. Both are rapidly metabolized to several active folates, including 5-methyltetrahydrofolate (5-CH3-THF). The dose of levoleucovorin is half of leucovorin.
After a DHFR inhibitor such as methotrexate inhibits the formation of tetrahydrofolic acid, the intracellular machinery for the synthesis of indispensible thymidylate and purine nucleotides comes to a halt, and DNA production ceases. Leucovorin and levoleucovorin are biologically active forms of folic acid and bypass this inhibition of DHFR caused by methotrexate.
Folate catalyzes the formation of carbon dioxide and water from formic acid, the final metabolic step in methanol elimination. Because there is no inhibition of the formation or recycling of active folate, either folic acid or leucovorin is beneficial.
Early investigations suggest that folic acid may aid in the methylation and subsequent elimination of arsenic. Folate supplementation in folate-deficient subjects was done to enhance the elimination of arsenic and potentially decreased chronic arsenic toxicity.5,7,8,30 Studies remain to be performed to determine whether folate supplementation in folate-replete individuals is beneficial.
Whereas leucovorin is naturally formed in the body as the active (l) isomer, the commercial preparation is the racemic mixture, which means it consists of equal amounts of the inactive (d) and active (l) isomers. The pharmacokinetics of the racemic mixture of leucovorin and its active metabolite were studied after a single intravenous (IV) infusion and as a constant infusion in normal human volunteers.39,40 During constant infusion of 500 mg/m2/d, the steady-state concentration for the active isomer was 2.33 µmol, the half-life was 35 minutes, and the volume of distribution (Vd) was 13.6 L. The active isomer is metabolized to an active metabolite (l-5-CH3-THF) that achieved a steady-state concentration of 4.85 µmol and a half-life of 227 minutes. Similar values were achieved for half-life and Vd after single IV doses ranging from 25 to 100 mg. The inactive d-isomer achieved higher concentrations and had a much longer half-life with oral administration, which is saturable and stereoselective, resulting in absorption of the active isomer that is four to five times greater than that of the inactive isomer. Studies of stereospecific oral absorption demonstrate that 100% of the l-leucovorin is absorbed, but only 20% of the d-leucovorin is absorbed at this dose.19 One study detected no adverse effects of the inactive isomer on the intracellular uptake of the active isomer and concluded that giving the active isomer provided no pharmacokinetic advantage over the racemic mixture.35
Two hundred milligrams of levoleucovorin was compared to 400 mg of leucovorin, each administered as a 2 hour IV infusion as a crossover study in 40 healthy volunteers. The area under the curve and the maximum serum concentrations of l-5-CH3-THF were similar for both.
Preliminary evidence suggests that l-5-CH3-THF enters the CSF after systemic administration of both leucovorin and levoleucovorin. However, the concentration achieved is one to three orders of magnitude less than is normally obtained after intrathecal methotrexate. Intrathecal administration of leucovorin and levoleucovorin is contraindicated.
The pharmacokinetics of IV leucovorin was compared with intramuscular (IM) and oral administration in male volunteers given 25 mg. The mean peak of the active l-5-CH3-THF concentration was 258 ng/mL (5.5 × 10–1µmol/L) at 1.3 hours after IV administration compared with 226 ng/mL at 2.8 hours for IM and 367 ng/mL at 2.4 hours for oral administration.
The pharmacokinetics of orally administered leucovorin was studied in healthy, fasted male volunteers in single doses ranging from 20 to 100 mg and 200 mg IV over 5 minutes compared with 200 mg orally.23,31 Bioavailability decreased from 100% for the 20-mg dose to 78% for the 40 mg dose and ultimately to 31% for the 200 mg dose. A microbiologic assay was used to measure total tetrahydrofolates (reduced and active folates). Normal serum folate concentrations are approximately 0.05 μmol/L.14 The 200 mg oral dose produced a peak serum concentration of 1.82 µmol/L compared with 0.66 µmol/L for the 20-mg oral dose and 27.1 µmol/L for the 200 mg IV dose.23,31