Fig. 1.

A diagram of the hypothesized biochemical basis for the antiproliferative and anti-inflammatory activities of MTX. Inside the cell, MTX and its polyglutamate metabolites bind with high affinity to DHFR, inhibiting the reduction of folic acid (FA) and DHF to tetrahydrofolate (THF), resulting in depletion of the reduced folate pool, including THF, methylene-tetrahydrofolate (CH₂-THF), and formyl-tetrahydrofolate (f-THF). Inhibition of enzymes of nucleotide biosynthesis, including ATase, GART, AICART, and TS, is inhibited through the following: 1) depletion of their respective reduced folate cofactors, 2) direct inhibition by MTX and its polyglutamate metabolites, and 3) direct inhibition by DHF. The antiproliferative activity of MTX is attributed to inhibition of purine and pyrimidine biosynthesis, whereas the anti-inflammatory activity is hypothesized to occur through inhibition of AICART, resulting in the accumulation of its substrate and anti-inflammatory mediator, ZMP. FGAR, formyl-glycinamide ribonucleotide; GAR, glycinamide ribonucleotide; PRPP, phosphoribosyl pyrophosphate.