Methotrexate

Methotrexate pharmacology extends far beyond simple DHFR inhibition when examined at the molecular level. After cellular uptake via the reduced folate carrier (RFC1/SLC19A1), methotrexate undergoes intracellular polyglutamation by folylpolyglutamate synthetase (FPGS), adding up to 5-6 glutamate residues. These polyglutamated forms (MTX-PG) are retained intracellularly with half-lives of weeks to months, creating a sustained pharmacological effect even with weekly dosing. MTX-PG directly inhibits multiple enzymes in the folate pathway: DHFR (blocking dihydrofolate to tetrahydrofolate conversion), thymidylate synthase (TS, blocking dTMP synthesis required for DNA replication), and critically, AICAR transformylase (aminoimidazole carboxamide ribonucleotide transformylase, also called ATIC). Inhibition of AICAR transformylase leads to intracellular accumulation of AICAR, which inhibits adenosine deaminase (ADA) and AMP deaminase, resulting in increased extracellular adenosine release. This adenosine pathway is now understood to be the PRIMARY anti-inflammatory mechanism of low-dose methotrexate in autoimmune disease. Adenosine binds to A2A and A3 receptors on inflammatory cells, suppressing NF-κB activation, reducing TNF-α, IL-6, and IL-8 production, inhibiting neutrophil adhesion, and promoting anti-inflammatory macrophage polarization (M2 phenotype). Pharmacogenomic variation significantly impacts methotrexate efficacy and toxicity. The MTHFR (methylenetetrahydrofolate reductase) C677T and...