The folate antagonist methotrexate (MTX) is the 'anchor-drug' in the treatment of patients with rheumatoid arthritis (RA) . The main target of MTX in intracellular folate metabolism is dihydrofolate reductase (DHFR) but several other targets have been described (e.g. thymidylate synthase [TS] and 5-amino-imidazole-4-carboxamide ribonucleotide [AICAR]). At present the exact mechanism of action of MTX in RA still remains elusive . Despite the potent anti-rheumatic capacity of MTX many patients (at least 50%) become resistant to MTX during long-lasting therapy. However, little is known about the mechanisms of resistance against MTX in RA patients .
From the field of oncology, where MTX is used against haematological malignancies, new anti-folate drugs were developed to circumvent MTX resistance . These new folate antagonists have the following characteristics: are better transported through the reduced folate carrier, are retained intracellular more efficiently by polyglutamylation via folylpolyglutamate synthetase, and/or have other targets in the folate pathway besides DHFR (e.g. TS).