Anti-inflammatory mechanisms of methotrexate in rheumatoid arthritis

Anti-inflammatory mechanisms of methotrexate in rheumatoid arthritis Methotrexate in rheumatoid arthritisMethotrexate (MTX) is a folate analogue originally synthesised in the 1940s and designed to inhibit dihydrofolate reductase. 1Reduced folate (tetrahydrofolate) is the proximal single carbon donor in several reactions involved in the de novo synthetic pathways for purine and pyrimidine precursors of DNA and RNA required for cell proliferation.Furthermore, tetrahydrofolate plays a part in a second important biochemical step: the methioninehomocysteine cycle, which is necessary to provide a methyl group for several downstream reactions such as methylation of DNA, RNA proteins, and others.Therefore, MTX has been used extensively for treatment of neoplastic diseases.][5][6][7][8] Finally, four well designed, blinded, placebo controlled studies published in 1984 and 1985 introduced the use of MTX in the treatment of RA. [9][10][11][12] The early indications for MTX use in the rheumatic diseases were first reported in a large review in 1984. 13From the considerable experience obtained over the past 15 years, several lines of evidence clearly suggest that MTX does not act simply as a cytotoxic (antiproliferative) agent for the cells responsible for the joint inflammation in RA. 14 As a matter of fact, it would be diYcult to understand how a drug that diminishes inflammation by preventing proliferation of immune cells might work at eVective concentrations for only a very short time and once a week.In addition, the rapid clinical remission and the short term eVect on the acute phase reactants, as seen with low dose MTX administration in most patients with RA, as well as the fast flare of disease after drug discontinuation, suggest that the mechanism of action of low dose MTX might be more anti-inflammatory than antiproliferative (immunosuppressive). 15 16 Recently, MTX has been shown to possess a variety of anti-inflammatory eVects. 17][19][20][21][22][23] These and other separate lines of evidence support the view that alternative mechanisms are responsible for the antirheumatic/anti-inflammatory eVects of MTX, which will be reviewed here. Cellular eVects of MTXMTX is a folate analogue with an amino group (NH 2 ), a methyl group (CH 3 ), and a fully oxidised pteridine ring, rendering the molecule inactive as cofactor. 1nce administered MTX is delivered to cells in the same way as the parenteral folates; 3-12% is hydroxylated in liver and circulates as 7-OH-MTX. 24xtracellular MTX is brought into the cell by the folate receptors (FR , FR ).Thereafter, a portion of intracellular MTX and 7-OH-MTX is metabolised to polyglutamates (MTX-glu) in the same manner as naturally occurring folates. 25MTX-glu represent long lived derivatives, which in rats may be detected in the skin for as long as two weeks after a single dose of the drug. 26ecause there is a latent period of weeks before the MTX eVects are appreciated in patients with RA, it may be the intracellular MTX-glu derivatives which are the true active anti-inflammatory agents.MTX binds dihydrofolate reductase (DHFR) with high aYnity.MTX-glu binds DHFR and has fairly high aYnity for enzymes that require folate cofactors, including thymidylate synthetase (TS) and 5-aminoimidazole-4carboxamide ribonucleotide (AICAR) transformylase.The inhibition of TS, induced by MTX, interferes with DNA synthesis in actively dividing cells, and the increase of AICAR enzyme system, which plays a key part in the purine metabolism of the cell, leads to enhanced release of adenosine into the blood. 23 27 28 In fact, a number of anti-inflammatory eVects exerted by MTX seem to be related to the extracellular adenosine increase and its interaction with specific cell surface receptors, with subsequent inhibition such as interleukin 8 (IL8) production by peripheral blood mononuclear cells (PBMC), IL6 secretion by human monocytes, leucotriene B4 synthesis in neutrophils, and decreased synovial collagenase gene expression. 14 29www.annrheumdis.