Methotrexate: its use in the rheumatic diseases

Abstract

Methotrexate in a low dose intermittent regimen has become popular as a therapeutic choice for refractory psoriatic arthritis, polymyositis, Reiter's disease and more recently rheumatoid arthritis. As a folate analogue, it inhibits the formation of reduced folate cofactors which participate in a host of important reactions including DNA synthesis. It has been shown to have both immunosuppressive and anti-inflammatory properties although its precise mechanism of action in these diseases is not known. It may be variably absorbed especially in psoriatics and its action may be antagonized by folate supplements. Its major route of metabolism appears to be via the enterohepatic circulation. Numerous drugs, including salicylates, may increase serum levels by displacing the drug from protein binding sites and by competing for renal excretion. It has fewer short term side effects than the other immunosuppressives used in rheumatic disease. Its major long term toxicity is liver fibrosis or cirrhosis which appears to increase with greater cumulative dosage and treatment duration. Several recent reports of hypersensitivity pneumonitis reinforce the previous literature on this topic. Pulmonary toxicity may be more common than suggested as more patients are treated with methotrexate for rheumatic diseases. Clinical studies in general have been uncontrolled and lacking in scope and size. Nevertheless, the literature to date appears to show this to be an excellent drug for use in the diseases mentioned. Prospective double blind controlled studies on psoriatic arthritis and rheumatoid arthritis should help establish this drug as the immunosuppressive of choice in these diseases.