THE ANTIINFLAMMATORY MECHANISM OF METHOTREXATE - INCREASED ADENOSINE RELEASE AT INFLAMED SITES DIMINISHES LEUKOCYTE ACCUMULATION IN AN IN-VIVO MODEL OF INFLAMMATION

Methotrexate, a folate antagonist, is a potent antiinflammatory agent when used weekly in low concentrations. We examined the hypothesis tha t the antiphlogistic effects of methotrexate result from its capacity to promote intracellular accumulation of 5-aminoimidazole-4-carboxamid e ribonucleotide (AICAR) that, under conditions of cell injury, increa ses local adenosine release. We now present the first evidence to esta blish this mechanism of action in an in vivo model of inflammation, th e murine air pouch model. Mice were injected intraperitoneally with ei ther methotrexate or saline for 3-4 wk during induction of air pouches . Pharmacologically relevant doses of methotrexate increased splenocyt e AICAR content, raised adenosine concentrations in exudates from carr ageenan-inflamed air pouches, and markedly inhibited leukocyte accumul ation in inflamed air pouches. The methotrexate-mediated reduction in leukocyte accumulation was partially reversed by injection of adenosin e deaminase (ADA) into the air pouch, completely reversed by a specifi c adenosine A, receptor antagonist, 3,7-dimethyl-1-propargylxanthine ( DMPX), but not affected by an adenosine Al receptor antagonist, 8-cycl opentyl-dipropylxanthine. Neither ADA nor DMPX affected leukocyte accu mulation in the inflamed pouches of animals treated with either saline or the potent antiinflammatory steroid dexamethasone. These results i ndicate that methotrexate is a nonsteroidal antiinflammatory agent, th e antiphlogistic action of which is due to increased adenosine release at inflamed sites.