Anti-arthritic effect of methotrexate: is it really mediated by adenosine?

The mechanism of action for the anti-arthritic effect of methotrexate (MTX) eras investigated in rats with antigen-induced arthritis (AIA). Arthritis intensity was quantified as area under the curve (AUC) for the joint swelli ng. The response to MTX was in several respects similar to what is seen in the clinic. The drug reduced the AUC in a dose-dependent manner after oral weekly (2-4 mg/kg/week) or daily (0.3 mg/kg/day) dosing. This effect was no t affected by supplementation with an equal dose of folate. The model thus seemed suitable for this type of study. Supplementation with folate in exce ss abolished the effect of MTX. A structurally similar antifolate, aminopte rin, also reduced the arthritis. The effect thus seemed to be due to folate antagonism although a complete inhibition of dihydrofolate reductase (DHFR ) might not be essential. Hence, it could be that the main target is a proc ess downstream of DHFR. It has been proposed that inhibition of AICAR-trans formylase induce the release of adenosine with anti-inflammatory properties . Here the adenosine antagonist R-PIA reduced the arthritis but when MTX wa s combined with adenosine antagonists no attenuation of the anti-arthritic effect was seen. On the contrary, three adenosine agonists (8-p-sulphopheny ltheophyllamine 30 mg/kg i.p. twice daily; 3,7-dimethyl- 1-propargylxanthin e, p.o. 3 mg/kg/day and 8-cyclopentyl-1,3-dipropylxanthine, 1.5 mg/kg/day p .o.) potentiated MTX. The specific thymidylate synthase inhibitor 5-fluouro uracil (0.3-3.0 mg/kg/day) had no anti-arthritic effect. Neither did our da ta support the hypotheses that syntheses of polyamines or cytokines were pr imary targets. It is thus possible that the mechanism of action is inhibiti on of a process downstream of DHFR but the release of adenosine seems not t o be important. (C) 2000 Published by Elsevier Science B.V. All rights rese rved.