The anti-inflammatory mechanism of sulfasalazine is related to adenosine release at inflamed sites.

Abstract

The anti-inflammatory mechanism of sulfasalazine is not well understood. It has recently been shown that sulfasalazine inhibits 5-aminoimidazole-4-carboxamidoribonucleotide (AICAR) transformylase, an enzyme involved in de novo purine biosynthesis. We recently demonstrated that methotrexate promotes intracellular AICAR accumulation, thereby increasing adenosine release and diminishing inflammation, so we tested the hypothesis that sulfasalazine similarly promotes intracellular AICAR accumulation. We studied adenosine release and the state of inflammation in in vitro and in vivo models of the inflammatory process. The adhesion of stimulated neutrophils (FMLP) to endothelial cells preincubated with sulfasalazine was inhibited in a dose-dependent manner. Elimination of extracellular adenosine by addition of adenosine deaminase or inhibition of adenosine by the adenosine A2 receptor antagonist 3,7-dimethyl-1-propargylxanthine (DMPX) completely reversed the anti-inflammatory effect of sulfasalazine (at concentrations <1 microM in this in vitro model. To determine whether this phenomenon was relevant to inhibition of inflammation in vivo, we studied the effect of sulfasalazine (100 mg/kg/day by gastric gavage for 3 days) on leukocyte accumulation in the murine air pouch model of inflammation. Treatment with sulfasalazine markedly decreased the number of leukocytes that accumulated in the inflamed (carrageenan, 2 mg/ml) air pouch. Injection of either adenosine deaminase or DMPX, but not the A1 receptor antagonist 8-cyclopentyl-dipropylxanthine, significantly reversed the anti-inflammatory effects of sulfasalazine treatment. Sulfasalazine increased the exudate adenosine concentration from 127 +/- 64 nM to 869 +/- 47 nM. Moreover, sulfasalazine treatment promoted a marked increase in splenocyte AICAR concentration from 35 +/- 6 to 96 +/- 3 pmols/10(6) splenocytes, which is consistent with the in vitro observation that sulfasalazine inhibits AICAR transformylase. These results indicate that sulfasalazine, like methotrexate, enhances adenosine release at an inflamed site and that adenosine diminishes inflammation via occupancy of A2 receptors on inflammatory cells. Our studies provide evidence that sulfasalazine and methotrexate may be described as a newly recognized family of anti-inflammatory agents that share the property of using adenosine as an antagonist of inflammation.