Adenosine A2A or A3 receptors are required for inhibition of inflammation by methotrexate and its analog MX‐68

Objective

Low‐dose weekly methotrexate therapy remains a mainstay in the treatment of inflammatory arthritis. Results of previous studies demonstrated that adenosine, acting at one or more of its receptors, mediates the antiinflammatory effects of methotrexate in animal models of both acute and chronic inflammation. We therefore sought to establish which receptor(s) is involved in the modulation of acute inflammation by methotrexate and its nonpolyglutamated analog MX‐68 (N‐[[4‐[(2,4‐diaminopteridin‐6‐yl)methyl]‐3,4‐dihydro‐2H‐1,4‐benzothiazin‐7‐yl]‐carbonyl]‐L ‐homoglutamic acid).

Methods

We studied the effects of low‐dose methotrexate (0.75 mg/kg intraperitoneally [IP] every week for 5 weeks), MX‐68 (2 mg/kg IP 2 days and 1 hour before induction of inflammation), dexamethasone (1.5 mg/kg IP 1 hour before induction of inflammation), or vehicle control on acute inflammation in an air‐pouch model in A_2A and A₃ receptor knockout mice.

Results

Low‐dose weekly methotrexate treatment increased the adenosine concentration in the exudates of all mice studied and reduced leukocyte and tumor necrosis factor α accumulation in the exudates of wild‐type mice, but not in those of A_2A or A₃ receptor knockout mice. Dexamethasone, an agent that suppresses inflammation by a different mechanism, was equally effective at suppressing leukocyte accumulation in A_2A knockout, A₃ knockout, and wild‐type mice, indicating that the lack of response was specific for methotrexate and MX‐68.

Conclusion

These findings confirm that adenosine, acting at A_2A and A₃ receptors, is a potent regulator of inflammation. Moreover, these results provide strong evidence that adenosine, acting at either or both of these receptors, mediates the antiinflammatory effects of methotrexate and its analog MX‐68.