Low-dose methotrexate kinetics in arthritis

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Abstract
Twelve patients with either rheumatoid or psoriatic arthritis were injected with a 10-mg bolus dose of methotrexate (MTX) either i.m. (n = 6) or i.v. (n = 6) and the MTX concentration in their sera was determined by radioimmunoassay. MTX concentration-time data fitted triexponential equations. Doses injected i.m. were rapidly and completely absorbed. There were no significant intergroup differences in drug mean t1/2 [half-life], volume of distribution and total body clearance. In 9 patients, serum MTX concentrations remained above the suggested critical level of 0.01 .mu.mol throughout the 24-h study irrespective of the route of administration, but MTX did not cumulate in the serum. Apparently, the behavior of low doses of MTX in patients with arthritis closely resembles the pattern in patients receiving intermediate and high doses for the treatment of neoplasia.