Pharmacokinetics of small doses of methotrexate in patients with psoriasis.

The pharmacokinetics of methotrexate was studied in 23 patients with chronic plaque psoriasis. Ten patients received a single oral does and 13 patients a single intramuscular injection of 25 mg methotrexate. Serum methotrexate was measured for 24 hours following administration of the drug using a magnetizable solid-phase radio-immunoassay. The disappearance of methotrexate from the serum fitted a two-compartment model with a distribution phase half-life of 1.18 +/- 0.12 h and an elimination phase half-life of 5.35 +/- 0.62 h following the oral dose and 1.45 +/- 0.22 h and 4.71 +/- 0.32 h, respectively following the intramuscular dose. Peak serum methotrexate concentrations varied greatly between patients but the mean area below the curve for the two routes of administration did not differ significantly. In a further 18 psoriatic patients receiving long-term maintenance methotrexate therapy there was no consistent relationship between salivary and serum methotrexate levels.