To determine the optimal time to administer methotrexate (MTX) in rheumatoid arthritis (RA).
In a crossover study 23 patients were administered MTX intramuscularly at either 10 AM or 6 PM. A 2 week interval separated the 2 injections. MTX concentrations were measured using a fluorescence polarization immunoassay. Pharmacokinetic variables were estimated using a Bayesian approach. The morning and evening schedules were compared using analysis of variance to determine the optimal time of injection.
No statistical difference was found in the pharmacokinetics of MTX according to hour of injection. A difference in the creatinine clearance, however, was observed in the samples obtained at noon and 8 PM, but clearance of MTX was unchanged.
Pharmacokinetic variables suggest that MTX can be administered either in the morning (10 AM) or evening (6 PM) in the treatment of RA.