Lack of interaction between flucloxacillin and methotrexate in patients with rheumatoid arthritis

1. The aim of the study was to examine the potential pharmacokinetic interaction between methotrexate and flucloxacillin. 2. Ten rheumatoid arthritis patients participated in the interaction study. Subjects were allocated to either methotrexate alone (5-15 mg per week) or methotrexate plus flucloxacillin (500 mg four times a day 48 h prior to sampling) in a random order. 3. There was a statistically, but not clinically, significant decrease in methotrexate AUC (1307 +/- 389 vs 1212 +/- 394 micrograms l-1 h) in the presence of flucloxacillin. Cmax and tmax parameters for methotrexate were not significantly altered in the presence of flucloxacillin. 4. Data from an additional 10 rheumatoid arthritis patients, starting on methotrexate, were added to the data from the placebo arm of the interaction study and a model dependent pharmacokinetic analysis was performed. The plasma concentration profiles were best described by a two-compartment model with a mean clearance of 11.9 (+/- 1.7) l h-1 and an initial volume of distribution of 31.2 (+/- 2.6) l. The pronounced intersubject variability in the pharmacokinetic parameters was not related to any of the available covariate information. 5. Our findings suggest that no important clinical interaction occurs between flucloxacillin and methotrexate in patients with rheumatoid arthritis.