Bayesian calculation of methotrexate clearance after low dose intramuscular administration in patients with rheumatoid arthritis.

Objective: To determine a pharmacokinetic procedure (Bayesian method) for estimation of methotrexate (MTX) clearance, using only 2 blood samples, in outpatients with rheumatoid arthritis treated with low dose intramuscular (i.m.) MTX.

Methods: Population pharmacokinetic parameters were obtained by the weighted least squares (WLSQ) method in plasma samples from 14 patients with rheumatoid arthritis (RA). In each patient, 11 samples were measured by fluorescence polarization immunoassay, at Time 0, 0.25, 0.5, 0.75, 1, 2, 4, 6, 8, 12, and 24 h after i.m. administration. These measures were validated by pharmacokinetic studies in 20 other patients with RA. Individual total body clearance of MTX was calculated using only 2 plasma samples (at 0.5 and 2 h after i.m. injection) by the Bayesian method using the population pharmacokinetic parameters. The clearance measures obtained by the Bayesian method were compared with those obtained by the WLSQ method.

Results: The pharmacokinetic variables (clearance, half-life, area under the curve) of 14 patients were determined, as well as the covariance and the mean values necessary to apply the Bayesian method. No significant difference was found between clearance values obtained by the Bayesian method compared to the WLSQ method, confirming the validity of the Bayesian values.

Conclusion: The present population pharmacokinetic parameters allowed the determination of individual clearance of MTX with only 2 plasma samples (0.5 and 2 h after administration) in patients treated with low dose im MTX. Individual clearance is used to modulate MTX administration in patients presenting adverse reactions in spite of good clinical response. Individual determination of MTX pharmacokinetics in patients at risk for adverse MTX reactions could be useful for adjustment of the drug regimen.