Red blood cell methotrexate polyglutamates emerge as a function of dosage intensity and route of administration during pulse methotrexate therapy in rheumatoid arthritis

Methods. MTX pharmacokinetics were evaluated in 47 MTX-naive patients enrolled in an MTX dose-escalation study for an average of 20 weeks and 223 patients enrolled in a cross-sectional study under long-term MTX therapy. Short-chain (MTXPG(1-2)), long-chain (MTXPG(3)) and very long-chain (MTXPG(4-5)) concentrations were measured in circulating red blood cells using liquid chromatography. Statistical analyses consisted of non-linear mixed models, multivariate regression analyses and Wilcoxon signed-rank test. Results. The accumulation of MTXPG(1-5) was sigmoidal and steady-state concentrations were achieved after 7 weeks of therapy. However, additional exposure and MTX dosage escalation produced a selective redistribution towards longer chain MTXPGs at the expense of shorter chain MTXPGs. Age, glomerular filtration rate and route of MTX administration were the most important predictors of MTXPG accumulation. In 10 patients, a switch from oral to parenteral MTX was associated with a 37% increase in long-chain MTXPGs, a 132% increase in very long-chain MTXPGs and a concomitant 31% reduction in disease activity (P < 0.02). Conclusion. The selective emergence of long-chain MTXPGs is function of dose, time of exposure and hence dosage intensity. Switching from oral to parenteral MTX produces a selective accumulation of longer chain MTXPGs that are known to be more potent inhibitors of de novo purine biosynthesis than shorter chain MTXPGs.