OBJECTIVES: Methotrexate (MTX) is a cornerstone in the treatment of rheumatoid arthritis (RA). Although in general MTX is very effective, the major drawback is the large inter-patient variability in clinical response. The circulating levels of MTX polyglutamates (MTXPGs) are supposed to correlate with clinical efficacy, therefore having a potential role in drug monitoring. However, there is a controversial discussion about the importance of methotrexate polyglutamates as outcome parameters in the therapy of rheumatoid arthritis. The aim of the present study was to investigate the formation and pharmacokinetics of MTXPGs and to correlate their concentration with clinical response in MTX-naïve patients. METHODS: The pharmacokinetics of erythrocyte MTXPGs was determined in samples of nineteen MTX-naïve patients by high pressure liquid chromatography (HPLC) using post-column photo-oxidation and fluorimetric detection. The relationship between erythrocyte concentrations of MTXPGs and the primary outcome parameter DAS-28 was assessed using the Spearman's correlation coefficient. RESULTS: The short-chain polyglutamate MTXPG2 revealed to be a potential marker for clinical outcome in rheumatoid arthritis with a statistically significant positive correlation of MTXPG2 Cmax levels and improvement in DAS-28 (+0.518, p=0.023) over 16 weeks. Furthermore, Cmax levels of MTXPG2 negatively correlated with basophils (-0.478, p=0.038) and eosinophils (-0.531, p=0.019), both pro-inflammatory cells involved in the disease. CONCLUSIONS: MTXPG2 seems to be a potential indicator for clinical response and may serve as a marker for drug monitoring.