Methotrexate polyglutamates as a marker of patient compliance and clinical response in psoriasis: a single-centre prospective study.

BACKGROUND: Methotrexate is activated by the sequential addition of glutamic acid residues to form methotrexate polyglutamates (MTXPG(1-5)). MTXPG(1-5) inhibit enzymes of the folate-purine-pyrimidine pathways, and longer-chain MTXPG(3-5) species are more active. OBJECTIVES: To determine the pattern of erythrocyte MTXPG(1-5) in patients initiated on oral methotrexate for psoriasis, and to investigate the potential utility of MTXPGs as markers of compliance and/or clinical response. METHODS: This was a single-centre, prospective study of 55 adult patients with chronic plaque psoriasis initiated on weekly oral methotrexate. Erythrocyte MTXPG(1-5) concentrations were measured (at weeks 4, 8, 12, 24 and 52) using high-performance liquid chromatography. Methotrexate responders achieved ≥ 50% improvement in Psoriasis Area and Severity Index or physician's global score of 'clear'/'nearly clear' at 24 weeks. RESULTS: MTXPG levels were measured in 14-33 patients at each time point. All MTXPG(1-5) species were detected at week 4 of therapy. Steady state for long-chain MTXPG(3-5) and total MTXPG(1-5) was achieved by week 24. MTXPG(3) emerged as the predominant MTXPG species (from week 12 onwards) and reflected overall polyglutamate status (correlating strongly with MTXPG(2-5) , MTXPG(3-5) and MTXPG(4-5) ; R = 0·76-0·95, P < 1·55 × 10(-5)). Age, renal function and sex were not significant determinants of MTXPG(3) concentration. No significant association was identified between MTXPG and adverse events or responder status. CONCLUSIONS: This is the first study to demonstrate the prospective accumulation of MTXPG(1-5) in patients with psoriasis. The detection of MTXPGs early in therapy and the establishment of a steady state with continuous treatment may offer measuring of MTXPG as a test to monitor patient compliance with therapy. Larger studies are required to determine the role of MTXPG as a potential biomarker of clinical response.