OA26 Erythrocyte mean corpuscular volume as a surrogate marker for methotrexate polyglutamation during early treatment in rheumatoid arthritis

Abstract

Abstract Background/Aims The long-term outcomes for patients with rheumatoid arthritis (RA) depend on early and effective disease control. Methotrexate remains the key first-line disease-modifying therapy for the majority of patients. The gradual onset of action of methotrexate has been proposed to be linked to the build-up of intracellular stores of the drug, achieved through the process of polyglutamation. This is a complex and expensive assay. Recent work has shown an association between the increase in mean corpuscular volume (MCV) of erythrocytes and improvements in disease activity. This study seeks to relate intracellular methotrexate and 5-methyl-tetrahydrofolate polyglutamate concentrations with changes in MCV and their association with early ‘steroid free’ remission in RA patients. Methods 68 drug-naïve, newly diagnosed RA patients were recruited form the Newcastle Early Arthritis Clinic (NEAC). These patients were followed up for 6 months with serial blood sampling and quantification of the erythrocyte methotrexate and 5-methyl-tetrahydrofolate polyglutamate levels by liquid chromatography-mass spectrometry (LC-MS). This was then related to the contemporaneous erythrocyte MCV values from the clinical monitoring bloods on those occasions and the treatment outcome at 6 months. The outcome measure used was 4 component DAS28CRP remission (<2.4) with only the baseline intramuscular steroid permitted for the definition of remission. Results The majority of patients displayed a gradual increase in MCV over the first 6 months of treatment (MCV change from patient baseline used to normalise the data). When grouped by the ‘steroid free’ remission outcome, there was a divergence in MCV change with greater MCV increases observed in those achieving remission from 4 months of treatment onward (p < 0.05). In the same patient group, the accumulation of longer chain methotrexate polyglutamates (chain length 3 to 5) was observed to follow a similar trend to MCV with higher polyglutamate concentrations in those achieving the remission outcome from 2 months of treatment (P < 0.05). When the contemporaneous MCV and MTX polyglutamate samples were compared, an association was observed between the longer chain MTX polyglutamates and the change in MCV values (R2 = 0.39, p < 0.001), but not between intracellular 5-methyl-tetrahydrofolate and MCV (R2 = 0.01, p = NS). Conclusion Change in MCV shows an association with early remission induction with MTX therapy in treatment-naïve RA patients. This change in MCV appears to be associated with the accumulation of intracellular MTX metabolites, rather than via depletion of intracellular folates. MCV may therefore represent a tractable proxy measure of methotrexate polyglutamation for application in larger studies or for the purpose of drug monitoring. Disclosure P.M. Brown: None. A.G. Pratt: None. A.E. Anderson: None. A.W. Morgan: None. J.D. Isaacs: None.