Background/Purpose: Optimal dosing of methotrexate (MTX) for individual rheumatoid arthritis (RA) patients to achieve adequate disease control remains challenging. Assessment of erythrocyte MTX-polyglutamates (PGs) levels has been used as a tool to monitor clinical response of RA patients in the first 3-12 months of treatment and MTX-PG_2-4 as well as total MTX-PGs were associated with lower DAS28 over 9 months. However, data per route of administration and from earlier time points, including MTX-PG₆ levels are lacking.

We investigated the pharmacokinetics and -dynamics of erythrocyte MTX-PG accumulation in RA patients receiving oral or subcutaneous MTX in the early phase (1, 2, 3 months) and later stage (6 months) of MTX treatment.

Methods: In a clinical prospective cohort study (MeMo study: NTR7149), newly diagnosed RA patients were treated with oral (n=24) or subcutaneous (n=22) MTX mostly according to the COBRA-light schedule (start 10 mg MTX, increased to 25 mg MTX in 8 weeks, with prednisolone). At 1, 2, 3 and 6 months after start of therapy, blood was collected and individual MTX-PGs (MTX-PG₁ – MTX-PG₆) were analyzed in erythrocytes using a validated UHPLC-MS/MS method with labeled internal standards. (De Rotte 2015) Dosing, concomitant treatments and DAS28-ESR assessments were in conformity with clinical practice. Adverse events were recorded. We used a linear mixed model analysis to assess the association between MTX-PG levels and DAS28-ESR (per group and total), with corrections for age, baseline DAS28, eGFR, MTX dose (1 month before sampling), smoking and BMI.

Results: 46 consecutive patients were included in this study of which 76% female, mean age was 57.8 years, BMI was 25.8, 20% were smokers, mean baseline DAS28-ESR was 3.5. MTX dose at baseline was 10.5 mg (SD: 1.5) for both groups, 15.4 (4.4) and 16.8 (1.8) at 1 month and 22.8 (3.9), 22.4 (5.2) at 2 months, 20.1 (6.3) and 20.8 (5.6) at 3 months and 19.0 (6.5) and 18.6 (8.6) at 5 months for oral and subcutaneous use, respectively. On average, patients starting subcutaneous MTX had significantly higher levels of long chain MTX-PGs (MTX-PG_3-6) in erythrocytes when compared to patients in the oral MTX group at 1 and 2 months (Figure 1B and 1C, Table 1). Similarly, erythrocyte MTX-PG_1-6 and MTX-PG₃ concentrations were significantly higher in subcutaneous MTX-users at month 1 compared to the oral group (median 68.6 nmol/L (IQR:40.5) vs 51.9 (55.6) and 17.4 (11.1) vs 11.2 (15.6), respectively (Figure 1B and 1C, Table 1). This difference remained present at month 2. Erythrocyte MTX-PG accumulation reached a plateau after 3 months.

Apart from small positive and negative correlations, no (clinically) significant relation between MTX-PG concentrations and DAS28 was observed during the first 6 months of treatment; DAS28 decreased with 1.2 in the oral group and 1.6 in the subcutaneous group.

Side effects, mostly headache and dizziness were similar in both groups and not correlated with MTX-PG levels.

Conclusion: This study demonstrated significantly higher accumulation of long-chain and total MTX-PGs following subcutaneous versus oral MTX administration in the first 3 months. Early phase erythrocyte MTX-PG analyses may hold promise for optimization of individual patient MTX dose scheduling.

Table 1. Linear regression of MTX-PG levels and administration route, corrected for age, baseline DAS28, smoking, BMI, eGFR and MTX dose.

Figure 1. Erythrocyte individual MTX-PG accumulation in RA patients (A) during the first 6 months of oral (B) or subcutaneous (C) MTX administration. At 6 months, 18 patients using oral and 18 patients using subcutaneous MTX were still continuing MTX treatment. Means (colored lines) and standard errors (shaded areas) are depicted (Loess regression).

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/erythrocyte-methotrexate-polyglutamates-are-substantially-higher-after-subcutaneous-methotrexate-treatment-in-rheumatoid-arthritis-patients-in-the-first-months-of-treatment/