Background/Purpose: In inflammatory rheumatic diseases, pharmacokinetics of MTX may be modified by patients’ weight [1]. The rheumatologists are though reluctant to prescribe a high dosage to patients with a low weight. On the other hand, international guidelines (ACR and EULAR [2, 3]) do not recommend any weight adaptation concerning MTX dose. Following which, rheumatologists generally do not increase MTX dosage above 25 mg/wk for patients with high weight. Our goal was to evaluate the MTX dosage used in routine practice to treat RA as compared the patients’ weight.

Methods: We used the baseline data of SELFi study. This phase III trial included patients treated by MTX (≥ 3 months, oral or SC) randomized in two arms: MTX in prefilled syringes or MTX autoinjector. The primary objective of the study was to compare the arms in terms of HAQ and treatment adhesion at 6 months. In this preliminary post-hoc analysis, we evaluated in four groups <50kg, 50-74,9 kg, 75-99,9 kg, and >100 kg the mean dosage of MTX in mg/kg, the proportion of patient receiving <10, 10 to 14.9, 15 to 19.9, ≥ 20 mg/wk, and the proportion of patients receiving < or ≥ 0.3 mg/kg/wk. We also evaluated the factors associated to the use of ≥ 0.3 mg/kg/wk using univariate or multivariate analyses.

Results: Between Sept 2015 and Sept 2016 SELFi study recruited 264 patients (192 women and 72 men); mean age: 58.4±13.2 years. Mean weight of women/men was 67.3±13.5 kg and 81.5±13.7 kg respectively (p<0.0001). The difference of BMI between men and women was statistically significant: 25.8±5.3 for women and 26,9±3.9 for men (p=0.014). Absolute value of MTX dosage was the same for both genders: 15.4 mg/wk in women vs 15.9 in men (p=0.36). Although, the proportion of women and men receiving ≥ 15mg/wk dosage is not statistically different (75.5% vs 84.7%, p=0.11), 17.7% of women vs only 2.9% of men received ≥0.3 mg/kg/wk (p=0.002). While the patients <50kg receive 0.29 mg/kg/wk for the other groups this parameter is decreased to 0.24, 0.19 et 0.16 mg/kg/wk for the tree other groups. Results of the univariate analysis show that the prescription of MTX ≥ 0.3 mg/kg/wk is associated with the female gender (p<0.002), the height (p<0.001) and the weight (p<0.0001). In multivariate analysis, only the weight remains associated (p<0.0001). However, the MTX dosage <0.3 mg/kg/wk was not associated with higher disease activity, but the study was designed to consider patients with low disease activity.

Conclusion: Weight does not seem to influence the MTX dosage in RA patients. Although the dosage of MTX in mg/wk seems slightly higher in men vs women, it is significantly lower in mg/kg/wk. These results raise the question of the utilization of mg/kg/wk MTX dosage rather than the absolute value in mg/wk. On this basis, it would be interesting to design a randomized controlled trial to further explore these findings.

References: 1 – Berthelot et al Rev Rhum, 2002, 69 : 72-83; 2 – Singh et al Arthritis Care Res 2016;68:1-25; 3 – Smolen et al doi: 10.1136/annrheumdis-2016-210715

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/is-it-necessary-to-weight-by-weight-the-dosage-of-mtx-in-ra-patients-results-from-obsevational-analysis-of-baseline-data-of-a-phase-iii-trial/