Long-term effect of first-line injectable multiple sclerosis treatments: Input of a time-dependent propensity score

The long-term effect of beta-interferon and glatiramer acetate on multiple sclerosis (MS) disability progression has resulted in controversial results, probably due to a lack of appropriate control of biases as raised in observational studies. In particular, the time of the therapeutic decision is difficult to define when the controls are not treated.

This retrospective observational study was based on a series of patients from the MS expert center in Rennes, France. We used a time-dependent propensity score defined as the linear predictor of a Cox model estimating the hazard of being treated at each time from MS onset. The matching procedure resulted in two groups: patients matched as treated and as not yet treated. The restricted mean times (RMST) to reach a moderate level of disability or worsening of the disability were compared between the two groups in an intention-to-treat analysis.

Of the 2383 patients included in the study, 556 were matched as treated. The matching procedure provided a good balance of both the time-fixed and the time-dependent covariates. A slight difference was observed for the time to reach a moderate level of disability, in favor of the "not yet treated" group (difference in the RMST: -0.62 [-0.91; -0.33]) while no difference was found in terms of worsening of the disability (-0.03 [-0.24; 0.33]).

This unexpected result is probably due to unmeasured confounders. However, this time-dependent PS warrants consideration in long-term effectiveness studies.