Effect of adherence to the first-generation injectable immunomodulatory drugs on disability accumulation in multiple sclerosis: a longitudinal cohort study

OBJECTIVE To examine the association between optimal adherence to the first-generation injectable immunomodulatory drugs (IMDs) for multiple sclerosis (MS) and subsequent disability accumulation. METHODS We accessed prospectively collected linked clinical and administrative health data from British Columbia, Canada. Subjects with MS treated with a first-generation injectable IMD at an MS clinic (1996-2004) were followed until their last clinic visit before 2009. Adherence was estimated using the proportion of days covered (PDC). The primary outcome was disability accumulation, defined as an increase in the Expanded Disability Status Scale (EDSS) score as recorded during each year of follow-up. Generalised estimating equation models, adjusted for baseline sex, age, EDSS and time between scores, were used to measure associations between optimal adherence (≥80% PDC) during the first year of treatment and subsequent disability accumulation. The relationship between early IMD adherence and the secondary outcome, time to sustained EDSS 6, was examined using Cox proportional hazards regression. RESULTS Among 801 subjects, 598 (74.7%) had optimal adherence over the first year of IMD treatment and 487 (39.0%) demonstrated one or more instances of disability accumulation. Early optimal adherence was not associated with disability accumulation (adjusted OR 0.94; 95% CI 0.78 to 1.15), nor with time to sustained EDSS 6 (adjusted HR 0.91; 95% CI 0.57 to 1.44). CONCLUSION Almost three-quarters of subjects with MS had optimal early adherence to their first-line injectable IMD. There was no evidence that this was associated with disability accumulation in the following years.