Comorbidity and persistence of disease-modifying therapy use in relapsing remitting multiple sclerosis

Background: Comorbidity decreases the likelihood of initiating disease-modifying therapy (DMT) for multiple sclerosis (MS). Our objective was to characterize the relationship between comorbidity and initial DMT persistence along with reasons for DMT discontinuation. Methods: We identified individuals with relapsing remitting MS or clinically isolated syndrome starting a platform DMT (interferon-beta, glatiramer acetate, dimethyl fumarate, teriflunomide) as initial therapy in the Canadian province of Nova Scotia from 2001 to 2016. Cases were identified using a clinic database for the only clinic providing specialty MS care in a province with universal publicly-funded health care. Comorbidity was determined by linkage of MS cases to provincial health administrative data using validated case definitions for mental health disorder, hypertension, hyperlipidemia, diabetes, chronic lung disease, ischemic heart disease, epilepsy, and inflammatory bowel disease. Cox proportional hazards models explored the relationship between comorbidity, as a count or individual comorbidities, and time to discontinuation of initial DMT. Logistic regression models explored reasons for DMT discontinuation. Results: Among 1464 individuals starting platform therapy as initial DMT, the median duration on first DMT was 4 years (95% CI 4 - 4). Comorbidity count (0, 1, >= 2) was not associated with time to discontinuation of initial DMT. However, the presence of a mental health disorder was associated with an increased hazard of discontinuing DMT (hazard ratio 1.22, 95% CI 1.03-1.44). Comorbidity count was not associated with discontinuation for lack of efficacy or lack of tolerability after adjusting for covariates. Conclusion: Individuals with mental health comorbidity may have unique challenges that affect persistence on DMT after treatment initiation.