Impact of adherence to interferons in the treatment of multiple sclerosis: a non-experimental, retrospective, cohort study.

Background: Relapsing-remitting multiple sclerosis (RRMS) is a chronic disease affecting about 400 000 people in the US characterized by increasing patient disability and burden on society. While there is no cure for multiple sclerosis (MS), pharmaceutical treatments exist that can limit the number of relapses a patient experiences, and slow disease progression. One such class of agents used to treat RRMS are the interferons: interferon-beta-1a (Rebif and Avonex and interferon-beta-1b (Betaseron and Extavia). Patients must take these injectable medications regularly to achieve the optimal outcomes. However, patient issues and potential adverse effects of the medication may prevent the patient from taking the medication as directed and lower adherence. To date, limited evidence exists regarding the effect of patient adherence to interferon-beta therapies on clinical and economic outcomes.

Objective: The purpose of this study was to explore the impact of patient adherence to interferon-beta therapy on MS relapse rates and healthcare resource utilization.

Methods: Using a non-experimental, retrospective cohort design, a sample population (n = 1606) was drawn from patients identified in a database that includes both pharmacy and medical claims data. The study population was separated into two groups based on a measure of medication possession ratio (MPR)-adherent and non-adherent patients, and adherence was defined as MPR > or =85% in a given year during the study period (2006-8). Key outcome variables included MS relapses and healthcare resource utilization. Data were analysed using parametric and non-parametric statistics, and regression modeling.

Results: During the study period, the average MPR for all patients on interferon-beta therapy varied from 72% to 76%. Only 27-41% of patients in each year were considered adherent (i.e. MPR > or =85%) and only 4% of patients had an MPR of > or =85% throughout the 3-year study period (2006-8). Patients who were adherent tended to have a lower risk of relapses over 3 years than non-adherent patients. A significantly lower risk of relapses was found in 2006 (risk ratio [RR] 0.89; 95% CI 0.81, 0.97). Furthermore, an increasingly larger effect emerged between adherence and relapses when comparing adherent patients (MPR > or =85%) with subgroups of non-adherent patients (<80%, <75%, <70%, <65% and <60%). The impact of adherence on emergency room (ER) visits also tended to suggest a lower risk during 2006, 2007 and 2006-8. During 2008, the risk for an ER visit was significantly lower for patients adherent in 2007 (RR 0.78; 95% CI 0.61, 0.99). Inpatient admissions followed the ER trends, as patients considered adherent in 2006 and 2007 tended to have a lower risk over 3 years. This result was significant for patients adherent in 2007 (RR 0.79; 95% CI 0.65, 0.98).

Conclusion: The findings of low patient adherence and the impact of adherence on relapses and healthcare resource utilization strongly suggest opportunities to reduce healthcare resource utilization and healthcare costs among RRMS patients taking interferon-beta therapy. Efforts should be undertaken to understand and improve medication-taking behaviour in this population so as to minimize the negative impacts of RRMS on patients while reducing unnecessary direct and indirect costs to treat disease exacerbations.