Adherence with intravenous zoledronate and intravenous ibandronate in the United States medicare population

OBJECTIVE: To evaluate adherence to drug therapy among new users of zoledronate and intravenous (IV) ibandronate among US Medicare enrollees. METHODS: We used data from the Medicare 5% random sample to evaluate new users of IV zoledronate and IV ibandronate with continuous Part A and Part B fee-for-service coverage. The outcome was adherence as quantified by the proportion of days covered (PDC) measured continuously and dichotomously (≥ 80%). Followup time extended from 18-27 months for all individuals. Factors associated with low adherence to zoledronate were evaluated with logistic regression. RESULTS: We identified 775 new users of zoledronate and 846 new users of IV ibandronate. For both drugs, 30-48% of the first infusions were given in an outpatient infusion center, not in a physician's office. The mean PDC for zoledronate users was 82%, which was greater than the mean PDC for the IV ibandronate users (58-62% depending on the time period; P < 0.0001). Approximately 30% of zoledronate users did not receive a second infusion. Factors associated with low adherence to zoledronate included older age and receipt of the first infusion in an outpatient infusion center rather than a physician's office. CONCLUSION: Less frequently dosed IV bisphosphonates have not resolved the problem of suboptimal adherence with prescription osteoporosis medications. Interventions continue to be warranted to improve long-term adherence to osteoporosis treatments.