Reference-based pricing of prescription drugs: exploring the equivalence of angiotensin-converting-enzyme inhibitors.

BACKGROUND: Reference-based pricing is a cost-containment policy applied to prescription drugs that are in the same class and deemed to be therapeutically equivalent. Recent reference-based pricing measures have targeted several drug classes, including angiotensin-converting-enzyme (ACE) inhibitors. The objective of this study was to assess whether patients treated for hypertension with various ACE inhibitors differed in their utilization of health care services and hence, whether the various ACE inhibitors should be considered therapeutically equivalent.
METHODS: A retrospective cohort was formed from 4709 Saskatchewan residents aged 40-79 years who initiated treatment for hypertension with 1 of the 3 most frequently prescribed ACE inhibitors (captopril, enalapril or lisinopril) between Jan. 1, 1991, and Dec. 31, 1993. Information obtained from universal insurance databases included prescription drug use, the number of visits to a general practitioner (GP) or specialist and the number of hospital admissions during the year before treatment was initiated and during a follow-up period of up to 4 years. Rates were statistically adjusted for potential confounding variables and compared across treatment groups.
RESULTS: Of the 4709 patients, 529 were prescribed captopril initially, 2939 enalapril and 1241 lisinopril. After treatment was initiated patients prescribed captopril were dispensed more medications on average, with an overall rate of 18.6 prescriptions per patient per year (v. 16.4 and 14.7 for enalapril and lisinopril users respectively); they were admitted to hospital more often, and they made more visits to GPs and specialists. The adjusted rate ratio of the number of visits to a GP for patients receiving enalapril, relative to captopril, was 0.84 (95% confidence interval [CI] 0.80-0.88), and for those receiving lisinopril it was 0.79 (95% CI 0.74-0.83). The adjusted rate ratios for the number of visits to a specialist were similar but lower, and for the number of hospital admissions they were 0.82 for patients prescribed enalapril initially (95% CI 0.73-0.93) and 0.65 (95% CI 0.56-0.75) for those prescribed lisinopril.
INTERPRETATION: Patients with hypertension who are initially prescribed captopril used health care services more than those initially prescribed enalapril or lisinopril. This suggests that ACE inhibitors may not be therapeutically equivalent.