A Bayesian analysis of bisphosphonate effects and cost-effectiveness in post-menopausal osteoporosis

Objective - Effects of oral bisphosphonates on the rate of vertebral fractu res in post-menopausal osteoporotic women have been found in clinical trial s. We wanted to compare the effects of two bisphosphonates, alendronate and etidronate, and calculate the price difference that would give the same co st-effectiveness for the two drugs. We also intended to give, by means of B ayesian statistics, probability distributions and point and interval estima tes for key parameters. Methods - We used published, double-blind, randomized placebo controlled st udies describing the results of occurrence of vertebral fractures at 3-year follow-up in post-menopausal women taking bisphosphonates. Four studies we re identified, including altogether 3510 women. The women had either suffer ed a fracture at entry or had a bone density at least 2.5 SD below the mean value for young women. Two of the studies dealt with alendronate and two w ith etidronate. Results - According to three of the studies, the number of women out of 100 avoiding vertebral fractures during a 3-year observation period varied fro m two to seven. The fourth study did not contain the necessary data. The fo ur studies showed that, for the incidence rate, the multiplicative treatmen t effects were respectively 0.45, 0.74, 0.40 and 0.36, where values less th an 1 indicate positive treatment effects. Using data from all four studies, a comparison of the two drugs gave a point estimate of 0.247 with 95% cred ibility interval (CI): -0.051 to 0.496 for a difference in effect in favour of alendronate measured in terms of risk ratio of fracture and 0.302 (CI: 0.099 to 0.539) measured as incidence rate ratio. Based on two studies, sho wing about the same prevalence of fractures in the control groups, the diff erence in the risk difference between the two drugs was 0.028 (CI: -0.039 t o 0.079). Conclusions - Bisphosphonates effectively reduce risk of new vertebral frac tures, but alendronate is somewhat more effective than etidronate. To obtai n equal cost-effectiveness alendronate should be priced 40-70% higher than etidronate. Copyright (C) 2000 John Wiley & Sons, Ltd.