Effect of oral bisphosphonates for osteoporosis on development of skeletal metastases in women with breast cancer: results from a pharmaco-epidemiological study.

BACKGROUND Treatment with bisphosphonates in women with breast cancer and established bone metastasis delays further skeletal-related events. Evidence is emerging that bisphosphonates are beneficial for secondary prevention of bone metastasis. The study aimed to estimate the effect of oral bisphosphonates for treatment or prevention of osteoporosis on development of bone metastasis in a population of women with breast cancer. METHODS A historical cohort of 21664 women diagnosed with breast cancer was created from health administrative data in Quebec, Canada. The primary outcome was time to develop bone metastasis; exposure was bisphosphonate use prediagnosis, postdiagnosis, both, or neither and a cumulative index of drug exposure. The sample was stratified according to stage (0-II or III) at time of diagnosis. Cox proportional hazards tested the effect of bisphosphonate use on time to develop bone metastases. RESULTS Taking bisphosphonates postdiagnosis of breast cancer only or continuing bisphosphonates started prior to diagnosis after diagnosis was associated with a reduction in risk of bone metastasis from 45% to 28% in women with local disease at diagnosis. In women with regional disease, postdiagnosis bisphosphonate use, with or without prediagnosis use, reduced risk by almost 50%. A statistically significant dose-response trend was observed relating increased use to lower risk (slope = 0.94, 95% confidence interval = 0.90 to 0.99). Bisphosphonates were also associated with a decreased risk of all-cause mortality similar to that of the development of bone metastasis. CONCLUSION Low-dose oral bisphosphonates administered for prevention or treatment of postmenopausal osteoporosis were associated with lower risk of skeletal metastasis in patients with early- or more advanced-stage breast cancer.