Lasofoxifene shown to sharply reduce incidence of breast cancer

The investigational selective estrogen-receptor modulator laso-foxifene is shaping up as a PEARL of a drug for the prevention of breast cancer.

Lasofoxifene slashed the incidence of estrogen receptor-positive breast cancer by 81 percent, compared with placebo, over a five-year period in the 8,556-woman, phase III PEARL (Postmenopausal Evaluation and Risk Reduction With Lasofoxifene) trial, reported at the San Antonio Breast Cancer Symposium.

That’s a considerably more dramatic preventive effect than the roughly 50 percent reductions seen in earlier placebo-controlled trials of tamoxifen and raloxifene, the two approved agents for primary prevention of breast cancer, added a professor of epidemiology at the University of Washington, Seattle.

Reduction of estrogen receptor-positive breast cancer was a coprimary end point in PEARL, together with nonvertebral fractures, which were decreased by a highly significant 24% in women randomized to lasofoxifene at 0.5 mg/day.

The PEARL participants (aged 59-80 years) were enrolled in 32 countries. All had osteoporosis at entry. They were randomized to placebo or lasofoxifene at 0.25 or 0.5 mg/day.

During five years of following-up 21 women in the placebo arm developed estrogen receptor-positive breast cancer, as did 11 in the lower-dose and four in the higher-dose lasofoxifene arms. The incidence was 0.3 cases per 1,000 patient-years in women on lasofoxifene at 0.5 mg/day, 0.9 cases per 1,000 patient-years with lasofoxifene at 0.25 mg/day, and 1.9 cases per 1,000 patient-years with placebo.

Women on lasofoxifene at 0.5 mg/day had a 35 percent reduction in breast biopsies, compared with the 2.8 percent incidence in the placebo arm. As expected, the SERM had no effect on the rate of estrogen receptor-negative breast cancers. The risk of ductal carcinoma in situ was similarly unaffected.

Noted that lasofoxifene had its greatest breast cancer reduction benefit among women with above-average levels of estradiol. Other benefits included a reduction of 42 percent in vertebral fractures, a decrease of 32 percent in major coronary heart disease events and a 36 percent reduction in strokes.

The chief risk associated with the SERM was a twofold increase in venous thromboembolic events (from 0.4 percent over five years in the placebo arm to 0.8 percent with 0.5 percent mg lasofoxifene).

In September 2008, the Food and Drug Administration’s Reproductive Health Advisory Committee recommended approval of application for an indication for lasofoxifene at the five-mg dose for treatment of osteoporosis. One oncologist in the San Antonio audience shrugged off failure to seek an additional indication for breast cancer prevention, noting that once lasofoxifene is approved for osteoporosis, physicians will readily be able to prescribe the drug in postmenopausal women for breast cancer prevention as well.