Lasofoxifene, a selective estrogen receptor modulator (SERM), appears to reduce the risk of breast cancer among postmenopausal women with osteoporosis. These findings were recently reported in the Journal of the National Cancer Institute.
As a SERM, the investigational drug lasofoxifene works to prevent breast cancer by blocking the effects of estrogen on breast tissue. This action is similar to that of the hormonal therapy tamoxifen, which is also used in breast cancer prevention. Another example of a SERM is raloxifene (Evista®), which is approved by the U.S. FDA for the treatment and prevention of osteoporosis and the prevention of breast cancer. Though tamoxifen and raloxifene may help prevent breast cancer among women at high risk, both therapies have limitations: tamoxifen is associated with a risk of endometrial cancer and other gynecological conditions, and raloxifene may not offer a broad enough spectrum of benefits.
Earlier results from the 8,556-woman Postmenopausal Evaluation and Risk-Reduction with Lasofoxifene (PEARL) trial have indicated that lasofoxifene could reduce the risk of estrogen receptor-positive (ER+) breast cancer as well as non-vertebral and vertebral fractures, coronary artery disease, and stroke.  Risks of lasofoxifene included an increased risk of blood clots.
The current analysis evaluated the effects of lasofoxifene on two other outcomes: 1) the risk of all breast cancers combined (invasive cancer as well as ductal carcinoma in situ, and ER+ or ER- cancers) and 2) the risk of ER+ invasive cancers.
- Compared with placebo, lasofoxifene (at a dose of 0.5 mg) significantly reduced the risk of all types of breast cancer combined—a reduction of 79%.
- Lasofoxifene reduced the risk of ER+ invasive breast cancer by 83%.
- The benefit of lasofoxifene appeared to be greatest among women who had the highest estrogen levels at the start of the study.
The researchers concluded that lasofoxifene reduces the risk of both total and ER+ invasive breast cancers. The risk reduction of lasofoxifene was similar to that of tamoxifen and raloxifene without the risk of endometrial cancer associated with tamoxifen and with a broader spectrum of benefits than raloxifene. The researchers add that further study of lasofoxifene will need to focus on the drug’s long-term effects.
 LaCroix AZ, Powles T, Osborne CK, et al. Breast cancer incidence in the randomized PEARL trial of lasofoxifene in postmenopausal osteoporotic women. Journal of the National Cancer Institute [early online publication]. November 4, 2010