Lasofoxifene: a new type of selective estrogen receptor modulator for the treatment of osteoporosis

Selective estrogen receptor modulators (SERMs) are structurally different compounds that interact with intracellular estrogen receptors in target organs as estrogen agonists and antagonists. Thus far SERMs have proven to be a highly versatile group and are being evaluated primarily for conditions associated with aging, including hormone-responsive cancer and osteoporosis. Tamoxifen and toremifene are currently used to treat advanced breast cancer and also have beneficial effects on bone mineral density and serum lipids in postmenopausal women. Raloxifene is the only SERM compound actually approved worldwide for the prevention and treatment of postmenopausal osteoporosis and fragility fractures. Unfortunately, although these SERMs possess many benefits, they are also responsible for some very serious side effects, such as thromboembolic disorders and, in the case of tamoxifen, uterine cancer. These contraindications represent a major concern for the type of long-term, chronic therapy that is required to prevent osteoporosis. Moreover, both preclinical and clinical reports suggest that these SERMs are considerably less potent than estrogen, probably due to their reduced bioavailability. Lasofoxifene (CP 336,156) is a naphthalene-derivative, third-generation SERM, structurally distinct from the first- and second-generation SERMs. This compound selectively binds to both estrogen receptor subtypes (estrogen receptor-alpha or -beta) with high affinity. It has a half-inhibition concentration similar to that seen with estradiol and thus at least 10-fold higher than those reported for raloxifene and tamoxifen. Moreover, due to increased resistance to intestinal wall glucuronidation, lasofoxifene has a remarkably improved oral bioavailability with respect to other SERMs. In both preclinical and short-term clinical studies lasofoxifene has shown a proven efficacy in preventing bone loss and lowering cholesterol levels. Dose modeling from phase II studies allowed the selection of lasofoxifene 0.25 mg/day as the lowest fully effective dose. The compound shows a favorable safety profile and is currently in phase III development for the prevention and treatment of osteoporosis in postmenopausal women.