New anticancer drugs are usually developed specially for the job, but occasionally they are borrowed from another field of medicine, and applied speculatively in cancer. Tamoxifen was designed as an anti-oestrogen, based on the observation that at least a third of breast cancers depend on female sex hormones such as oestrogen for survival. Tamoxifen has shown to be an exceptionally effective molecule in cancer treatment; It was never planned to be a preventive agent, but so it has proved to be! It is now licensed to be used to prevent breast cancer in certain women at high risk of the disease.
Contrast this with raloxifene, a drug first developed to treat osteoporosis in women. A selective benzothiophene oestrogen receptor modulator (SERM), raloxifene binds to oestrogen receptors as a mixed oestrogen and anti-oestrogen effect. It functions as an oestrogen sometimes (in bones and on lipid metabolism) and as an anti-oestrogen in other target tissues (endometrium and breast). So, it has the potential for producing some of oestrogen's beneficial effects without producing its adverse effects. In a trial of its use in osteoporosis, it appeared to have another completely different effect, namely prevention of new hormone dependant breast cancers.
Results from the MORE (The Multiple Outcomes of Raloxifene Evaluation) study of 7,705 women that were randomised to raloxifene or placebo demonstrated that among postmenopausal women with osteoporosis, the risk of invasive breast cancer was decreased by 76% during three years of treatment with raloxifene.
Stronger evidence on the safety and efficacy of raloxifene is awaited from the STAR Trial. This trial includes almost 20,000 postmenopausal women in the US who are at increased risk of breast cancer to determine whether raloxifene is as e
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