Tissue-Selective Agents: Selective Estrogen Receptor Modulators and the Tissue-Selective Estrogen Complex

Abstract

Menopause has been associated with vasomotor symptoms, vulvar-vaginal atrophy and osteoporosis. One of the goals in exploring the potential of selective estrogen receptor modulators (SERMs) was to determine if they could prevent fractures, reduce menopausal symptoms and treat vaginal atrophy, while being neutral or protective on the uterus, breast and cardiovascular system. However, no SERM to date has achieved this goal. More recently, the idea of pairing a SERM with estrogen(s), known as a tissue-selective estrogen complex (TSEC), has been studied in postmenopausal women. A TSEC combines the complementary tissue-selective activities of a SERM and estrogen(s), in an attempt to gain the benefits of each with better overall tolerability. The Selective estrogen Menopause And Response to Therapy (SMART) trials were multicentre, randomized, double-blind, placebo- and active-controlled phase 3 studies evaluating the safety and efficacy of the SERM, bazedoxifene (BZA) paired with conjugated estrogens (CEs) in healthy postmenopausal women. In the first SMART trial, BZA/CE protected the endometrium from estrogenic stimulation, relieved hot flushes and maintained bone mass, with rates of amenorrhea, breast pain and overall adverse events similar to those with placebo in more than 3400 women over two years. BZA 20 mg was the lowest effective dose of BZA in BZA/CE to protect the endometrium and maintain bone mass when paired with CE 0.625 mg and CE 0.45 mg. In SMART-2, these BZA/CE doses significantly reduced the frequency and severity of hot flushes over 12 weeks. Collectively, these data support the TSEC containing BZA/CE as a new paradigm for treating menopausal symptoms and preventing osteoporosis while protecting the endometrium from unopposed estrogenic stimulation.