In this educational content, we explore Selective Estrogen Receptor Modulators (SERMs) and Aromatase Inhibitors, primarily used in the prevention and treatment of estrogen-receptor positive breast cancer. Estrogens are steroids involved in numerous physiological processes including reproduction and sex determination, and are synthesized from precursor androgens via the Aromatase Enzyme. This enzyme is represented by an aromatic-smelling flower, highlighting its role in converting androgens to estrogen. Estrogens exert genomic and non-genomic biological effects through interaction with either of two receptor isoforms, Estrogen Receptor Alpha (ER-alpha), or Estrogen Receptor Beta (ER-beta). These receptors, predominantly expressed in the nucleus and the cytoplasm of cells, determine which type of sex hormone stimulates tumor cell growth, leading to transcriptional regulation of genes involved in cell survival and proliferation.
Breast cancers can be categorized into various types, with the most common being Hormone Receptor-Positive Breast Cancers. In cases where cells express estrogen receptors, these cells are stimulated by estrogen, making this signaling pathway a strategic therapeutic target for treating hormone receptor-positive breast cancer. SERMs, such as Tamoxifen and Raloxifene, compete with estrogen for receptor binding thereby inhibiting tumor growth. However, it's important to note that SERMs aren't pure antagonists and exhibit partial agonist activity on the endometrium, skeletal, and cardiovascular systems. The other therapeutic targets are Aromatase Inhibitors, including Anastrozole, Letrozole, and Exemestane which drastically reduce tissue estrogen concentration. Both SERMs and Aromatase Inhibitors have potential side effects including hot flashes, and vaginal dryness. In addition, Aromatase Inhibitors can lead to joint, muscle, and back soreness, and reduced bone mineral density. On the other hand, SERMs are commonly associated with gastrointestinal upset.
Estrogens, synthesized from precursor androgens by the aromatase enzyme (CYP19A1) found in fat, liver, muscle, and breast tissue of postmenopausal women, interact with estrogen receptor Œ± (ERŒ±) or estrogen receptor Œ≤ (ERŒ≤) to exert both genomic and non-genomic effects. Hormone receptor-positive breast cancers are reliant on estrogen and progesterone, and make up the majority of cases. SERMs, such as tamoxifen and raloxifene, compete with estrogen for receptor binding, hindering tumor growth. Conversely, aromatase inhibitors like anastrozole, letrozole, and exemestane block the conversion of precursor androgens to estrogens, depriving cancer cells of their growth stimulus.
Beyond their anti-cancer properties, SERMs showcase partial agonist activities on bone and lipids. This makes them effective in mitigating postmenopausal bone loss and optimizing cholesterol, lipid, and lipoprotein profiles. Notably, tamoxifen also acts as a partial agonist on the endometrium. While tamoxifen is employed in treating estrogen-receptor-positive breast cancer and in prevention among high-risk populations, raloxifene serves in preventing invasive breast cancer in high-risk individuals and in addressing postmenopausal osteoporosis. Aromatase inhibitors, however, might diminish bone mineral density (BMD), warranting periodic BMD evaluations and potential supplementation with vitamin D and calcium.
Administering tamoxifen typically spans up to 10 years following initial breast cancer therapy. Some patients may switch to an aromatase inhibitor after 5 years to complete adjuvant therapy.
SERMs, such as tamoxifen, can elevate the risk of endometrial cancer, especially amidst pre-existing endometrial hyperplasia. This makes it vital to assess any unusual vaginal bleeding. Furthermore, SERMs can heighten the risk of thromboembolic events; hence, a thorough risk-benefit evaluation is essential for those with a history of these conditions. On the other hand, aromatase inhibitors can contribute to a decrease in BMD. Both drug classes can induce side effects like hot flashes and vaginal dryness. Joint, muscle, and back discomfort are notably prevalent with aromatase inhibitors, while nausea is more common with SERMs.