Selective androgen receptor modulator

Selective androgen receptor modulators (SARMs) are a class of drugs that selectively activate the androgen receptor in specific tissues, promoting muscle and bone growth while having less effect on male reproductive tissues like the prostate gland.

Non-selective steroidal drugs, called anabolic androgenic steroids (AAS), have been used for various medical purposes, but their side effects limit their use. In 1998, researchers discovered a new class of non-steroidal compounds, the SARMs. These compounds selectively stimulate the androgen receptor, offering potent effects on bone and muscle to increase bone density and lean body mass while having minimal impact on reproductive tissues.

SARMs have been investigated in human studies for the treatment of osteoporosis, cachexia (wasting syndrome), benign prostatic hyperplasia, stress urinary incontinence, and breast cancer. As of 2023^[update], there are no SARMs which have been approved by the United States Food and Drug Administration or the European Medicines Agency. Although adverse effects in clinical studies have been infrequent and mild, SARMs can cause elevated liver enzymes, reduction of HDL cholesterol levels, and hypothalamic–pituitary–gonadal axis (HPG axis) suppression, among other side effects.

Since the early twenty-first century, SARMs have been used in doping; they were banned by the World Anti-Doping Agency in 2008. SARMs are readily available on internet-based gray markets and are commonly used recreationally to stimulate muscle growth.

Anabolic androgenic steroids (AAS) are used to treat a variety of medical conditions, but their side effects have fueled a search for a new class of drugs, with a better separation between desirable anabolic and undesirable androgenic effects. The first clinically used AAS was testosterone which was discovered in 1935 and first approved for medical use in 1939. AAS including those produced endogenously such as testosterone and dihydrotestosterone (DHT), bind to and activate the androgen receptor (AR) to produce their effects. AAS effects can be separated into androgenic (the development and maintenance of male sexual characteristics) and anabolic (increasing bone density, muscle mass and strength). AAS also affect hematopoiesis, coagulation, metabolism, and cognition. For most medical applications, an AAS with potent anabolic and minimal androgenic and cardiovascular effects would be an advantage.

In the 1930s, 17α-alkylated anabolic steroids were discovered. These have increased metabolic stability and are orally active, but are not tissue selective. These alkylated anabolic steroids still have significant androgenic effects, and are also hepatotoxic. In 1950, nandrolone (19-nortestosterone) was first synthesized, which is sometimes considered a SARM due to greater tissue selectivity than testosterone. In addition, 7α-alkyl substitution of testosterone (for example trestolone) has also been reported to increase its anabolic effects. However, efforts to develop a steroid with anabolic but minimal androgenic effects were not successful.

Interest in nonsteroidal AR mixed agonists/antagonists increased after the therapeutic uses of selective estrogen receptor modulators (SERMs) became evident. The first SERM, tamoxifen, was originally developed as an anti-estrogen contraceptive. However, it was discovered it promoted ovulation in humans by acting as an agonist in ovaries. The drug was then successfully repurposed as a treatment for breast cancer where it was found to act as a full antagonist in breast tissue. Somewhat unexpectedly, it was also discovered that tamoxifen preserves bone density by acting as an agonist in bone resorbing osteoclasts. The clinical success of SERMs stimulated interest in analogous tissue selective drugs that target the AR.

The chemical starting point for AR mixed agonist/antagonists were nonsteroidal AR antiandrogens such as flutamide, nilutamide, bicalutamide. These antagonists work by binding to the AR to prevent androgenic action; this class of chemicals dates to the 1970s. The discovery of arylpropionamides, which share structural similarity with bicalutamide and hydroxyflutamide, suggested a way to make compounds that bind to the AR and produce both anabolic and antiandrogenic effects. Selective androgen receptor modulators (SARMs) were developed out of a desire to maintain the anabolic effects of androgens on muscle and bone, while avoiding side effects on other tissues such as the prostate and cardiovascular system.