Myostatin inhibitors are a class of drugs that work by blocking the effects of myostatin, which inhibits muscle growth. In animal models and limited human studies, myostatin inhibitors have increased muscle size. They are being developed to treat obesity, sarcopenia, muscular dystrophy, and other illnesses.
Myostatin, a member of the transforming growth factor superfamily, is a negative regulator of bone and muscle growth. It may also play a role in obesity, insulin resistance, cardiovascular disease, and chronic kidney disease. [1] [2]
Follistatin is an endogenous protein that negatively regulates myostatin. [3]
Reduction of myostatin expression is one of the mechanisms for the effects of androgens in promoting muscle growth. Androgens both regulate myostatin expression directly and upregulate follistatin expression. [3] YK-11, a selective androgen receptor modulator, is also a myostatin inhibitor. [4] [5]
Resistance training reduces myostatin activity and increases follistatin activity. [6] Pharmacological myostatin inhibitors can therefore be considered exercise mimetics. [7] Creatine, a popular workout supplement, has shown some myostatin inhibitory effects in preclinical studies. [6]
Many drugs in development as myostatin inhibitors also reduce the activity of related proteins such as GDF11, activins, and bone morphogenetic proteins. While this off target activity can increase their effectiveness in promoting anabolism, it also increases the risk of adverse effects. [8]
Monoclonal antibodies have been developed that disable myostatin, including apitegromab, domagrozumab, landogrozumab, and stamulumab. [9] Another form of myostatin inhibition is gene therapy. [10]
Another monoclonal antibody, bimagrumab, works as an antagonist of the ACVR2 and ACVR2B receptors, preventing myostatin and activin A from binding. [11] Because activin A reduces erythropoiesis, targeting the ACVR receptors and inhibiting activin A activity can increase the risk of venous thromboembolism in patients who are not anemic. [12]
Clinical trials of myostatin inhibitors for muscular dystrophy have not proven successful in generating functional improvements compared to placebo. Gains of muscle mass were small to non-existent in this population. [13] Research is ongoing on the potential use of myostatin inhibitors for motor neuron diseases like spinal muscle atrophy and amyotrophic lateral sclerosis. [14] Due to myostatin's effect as a negative regulator of bone, its inhibition has also been considered for orthopedic diseases such as rheumatoid arthritis. [15]
Myostatin inhibitors were generally able to increase lean body mass and reduce body fat in people with sarcopenia, but the extent to which this translated into functional improvements varied. [11]
Bimagrumab showed effectiveness in increasing lean mass and reducing fat mass in obese individuals in a clinical trial. [11]
It is hypothesized that myostatin inhibitors have an ergogenic effect due to promoting muscle growth. [16] Myostatin inhibitors are banned by the World Anti-Doping Agency. [9]
Cachexia is a complex syndrome associated with an underlying illness, causing ongoing muscle loss that is not entirely reversed with nutritional supplementation. A range of diseases can cause cachexia, most commonly cancer, congestive heart failure, chronic obstructive pulmonary disease, chronic kidney disease, and AIDS. Systemic inflammation from these conditions can cause detrimental changes to metabolism and body composition. In contrast to weight loss from inadequate caloric intake, cachexia causes mostly muscle loss instead of fat loss. Diagnosis of cachexia can be difficult due to the lack of well-established diagnostic criteria. Cachexia can improve with treatment of the underlying illness but other treatment approaches have limited benefit. Cachexia is associated with increased mortality and poor quality of life.
Atrophy is the partial or complete wasting away of a part of the body. Causes of atrophy include mutations, poor nourishment, poor circulation, loss of hormonal support, loss of nerve supply to the target organ, excessive amount of apoptosis of cells, and disuse or lack of exercise or disease intrinsic to the tissue itself. In medical practice, hormonal and nerve inputs that maintain an organ or body part are said to have trophic effects. A diminished muscular trophic condition is designated as atrophy. Atrophy is reduction in size of cell, organ or tissue, after attaining its normal mature growth. In contrast, hypoplasia is the reduction in the cellular numbers of an organ, or tissue that has not attained normal maturity.
Myostatin is a protein that in humans is encoded by the MSTN gene. Myostatin is a myokine that is produced and released by myocytes and acts on muscle cells to inhibit muscle growth. Myostatin is a secreted growth differentiation factor that is a member of the TGF beta protein family.
Spinal and bulbar muscular atrophy (SBMA), popularly known as Kennedy's disease, is a rare, adult-onset, X-linked recessive lower motor neuron disease caused by trinucleotide CAG repeat expansions in exon 1 of the androgen receptor (AR) gene, which results in both loss of AR function and toxic gain of function.
Sarcopenia is a type of muscle loss that occurs with aging and/or immobility. It is characterized by the degenerative loss of skeletal muscle mass, quality, and strength. The rate of muscle loss is dependent on exercise level, co-morbidities, nutrition and other factors. The muscle loss is related to changes in muscle synthesis signalling pathways. It is distinct from cachexia, in which muscle is degraded through cytokine-mediated degradation, although the two conditions may co-exist. Sarcopenia is considered a component of frailty syndrome. Sarcopenia can lead to reduced quality of life, falls, fracture, and disability.
Muscle atrophy is the loss of skeletal muscle mass. It can be caused by immobility, aging, malnutrition, medications, or a wide range of injuries or diseases that impact the musculoskeletal or nervous system. Muscle atrophy leads to muscle weakness and causes disability.
Bone morphogenetic protein 4 is a protein that in humans is encoded by BMP4 gene. BMP4 is found on chromosome 14q22-q23.
Follistatin, also known as activin-bindings protein, is a protein that in humans is encoded by the FST gene. Follistatin is an autocrine glycoprotein that is expressed in nearly all tissues of higher animals.
Muscle hypertrophy or muscle building involves a hypertrophy or increase in size of skeletal muscle through a growth in size of its component cells. Two factors contribute to hypertrophy: sarcoplasmic hypertrophy, which focuses more on increased muscle glycogen storage; and myofibrillar hypertrophy, which focuses more on increased myofibril size. It is the primary focus of bodybuilding-related activities.
Growth differentiation factor 11 (GDF11), also known as bone morphogenetic protein 11 (BMP-11), is a protein that in humans is encoded by the growth differentiation factor 11 gene. GDF11 is a member of the Transforming growth factor beta family.
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.
Enobosarm, also formerly known as ostarine and by the developmental code names GTx-024, MK-2866, and S-22, is a selective androgen receptor modulator (SARM) which is under development for the treatment of androgen receptor-positive breast cancer in women and for improvement of body composition in people taking GLP-1 receptor agonists like semaglutide. It was also under development for a variety of other indications, including treatment of cachexia, Duchenne muscular dystrophy, muscle atrophy or sarcopenia, and stress urinary incontinence, but development for all other uses has been discontinued. Enobosarm was evaluated for the treatment of muscle wasting related to cancer in late-stage clinical trials, and the drug improved lean body mass in these trials, but it was not effective in improving muscle strength. As a result, enobosarm was not approved and development for this use was terminated. Enobosarm is taken by mouth.
Activin and inhibin are two closely related protein complexes that have almost directly opposite biological effects. Identified in 1986, activin enhances FSH biosynthesis and secretion, and participates in the regulation of the menstrual cycle. Many other functions have been found to be exerted by activin, including roles in cell proliferation, differentiation, apoptosis, metabolism, homeostasis, immune response, wound repair, and endocrine function. Conversely, inhibin downregulates FSH synthesis and inhibits FSH secretion. The existence of inhibin was hypothesized as early as 1916; however, it was not demonstrated to exist until Neena Schwartz and Cornelia Channing's work in the mid-1970s, after which both proteins were molecularly characterized ten years later.
A myokine is one of several hundred cytokines or other small proteins and proteoglycan peptides that are produced and released by skeletal muscle cells in response to muscular contractions. They have autocrine, paracrine and/or endocrine effects; their systemic effects occur at picomolar concentrations.
LGD-4033, also known by the developmental code name VK5211 and by the black-market name Ligandrol, is a selective androgen receptor modulator (SARM) which is under development for the treatment of muscle atrophy in people with hip fracture. It was also under development for the treatment of cachexia, hypogonadism, and osteoporosis, but development for these indications was discontinued. LGD-4033 has been reported to dose-dependently improve lean body mass and muscle strength in preliminary clinical trials, but is still being developed and has not been approved for medical use. The drug is taken by mouth.
Vosilasarm, also known by the development codes RAD140 and EP0062 and by the black-market name Testolone or Testalone, is a selective androgen receptor modulator (SARM) which is under development for the treatment of hormone-sensitive breast cancer. It is specifically under development for the treatment of androgen receptor-positive, estrogen receptor-negative, HER2-negative advanced breast cancer. Vosilasarm was also previously under development for the treatment of sarcopenia, osteoporosis, and weight loss due to cancer cachexia, but development for these indications was discontinued. The drug is taken by mouth.
TFM-4AS-1 is a dual selective androgen receptor modulator (SARM) and 5α-reductase inhibitor. It is a potent and selective partial agonist (Emax = 55%) of the androgen receptor (IC50 = 30 nM) and inhibitor of 5α-reductase types I and II (IC50 = 2 and 3 nM, respectively). TFM-4AS-1 shows tissue-selective androgenic effects; it promotes the accumulation of bone and muscle mass and has reduced effects in reproductive tissues and sebaceous glands. In an animal study, TFM-4AS-1 stimulated sebaceous gland formation only 31% as much as dihydrotestosterone (DHT) at doses that were as anabolic or more so than DHT. In addition, TFM-4AS-1 only weakly promoted growth of the prostate gland and it partially antagonized the actions of DHT in the seminal vesicles and endogenous androgens in the prostate gland. Structurally, TFM-4AS-1 is a 4-azasteroid. A structurally related and more advanced version of TFM-4AS-1, MK-0773, was developed and pursued for potential pharmaceutical use.
MK-0773, also known as PF-05314882, is a steroidal, orally active selective androgen receptor modulator (SARM) that was under development by Merck and GTx for the treatment of sarcopenia in women and men. Clinical trials for sarcopenia began in late 2007 but the collaboration between Merck and GTx ended in early 2010 and GTx terminated development of MK-0773 shortly thereafter. MK-0773 was developed as a more advanced version of the related compound TFM-4AS-1.
GSK2881078 is a drug which acts as a selective androgen receptor modulator (SARM). It was developed for the prevention of muscle wasting and sarcopenia in elderly people.
Apitegromab (SRK-015) is a fully human monoclonal antibody developed to treat spinal muscular atrophy. It works by binding to and inhibiting promyostatin, a precursor to myostatin, which limits the size of skeletal muscle tissue, as well as inactive myostatin. It does not bind to active myostatin, activin A, active BMP9/10 or TGFβ1 that all operate on the activin type 2 receptors.