Advances in research on pharmacotherapy of sarcopenia

Abstract Sarcopenia is a comprehensive degenerative disease with the progressive loss of skeletal muscle mass with age, accompanied by the loss of muscle strength and muscle dysfunction. As a new type of senile syndrome, sarcopenia seriously threatens the health of the elderly. The first‐line treatment for sarcopenia is exercise and nutritional supplements. However, pharmacotherapy will provide more reliable and sustainable interventions in geriatric medicine. Clinical trials of new drugs targeting multiple molecules are ongoing. This article focuses on the latest progress in pharmacotherapeutic approaches of sarcopenia in recent years by comprehensively reviewing the clinical outcomes of the existing and emerging pharmacotherapies as well as the molecular mechanisms underlying their therapeutic benefits and side effects.


| INTRODUC TI ON
Sarcopenia is a comprehensive degenerative disease with the progressive loss of skeletal muscle mass with age, accompanied by the loss of muscle strength and muscle dysfunction. As a kind of senile disease, sarcopenia seriously affects the health and quality of life of the elderly. Patients with sarcopenia have reduced muscle function, restricted mobility, are prone to falls and fractures, and may induce diabetes and other chronic non-communicable diseases, and even increase the risk of death. How to better prevent and treat sarcopenia has become one of the frontiers of global geriatric research.
Over the last decade, there has been a remarkable increase in the understanding of the molecular mechanism of the pathogenesis of sarcopenia. For example, the role of cell signaling regulating protein synthesis and degradation in sarcopenia onset and development has been illustrated. Many molecules have been identified as therapeutic targets, bringing a booming of drug investigation targeting this disease. Here, we will focus on the molecular mechanisms ( Figure 1) and the preclinical and clinical approaches (Table 1) of existing and emerging pharmacotherapies designed or repurposed for treating sarcopenia.

| DRUG S TARG E TING MYOS TATIN S IG NALING
Myostatin (MSTN), also known as growth differentiation factor-8 (GDF-8), belongs to the transforming growth factor β (TGFβ) superfamily and functions as a negative regulator of muscle mass. 1,2 As a secreted hormone, it binds to activin type 2 receptors (ACVR2s) on muscle fiber membranes, which recruits and activates activin type I receptor-like kinases, ALK4 and ALK5 (also known as ACVR1B and TGFBR1) to phosphorylate SMAD2 and SMAD3. Phosphorylated SMAD2 and SMAD3 form a complex with SMAD4, which consequently translocates into nuclear to promote atrophy-related gene expression. 3 Moreover, activated ALK4 and ALK5 can also impact some SMAD-independent pathways, such as ERK, JNK, and p38 MAPK to regulate muscle growth, proliferation, and differentiation. 4 At the same time, MSTN inhibits muscle growth by reducing Akt Besides biochemical and cell biological evidence, the importance of MSTN signaling in muscles is further confirmed in genetic studies. The polymorphisms of genes involved in the pathway, including MSTN, ACVR2B, ACVR1B, and follistatin, are highly associated with individual variation for muscle strength and some muscle diseases, reviewed in ref. 6 In the mouse model, gene knockout or mutation of MSTN can cause muscle hypertrophy, whereas the increase of MSTN level will induce muscle atrophy. 7 Moreover, the MSTN inhibitor, PF-354, can effectively increase muscle mass and muscle function in mice. 8 The inhibition of the MSTN pathway has become an attractive therapeutic strategy for stimulating muscle growth and/or preventing muscle wasting in many muscle-wasting diseases, including sarcopenia, comprehensively reviewed in ref. 2. There are several types of drugs targeting this pathway progress in clinical trials, including MSTN inhibitors, activin receptor antagonists, and follistatin-based drugs. However, some of these drugs also impact the activities of other closely related TGFβ family members, such as GDF11, activins, and bone morphogenetic proteins (BMPs), which potentially bring unwanted side effects. For example, follistatin (FST), an endogenous antagonist of myostatin, can block both MSTN and GDF11 interacting with ACVR2B. Overexpression of FST increases skeletal muscle mass by suppressing the activity of myostatin, but diminishes BMP and induces bone fractures likely through repressing the activity of GDF11. 9 Therefore, we need to carefully distinguish the molecular activities and adverse clinical effects of these drugs. In the following sections, we will focus on the clinical trials in sarcopenia, as well as other muscle-wasting diseases whose data provide some transferable information for future sarcopenia studies.

| MSTN inhibitor
Landogrozumab (LY-2495655) is a humanized monoclonal antibody targeting myostatin. In its phase II trial in patients with sarcopenia (NCT01604408, completed in 2013), landogrozumab treatment significantly increased muscle mass and partially improved muscle function and mobility of older people aged 75 years or over. 10 The treatment continued for 20 weeks with a 315 mg of landogrozumab injection every 4 weeks, followed by 16 weeks of observation.
Interestingly, the treatment only significantly improved some physical performances measured by stair climbing time, the ability of hand rise with arms, fast gait speed, but not others measured by usual gait speed, 6-minute walking distance, handgrip strength, and isometric leg extension strength.
Trevogrumab (REGN-1033) is another monoclonal anti-myostatin antibody, which specifically binds myostatin but not GDF11 or F I G U R E 1 Signaling of the pharmacotherapies of sarcopenia. By targeting multiple pathways, such as myostatin (MSTN), reninangiotensin system (RAS), androgen receptor (AR), activated protein kinase (AMPK) signaling, potential drugs rebalance protein synthesis and degradation, reshape the endocrine system, reduce oxidative stress and promote mitochondrial function, result in beneficial effects in muscle hypertrophy. Green and red arrows demonstrate promoting and inhibiting effects, respectively. Yellow arrows indicate the pathway of side effect. The indirect impacts are illustrated with dash lines.

TA B L E 1 (Continued)
activin, and has effectively increased muscle mass and improved isometric force production in a mouse model. 11

| DRUG TARG E TING RENIN -ANG IOTENS IN SYS TEM
The Renin-angiotensin system (RAS) is known for its robust effect on blood pressure and fluid homeostasis. In recent years, emerging evidence has clarified the role of RAS in promoting muscle atrophy in response to different chronic diseases, such as congestive heart failure, chronic kidney disease, and ventilator-induced diaphragmatic wasting (comprehensively reviewed in ref. 34). Briefly, in a classical pathway, angiotensin I (AngI), cleaved from angiotensinogen by renin, is further converted to angiotensin II (AngII) by angiotensin converting enzyme (ACE). AngII binds angiotensin II type 1 receptor (AT1R) and activates the downstream PKC and/or Src pathway, resulting in the activation of NADPH oxidase II (Nox2), which upregulates the production of reactive oxygen species (ROS). Subsequently, this oxidative stress leads to muscle atrophy by accelerating protein degradation and depressing protein synthesis. In the nonclassical pathway, Ang 1-7, generated from Ang1-9, AngI, and AngII, binds and activates the mitochondrial assembly receptor (MASR). The activation of MASR conversely inhibits AT1R activation as well as its downstream effects ( Figure 1). Moreover, a high circulating level of AngII induces high plasma levels of glucocorticoids, interleukin 6 (IL6), and serum amyloid A (SAA), inhibits the expression of IGF-1 but promotes the expression of MSTN, which promotes skeletal muscle atrophy through AT1 receptor-independent signaling (Figure 1).
RAS receptors not only locate on cell membranes but also locate on nuclear membranes and mitochondrial membranes.
Mitochondrial RAS signaling is coupled to mitochondrial nitric oxide production and can modulate respiration. The aging-related change in Ang receptor expression may impact mitochondrial dysfunction associated with aging. 35 Inhibition of RAS signaling will inhibit muscle atrophy and potentially leads to a therapeutic application to treat muscle-wasting conditions, such as sarcopenia. Currently, there are three types of drugs targeting the RAS signaling process in clinical trials: ACE inhibitors, AT1 receptor antagonists, and MASR agonists.

| ACE inhibitors
ACE inhibitors block the production of AngII and potentially inhibit the development of sarcopenia by inhibiting AngII mediated muscle atrophy. ACE inhibitors are common drugs to treat cardiovascular diseases and prevent strokes for many years, during which their effect on promoting muscle function has been exposed.

| Testosterone
As one of the sex steroids, testosterone plays an important role in the maintenance of muscle mass and function and has been extensively examined for the prevention of muscle wasting associated with aging and chronic disease. 47 Testosterone binds to the androgen receptor (AR), which leads AR to translocate from the cytoplasm to nuclear to regulate myogenic gene expression. 48 This interaction also intrigues a series of cellular signal transduction. In cell and mice models, testosterone treatment stimulates Akt/mTORC1 to promote protein synthesis and suppresses FoxO-targeted gene expression to inhibit protein degradation. 49 By suppressing the myostatin expression, testosterone treatment activates Akt/Notch signaling to promote activation and proliferation of satellite cells for muscle repairment and regeneration; it also inhibits JUN kinase-regulated cell apoptosis. 50 In the first testosterone trial in sarcopenia (NCT00240981, terminated in 2009), the testosterone supplementation group significantly increase maximal voluntary muscle strength in older men with low testosterone levels and mobility limitations. However, the trial was terminated due to a higher rate of adverse cardiovascular events. 51 Later on, more clinical evidence has shown the beneficial effect of testosterone in improving muscle function. For example, testosterone treatment improved self-reported walking ability and 6-minute walk test distance but did not affect the rate of falls in elderly men with low testosterone in a phase III trial (NCT00799617; completed in 2014). 52 In another phase II trial (NCT00104572; completed in 2015), the treatment improved fast gait speed at 3 and 12 months and knee strength at 12 months compared to the placebo group. 53 However, it also leads to prostatic hyperplasia and lower urinary tract symptoms. 54 With more side effects showing up, such as allergic reactions, thrombosis, and prostate cancer, whether the benefit overcomes the risk remains undebated. 55,56 One possible solution is to combine the testosterone treatment

| Selective androgen receptor modulators
The selective androgen receptor modulators (SARMs) are a group of chemically synthesized small molecules that function as agonists/antagonists of AR. Similar to testosterone, the interaction of SARM and AR intrigues the translocation of the complex to nuclear.

| G HRELIN AND ITS MIME TI C S
Ghrelin is a peptide hormone secreted predominantly from the stomach. As the ligand of growth hormone (GH)-secretagogue receptor (GHS-R), it elicits multiple endocrine effects. It promotes growth hormone secretion, at the same time, it inhibits the production of inflammatory factors IL-1β, IL-6, and TNFα, 67  It can also suppress the development of sarcopenia by inhibiting NF-kappaB mediated inflammation and oxidation response. 76 In several model systems, metformin can extend the lifespan and improve physical performance. 77 Metformin was hypothesized to improve or argue the exercise training effect in seniors, 78 however, recent studies have presented a more complex scenery when combining metformin and exercise.
For example, although both metformin and exercise improved insulin sensitivity in individuals with prediabetes, combined treatments did not provide additional effects. 79 New data from a phase I trial (NCT02308228, completed in 2018) found that 14-week treatment with 1700 mg/day of metformin blunts muscle hypertrophy in response to progressive resistance exercise training in older adults. 80 Recently, a study in the sarcopenia mice model seems to settle the great controversy if metformin treatment improves or nullifies exercise training. It has shown a long-term metformin treatment did not eliminate the beneficial effect given by exercise and supported the use of metformin for anti-aging effects. 81 More clinic trials conducted in elderly adults with prediabetes to evaluate the impacts of metformin on muscle size, strength, and physical function are currently ongoing or under data evaluation stage, including a phase I/II trial (NCT01804049, completed in 2018), a phase II trial (NCT02570672, ongoing), and a phase II trial (NCT03309007, ongoing).

| CON CLUS ION
The  82 Moreover, a consensus of the conduct of clinical trials for sarcopenia has been formulated. 83 This will improve the methodological robustness and comparability of the clinical trials, in turn, exceedingly accelerating the drug development.

CO N FLI C T O F I NTE R E S T
Nothing to declare.

AUTH O R CO NTR I B UTI O N S
Yang Feike completed the collection and analysis of relevant literature and drafted the manuscript as the main writer of the review.
Liu Zhijie and Chen Wei participates in the analysis and sorting of literature materials. All authors have read and approved the content of the manuscript.