Pharmacologic Treatments for the Preservation of Lean Body Mass During Weight Loss

Insulin signaling plays an important role in glucose disposal and skeletal muscle anabolism and maintenance. However, in the setting of decreased nutrient intake or insulin resistance, the muscle-protective effects of insulin signaling are lost. In patients participating in weight loss regimens—whether through caloric restriction, bariatric surgery, or pharmacologic methods—the perfect storm exists where fewer calories are being consumed, resulting in a decreased need for insulin release and, thus, lower insulin; combined with this is the potential insulin resistance seen in overweight and obesity where the response to insulin is lost, leading to decreased skeletal muscle preservation signaling.

Insulin elicits its effects by binding to the insulin/IGF (insulin-like growth factor) receptor in skeletal muscle, thus activating several downstream signaling pathways, including phosphatidylinositol-3 kinase (PI3K) and Akt (also known as Akt serine/threonine kinase), which modulate the mechanistic target of rapamycin complex 1 (mTORc1) [19,20]. A lack of insulin or insulin resistance prevents the activation of PI3K and Akt, and thus decreases mTORc1 signaling, which is a key anabolic signal in driving protein synthesis and cell growth through the activation of translational regulators [19,20]. Loss of insulin/PI3K/Akt/mTORc1 signaling is largely responsible for the loss of LBM seen with weight loss [19,20].

AMP-activated protein kinase (AMPK) is an energy-sensing protein in cells that is activated during low-energy states, such as during weight loss, when nutrient intake is low [21,22]. When AMPK is activated, it prevents mTORc1 activation by phosphorylating two of the regulatory proteins of mTORc1, Raptor and tuberous sclerosis complex 2 (TSC2) [21,22]. With AMPK-mediated phosphorylation of Raptor, part of the mTORc1 complex, mTORc1, is directly inhibited, preventing the anabolic and cell growth signaling that mTORc1 is responsible for driving [21,22]. AMPK also indirectly regulates mTORc1 by phosphorylating and activating TSC2 [21,22]. TSC2 is responsible for turning off the small GTP-ase, Rheb, another mTORc1 activator [21,22]. Thus, without mTORc1 signaling, the balance is shifted from anabolic protein synthesis to catabolic autophagy mechanisms, leading to loss of skeletal muscle. Additionally, AMPK signaling occurs alongside other atrophy pathways and they often converge.

Two pathways that are active in muscle atrophy and protein breakdown are the ubiquitin–proteosome and autophagy–lysosomal pathways [23]. The signaling of forkhead transcription factors 1 and 3 (FoxO1/3) is usually suppressed by Akt. However, when Akt activity is decreased in conditions of decreased insulin signaling, FoxO1/3 are upregulated, leading to the expression of E3 ligases Atrogin1 and MuRF1 (muscle ring finger 1), which target proteins for proteasomal degradation [23]. Additionally, when AMPK is activated, it differentially phosphorylates FoxO1/3, leading to the activation and downstream transcription of Atrogin1 and MuRF1, resulting in protein degradation [23]. Thus, the activation of FoxO1/3 leads to increased proteolysis and a net breakdown of skeletal muscle, leading to atrophy. FoxO1/3 signaling converges with insulin-Akt and AMPK signaling pathways.

Myostatin, predominately expressed in skeletal muscle, is a negative regulator of muscle mass. Mutations in the myostatin gene lead to a hyper-muscular phenotype [24]. The negative regulatory effects of myostatin occur via signaling through the activin receptor type II (ActRIIB), leading to downstream signaling through SMAD2/3 (small mother against decapentaplegic 2/3), which promotes protein degradation, muscle loss, and atrophy [25]. The activation of SMAD2/3 leads to the transcription of atrophy-related genes such as Atrogin1 and MuRF1, similar to FoxO1/3 [25]. At the same time, myostatin decreases the transcription of genes leading to myogenesis, including MyoD (myoblast determination protein 1), MyoG (myogenin), MyHC (myosin heavy chain), and Pax7 [25]. Thus, the effects of myostatin are two-fold in that they increase atrophy while decreasing myogenesis; this is why this pathway has been the target for many drug developers aiming to preserve LBM.

Importantly, when initially transcribed, myostatin is released as a pro-myostatin [26]. Extracellular matrix proteases cleave the pro-myostatin, resulting in latent myostatin, which is also inactive and can be detected in serum. Following a final cleavage event, latent myostatin is transformed into a fully active mature myostatin which can bind to ActRIIB [24,26]. Various compounds have been developed to target the various forms of myostatin, which may lead to the preservation of LBM by the attenuation of protein degradation signaling and disinhibition of myogenic signaling [27].

Cortisol binding to the glucocorticoid receptor (GR) in skeletal muscle leads to downstream disinhibition of the previously mentioned FoxO1/3, which can enter the nucleus and upregulate the transcription of E3 ligases Atrogin1 and MuRF1 (muscle ring finger 1), as well as several autophagy-related genes, leading to increased protein degradation [28]. By doing so, leucine and alanine can be released to be used by the liver for gluconeogenesis in conditions of low nutrients, such as during weight loss. Additionally, the caloric deficit is seen as a stressor, and the body responds by increasing the levels of circulating cortisol. The increase in cortisol signaling increases skeletal muscle protein breakdown, leading to skeletal muscle atrophy overtime.

The autophagy–lysosomal protein degradation pathway has been noted to play a role in skeletal muscle atrophy in weight loss, aging, cachexia, disuse, inflammatory conditions, and chronic disease. Autophagy is mediated by numerous proteins, including LC3 (microtubule-associated protein 1 light chain 3), Beclin-1, and ULK1 (Unc-51-like kinase 1). These proteins play a role in shuttling proteins into the autophagosome as well as autophagosome lysosome fusion [29]. In the presence of AMPK signaling during weight loss, autophagy is upregulated; thus, there is breakdown of more proteins to release amino acids for gluconeogenesis, leading to loss of LBM overtime [29].

Skeletal muscle is a major target for the thyroid hormone and expresses deiodinases to convert T4 into active T3. Once inside the cell, T3 can bind to proteins or the thyroid hormone receptor to elicit non-transcription- or transcription-related changes, respectively. The thyroid hormone upregulates proteins involved in the formation of slow twitch and oxidative muscle fibers by upregulating peroxisome proliferator-activated receptor coactivator 1α (PGC-1α) expression, which increases mitochondrial biogenesis [30]. Other myogenic signaling is also affected, leading to an imbalance between skeletal muscle generation and breakdown, producing muscle atrophy that presents as proximal muscle weakness [30].

PGC-1α (peroxisome proliferator-activated receptor gamma coactivator 1-alpha), as mentioned, is a regulator or mitochondrial function. PGC-1α controls transcription factors NRF-1 (Nuclear respiratory factor 1) and NRF-3 (Nuclear respiratory factor 3), which are responsible for increasing the expression of TFAM (Mitochondrial transcription factor A), ultimately leading to mitochondrial biogenesis, protecting from muscle atrophy [31]. Additionally, by suppressing FoxO3, PGC-1α prevents the transcription of Atrogin1 and MuRF-1, thus preventing muscle atrophy [31]. Under conditions of elevated AMPK activity, such as low energy states and insulin resistance, AMPK drives PGC-1α to be overexpressed [29,31]. This overexpression of PGC-1α leads to substantial mitochondria biogenesis, which causes mitochondrial uncoupling, leading to a significant decrease in ATP (Adenosine triphosphate) production [29,31]. Without ATP to support the muscle, atrophy increases and LBM is lost.

Inflammation from various causes leads to increases in circulating inflammatory cytokines, including two major ones, tumor necrosis factor α (TNF-α) and interleukin 6 (IL-6). The expression of TNF-α and IL-6 are increased during chronic diseases such as diabetes [32]. Signaling through the TNF-α and IL-6 receptors converges on a downstream signaling factor, nuclear factor-κB (NF-κB), which acts as a nuclear transcription factor [29,32]. Once in the nucleus, NF-κB drives the transcription of genes of the ubiquitin–proteosome system, inflammatory mediators, and inhibitors of myogenesis [29,32]. Activation of the ubiquitin–proteosome system increases protein degradation; increased expression of inflammatory mediators causes increased infiltration of immune cells and myocyte death, increased protein degradation, and fibrosis and inhibition of myogenic signaling decreases muscle protein synthesis [32]. Overall, this leads to a phenotype of atrophy and LBM loss in a variety of conditions with increased proteolysis and decreased myogenesis.

Leptin is a signaling molecule released from adipose tissue, known as an adipokine. Under conditions of decreased nutrient intake or leptin resistance, a decrease in leptin signaling drives "starvation" signaling, leading to the increased breakdown of skeletal muscle proteins and, ultimately, skeletal muscle atrophy through decreases in energy expenditure [33]. Signaling through the leptin receptor in the hypothalamus increases JAK/STAT3 (Janus Kinase (JAK)/Signal Transducer and Activator of Transcription (STAT) 3) signaling and neuroendocrine regulation [33]. As a neuroendocrine molecule, decreases in leptin signaling can decrease various hypothalamus–pituitary signaling axes, which can also contribute to muscle atrophy. The effect on two of these hypothalamic–pituitary axes, thyroid and growth hormone/IGF-1, were discussed previously [33].

Under conditions of low testosterone, the normal muscle preserving signaling via the androgen receptor decreases, leading to muscle atrophy. When testosterone is absent, there is a decrease in the protein anabolic signaling pathway PI3K-Akt-mTORc1 [34,35]. Additionally, testosterone inhibits myostatin signaling, so in its absence, there is more myostatin-driven proteolysis and thus increased muscle atrophy [34,35]. Overall testosterone is necessary to promote muscle growth and maintenance and prevent muscle atrophy.

The drugs in this class use various mechanisms to target and prevent myostatin signaling with the goal of preserving lean body mass by inhibiting the catabolic effects of myostatin. Bimagrumab, trevogrumab, garetosmab, apitegromab, and landogrozumab are all promising, fully humanized monoclonal antibodies to various components of the myostatin–activin signaling pathway. RG6237 (also known as GYM329 or embrobart) is an antibody–antigen complex designed to bind latent myostatin, then, by binding the FcγRllb receptor, be internalized and degraded in a "sweeping" mechanism. Additionally, HS235 is a soluble receptor that binds and traps activin A.

By binding to activin type II receptors A and B (ActRIIA and ActRIIB), bimagrumab prevents binding of endogenous ligands myostatin, activin A, and growth differentiation factor 11 (GDF11), thus preventing activation of downstream Smad 2/3 muscle atrophy signaling. A phase II clinical trial with bimagrumab alone in patients with diabetes and a BMI between 28 and 40 showed positive outcomes [36]. Patients in the bimagrumab group had a decrease in total fat mass of 20.5% compared to just 0.5% in the placebo group, and all patients in the bimagrumab group lost at least 5% of total body fat mass [36]. The bimagrumab group also had a 3.6% increase in LBM compared to the placebo group, which had a −0.8% loss of LBM. The phase IIb BELIEVE trial looked at the effects of bimagrumab alone and in combination with semaglutide in adults with overweight or obesity [37]. When bimagrumab was used in combination with semaglutide, 92.8% of total weight loss was from fat mass compared to just 71.8% of weight loss being attributed to fat mass when semaglutide was used alone [37].

Trevogrumab and garetosmab target myostatin and activin A, respectively. While myostatin is an established negative muscle regulator, in experiments in which its receptor ActRIIB was blocked, muscle growth was greater than myostatin blockade. This suggests the presence of other growth factors signaling through ActRIIB also working to negatively regulate skeletal muscle. This additional molecule was later found to be activin A [38]. Figure 1 below depicts the mechanism of action of pharmacological agents acting via the myostatin/activin/ActRII pathway.

In preclinical studies, trevogrumab was shown to be specific for both the pro- and latent forms of myostatin with little cross reactivity to GDF-11, which is the closest cousin to myostatin [39]. Garetosmab showed high affinity and specificity for activin A without cross reactivity as well [38]. Phase II clinical trials have been conducted administering trevogrumab alone or in combination with garetosmab in patients taking semaglutide for weight loss [40]. The phase II COURAGE trial showed that in the triplet group taking trevogrumab and garetosmab with semaglutide, there was a 27.1% decrease in fat mass compared to 15.7% with semaglutide alone and only a 2.0% change in LBM compared to 6.5% change in LBM with semaglutide alone [41]. These data show that by targeting both myostatin and activin A, greater weight loss can be achieved while preserving a greater percentage of LBM.

Similarly to others in this class, RG6237/GYM329 (emubrobart) is a promising latent myostatin-specific antibody engineered to include a novel sweeping function to continually clear myostatin from the plasma and muscle tissue while recycling the antibody [42]. Currently, a phase II clinical trial is underway to assess RG6237/GYM329 in combination with tirzepatide in patients with obesity or overweight with at least one weight-related comorbidity [43].

Apitegromab/SRK-015 is an anti-pro/anti-latent myostatin biologic being explored as a potential treatment for the preservation of LBM in obese or overweight patients taking GLP-1 RAs [44]. Apitegromab binds to the pro domain of myostatin and prevents extracellular proteolytic cleavage and subsequent activation of pro- and latent myostatin, thus preventing its binding to ActRIIB and subsequent downstream negative regulation of skeletal muscle [45]. The phase II randomized, double-blind, placebo-controlled, proof-of-concept trial EMBRAZE tested apitegromab for the preservation of LBM during weight loss with tirzepatide in non-diabetic patients with overweight or obesity [44,46]. EMBRAZE showed a significant increase in LBM compared to tirzepatide alone, with a difference of 1.9 kg of LBM [46]. There was a 15.8% greater mass loss due to fat in the apitegromab and tirzepatide group compared to the tirzepatide-only group, with no significant difference in total weight loss during the 24-week trial, indicating higher-quality weight loss when apitegromab is given with tirzepatide [46].

Landogrozumab is a monoclonal antibody that neutralizes myostatin. While there are not any trials looking at landogrozumab in overweight and obesity or in conjunction with GLP-1 RAs or GIP/GLP-1 RAs, landogrozumab has been looked at in other conditions of muscle atrophy, including sarcopenia and frailty. Patients who were 75 or older and had a fall in the past year who were randomized to landogrozumab showed a 0.43 kg increase in appendicular LBM [47]. In addition to the increases in LBM, they showed that these increases led to significant functional improvements, as assessed via stair climbing time, chair rise with arms, and gait speed [47]. These results indicate that landogrozumab may be beneficial in increasing functional LBM in other conditions besides sarcopenia.

With myostatin's only known role being skeletal muscle atrophy, this pathway is a safe and effective target for the prevention of LBM loss with weight loss and other conditions of atrophy, with little risk of off-target effects. The promising results presented here show that the myostatin blockade can indeed preserve LBM in the setting of pharmacologic weight loss, which is of great importance and benefit to patients.

Drugs targeting myostatin are likely to continue to show benefits in preserving LBM during weight loss in future trials, which is of great interest for patients who experience substantial weight loss with GLP-1 RAs and GIP/GLP-1 RAs. Documented adverse events have been mild-to-moderate and include muscle spasms, diarrhea, nausea, and acne. These promising safety and tolerability profiles make them promising treatment options; however, these biologics are designed to be given as an IV infusion, thus having the downfall that patients go to a clinic for their treatment. Table 2 below shows the pharmacological agents in the class of the myostatin/activin/ActRII pathway, their downstream effects, and progress in research development, along with the area/subject they are being assessed for.

Enobosarm (GTx-024) is a selective androgen receptor modulator (SARM) that selectively targets muscle [48]. Designed to mimic testosterone, enobosarm acts to promote skeletal muscle growth and maintenance by acting through PI3K-Akt-mTORc1 anabolic signaling [48]. In the phase IIb QUALITY clinical trial, enobosarm's use was associated with a 71% reduction in LBM loss and a 27% increase in fat mass loss, when taken in combination with semaglutide [49]. In a maintenance extension of this phase IIb trial, it was seen that enobosarm use prevented more fat mass regain compared to the placebo [50]. With no serious adverse events reported and the most common mild adverse events being headache and back pain, enobosarm has a good safety profile [48]. Phase III clinical trials for cachexia and metastatic breast cancer have been performed but enobosarm has not yet received FDA approval. With its promising efficacy and safety, it is likely that enobosarm will continue to be researched for development in the arena of preservation of LBM loss during pharmacologic weight loss.

Tesamorelin (TH99507) is an FDA-approved growth hormone-releasing hormone (GHRH) analog. It promotes the release of growth hormone and leads to a subsequent increase in insulin-like growth factor 1 (IGF-1) [51]. Downstream IGF-1 signaling leads to increased activation of PI3K-Akt-mTORc1 anabolic signaling. Its approved indication is HIV (human immunodeficiency virus)-induced lipodystrophy, where it has been shown to reduce visceral and liver adipose tissue [51]. Tesamorelin has started being explored for use in overweight and obesity, insulin resistance, and nonalcoholic fatty liver disease [52]. It has been associated with an increase in abdominal muscle density and area along with a decrease in intramuscular and visceral adipose tissue [52]. Side effects are generally mild and include arthralgia, myalgia, and peripheral edema [51,52].

Anamorelin is a ghrelin agonist that is under phase III clinical investigation in patients with non-small cell lung cancer and cachexia. Ghrelin is known as the hunger hormone, as it is released from the empty stomach to reduce satiety signaling and promote feeding [53]. Once released, ghrelin binds to the growth hormone secretagogue receptor type 1a (GHS-R1a) in the hypothalamus, leading to the activation of NPY neurons, which promotes hunger [53]. In addition to promoting hunger, ghrelin signaling also promotes the release of growth hormone from the pituitary [53]. Once released, as previously discussed, growth hormone increases IGF-1 release via signaling through the insulin/IGF-1 pathway, promoting mTORc1 activation.

Phase III ROMANA 1 and 2 studies of anamorelin in patients with non-small cell lung cancer showed significant increases in LBM; however, muscle function did not improve, even with the increases in LBM [54]. Adverse side effects included hyperglycemia and diabetes [54]. While not yet approved by the FDA, anamorelin has been approved in Japan for use in improving cachexia. Anamorelin has not yet been explored for use in preserving LBM with weight loss.

Mecasermin (rhIGF-1) is a synthetic IGF-1 that signals through the IGF-1-mTORc1 pathway to promote LBM. Mecasermin was approved by the FDA in 2005 for the treatment of growth failure in children with IGF-1 deficiency and Rett Syndrome. Investigations showed that its use was associated with significant increases in LBM [55]. Adverse side effects of mecasermin include hypoglycemia, tissue overgrowth, and potential cancer. Investigations of mecasermin use with weight loss have not been performed; however, the risk of hypoglycemia may limit the use of mecasermin with GLP-1 RAs and GIP/GLP-1 RAs [55].

Tocilizumab is an FDA-approved monoclonal antibody that targets and blocks IL-6 receptor, which reduces downstream inflammatory signaling. As mentioned above, the presence of inflammatory signaling stops myogenic processes and activates muscle atrophy. Tocilizumab has been shown to increase LBM in trials for rheumatoid arthritis, where participants taking tocilizumab had increased appendicular LBM without changes noted in fat mass [56]. The increases seen in LBM points toward tocilizumab being a potential target agent to be used alongside GLP-1 RAs and GIP/GLP-1 RAs for preservation of LBM with weight loss; however, a trial has yet to be completed to investigate the potential outcome. Due to its inhibitory effect on inflammation, tocilizumab may be linked with an increased risk of infections. Potential for liver damage and changes in blood cell counts and cholesterol are all possible adverse effects as well.

Another potent target for reducing inflammation, which leads to muscle atrophy, is the blockade of tumor necrosis factor-alpha (TNF-α). Several TNF-α-blocking drugs have been approved by the FDA for various inflammatory and autoimmune conditions. These agents include etanercept, infliximab, adalimumab, and certolizumab. By blocking TNF-α-driven inflammatory signaling, myogenic signaling is preserved and muscle atrophy is diminished through decreased activation of NFκB. Following one year of treatment, patients with rheumatoid arthritis taking anti-TNF-α monoclonal antibodies showed increased total LBM, fat-free mass, and skeletal muscle mass with no changes in fat mass [57]. It was shown that the increases in LBM were associated with improved strength and walking [57]. While there have not been any direct studies performed combining anti-TNF-α agents with GLP-1 RAs or GIP/GLP-1 RAs, it has been shown that GLP-1 RAs alone seem to have some anti-inflammatory properties and they are associated with decreased levels of TNF-α [58]. This suggests that GLP-1 RAs or GIP/GLP-1 RAs and anti-TNF-α biologics may work synergistically together to induce weight loss and potentially spare LBM. Similarly to tocilizumab, other anti-TNF-α agents also have increased risk of infection, along with other side effects such as worsening of congestive heart failure and neurologic problems [59].

Table 3 below provides names, mechanisms, and the research progress for pharmacological agents targeting other muscle loss signaling pathways. Table 4 below lists key clinical trials that have been undertaken for the investigation of preservation of lean body mass with weight loss.