Novel Potential Targets for Function-Promoting Therapies: Orphan Nuclear Receptors, Anti-inflammatory Drugs, Troponin Activators, Mas Receptor Agonists, and Urolithin A

Abstract

In recent years, several new classes of therapies have been investigated with their potential for restoring or improving physical functioning in older adults. These have included Mas receptor agonists, regulators of mitophagy, skeletal muscle troponin activators, anti-inflammatory compounds, and targets of orphan nuclear receptors. The present article summarizes recent developments of the function-promoting effects of these exciting new compounds and shares relevant preclinical and clinical data related to their safety and efficacy. The development of novel compounds in this area is expanding and likely will need the advent of a new treatment paradigm for age-associated mobility loss and disability.

There is a strongly active search for new treatments that can improve physical function in older persons through different mechanisms. Beyond the steroidal and nonsteroidal selective androgen receptor (AR) modulators, a number of exciting novel compounds with potential function-promoting efficacy have been developed (Table 1). Most orphan nuclear receptors (ONRs) have structural features similar to steroid receptors but with unknown natural ligands. Their activation has major effects on skeletal muscle homeostasis, via direct and indirect actions.

Table 1.

List of Novel Candidate Function-Promoting Therapies Presented in This Review

Molecule	Molecular Target	Indication	Phase	Endpoint	Outcome/Status	ClinicalTrial.gov
BIO101	MAS receptor	Sarcopenia	2	400MWT gait speed	Completed	NCT03452488
Urolithin A	Mitophagy mitochondria dysfunction	Adults between 40 and 64 y of age, overweight, and having low physical endurance and of low mitochondrial function	NA	Aerobic endurance measures linked to endurance and peak power output (PPO)	Completed	NCT03464500
Canakinumab	Interleukin-1β	Patients with previous myocardial infarction and a high-sensitivity C-reactive protein level of 2 mg or more per liter	3	Analysis of core phase first CEC confirmed major adverse cardiovascular events (MACE)	Completed	NCT01327846
Reldesemtiv	Troponin activator	Amyotrophic lateral sclerosis (ALS); sarcopenia	2	Percent-predicted slow vital capacity (SVC)	Completed	NCT03160898

For example, orphan receptors such as peroxisome proliferator activator receptors (PPARs), estrogen-related receptors (ERRs), and Rev-erbs receptors and nuclear receptor 4 A subfamily (NR4As) are expressed in the myocytes, imposing transcriptional remodeling of muscle architecture and function.

Moreover, these receptors are also expressed in other resident nonmuscle cells such as macrophages, intramuscular adipocytes, and endothelial cells through which they can indirectly contribute to muscle function; however, this needs further exploration. Urolithin A (UA) is a naturally occurring molecule produced by the gut microbiome from ingested ellagitannins (ETs) and ellagic acid, complex polyphenols abundant in foods such as pomegranate, berries, and nuts. UA enhances cellular health by increasing mitophagy and mitochondrial function and reducing detrimental inflammation (1).

Modulators of the skeletal muscle troponin and tropomyosin complex including reldesemtiv, formerly known as CK-107, have been developed in the past years. Troponin and tropomyosin, the myosin motor, and the actin filaments are major elements of the sarcomere, the basic contractile unit of skeletal muscle. The troponin and tropomyosin complex governs the interaction between actin and myosin, rendering it sensitive to calcium. The development of muscle-type selective modulators of the troponin complex and tropomyosin stems from each muscle type having tissue-specific isoforms of the regulatory complex that can be leveraged to develop selective potential therapeutics.

The renin–angiotensin system (RAS) is made of several components including enzymes, peptides, and receptors organized in 2 main arms. The classical arm comprising angiotensin-converting enzyme (ACE), angiotensin II (AngII), and angiotensin receptor 1 (AT1R) elicits vasoconstriction, oxidative stress, fibrosis, and inflammation when activated, whereas countering functions are induced by the protective arm—including exerting cardioprotective effects—which is composed of ACE2, angiotensin 1–7 (Ang 1–7), and Mas1 receptor (MasR) pathway (2). MasR agonists, including BIO101, activate MasR and inhibit downstream signaling of the AT1 receptor resulting in the attenuation of muscle atrophy.

Orphan Nuclear Receptors as Function-Promoting Therapeutic Targets

Steroid hormones such as testosterone and estrogen have long been known to regulate skeletal muscle homeostasis (3). These steroids regulate myofiber type, metabolism, and mass ultimately affecting contractile function, exercise tolerance, strength, and mobility. Steroid hormones elicit their effects through gene regulation by activating nuclear receptors in the skeletal muscle (eg, AR and ERs). Nuclear receptors are transcriptional factors that have evolved to structurally contain unique ligand-binding pockets through which the activity of these receptors can be modulated (4). Other structural domains of the nuclear receptors are the N-terminal AF1, DNA-binding domain, hinge region, and C-terminal AF2 domain. Over the last decade, ONRs including PPARs, ERRs, and Rev-erbs receptors, and NR4As receptors (Table 2) have been found to be expressed in the skeletal muscle and have emerged as important players in the regulation of muscle function. The majority of the ONRs share the same structural features as steroid receptors; however, natural ligands for many of these receptors are unknown. PPARs, particularly PPARα and PPARδ, have been implicated in skeletal muscle glucose metabolism, fatty acid metabolism, and mitochondrial biogenesis, as well as myofiber type switch to an oxidative phenotype (5). Prototypical synthetic PPAR agonists have weight-reducing and antidiabetic effects in preclinical diabetes models, as well as performance-enhancing effects in exercise training (6). Similar to PPARs, ERRs regulate fatty acid oxidation, mitochondrial biogenesis, and oxidative phosphorylation, as well as oxidative myofiber type switch, leading to improved exercise endurance (7). In addition, ERRs promote muscle vascularization, which has implications for muscle ischemic disease (8). NR4As are highly inducible orphan receptors and regulate glucose metabolism, mitochondrial biogenesis, and fatty acid oxidation in the skeletal muscle (9). Rev-erbs act as transcriptional repressors and positively regulate fatty acid oxidation and mitochondrial respiration in the skeletal muscle. Rev-erb agonists have been shown to be effective in improving skeletal muscle metabolism, exercise endurance, and insulin sensitivity (10). Several orphans (eg, PPARδ, ERRγ, and Rev-erbs) also regulate muscle regeneration and have beneficial implications in muscular dystrophies (11). It is clear from these preclinical studies that ONRs warrant in-depth exploration as targets for function-promoting therapies in muscle diseases and aging. Several unexplored areas of research remain. Endogenous and/or synthetic ligands for most ONRs are unknown. Transcriptional mechanisms through which ONRs regulate gene expression, as well as upstream signaling pathways impinging on ONRs, remain unexplored. ONR signaling for the most part has not been investigated in aging, where there is a major decline in skeletal muscle capacity and mass. Several other orphans are expressed in the skeletal muscle but remain to be fully investigated. One of the major strengths of advancing this area of muscle biology research is the potential for developing pharmacological interventions of promoting muscle function. Due to the infancy of this field, a major current weakness, as described earlier, is underdeveloped genetic and chemical tools for targeting orphan receptors in the skeletal muscle. For example, most orphans currently lack well-developed pharmacological modulators to appraise the ultimate translation value of the receptors. However, the future remains exciting, as new studies should expand our understanding of ONRs’ impact on skeletal muscle health and disease.

Table 2.

ONRs in Skeletal Muscle

ONR	Targeting Strategies	Phenotype	Disease Models
PPARs	SkM-OE, SkM-KO, sc-KO, and pharmacological agonists	Fatty acid oxidation, glucose metabolism, mitochondrial biogenesis, fiber type switch, regeneration, size, exercise tolerance, and insulin/glucose tolerance	DIO, DMD, PAD, and atrophy/wasting
Rev-Erbs	SkM-KO and pharmacological antagonists	Fatty acid oxidation, glucose metabolism, mitochondrial biogenesis, myofiber type switch, regeneration, size, and insulin/glucose tolerance	DIO, DMD, and atrophy/ wasting
NR4As	SkM-OE, SkM-KO, and global KO	Oxidative metabolism, glucose metabolism, and myofiber type switch	DIO
ERRs	SkM-OE, global KO, and SkM-KO	Fatty acid oxidation, mitochondrial biogenesis, myofiber type switch, and muscle vascularization	DMD and PAD

Notes: Several ONRs have now been studied in the skeletal muscle. Due to the lack of endogenous ligands, various strategies such as SkM-OE and SkM-KO, sc-KO, global knockout, and pharmacological manipulation have been used. ONRs are generally implicated in regulating skeletal muscle metabolism, vascularization, and regeneration, and have been studied in models of DIO, muscular dystrophies (DMD), PAD, and muscle wasting. DIO = diet-induced obesity; DMD = Duchenne muscular dystrophy; ERR = estrogen-related receptor; NR4A = nuclear receptor 4 A subfamily; ONR = orphan nuclear receptor; PAD = peripheral arterial disease; PPAR = peroxisome proliferator activator receptor; sc-KO = satellite-cell-specific knockout; SkM-OE = skeletal-muscle-specific overexpression; SkM-KO = skeletal-muscle-specific knockout.

Urolithin A: A Natural Mitophagy Activator to Boost Mitochondrial Health and Address 
Age-Related Muscle Wasting

UA is a postbiotic compound produced by the gut microflora. UA is not directly available through the foods we eat, instead, our gut microbiome converts dietary polyphenolic compounds called ETs found in abundance in fruits and nuts such as pomegranates, berries, walnuts, and pecans into UA. However, research has demonstrated that only about one third (~30%–40%) of the adult human population has the ability to convert these dietary precursor molecules into UA (12).

Mitochondrial dysfunction is a key hallmark of aging, and mitophagy plays a key role in this context. Over time, mitochondria accumulate free radicals and get damaged as the pathway of mitophagy declines. The body cannot sufficiently clean up the poorly functioning mitochondria, which leads to inflammation, and in turn, a lack of muscle strength and energetics. A number of studies have associated impaired mitochondrial function with a decline in walking speed, muscle fatigue, loss of strength, and ultimately, the development of age-related muscle decline manifesting as sarcopenia and frailty (13). The rate of mitophagy also declines with aging and this has been linked to several other age-related disorders such as Parkinson’s and Alzheimer’s diseases (14). Improving mitochondrial health by boosting mitophagy is, therefore, a viable strategy to improve muscle health with aging. UA has been shown to activate mitophagy in skeletal muscle and affect cellular and mitochondrial health.

Oral intervention studies with UA have shown that nematode worms live longer, and aged rodents show increased endurance and grip strength (15). UA affects not only muscle but also other tissues that have abundant mitochondria, such as the brain. In addition, mitophagy activation by UA in different cell types such as immune cells and chondrocytes shows a global impact on mitochondrial health via mitophagy activation (16,17). A recent paper provided evidence that UA reduces inflammation and amyloid-beta aggregation, thereby improving cognitive function in mouse models of Alzheimer’s disease (18). How biological pathways that link mitophagy activation and dampening of inflammatory responses interact together in both preclinical models and clinical studies via UA is an area that needs further exploration.

Clinical data (Table 3) from recent randomized controlled trials (12,19) with a purified form of UA (Mitopure) have shown early promising signals of improvement on endpoints linked to physical performance (peak VO₂, 6-minute walking distance), muscle function (muscle strength and resistance-to-fatigue), and circulating biomarkers of inflammation (C-reactive protein [CRP] and proinflammatory cytokines). Future, well-powered clinical studies are required to further confirm these effects in relevant study in older impaired populations.

Table 3.

Key Studies on Muscle Health with UA in the Relevant Preclinical Models of Muscle Wasting

Study	Model/Population	Dose of UA/Length of Administration	Outcome
Aged rodent model (15)	16 mo old fed a high-fat diet 22 mo old	50 mg/kg/d of UA for 8 mo 50 mg/kg/d of UA for 6 wk	Improved endurance (~57% improvement) and higher grip strength (~9%) >42% endurance
DMD rodent animal model (18)	3-wk-old mdx mice	50 mg/kg/d of UA for 10 wk	Increased grip strength, muscle force, and running endurance
RCT—Sedentary older adults (20)	>60+ y of age, low physical activity levels	500 mg and 1 000 mg of UA, 4 wk of oral administration	Safe, bioavailable, improved cellular (acylcarnitines), and mitochondrial health in muscle (increase in mitophagy and biogenesis gene expression)
RCT—Healthy older adults with low-average mitochondrial function (19)	40–64 y of age, BMI ~29 kg/ m2, VO2max~24 mL/kg/min	1 000 mg of UA, 2 and 4 mo of oral administration	~20% improvement in muscle endurance at 2 mo in hand and leg muscle function tests (resistance- to-fatigue), lower CRP, and lower ceramides and acylcarnitines

Notes: Completed randomized, placebo-controlled clinical studies in different populations and their key findings are summarized. CRP = C-reactive protein; DMD = Duchenne muscular dystrophy; RCT = randomized controlled trial; UA = Urolithin A.

Targeting Inflammation to Improve Health in Older Persons

Whether blocking inflammation prevents decline or improves physical function in older persons has not been determined. Several strategies are theoretically available that can be used to reduce inflammation. Nonsteroid anti-inflammatory drugs actively modulate main inflammatory pathways; treatment of traumatic brain injury with melatonin that enhances the clearance of impaired mitochondria via mitophagy appears to reduce inflammation; the elimination of senescent cells through senolytics prevents mt-DNA-induced inflammation and promotes the survival of aged organs following transplantation (21–23). However, no properly sized randomized controlled trials definitively established whether reducing inflammation positively affects health and physical function in middle-aged and older individuals.

The epidemiological evidence that the proinflammatory state of aging is prospectively associated with the development of chronic medical conditions, disability, and frailty is strong and confirmed by multiple studies performed in several large populations.

In the established population for epidemiological studies in the elderly, older persons with high interleukin-6 (IL-6) serum levels showed a higher risk of developing mobility disability and disability in activities of daily living. In a cohort study of older women (Women’s Health and Aging Study) with moderate to severe disability and for whom information on baseline IL-6 serum level was available, IL-6 level was found to cross-sectionally associate with lower walking speed and with future accelerated decline of walking speed over a 3-year follow-up. Interestingly, the decline in mobility was explained in part by the parallel decline of muscle strength, suggesting that the effect of IL-6 is mediated by sarcopenia. There is also evidence that IL-6 level is associated with higher multimorbidity and accelerated decline of multimorbidity over time. These are just a few examples of the enormous literature that connects inflammation with several aspects of accelerated health decline with aging. From these data, it is reasonable to hypothesize that if inflammation is effectively targeted through interventions that selectively modulate its deleterious effect while maintaining its role in defense, repair, and regeneration, it would result in the expansion of health span and preservation of functional status in old age.

Several attempts to reduce inflammation through the administration of aspirin, sodium salicylate, losartan, and fish oil have not been successful, although it should be noticed that many of these studies were conducted in relatively small populations (24).

The Canakinumab Anti-inflammatory Thrombosis Outcome Study (CANTOS) trial remains the strongest example that demonstrated that reducing inflammation may prevent “hard” medical outcomes. In particular, the CANTOS demonstrated that blocking interleukin-1 by the monoclonal antibody Canakinumab significantly reduced cardiovascular events in individuals with prior history of myocardial infarction and negative cardiovascular risk profile. The effect was found to be much superior among those who, after receipt of the first dose, exhibited a decline in IL-6 and CRP. In reanalyses of the CANTOS database, Canakinumab has also been found to reduce hospitalizations for heart failure and incidence of anemia, as well as to affect insulin resistance (25). Colchicine is another drug with anti-inflammatory properties that was effective in the secondary prevention of new CVD events in patients with coronary artery disease and myocardial infarction (26,27). Whether colchicine may prevent other noncardiovascular outcomes is unknown.

Canakinumab is not the only monoclonal antibody that can modulate inflammation. Tumor necrosis factor (TNF) alpha is a trigger to nuclear factor kappa B (NF-κB), functioning as the gate to all the proinflammatory cytokines and many of the inflammatory signals. There is a strong rationale to hypothesize that blocking TNF-alpha may improve muscle function and mobility, especially in frail older persons who characteristically have an inflammatory component. In mice, pharmacological TNFα blockade with weekly subcutaneous injections of etanercept from 16 to 28 months of age prevented atrophy and loss of type II fibers and led to significant improvements in muscle function and life span. The investigators suggested that such results are evidence of proof of principle that endogenous TNFα is sufficient to cause sarcopenia and to reduce animal survival and open a new perspective on novel potential pharmacological treatment strategies based on TNFα blockade to prevent the harmful events associated with aging (28). Unfortunately, etanercept or other monoclonal antibodies that selectively block major inflammatory mediators, such as tocilizumab, sarilumab, and siltuximab that interfere with IL-6 signaling; anakinra and Rilonacept that block IL-1 signaling; and Infliximab and Certolizumab pegol that block TNF-alpha signaling, just to mention a few that show effectiveness in overt inflammatory diseases, have never been tested for their effectiveness to improve health and function in the older population.

It is important to recognize that “treating” inflammation is intrinsically complex. Because inflammation is a defense reaction elicited by either an attack (eg, viruses and bacteria) or a damaged structure (that needs to be eliminated or repaired), anti-inflammatory drugs are likely to have unwanted consequences. This is especially true if inflammation is blocked at the hubs of these important defensive mechanisms, as strategy that is shared by many of the agents that are used nowadays and that were developed to fight overt inflammatory conditions. However, investigating the specific causes and mechanisms that trigger inflammaging may provide clues about new most effective targets that are less prone to unwanted effects (29). Recent studies point to the effect of mitochondria, including mitochondria’s role in regulating the innate immune system, the mechanisms linking mitochondrial quality control to age-dependent pathology, and the possibility that mitochondrial-to-nuclear signaling might regulate the rate of aging. Further investigation is needed, not only to better ascertain the role of mitochondria in causing the proinflammatory state of aging but also to dissect mitochondrial signaling pathways that are amenable to therapeutic manipulation (30).

Another promising approach is the interference with cellular senescence, either by eliminating them through senolytics or by blocking the production of the senescence-associated secretory phenotypes. There is evidence that the accumulation of senescent cells in tissues is a major cause of inflammaging. In the animal model, senolytics like dasatinib and quercetin were able to not only remove senescent cells but also reduce the increase in the levels of systemic cytokines in the muscle and in other tissues (31). The numerous ongoing clinical trials in place should provide answers to this hypothesis.

Fast Skeletal Muscle Troponin Activation as a Potential Therapeutic Approach for Neuromuscular and Non-neuromuscular Skeletal Muscle Dysfunction

The sarcomere occupies much of the volume of skeletal muscle. The sarcomere consists of the myosin motor and actin filaments, plus a regulatory complex that governs the interaction between actin and myosin, rendering it sensitive to calcium. This complex consists of the troponin complex and tropomyosin. Cytokinetics has developed selective modulators of this structure based on the fact that each of the components of the regulatory complex has tissue-specific isoforms that can be leveraged to develop selective potential therapeutics.

Reldesemtiv, formerly known as CK-107, is a selective fast skeletal troponin activator (FSTA) that leverages the fact that all the subunits of troponin in fast skeletal muscle are unique to fast skeletal muscle fibers. This molecule’s main pharmacologic activity in terms of how it changes sarcomere function is to increase the affinity of troponin for calcium, resulting in calcium sensitization. This increase in calcium sensitivity translates into 3 properties in preclinical models of muscle function (nerve to rat extensor digitorum longus stimulated in situ). Reldesemtiv amplifies the response motor neuron input. Neuromuscular input causes calcium release in the muscle, and the faster the nerves fire, the more calcium is released. As reldesemtiv makes the sarcomere more sensitive to calcium, its response to calcium is amplified. What naturally follows is a shifting in the force–frequency relationship, with more force produced at lower stimulation frequencies in the rat model. As the nerves innervating the neuromuscular junction can operate at lower firing frequency and the muscle can operate at lower cytosolic calcium concentrations to produce the same amount of force, there is less run down of the nerve and muscle with repetitive stimulation which leads to an increase in muscle power and improvement in muscle endurance. There is no direct effect of skeletal troponin activators on the nerve, but the mechanism of action is energetically favorable to both the nerve and the muscle, which may be of consequence when the health of either the nerve or the muscle is compromised.

Amyotrophic lateral sclerosis (ALS) is a disease where neuromuscular input diminishes over time for reasons that are still not well understood. Rather than focusing on the nerve-related aspects of the disease, researchers investigated whether a muscle-based therapeutic might impact the course of ALS. The Phase 2 study called FORTITUDE-ALS was a multicenter, double-blind, randomized, dose-ranging, placebo-controlled study to evaluate efficacy, safety, and tolerability of reldesemtiv in patients with ALS. The key efficacy endpoints looked at the change in percent-predicted vital capacity from baseline to week 12, a change from baseline to week 12 in the ALS functional rating scale-revised (ALSFRS), the slope of muscle strength mega-score from baseline to week 12, and safety and pharmacokinetics.

Results of FORTITUDE-ALS include:

1. Patients on all doses of reldesemtiv declined less than patients on placebo for slow vital capacity (SVC) and ALSFRS-R, with larger differences emerging over time.

2. Reldesemtiv showed potentially meaningful effects in reducing the decline of SVC (27%, p = .1) and ALSFRS-R (25%, p = .01) for active groups combined versus placebo.

3. The incidences of early treatment discontinuations, serious adverse events, and clinical adverse events were similar between placebo and active arms.

The results support the investigation of reldesemtiv in a pivotal Phase 3 clinical trial underway called COURAGE-ALS. This ongoing trial will enroll 555 patients, with the treatment arm of reldesemtiv being dosed at 300 mg twice a day for up to a year and the placebo arm for 24 weeks, with transition to reldesemtiv for an additional 24 weeks. The primary endpoint is the change in the ALSFRS-R from baseline to week 24.

ALS and the loss of muscle mass with aging, which is largely due to the progressive loss of motor neurons, are associated with reductions in the number and size of muscle fibers. One of the challenges in ALS and sarcopenia is the lack of functional measures affected by therapeutics. It is indeed a challenge to measure changes in voluntary muscle function in a small number of people in Phase 2 that would allow progression into a larger Phase 3 trial.

The potential therapeutic applications of FSTAs are conditions that might benefit from sensitization of the muscle to nerve stimulation to increase the force at submaximal forces, such as ALS, SMA, and myasthenia gravis. Conditions that might benefit from the increase in submaximal shortening velocity and power, include multiple conditions that result in weakness and frailty. All these conditions may benefit from the improvement in muscle fatigability often observed in atrophied and hypoxia muscle. The challenge in these varied conditions is the lack of appropriate endpoints and predictive models to guide clinical development.

Mas Receptor Agonists for Age-Related Mobility Limitation

The RAS is a complex system composed of bioactive peptides, enzymes, and receptors organized in 2 main axes. The classical axis with ACE, AngII, and AT1R elicits vasoconstriction, oxidative stress, fibrosis, and inflammation when activated, whereas countering functions—including exerting cardioprotective effects—are linked to ACE2, Ang 1–7, and MasR protective axis (2,32). Active classical RAS axis has deleterious effects on the skeletal muscle; therefore, its inhibition has been important in the treatment of several pathologies affecting the skeletal muscle (eg, insulin resistance, muscle atrophy, and fibrosis (33)).

Skeletal muscle can be severely affected by RAS dysregulation as a result of chronic pathologies or through infections (eg, with severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2]) that could induce loss of muscle mass, strength, and physical function, which may delay and interfere with the recovery process of patients with COVID-19 (34). Modulating MasR with Ang1-7 pharmacological approach could protect skeletal muscles against AngII-induced muscle wasting through the activation of the nonclassical pathway of RAS signaling (35,36) Ang1–7 activates MasR signaling and protects against AngII-induced muscle atrophy (37). Moreover, skeletal muscle function and tissue levels of Ang1–7 decrease with age in mice (38) concomitant with an enhancement in AngII concentration suggesting a development of RAS system imbalance in skeletal muscles as mice age. These changes cause the decreased performance of the protective ACE2/Ang1–7 axis and an increase in the deleterious activity of the ACE/AngII pathway. Despite evidence in animal studies, strong confirmation that infusion of Ang1–7 can protect against aging-induced mobility limitation and muscle wasting in humans is still lacking.

Indeed, the RAS is a relevant target for age-related mobility limitations because hypertension and sarcopenia are commonly seen in older. An imbalance in RAS may contribute to the development of both diseases (39). Moreover, in older adults with hypertension, sarcopenia is less prevalent (p = .020) in patients taking ACE inhibitors (9.1%) compared to users of angiotensin receptor blockers (ARBs, 40.5%) or other antihypertensive drugs (42.9%) (39).

Early clinical studies conducted with 130–1 268 older patients (Table 4) have suggested that ACE inhibitors (ACEi) use was associated with improved muscle strength and physical function (walking distance/speed or short physical performance battery [SPPB]), and with positive effects in sarcopenia (39). Other studies with 639 and 257 older patients on grip strength and physical function (SPPB and gait speed) have not supported this association (40,41) and even, 2 studies conducted with 281 patients (42,43) have shown an association between ACEi use and reduced physical functions (isokinetic knee extensor strength, hand grip strength, 8 and 20 ft walk tests, and self-reported physical activity engagement).

Table 4.

Key Clinical Studies Targeting the RAS

References	Intervention/Observation	Patients	Outcome
Onder et al. (44)	ACEi continuous vs intermittent or nonusers	641 women with hypertension 77–80 y old	ACEi group continuously had a lower mean 3-y decline in muscle strength (−1 kg) and in walking speed (−1.7 cm/s)
Buford et al. (45)	ACEi vs other antihypertensive drug, vs nonusers under physical activity (PA) or successful aging (SA)	424 older patients (70–89 y old) With mild or moderate functional impairment	PA significantly improved the adjusted walking speed of ACEi users (p <.001) but did not of nonusers.PA improved the adjusted SPPB score of ACEi users
Sumukadas et al. (46)	Perindopril vs placebo	130 olderpatients (mean 78.7 y old) with problems in mobility or functional activities of daily living	Improved 6-min walking distance in ACE inhibitors group: +31.4 m
Coelho et al. (47)	ACEi and ARBs vs placebo	407 older adults (mean age of 69.6 y old)	ACEi and ARBs groupsshowed higher predicted values in the 6MWT than control. ACEi and ARBs groups showed higher predicted values of musclestrength than control group patients (mean: 98.15% ± 18.77%)
Di Bari et al. (48)	ACEi vs other antihypertensive drugs and nonusers	2 431 olderwomen (mean 73 y old) cross-sectional analysis of data from the health, aging, and body composition (Health ABC) Study	ACEi associated cross-sectionally with larger low extremity muscle mass
Nguyen et al. (49)	ACEi, ARB, and other antihypertensive drugs	1 268 olderparticipants (mean age 62.7–67.8 y old) in the Singapore Longitudinal Ageing Study (SLAS-2)	The use of ARBs is associated with a reduction in frailty and age-related loss of muscle mass and strength MMSz-scores: 0.329 vs 0.076, p= .022

Notes: Interventional and observational clinical studies using antihypertensive drugs targeting the RAS and evaluating physical function in older patients. ACE = angiotensin-converting enzyme; ACEi = ACE inhibitors; AngII = angiotensin II; ARB = angiotensin receptor blocker; RAS = renin–angiotensin system.

ARBs showed a reduction in frailty and age-related loss of muscle mass and strength (49) conducted on 1 268 older patients. However, in a Phase 2 trial (ENERGISE; NCT02676466) losartan treatment failed to prevent mobility loss in 289 older adults with low-grade inflammation and mobility limitations (50). Likewise, losartan (Phase 2 trial: NCT01989793) did not show a difference between treatment and placebo in preventing muscle strength loss associated with aging (51) in 37 older adults, although such a small sample size may indicate underpowered trial.

These results confirmed that, despite many clinical studies conducted using ACEi or ARB, there are still conflicting results on preventing age-related mobility disability or muscle strength loss. Differences in sarcopenia or age-limited mobility definitions and/or heterogeneity of hypertension treatment render the effects of antihypertensive drugs on sarcopenia difficult to assess.

Further longitudinal studies are needed to understand the effects of RAS activation treatments in age-related sarcopenia. However, there is still room for investigating ACEi, ARBs, and MasR agonists for the treatment of age-related mobility limitations including sarcopenia. Ang1–7 is the endogenous ligand of MasR (52) and the activation of its receptor inhibits downstream signaling of the AT1 receptor resulting in the attenuation of muscle atrophy. BIO101 is an oral investigational drug with 20-hydroxyecdysone (20E) as the active pharmaceutical ingredient. 20E is a polyhydroxylated phytosteroid pharmacologically active in mammals and belonging to a group of structurally related compounds (ecdysteroids), of which more than 500 members have been described (53). BIO101 does not bind to AR but activates MasR as demonstrated using pharmacological (antagonists) and siRNA approaches in a mouse myoblast cell line (C2C12) using the inhibition of myostatin (a negative regulator of muscle growth) gene expression as a biomarker of anabolic activity (54). BIO101 effect was demonstrated in an aged mice model after 14-week treatment, by reducing the significant age-related loss of running velocity (55). The remarkable safety profile of BIO101 was assessed in the Phase 1 study on healthy young and older volunteers at doses up to 1 400 mg/d. This allowed to select doses subsequently tested in sarcopenic older patients in the SARA-INT Phase 2 clinical trial that was aimed at evaluating the efficacy on mobility and safety (ClinicalTrials.gov: NCT03452488). The study was recently completed and showed very promising results on the 400 Meter Walk Test (400MWT) gait speed as the primary endpoint. These results are paving the way for a Phase 3 study in sarcopenic patients.

The main strength of this section is that by providing state-of-the-art clinical studies targeting RAS agonists, it unravels the complexity of obtaining a unique view on MasR agonists effects on mobility limitations.

A major limitation is the lack of studies on MasR agonists in patients suffering from sarcopenia and hypertension, to evaluate the link between both indications. Indeed, MasR agonists and other antihypertensive products were first developed in hypertension but subsequently tested in age-related physical function limitation.

Overall, it is very difficult to assess the effect in age-related mobility limitation given the diversity of products targeting the RAS (ACEi; ARBs; alpha/beta blockers, diuretics, and calcium channel blockers) and their often combination in the clinical studies.

Summary and Conclusion

As summarized in Figure 1, the RAS ligands for classical or protective arms interact with their receptors to trigger downstream pathways for protein degradation/atrophy gene activation or protein synthesis/myogenic gene activation, respectively (51,56). Likewise, specific ligands elicit their effects through gene regulation by activating ONRs, leading to an increase in skeletal muscle protein synthesis. UA activates mitophagy in skeletal muscle as a process to improve cellular and mitochondrial health and therefore muscle health during aging.

Figure 1.

Main interactions between ligands and receptors as well as cell signaling pathway for potential targets of function-promoting therapies. Adapted from literature (51,56,57). ACEi = angiotensin-converting enzyme inhibitor; Ang1 = angiotensin 1; AngII = angiotensin 2; Ang1–7 = angiotensin 1–7; AT1 = angiotensin II receptor type 1; ONR = orphan nuclear receptors.

There are multiple exciting and novel therapeutic pathways that are leading to the development of compounds targeting functional limitations in older adults. The recent past has seen an emergence of less traditional candidate pathways for intervention as diverse as skeletal muscle excitation–contraction coupling, mitophagy, Mas receptor agonism, and many more. With the continued interest in the development of therapeutic targets aimed at restoring mobility and improving physical functioning in older adults, novel target identification is essential. The target pathways presented here are probably only the tip of the iceberg and other targets may emerge. All molecules listed in this review including UA (NCT02655393), BIO101 (NCT03452488), reldesemtiv (NCT03160898), and canakinumab (NCT00900146, NCT01068860, NCT00900146, EUDRACT No. 2007-003729-26) did show good safety profiles in Phase 1/2 studies. We remain in the very early days in the identification of appropriate therapeutic targets in aging otherwise pathologically affected skeletal muscle.

However, there is more consensus now regarding participant selection, uniformity of outcomes chosen, and a therapeutic area, especially with the recent multicomponent interventional studies and recommendations of consortia in sarcopenia field (SPRINTT study (58), ESCEO (59), and SDOC (60)). Indeed, clinical trials of function-promoting therapies in older adults continue to mature and the future of discovery is very promising.

Funding

R.A.F. was supported by the Boston Claude Pepper Older Americans Independence Center (P30AG031679) and the U.S. Department of Agriculture, under agreement No. 58-1950-4-003.

This supplement is sponsored by the National Institute on Aging at the National Institutes of Health.

Any opinions or recommendations expressed in this publication are those of the author (s) and do not necessarily reflect the views of the National Institute on Aging, National Institutes of Health, or the U.S. Department of Health and Human Services and the U.S. Department of Agriculture.

Conflict of Interest

None declared.