RAD140 (CAS 1182367-47-0), also incorrectly referred to by invented names “testolone” or “radarine,” is a SARM (selective androgen receptor modulator) first characterized in 2010 and currently under development by Radius Health, Inc. for possible applications including: neuroprotective applications in Alzheimer’s disease or other degenerative CNS conditions; as a treatment for breast cancer; as a treatment for wasting states including HIV/AIDS and cancer cachexia; as an adjunct treatment for prostate cancer and other related pathology of the prostate; as an osteoanabolic agent in osteoporosis and a muscle-anabolic agent in the elderly experiencing sarcopenia (aging-related loss of muscle mass); as an alternative to testosterone replacement therapy for aging men; and other possible applications including male contraception alone or alongside exogenous testosterone or other anabolic-androgenic steroids. Like other anabolic agents, SARMs including RAD140 are considered a possible “adverse finding” on doping tests, reflecting their perceived utility for those athletes seeking ever-more-potent performance-enhancing drugs (PEDs).[a][a1][b][c][c1][d][e]
This educational paper is presented as a digest of high-quality research, with follow-up commentary and synthesis of multiple perspectives. The paper concerns itself with defining, and providing physiological and pharmacological/medical context for current and future development of SARMs, mainly RAD140, as well as comparing and contrasting SARMs/RAD140 with alternatives (mainly anabolic-androgenic steroids or AAS), and examining the preliminary/preclinical data and promising or possible future directions for SARM development.
A number of key unique features of SARMs are highlighted and discussed such as: favorable safety profile (especially compared to traditional anabolic-androgenic agents and in light of the potent effects of muscle-building and increases in muscular strength), favorable effects on the central nervous system, benign/neutral or even pro-safety effects on the prostate, and SARMs as a promising alternative to drugs like testosterone in multiple applications.
SARMs exert a wide variety of physiological effects, but most such effects are through AR-involved pathways (or very weakly through other steroid receptor pathways).
The development and potential clinical use of tissue-selective androgen receptor modulators (SARMs) have advanced tremendously over the past few years. A key aspect of SARMs is the ability to clearly differentiate between the anabolic and androgenic activities. SARMs provide therapeutic opportunities in a variety of diseases, including muscle wasting associated with burns, cancer, end-stage renal disease, osteoporosis, frailty and hypogonadism.[a1]
The concept of a tissue-selective AR modulator (SARM) was first documented in 1999. An ideal SARM is expected to have: 1) high specificity for the AR; 2) desirable oral bioavailability and pharmacokinetic profile; and, most importantly, 3) desirable, tissue-selective pharmacological activities. The major discriminating criterion is tissue selectivity of the ligand in vivo. With much improved tissue selectivity, these ligands should allow previously untenable therapeutic applications. For example, anabolic androgens [(meaning SARMs)] could be used for the treatment of osteoporosis, muscle wasting conditions such as frailty and those caused by severe burn injury, cancer, and end-stage renal disease and AIDS. They could also be used for hormone replacement therapy in elderly men, and even in women, without concerns related to virilizing effects. On the other hand, tissue-selective antiandrogens could be advantageous for the treatment of BPH and prostate cancer, by specifically blocking androgenic actions in the prostate without abolishing needed effects on muscle, bone, or libido.(emphasis added)[c1]
Overall, the discovery and development of SARMs is at an early stage, with many compounds still under preclinical development and a handful now completing either phase I or phase II clinical trials. No SARM has entered the market to date. Most of the SARMs identified to date are nonsteroidal anabolic agents, with the first generation [aryl propionamide and quinoline analogs] reported in 1998. The aryl propionamide SARMs were the first to demonstrate tissue selectivity in vivo in 2003, followed later that year by a tetrahydroquinoline (THQ) SARM, quinoline SARM in 2006, and hydantoin SARM in 2007. All of these anabolic SARMs demonstrate some degree of tissue selectivity in castrated animal models, with stronger agonist activities in anabolic tissues (e.g., levator ani muscle) than in androgenic tissues (e.g., prostate).
[A potent SARM] also potently suppresses serum levels of luteinizing hormone (LH), the gonadotropin responsible for stimulating testicular production of testosterone…. [per these 2007 preliminary findings] the ED50 for BMS-564929 for LH suppression (0.008 mg/kg) was significantly lower than the doses required for 100% muscle stimulation (0.1 mg/kg), indicating that profound suppression of endogenous LH levels and testosterone production occurs within the range of doses required for anabolic activity. Selectivity with regard to gonadotropin suppression represents a significant barrier to the clinical use of SARMs. Clinical studies...will undoubtedly need to address this issue in humans, particularly as related to its intended use in the treatment of andropause and age-related functional decline, where further declines in endogenous testosterone levels owing to drug treatment would likely be deemed clinically unacceptable.(emphasis added)[c1]
Further complicating our understanding of the origin of SARM selectivity is the “bio-amplification” of the primary endogenous androgen testosterone. Interestingly, the endogenously produced and very important androgen testosterone serves as a type of “anti-SARM” or “inverse SARM” because its androgenic activity is increased by conversion to the more potent 5α-dihydrotestosterone by the 5α-reductase enzyme in certain tissues including the scalp and prostate (but not in muscle or bone)[*]. As a result, androgens that do not undergo such bioamplification in the prostate will demonstrate improved selectivity regarding muscle vs prostate when compared to a testosterone-treated control or an intact animal whose primary endogenous androgen is testosterone More broadly put, one might appreciate that metabolic differences between endogenous androgens such as testosterone or dihydrotestosterone and SARMs can also vouch for at least some selectivity differences.[a]
Perspectives differ considerably within published literature as to the role of the receptor and compound, etc, in enabling relative potency differences in different tissues.
*Several models have been invoked to explain the role of DHT in men. [First is the] widely held view...that conversion to DHT is obligatory for mediating testosterone's effects in some tissues with high 5α-reductase activity, such as prostate and skin, but not in others, such as skeletal muscle and bone. [Second] it is possible that conversion of testosterone to DHT is not obligatory, but that it amplifies the effects of testosterone in tissues with high 5α-reductase activity such as the prostate and skin, but not in tissues with low 5α-reductase activity such as skeletal muscle and bone. [And] a third possibility is that 5α-reduction of testosterone is not obligatory for mediating its effects in any tissue in men, but that testosterone and DHT can both exert androgenic effects in all androgen-sensitive tissues, and their relative effects in any tissue are contingent upon their relative concentrations and potency.
...Our data, when viewed together with published literature on the effects of 5α-reductase inhibitors in patients with benign prostatic hyperplasia, are the most consistent with the second model in which DHT amplifies the effects of testosterone in tissues with high 5α-reductase activity. ...
...steroid 5α-reductase plays an essential role in the development of prostate and phallus by providing local amplification of an androgenic signal without systemic hyperandrogenemia during critical periods of sexual differentiation, illustrating nature's extraordinary ingenuity in creating mechanisms for tissue-selective amplification during development. We speculate that in adult men, in whom this tissue-specific amplification is not essential because the circulating testosterone concentrations are substantially higher than those in the fetus, testosterone and DHT can interchangeably subserve many androgenic functions[**]
SARMs will likely enable a better understanding of selectivity, bio-amplification, and tissue-specificity of anabolic-androgenic compounds.
The androgen receptor (AR) is a member of the steroid hormone nuclear receptor superfamily that includes estrogen, progestin, glucocorticoid and mineralocorticoid receptors. The binding of the prototypical, endogeneously[sic] produced androgen testosterone and the important active metabolite dihydrotestosterone to AR initiates a remarkably diverse array of biological activities that can vary according to a subject’s sex, age and hormonal status.[a]
Here, “prototypical” testosterone and DHT’s actions at the AR are provided as an example that even relatively mundane endogenous steroids “initiate a remarkably diverse array…” of activities.
Further factors in tissue specificity: differential tissue distribution, tissue-specific expression of 5alpha-reductase, ligand-specific regulation of gene expression and AR interactions with tissue-specific coactivators[a].
The activity of AR is critical to normal human sexual development and function, but beyond this signature role, AR activation also has important effects on diverse targets such as bone, liver, muscle and the central nervous system. The therapeutic potential of androgen signaling is well-appreciated in the medicinal chemistry community, and for quite some time, chemists have sought compounds that selectively stimulate muscle and bone growth while minimizing the proliferative and/or hypertrophic effects on sex tissues such as the prostate in males and clitoris in females. Such compounds have been termed selective androgen receptor modulators or SARMs. In this regard, the prototypical and endogenous androgen, testosterone, is considered to be a logical benchmark comparator. [a]
The possibility of obtaining compounds having tissue-selective activities that are different from that of the endogenous benchmark testosterone might derive from the fact that typical AR receptor activation, which is initiated by the binding of a molecule with affinity for the AR to the AR ligand binding domain, is then followed by a rather remarkable, coordinated series of interactions:
These may include a change in receptor topology, dissociation of heat shock proteins, receptor dimerization, receptor phosphorylation, rapid-signaling events, translocation to the nucleus (AR), association with many different coregulatory proteins to form a transcriptional complex that results in the activation or suppression of RNA synthesis from AR-modulated genes, and finally receptor degradation. Since each receptor−ligand complex topology is unique to that ligand structure, one can appreciate that the interaction of any particular ligand−receptor complex with coregulatory proteins is likely to be unique to that ligand as well.
Furthermore, because the expression level of AR, the constellation and expression level of coregulatory proteins, and the patterns of post-transcriptional regulatory events differ in each type of androgen target cell, and the topography of AR regulatory sites in the genome differs at each gene, this remarkable choreography of events and interactions provides a rich environment within which one might search for SARMs having a desirable pattern of tissue-selective pharmacology, such as high anabolic but limited androgenic activity.[a]
The androgen receptor (AR) is a member of the nuclear receptor superfamily, and an important drug target. Various AR ligands have been discovered and developed for the treatment of male hypogonadism, prostate cancer, muscle wasting, anemia, and benign prostate hyperplasia (BPH). AR ligands can be classified as agonists (androgens) or antagonists (antiandrogens), based on their pharmacological activity (i.e., ability to activate or inhibit the transcription of AR target genes); or as steroidal and nonsteroidal ligands based on structure. Endogenous androgens include testosterone and dihydrotestosterone (DHT); both are steroidal AR agonists. Testosterone is the major circulating androgen and is converted, in a tissue-specific manner, to DHT by 5α-reductase (in prostate and skin) or to estrogen by aromatase (in adipose tissue, bone, and CNS). Pharmacologically, androgen actions in reproductive tissues, including the prostate, seminal vesicle, testis, and accessory structures, are commonly referred to as androgenic effects, whereas the growth-promoting effects of androgens in muscle and bone are recognized as anabolic effects.[c1]
Within the preclinical pipeline of owner company Radius Health Inc, RAD140 is touted as a potential breast cancer drug:
RAD140 is a nonsteroidal selective androgen receptor modulator. The androgen receptor (AR) is highly expressed in many estrogen receptor (ER)-positive, ER-negative, and triple-negative receptor breast cancers. Because of its receptor and tissue selectivity, potent activity, oral bioavailability, and long half-life, RAD140 could have clinical potential in the treatment of breast cancer. RAD140 resulted from an internal drug discovery program focused on the androgen receptor pathway, which is highly expressed in many breast cancers. [(investors.radiuspharm.com/releasedetail.cfm?releaseid=987014)]
We have reported that RAD140 in preclinical xenograft models of breast cancer has demonstrated potent tumor growth inhibition when administered alone or in combinations with CDK4/6 inhibitors. It is estimated that 77% of breast cancers show expression of the androgen receptor. Our preclinical data suggest that RAD140 activity at the androgen receptor stimulates up-regulation of a tumor suppression pathway. We expect to provide an update on the RAD140 program at an upcoming scientific meeting. [(investors.radiuspharm.com/releasedetail.cfm?releaseid=982846)]
Early findings included favorable ex vivo and in vivo properties of RAD140:
The stability of RAD140 was high ... in incubations with rat, monkey, and human microsomes, and it also had good bioavailability in rats (F = 27−63%) and monkeys (65−75%). RAD140 demonstrated excellent affinity for the androgen receptor ([inhibition coefficient] = 7 nM vs 29 nM for testosterone and 10 nM for DHT) as well as good selectivity over other steroid hormone nuclear receptors, with the closest off target receptor being the progesterone receptor (IC50 = 750 nM vs 0.2 nM for progesterone). In vitro functional androgen agonist activity was confirmed in the C2C12 osteoblast differentiation assay, where an EC50 of 0.1 nM was shown (DHT = 0.05 nM).
RAD140 was characterized in a number of in vivo assays to determine its oral efficacy on a number of parameters associated with androgenic activity in preclinical models. For example, RAD140 was dosed in both young castrated and intact male rats in order to assess its effects through a range of endogenous androgenic signaling backgrounds. The young castrated rat provides a very sensitive in vivo assay for androgenic activity because the animal is relatively androgen-naïve; thus, any signaling activity from an exogenously administered androgen is superimposed on an essentially blank background….increasing doses of orally administered RAD140 (0.5% methylcellulose) on levator ani bulbocavernosus muscle (“levator ani” or “LABC”) weight and prostate weight [were analyzed] relative to vehicle (castrated control), sham (noncastrated control), and testosterone propionate (TP) dosed subcutaneously at 1 mg/kg in corn oil. As can be seen, RAD140 stimulates the levator ani muscle beginning at a dose of 0.03 mg/kg (po) and reaches a level of efficacy equivalent to the sham-operated animal at 0.3 mg/kg.[a]
RAD140 was the “winner” chosen from among seven similar compounds as a next-generation SARM.[a] The next step was a trial comparing RAD140 to testosterone in several mammal models including a primate model:
Since these were young, intact male cynomolgous[sic] monkeys (3 to 4 years of age), they had fairly high endogeneous total plasma testosterone at day −1 (approximately 600−800 ng/dL), which is similar to the approximately 600 ng/mL that [healthy adult] human males have.... After 28 days of dosing with RAD140, the testosterone levels in all three groups was suppressed to approximately 200−300 ng/dL, with similar suppression in all three groups, although testosterone levels were significantly different for only the 0.01 mg/kg group (p < 0.05). Although this measurement did not account for possible diurnal variations in the animals and LH levels were not definitive, since they were below the level of detection in most pre- and postdose groups (LH < 0.8 ng/mL), one might still consider the possibility that even the 0.01 mg/kg dose was a fully effective, testosterone replacement dose, since body weight and lean mass were at least maintained (if not increased) in the low dose group despite significant testosterone suppression. Beyond this finding, we do not know whether testosterone suppression is a proxy for other CNS-related androgen effects beyond LH interference, such as mood, libido, and cognition, but we do believe a SARM with potent androgen agonist, CNS-type activity would be an interesting tool for that sort of exploration.(footnote 27)[a]
Reference material was synthesized according to established protocols and dissociation pathways of RAD140...were elucidated [for purposes of analytical findings using doping control samples] with liquid chromatography/electrospray ionization quadrupole/time-of-flight or iontrap/orbitrap and gas chromatography/electron ionization quadrupole/time-of-flight high resolution/high accuracy mass spectrometry.[c]
The principal characteristic of a SARM is to “elicit muscle-anabolic effects while only sparingly affecting reproductive tissues” RAD140 and other SARMs such as ostarine/enobosarm and andarine/S4 meet this criteria [z3][z4].
And like other SARMs RAD140 is a promising candidate for cachexia (wasting syndrome) associated with some cancers:
The availability of approved drugs for cancer cachexia is very slim, meaning that if enobosarm or other SARMs are approved, they will be a first-in-class; Madeddu and Montovani have suggested that for patients outside of clinical trials, a multi-prong approach incorporating more than one therapy, even more than one cutting-edge therapy, would be appropriate [z5]. [f - ostarine]
RAD140 compares favorably to other SARMs in general, as well as to testosterone and AAS.
Before the advantages of SARMs, a discussion of the disadvantages of traditional androgen receptor agonists/antagonists:
A variety of testosterone preparations (e.g., transdermal patches and injectable esters) and steroidal analogs (e.g., 17α-alkylated androgens and 19-norandrogens) have been developed. However, virilizing androgenic side effects, including acne and hirsuitism[sic], concerns related to hepatotoxicity (e.g., 17α-alkylated androgens), serum lipid profiles and the incidence or severity of prostate diseases, and less than optimal pharmacokinetic characteristics (e.g., testosterone is not orally available, and has short half life in vivo) have limited their clinical application. Various nonsteroidal pharmacophores were introduced in the 1970s as a means to overcome drug design limitations imposed by the rigid steroidal plane. Nonsteroidal AR antiandrogens, including bicalutamide, flutamide, and nilutamide, remain an important and widely used treatment in the management of prostate cancer. Although these nonsteroidal anti-androgens exhibit high specificity for AR and are orally available, they do not possess meaningful tissue selectivity. Along with the blockade of AR action in the prostate, antiandrogens also block AR actions in other target tissues, including anabolic tissues (e.g., skeletal muscle and bone) and the hypothalamus-pituitary-testis axis.[c1]
SARMs offer several theoretical advantages as a therapeutical alternative over traditional compounds such as testosterone or related androgenic-anabolic steroids (AAS). SARMs are orally active, non-hormonal small molecule compounds meaning that unlike most AAS, they do not present issues of requiring injection or concerns over toxicity to organs such as the liver. As such, in studies SARMs can be self-administered and given to populations that would not normally be appropriate for anabolic-androgenic steroids despite potential therapeutic benefit. SARMs do not convert enzymatically to hormones, and have more specific effects than the vast majority of AAS. The selectivity for the androgen receptor means a theoretically improved safety and efficacy profile compared to other compounds with the same class of therapeutic uses.[f1- sarms s4]
Because we consistently observed that RAD140 failed to achieve a level of prostate or seminal vesicle stimulation equal to TP (testosterone propionate) at 1 mg/kg (no matter how high the dose of RAD140), we decided to test whether RAD140 could antagonize the effect of TP on rat prostate and seminal vesicles and, at the same time, determine what effect the coadministration of RAD140 and TP might have on the levator ani muscle. ...it is apparent that a high dose of RAD140 (10 mg/kg, po) actually antagonizes the effect of TP at 1 mg/kg on the seminal vesicles but adds to the effect of TP on the levator ani muscle [emphasis added]. We were able to ascertain that the effective dose for achieving antagonism by RAD140 is 0.3−1 mg/kg (po) for 1 mg/kg TP (sc) . In the prostate, RAD140 also caused a downward trend in the stimulation by TP, but the change did not reach statistical significance. Thus, in the young castrate male rat model, RAD140 appears to be a potent and complete androgen agonist on the levator ani, but a weaker, partial antagonist on the seminal vesicle and possibly the prostate.[a]
To better understand how this group might respond, we decided to look at young intact male rats, since they have endogenous testosterone but at somewhat reduced levels. Therefore, they retain prostate sensitivity to an androgenic compound but at the same time have a baseline stimulation that is more similar to the target population than castrated animals….RAD140 increased the weight of the levator ani muscle above that of the intact control starting with the lowest tested dose (0.1 mg/kg). Interestingly, RAD140 demonstrated no stimulation of the prostate above the intact animal control level until the highest dose tested, 30 mg/kg. At 0.3 mg/kg, RAD140 demonstrated muscle efficacy similar to TP at 0.5 mg/kg, but a dose of 30 mg/kg of RAD140 was required to approximate the prostate efficacy of 0.5 mg/kg TP. From this study it is apparent that in young intact male rats RAD140 has a very wide range of selectivity relative to both TP-treated rats as well as sham-control rats.[a]
Finally, we were interested in evaluating the effect of RAD140 in young, male cynomolgous[sic] monkeys to establish efficacious dosing levels in what we considered to be a more relevant preclinical species. We performed a relatively simple, nonterminal study that still allowed us to evaluate anabolic as well as lipid and other clinical chemistry parameters. To assess anabolic activity, we first looked at gross body weight, which we knew to be a sensitive marker of anabolic androgen action in young nonhuman primates. The results on animal body weight of 28-day dosing with RAD140 at 0.01 mg/kg, 0.1 mg/kg, and 1 mg/kg [are published]...In this study, a mean weight gain of greater than 10% in just 28 days of dosing was achieved at a dose of just 0.1 mg/kg, with a similar effect observed at the 1.0 mg/kg dosing group.
Dual energy X-ray absorptiometry (“DEXA”) scans of all monkeys were taken two days before dosing began and one day after the final dose (day −2 and day 29) in order to determine the effects of RAD140 on lean tissue and fat...there was no consistent effect on absolute fat mass, whereas muscle showed a qualitative trend that increases with dose.
...Clinical chemistry indicated the expected lowering of lipids (LDL, HDL, triglycerides)... Despite the rather dramatic increases in body weight over such a short time, there was no elevation of liver enzyme transaminase levels in any animal at any dose >2 fold over its baseline value…. Given the well-established relationship between oral androgen use and liver stress indicators, we were quite pleased that at a dose 10-fold greater than the fully effective dose we saw minimal liver enzyme elevations....
Taken in sum, RAD140 has all the hallmarks of a SARM. It is potency selective, since it stimulates muscle weight increases at a lower dose than that required to stimulate prostate weight increases. Moreover, it is also efficacy selective, because it is fully anabolic on muscle but demonstrates less than complete efficacy on the prostate and seminal vesicles and, in fact, can partially antagonize the stimulation of the seminal vesicles induced by testosterone. RAD140 has excellent pharmacokinetics and is a potent anabolic in nonhuman primates as well. We believe the overall preclinical profile of RAD140 is very good, and the compound has completed preclinical toxicology in both rats and monkeys. We are currently preparing RAD140 for phase I clinical studies in patients suffering from severe weight loss due to cancer cachexia[a]
In some cases SARMs replicate lesser-known benefits of steroid hormones. It is not known the extent to which this is the case in CNS tissue. It is known that the AR is an important regulator of cell activity in some CNS tissue types. Some evidence (below, end of section) shows SARM modulation of this cell activity through the AR.
Gonadal steroids are potent regulators of adult neurogenesis. We previously reported that androgens, such as testosterone (T) and dihydrotestosterone (DHT), but not estradiol, increased the survival of new neurons in the dentate gyrus of the male rat. These results suggest androgens regulate hippocampal neurogenesis via the androgen receptor (AR). To test this supposition, we examined the role of ARs in hippocampal neurogenesis using 2 different approaches….chronic T increased hippocampal neurogenesis in wild-type males but not in androgen-insensitive testicular feminization mutation males….DHT increased hippocampal neurogenesis via cell survival, an effect that was blocked by concurrent treatment with flutamide. DHT, however, did not affect cell proliferation. ...these studies provide complementary evidence that androgens regulate adult neurogenesis in the hippocampus via the AR but at a site other than the dentate gyrus. Understanding where in the brain androgens act to increase the survival of new neurons in the adult brain may have implications for neurodegenerative disorders.
…adult hippocampal neurogenesis is an important factor in learning, memory and depression.
Neurogenesis is a multistep process that begins with the proliferation of cells, differentiation into neurons, migration, survival, and integration of these new neurons into the circuitry. Adult hippocampal neurogenesis can be increased via independent modulation of any one of these stages. For example, spatial learning increases neurogenesis in the hippocampus via enhancement in cell survival independent of cell proliferation. Conversely, chronic antidepressant treatment increases neurogenesis in the hippocampus via increasing cell proliferation independent of cell survival.
Androgens and estrogens are potent modulators of adult hippocampal neurogenesis. Chronic estradiol decreases neurogenesis in adult female rats. In males, however, chronic testosterone (T) or dihydrotestosterone (DHT), but not estradiol, increases hippocampal neurogenesis via cell survival. Given that T can be converted to estradiol via aromatase or to the androgen, DHT via 5α reductase, this previous finding suggests that androgens mediate hippocampal neurogenesis via binding to the androgen receptor (AR) in male rats. However, this is somewhat equivocal because DHT can be reduced to another metabolite, 5α-androstane-3α,17β-diol that may act via a non-AR mediated mechanism. Thus, it is unclear whether androgens mediate neurogenesis via ARs or through some other mechanism. ...it is possible that there is transient AR expression in immature neurons, which may suggest a direct mechanism for androgens to promote survival of immature neurons.[d]
This is the first study to provide direct evidence that androgens affect survival of newborn neurons in the hippocampus of adult male rats by acting directly through the AR using both genetic and pharmacological methods. [replicating earlier results, we confirmed] systemic DHT treatment increased the survival of new neurons in the DG. Second, we extend these results by showing that the survival promoting effects of DHT on neurogenesis can be blocked via systemic injection of the AR-specific antagonist, flutamide...
The duration of androgen administration is an important factor in promoting the survival of new neurons. We found that 30 days of systemic androgen treatment increased cell survival in the DG of adult male rats..However, a shorter duration (15 days) of androgen treatment did not enhance DG [dentate gyrus of hippocampus] neurogenesis, suggesting that the optimal time period for androgens to increase cell survival is between 16 and 30 days and, given the time frame involved, likely via a genomic mechanism.[d]
Androgens act as endogenous modulators of neuron viability, a function that may be compromised in aging men as a consequence of normal, age-related androgen depletion.[e]
Clinical utility of SARMs for neural disorders requires that they mimic androgen actions in brain. ...we evaluated the neuroprotective efficacy of the SARM RAD140 using in vitro and in vivo paradigms previously demonstrated to be androgen responsive. RAD140 is a novel SARM with high affinity and specificity for AR, is orally available, and exhibits potent anabolic effects in rodents and nonhuman primates . We determined the effects of RAD140 against toxic insults in both primary neuron cultures and the rat kainate lesion, an animal model of hippocampal neuron loss relevant to neurodegenerative diseases , which has previously been established to respond to androgen neuroprotection. [b]
In this study, we report the first findings of neuroprotective actions by SARMs in both cell culture and in vivo. Our results show in primary neuron cultures that the SARM RAD140 increases cell viability against Aβ [amyloid-beta] toxicity in a concentration-dependent manner. The neuroprotective effects of RAD140 are specific to apoptotic insults and dependent upon a MAPK-signaling pathway. ...RAD140 induces androgenic responses in muscle and brain, but not in reproductive tissues. Moreover, RAD140 treatment significantly protects hippocampal neurons
The data presented here are among the first highlighting the potential efficacy of SARMs for neural endpoints. Our finding that RAD140 can exert androgenic actions in brain at a dose that retains peripheral tissue selectivity is consistent with prior observations in rodents that the SARM ACP-105 can ameliorate cognitive deficits associated with apolipoprotein E and irradiation.
Androgens exert numerous beneficial actions in brain by several distinct mechanisms. Many, but not all, neural androgen actions involve AR activation, which triggers a wide range of rapid cell-signaling pathways as well as classic genomic regulation. Because SARMs can interact with AR differently than endogenous androgens, the efficacy of specific SARMs in activating defined androgenic pathways is a key consideration in pursuing translational goals.[b]
In terms of androgen neuroprotection, our prior work has defined a mechanism that is both dependent upon MAPK/ERK signaling pathway and limited in protective efficacy to apoptosis.
Our findings with RAD140 and the related compound RAD192 demonstrate that both SARMs mimic this established mechanism of neuroprotection in cultured neurons: they activate MAPK/ERK signaling as evidenced by ERK phosphorylation, their neuroprotection is blocked by pharmacologic inhibition of MAPK signaling, and they protect against 2 apoptotic insults but not a nonapoptotic insult. MAPK/ERK signaling is also known to contribute to androgen protection in non-neural cells. In adult male rats depleted of endogenous androgens by GDX, RAD140 matched the neuroprotection observed with T against kainate, a neurotoxin known to kill hippocampal neurons via apoptosis. ...MAPK/ERK signaling is involved in several important functions in the brain including neurogenesis, differentiation, synaptic plasticity, memory formation, and cell survival. However, additional signaling pathways likely contribute to androgen actions in brain. in addition to MAPK/ERK, androgens also rapidly activate cAMP response element binding protein signaling in neurons by a protein kinase C-dependent mechanism. In addition to rapid cell-signaling pathways, many neural androgen effects involve classic genomic responses. For example, androgen regulation of gene expression is implicated in protecting against AD pathology by reducing Aβ accumulation. Specifically, androgens reduce the expression of the proamyloidogenic enzyme β-secretase and increase expression of the Aβ degrading enzyme neprilysin by an AR-dependent genomic mechanism . Androgen-induced neurogenesis is also AR dependent in male rats and, at least in songbirds, involves genomic mechanisms including up-regulation of matrix metalloproteinases. Similarly, androgen-mediated increases in dendritic spines in mouse hippocampus has been linked to up-regulation of brain-derived neurotrophic factor and postsynaptic density protein 95.[b]
The decline in testosterone levels in men during normal aging increases risks of dysfunction and disease in androgen-responsive tissues, including brain. The use of testosterone therapy has the potential to increase the risks for developing prostate cancer and or accelerating its progression. To overcome this limitation, novel compounds termed "selective androgen receptor modulators" (SARMs) have been developed that lack significant androgen action in prostate but exert agonist effects in select androgen-responsive tissues. The efficacy of SARMs in brain is largely unknown. In this study, we investigate the SARM RAD140 in cultured rat neurons and male rat brain for its ability to provide neuroprotection, an important neural action of endogenous androgens that is relevant to neural health and resilience to neurodegenerative diseases. In cultured hippocampal neurons, RAD140 was as effective as testosterone in reducing cell death induced by apoptotic insults. Mechanistically, RAD140 neuroprotection was dependent upon MAPK signaling, as evidenced by elevation of ERK phosphorylation and inhibition of protection by the MAPK kinase inhibitor U0126. Importantly, RAD140 was also neuroprotective in vivo using the rat kainate lesion model. In experiments with gonadectomized, adult male rats, RAD140 was shown to exhibit peripheral tissue-specific androgen action that largely spared prostate, neural efficacy as demonstrated by activation of androgenic gene regulation effects, and neuroprotection of hippocampal neurons against cell death caused by systemic administration of the excitotoxin kainate. These novel findings demonstrate initial preclinical efficacy of a SARM in neuroprotective actions relevant to Alzheimer's disease and related neurodegenerative diseases.[emphasis added] [b]
The normal age-related decline in testosterone in men can increase the risks for dysfunction and disease in several androgen-responsive tissues throughout the body. In brain, low testosterone is an established factor for the development of Alzheimer's disease (AD). Circulating and brain levels of testosterone are lower in men with AD, and this androgen depletion occurs prior to clinical and neuropathological diagnoses of the disease, suggesting that low testosterone contributes to AD pathogenesis. In transgenic mouse models of AD, depletion of endogenous androgens by surgical or chemical castration accelerates development of AD-like pathology whereas elevation of endogenous testosterone above normal levels significantly impedes pathology development.
Androgens induce numerous beneficial neural effects relevant to a protective role against AD, including reduction of the AD-related protein β-amyloid (Aβ) and promotion of synapse formation, neurogenesis and specific aspects of cognition. An androgen action particularly important to neurodegenerative diseases is neuroprotection. Testosterone can increase neuron survival in several cell culture and animal models of injury. Although testosterone neuroprotective actions are largely androgen receptor (AR) dependent, testosterone is metabolized to several steroids that can act through other mechanisms. For example, testosterone is metabolized to dihydrotestosterone (DHT) by the enzymatic actions of 5α-reductase. Because DHT is a more potent androgen than testosterone at AR, the conversion of testosterone to DHT results in more robust androgen signaling in tissues like prostate. DHT is metabolized to other steroids, including 5α-androstane-3 α,17βdiol, which can reduce some forms of neural injury. Testosterone is also converted by the enzyme aromatase to 17β-estradiol (E2), which signals through estrogen receptors (ER). In some paradigms, testosterone neuroprotection is dependent upon conversion to E2. Thus, neural benefits of testosterone can be mediated largely by AR, ER, or a combination of both. For example, both AR and ER are implicated in testosterone reduction of Aβ levels, whereas AR, but not ER, mediates testosterone increases in spine density. These and other experimental data predict that androgen-based hormone therapy may be an effective approach for the prevention of AD and related neurodegenerative disorders in aging men.
One significant limitation of androgen therapy is the potential for increased risk of developing prostate cancer and or accelerated growth of existing prostate tumors. To overcome this problem, new classes of synthetic testosterone-like compounds, called “selective androgen receptor modulators” (SARMs), have been developed. SARMs are l