As the scientific community adjusts to moving forward without LGD-4033, synthetic chemists are increasingly reevaluating how to push the boundaries of SARM development to maximize potency, biodistribution, half-life, and tissue selectivity, while still minimizing off-target/adverse effects. The most cutting-edge research SARMs undergoing development are those which are rationally designed using the cumulative pharmacological insights from over 30 years of androgen research, rather than discovered via screening or dumb luck, which was typical of SARM development early on.
OTR-AC, or MK-2866 Ester, belongs to class of compounds known as “anabolic esters” due to esterification during chemical synthesis. Historically, esterification of testosterone was pursued in order to permit more stable serum testosterone levels. Now, synthetic chemists are applying these insights to SARM research and development to achieve a similar goal: more stable and durable serum SARM levels.
Fig. 1. Timeline of half-lives for three different testosterone esters. Each ester has an increased half-life compared to the one before it, and all three have better stability than non-esterified testosterone, which has a very short half-life of only 10 to 100 minutes. Adapted from Nieschlag et al. Andrology: Male Reproductive Health and Dysfunction. Springer Science (2010).
Superficially, it seems reasonable that employing bioidentical testosterone would be the most rational approach increase serum testosterone. However, this is not the case, mainly due to the issue of half-life (ie. stability). Unmodified testosterone has a very short half-life, resulting in spikes which are associated with undesirable mood swings and other consequences. The downsides and adverse effects from testosterone spikes that result from using non-esterified anabolic agents have always prevented their widespread adoption and acceptance.
To overcome this issue, scientist began to modify testosterone to make it more stable in the body. The chemical synthesis process by which any variety of ester groups are attached to carbon-based molecule is call esterification, and it has been a common functional modification to anabolic compounds since the discovery of the original androgens and estrogens. Esterification can serve to enhance stability (chemical or metabolic), change bioavailability, increase absorption, among other pharmacological goals.
For instance, one of the most commonly prescribed testosterone replacement formulations is Sustanon, which is composed of four testosterone esters (testosterone propionate, phenylpropionate, isocaproate and decanoate). Each of these esters have different half-lives (yet all of which are longer-lasting than free testosterone) which yields more consistent serum testosterone levels. For example, testosterone propionate has one of the shortest half-lives due to its short ester side chain, whereas testosterone buciclate has an incredibly extended half-life.
Anabolic Esters As Prohormones
The development of prohormone supplements was initially a very exciting development . Esterifcation of anabolic agents into degradation-resistant prohormones permitted a number of potential benefits: increased oral bioavailability, increased lipophilicity, resistance to degradation in vivo, and slower absorption. These benefits were anticipated due to the fact that esterified anabolic agents often require metabolic pre-processing before maximum utility, thus reducing the immediate and undesirable surge of anabolic function immediately upon entry to the circulatory system.
Unfortunately, anabolic prohormones did not live up to their hype, and are no longer the subject of groundbreaking research . In fact, the bulk of contemporary research reveals that the use of prohormone nutritional supplements (ie. DHEA, androstenedione, androstenediol, etc) does not produce sufficient anabolic or ergogenic effects in men, and many unfortunately retain their aromatizing properties that lead to increased estrogen. Furthermore, the use of prohormone nutritional supplements may actually increase the risk of experiencing negative health effects. Thus, even though prohormone esterification results in better stability, there are simply too many unrelated drawbacks to support continued research and development of ester prohormones because the risk to benefit ratio of using these substances orally is unfavorable . On the other hand, designer SARM esters like OTR-AC (MK-2866 Ester) are starting to generate plenty of excitement in the research community for the potential to overcome the drawbacks of prohormone esters.
Anabolic Ester Safety
There are numerous instances of improved safety profiles for anabolic esters compared to their parent compounds. For instance, the safety profile of testosterone undecanoate, an esterified form of testosterone, is excellent due in part to the continuous normalization of plasma testosterone levels. No polycythemia (ie. increased red blood cells) has been observed—which is common with other short-acting androgens , and no adverse effects on lipid profiles have been recorded. Prostate safety parameters remain within reference limits, and there is no reported impairment in urinary flow .
Another example of improved safety via esterification is nandrolone decanoate, which is an ester prodrug of nandrolone with an exceptional half-life in the body, and it is one of the most widely prescribed legal anabolic-androgen steroid (AAS) in the world. In fact, its mild side effect profile of this nandrolone ester (compared to its parent compound) enabled it to be prescribed to individuals with compromised kidney function –, women with osteoporosis , and children with growth failure 
However, it is essential to note that despite the improved safety profile of testosterone esters and nandrolone esters relative to their parent compounds, there still exists adverse reactions to these conventional steroidal drugs, thus highlighting the need for continued research and development of novel SARM esters like OTR-AC (MK-2866 Ester).
Nevertheless, the examples above demonstrate that there are numerous precedents for the increased safety of esterified anabolic agents like OTR-AC (MK-2866 Ester), although researchers are cautioned that anabolic ester PK/PD cannot always be predicted a priori, so empirical validation is necessary.
Anabolic Ester Pharmacokinetics
As a general rule, esterification of anabolic agents results in at least a 10-fold increase in half-life, meaning that the active substance has the potential to remain bioactive for ten times as long . This results in numerous benefits, such as decreased frequency of dosing and lack of “roller coaster” hormone response.
Since OTR-AC is an esterified form of MK-2866, it is anticipated to offer more durable pharmacokinetics in the body and thus dosing frequency may be reduced if appropriate, which would make it more economical in the long run. An extended half-life also provides a buffer against a drop in anabolic response after missed doses due to unexpected interruption of experimentation.
There is already some precedent for improved pharmacokinetics of a SARM ester: YK-11. Metabolites of this SARM ester remain in the body beyond 48 hours , which is on par with some of the earliest innovations in testosterone ester chemistry.
OTR-AC (MK-2866 Ester) is the culmination of years of pharmacological optimization of one of the most sought-after SARMs, and it promises an exciting new avenue for SARM research and dosing optimization at the preclinical stage.
*This information is for educational purposes only. THE PRODUCTS DESCRIBED HEREIN ARE FOR RESEARCH USE ONLY. All clinical research must be conducted with oversight from the appropriate Institutional Review Board (IRB). All preclinical research must be conducted with oversight from the appropriate Institutional Animal Care and Use Committee (IACUC) following the guidelines of the Animal Welfare Act (AWA).
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Shipping Conditions: Ambient temperature.
Storage: Use within 12 months. Keep in a cool and dark location.
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