Andarine (S-4) is a selective inhibitor of AR with Ki value of 4 nM [1].

Androgen receptor (AR), also known as nuclear receptor subfamily 3, group C, member 4, is a type of nuclear receptor and plays an important role in DNA-binding transcription factor that regulates gene expression [2].

Andarine is a potent AR inhibitor and has a higher affinity with the reported AR inhibitor S-1, S-2 and S-3. Using a cotransfection system, it was shown that Andarine stimulated AR-mediated transcription as high as 93% at the concentration of 10 nM. [1].

In male Sprague-Dawley rat model, administration of Andarine resulted in the growth in prostate, seminal vesicles, and levator ani muscle at a dose-dependent manner and promoted their weight being maximally to 33.8, 28.2 and 101% of intact controls, respectively. Further, it was revealed that the ED50 value of Andarine functioning on prostate, seminal vesicles, and levator ani muscle was 0.43±0.01, 0.55±0.02, and 0.14±0.01 mg/day [1].

References:
[1].  Yin, D., et al., Pharmacodynamics of selective androgen receptor modulators. J Pharmacol Exp Ther, 2003. 304(3): p. 1334-40.
[2].  Thevis, M., et al., Mass spectrometric characterization of urinary metabolites of the selective androgen receptor modulator S-22 to identify potential targets for routine doping controls. Rapid Commun Mass Spectrom, 2011. 25(15): p. 2187-95.