[H-3] Methyltrienolone

Methyltrienolone (R1881, metribolone) is a potent synthetic 19-nortestosterone derivative and non-aromatizable androgenic-anabolic steroid. It binds directly and with extremely high affinity to the ligand-binding domain of the androgen receptor (AR). This binding triggers receptor dimerization, nuclear translocation, and robust transcriptional activation of androgen-responsive genes.

Its extreme potency stems from a structural design that combines a 17α-methyl group with a triene steroid nucleus, preventing metabolic inactivation and aromatization into estrogens. In addition, R1881 does not bind significantly to Sex Hormone-Binding Globulin (SHBG), leaving nearly all circulating compound biologically active to continuously stimulate androgen receptors.

Depending on the specific species, tissue preparation, and assay conditions, the reported inhibition constant (Ki) and dissociation constant (Kd) of methyltrienolone for the androgen receptor typically range from 0.4 to 0.6 nM. This sub-nanomolar range reflects an extraordinarily tight, high-affinity binding interaction.

Methyltrienolone exhibits significantly higher binding affinity and functional potency for the androgen receptor than both testosterone and dihydrotestosterone (DHT). Unlike testosterone, R1881 cannot be converted into estrogen by aromatase, and unlike DHT, it is resistant to rapid enzymatic inactivation by 3α-hydroxysteroid dehydrogenases in target tissues.

Tritiated methyltrienolone serves as the gold-standard reference radioligand in radioligand binding assays and autoradiography to quantify androgen receptor density (Bmax) and affinity (Kd). It is widely used in prostate cancer research, endocrine disruptor screening, and competitive binding assays for novel pharmaceutical candidates.

Yes, methyltrienolone lacks strict selectivity for the androgen receptor and exhibits cross-reactivity with other nuclear steroid receptors. It binds with high affinity to progesterone receptors and shows measurable cross-affinity for mineralocorticoid receptors and glucocorticoid receptors.

In clinical trials, methyltrienolone demonstrated severe, dose-dependent hepatotoxicity (cholestatic jaundice), making it far too toxic for human therapeutic use. In laboratory environments, researchers must wear personal protective equipment, avoid skin contact or inhalation, and use proper containment measures due to its high hormonal potency and risks of endocrine disruption.