Receptor Binding Affinity

Receptor binding affinity (Ki or IC50) measures the concentration at which a ligand occupies half of available receptor sites. For AAS the primary target is the androgen receptor (AR); higher affinity produces stronger transcriptional signal per molecule, but almost always co-drives androgenic side-effect expression.

Relative AR binding affinity (testosterone = 1.0):

– Trenbolone: 5× — the highest-affinity AR ligand in routine use
– Methyltrienolone (R1881, research standard): 4×
– Dihydrotestosterone: 3×
– Oxandrolone: 3×
– Testosterone: 1.0 reference
– Methandrostenolone: 0.5
– Nandrolone: 0.5
– Stanozolol: 0.25
– Boldenone: 0.3

Affinity is necessary but not sufficient — three moderating factors:

Dwell time at the receptor. Trenbolone’s 5× affinity is amplified clinically by prolonged AR residency — once bound, it dissociates slowly, sustaining transcriptional activation longer per binding event than testosterone. Ki measures binding equilibrium at a snapshot; dwell time measures the integral across time. Clinical effect tracks the integral, not the snapshot.

Tissue selectivity. AR is expressed across skeletal muscle, prostate, sebaceous gland, and CNS with different densities and coactivator profiles. A compound with systemic high affinity but prostate-sparing metabolism (stanozolol is not reduced by 5α-reductase because it is already a DHT derivative) produces a different tissue fingerprint than one with equal systemic affinity but high prostate penetration.

Non-AR mechanisms. Stanozolol shows modest AR affinity but exerts strong clinical anabolism through SHBG suppression, glucocorticoid-receptor antagonism, and direct effects on protein-synthesis regulatory pathways. Ki alone dramatically underestimates the effect. The mirror case is oxandrolone — strong AR affinity, but cortisol-antagonist activity drives a substantial fraction of the clinical response beyond what AR binding alone predicts.

Coactivator recruitment — where current research is moving:

AR transcriptional activity requires coactivator proteins (ARA70, SRC-1, p300). Different ligands recruit coactivators with different efficiency after receptor binding — a phenomenon termed “biased agonism.” This is why two compounds with identical Ki can produce divergent downstream transcription patterns. The AR affinity table is a useful first approximation, not a final answer.

Clinical translation:

Affinity sets the floor of possible effect. Tissue distribution, dwell time, coactivator recruitment, and non-AR mechanism determine where within that ceiling the compound actually lands. A Ki value on a datasheet is the starting point of the question, not the answer.