Ki, the inhibition constant, is the concentration of a test compound at which it occupies half the receptors in a competitive binding assay, after accounting for the concentration and affinity of the radioligand used. It is the most commonly reported metric of receptor binding affinity.

Kd, the dissociation constant, is the concentration of a ligand at which half the receptors are bound in a saturation-binding assay. It is conceptually similar to Ki but measured directly rather than competitively.

Why is lower Ki better?

A lower Ki value means the compound binds the receptor more tightly - fewer molecules are needed to occupy the same fraction of receptors. "Better" depends on context, however; tighter binding does not by itself indicate functional activity.

What is the published MOR affinity of 7-OH compared to mitragynine?

Across published in vitro assay systems, 7-hydroxymitragynine binds the mu-opioid receptor with affinity roughly 5-22 times higher than mitragynine. The exact ratio varies with assay conditions and reference ligand.

Is affinity the same as potency?

No, but they are related. Affinity (Ki/Kd) is the binding parameter - how tightly a ligand binds the receptor. Potency (often expressed as EC50 in functional assays) is the concentration at which the ligand produces 50% of its maximum functional response. A ligand can have high affinity but be a partial agonist with low maximum efficacy.

Receptor Pharmacology

Receptor Binding Affinity

At-a-glance comparison

Spec	Value
What it measures	How tightly a ligand binds a receptor
Common metrics	Ki (inhibition constant) and Kd (dissociation constant)
Units	Molar concentration (nM, μM); lower = tighter binding
Measurement methods	Radioligand binding assay; fluorescence-based binding; surface plasmon resonance (SPR); cellular thermal shift
7-OH MOR Ki (published)	Approximately 20-80 nM range across published assay systems
Mitragynine MOR Ki (published)	Approximately 200-700 nM range across published assay systems
Important distinction	Affinity (Ki/Kd) is independent of efficacy (Emax); a high-affinity ligand can be a partial agonist or antagonist

What is receptor binding affinity?

Receptor binding affinity quantifies how tightly a ligand binds to a receptor. The most commonly reported affinity metric is the inhibition constant Ki, derived from a competitive radioligand-binding assay in which the test compound competes with a labeled reference ligand for receptor occupancy. The dissociation constant Kd, derived from a saturation-binding assay, is conceptually similar but measured directly. Both are reported in molar concentration units, with smaller values indicating tighter binding - a Ki of 10 nanomolar represents tighter binding than a Ki of 100 nanomolar.

Affinity is a fundamental parameter for comparing ligands at the same receptor. When one ligand binds with affinity 10 times that of another, fewer molecules are needed to occupy the same fraction of receptors. This is why receptor-binding affinity is one of the first numbers reported in any new pharmacology characterization.

How is affinity measured?

The most common measurement is the radioligand competition-binding assay. A radioactively labeled reference ligand of known affinity (for example, [3H]DAMGO at the mu-opioid receptor) is incubated with membrane preparations expressing the receptor of interest. Increasing concentrations of the test compound are added, and the test compound competes with the radioligand for binding. The concentration at which the test compound displaces 50% of the radioligand (the IC50) is converted, using the Cheng-Prusoff equation, to a Ki value that can be compared across assays.

Modern alternatives include fluorescence-based binding assays (avoiding radioactivity), surface plasmon resonance (real-time binding kinetics), and cellular thermal shift assays (binding in living cells). Each method has trade-offs in sensitivity, throughput, and physiological relevance, and reported Ki values can differ across methods, so cross-publication comparison requires care.

Affinity is not the same as efficacy

An important distinction in receptor pharmacology: binding affinity is independent of functional efficacy. A ligand can have high affinity for a receptor (binds tightly) but produce little or no functional response (low Emax) - that combination describes either an antagonist or a low-efficacy partial agonist. Conversely, a ligand can have lower affinity but produce a maximal response (full agonist) at sufficient concentration.

For 7-hydroxymitragynine and mitragynine at the mu-opioid receptor, both compounds are described as partial agonists. Their published binding affinities differ markedly - 7-OH binds tighter - but their functional efficacy as partial agonists is what determines the maximum response they can produce. The dedicated Partial Agonist and Biased Agonism glossary entries explain how these parameters work together to define a complete pharmacological profile.

Common questions about receptor binding affinity

What does Ki mean?: Ki, the inhibition constant, is the concentration of a test compound at which it occupies half the receptors in a competitive binding assay, after accounting for the concentration and affinity of the radioligand used. It is the most commonly reported metric of receptor binding affinity.
What does Kd mean?: Kd, the dissociation constant, is the concentration of a ligand at which half the receptors are bound in a saturation-binding assay. It is conceptually similar to Ki but measured directly rather than competitively.
Why is lower Ki better?: A lower Ki value means the compound binds the receptor more tightly - fewer molecules are needed to occupy the same fraction of receptors. "Better" depends on context, however; tighter binding does not by itself indicate functional activity.
What is the published MOR affinity of 7-OH compared to mitragynine?: Across published in vitro assay systems, 7-hydroxymitragynine binds the mu-opioid receptor with affinity roughly 5-22 times higher than mitragynine. The exact ratio varies with assay conditions and reference ligand.
Is affinity the same as potency?: No, but they are related. Affinity (Ki/Kd) is the binding parameter - how tightly a ligand binds the receptor. Potency (often expressed as EC50 in functional assays) is the concentration at which the ligand produces 50% of its maximum functional response. A ligand can have high affinity but be a partial agonist with low maximum efficacy.

References

Cheng Y, Prusoff WH. (1973). Relationship between the inhibition constant (K1) and the concentration of inhibitor which causes 50 per cent inhibition (I50) of an enzymatic reaction. Biochemical Pharmacology.
Varadi A, Marrone GF, Palmer TC, et al. (2016). Mitragynine/Corynantheidine pseudoindoxyls as opioid analgesics. J Med Chem. PMID 27513560.
Kenakin T. (2019). A Pharmacology Primer (5th ed.). Academic Press.
International Union of Basic and Clinical Pharmacology. https://www.guidetopharmacology.org

Important safety information:

Products containing 7-hydroxymitragynine (7-OH) are sold for adult use only (21+). These statements have not been evaluated by the U.S. Food and Drug Administration. Products are not intended to diagnose, treat, cure, or prevent any disease. The FDA has raised safety concerns regarding concentrated 7-OH products; consult a qualified healthcare professional before use. Do not operate vehicles or machinery after use. Keep out of reach of children and pets. Laws vary by state, buyers are responsible for knowing applicable law.