Pharmacodynamics

Affinity

TRAfinite
Also known as:Binding affinity · Receptor affinity

The strength of binding between a drug and its receptor or target, determined by the rates of association and dissociation and usually expressed as the equilibrium dissociation constant (Kd) or its negative logarithm (pKd).

Formula

Kd = koff / kon pKd = −log Kd

In depth

The lower the Kd, the higher the affinity; at a free drug concentration equal to Kd, half of the receptors are occupied at equilibrium. Kd equals the ratio of the dissociation (koff) and association (kon) rate constants. Affinity is measured in radioligand saturation or competition binding assays (Kd, Ki) and, for competitive antagonists, functionally by Schild analysis (pA2, KB).

Affinity is distinct from efficacy: an antagonist may have very high affinity but no efficacy. Potency depends on both affinity and efficacy as well as on tissue factors such as receptor number, so a high-affinity drug is not necessarily more effective clinically.

Clinical pearl

A high-affinity ligand can displace a lower-affinity one: buprenorphine binds μ-opioid receptors so avidly that it can displace full agonists, and naloxone reversal of its respiratory depression may be incomplete or need higher or repeated doses.

Example drugs

Buprenorphine
TRIn Turkish

Afinite

Bir ilaç ile reseptörü veya hedefi arasındaki bağlanmanın gücüdür; birleşme ve ayrışma hızlarıyla belirlenir ve genellikle denge disosiyasyon sabiti (Kd) ya da bunun negatif logaritması (pKd) ile ifade edilir.

Kd ne kadar düşükse afinite o kadar yüksektir; serbest ilaç konsantrasyonu Kd'ye eşit olduğunda dengede reseptörlerin yarısı işgal edilmiş olur. Kd, ayrışma (koff) ve birleşme (kon) hız sabitlerinin oranına eşittir. Afinite, radyoligand doygunluk veya yarışma bağlanma deneyleriyle (Kd, Ki), yarışmalı antagonistlerde ise işlevsel olarak Schild analiziyle (pA2, KB) ölçülür.

Afinite etkinlikten farklıdır: bir antagonistin afinitesi çok yüksek, etkinliği ise sıfır olabilir. Potens hem afiniteye hem etkinliğe hem de reseptör sayısı gibi doku etkenlerine bağlıdır; bu nedenle yüksek afiniteli bir ilaç klinik olarak mutlaka daha etkili değildir.

Yüksek afiniteli ligand, düşük afiniteli olanı yerinden edebilir: buprenorfin μ-opioid reseptörlerine tam agonistleri uzaklaştıracak kadar güçlü bağlanır; yol açtığı solunum depresyonu naloksonla tam geri döndürülemeyebilir veya yüksek ya da tekrarlanan doz gerekebilir.

Diğer adları:Bağlanma afinitesi · Reseptör afinitesi
Updated 24.09.2026 1 views Educational reference — verify against current sources before clinical use.

Read next

Dissociation ConstantDisosiyasyon Sabiti

The equilibrium constant for dissociation of a drug–receptor complex, equal to the free drug concentration at which half of the receptors are occupied at equilibrium; a lower Kd means higher affinity.

 Inhibition Constantİnhibisyon Sabiti

The equilibrium dissociation constant of an inhibitor from its enzyme, receptor or transporter; an assay-independent measure of inhibitor affinity, with lower Ki values indicating more potent inhibition.

 EfficacyEtkinlik

The capacity of a drug to produce a response once bound to its receptor; in clinical terms, the maximal effect (Emax) a drug can produce, however much the dose is increased.

 PotencyPotens

The amount or concentration of a drug needed to produce an effect of a given intensity, usually expressed as the EC50 or ED50; the lower the value, the more potent the drug.

 ReceptorReseptör

A macromolecule, usually a protein, to which a drug or endogenous ligand binds specifically and thereby initiates a change in cell function that produces a biological response.

 Intrinsic Activityİntrinsik Aktivite

The ability of a drug, once bound, to activate its receptor and produce a response, expressed relative to a full agonist on a scale from 0 (antagonist) to 1 (full agonist); inverse agonists have negative values.