Dissociation Constant (Kd)
Definition
The concentration at which half the available binding sites are occupied at equilibrium. A lower value means tighter binding, and unlike IC50 it describes the interaction rather than the readout used to measure it.
Terms used in this definition
Other entries this definition refers to.
Where this term comes up
Entries whose definition mentions Dissociation Constant (Kd).
What this means for research sourcing
Mechanistic terms like this describe how research compounds behave in vitro. See the mechanism overviews for compounds Volta stocks.
Related Terms in Pharmacology
Uptake
Movement of a substance from outside a cell or compartment to inside it, whether passively or by a transporter. It is measured in a defined system, and an uptake result in cultured cells does not establish what happens in an organism.
Bioavailability
The fraction of an administered amount that reaches the circulation intact and is therefore available to act. Intravenous delivery is 100% by definition; everything else is measured against it.
Oral Bioavailability
Bioavailability by mouth, which for most peptides is under 1%: gastric acid and intestinal proteases degrade them and the surviving fraction struggles to cross the epithelium. Overcoming it takes permeation enhancers, enteric protection or a very short sequence.
Half-Life
The time taken for the amount in the body to fall by half. It sets how often a compound has to be given and how long elimination takes, about four to five half-lives for the bulk of it to clear.
Clearance
The rate at which the body removes a compound from blood, through the kidneys, the liver, or enzymatic breakdown in plasma. Clearance together with volume of distribution is what produces the half-life, rather than the other way round.
Steady State
The point in repeated administration where the amount going in matches the amount being cleared, so peaks and troughs stop climbing. It takes roughly four to five half-lives to arrive, whatever the interval.
Area Under the Curve (AUC)
Total exposure over time, the integral of plasma concentration against time. Two regimens can share a peak concentration and differ substantially in AUC, which is why trials report both.
Pharmacokinetics
What the body does to a compound: absorption, distribution, metabolism and excretion. Half-life, peak concentration and dosing interval all come out of pharmacokinetic work.