First-Pass Metabolism
Definition
The phenomenon where a drug is metabolized by the liver before reaching systemic circulation, significantly reducing bioavailability. This is a primary reason most peptides are administered by injection rather than orally.
Related Terms in Pharmacology
Bioavailability
The proportion of a substance that enters the circulation when introduced into the body and is able to have an active effect. Different administration routes affect bioavailability.
Half-Life
The time required for the concentration of a substance in the body to decrease by half. Determines dosing frequency and accumulation patterns.
Pharmacokinetics
The study of how a drug moves through the body — absorption, distribution, metabolism, and excretion (ADME).
Pharmacodynamics
The study of what a drug does to the body — its mechanism of action, therapeutic effects, and dose-response relationships. Complementary to pharmacokinetics.
Receptor Agonist
A substance that binds to a receptor and activates it, mimicking the action of a naturally occurring substance. Many therapeutic peptides are receptor agonists.
Receptor Antagonist
A substance that binds to a receptor and blocks its activation, preventing the natural ligand from exerting its effect.
Desensitization
A reduction in receptor response after prolonged or repeated exposure to an agonist. Can lead to reduced effectiveness of some peptides over time.
Tachyphylaxis
A rapid and acute decrease in response to a drug after repeated doses in a short time period. Distinct from slower desensitization, tachyphylaxis can occur after just a few doses. Melanotan II and some GHRPs are susceptible to tachyphylaxis without proper cycling.