Peptide pharmacokinetics glossary

37 terms used in half-life charts, lab reports and dosing labels, in plain language.

Absorption: Movement of a drug from where it was administered (for example under the skin) into the bloodstream.
Accumulation ratio: How much higher steady-state levels are than after a single dose, set by the dosing interval relative to the half-life: 1 / (1 − 2^(−interval/half-life)). Half-life explained
Apparent half-life: The half-life observed after a non-intravenous dose. When absorption is slower than elimination, the decline reflects absorption and the apparent half-life is longer than the true elimination half-life.
Area under the curve (AUC): The total exposure to a drug over time, calculated as the area under the concentration-time curve.
Bacteriostatic water: Sterile water containing 0.9% benzyl alcohol, which inhibits bacterial growth, commonly used to reconstitute multi-use vials.
Bioavailability: The fraction of an administered dose that reaches the bloodstream unchanged. Intravenous doses have 100% bioavailability by definition.
Certificate of Analysis (COA): A lab report for a specific batch that documents identity, purity and any contaminant tests performed. How to read a COA
Clearance: The volume of plasma cleared of a drug per unit of time; together with volume of distribution it determines the half-life.
Cmax (peak): The highest concentration reached after a dose.
Cmin (trough): The lowest concentration reached, typically just before the next dose.
Elimination rate constant (k): The fraction of drug removed per unit of time in first-order elimination, equal to ln 2 divided by the half-life.
Endotoxin: Lipopolysaccharide from the outer membrane of gram-negative bacteria. It survives filtration and freeze-drying and is measured with the LAL test in endotoxin units (EU) per mg.
First-order elimination: Elimination in which a constant fraction of the drug is removed per unit of time, so the amount halves every half-life.
GH secretagogue: A compound that stimulates the pituitary to release growth hormone, such as ghrelin-receptor agonists (ipamorelin, GHRP-2) or GHRH analogues (sermorelin).
GHRH analogue: A synthetic version of growth hormone-releasing hormone, such as sermorelin, CJC-1295 or tesamorelin.
GLP-1 receptor agonist: A drug that activates the glucagon-like peptide-1 receptor, such as semaglutide or liraglutide; some newer drugs also target GIP or glucagon receptors.
Half-life (t½): The time for the amount of a drug in the body to fall by half. Half-life explained
HPLC: High-performance liquid chromatography: separates a sample into components to measure the purity of the target peptide relative to related impurities.
International unit (IU): A unit of biological activity defined against a reference standard. Its relationship to mass differs for each substance. Unit conversions
Intramuscular (IM): Injection into a muscle, which is usually absorbed faster than a subcutaneous injection.
Intranasal: Administration through the nasal lining, used for some small peptides such as Semax and Selank.
LAL test: The Limulus amebocyte lysate assay, the standard test for bacterial endotoxin.
Loading dose: A larger initial dose used to reach target levels faster than waiting for steady state.
Lot number: The identifier for one manufacturing batch. A COA applies only to the lot it names.
Lyophilized: Freeze-dried. Most research peptides are supplied as a lyophilized powder for stability.
Mass spectrometry (MS): A technique that measures molecular weight, used on COAs to confirm a peptide's identity.
Net peptide content: The share of a powder's weight that is actually peptide, as opposed to water and counter-ions such as acetate or TFA.
One-compartment model: A simplified model that treats the body as a single well-mixed space with first-order elimination. Kinetic uses it for its estimates.
Pharmacokinetics (PK): How the body absorbs, distributes, metabolizes and eliminates a drug over time.
Reconstitution: Dissolving a lyophilized powder in a measured volume of liquid; the concentration is amount divided by volume.
Research use only (RUO): A label for products sold for laboratory research that are not approved for diagnosing, treating or preventing disease, or for human consumption.
Steady state: The plateau reached with repeated dosing when elimination between doses equals the amount given, after about 4 to 5 half-lives.
Sterility testing: Testing for living microorganisms, required for pharmaceutical injectables but usually not performed on research peptides.
Subcutaneous (SC): Injection into the fat layer beneath the skin, the usual route for most peptide drugs.
Tmax: The time after a dose at which the peak concentration occurs.
Volume of distribution: A proportionality factor relating the amount of drug in the body to its plasma concentration, reflecting how widely it spreads into tissues.
Washout period: Time allowed for a drug to clear before starting another or taking measurements, commonly about five half-lives.