Peptide glossary

Bacteriostatic water for injection is sterile water with benzyl alcohol added as a preservative, supplied in a multiple-dose vial so a drug can be diluted or dissolved for injection. The Hospira label specifies 0.9% (9 mg/mL) or 1.1% benzyl alcohol, and states it is not for use in neonates.

Bioavailability is the rate and extent to which the active ingredient of a drug product is absorbed and becomes available at the site of action. That is the FDA definition in 21 CFR 314.3. For an injected peptide, the route changes that rate and extent. Intravenous delivery is the reference other routes are compared against.

Growth hormone-releasing hormone (GHRH) is the hypothalamic peptide that signals the pituitary to release growth hormone. It is one endogenous route to that release. Ghrelin, acting through the secretagogue receptor, is a different route. GHRH is not growth hormone, and it is not a growth-hormone secretagogue.

Plasma half-life is the time required for the plasma concentration of a substance to fall by half after pseudo-equilibrium. It is not the time required to eliminate half the administered dose, and it is not the duration of a biological effect.

Lyophilization, also called freeze-drying, removes water from a frozen solution by sublimation under vacuum and leaves a dry solid. For peptides and proteins it is the usual way to make a solid that can be stored longer than the same molecule dissolved in water.

Reconstitution is the step that returns a dry, usually lyophilized, peptide or protein to solution by adding a diluent. It is a preparation step, not a dose. The label of the solute, not the nickname of the diluent, governs whether that solution may be stored.

A secretagogue is a substance that stimulates a cell or gland to release a hormone it already produces. A growth-hormone secretagogue prompts the pituitary to release growth hormone. It is not growth hormone itself, and it is not growth hormone-releasing hormone.

Subcutaneous administration places a solution into the tissue under the skin and above the muscle. For peptide and protein drugs it is a common injection route. Absorption from that tissue is relatively slow and, for many biotherapeutics, mostly incomplete compared with intravenous delivery.