Prepared by: Research Desk. Sources are linked for verification.

This peptide glossary defines the technical terms used across Peptide Infinity entries. It covers chemistry, pharmacology, hormone signalling, U.S. regulation and research evidence. Definitions are short and neutral. Where a real example makes a term clearer, it links to a public FDA or PubMed record.

How to use this glossary

Terms are grouped into five themes and listed alphabetically within each theme. Cross-references appear in italics.

• For amino acids, chains, peptide types and the drug development pathway, see what peptides are.

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• For sourcing standards and the corrections policy, see how this site works.

This glossary defines words. It does not describe how any substance is used, prepared or administered, and it gives no amounts.

Chemistry terms

Acetylation. Addition of an acetyl group, usually to the N-terminus (the starting end) of a chain. It can make a molecule less vulnerable to enzymes that cut chains from the end. See also amidation.

Amidation. Conversion of the C-terminal carboxyl group (the finishing end) into an amide. Many natural hormones are amidated, and synthetic designs often copy this feature to increase stability.

Amino acid. The building block of peptides and proteins, with an amino group, a carboxyl group and a distinctive side chain. Twenty standard amino acids are encoded by the genetic code, each abbreviated with one or three letters (G or Gly for glycine).

Analogue (analog). A molecule designed to resemble a natural one while differing in sequence, length or chemical modification, usually to keep receptor activity while changing properties such as stability. Sermorelin acetate and tesamorelin acetate are both analogues of growth hormone-releasing hormone (GHRH), and both appear in Drugs@FDA [1][2].

Cyclic peptide. A chain whose ends, or two internal points, are joined into a ring. Cyclization can restrict shape and reduce enzymatic breakdown.

Lipidation. Attachment of a fatty-acid chain. A lipidated molecule can bind albumin, a common blood protein, which slows its removal from circulation and lengthens its half-life.

Peptide bond. The amide bond joining the carboxyl group of one amino acid to the amino group of the next. Water is released as each bond forms.

Residue and sequence. A residue is an amino acid once incorporated into a chain; a "29-residue peptide" contains 29 amino acids. The sequence is the order of residues from N-terminus to C-terminus. Two chains with the same amino acids in a different order are different molecules.

Pharmacology terms

Affinity. How strongly a molecule binds its target. High affinity alone does not indicate whether binding activates or blocks the target.

Agonist. A molecule that binds a receptor and activates it, producing a response similar to the natural signal. A partial agonist produces a weaker maximum response than a full agonist.

Antagonist. A molecule that binds a receptor without activating it, blocking the natural signal.

Bioavailability. The fraction of a substance that reaches systemic circulation unchanged. Digestive enzymes break down most natural peptides, one reason peptide drug development is technically demanding.

Half-life. The time for the plasma concentration of a substance to fall by half. Unmodified natural peptides often have short half-lives; modifications such as lipidation can extend this.

Mimetic. A molecule that imitates the activity of a natural substance without necessarily sharing its structure. A 2009 rodent study describes ipamorelin as "a novel ghrelin mimetic" [3].

Receptor. A protein, usually on the cell surface, that recognises a specific signalling molecule and converts binding into a cellular response. Many peptide hormones act on G protein-coupled receptors.

Secretagogue. A substance that causes a gland or cell to release another substance. A growth hormone secretagogue prompts the pituitary to release growth hormone. GHRH analogues and ghrelin-receptor agonists are two distinct classes because they act through different receptors.

Selectivity. The degree to which a molecule acts on its intended receptor rather than related ones. Off-target activity can produce unwanted effects, so selectivity is a key design goal.

Hormone and signalling terms

Anterior pituitary. The front lobe of the pituitary gland. It releases growth hormone and several other hormones in response to hypothalamic signals.

GHRH (growth hormone-releasing hormone). A hypothalamic peptide hormone that stimulates growth hormone release from the anterior pituitary. Also called GRF (growth hormone-releasing factor).

GHRH analogue. A synthetic molecule modelled on GHRH that acts at the GHRH receptor. Two with U.S. application records are sermorelin acetate (NDA 019863) [1] and tesamorelin acetate (application 022505) [2]. Their current statuses differ; see discontinued and BLA.

Ghrelin. A peptide hormone produced mainly in the stomach. It acts on the growth hormone secretagogue receptor (GHS-R1a) and is involved in appetite signalling and growth hormone release.

Ghrelin-receptor agonist. A molecule that activates the ghrelin receptor. Ipamorelin belongs to this class and was studied in a rodent model of postoperative ileus, a slowing of gut movement after abdominal surgery [3]. That evidence is preclinical (see animal model).

Growth hormone axis. The signalling chain from hypothalamus to pituitary to tissues. GHRH stimulates growth hormone release, somatostatin inhibits it, and growth hormone stimulates production of insulin-like growth factor 1 (IGF-1), mainly in the liver. IGF-1 feeds back to dampen the system.

Somatostatin. A peptide hormone that inhibits growth hormone release, counterbalancing GHRH.

Regulatory terms

503A. Section 503A of the U.S. Federal Food, Drug, and Cosmetic Act, which sets conditions for compounding by licensed pharmacies. Compounded drugs are not FDA-approved.

503A bulks list. FDA's list of bulk drug substances that may be used in 503A compounding even though they lack a USP monograph and are not components of an approved drug. FDA's page also explains how nominated substances are evaluated [4]. This is separate from whether any approved product contains the substance.

BLA (Biologics License Application). The application used to seek approval for a biological product. Some products originally approved under NDAs are now listed as BLAs. The Drugs@FDA record for tesamorelin acetate notes that the former NDA "was deemed to be a BLA on March 23, 2020" [2].

Compounding. Preparation of a medication by a licensed pharmacist by combining or altering ingredients. Section 503A sets the conditions for licensed pharmacies, including which bulk substances may be used [4].

Discontinued. A Drugs@FDA marketing status meaning an approved product is no longer sold. The sermorelin acetate record (NDA 019863) is listed as discontinued, with a "Federal Register determination that product was not discontinued or withdrawn for safety or effectiveness reasons" [1].

Drugs@FDA. FDA's public database of approved drug and biological products, showing application numbers, sponsors and marketing status [1][2].

Investigational. Describes a substance being studied in registered clinical trials without approval for the use under study.

NDA (New Drug Application). The application used to seek FDA approval for a new drug. Sermorelin acetate was approved under NDA 019863 [1].

Off-label. Use of an approved product for a purpose, population or form not included in its approved labelling.

PCAC (Pharmacy Compounding Advisory Committee). An FDA advisory committee that gives advice on scientific, technical and medical questions about compounding, including bulk substances nominated for the 503A bulks list [5].

USP monograph. A United States Pharmacopeia quality standard defining a substance's identity, strength and purity. Having a monograph is one route to 503A eligibility for a bulk substance [4].

Evidence terms

Animal model. A laboratory animal setup designed to reproduce features of a human condition. The ipamorelin study used rats to model postoperative ileus [3]. Animal results generate hypotheses; they do not establish effects in people.

Endpoint. A pre-specified outcome a study measures. The primary endpoint is the main one the study is powered to detect; secondary endpoints are additional. A surrogate endpoint is a laboratory or imaging measure used in place of a direct outcome.

In vitro. Experiments in cells or tissues outside a living organism ("in glass").

Phase 1, 2 and 3. Sequential stages of human clinical development, from small early-safety studies to large confirmatory trials. The full pathway from preclinical testing to regulatory review is set out in how a peptide becomes an approved medicine.

Preclinical. Research done before human trials, in cells (in vitro) or animals (in vivo). Many peptides discussed online have only preclinical evidence.

Randomized, placebo-controlled, double-blind. Three common trial safeguards. Randomization allocates participants by chance, balancing known and unknown factors. A placebo control gives one group an inactive look-alike, separating effects from chance and expectation. Double-blinding keeps both participants and investigators unaware of group allocation, reducing expectation bias.

Tools and further reading

A web glossary covers the terms that recur on this site. A dedicated reference book adds full structural diagrams, systematic nomenclature and cross-referencing. These neutral criteria help when comparing titles:

• Edition date. Pharmacology and regulatory vocabulary changes, so a recent edition is more likely to be current.

• Scope. A general science dictionary gives breadth; a biochemistry or pharmacology dictionary gives depth on receptors, enzymes and nomenclature.

• Publisher. University presses and established scientific publishers typically use specialist contributors and formal editing.

• Format. Print suits browsing; e-books add search. Confirm diagrams render clearly in digital editions.

• Price transparency. Compare list price across retailers by ISBN so the exact edition is clear before purchase.

For a first purchase, a recent-edition dictionary of biochemistry or pharmacology from a scientific publisher covers most of the vocabulary above in more depth.

Last updated: 18 October 2026.

Not medical advice. This page reports published research and regulatory records for information only.

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