Peptide Research Glossary
Key terms in peptide research, pharmacology, and clinical science. Definitions focus on how these terms are used in the research literature.
A
- Amino acid(amino acids)
- The molecular building block of peptides and proteins. Twenty standard amino acids are encoded by the human genome; each has a common backbone structure and a unique side chain that determines its chemical properties.
- See: Amino Acid Chemistry →
B
- Bioavailability(bio-availability, oral bioavailability)
- The fraction of an administered dose that reaches systemic circulation in an active form. Oral bioavailability of most research peptides is very low due to gastrointestinal enzyme degradation. Subcutaneous injection typically achieves 70–100% bioavailability.
- See: Peptide Bioavailability Guide →
- BPC-157(BPC 157, PL 14736)
- A synthetic pentadecapeptide (15 amino acids) derived from a fragment of human gastric juice protein, studied in animal models for effects on tissue repair, angiogenesis, and gastrointestinal healing. It has no completed human randomized controlled trials and is not FDA-approved for any use; FDA's Pharmacy Compounding Advisory Committee gave it a favorable recommendation for compounding eligibility in July 2026.
- See: BPC-157 Research and Mechanisms →
D
- DPP-4(dipeptidyl peptidase-4, dipeptidyl peptidase IV)
- A serine protease present in plasma and on cell surfaces that cleaves dipeptides from the N-terminus of peptides with proline or alanine at position 2. Responsible for the rapid degradation of native GLP-1 and GHRH. Inhibited by the gliptin class of diabetes drugs.
F
- FDA(U.S. Food and Drug Administration, Food and Drug Administration)
- The U.S. Food and Drug Administration, the federal agency responsible for regulating drugs, biologics, and — through Section 503A of the FD&C Act — pharmacy compounding. FDA evaluates candidate bulk drug substances for compounding eligibility with the advice of the Pharmacy Compounding Advisory Committee (PCAC), a separate and less rigorous process than full drug approval.
- See: Introduction to Peptide Compounding & FDA Regulation →
G
- GHK-Cu(GHK-Copper, copper peptide)
- A naturally occurring copper-binding tripeptide (glycyl-L-histidyl-L-lysine) studied for signaling effects linked to collagen synthesis, wound healing, and antioxidant activity in skin. Research is concentrated in cell-culture, animal, and topical cosmetic studies rather than large randomized human trials. GHK-Cu is used in cosmetic formulations and has also been discussed as a nominated substance in FDA's peptide compounding review process.
- See: Copper Peptides (GHK-Cu) Research →
- GLP-1(glucagon-like peptide-1, GLP-1 receptor agonist)
- Glucagon-like peptide-1, an incretin hormone produced in the gut in response to food intake. GLP-1 stimulates insulin secretion, suppresses glucagon, slows gastric emptying, and reduces appetite. It is the biological target of a class of diabetes and obesity medications including semaglutide, tirzepatide, and retatrutide.
- See: GLP-1 Receptor Agonists Research →
- GPCR(G protein-coupled receptor, G protein coupled receptor)
- A large family of cell-surface receptors that transmit signals through intracellular G proteins. Most peptide hormones and research peptides act through GPCRs. Named for their characteristic seven-transmembrane domain structure.
- Growth hormone(GH, hGH)
- A peptide hormone produced by the pituitary gland that stimulates growth, cell reproduction, and metabolic regulation. In adults, growth hormone governs body composition, bone density, and fat metabolism. It is the downstream target of growth hormone-releasing hormone (GHRH) and the goal of growth hormone secretagogue research.
- See: Growth Hormone Secretagogues Overview →
- Growth hormone secretagogue(GHS, growth hormone secretagogues)
- A class of compounds studied for their ability to stimulate release of growth hormone from the pituitary gland, acting through the GHS receptor (GHS-R1a). Distinct from GHRH analogs, which act through a different receptor.
- See: GHS Overview →
H
- Half-life(t½, elimination half-life)
- The time required for the plasma concentration of a compound to fall by 50% after administration. Most unmodified natural peptides have half-lives measured in minutes; synthetic modifications can extend this to hours or days.
- See: Peptide Half-Life and Stability →
I
- In vitro(in-vitro, cell culture)
- Literally 'in glass' — referring to experiments conducted in cell cultures, test tubes, or other laboratory containers outside of a living organism. In vitro findings provide mechanistic hypotheses but do not establish that effects occur in a living system.
- See: How to Read a Research Study →
- In vivo(in-vivo, animal study)
- Literally 'within the living' — referring to experiments conducted in a living organism. Animal in vivo studies are closer to biological reality than in vitro work but do not automatically predict human outcomes.
M
- Melanocortin(melanocortin receptor, MC receptor)
- A family of peptide hormones (including alpha-MSH and ACTH) derived from the POMC precursor protein, and the five G protein-coupled receptors they activate (MC1R–MC5R). Different receptor subtypes are linked to different physiology — pigmentation (MC1R), energy homeostasis (MC3R/MC4R), and exocrine function (MC5R) — which is why melanocortin-targeting research peptides can produce effects across multiple, seemingly unrelated systems.
- See: Introduction to Melanocortin Research →
P
- PCAC(Pharmacy Compounding Advisory Committee)
- The Pharmacy Compounding Advisory Committee, a standing FDA advisory committee of outside experts who review nominated bulk drug substances — including research peptides — and give the agency nonbinding recommendations on compounding eligibility. PCAC does not vote on drug approval and has no independent authority to add a substance to FDA's 503A Bulks List.
- See: Introduction to Peptide Compounding & FDA Regulation →
- Peptidase / Protease(peptidase, protease)
- Enzymes that cleave peptide bonds. Peptidases act on short peptides; proteases typically refer to enzymes acting on proteins. The gastrointestinal tract and blood plasma both contain active peptidases, which is why most peptides have poor oral bioavailability.
- Peptide(peptides)
- A short chain of amino acids linked by peptide bonds. Typically defined as 2–50 amino acids in length. Longer chains that adopt stable three-dimensional structures are generally called proteins.
- See: What Are Peptides? →
- Peptide bond(amide bond, peptide bonds)
- The covalent bond that links the carboxyl group of one amino acid to the amino group of the next. Formed by condensation (release of water). Cleaved by enzymes called peptidases or proteases.
R
- Randomized controlled trial(RCT, randomized trial)
- A clinical study design in which participants are randomly assigned to treatment or control groups. When properly blinded and pre-registered, the RCT is the strongest design for establishing whether an intervention causes a clinical effect.
- See: Understanding Clinical Trials →
- Receptor agonist(agonist, full agonist)
- A compound that binds to a receptor and activates it, producing a biological response. A full agonist produces the maximum response achievable for that receptor; a partial agonist produces a submaximal response even at full receptor occupancy.
- See: Peptide Receptor Binding →
- Receptor antagonist(antagonist, receptor antagonists)
- A compound that binds to a receptor without activating it, thereby blocking the receptor from being activated by endogenous ligands or other agonists. Does not produce a direct biological response but inhibits the response of other compounds.
S
- Solid-phase peptide synthesis(SPPS, peptide synthesis)
- The standard laboratory method for manufacturing synthetic peptides. Amino acids are added sequentially to a solid polymer resin support. The primary method used to produce research-grade peptides.
- See: How Peptides Are Synthesized →
T
- TB-500(TB 500, thymosin beta-4 fragment)
- A synthetic fragment of thymosin beta-4, a naturally occurring protein involved in actin regulation and cell migration. Research has examined TB-500 in animal models for effects on wound healing and soft tissue repair. It has limited published human trial data and is not FDA-approved; FDA's Pharmacy Compounding Advisory Committee gave it a favorable recommendation for compounding eligibility in July 2026.
- See: TB-500 (Thymosin Beta-4) Research →