Peptides 101: A Clinician's Reference

1. What a peptide is and isn't

A peptide is a polymer of amino acids linked by amide bonds. The boundary between a 'peptide' and a 'protein' is conventional rather than absolute; biochemists often use a chain length of approximately fifty residues as the dividing line, but the threshold is not standardised. Insulin, at fifty-one residues across two chains joined by disulphide bonds, is variously called a 'peptide hormone' and a 'small protein' depending on context. (see IUPAC Compendium of Chemical Terminology)

Three categories of molecule are commonly conflated in popular literature and should be kept distinct in clinical reading:

• Endogenous signalling peptides — naturally occurring molecules such as insulin, glucagon, GLP-1, GIP, vasopressin, oxytocin, and somatostatin. These have well-characterised receptors and physiological roles.
• Therapeutic peptide drugs — synthetic or recombinant analogues of endogenous peptides that have completed regulatory review and are approved for specific indications. Semaglutide, liraglutide, octreotide, leuprolide, teriparatide, and many others fall in this category.
• Investigational and research-grade peptides — sequences that are studied in academic, preclinical, or early-clinical contexts but are not approved for human therapeutic use. The compounds catalogued at /products/ are predominantly in this category and are supplied strictly For Research Use Only.

The clinical caution implicit in this framing — that 'peptide' alone tells you nothing about whether a compound is safe, effective, or legal for human administration — is the single most important orientation point of this guide.

2. Functional classes of peptides

Peptides are conveniently grouped by physiological action. The classes below are not mutually exclusive and are intended as a high-level map for navigation rather than an exhaustive taxonomy.

Incretin and metabolic peptides

Includes GLP-1 receptor agonists, GIP receptor agonists, dual and triple agonists (tirzepatide, retatrutide), and amylin analogues (cagrilintide). These are the most commercially developed peptide class today and the focus of a dedicated pillar guide.

Growth-hormone secretagogues and growth-axis peptides

Includes growth-hormone-releasing hormone analogues (sermorelin, tesamorelin, CJC-1295) and growth-hormone-releasing peptides (ipamorelin, GHRP-2, GHRP-6, hexarelin). Tesamorelin is FDA-approved for HIV-associated lipodystrophy; the others are predominantly research compounds. See /docs-tesamorelin-15mg/ and related docs pages.

Tissue-repair and recovery peptides

BPC-157, TB-500 (thymosin beta-4 fragment), and copper-peptide complexes such as GHK-Cu fall in this group. Mechanism literature is variably mature; clinical evidence in approved indications is sparse to absent. See /docs/ for per-compound literature summaries.

Mitochondrial and longevity peptides

Includes MOTS-c, humanin, and epitalon. MOTS-c was characterised by Lee et al. in 2015 (PMID 25738459). (Lee et al., 2015, Cell Metabolism — PMID 25738459) These compounds are at varying stages of mechanistic and translational evidence.

Neuropeptides and cognition

Selank, semax, dihexa, cerebrolysin, and DSIP. Most are early-stage or country-specific; clinical translation in the U.S. is limited.

Immune and repair peptides

Thymosin alpha-1, KPV (an α-MSH fragment), LL-37, ARA-290. Thymosin alpha-1 has approval in some non-U.S. jurisdictions for hepatitis indications.

Cosmetic / dermal peptides

Argireline (acetyl hexapeptide-8), copper peptides (GHK-Cu), Matrixyl. Predominantly topical formulations; regulated as cosmetics rather than drugs in most jurisdictions.

3. Pharmacokinetics — why peptides are engineered

The pharmacokinetic limitations of native peptides are the single greatest driver of how therapeutic peptides are designed. Three problems dominate.

Proteolytic degradation

Endopeptidases and exopeptidases in plasma, on cell surfaces (notably DPP-4, neprilysin), and in tissues rapidly cleave native peptide sequences. The plasma half-life of native GLP-1 is approximately 1.5 to 2 minutes for this reason. Engineering strategies include N-terminal substitutions that block exopeptidase recognition, D-amino-acid substitutions, backbone modifications, and cyclisation.

Renal clearance

Small peptides are filtered at the glomerulus and partially reabsorbed and degraded in the proximal tubule. Increasing molecular size or promoting reversible plasma-protein binding (e.g., albumin) reduces renal clearance and extends half-life.

Oral bioavailability

Native peptides are typically destroyed by gastric acid and pancreatic proteases and have poor permeability across the intestinal epithelium. Most peptide therapeutics are therefore delivered by injection. Oral peptide formulations such as oral semaglutide require permeation enhancers (SNAC) and strict fasting administration to achieve clinically relevant exposure.

Common pharmacokinetic engineering strategies

• Fatty-acid acylation — attach a long-chain fatty acid (C16–C20) via a glutamic-acid spacer to promote albumin binding (semaglutide, liraglutide, retatrutide).
• PEGylation — covalent conjugation of polyethylene glycol to increase hydrodynamic radius and reduce renal clearance.
• Fc fusion — fusion to an antibody Fc fragment to leverage neonatal Fc receptor recycling (dulaglutide).
• D-amino-acid substitution — replace stereochemistry at proteolytically vulnerable residues to block enzymatic recognition.
• Cyclisation — head-to-tail or disulphide-bridged cyclic peptides resist exopeptidase cleavage and often gain conformational selectivity.

4. Manufacturing and quality control

Most therapeutic and research peptides under fifty residues are manufactured by solid-phase peptide synthesis (SPPS), the technique pioneered by Bruce Merrifield. Sequences longer than approximately fifty residues, or sequences requiring complex disulphide patterns, are increasingly produced by recombinant expression in E. coli, yeast, or mammalian systems.

Solid-phase peptide synthesis

SPPS proceeds by stepwise coupling of N-α-protected amino-acid building blocks to a growing chain anchored to a solid resin. The two principal protecting-group strategies are Boc (tert-butoxycarbonyl) and Fmoc (9-fluorenylmethoxycarbonyl); modern commercial production is dominated by Fmoc chemistry. After full chain assembly, the peptide is cleaved from the resin and global side-chain deprotection is performed. Crude product is purified by reversed-phase HPLC and characterised by mass spectrometry.

Quality attributes that matter for research-grade material

• Purity by HPLC — typically reported as area-percent at a defined wavelength (commonly 220 nm). Research-grade material is generally specified at ≥98% HPLC purity. (see USP General Chapters and current peptide-CMC industry guidance)
• Identity by mass spectrometry — observed mass within a defined tolerance of theoretical mass.
• Counter-ion content — most synthetic peptides are isolated as trifluoroacetate (TFA) or acetate salts; the salt form is part of the specification.
• Water content — Karl Fischer titration; affects per-mg dose calculations.
• Endotoxin — important for any peptide intended for cell culture or animal research; Limulus amebocyte lysate (LAL) testing per USP <85> or equivalent.
• Residual solvents — gas chromatography, per ICH Q3C.

Each lot should be accompanied by a Certificate of Analysis (CoA). A pillar guide on how to read a CoA details what each line item means and what to look for when comparing suppliers.

5. Regulatory framing

The regulatory status of a peptide is independent of its biological plausibility. A peptide may have a well-characterised receptor, a coherent mechanism, and supportive preclinical data, and yet be unapproved for any human indication. Three regulatory categories are most relevant in U.S. practice:

• FDA-approved drug products — peptides that have completed New Drug Application or Biologics License Application review and are marketed for specific indications. Examples include semaglutide, liraglutide, tesamorelin, octreotide, teriparatide, and leuprolide.
• Compounding categories — under sections 503A and 503B of the Federal Food, Drug, and Cosmetic Act, certain bulk substances may be compounded by licensed pharmacies under defined conditions. The eligibility of specific peptides for 503A compounding is governed by the FDA's bulk-substance review process and has changed over time. (see FDA.gov bulk-substance category-1 / category-2 nominations)
• Research-use-only (RUO) materials — peptides supplied for in vitro and laboratory research, not for human or veterinary administration, diagnostic, or therapeutic use. RUO labelling is not a regulatory pathway to clinical use; it is a compliance posture for material supplied to qualified researchers.

Statements in popular media that a peptide is 'legal' or 'approved' should be checked against the current FDA database for the specific compound and indication.

6. How to read the peptide literature

Peptide research spans rigorous, well-controlled clinical trials at one end and underpowered, overinterpreted preliminary studies at the other. A few simple discipline points dramatically improve the value of literature review:

• Source the original publication. Many peptide claims propagate through secondary summaries that distort or omit limitations. Always trace a claim to a PMID or DOI you can read.
• Note the model. In vitro, rodent, and human data are not interchangeable. A claim grounded only in cell culture or rodent work should be reported as such.
• Note the dose and route. Weight-of-evidence claims that ignore the administered dose, route (intravenous, subcutaneous, oral, intranasal), and duration are uninformative.
• Note the funding and author affiliation. Industry-sponsored work is not invalid, but disclosure context matters.
• Distinguish association from causation. Observational and registry data establish association; randomised trials with intention-to-treat analysis establish efficacy.
• Look for replication. A single small trial is a hypothesis, not a conclusion.

The Research News feed on this site is curated from PubMed, FDA press releases, and ClinicalTrials.gov, and is intended to surface primary literature rather than secondary commentary.

References

This guide cites primary peer-reviewed and regulatory sources where the underlying claim has been verified. A small number of supporting items remain in active verification and are listed below for transparency.

Verified primary references

1. Lee C et al. The Mitochondrial-Derived Peptide MOTS-c Promotes Metabolic Homeostasis. Cell Metabolism 2015. PMID 25738459.

Still seeking primary citation

• Conventional residue threshold separating peptides from proteins — IUPAC reference
• USP / industry standard for research-grade HPLC purity threshold
• Current FDA list of peptides eligible / ineligible for 503A bulk-substance compounding (FDA.gov)