Peptide classes overview: incretins, secretagogues, repair, melanocortin and more

Why classify peptides by function

The word 'peptide' describes a chemical structure — a chain of amino acids — not a category of biological effect. A peptide hormone that regulates blood glucose (like GLP-1) has essentially nothing pharmacologically in common with a peptide fragment studied for tissue repair (like BPC-157) beyond the fact that both are peptides. Grouping peptides by their receptor target and physiological system, rather than by chemistry alone, is far more useful for understanding what a given molecule plausibly does and how strong the supporting evidence is.

This guide organizes peptides referenced across GLPWiki into functional classes. For each class, it names the receptor or mechanism involved, gives examples with links to their dedicated pages, and states plainly where evidence is mature (large randomized trials, regulatory approval) versus where it is early or preclinical.

Incretins and related metabolic hormone mimetics

This is the most clinically mature class covered here. Incretin hormones are gut-derived signals released after eating that regulate insulin secretion, appetite, and gastric emptying. Synthetic and engineered analogues of these hormones now form the backbone of modern obesity and type 2 diabetes pharmacotherapy.

• GLP-1 receptor agonists: the original class, including semaglutide, liraglutide, dulaglutide, and exenatide. Multiple agents are FDA-approved for type 2 diabetes and/or chronic weight management, backed by large Phase 3 programmes (STEP, SUSTAIN, LEAD, AWARD, and others).
• Dual and multi-agonists: tirzepatide (GLP-1/GIP), retatrutide (GLP-1/GIP/glucagon), survodutide and mazdutide (GLP-1/glucagon), and efinopegdutide (GLP-1/glucagon) represent an active area of engineering additional receptor targets onto a GLP-1 backbone. Evidence maturity ranges from full approval (tirzepatide) to Phase 2/3 (retatrutide, survodutide, mazdutide, efinopegdutide).
• GIP as a standalone target: GIP receptor biology is mostly studied in combination with GLP-1 rather than alone; understanding of its independent contribution to weight and glycemic effects is still evolving.
• Oral non-peptide alternatives: orforglipron is a small-molecule (not a peptide) GLP-1 receptor agonist included here for taxonomic completeness given how often it is compared against peptide incretins; its Phase 3 programme is ongoing.
• GLP-2 and gut-trophic analogues: teduglutide and glepaglutide act on the GLP-2 receptor to promote intestinal adaptation, an entirely different clinical use case (short bowel syndrome) from GLP-1's metabolic effects, despite the similar name.
• Oxyntomodulin: a naturally occurring dual GLP-1/glucagon receptor agonist that served as an early proof-of-concept for the multi-agonist approach later engineered into synthetic drugs like survodutide.

Amylin analogues

Amylin is a hormone co-secreted with insulin from pancreatic beta cells that independently regulates satiety and gastric emptying through amylin receptors, which are distinct from GLP-1 receptors. Cagrilintide is a long-acting amylin analogue studied both alone and, more prominently, in fixed combination with semaglutide (as CagriSema) to test whether combining amylin and GLP-1 receptor agonism produces additive weight-loss effects. Amylin-class evidence in combination therapy is at Phase 3 for the leading combination candidates, while standalone amylin analogue data is comparatively less prominent in current peptide research discussion.

Growth-hormone secretagogues: two distinct receptor families

Peptides that stimulate growth hormone release are often lumped together, but they act through two pharmacologically distinct receptor systems.

Repair and regenerative research peptides

This class includes peptides studied, largely in preclinical animal models, for potential roles in tissue healing, angiogenesis, and inflammation modulation. BPC-157 (a synthetic fragment derived from a naturally occurring gastric protective protein) and TB-500 (a synthetic fragment related to the protein thymosin beta-4) are the most frequently discussed members of this class. Honest framing matters here: the large majority of available evidence for both compounds comes from rodent and in vitro studies; robust randomized controlled human trial data establishing efficacy and safety for tissue repair indications is not currently available in the way it is for approved incretin drugs. Claims about human efficacy for these two peptides should be treated as extrapolations from preclinical data, not established clinical fact.

GHK-Cu, a naturally occurring copper-binding tripeptide, is studied for skin and wound-related biology, with more topical/dermatologic human data (including some clinical cosmetic research) than BPC-157 or TB-500, though it still falls well short of the trial density seen in the incretin class.

Immune and thymic peptides

Thymosin alpha-1 is a peptide derived from thymic tissue that has been studied internationally, including in some countries where it holds approval for specific indications such as certain chronic infections and as an immune adjunct, giving it a comparatively longer clinical research history than many peptides in the repair or longevity categories. This does not mean thymosin alpha-1 is universally approved or that its evidence base matches that of incretin drugs; regulatory status varies significantly by country and indication, and this variability should be checked rather than assumed.

KPV, a tripeptide fragment derived from alpha-melanocyte-stimulating hormone, is studied preclinically for anti-inflammatory effects, particularly in gut inflammation models. Human clinical trial data for KPV specifically remains limited, and most published evidence is preclinical.

Melanocortin receptor peptides

The melanocortin receptor family (MC1R through MC5R) mediates diverse effects from pigmentation to sexual function to appetite regulation, and different peptides in research and clinical use target different receptor subtypes within this family.

• PT-141 (bremelanotide) is an MC4R-preferential agonist FDA-approved (under the brand Vyleesi) for hypoactive sexual desire disorder in premenopausal women, giving it a genuine regulatory approval within this class.
• Melanotan II is a non-selective melanocortin receptor agonist studied for pigmentation and libido effects; it is not FDA-approved for any indication, and its safety and impurity profile in unregulated preparations is a distinct concern raised in the safety literature independent of receptor pharmacology.

Longevity and mitochondrial peptides

This is among the least clinically mature classes discussed here. Epitalon (a synthetic tetrapeptide related to epithalamin, a pineal gland extract) has been studied, primarily in older Russian-language research literature and some more recent preclinical work, for potential effects on telomerase activity and aging biomarkers; large-scale, peer-reviewed randomized human trials are not currently available. MOTS-c is a mitochondrial-derived peptide studied preclinically for metabolic and exercise-related effects; human trial data is sparse relative to preclinical mechanistic work. Statements about anti-aging or longevity benefits for either peptide should be treated as hypotheses supported by early-stage or preclinical data rather than established outcomes.

Nootropic peptides and additional research categories

Semax and selank are peptides developed and studied predominantly within Russian pharmacological research, proposed to have effects on cognition, mood, and neurotrophic signaling (including modulation of BDNF expression in some preclinical work). Human trial data exists primarily from Russian-language clinical research with methodological standards and regulatory oversight that differ from Western Phase 3 trial norms, and independent replication in large randomized trials meeting typical Western regulatory standards is limited. Both peptides lack FDA approval for any indication.

AOD-9604 is a modified fragment of human growth hormone studied for potential fat-metabolism effects without the broader growth-promoting effects of full-length GH; human trial data specifically supporting meaningful fat-loss efficacy has been mixed to limited, and it is not FDA-approved as a weight-management therapy. IGF-1 LR3 is a long-acting analogue of insulin-like growth factor 1 studied primarily in preclinical and research contexts for muscle and tissue growth signaling; it carries substantial theoretical safety considerations related to unregulated IGF-1 receptor activation that are more thoroughly documented in mechanistic and case-report literature than in controlled human efficacy trials.

How to read evidence maturity across classes

When comparing peptides across these classes, it helps to ask a consistent set of questions: Has this molecule completed a randomized, placebo-controlled human trial? Is there a Phase 3 programme, and has it read out? Has any regulatory agency approved it for a specific indication, and if so, which one exactly — approval for one narrow indication (like tesamorelin for lipodystrophy or PT-141 for a specific sexual dysfunction diagnosis) does not imply broader efficacy or safety for other uses some communities have extrapolated to.

The incretin class is unusual in how much Phase 3, multi-thousand-participant trial data exists across multiple molecules. Most other classes surveyed here — repair peptides, longevity peptides, nootropic peptides, and several growth-hormone secretagogues — have comparatively thin human trial data, with much of the mechanistic rationale resting on preclinical or small early-phase studies. This is not a criticism of the biology being uninteresting; it is a statement about where each class currently sits on the evidence maturity spectrum, which is exactly the kind of distinction a citation-first reference should make explicit rather than blur.