Compound Classification & Category Terms

While ADCs use antibodies rather than peptides as targeting agents, the conjugation chemistry, linker design, and targeted delivery concepts are shared with peptide-drug conjugates. ADC components: monoclonal antibody (targeting) + linker (cleavable — protease, acid, or glutathione-sensitive; or non-cleavable) + cytotoxic payload (typically potent microtubule inhibitors or DNA-damaging agents). The drug-to-antibody ratio (DAR) affects efficacy and tolerability. For peptide-based targeted delivery, the smaller size of peptides (2-5 kDa vs ~150 kDa for antibodies) offers advantages in tumour penetration (faster diffusion through interstitial space) and manufacturing (chemical synthesis vs biological production). However, peptides have shorter half-lives than antibodies, requiring half-life extension strategies.

Approved antimicrobial peptides span multiple structural classes and mechanisms: glycopeptides (vancomycin, telavancin, dalbavancin, oritavancin — inhibit cell wall synthesis by binding D-Ala-D-Ala), lipopeptides (daptomycin — calcium-dependent membrane disruption; colistin/polymyxin B — electrostatic disruption of gram-negative outer membrane), cyclic peptides (bacitracin — interferes with undecaprenyl pyrophosphate recycling in cell wall synthesis), linear peptides (gramicidin — forms ion channels in bacterial membranes), and echinocandin-related (rezafungin — inhibits fungal beta-1,3-glucan synthase). The rise of multidrug-resistant bacteria (MRSA, VRE, carbapenem-resistant Enterobacteriaceae) has renewed interest in peptide antibiotics. Novel AMP development strategies include: peptide engineering for improved selectivity, combination therapy approaches, and anti-biofilm peptides.

Approved drug status means: completed Phase I-III clinical trial programme, met safety and efficacy standards established by the regulatory authority, received marketing authorisation (NDA/BLA approval in US, MA in EU, NOC in Canada), manufactured under cGMP conditions, subject to ongoing pharmacovigilance and post-marketing requirements, and covered by product liability regulations. The 73 approved peptide compounds tracked by PeptideTrace have varying evidence bases — from blockbusters with extensive trial programmes (semaglutide: >10 Phase III trials, >10,000 participants) to rare disease drugs with smaller programmes (vosoritide, trofinetide). Approval in one jurisdiction does not guarantee approval in others — regulatory standards, review timelines, and clinical data requirements differ.

Biological products are distinguished from conventional drugs by: molecular complexity (large, heterogeneous molecules that cannot be fully characterised by physicochemical methods alone), sensitivity to manufacturing conditions (the product is defined partly by its manufacturing process — 'the process is the product'), and biological source (produced by living systems). The Biologics Price Competition and Innovation Act (BPCIA, 2010) created the US biosimilar pathway (351(k)). Products transitioned from NDA to BLA regulation: insulin, growth hormone, and other protein products were reclassified as biologics under the BPCIA 'deemed to be a license' provision (effective March 2020). This transition affects market exclusivity provisions, patent challenge procedures, and generic/biosimilar pathways. The distinction between small molecule peptides (NDA) and biological peptides (BLA) is based on molecular complexity and manufacturing method rather than a strict size cutoff.

BPC-157 (GEPPPGKPADDAGLV) is a 15 amino acid sequence derived from a larger protective protein (BPC) found in human gastric juice. Preclinical literature: over 100 published studies, primarily from a single research group (University of Zagreb, Croatia). Investigated effects include: tendon, ligament, and muscle healing (in rat models of Achilles tendon transection, MCL rupture, muscle crush injury), gastrointestinal protection (cytoprotection against various GI insults), wound healing, bone healing, neuroprotection, and vascular effects. Proposed mechanisms: nitric oxide system modulation, VEGF upregulation (promoting angiogenesis), FAK-paxillin pathway activation, and interaction with the dopaminergic system. Critical assessment: evidence is almost entirely from a single research group in animal models; no completed human clinical trials registered on ClinicalTrials.gov; the lack of independent replication and human data is a significant limitation.

LL-37 (37 amino acids beginning with Leu-Leu) is the only human cathelicidin, cleaved from the precursor protein hCAP-18 by proteinase 3. LL-37 has: broad-spectrum antimicrobial activity (gram-positive and gram-negative bacteria, fungi, viruses — through membrane disruption), anti-biofilm activity (disrupting bacterial biofilms at sub-antimicrobial concentrations), wound healing promotion (stimulating re-epithelialisation and angiogenesis), and immunomodulatory effects (modulating cytokine production, promoting chemotaxis, and enhancing phagocytosis). Vitamin D upregulates hCAP-18/LL-37 expression, linking vitamin D deficiency to increased infection susceptibility. Therapeutic applications under investigation: topical LL-37 for chronic wounds and diabetic ulcers (Phase I/II trials), LL-37 derivatives with improved selectivity and reduced haemolytic activity, and LL-37-based anti-biofilm strategies for device-related infections.

CPPs are typically 5-30 amino acid cationic or amphipathic peptides that cross cell membranes through: direct penetration (creating transient pores or inverted micelles in the lipid bilayer), macropinocytosis (stimulating cellular uptake through membrane ruffling), or endocytosis (receptor-mediated or adsorptive internalisation). Classic CPPs: Tat peptide (GRKKRRQRRRPPQ, from HIV Tat protein), penetratin (RQIKIWFQNRRMKWKK, from Antennapedia homeodomain), and polyarginine (R8-R12). CPPs can deliver diverse cargo: peptide therapeutics, proteins, nucleic acids, nanoparticles, and imaging agents. Limitations: poor selectivity (CPPs enter all cell types, not just target cells), endosomal trapping (cargo may be degraded in lysosomes after endocytic uptake), and potential membrane toxicity at high concentrations. Targeted CPPs (conjugated to cell-specific ligands) address the selectivity limitation.

Cyclic peptide drug design strategies: head-to-tail cyclisation (N→C backbone linkage — cyclosporine's 11 amino acid ring), disulphide cyclisation (Cys-Cys bridge — octreotide has D-Phe-Cys-Phe-D-Trp-Lys-Thr-Cys-Thr-ol with Cys2-Cys7 bridge; oxytocin has Cys1-Cys6 bridge), lactam bridge (side chain Lys-Asp/Glu linkage), thioether bridge (lanthionine — natural in lantibiotics), and stapled peptides (hydrocarbon bridge across helix turns). Cyclic peptide advantages: constrained conformation enhances receptor binding (pre-organisation reduces entropic cost of binding), improved metabolic stability (no free termini for exopeptidases, reduced endopeptidase access), and potentially improved membrane permeability (cyclisation can mask polar groups). Rule of 5 violations are tolerated in cyclic peptides — cyclosporine (MW 1202, 7 H-bond donors) achieves oral bioavailability through chameleonic conformational flexibility.

Human defensins: alpha-defensins (HNP-1 through HNP-4, found in neutrophil granules; HD-5, HD-6, found in Paneth cells of the small intestine) and beta-defensins (HBD-1 through HBD-4, expressed by epithelial cells of skin, airways, and urogenital tract). Structure: 18-45 amino acids with 6 conserved cysteines forming 3 disulphide bonds in a characteristic beta-sheet scaffold. Mechanism: electrostatic attraction to negatively charged bacterial membranes (microbial membranes have more anionic phospholipids than human cell membranes), followed by membrane disruption through pore formation or carpet model. Defensins also modulate adaptive immunity — chemotactic for dendritic cells and T cells, linking innate and adaptive responses. Therapeutic defensin development faces challenges: production cost (multiple disulphide bonds complicate synthesis), potential cytotoxicity at antimicrobial concentrations, and salt sensitivity.

Echinocandins are cyclic hexapeptides with a fatty acid side chain that inhibit beta-1,3-D-glucan synthase — an enzyme essential for fungal cell wall integrity that has no mammalian counterpart (explaining the favourable safety profile). Rezafungin is a novel long-acting echinocandin (half-life ~133 hours) approved for invasive candidiasis, enabling once-weekly IV dosing vs daily dosing for older echinocandins (caspofungin, micafungin, anidulafungin). Rezafungin's structural modification (a choline ether at the C5 ornithine position) increases stability and extends half-life. The echinocandin class illustrates how peptide-based antifungals can target pathogen-specific pathways (beta-1,3-glucan is absent in human cells) to achieve selective toxicity. Resistance through FKS gene mutations (encoding glucan synthase subunits) is clinically significant in Candida species.

Fusion protein/peptide design strategies: Fc fusion (therapeutic peptide fused to the Fc region of IgG antibody — provides half-life extension through FcRn-mediated recycling; dulaglutide = GLP-1 analogue-Fc fusion, romiplostim = thrombopoietin-binding peptide-Fc fusion), albumin fusion (peptide fused to human serum albumin — extending half-life to albumin's ~19-day half-life), CTP fusion (C-terminal peptide of hCG fused to therapeutic protein — somatrogon uses CTP technology for long-acting GH), and targeting fusion (combining a targeting peptide with a therapeutic payload). The linker between fusion partners is critical — it must be long enough to allow independent folding and function of both domains while being stable enough to resist proteolytic cleavage. Rigid vs flexible linker design affects the spatial relationship between fusion partners.

Glycopeptide mechanism: the peptide backbone forms 5 hydrogen bonds with the D-Ala-D-Ala terminus of peptidoglycan precursors (lipid II), physically blocking transglycosylation and transpeptidation steps of cell wall synthesis. Vancomycin resistance (VRE): bacteria modify the target to D-Ala-D-Lac, reducing binding affinity 1000-fold. The vanA and vanB gene clusters encode resistance enzymes. Newer glycopeptides address resistance partially: telavancin has additional membrane depolarisation activity; dalbavancin and oritavancin have enhanced lipophilic side chains enabling membrane anchoring and dimerisation, increasing binding avidity and providing activity against some VanB-type VRE. Dalbavancin's ultra-long half-life (~346 hours) enables single-dose or two-dose treatment of skin infections — the longest dosing interval of any approved antibiotic.

GnRH (pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2) is a 10 amino acid peptide with both terminal modifications (N-terminal pyroglutamate, C-terminal amidation). The GnRH receptor is unique among GPCRs — it lacks a cytoplasmic tail, preventing beta-arrestin-mediated desensitisation. Instead, desensitisation relies entirely on receptor internalisation and transcriptional downregulation, which takes 1-2 weeks. Agonist modifications: position 6 substitution (D-amino acid — D-Ser(tBu), D-Leu, D-Trp, D-His(Bzl) — resists enzymatic cleavage and enhances receptor affinity), position 10 modification (des-Gly10 with ethylamide — enhances potency). These modifications increase half-life from ~4 minutes (native GnRH) to hours. The agonist class (goserelin, leuprolide, triptorelin, histrelin, nafarelin) shares this design strategy. Antagonist design requires bulkier substitutions at multiple positions to prevent receptor activation while maintaining binding.

GHRPs are typically 6-7 amino acid synthetic peptides that activate the ghrelin receptor (GHS-R1a) on pituitary somatotrophs. Key GHRPs and their distinguishing features: GHRP-6 (His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 — the first GHRP, strong appetite stimulation via GHS-R, increases cortisol and prolactin), GHRP-2 (D-Ala-D-βNal-Ala-Trp-D-Phe-Lys-NH2 — most potent GHRP, moderate appetite/cortisol effects), ipamorelin (Aib-His-D-2-Nal-D-Phe-Lys-NH2 — most selective, minimal appetite/cortisol/prolactin elevation, considered cleanest GH stimulus), hexarelin (His-D-2MeTrp-Ala-Trp-D-Phe-Lys-NH2 — high potency, significant cortisol/prolactin effects, notable tachyphylaxis with chronic use). Ipamorelin's selectivity profile has made it the most discussed GHRP in research peptide communities. None are approved for therapeutic use in major markets. MK-677 (ibutamoren) is often discussed alongside GHRPs but is a non-peptide oral GHS-R agonist.

Cyclosporine mechanism: binds to the intracellular protein cyclophilin → the cyclosporine-cyclophilin complex inhibits calcineurin (a serine-threonine phosphatase) → prevents dephosphorylation of NFAT (nuclear factor of activated T-cells) → NFAT cannot translocate to the nucleus → IL-2 gene transcription is blocked → T-cell activation and proliferation are inhibited. Cyclosporine's discovery (from the fungus Tolypocladium inflatum) and development in the 1980s revolutionised organ transplantation by enabling effective immunosuppression. Glatiramer acetate mechanism: a random polymer of L-alanine, L-glutamic acid, L-lysine, and L-tyrosine that modulates the immune response in MS through multiple mechanisms — promoting anti-inflammatory Th2 cells, inducing regulatory T cells, and competing with myelin antigens for MHC binding. Corticotropin (ACTH) has immunomodulatory effects beyond adrenal steroid stimulation, acting directly on melanocortin receptors on immune cells.

Investigational compound status indicates: formal IND application filed with regulatory authority, at least one clinical trial registered and conducted under regulatory oversight, defined protocol with ethics committee approval, and safety monitoring in place. This distinguishes investigational compounds from research compounds, which may have only preclinical data without regulatory involvement. PeptideTrace classifies 20 compounds as investigational — primarily in the weight management pipeline (next-generation GLP-1 RAs, triple agonists such as retatrutide, amylin analogues such as cagrilintide). The investigational classification may change as compounds advance (moving to 'approved' upon approval) or fail (potentially reclassified or removed). Key investigational endpoints (Phase II/III data) are tracked for these compounds as they progress toward potential regulatory submissions.

Lantipeptides are ribosomally synthesised and post-translationally modified peptides (RiPPs) containing lanthionine (Lan) and/or methyllanthionine (MeLan) bridges — thioether crosslinks formed by post-translational dehydration and cyclisation of serine/threonine with cysteine. These crosslinks create conformationally constrained ring structures. Nisin (34 amino acids, 5 ring structures) is the most extensively studied lantibiotic — used as a food preservative (E234) for over 50 years with no significant resistance development. Nisin's mechanism: binds lipid II (the same target as vancomycin) with high affinity and forms pores in the bacterial membrane — a dual mechanism. Pharmaceutical lantibiotic development targets: MRSA, VRE, and other drug-resistant gram-positive pathogens. The complexity of lanthionine bridge formation makes chemical synthesis challenging — production typically uses fermentation of the producing organism.

Linear peptide stabilisation strategies: N-terminal modifications (acetylation blocks aminopeptidases, pyroglutamate forms naturally), C-terminal modifications (amidation blocks carboxypeptidases), internal modifications (D-amino acid substitution at protease-sensitive sites, N-methylation reducing protease recognition, Aib substitution for DPP-4 resistance), half-life extension (fatty acid conjugation for albumin binding — semaglutide, liraglutide; PEG conjugation — palopegteriparatide; Fc fusion — dulaglutide, romiplostim), and formulation-based approaches (depot formulations, implants). Linear peptide drug examples span diverse lengths: TRH (3 aa), leuprolide (9 aa), goserelin (10 aa), octreotide active sequence analogue alternatives (8-14 aa), semaglutide (31 aa), teriparatide (34 aa), and tesamorelin (44 aa). Each requires a tailored stability strategy based on its specific degradation profile.

Lipoglycopeptides combine the glycopeptide core (vancomycin-like binding to D-Ala-D-Ala) with a lipophilic side chain providing additional mechanisms: membrane anchoring (the lipid tail inserts into the bacterial membrane, positioning the glycopeptide for enhanced target access), membrane depolarisation (direct disruption of membrane potential), and increased membrane permeability. This dual mechanism (cell wall synthesis inhibition + membrane disruption) provides bactericidal activity against some organisms where vancomycin is only bacteriostatic. Pharmacokinetic advantages of lipophilicity: telavancin concentrates in epithelial lining fluid (relevant for pneumonia); dalbavancin has a half-life of ~14 days (enabling single-dose treatment); oritavancin has a half-life of ~245 hours with extensive tissue distribution. These PK properties reduce healthcare burden through less frequent dosing.

Lipopeptide structural biology: the peptide component provides receptor/target specificity while the lipid component facilitates membrane interaction. Daptomycin's mechanism: in the presence of calcium ions, daptomycin oligomerises and inserts its lipid tail into the bacterial cytoplasmic membrane, forming pores that depolarise the membrane and disrupt ion gradients — causing rapid cell death without lysis (this may reduce endotoxin release compared to lytic antibiotics). The lipidation strategy in drug design: fatty acid chain length affects pharmacokinetics — C-16 palmitic acid (liraglutide, half-life ~13h) vs C-18 fatty diacid (semaglutide, half-life ~165h). Longer fatty acids generally provide stronger albumin binding but may reduce aqueous solubility. The position and chemistry of fatty acid attachment (direct vs via linker) also affect receptor binding and albumin interaction.

Teriparatide (PTH 1-34) contains the biologically active N-terminal fragment — the first 34 amino acids are sufficient for full PTH1R activation. Daily SC injection produces a brief pulse of PTH1R activation (hours, not continuous) that preferentially activates the Wnt/β-catenin pathway → increased osteoblast differentiation and activity → net bone formation (the anabolic window). Continuous PTH exposure shifts the balance toward RANKL upregulation → osteoclast activation → net bone resorption. Abaloparatide is an analogue of PTHrP (parathyroid hormone-related protein) 1-34, with amino acid modifications that produce more transient PTH1R signalling (preferential RG vs R0 receptor conformation binding), potentially providing similar bone formation with less bone resorption and hypercalcaemia. Palopegteriparatide uses TransCon technology — PTH linked to PEG via a cleavable linker that releases active PTH after SC injection for sustained daily exposure.

Peptide-drug conjugates (PDCs) use the peptide as a targeting moiety to deliver cytotoxic or therapeutic payloads specifically to cells expressing the target receptor. Components: targeting peptide (binds receptor on target cell surface) + linker (cleavable or non-cleavable, controlling payload release) + payload (cytotoxic drug, radionuclide, fluorescent label, or other therapeutic agent). Lu-177 dotatate is essentially a PDC: targeting peptide (dotatate, a somatostatin analogue) + chelator linker (DOTA) + radioactive payload (Lu-177). Peptide-fluorophore conjugates are used for fluorescence-guided surgery — pegulicianine is an approved peptide-fluorescent dye conjugate for identifying cancer margins during breast surgery. The PDC approach combines the targeting specificity of peptides with the potent biological effects of conjugated payloads.

Peptide vaccines use short synthetic peptide sequences (epitopes) that represent fragments of target antigens — typically 8-11 amino acids for MHC class I presentation (CD8+ cytotoxic T cell response) or 13-25 amino acids for MHC class II presentation (CD4+ helper T cell response). Applications: cancer immunotherapy (tumour-associated antigen peptides to prime anti-tumour immunity), infectious disease (pathogen-derived epitopes), and autoimmune disease (tolerogenic peptides to induce immune tolerance). Advantages: precise targeting of immune response, synthetic manufacturing (consistent quality, rapid production), and safety (no risk of reversion to virulence). Limitations: HLA restriction (individual MHC genotype determines which peptides are presented — a single epitope won't work for all patients), weak immunogenicity (short peptides are poorly immunogenic alone, requiring adjuvants and delivery systems), and immune evasion (tumours/pathogens may lose the targeted epitope).

Polymyxins are cyclic lipopeptides with a cationic peptide ring (positively charged diaminobutyric acid residues) linked to a fatty acid tail. Mechanism: electrostatic binding to lipopolysaccharide (LPS) in the gram-negative outer membrane → displacement of divalent cations (Mg2+, Ca2+) that stabilise LPS → outer membrane destabilisation → increased permeability → cell death. Colistin (polymyxin E, discovered 1949, reintroduced clinically after 2000 due to resistance crisis) is available as colistimethate sodium (inactive prodrug for IV/inhaled use) and colistin sulphate (for topical/oral use). Polymyxin B is used directly. Both have significant nephrotoxicity (incidence 20-60%) and neurotoxicity (paraesthesias, dizziness). Therapeutic drug monitoring of colistin is recommended due to the narrow therapeutic window. Polymyxin resistance (mcr genes encoding phosphoethanolamine transferases that modify LPS) is an emerging global concern.

The 26S proteasome consists of a 20S core particle (barrel-shaped, contains catalytic sites with chymotrypsin-like, trypsin-like, and caspase-like activities) and two 19S regulatory particles (recognise ubiquitinated proteins, unfold and thread them into the 20S core). Bortezomib (boronic acid dipeptide) reversibly inhibits the chymotrypsin-like site of the 20S proteasome. Carfilzomib (epoxyketone tetrapeptide) irreversibly inhibits the same site with greater selectivity. Cancer cell sensitivity: myeloma cells produce massive amounts of immunoglobulin, generating extraordinary levels of misfolded protein waste that must be proteasomally degraded. Proteasome inhibition overwhelms this already-stressed system → ER stress → unfolded protein response → apoptosis. Normal cells, with lower protein synthesis rates, are more tolerant. Bortezomib-induced peripheral neuropathy is an important dose-limiting toxicity; carfilzomib has less neurotoxicity but greater cardiotoxicity risk.

Peptide receptor radionuclide therapy (PRRT) concept: a tumour-targeting peptide (delivering specificity) is conjugated via a chelator (DOTA, DTPA) to a therapeutic radionuclide (delivering cytotoxicity). Lutetium-177 (Lu-177): beta-emitter, half-life 6.7 days, tissue penetration 0.25-2mm, suitable for small-to-medium tumour deposits. Yttrium-90 (Y-90): beta-emitter, half-life 2.7 days, tissue penetration up to 11mm, suitable for larger tumours. Lu-177 dotatate (Lutathera): DOTA-Tyr3-octreotate chelated to Lu-177, targeting SSTR2-positive NETs. The NETTER-1 Phase III trial showed significant PFS benefit vs high-dose octreotide LAR. Lu-177 vipivotide tetraxetan (Pluvicto): targets PSMA on prostate cancer cells. Selection requires diagnostic imaging with Ga-68 DOTATATE PET (for NETs) or Ga-68 PSMA PET (for prostate cancer) to confirm target expression — a theranostic approach (diagnosis and therapy using the same peptide targeting moiety with different radionuclides).

Research compounds (92 in the PeptideTrace database) have scientific literature and/or community interest but have not entered formal clinical development under regulatory oversight. Evidence types available: in vitro receptor binding/activity data, animal model efficacy studies, animal PK studies, and sometimes small case series or published human observations (outside formal trials). Key limitations: no GMP manufacturing requirement (quality varies between suppliers), no systematic human safety assessment, no established dose-response data in humans, and no regulatory benefit-risk evaluation. Research compounds span all PeptideTrace research areas: tissue repair (BPC-157, TB-500), GH secretagogues (GHRP-2, GHRP-6, ipamorelin, CJC-1295), cognitive (selank, semax, dihexa), immune (thymosin alpha-1 in non-approved markets, thymosin beta-4), and others.

Secretagogues stimulate hormone release through receptor activation on the secreting cell. Growth hormone secretagogues: GHRH receptor agonists (tesamorelin, Mod GRF 1-29) stimulate GH via cAMP/PKA pathway on somatotrophs; GHS-R1a agonists (GHRPs, ipamorelin) stimulate GH via IP3/DAG/calcium pathway. The two pathways are synergistic — combined GHRH + GHRP produces greater GH release than either alone (this synergy is exploited in some research peptide protocols). Insulin secretagogues: GLP-1 RAs stimulate insulin release by potentiating glucose-stimulated insulin secretion via cAMP/Epac2 on beta cells — crucially, this is glucose-dependent (no insulin release at low glucose), reducing hypoglycaemia risk vs sulfonylureas (which stimulate insulin release regardless of glucose level). Calcitonin secretagogues: GLP-1R activation on rodent C-cells stimulates calcitonin release — this is the mechanism behind the C-cell tumour signal in rodent studies.

Small molecules (<900 Da) differ from peptides in key properties: oral bioavailability (typically achievable for small molecules, rarely for peptides), manufacturing (chemical synthesis with exact molecular replication vs biological production with inherent variability), stability (generally more stable than peptides), and regulatory pathway (NDA for small molecules, potentially BLA for biological peptides). Some drugs that target peptide receptors are themselves small molecules rather than peptides: relugolix (oral GnRH antagonist, MW 623 Da) and elagolix (oral GnRH antagonist, MW 631 Da) are non-peptide small molecules designed from the GnRH receptor pharmacophore — they achieve oral bioavailability that peptide GnRH analogues cannot. DPP-4 inhibitors (sitagliptin, saxagliptin) are also small molecules targeting peptide biology. The peptide-to-small-molecule transition represents one endpoint of the peptidomimetic design continuum.

Stapled peptides use olefin metathesis chemistry (ruthenium-catalysed ring-closing metathesis, typically using Grubbs catalysts) to create an all-hydrocarbon crosslink between non-natural amino acids (typically alpha-methyl, alpha-alkenyl amino acids) placed at positions i, i+4 (one helix turn) or i, i+7 (two helix turns) on the same face of an alpha-helix. The hydrocarbon staple: constrains the peptide in its alpha-helical bioactive conformation (increasing binding affinity by reducing the entropic cost of binding), enhances protease resistance (the crosslink shields adjacent peptide bonds), and can improve cell membrane permeability (the staple increases hydrophobicity and masks backbone polarity). Aileron Therapeutics' ALRN-6924 (targeting MDM2/MDMX to reactivate p53 in cancer) is the most advanced clinical-stage stapled peptide. The technology is particularly valuable for disrupting protein-protein interactions — traditionally considered 'undruggable' targets.

Tα1 clinical evidence: approved in China for chronic hepatitis B (based on Chinese trials showing improved HBV DNA clearance and HBeAg seroconversion when combined with interferon-alpha), and used in various countries for immune support. Meta-analyses of Tα1 for hepatitis B show modest benefit as adjunct therapy. Investigated for: cancer immunotherapy enhancement (improving response rates to chemotherapy/immunotherapy in preliminary studies), sepsis-associated immunosuppression (improving immune cell function in critically ill patients), and COVID-19 (explored during the pandemic for immune modulation in severe cases). Limitations of the evidence base: many studies are small, single-centre, and conducted in countries with different regulatory standards; large, multicentre Phase III trials meeting FDA/EMA design standards have not been completed for most proposed indications.

Tβ4 (SDKPDMAEIEKFDKSKLKKTETQEKNPLPSKETIEQEKQAGES, 43 aa) is the most abundant actin-sequestering peptide in mammalian cells. Preclinical wound healing data: Tβ4 promotes corneal wound healing (accelerating re-epithelialisation in multiple animal models), cardiac repair after myocardial infarction (activating epicardial progenitor cells in mouse models), and dermal wound healing (accelerating closure and reducing scarring in rodent models). TB-500 is typically described as a synthetic fragment of Tβ4, commonly the 17 amino acid active region. RegeneRx Biopharmaceuticals developed RGN-259 (Tβ4 eye drops) for neurotrophic keratopathy and dry eye — Phase III trials were conducted but the programme did not achieve approval. The broader tissue repair claims for Tβ4/TB-500 in the research peptide space are based primarily on preclinical models without adequate human clinical trial validation.

Thymosin alpha-1 (Tα1, thymalfasin): 28 amino acid N-terminally acetylated peptide, produced commercially by SPPS. Mechanism: activates toll-like receptors (TLR2, TLR9) on dendritic cells, promotes T-cell maturation and differentiation (especially CD4+ and CD8+ T cells), enhances NK cell activity, and modulates cytokine production (increasing IL-2, IFN-α/γ). Approved in approximately 35 countries (China, Italy, South Korea, among others) for: hepatitis B/C treatment (as adjunct to interferon), immunodeficiency, and cancer immunotherapy adjunct. Not approved in US/EU/UK (where regulatory standards require larger, more rigorously designed clinical trials). Thymosin beta-4 (Tβ4): 43 amino acid peptide, primary intracellular function is actin sequestration (binds G-actin monomers, regulating actin polymerisation and cell motility). TB-500 is a synthetic fragment of Tβ4 commonly used in research peptide settings.

Animal venoms are rich sources of bioactive peptides optimised by millions of years of evolution for potency and selectivity. Approved venom-derived drugs: exenatide (from Gila monster Heloderma suspectum saliva — exendin-4 peptide, the first GLP-1 RA), ziconotide (from cone snail Conus magus — omega-conotoxin MVIIA, a voltage-gated calcium channel blocker for severe pain), eptifibatide (from pygmy rattlesnake Sistrurus miliarius — barbourin peptide derivative, platelet glycoprotein IIb/IIIa antagonist), and bivalirudin (inspired by hirudin from medicinal leech Hirudo medicinalis — direct thrombin inhibitor). Venom peptide research continues to yield drug candidates: spider venom peptides targeting ion channels, scorpion venom peptides for antimicrobial and anticancer applications, and additional cone snail conotoxins for pain management. The diversity of venom peptide targets and mechanisms makes venomics an active drug discovery field.