Peptide research glossary: terminology for reading the literature

The peptide literature sits across at least five technical vocabularies — clinical trial design, statistics, pharmacokinetics, peptide chemistry, and receptor biology — and a study abstract typically borrows from three of them in the same paragraph. A curious reader who opens Jastreboff et al. 2022 and meets intention-to-treat, MMRM, Cmax, Emax, DPP-4 cleavage, and body-composition substudy in five sentences is reading five distinct fields stitched into one trial report. The papers do not pause to define the vocabulary. This glossary does.

Entries are 2–4 sentences each — long enough to anchor the term, short enough to scan. Where a term appears in context on this site, the entry cross-links to the peptide page, research entry, or dossier where the concept is doing work. Where a term has a canonical external source (CTCAE published by the NCI; PRISMA at prisma-statement.org; ICH E6(R3) Good Clinical Practice as of 2025), the entry points there. The audience is the reader who wants to read primary sources directly rather than relying on someone else's summary; this page is the lookup table that makes that possible.

The structure is six thematic sections rather than a single A–Z roll. The categories: trial design and statistics, pharmacokinetics and pharmacodynamics, peptide chemistry and manufacturing, receptor and mechanism vocabulary, regulatory and legal, and the site-specific evidence-quality terminology that anchors this corpus's claims.

A. Trial design and statistics

Randomized controlled trial (RCT)

A trial in which participants are randomly assigned to intervention or comparator (placebo or active control), so that any systematic baseline difference between the groups is reduced to chance. The RCT is the design that supports the strongest causal inference in clinical research; absence of randomization (cohort studies, case series, observational data) means baseline differences may confound any observed effect. The SURMOUNT, STEP, SELECT, and SURPASS programs that define the modern GLP-1 receptor pharmacology dossier evidence base are all RCTs.

Crossover trial vs. parallel-group

Two ways to structure an RCT. In a parallel-group trial, each participant is assigned to one arm and stays there; in a crossover trial, each participant receives both intervention and control in sequence (typically with a washout period between), serving as their own comparator. Crossover designs are statistically more powerful per participant but are infeasible when the intervention has lasting effects, when the condition changes over time, or when the trial is too long to expect retention. Most large peptide trials are parallel-group; smaller pharmacokinetic and mechanism studies often use crossover designs.

Blinding (single, double, triple)

Concealment of group assignment to reduce bias from expectation. Single-blind hides assignment from the participant; double-blind hides it from both participant and investigators (including those administering treatment and assessing outcomes); triple-blind extends the concealment to the statistician analyzing the data. Effective blinding in a subcutaneous peptide trial requires a matched-volume placebo injection and is harder to maintain when the active drug produces obvious effects (nausea, weight loss, skin reactions); SELECT and STEP report adverse-event-pattern divergence that likely unblinded some participants.

Intention-to-treat (ITT) vs. per-protocol (PP)

Two analysis populations. Intention-to-treat analyzes participants in the group they were randomized to, regardless of whether they completed the protocol, switched arms, or were lost to follow-up — preserving the randomization and yielding a real-world effect estimate. Per-protocol restricts the analysis to participants who completed the assigned intervention as specified, yielding the maximum-fidelity effect estimate but breaking the randomization. The two analyses are reported together when discontinuation rates differ between arms; the Jastreboff et al. 2022 SURMOUNT-1 trial is a well-executed example of ITT-anchored reporting with PP sensitivity analyses.

Primary endpoint vs. secondary endpoint

The primary endpoint is the single outcome the trial is statistically powered to detect — chosen in advance, registered before unblinding, and protected by the trial's alpha allocation. Secondary endpoints are additional outcomes of interest, analyzed only after the primary endpoint is established and typically without the same statistical protection from false-positive findings. STEP 1 and SURMOUNT-1 used percentage change in body weight as the primary endpoint; cardiovascular events, glycemic markers, and body-composition substudies were secondary or exploratory.

Non-inferiority margin

The pre-specified largest acceptable difference between a new intervention and an active comparator below which the new intervention is considered "not worse than" the comparator. Non-inferiority trials are common when the placebo-controlled question has been answered and the relevant clinical question is whether a new agent matches an existing one. The Frías et al. 2021 SURPASS-2 trial was structured as a non-inferiority comparison of tirzepatide against semaglutide; tirzepatide met non-inferiority and then crossed into demonstrated superiority on the primary HbA1c endpoint.

Responder analysis

An analysis that reports the proportion of participants reaching a pre-specified threshold (for example, ≥5% weight loss, ≥10%, ≥15%) rather than only reporting the mean. Responder analyses surface distributional information that the mean obscures — a trial in which everyone loses 10% and a trial in which half lose 20% and half lose nothing have the same mean but very different clinical meaning. STEP 1 reported 86.4%, 69.1%, and 50.5% of participants reaching the 5%, 10%, and 15% thresholds respectively; that distribution is more useful than the 14.9% mean alone.

ANCOVA, MMRM, LOCF

Three statistical methods for handling repeated measurements and missing data in longitudinal trials. ANCOVA (analysis of covariance) compares group means at a single time point while adjusting for baseline values. MMRM (mixed-model repeated-measures) is the modern default for chronic-disease trials — it models the trajectory across all visits and uses all available data without imputing missing values, under a missing-at-random assumption. LOCF (last-observation-carried-forward) imputes missing data by carrying the last available value forward; it tends to bias toward the null in placebo arms (where dropouts often leave because of no benefit) and is largely deprecated in modern trial reporting. The contemporary obesity-pharmacotherapy program reports MMRM as the primary analysis with sensitivity analyses across alternative imputation methods.

Effect size, confidence interval, p-value

Three distinct quantities. Effect size describes the magnitude of the difference between groups (a 14% weight-loss difference, an HbA1c reduction of 1.5 percentage points). Confidence interval describes the precision of that estimate — a 95% confidence interval is the range within which the true effect lies with 95% probability under the trial's statistical model. P-value describes the probability of observing the effect or a larger one if the null hypothesis (no difference) were true; a p-value of 0.05 means there is a 5% probability the observed effect arose by chance alone. The three together describe what the trial found; the p-value alone does not.

Open-label extension (OLE)

A follow-on study that continues participants from a randomized trial on the active intervention without blinding, intended to capture longer-duration safety and efficacy data than the parent trial. OLE data is informative but limited — without the randomized comparison, secular trends and dropout-related selection bias can produce apparent effects that the parent trial would not have supported. Long-term GLP-1 and incretin data has accrued largely through OLE programs after the pivotal trials closed.

Phase I / II / III / IV

The four-stage structure of drug development. Phase I tests safety, tolerability, and basic pharmacokinetics, typically in small healthy-volunteer cohorts (Teichman et al. 2006 is the Phase I for CJC-1295). Phase II tests efficacy signal and dose-response in moderate-size patient cohorts (Jastreboff et al. 2023 is the Phase II for retatrutide). Phase III is the pivotal large-scale efficacy and safety trial used for regulatory approval (Wilding et al. 2021, Jastreboff et al. 2022). Phase IV is post-approval surveillance and real-world studies after the drug is marketed.

Pharmacovigilance and FAERS

Pharmacovigilance is the systematic monitoring of drug safety after approval, intended to detect rare adverse events that pre-approval trials are too small to catch. The FDA Adverse Event Reporting System (FAERS, transitioning into the FDA Adverse Event Monitoring System / AEMS as of March 2026) is the U.S. repository of voluntary post-marketing adverse-event reports. FAERS data is hypothesis-generating, not hypothesis-confirming — reports are not adjudicated, denominators are unknown, and reporting rates vary by drug, indication, and media coverage. Several emerging GLP-1-class safety signals (NAION, gastroparesis, suicidal ideation) entered the literature through FAERS pattern analysis before any controlled investigation; the Akbar 2026 and Aboukaoud 2026 analyses on this site sit in that tradition.

CTCAE grades (1–5)

The Common Terminology Criteria for Adverse Events, published by the National Cancer Institute (current version: CTCAE v6, 2025, with CTEP implementation targeted for 2026). CTCAE assigns a standardized severity grade to every adverse event in oncology and (by adoption) most contemporary trials: Grade 1 (mild, asymptomatic or mild symptoms, no intervention indicated), Grade 2 (moderate, minimal local or noninvasive intervention indicated), Grade 3 (severe or medically significant but not immediately life-threatening), Grade 4 (life-threatening, urgent intervention indicated), Grade 5 (death related to the adverse event). Peptide-trial reporting uses CTCAE grading even for non-oncology indications; "Grade 2 nausea" in a GLP-1 trial maps to the same definition as in a chemotherapy trial.

HEDIS endpoints

Healthcare Effectiveness Data and Information Set — a payer-side quality-measurement framework published by the U.S. National Committee for Quality Assurance, mostly relevant for chronic-disease management rather than peptide research per se. HEDIS endpoints (HbA1c control thresholds in diabetes, BMI documentation, cardiovascular-risk surveillance) are sometimes invoked in real-world-evidence studies of incretin therapy because they describe the population-health metrics health systems already collect; not central to mechanism or efficacy reporting but useful for understanding how the cardiometabolic literature is consumed downstream.

Meta-analysis vs. systematic review (PRISMA)

A systematic review is a structured synthesis of all published evidence on a specific question, following pre-registered methods for search, inclusion, quality assessment, and analysis. A meta-analysis is a quantitative subset — a statistical pooling of effect estimates across studies to produce a combined estimate, typically embedded within a systematic review. The PRISMA 2020 statement is the canonical reporting framework (DOI 10.1136/bmj.n71); a PRISMA flow diagram is the visual summary of how many studies entered the search, were screened, met inclusion, and contributed to the final analysis. The Batsis et al. 2026 systematic review on incretin body-composition outcomes is a worked example.

B. Pharmacokinetics and pharmacodynamics

Half-life (t½) — plasma vs. biological

The time required for a drug's plasma concentration (or biological effect) to fall to half its peak value. Plasma half-life describes the disposition of the drug itself; biological half-life describes how long the drug's effect persists, which can be much longer if the drug acts through a stable downstream messenger or covalent receptor modification. Native GLP-1 has a plasma half-life of ~2 minutes; semaglutide's structural modifications and albumin-binding fatty diacid produce a plasma half-life of about a week, which is the design feature that enables once-weekly dosing.

AUC (area under the curve)

The integral of plasma concentration over time, typically reported in units of concentration × time (ng·hr/mL). AUC describes total drug exposure across a dosing interval — a more complete summary than peak concentration alone, because two compounds with the same peak can produce very different AUCs if their elimination rates differ. AUC is the canonical pharmacokinetic comparison metric for bioequivalence assessments and for comparing different routes, doses, or formulations of the same molecule.

Cmax (peak concentration)

The maximum plasma concentration achieved after dosing. Cmax determines whether the drug exceeds the receptor-engagement threshold and is the dominant driver of receptor-occupancy-related side effects that scale with peak exposure (nausea on rapid GLP-1 receptor activation, transient cardiovascular effects on PT-141 dosing). Tmax — the time at which Cmax is reached — is the companion parameter; subcutaneous Tmax for most peptide therapeutics sits in the 2–8 hour range.

Bioavailability (F%)

The fraction of an administered dose that reaches systemic circulation in active form. By definition, intravenous bioavailability is 100%; subcutaneous and intramuscular bioavailability is typically 60–95% for peptides; oral bioavailability for unmodified peptides is essentially zero due to gastric digestion and intestinal-wall transit (oral semaglutide solves this through co-formulation with an absorption enhancer). Intranasal bioavailability varies widely by molecule and formulation — PT-141 is reported at ~20% intranasal vs. ~100% subcutaneous, which is part of why the subq autoinjector became the FDA-approved route.

Volume of distribution (Vd)

The apparent volume in which a drug appears to be distributed, calculated as dose divided by plasma concentration. A small Vd (close to plasma volume, ~5 L for an adult) suggests the drug stays largely in circulation; a large Vd (hundreds of liters) suggests extensive tissue binding. Peptides with albumin-binding modifications (semaglutide, tirzepatide, CJC-1295 with DAC) have Vds that track the albumin distribution space, which is part of why their elimination kinetics are predictable.

Steady state

The condition reached after repeated dosing when the rate of drug input equals the rate of elimination, so plasma concentrations stabilize at a predictable peak-trough oscillation around a mean. Steady state is typically achieved at 4–5 elimination half-lives; for a once-weekly peptide with a one-week half-life, that is approximately 4–5 weeks of consistent dosing. Trial endpoints assessed before steady state may underestimate the eventual effect; this is part of why the dose-escalation phase of a GLP-1 program runs 16–20 weeks before efficacy is read.

First-pass metabolism

Hepatic and intestinal metabolism that drug substances undergo before entering systemic circulation after oral administration. Most peptides are degraded by gastric and intestinal proteases long before first-pass metabolism is relevant; the few orally bioavailable peptide therapeutics use formulation strategies (permeation enhancers, enteric coatings, prodrug modifications) to survive the gut, after which hepatic first-pass becomes the next attrition step.

Dose-response curve

The mathematical relationship between drug dose and observed effect, typically plotted as effect-on-y-axis versus log-dose-on-x-axis. A sigmoidal dose-response curve — the canonical receptor-pharmacology shape — has a defined threshold below which no effect is observed, a steep middle range where effect scales with dose, and a plateau where additional dose adds no additional effect (the Emax). A linear dose-response, in contrast, scales effect proportionally with dose across the studied range without obvious threshold or ceiling. Most receptor-mediated peptide effects are sigmoidal; the question in trial design is which part of the curve the studied doses are exploring.

Receptor pharmacology — affinity (Kd), efficacy (Emax), EC50

The three numbers that define a drug-receptor interaction. Affinity (Kd, the dissociation constant) describes how tightly the drug binds the receptor — a lower Kd means tighter binding. Efficacy (Emax) describes the maximum effect the drug can produce at receptor saturation. EC50 is the drug concentration that produces half the maximum effect — the practical metric for comparing potency across compounds. Two drugs with similar Kd can have very different Emax (one a full agonist, one a partial agonist); two drugs with similar Emax can have very different EC50 (one ten times more potent than the other). The Raun et al. 1998 paper on ipamorelin reports all three for GH release versus the off-target receptors.

Tachyphylaxis vs. tolerance

Two patterns of diminishing drug response on repeated exposure. Tachyphylaxis is a rapid decrease in response across successive doses given over hours to days, typically driven by receptor desensitization or depletion of a downstream messenger; tolerance is a slower, longer-timescale loss of response across weeks to months, often involving receptor downregulation or counter-regulatory adaptations. PT-141 shows clear tachyphylaxis on close-spaced dosing (which is why community protocols cap frequency at 1×/24h with self-imposed monthly limits); chronic GLP-1 dosing produces modest receptor downregulation that does not appear to meaningfully erode clinical effect at maintenance doses.

β-arrestin and receptor desensitization

A receptor-regulation pathway in which agonist-bound G-protein-coupled receptors are phosphorylated, bound by β-arrestin proteins, uncoupled from G-protein signaling, and internalized into the cell — temporarily or permanently removing them from the cell surface. β-arrestin signaling itself has independent downstream effects beyond receptor regulation; the GLP-1 receptor field has explored biased agonism (compounds that activate G-protein signaling more than β-arrestin signaling) as a route to reduce nausea while preserving glycemic effect. Whether biased agonism produces clinically distinct outcomes remains an open mechanistic question.

Allosteric modulation

A pharmacological mechanism in which a compound binds a receptor site distinct from the natural ligand-binding site (the orthosteric site), modulating the receptor's response to its native ligand without directly activating it. Positive allosteric modulators amplify the native ligand's effect; negative allosteric modulators dampen it. Allosteric ligands typically produce more physiological effects than orthosteric agonists because they preserve the endogenous signaling pattern; the approach is being explored across several peptide-receptor families but is not yet load-bearing in the marketed peptide therapeutic class.

Receptor internalization

The cellular process by which activated receptors are removed from the plasma membrane and brought into intracellular compartments, either for recycling back to the surface, for degradation, or for sustained intracellular signaling. Internalization is a major mechanism of acute receptor desensitization and ties into the β-arrestin pathway. GLP-1 receptor internalization rates differ across agonists and are part of the mechanistic explanation for why some compounds produce more durable signaling than others at equivalent receptor-binding profiles.

C. Peptide chemistry and manufacturing

Solid-phase peptide synthesis (SPPS)

The dominant method for manufacturing synthetic peptides, in which the growing peptide chain is anchored to an insoluble polymer resin while amino acids are added one at a time from C-terminus to N-terminus. After each coupling cycle, excess reagents are washed away and the next protected amino acid is added; the completed peptide is cleaved from the resin and deprotected at the end. SPPS scales from milligrams (research) to kilograms (industrial); the synthesis cost of a 30-amino-acid peptide is dominated by the per-cycle yield (each coupling step introduces small losses that compound across the length of the peptide).

Fmoc and tBOC strategies

Two protecting-group chemistries used in SPPS. Fmoc (9-fluorenylmethyloxycarbonyl) is the modern default — protection groups are removed with mild base, allowing acid-labile side-chain protections and a relatively mild final cleavage. tBOC (tert-butoxycarbonyl) is the older approach — protection groups are removed with trifluoroacetic acid at each cycle, with a final cleavage using anhydrous hydrogen fluoride. Fmoc dominates current manufacturing; tBOC remains in use for some specialty syntheses where Fmoc-incompatible chemistry is required.

HPLC purity

High-performance liquid chromatography separates molecules by physicochemical properties; the chromatogram integrates the signal from each separated peak to produce a purity percentage. A "99% HPLC purity" certificate states that 99% of the UV-absorbing material in the sample eluted as the target peptide peak. The certificate does not measure: (1) impurities that do not absorb at the detection wavelength, (2) endotoxin content, (3) microbial contamination, (4) the actual sequence identity of the main peak (which requires mass spectrometry), or (5) sterility of the final formulation. HPLC purity is a necessary but radically insufficient quality metric.

Lyophilization

Freeze-drying — the process by which water is removed from a frozen solution under vacuum by sublimation, producing a stable solid powder. Most peptide therapeutics ship as lyophilized powder because peptides in aqueous solution degrade through hydrolysis and oxidation at storage temperatures; the dried powder is reconstituted before use. Lyophilized peptides typically tolerate room-temperature shipping for short windows but require refrigerated long-term storage; once reconstituted, the dating runs much shorter.

Reconstitution chemistry (bacteriostatic water)

The diluent used to bring a lyophilized peptide back into solution. Bacteriostatic water for injection is sterile water containing 0.9% benzyl alcohol, which inhibits bacterial growth and extends the in-use dating of a multi-dose vial. Sterile water without preservative is used for single-dose applications or when the peptide is sensitive to benzyl alcohol. Saline is rarely used for peptide reconstitution because salt concentration can affect peptide stability and aggregation; pH of the final solution matters for some peptides (the GHK-Cu copper coordination is pH-sensitive).

Salt forms (acetate, citrate, trifluoroacetate)

The counterion attached to charged amino acid residues in the final peptide product, set by the manufacturing and purification process. Acetate is the standard pharmaceutical salt form. Trifluoroacetate (TFA salt) is residual from TFA-based HPLC purification and is non-pharmacopeial; TFA-salt peptides are common in research-grade material but are not the form used in approved therapeutics because TFA itself has biological activity at higher exposures. Citrate and other salts appear in some specialty formulations. Salt content affects mass measurement: an "acetate salt" peptide is a few percent heavier per mole than the free peptide, which matters for dose calculations.

Disulfide bonds

Covalent S-S bonds formed between cysteine residues, which fold and stabilize peptide tertiary structure. Intra-chain disulfides connect cysteines within a single peptide chain; inter-chain disulfides connect two separate chains. Selank (Thr-Lys-Pro-Arg-Pro-Gly-Pro) has no disulfides; insulin and many growth-factor mimetics rely on multiple intra-chain disulfides for activity. Correct disulfide formation is a manufacturing quality concern; mis-paired or unformed disulfides produce inactive isomers that can be hard to separate from the active form by HPLC alone.

Glycosylation, PEGylation, DAC modifications

Three covalent modifications that extend peptide half-life or modulate distribution. Glycosylation attaches carbohydrate side chains to peptide backbone amino acids; relevant for some endogenous peptide hormones but uncommon in synthetic-peptide therapeutics. PEGylation covalently attaches polyethylene glycol chains, increasing molecular weight to slow renal filtration and reducing immunogenicity. DAC (drug affinity complex) is the covalent albumin-binding strategy used in CJC-1295 — a maleimidopropionic acid linker covalently binds cysteine-34 of plasma albumin in vivo, producing a multi-day plasma half-life from a single injection.

Endotoxin testing (LAL)

The Limulus amebocyte lysate assay detects lipopolysaccharide endotoxin (a cell-wall component of gram-negative bacteria) at picogram-per-milliliter sensitivity, using clotting factors from horseshoe crab blood. Endotoxin contamination of injectable peptides produces systemic inflammatory responses (fever, headache, hypotension) at very low doses; LAL testing is a standard release specification for parenteral pharmaceutical-grade peptides. Synthetic-route peptides generally have lower endotoxin risk than fermentation-route peptides; the risk re-enters through reconstitution and storage if aseptic technique fails.

Sterile filtration (0.22 μm)

Passage of a peptide solution through a 0.22 μm membrane to remove bacteria and most fungi before final fill, container-closure, or self-administration. 0.22 μm filtration is the canonical pharmaceutical-grade sterile-filtration specification (smaller than the smallest known bacteria); 0.45 μm filtration is bioburden-reduction rather than sterilization. Filter-sterilization does not remove endotoxin (which is small and passes the filter) or viruses; sterile filtration is one step in a broader aseptic-processing chain rather than a complete sterility assurance.

Mass spectrometry confirmation

Mass spectrometry measures the molecular weight of a peptide to high precision (typically within 0.1 Da for small peptides), confirming the sequence assembly was correct. MS confirmation is the orthogonal check on HPLC purity — an HPLC peak at the right retention time but the wrong mass indicates a synthesis error (deletion sequence, racemization, side-chain modification) that HPLC alone cannot distinguish. Pharmaceutical-grade peptide release specifications include MS identity confirmation alongside HPLC purity; research-grade certificates of analysis often report HPLC but not MS.

D. Receptor and mechanism vocabulary

GPCR (G-protein-coupled receptor)

The largest family of cell-surface receptors, characterized by a seven-transmembrane-helix structure that, on ligand binding, activates intracellular heterotrimeric G proteins to relay the signal. Most peptide-hormone receptors are GPCRs — GLP-1R, GIP-R, glucagon-R, GHSR-1a, the melanocortin receptors, the opioid receptors, and many others. The GPCR superfamily is one of the most-drugged target classes in pharmacology; the modern peptide therapeutic class targets it almost exclusively.

Ghrelin receptor (GHSR-1a)

The G-protein-coupled receptor for the endogenous hormone ghrelin, expressed on pituitary somatotrophs (where activation drives GH release), hypothalamic neurons (appetite stimulation), and gastric tissue. Ipamorelin and MK-677 are GHSR-1a agonists used for GH-axis modulation; the broader story is summarized in the GH-axis dossier. Ghrelin-receptor agonism is functionally distinct from GHRH-receptor agonism — the two pathways converge on GH release but engage somatotrophs through different second-messenger cascades and can summate or amplify.

GLP-1R, GIP-R, glucagon-R

The three incretin-and-glucagon-class receptors that anchor the modern metabolic-peptide field. GLP-1R — the receptor for glucagon-like peptide-1, the target of semaglutide, liraglutide, and the GLP-1 component of multi-agonist designs. GIP-R — the receptor for glucose-dependent insulinotropic polypeptide, the second target engaged by tirzepatide. Glucagon-R — the receptor for glucagon (counter-regulatory to insulin on hepatic glucose output), the third target engaged by retatrutide. The receptor-breadth progression from mono- to dual- to triple-agonist tracks the empirical effect-size progression in obesity weight loss; the GLP-1 receptor pharmacology dossier details how each receptor contributes.

Melanocortin receptors (MC1-5R)

A family of five GPCRs (MC1R through MC5R) responding to melanocortin peptides (α-MSH, β-MSH, ACTH). MC1R drives skin pigmentation; MC2R is the ACTH receptor on the adrenal cortex; MC3R and MC4R mediate energy balance and feeding behavior; MC4R is the dominant central appetite-suppression receptor and the genetic basis of monogenic obesity in MC4R-deficiency syndromes. PT-141 / bremelanotide is a non-selective melanocortin agonist with activity at MC3R and MC4R; the FDA-approved indication is hypoactive sexual desire disorder. KPV is a tripeptide derived from α-MSH with anti-inflammatory activity through melanocortin pathway modulation.

VEGFR2 and the eNOS-NO axis

VEGFR2 (vascular endothelial growth factor receptor 2) is the principal receptor mediating VEGF-driven angiogenesis and endothelial activation. eNOS (endothelial nitric oxide synthase) produces nitric oxide in vascular endothelium; eNOS activation downstream of VEGFR2 is part of the vasoreactivity and angiogenesis cascade. BPC-157's rodent-mechanism literature centers on the VEGFR2-eNOS-NO axis as the proposed pathway for its tissue-healing effects; the healing and angiogenesis dossier details the Sikiric 2018 review of this work.

IGF-1 and the GH receptor

IGF-1 (insulin-like growth factor 1) is the principal endocrine messenger of growth hormone signaling — synthesized in the liver in response to GH receptor activation, IGF-1 circulates with a much longer plasma half-life than GH itself and is the steady-state signal most peripheral tissues read. GH receptor activation also produces direct, IGF-1-independent effects on adipose lipolysis. The pulsatile-versus-tonic distinction in GH-axis pharmacology is about IGF-1 elevation pattern, which is part of why GHRH analogs and ghrelin-receptor agonists with different durations of action carry different long-term risk frames.

Innate Repair Receptor (CD131 + EPOR heterodimer)

A heterodimeric receptor complex formed by CD131 (the common β-chain shared with GM-CSF receptors) and EPOR (the erythropoietin receptor), which mediates the tissue-protective non-erythropoietic effects of erythropoietin and EPO-derived peptides. The Innate Repair Receptor pathway is the target of ARA-290 (cibinetide), a peptide engineered to engage the IRR without engaging the homodimeric EPOR that drives erythropoiesis. The therapeutic intent is anti-inflammatory and tissue-protective effects without the hematocrit-elevation safety risk of full EPO agonism.

BDNF and TrkB

Brain-derived neurotrophic factor is a growth-factor family member secreted in the central nervous system that supports neuronal survival, synaptic plasticity, and learning. BDNF signals through TrkB (tropomyosin receptor kinase B), a receptor tyrosine kinase. Semax is reported to upregulate BDNF expression in rodent and limited human work; the mechanism is part of the rationale for nootropic and neuroprotection indications. The BDNF/TrkB axis is also implicated in antidepressant response, exercise-induced cognitive resilience, and aging-related cognitive decline.

PepT1 transporter

PepT1 (peptide transporter 1) is an intestinal-epithelium apical-membrane transporter that uptakes di- and tripeptides from the gut lumen, distinct from the amino-acid transporter system. Tripeptide therapeutics targeted to gut epithelium can be designed to use PepT1 for transit; KPV is hypothesized to engage gut epithelium through PepT1-mediated uptake, contributing to its oral bioavailability for colitis indications. PepT1 is also relevant for some peptidomimetic drug designs that exploit transporter recognition to achieve oral bioavailability.

NF-κB pathway

Nuclear factor kappa B is a transcription-factor family that drives inflammatory gene expression; NF-κB activation downstream of TLR4, TNF-α, and other inflammatory stimuli is a central node in the inflammatory response. Multiple peptide therapeutics with anti-inflammatory indications act through NF-κB suppression — α-MSH and its derivatives, KPV, and several thymic peptides modulate NF-κB activation in immune cells. The Manna 1998 paper on α-MSH and NF-κB is the load-bearing mechanism reference for the melanocortin-class anti-inflammatory work.

E. Regulatory and legal

FDA, EMA, MHRA, TGA, PMDA

The five major drug regulators in English-speaking and Pacific-Rim pharmaceutical markets. FDA (U.S. Food and Drug Administration); EMA (European Medicines Agency, covering EU member states); MHRA (Medicines and Healthcare products Regulatory Agency, U.K., post-Brexit); TGA (Therapeutic Goods Administration, Australia); PMDA (Pharmaceuticals and Medical Devices Agency, Japan). Approval pathways and labeled indications differ across jurisdictions — semaglutide's approved indications and dose ceilings differ between the FDA, EMA, and PMDA. The "approved in the U.S." framing common in peptide discourse refers to FDA approval; the same molecule may be unapproved, off-label, or differently labeled elsewhere.

Investigational New Drug (IND), New Drug Application (NDA)

The two principal U.S. regulatory filings in drug development. An IND (Investigational New Drug application) is filed before clinical trials begin and authorizes human testing under defined protocols; INDs cover Phase I, II, and III trials. An NDA (New Drug Application) is filed after Phase III trial completion and requests marketing approval; the NDA review by the FDA produces either approval, request for additional data, or rejection. Biologics — including most peptide therapeutics manufactured by recombinant means — file BLAs (Biologics License Applications) rather than NDAs, though the process is structurally parallel.

Section 503A vs. 503B compounding

Two regulatory pathways for U.S. pharmaceutical compounding. Section 503A pharmacies compound prescriptions for individual patients based on a specific prescription, with state-board oversight but not full FDA cGMP compliance. Section 503B outsourcing facilities operate at a larger scale, register directly with FDA, follow cGMP standards, and can produce compounded products for office stock without patient-specific prescriptions. The 503A-vs-503B distinction is relevant for the GLP-1 compounding landscape that emerged during the 2022–2024 semaglutide and tirzepatide shortages; the FDA's removal of those compounds from the drug shortage list in 2024–2025 ended the 503B production authority for those molecules.

Drug shortage list mechanism

A regulatory designation by the FDA (and equivalents elsewhere) recognizing that demand for an approved drug exceeds available supply. Drug shortage status enables 503B outsourcing facilities to legally compound the drug, allows hospitals additional flexibility on alternatives, and triggers structured FDA monitoring. The mechanism is the policy lever that made commercially compounded semaglutide and tirzepatide briefly legal during the GLP-1 supply shortage; the resolution of the shortage by manufacturer ramp-up ended the compounding authority and reshaped the compounded-GLP-1 market.

Orphan drug designation

A regulatory status granted by FDA (and equivalents) to drugs intended for rare diseases — formally, those affecting fewer than 200,000 patients in the U.S. Orphan designation provides tax credits, fee waivers, and a seven-year market exclusivity period upon approval. Tesamorelin's original approval for HIV-associated lipodystrophy carried orphan designation; the indication's rarity is part of why a Phase III peptide therapeutic could be developed for a population numbering in the tens of thousands rather than the tens of millions.

Schedule I-V classification

The U.S. Controlled Substances Act tiers, administered by the DEA. Schedule I substances are deemed to have no accepted medical use and high abuse potential (heroin, LSD, cannabis at federal level); Schedule II have accepted medical use but high abuse potential (opioid analgesics, amphetamines); Schedules III, IV, V have progressively lower abuse-potential designations. No peptide on this site is currently scheduled; peptides sit outside the controlled-substances framework but inside the FDA's drug-and-biologic regulatory framework, which is a different regulatory lane.

Research-only and "Not for human use" labeling

Labels affixed to peptide products sold outside the FDA pharmaceutical pathway, typically intended for in vitro research or animal-study use. The label is the manufacturer's regulatory positioning — research-only material is not subject to pharmaceutical-grade manufacturing, sterility, or purity standards, and is not marketed for human use even when downstream buyers self-administer. The label is also a legal-liability disclaimer; it does not change the chemistry of the product but does change the regulatory pathway under which the product moves.

WADA Prohibited List

The World Anti-Doping Agency Prohibited List, updated annually, defines substances and methods banned in competitive sport. Peptide-relevant categories include S0 (non-approved substances — a catch-all category covering compounds with no governmental health authority approval, which captures many research-grade peptides), S1 (anabolic agents), S2 (peptide hormones, growth factors, related substances, and mimetics, which covers GHRH analogs, GH secretagogues, EPO and EPO mimetics, and IGF-1 analogs), and S4 (hormone and metabolic modulators). Competitive athletes face routine testing for these categories; out-of-competition testing for S2 substances is part of standard anti-doping protocol.

F. Evidence-quality terminology specific to this site

Tier 1–4 source grading

The methodology page explains the four-tier source-grading system in detail; the summary is that Tier 1 is peer-reviewed primary research (RCT, mechanistic study, cohort), Tier 2 is peer-reviewed secondary (systematic review, meta-analysis), Tier 3 is expert primary (preprint, conference, named-author non-peer-reviewed), and Tier 4 is practitioner-grade (case reports, community aggregations, podcasts). Every research entry on the site carries a tier label visible in the frontmatter and on the page. The site does not aggregate Tier 4 sources into Tier 1 conclusions — five forum posts do not become an RCT.

Strength labels: strong, moderate, suggestive, anecdotal

Independent of tier, every claim carries a strength label that reflects the robustness of the evidence supporting it. Strong — large, well-designed, well-replicated; effect direction and magnitude are reliable. Moderate — multiple consistent studies, or a single high-quality study with mechanistic support. Suggestive — early-stage or limited evidence; hypothesis-generating rather than hypothesis-confirming. Anecdotal — case reports, community observations, expert opinion absent controlled data. A Tier 1 RCT can produce suggestive evidence if it is small or methodologically constrained; a Tier 4 community aggregate can produce moderate evidence if the replication pattern is consistent across many independent observations.

The k≥5 anonymity floor

The community-aggregation discipline that defines what cycle, biomarker, and adverse-event reports can be published from the member registry. No aggregate is shown publicly (or to members) unless at least five independent reports contribute to the bucket; reports drop out of the aggregation if the contributing population would reveal identifying information. The k≥5 floor is the privacy invariant that allows the site to surface community-validated patterns without exposing individual member data — a structural rather than a policy commitment, enforced at the query layer.

AI-translated provenance flag

The provenance label affixed to research entries derived from non-English primary literature passed through frontier-AI translation and editorial QC. The site's Russian / Eastern European peptide literature corpus — Selank, Semax, Epitalon, and adjacent compounds — sits largely in Russian-language journals; the AI-translated provenance flag identifies entries that originated in non-English source material and were rendered by AI assistance rather than by a human translator's manuscript. The flag is a transparency commitment; the editorial QC pass is what makes the resulting summaries citable rather than a downstream layer of error.

How to use this glossary

The glossary is intended as a lookup table while reading primary literature or this site's research entries. Three patterns of use:

When an unfamiliar term appears in a paper abstract or research summary, search this page for the term. If the term has a cross-link to a peptide page, dossier, or research entry that uses the concept in context, follow that link to see the term doing work in an actual case. When the term has a canonical external source (CTCAE, PRISMA, WADA Prohibited List, ICH E6(R3) Good Clinical Practice), the entry points to the authoritative reference rather than reproducing the full specification here.

When evaluating a study claim, the trial-design and statistics section is the most load-bearing — terms like intention-to-treat, primary endpoint, responder analysis, and MMRM are where most reading errors enter. A study reporting a strong effect on a per-protocol analysis of a secondary endpoint with LOCF imputation is a fundamentally different evidence claim than one reporting the same effect on an intention-to-treat analysis of a pre-specified primary endpoint with MMRM. Both reports may use the word "significant"; the underlying claim quality is not the same.

When evaluating a peptide manufacturing or sourcing claim, the chemistry and manufacturing section maps the terminology in vendor-side certificates of analysis. HPLC purity and MS identity confirmation answer different questions; bacteriostatic water and sterile water are not interchangeable; acetate salt and trifluoroacetate salt are not pharmacopeial equivalents. The vocabulary is the floor for evaluating the claims a research-chemical supplier puts on a label.

This page updates as the literature shifts and as new terms become load-bearing in the corpus. The current version is dated 2026-05-18.

Sources cited

External canonical references:

• PRISMA 2020 statement — systematic-review reporting framework (DOI 10.1136/bmj.n71)
• CTCAE v6 (2025) — NCI Common Terminology Criteria for Adverse Events
• WADA Prohibited List — World Anti-Doping Agency banned substances and methods
• FDA Adverse Event Reporting System (FAERS) — U.S. post-marketing pharmacovigilance database

In-corpus cross-references:

• Raun et al. 1998 — Ipamorelin selective-secretagogue paper (worked example of Kd / Emax / EC50 reporting)
• Teichman et al. 2006 — CJC-1295 Phase I (worked example of Phase I PK design)
• Manna 1998 — α-MSH NF-κB inhibition mechanism
• Sikiric 2018 — BPC-157 VEGFR2-eNOS-NO axis review
• Wilding et al. 2021 — STEP 1 pivotal Phase III (worked example of ITT, responder analysis, MMRM)
• Frías et al. 2021 — SURPASS-2 (worked example of non-inferiority design)
• Jastreboff et al. 2022 — SURMOUNT-1 pivotal Phase III
• Jastreboff et al. 2023 — Retatrutide Phase II
• Batsis et al. 2026 — incretin body-composition systematic review (PRISMA-structured)

In-corpus context:

• GH-axis dossier — pulsatile vs. tonic GH signaling and IGF-1 dynamics
• GLP-1 receptor pharmacology dossier — receptor breadth and mono- / dual- / triple-agonist progression
• Healing and angiogenesis dossier — VEGFR2 / eNOS / NO pathway in soft-tissue context
• Methodology page — full four-tier source grading and four strength labels