Glossary

Metabolic peptide glossary

50 essential terms for UK researchers reading the metabolic peptide literature.

By Metabolic Peptides Editorial TeamReviewed by Scientific Review PanelLast updated 16 May 2026

How this glossary is scoped

Each term is chosen to unlock a specific reading task in the metabolic-peptide literature: interpreting a trial protocol, reading an SmPC or EPAR, following a mechanism explainer, or parsing a regulatory statement. Definitions are short, evidence-led, and cross-linked to the relevant hub page (peptides, trials, mechanisms, safety, conditions or UK regulatory status) so the term functions as a bridge rather than an endpoint.

Where a term corresponds to an established entity in a public knowledge graph — Wikidata, MeSH, SNOMED CT — the glossary entry carries the corresponding identifier. That is what powers the sameAs array in the emitted DefinedTerm JSON-LD: AI search agents and semantic-web crawlers can resolve the term to its canonical entity rather than re-deriving it from surrounding text. Terms without a matched public-graph entity emit withoutsameAs rather than pointing at a speculative match.

Nothing in the glossary is prescriptive. Trial names are described in the abstract; the trial pages themselves at /trials carry the population, comparator, endpoint and interpretation. Regulatory concepts are defined neutrally; the UK regulatory framing lives at /uk-regulatory-status.

86 of 86 term(s) shown

Hormones & receptors

Glucagon-like peptide-1also GLP-1: An incretin hormone secreted by L-cells of the distal small intestine and colon in response to nutrient ingestion. GLP-1 augments glucose-stimulated insulin secretion, suppresses glucagon release, slows gastric emptying, and signals satiety via the hypothalamus and area postrema. Native GLP-1 has a plasma half-life of roughly two minutes owing to rapid cleavage by DPP-4 and renal clearance, which drove the development of longer-acting receptor agonists used in metabolic research.
Glucose-dependent insulinotropic polypeptidealso GIP: An incretin hormone secreted by K-cells of the duodenum and proximal jejunum following fat and carbohydrate ingestion. GIP amplifies glucose-stimulated insulin secretion and, at the adipose level, promotes lipid storage and may influence energy expenditure. Dual GLP-1/GIP receptor agonism is the pharmacological basis of tirzepatide. GIP is less susceptible to DPP-4 degradation than GLP-1 and has a native half-life of approximately seven minutes.
Glucagonalso GCG: A 29-amino-acid peptide hormone secreted by alpha-cells of the pancreatic islets of Langerhans in response to hypoglycaemia, prolonged fasting, and exercise. Glucagon promotes hepatic glycogenolysis and gluconeogenesis, elevating blood glucose. In the context of metabolic peptide research, glucagon receptor agonism is a component of triple-agonist molecules such as retatrutide, where it is thought to increase energy expenditure and hepatic fat oxidation alongside GLP-1 and GIP activity.
Growth hormone-releasing hormonealso GHRH, GRF: A hypothalamic neuropeptide that stimulates somatotroph cells of the anterior pituitary to synthesise and release growth hormone (GH). GHRH acts through a G-protein-coupled receptor to activate adenylyl cyclase. Synthetic GHRH analogues and modified peptide fragments are used in research contexts to interrogate the GH axis, assess pituitary reserve, and explore effects on body composition. Tesamorelin, a GHRH analogue, has received FDA approval for HIV-associated lipodystrophy.
Growth hormone secretagogue receptoralso GHSR, ghrelin receptor: A G-protein-coupled receptor expressed predominantly in the hypothalamus, pituitary, and vagal afferents. GHSR mediates the growth hormone-releasing and orexigenic effects of ghrelin. Two splice variants exist: GHSR-1a (functional) and GHSR-1b (truncated). Synthetic growth hormone secretagogues, including hexarelin and ipamorelin, bind GHSR-1a to stimulate GH secretion without the cortisol and prolactin surges associated with some other secretagogues, making them subjects of active body-composition research.
Ghrelin: An acylated 28-amino-acid peptide produced principally by X/A-like cells of the gastric fundus, often described as the hunger hormone. Circulating levels rise before meals and fall postprandially. Ghrelin binds GHSR-1a to stimulate GH release and enhance appetite through orexigenic pathways in the arcuate nucleus. Acylation of serine-3 by ghrelin O-acyltransferase (GOAT) is required for receptor activation; des-acyl ghrelin is biologically inactive at GHSR-1a but has independent metabolic effects.
Leptin: An adipokine encoded by the LEP gene and secreted in proportion to adipose tissue mass, conveying a signal of energy sufficiency to the hypothalamus. Leptin acts on arcuate nucleus neurons to suppress appetite and increase energy expenditure via anorexigenic POMC/CART pathways. In common obesity, leptin resistance rather than deficiency is the norm, with hypothalamic signalling impaired despite elevated circulating levels. Understanding leptin resistance is relevant to interpreting the additive satiety effects of GLP-1 receptor agonists.
Insulin: A 51-amino-acid anabolic hormone produced by pancreatic beta-cells in response to elevated blood glucose. Insulin promotes glucose uptake into muscle and adipose tissue, suppresses hepatic glucose output, and stimulates lipogenesis. In metabolic peptide research, insulin secretion dynamics (measured via C-peptide or HOMA-IR) serve as key secondary endpoints in trials of GLP-1 receptor agonists, and insulin resistance underpins the pathophysiology targeted by many investigational compounds.
Amylinalso IAPP, islet amyloid polypeptide: A 37-amino-acid peptide co-secreted with insulin by pancreatic beta-cells. Amylin slows gastric emptying, suppresses postprandial glucagon, and reduces food intake via receptors in the area postrema. Aggregated amylin forms the toxic islet amyloid deposits characteristic of type 2 diabetes. Synthetic analogues such as pramlintide are used therapeutically; newer dual amylin and calcitonin receptor agonists (DACRAs) combined with GLP-1 agonism are under active clinical investigation for obesity.

Pharmacology

Half-lifealso t½, elimination half-life: The time required for the plasma concentration of a compound to reduce by 50 % following distribution. For metabolic peptides, engineering an extended half-life is critical to clinical utility: native GLP-1 is inactivated within minutes, whereas semaglutide achieves a half-life of approximately one week through albumin binding and fatty-acid acylation, enabling once-weekly dosing. Half-life governs dosing interval, accumulation, and the time required to reach steady-state plasma concentrations.
Area under the curvealso AUC, AUC0–∞: A pharmacokinetic parameter representing the integral of plasma drug concentration over time, typically expressed in ng·h/mL or pmol·h/L. AUC reflects total systemic exposure to a compound following a dose and is used to assess bioavailability and compare formulations. In metabolic peptide trials, AUC for glucose (OGTT-derived) is also employed as a pharmacodynamic measure of glycaemic control, distinct from the drug-concentration AUC used in pharmacokinetic analyses.
Maximum plasma concentrationalso Cmax, peak concentration: The highest plasma drug concentration achieved following a single dose, typically occurring at time Tmax. Cmax is a key pharmacokinetic parameter used to assess peak exposure and guide safety thresholds. For subcutaneously administered peptides such as semaglutide and tirzepatide, Cmax is lower and Tmax longer than for intravenous administration, reflecting the absorption phase from the injection depot. Cmax is relevant to tolerability: gastrointestinal adverse effects often correlate with rapid concentration peaks.
Bioavailabilityalso F, absolute bioavailability: The fraction of an administered dose that reaches systemic circulation unchanged, expressed as a percentage relative to intravenous administration (absolute bioavailability). Subcutaneous peptides typically achieve 50–100 % bioavailability; oral semaglutide (Rybelsus) achieves only ~1 % due to proteolytic degradation in the gastrointestinal tract, necessitating SNAC co-formulation and strict fasting administration. Bioavailability data inform dose selection and regulatory submissions.
Albumin bindingalso plasma protein binding: Non-covalent association of a drug or peptide with serum albumin, the most abundant plasma protein. Albumin binding protects peptides from renal filtration and proteolytic degradation, markedly extending circulating half-life. Semaglutide and liraglutide employ a fatty-acid side chain that mediates reversible, high-affinity binding to albumin (>99 % bound). Only the free, unbound fraction is pharmacologically active; albumin binding therefore creates a reservoir that sustains prolonged action.
Fatty-acid acylationalso lipidation, C18 fatty diacid modification: A chemical modification strategy in which a fatty acid chain is covalently attached to a peptide, typically via a lysine residue and a linker. In GLP-1 receptor agonist design, fatty-acid acylation enables high-affinity albumin binding that protects the peptide from DPP-4 cleavage and renal clearance, extending half-life from minutes to days or weeks. Semaglutide carries a C18 fatty diacid chain attached at lysine-26 via a hydrophilic linker, which is central to its once-weekly pharmacokinetic profile.
Dipeptidyl peptidase-4also DPP-4, DPP-IV, CD26: A serine protease expressed on vascular endothelial cells and in plasma that rapidly cleaves the two N-terminal amino acids from peptides with a penultimate proline or alanine. DPP-4 is the primary enzyme responsible for inactivating native GLP-1 and GIP, giving them plasma half-lives of under two minutes. DPP-4 inhibitors (gliptins) are an established class of oral antidiabetic drugs; GLP-1 receptor agonists engineered with N-terminal modifications or albumin binding resist DPP-4 cleavage.
Tachyphylaxis: A rapid and reversible reduction in response to a pharmacological agent following repeated or continuous exposure, occurring over hours to days. Tachyphylaxis is mechanistically distinct from tolerance (which develops over weeks) and desensitisation. In metabolic peptide research, tachyphylaxis is discussed in the context of growth hormone secretagogues such as GHRP-6, where repeated bolus dosing can attenuate GH release. It is less of a concern for GLP-1 receptor agonists at therapeutic doses due to their gradual dose-escalation protocols.
Receptor downregulationalso receptor internalisation, desensitisation: A reduction in the number or functional activity of cell-surface receptors following sustained agonist exposure, typically mediated by phosphorylation, internalisation via endosomes, and lysosomal degradation. For GLP-1 receptors, prolonged agonism promotes internalisation; however, clinically meaningful downregulation does not appear to diminish the sustained glycaemic or weight-loss efficacy of once-weekly agonists. Receptor downregulation is an important consideration when interpreting acute versus chronic pharmacodynamic readouts in preclinical studies.
Pharmacokineticsalso PK: The discipline that describes what the body does to a drug over time, encompassing absorption, distribution, metabolism, and excretion (ADME). For subcutaneously injected metabolic peptides, key PK parameters include time to Cmax (Tmax), elimination half-life, volume of distribution, and clearance. Peptide PK is heavily influenced by molecular engineering choices such as fatty-acid acylation, PEGylation, and amino-acid substitutions that confer resistance to proteolytic degradation.
Pharmacodynamicsalso PD: The study of the biochemical and physiological effects of a drug on the body and the mechanisms by which those effects are produced. For GLP-1 receptor agonists, key pharmacodynamic readouts include glucose-lowering (assessed by HbA1c and OGTT), gastric emptying rate, body weight, appetite-suppressive visual analogue scores, and insulin secretion indices. PK/PD modelling integrates concentration-time profiles with effect-time profiles to predict therapeutic response across dose ranges.
Receptor agonistalso agonist: A molecule that binds to a receptor and activates it, producing a biological response. Full agonists produce a maximal response; partial agonists produce a sub-maximal response even at receptor saturation. GLP-1 receptor agonists (semaglutide, liraglutide, dulaglutide) activate the GLP-1 receptor; dual agonists activate two receptors (tirzepatide on GLP-1 and GIP); triple agonists activate three (retatrutide on GLP-1, GIP and glucagon).
Co-agonistalso dual agonist: A single molecule designed to activate two distinct receptors. Tirzepatide is a co-agonist (GLP-1 and GIP). Survodutide, pemvidutide, mazdutide and cotadutide are GLP-1/glucagon co-agonists. The integrated pharmacology of co-agonism is not a simple sum of separate mono-agonist effects.
Tri-agonistalso triple agonist: A single molecule designed to activate three distinct receptors. Retatrutide is a tri-agonist activating GLP-1, GIP and glucagon receptors. The pharmacological rationale is that combining glucagon-receptor activity (energy expenditure, hepatic fat oxidation) with incretin-receptor activity (appetite suppression, insulin secretion) yields complementary metabolic effects.

Anatomy & physiology

Adipose tissuealso body fat, fat tissue: A specialised connective tissue comprising lipid-laden adipocytes, stromal-vascular cells, immune cells, and a supporting extracellular matrix. Beyond energy storage as triglycerides, adipose tissue is an endocrine organ secreting adipokines including leptin, adiponectin, and resistin. Two principal anatomical depots — visceral and subcutaneous — have distinct metabolic properties and differ in their contribution to cardiometabolic risk. GLP-1 receptor agonists preferentially reduce visceral adipose tissue in clinical trials.
Visceral adipose tissuealso VAT, intra-abdominal fat: Adipose tissue located within the peritoneal cavity, surrounding intra-abdominal organs including the liver, intestines, and kidneys. Visceral fat is metabolically active, releasing free fatty acids and pro-inflammatory adipokines directly into the portal circulation. Elevated VAT correlates strongly with insulin resistance, type 2 diabetes, hypertension, and cardiovascular disease, independently of total body weight. In metabolic peptide trials, VAT is quantified by MRI or CT and serves as a clinically meaningful secondary endpoint alongside body weight.
Subcutaneous fatalso SAT, subcutaneous adipose tissue: Adipose tissue located beneath the dermis and above the deep fascia, distributed across the trunk, limbs, and gluteo-femoral regions. Subcutaneous fat is generally considered metabolically less harmful than visceral fat; gluteo-femoral depots may even exert protective cardiometabolic effects. DXA distinguishes subcutaneous from total fat mass, while MRI provides compartment-specific quantification. Loss of subcutaneous fat during GLP-1 receptor agonist therapy contributes to overall body weight reduction but is typically proportionally less than visceral fat loss.
Hypothalamus: A diencephalic brain region situated below the thalamus that integrates hormonal and neuronal signals to regulate energy homeostasis, food intake, thirst, body temperature, and neuroendocrine function. The hypothalamus contains several nuclei critical to appetite regulation, including the arcuate nucleus, which senses circulating hormones such as leptin, insulin, ghrelin, and GLP-1. Chronic inflammation or lipotoxicity of hypothalamic neurons is implicated in the development of leptin resistance and obesity.
Arcuate nucleusalso ARC, arcuate nucleus of the hypothalamus: A hypothalamic nucleus situated at the base of the third ventricle, adjacent to the median eminence, where the blood-brain barrier is fenestrated, allowing direct access to circulating hormones and metabolites. The arcuate nucleus contains two functionally opposing neuronal populations: orexigenic NPY/AgRP neurons, which promote feeding, and anorexigenic POMC/CART neurons, which suppress appetite. GLP-1, leptin, and insulin converge on arcuate neurons to modulate energy balance and body weight.
Area postremaalso AP, chemoreceptor trigger zone: A circumventricular organ on the dorsal surface of the medulla oblongata where the blood-brain barrier is absent, enabling detection of bloodborne emetic toxins, hormones, and peptides. The area postrema expresses GLP-1 receptors, amylin receptors, and cholecystokinin receptors, and contributes to satiety signalling and the nausea associated with high-dose GLP-1 receptor agonist therapy. Communication between the area postrema and adjacent nucleus tractus solitarius integrates peripheral and central satiety cues.
Pancreatic beta-cellsalso β-cells, islet beta cells: Insulin-secreting cells located in the islets of Langerhans within the exocrine pancreas, constituting approximately 70 % of islet mass. Beta-cells sense blood glucose via glucokinase-mediated ATP generation, triggering membrane depolarisation and calcium-dependent exocytosis of insulin granules. GLP-1 and GIP potentiate glucose-stimulated insulin secretion through cAMP/PKA signalling. Progressive beta-cell dysfunction and loss are central to the pathogenesis of type 2 diabetes and are endpoints in some investigational metabolic peptide trials.
Hepatic steatosisalso fatty liver, liver fat accumulation: The abnormal accumulation of triglycerides within hepatocytes, defined histologically as lipid droplets occupying >5 % of liver volume by weight. Hepatic steatosis represents the earliest stage of the metabolic dysfunction–associated steatotic liver disease spectrum. It is strongly associated with visceral obesity, insulin resistance, and dyslipidaemia. In metabolic peptide trials, liver fat is quantified non-invasively by MRI-PDFF or controlled attenuation parameter (CAP) on FibroScan, and reduction in hepatic steatosis is an emerging secondary endpoint.
Non-alcoholic fatty liver diseasealso NAFLD, MAFLD, MASLD: A spectrum of liver disease ranging from simple hepatic steatosis to non-alcoholic steatohepatitis (NASH/MASH), fibrosis, cirrhosis, and hepatocellular carcinoma, occurring in the absence of significant alcohol intake. The condition is now frequently reclassified as metabolic dysfunction-associated steatotic liver disease (MASLD) in updated nomenclature. GLP-1 receptor agonists and triple-agonist compounds demonstrate significant reductions in hepatic fat and, in some trials, histological improvement in steatohepatitis, positioning them as candidate treatments for NAFLD/MASLD.
Non-alcoholic steatohepatitisalso NASH, MASH: The progressive inflammatory form of non-alcoholic fatty liver disease, characterised histologically by hepatic steatosis accompanied by lobular inflammation, hepatocyte ballooning, and varying degrees of fibrosis. NASH/MASH carries significant risk of progression to cirrhosis and hepatocellular carcinoma. Definitive diagnosis requires liver biopsy with NAS scoring. Several GLP-1 and dual-agonist trials have reported liver biopsy-confirmed NASH resolution as a primary endpoint, representing a major unmet therapeutic need.
Lean massalso fat-free mass, FFM: The non-fat component of body composition: skeletal muscle, organs, bone, body water and connective tissue. Lean mass is quantified by DXA (whole-body and regional), bioelectrical impedance, or hydrodensitometry. All weight reduction — diet, surgery, pharmacotherapy — loses some lean mass alongside fat mass; the clinical question with incretin therapy is whether the proportion of lean-mass loss is greater than expected and how to support muscle and bone during therapy.

Trials & methodology

Randomised controlled trialalso RCT: The gold-standard experimental design in clinical research, in which participants are randomly allocated to intervention and control (placebo or active comparator) groups to minimise selection bias and confounding. Randomisation ensures balanced baseline characteristics, enabling causally interpretable estimates of treatment effect. Pivotal metabolic peptide trials — such as SURMOUNT, STEP, and SURPASS — are phase 3 RCTs with large sample sizes, typically exceeding 1,000 participants, double-blinded and with 52–72-week primary endpoints.
Major adverse cardiovascular eventsalso MACE, 3-point MACE, 4-point MACE: A composite cardiovascular endpoint typically comprising non-fatal myocardial infarction, non-fatal stroke, and cardiovascular death (3-point MACE). Regulatory agencies require MACE trials (cardiovascular outcomes trials, CVOTs) to demonstrate non-inferiority or superiority for metabolic agents before or shortly after approval. Semaglutide demonstrated cardiovascular superiority in SUSTAIN-6 and SELECT. Tirzepatide's SURPASS-CVOT is ongoing. MACE is adjudicated by an independent clinical events committee blinded to treatment allocation.
Glycated haemoglobinalso HbA1c, A1c: A measure of average blood glucose over the preceding 8–12 weeks, reflecting the proportion of haemoglobin with a non-enzymatically glycated N-terminal valine on the beta chain. Expressed as a percentage (NGSP/DCCT scale) or in mmol/mol (IFCC scale). HbA1c is the primary glycaemic endpoint in regulatory submissions for antidiabetic agents and is frequently a co-primary endpoint in GLP-1 receptor agonist trials alongside body weight change. Values are influenced by haemolytic conditions, haemoglobin variants, and recent transfusion.
Oral glucose tolerance testalso OGTT, 75 g OGTT: A standardised diagnostic and research test in which a fasting subject ingests 75 g of glucose dissolved in water, with venous blood sampled at 0, 60, and 120 minutes for plasma glucose and insulin measurement. The 2-hour plasma glucose threshold of ≥11.1 mmol/L indicates type 2 diabetes; 7.8–11.0 mmol/L indicates impaired glucose tolerance. In metabolic peptide trials, OGTT-derived insulin secretion indices (insulinogenic index, disposition index) and glucose AUC provide mechanistic pharmacodynamic data.
Intention-to-treat analysisalso ITT, modified ITT, mITT: A statistical analysis strategy in which all randomised participants are analysed in the group to which they were assigned, regardless of whether they completed the assigned intervention or deviated from the protocol. ITT preserves the benefits of randomisation and provides a conservative, real-world estimate of treatment effect by including dropouts. Most pivotal GLP-1 receptor agonist trials use a modified ITT (mITT) population — all randomised patients who received at least one dose — as the primary analysis set.
Dual-energy X-ray absorptiometryalso DXA, DEXA: An imaging technique that uses two X-ray beams at different energies to measure bone mineral density, lean mass, and fat mass with high precision and low radiation exposure. In metabolic peptide trials, DXA provides whole-body and regional (trunk, limb) fat and lean mass data, allowing assessment of whether weight loss is primarily adipose- or muscle-derived. Total body DXA cannot distinguish visceral from subcutaneous fat, so MRI or CT is required for depot-specific quantification.
MRI proton density fat fractionalso MRI-PDFF, PDFF: A non-invasive quantitative MRI technique that measures liver fat content as the ratio of the MRI signal from fat protons to total proton signal, expressed as a percentage. MRI-PDFF has high reproducibility and is accepted by regulators as a surrogate endpoint for hepatic steatosis in phase 2 and phase 3 NASH/MASLD trials. A threshold of ≥5 % liver fat fraction by MRI-PDFF is consistent with hepatic steatosis. It is increasingly preferred over liver biopsy for longitudinal fat quantification due to its non-invasive nature.
Body mass indexalso BMI: An anthropometric index calculated as weight in kilograms divided by height in metres squared (kg/m²), widely used to classify weight status: underweight (<18.5), normal (18.5–24.9), overweight (25–29.9), obesity class I–III (≥30). BMI is a primary eligibility criterion for most obesity pharmacotherapy trials, typically requiring ≥30 kg/m² or ≥27 kg/m² with weight-related comorbidities. Despite limitations — it does not distinguish fat from lean mass or account for fat distribution — BMI remains the standard trial entry and efficacy reporting metric.
Obstructive sleep apnoeaalso OSA, sleep apnoea: A disorder characterised by repetitive partial or complete collapse of the upper airway during sleep, resulting in oxyhemoglobin desaturation, sleep fragmentation, and daytime somnolence. Severity is graded by the apnoea-hypopnoea index (AHI): mild (5–14 events/hour), moderate (15–29), and severe (≥30). OSA is common in severe obesity. The SURMOUNT-OSA trial demonstrated that tirzepatide significantly reduced AHI, establishing OSA improvement as a registerable metabolic endpoint and securing a US FDA label expansion.
Placebo-controlled trialalso double-blind placebo-controlled: A trial design in which the control group receives an inert preparation (placebo) that is indistinguishable from the active intervention, enabling assessment of the true pharmacological effect above the placebo response. Double-blinding — concealing allocation from both participants and investigators — minimises bias in outcome assessment. For injectable metabolic peptides, matching saline or vehicle injections serve as placebos. Placebo response in appetite and weight trials can be substantial (2–3 % body weight), underscoring the importance of this design.
Hazard ratioalso HR: A measure of relative effect from time-to-event analyses such as Cox proportional-hazards regression. A hazard ratio of 0.80 means the rate of the event of interest is 20% lower in the active arm than in the comparator over the follow-up period; HR of 1.00 indicates no difference. Hazard ratios are typically reported with a 95% confidence interval; if the interval crosses 1.00, the result is not statistically significant at conventional thresholds. In metabolic peptide outcome trials (SELECT, FLOW, LEADER, SURPASS-CVOT) hazard ratios summarise the cardiovascular or kidney effect relative to comparator.
Confidence intervalalso CI, 95% CI: The range of values within which the true population parameter is plausibly located given the trial data, at a stated confidence level (conventionally 95%). For a hazard ratio of 0.80 with a 95% CI of 0.72-0.90, the interval excludes 1.00 — the result is statistically significant at conventional thresholds. Narrower intervals indicate more precise estimates. Confidence intervals are not probability statements about the parameter itself but about the procedure that produced them.
Non-inferiorityalso NI, non-inferiority trial: A trial design and analytic objective in which the goal is to demonstrate that the active treatment is not unacceptably worse than the comparator, by a pre-specified margin. Non-inferiority is not the same as equivalence and is not the same as superiority. SURPASS-CVOT, for example, was designed for non-inferiority of tirzepatide versus dulaglutide on MACE-3; a non-inferiority result does not establish superiority. The non-inferiority margin and analytic population (ITT vs per-protocol) are critical design details.
Superiorityalso superiority trial: A trial design and analytic objective in which the goal is to demonstrate that the active treatment is better than the comparator on the primary endpoint. SELECT demonstrated superiority of semaglutide 2.4 mg over placebo for MACE-3 in adults with overweight or obesity and pre-existing CVD without diabetes. Editorial framing should be careful not to imply superiority from a non-inferiority result.
Endpointalso primary endpoint, secondary endpoint: A pre-specified outcome measured in a trial to evaluate the treatment effect. The primary endpoint is the statistical focus on which the trial is powered (e.g., percent change in body weight at week 72 for SURMOUNT-1; time to first MACE-3 for SELECT). Secondary endpoints add context but do not, on their own, support regulatory claims. The hierarchy of primary and secondary endpoints and the statistical testing plan are declared in the protocol before unblinding.
Surrogate endpointalso surrogate marker: A measurable biomarker or physiologic variable expected to predict a clinical outcome of interest. HbA1c predicts diabetes complications; body weight predicts cardiometabolic risk; MRI-PDFF predicts steatohepatitis trajectory; eGFR slope predicts kidney failure. Surrogate-outcome relationships are population-derived and probabilistic, not deterministic. Regulatory acceptance of a surrogate varies by indication; outcome trials remain the standard for outcome claims.
Clinical outcomealso hard outcome, patient-relevant outcome: An event that patients and clinicians directly care about — death, myocardial infarction, stroke, kidney failure, hospitalisation, symptom improvement — as distinct from a biomarker change. Trials with clinical-outcome endpoints (CVOTs, kidney-outcome trials, MASH histology endpoints) provide the strongest evidence for outcome claims. SELECT, FLOW, SUMMIT and SURPASS-CVOT are clinical-outcome trials in the metabolic peptide class.
Per-protocol analysisalso PP: A statistical analysis strategy in which only participants who completed the protocol as intended are analysed. Per-protocol analysis is more sensitive to a true biological effect but breaks the randomisation guarantee — it is vulnerable to selection bias because dropouts (often differential between arms) are excluded. Non-inferiority trials commonly report both ITT and per-protocol analyses, since per-protocol is conservative against an erroneous non-inferiority conclusion.
Randomised withdrawal trial designalso randomised withdrawal, randomised discontinuation: A trial design in which all participants first receive open-label active therapy in a lead-in period (typically to allow titration and identify responders/tolerators), after which responders are randomised to continue active therapy or withdraw to placebo. SURMOUNT-4 used this design to characterise weight regain after tirzepatide discontinuation. The design directly addresses the durability question but generalises to the enriched responder/tolerator population, not to the original ITT cohort.
Comparatoralso control arm, active comparator, placebo comparator: The arm against which the active treatment is measured. Placebo comparators measure effect over the baseline of natural history; active comparators (e.g., dulaglutide in SURPASS-CVOT, semaglutide 2.4 mg in SURMOUNT-5) measure relative effect against an established standard. The comparator choice profoundly shapes how trial results should be interpreted: non-inferiority versus an active comparator is not the same as superiority over placebo.
Adverse eventalso AE, treatment-emergent adverse event, TEAE: Any untoward medical occurrence in a clinical trial participant, regardless of whether it is causally related to the intervention. Adverse events are coded using systems such as MedDRA and reported in trial publications by frequency, severity and relationship to treatment. The term covers everything from mild nausea to serious events; severity grading (typically mild / moderate / severe) is separate from seriousness.
Serious adverse eventalso SAE: An adverse event meeting at least one of: death, life-threatening, hospitalisation or prolongation of existing hospitalisation, persistent or significant disability/incapacity, congenital anomaly/birth defect, or other medically important event. SAE rates are a key safety endpoint and are reviewed by independent data and safety monitoring boards during ongoing trials.
Discontinuation ratealso dropout rate, treatment discontinuation: The proportion of randomised participants who stop the assigned intervention before completing the trial. In incretin trials, gastrointestinal adverse events are the leading cause of treatment discontinuation; the rate varies by molecule, dose-escalation speed and population. Discontinuation is reported overall and by reason (adverse event, withdrawal of consent, lost to follow-up, etc.).
Phase 1 trialalso first-in-human, FIH: The earliest stage of human testing, characterising pharmacokinetics, tolerability, dose-range safety and (for metabolic peptides) initial pharmacodynamic readouts. Phase 1 trials are typically small (tens of participants), often in healthy volunteers, and are not designed to demonstrate clinical efficacy. A phase 1 readout for an incretin programme establishes the dose range that will be explored in phase 2 but does not establish weight-management or glycaemic benefit.
Phase 2 trialalso proof-of-concept trial, dose-finding trial: Mid-stage human trial designed to demonstrate proof of concept, refine dose selection, and characterise the safety profile in the target population. Phase 2 trials are typically a few hundred participants and powered to detect dose-response, not population-level treatment effect with high precision. Retatrutide's phase 2 obesity trial (NEJM 2023) is a recent example: large mean body-weight reduction in the studied cohort, but phase 3 confirmation pending.
Phase 3 trialalso pivotal trial, registration trial: Pivotal large-scale trial designed to confirm efficacy and characterise safety in the intended-use population. Phase 3 trials are typically in the thousands of participants and are powered against the regulatory standard for the indication. SURMOUNT-1, STEP-1, SCALE Obesity and Prediabetes, SURPASS-2 are examples of pivotal phase 3 trials in the incretin class.
Phase 4 trialalso post-marketing study, post-authorisation safety study: Studies conducted after regulatory approval and marketing, often as a regulatory commitment (post-authorisation safety study) or to extend evidence into new populations, comparators or real-world settings. Phase 4 trials and post-marketing pharmacovigilance (MHRA Yellow Card, FDA FAERS) refine the safety profile of licensed incretin medicines beyond the controlled trial environment.
Atherosclerotic cardiovascular diseasealso ASCVD: A clinical entity comprising coronary artery disease, cerebrovascular disease and peripheral arterial disease attributable to atherosclerosis. Established ASCVD is a common inclusion criterion in cardiovascular outcomes trials (e.g., SELECT, SURPASS-CVOT, LEADER) and a key risk-stratification consideration in UK type 2 diabetes and obesity pathways. ASCVD risk in primary prevention is estimated by tools such as QRISK in the NHS.
Cardiometabolic riskalso cardio-metabolic risk: The integrated risk of cardiovascular events and metabolic disease arising from the cluster of obesity, dysglycaemia, dyslipidaemia, hypertension and insulin resistance. Cardiometabolic risk is the umbrella concept that frames why a metabolic intervention (incretin therapy, bariatric surgery) can plausibly influence cardiovascular outcomes — a relationship demonstrated in SELECT, LEADER, SUSTAIN-6 and SUMMIT.

Regulatory

Medicines and Healthcare products Regulatory Agencyalso MHRA: The UK government agency responsible for regulating medicines, medical devices, and blood components, operating under the Medicines Act 1968 and retained EU law. Following Brexit, the MHRA operates an independent regulatory pathway and issues its own marketing authorisations. It runs the Yellow Card pharmacovigilance scheme, publishes public assessment reports, and has introduced Project Orbis collaborative reviews with international partners. Compliance with MHRA requirements is mandatory for any product sold or marketed for therapeutic use in the United Kingdom.
European Medicines Agencyalso EMA: The decentralised agency of the European Union responsible for the scientific evaluation, supervision, and safety monitoring of medicines throughout the EU and European Economic Area. The EMA's Committee for Medicinal Products for Human Use (CHMP) conducts centralised assessments and issues opinions that form the basis of European Commission marketing authorisations. EMA publishes European Public Assessment Reports (EPARs) containing comprehensive scientific summaries. Post-Brexit, EMA decisions are not automatically valid in the UK, requiring separate MHRA review.
Food and Drug Administrationalso FDA, US FDA: The US federal agency within the Department of Health and Human Services responsible for regulating food, drugs, biologics, medical devices, and cosmetics. FDA approval via New Drug Application (NDA) or Biologics Licence Application (BLA) is a prerequisite for commercial sale in the United States. FDA New Molecular Entity designations, Breakthrough Therapy status, and Accelerated Approval pathways are significant for pipeline metabolic peptides. FDA label indications for obesity agents (SURMOUNT programme) set international precedents.
British National Formularyalso BNF, BNFC: A joint publication of the Royal Pharmaceutical Society and the British Medical Association, providing authoritative guidance on prescribing, dispensing, and administering medicines in the United Kingdom. The BNF is updated twice-yearly in print and continuously online via NICE Evidence. It lists licensed indications, dosing regimens, contraindications, interactions, and costs. GLP-1 receptor agonists approved by the MHRA appear in the BNF under antidiabetics and, increasingly, anti-obesity medicines, with NICE technology appraisals governing NHS eligibility.
Yellow Card Schemealso Yellow Card, MHRA Yellow Card: The UK's national pharmacovigilance reporting system, operated by the MHRA, enabling healthcare professionals, patients, and carers to report suspected adverse reactions to medicines, vaccines, and medical devices. Reports can be submitted online, via the Yellow Card app, or on paper. Signal detection from Yellow Card data informs regulatory action including label updates, Dear Healthcare Professional letters, and product withdrawals. The scheme has been integral to monitoring the real-world safety of GLP-1 receptor agonists, particularly regarding thyroid and pancreatic signals.
Marketing authorisationalso MA, product licence, PL: Formal regulatory approval permitting a pharmaceutical product to be placed on the market in a given territory, issued following satisfactory review of quality, safety, and efficacy data. In the UK, marketing authorisations are granted by the MHRA via the national, decentralised, or international recognition procedures. The summary of product characteristics (SmPC) and patient information leaflet are legally binding documents attached to the authorisation. Use of a product outside its MA constitutes off-label use.
Off-label prescribingalso unlicensed use, off-licence use: The use of a licensed medicine outside the terms of its marketing authorisation — including for a different indication, patient population, dose, or route of administration. Off-label prescribing is legal in the UK and is common practice, but it transfers greater clinical responsibility to the prescriber and requires informed patient consent. In metabolic medicine, tirzepatide is sometimes prescribed off-label for weight management pending full obesity licence approval, and several GLP-1 agonists are used off-label for non-diabetic obesity pending NICE appraisal.
Schedule 4 controlled drugalso Schedule 4 CD, Sched 4 CD: A classification under the UK Misuse of Drugs Regulations 2001 applying to substances with lower abuse potential than Schedules 1–3. Schedule 4 is divided into Part I (anabolic steroids, certain benzodiazepines) and Part II (most benzodiazepines). Some growth hormone-releasing peptides and their analogues used in bodybuilding or body-composition research may be subject to scheduling. Practitioners should verify the current scheduling status of any investigational peptide with the MHRA or Home Office Drugs Licensing before supply.
Compounded medicinealso specials, unlicensed specials: A medicine prepared by a licensed pharmacist or manufacturer to meet the specific clinical needs of an individual patient when no licensed product is available or suitable. UK 'specials' are permitted under the Medicines Act 1968 and require a valid prescription from a registered prescriber. Compounded semaglutide and tirzepatide have proliferated during supply shortages, raising MHRA safety concerns regarding dose accuracy, sterility, and the absence of pharmacovigilance data. The MHRA has issued guidance discouraging compounded versions of approved GLP-1 agonists.
Prescription Only Medicinealso POM, Rx-only: A classification under the Human Medicines Regulations 2012 designating medicines that may only be supplied to a patient on the authorisation of an appropriate practitioner (doctor, dentist, or authorised non-medical prescriber). All currently approved GLP-1 receptor agonists and metabolic peptide therapeutics in the UK hold POM status. Supply without a valid prescription is a criminal offence. Online clinics operating lawfully must conduct genuine clinical assessments before issuing prescriptions for POM metabolic agents.
European Public Assessment Reportalso EPAR: A publicly available document published by the European Medicines Agency following a positive CHMP opinion, providing a comprehensive scientific assessment of a medicine's quality, safety, and efficacy data package. EPARs include the product information (SmPC and package leaflet), the scientific discussion, and post-authorisation updates. For approved metabolic peptides such as semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro), EPARs serve as primary reference documents for understanding the evidence base and approved indications across EU member states.

Molecules & chemistry

Peptidealso polypeptide: A molecule comprising two or more amino acids linked by amide (peptide) bonds between the carboxyl group of one residue and the amino group of the next. Peptides are conventionally distinguished from proteins by size, with the boundary loosely placed at 50 amino acid residues or a molecular weight of approximately 5 kDa, although usage varies. Therapeutic metabolic peptides such as semaglutide (31 residues), tirzepatide (39 residues), and retatrutide (36 residues) are mid-size peptides requiring parenteral or engineered oral delivery due to poor GI absorption.
Amino acidalso AA: An organic molecule containing both an amino group (–NH₂) and a carboxyl group (–COOH) attached to a central alpha-carbon, along with a variable side chain (R group) that determines chemical character. Twenty standard proteinogenic amino acids are encoded by the genetic code and constitute the building blocks of peptides and proteins. Non-standard amino acids — such as Aib (alpha-aminoisobutyric acid) — are incorporated into synthetic therapeutic peptides to resist proteolytic degradation and alter receptor binding kinetics.
Residuealso amino acid residue: The portion of an amino acid that remains after condensation into a peptide chain, having lost a water molecule through peptide bond formation. The term is used in peptide sequence notation (e.g., lysine-26 of semaglutide, where the fatty diacid chain is attached). Modification of specific residues — by acylation, PEGylation, or substitution with non-natural analogues — is a central strategy in therapeutic peptide engineering to optimise potency, selectivity, and pharmacokinetic properties.
Fatty diacidalso diacid fatty acid, alpha-omega dicarboxylic acid: A fatty acid bearing carboxyl groups at both ends of the aliphatic chain, enabling covalent coupling at one terminus and albumin binding at the other. Semaglutide employs a C18 fatty diacid attached via a two-OEG (mini-PEG) linker to lysine-26 of the GLP-1 backbone. The diacid structure provides superior albumin binding affinity compared with monocarboxylic fatty acids used in earlier analogues such as liraglutide, contributing to semaglutide's extended half-life of approximately one week.
C18 fatty acidalso octadecanoic acid, stearic acid derivative: An 18-carbon aliphatic chain used as the lipid moiety in fatty-acid-acylated GLP-1 receptor agonists. In semaglutide, the C18 fatty diacid (C18 diacid) is the albumin-binding element that enables high-affinity, reversible association with serum albumin, protecting the peptide from DPP-4 degradation and renal clearance. The chain length of 18 carbons was selected through systematic structure-activity studies to balance albumin affinity, aqueous solubility, and manufacturing feasibility.
Recombinant DNA technologyalso rDNA, recombinant protein expression: A set of molecular biology techniques used to introduce foreign DNA sequences into a host organism (bacteria, yeast, or mammalian cells) to produce a desired protein or peptide at scale. GLP-1 receptor agonist backbones are typically produced by recombinant expression in microbial systems, followed by chemical modification (acylation, PEGylation) to introduce pharmacokinetic-enhancing groups. Recombinant production ensures high sequence fidelity and scalability, contrasting with solid-phase peptide synthesis used for smaller research-grade compounds.
Synthetic peptidealso chemically synthesised peptide, SPPS peptide: A peptide assembled stepwise using solid-phase peptide synthesis (SPPS), in which amino acids are sequentially coupled onto a resin support in a defined order and then cleaved and purified. Synthetic peptides are used extensively in research settings for shorter sequences (typically fewer than 50 residues) and for the incorporation of non-natural amino acids, isotopic labels, or chemical modifications. Research-grade synthetic peptides are characterised by HPLC purity and mass spectrometry, with ≥98 % purity generally required for biological assays.
Lyophilisedalso lyophilized, freeze-dried: Describing a substance that has been processed by lyophilisation (freeze-drying): frozen under vacuum so that ice sublimes directly to vapour, leaving a dry, stable solid residue. Lyophilisation substantially extends shelf-life and permits storage at ambient or refrigerated temperatures without the cold chain required for peptide solutions. Research-grade peptides are routinely supplied as lyophilised powders and must be reconstituted with an appropriate solvent — typically bacteriostatic water or sterile water for injection — before use.
Bacteriostatic wateralso BAC water, bacteriostatic water for injection: Sterile water for injection containing 0.9 % benzyl alcohol as a bacteriostatic preservative, which inhibits microbial growth, permitting multi-dose use of reconstituted peptide vials when stored at 2–8 °C. Bacteriostatic water is the standard diluent for research-grade lyophilised peptides; it enables repeated withdrawal from a single vial over several days without risk of microbial contamination. It should not be used for neonates or paediatric patients owing to benzyl alcohol toxicity risks, though this is not relevant in pure research contexts.
Reconstitutionalso peptide reconstitution: The process of dissolving a lyophilised peptide powder in a specified volume of diluent (typically bacteriostatic water or sterile saline) to produce a solution of known concentration for use in research. Accurate reconstitution requires knowledge of the vial's peptide mass and desired final concentration, with the diluent introduced slowly down the vial wall to minimise foaming and aggregation. Reconstituted peptide solutions should be stored at 2–8 °C and used within the manufacturer-specified stability window, typically 28–30 days.
HPLC purityalso RP-HPLC purity, chromatographic purity: A quantitative measure of a peptide's chemical purity determined by reversed-phase high-performance liquid chromatography (RP-HPLC), expressed as the area percentage of the principal peak relative to all detected peaks at a defined UV wavelength (typically 214 nm). A purity of ≥98 % is conventionally required for research-grade peptides intended for cell-based or animal studies. Certificates of analysis from reputable suppliers include HPLC chromatograms and mass spectrometry data confirming identity alongside purity.
Analoguealso peptide analogue, amino-acid analogue: A molecule whose structure has been modified relative to a natural reference (typically an endogenous hormone or peptide). Modifications can include amino-acid substitutions, addition of non-natural amino acids (e.g., Aib in semaglutide and tirzepatide), or attachment of fatty-acid moieties for albumin binding. Analogue design optimises receptor selectivity, potency, half-life and resistance to proteolytic degradation.