GLP-1 Research: Mechanism, History, and Current Trials

The GLP-1 research programme is the most significant transformation in metabolic pharmacology of the last decade. From a minor gut peptide with a two-minute half-life, researchers have built a class of compounds that reduce body weight by 15 to 24 percent at Phase 3 scale, reduce cardiovascular events in multiple populations, and are reshaping the clinical landscape of obesity, diabetes, and cardiometabolic risk. This article walks through the scientific and clinical history, the receptor-level pharmacology, and the current state of the class including semaglutide, tirzepatide, and retatrutide.

Discovery: incretin biology and the identification of GLP-1

The incretin concept dates to the 1960s: the observation that oral glucose produces a larger insulin response than intravenous glucose at the same blood glucose level, implying a gut-derived factor that amplifies insulin secretion. This “incretin effect” was biochemically elusive for two decades. The relevant factors were identified as gastric inhibitory polypeptide (GIP) in the 1970s and glucagon-like peptide-1 (GLP-1) in the 1980s.

GLP-1 was identified as a cleavage product of the preproglucagon gene, processed differently in intestinal L-cells than in pancreatic α-cells. The same gene produces glucagon (α-cells, metabolic stress response) and GLP-1 (L-cells, postprandial response) through tissue-specific differential processing. Jens Holst’s group in Copenhagen was central to characterising GLP-1’s physiological role; Svetlana Mojsov at Massachusetts General Hospital separately characterised the bioactive form (GLP-1(7-37) and GLP-1(7-36)amide).

Baggio and Drucker’s 2007 Gastroenterology review is the canonical reference for the incretin biology of the pre-therapeutic era ². The review covered:

• GLP-1 biosynthesis in L-cells
• Postprandial secretion patterns
• GLP-1R distribution (β-cells, hypothalamus, gastric smooth muscle, cardiac tissue)
• Physiological actions: glucose-dependent insulin secretion, glucagon suppression, gastric emptying delay, appetite reduction
• GIP biology as the parallel incretin with partially overlapping but distinct pharmacology

The physiological story established by Baggio and Drucker framed every subsequent therapeutic development.

The engineering challenge: from native GLP-1 to long-acting analogs

Native GLP-1 has a circulating half-life of approximately 2 minutes due to rapid N-terminal degradation by DPP-IV (dipeptidyl peptidase-4). For therapeutic use, the molecule needed to be re-engineered to resist DPP-IV and extend half-life to practical dosing intervals.

Three engineering strategies emerged, in rough chronological order:

1. DPP-IV-resistant mutations. Exenatide (a natural exendin-4 analog from Gila monster saliva) and later synthetic analogs substituted the DPP-IV-cleavage residues. Exenatide was the first GLP-1 receptor agonist approved (2005), with half-life ~2.4 hours requiring twice-daily dosing.

2. Albumin-binding extensions. Liraglutide (approved 2010) added a fatty acid chain that binds albumin, extending half-life to ~13 hours and enabling once-daily dosing. This was a substantial improvement but still required daily injection.

3. Further albumin optimisation. Semaglutide (approved 2017) used a different fatty acid spacer and extensive DPP-IV-resistant backbone modifications, extending half-life to ~165 hours (~7 days) and enabling once-weekly dosing. This is the current state of the art in GLP-1R single-agonist pharmacology.

Each half-life extension required protein engineering iterations that preserved GLP-1R binding and signalling efficacy while adding the pharmacokinetic modification. Drucker’s 2018 Cell Metabolism review is the definitive modern reference for the therapeutic application of GLP-1R agonists and covers the engineering history in detail ¹.

GLP-1 receptor pharmacology

GLP-1R is a class B G-protein-coupled receptor with tissue distribution broader than the original “pancreatic β-cell receptor” framing suggested. Current understanding of the therapeutic target distribution:

• Pancreatic β-cells. GLP-1R activation drives glucose-dependent insulin secretion. “Glucose-dependent” is critical: GLP-1 does not produce insulin secretion at normal fasting glucose, which is why GLP-1R agonists have minimal hypoglycaemia risk compared to older diabetes drugs.
• Pancreatic α-cells. GLP-1R activation suppresses glucagon secretion, contributing to glycaemic control in type 2 diabetes.
• Hypothalamic arcuate nucleus. GLP-1R activation on POMC/CART neurons drives appetite reduction. This is the mechanism underlying the body-weight endpoints in the modern trials.
• Gastric smooth muscle. GLP-1R activation delays gastric emptying, contributing both to glycaemic control (slower glucose absorption) and to satiety.
• Cardiac tissue. GLP-1R is expressed in cardiomyocytes and vascular tissue. The pharmacological relevance of this is still being characterised; some cardiovascular benefits may be direct, others are secondary to weight loss and glycaemic control.

The signalling cascade downstream of GLP-1R activation involves Gs-coupled cAMP elevation, PKA activation, and a characteristic set of downstream transcriptional and membrane-level effects. The cAMP mechanism overlaps with GHRH-R signalling (see the Ipamorelin vs CJC-1295 article for GHRH-R discussion), which explains why GLP-1R and GHRH-R agonists have some overlapping effects on cAMP-responsive transcription despite addressing different physiological systems.

The semaglutide clinical programme: STEP, SUSTAIN, SELECT

Semaglutide’s clinical trial programme set the modern evidence standard for GLP-1R agonism. Three programme tracks matter for research framing:

SUSTAIN (type 2 diabetes glycaemic control): SUSTAIN-6, published by Marso and colleagues in NEJM 2016, established cardiovascular-outcome benefit in patients with type 2 diabetes and high cardiovascular risk ⁶. This was the first major cardiovascular-outcomes trial for a GLP-1R agonist and moved the class beyond glycaemic control to cardiovascular prevention.

STEP (obesity without diabetes): STEP 1, published by Wilding and colleagues in NEJM 2021, was the pivotal obesity trial ⁵. Adults with overweight or obesity (BMI ≥ 27 with weight-related comorbidity, or BMI ≥ 30) without diabetes received semaglutide 2.4 mg/week SC for 68 weeks. Primary endpoint: percentage change in body weight. Results: approximately 15% mean body-weight reduction in the semaglutide arm vs. approximately 2.4% in placebo. This established semaglutide as the first GLP-1R agonist with obesity-specific regulatory approval at that dose.

SELECT (cardiovascular outcomes in obesity without diabetes): SELECT, published by Lincoff and colleagues in NEJM 2023, extended the cardiovascular-outcome benefit to obese patients without diabetes ⁷. Semaglutide 2.4 mg/week reduced major adverse cardiovascular events (composite of CV death, non-fatal MI, non-fatal stroke) by approximately 20% compared to placebo in obese patients with established cardiovascular disease but without diabetes. This confirmed that the cardiovascular benefit is not contingent on diabetes.

The sequence SUSTAIN-6 → STEP 1 → SELECT is the mainstream evidence foundation for semaglutide in any modern research discussion.

GIP biology: the companion incretin

GIP (gastric inhibitory polypeptide, later renamed glucose-dependent insulinotropic polypeptide) is the other incretin, secreted from intestinal K-cells. Historically, GIP received less therapeutic attention than GLP-1 because:

1. GIP appeared to have minimal insulin-secretory effect in type 2 diabetes (versus GLP-1’s preserved effect)
2. GIP’s role in body weight regulation was less clear than GLP-1’s appetite-reducing effect
3. Some preclinical work suggested GIP might promote obesity rather than reduce it

This older framing of GIP as “less useful than GLP-1” was revised substantially in the 2010s. Samms and colleagues’ 2020 Trends in Endocrinology and Metabolism review consolidated the emerging evidence ³: GIP acts on the hypothalamus, adipose tissue, and pancreas through mechanisms that complement rather than duplicate GLP-1. At adipose tissue specifically, GIP receptor activation has effects on lipid handling that may actually contribute to body-weight regulation rather than opposing it.

The revised GIP biology opened the therapeutic question: would dual-agonism at GLP-1R + GIP-R produce greater effects than GLP-1R agonism alone? This question was answered clinically by tirzepatide.

Dual agonism: the tirzepatide story

Tirzepatide was developed by Eli Lilly as a single-molecule dual agonist at GLP-1R and GIP-R. Coskun and colleagues’ 2018 Molecular Metabolism paper described the discovery, characterisation, and early clinical data ⁴. The design strategy: a 39-amino-acid peptide with residues optimised for balanced receptor binding at both GLP-1R and GIP-R, plus fatty-acid modifications for albumin binding and extended half-life.

Coskun 2018 established three points relevant to the research framework:

• Tirzepatide produces balanced co-activation of GLP-1R and GIP-R rather than selective activation of either
• The co-activation produces metabolic effects that exceed GLP-1R monotherapy at equivalent plasma levels
• The half-life (~5 days) supports once-weekly dosing, matching semaglutide’s dosing interval

The SURPASS programme (type 2 diabetes) and SURMOUNT programme (obesity) followed. SURPASS-2, published by Frías and colleagues in NEJM 2021, was the pivotal head-to-head against semaglutide ⁸. Tirzepatide produced larger reductions in HbA1c and body weight than semaglutide at the clinical doses compared, establishing the dual-agonist class as pharmacologically superior on glycaemic and weight endpoints.

SURMOUNT-1, published by Jastreboff and colleagues in NEJM 2022, established the obesity endpoints ⁹. Tirzepatide at the highest dose (15 mg/week) produced approximately 21% body-weight reduction at 72 weeks in adults with obesity without diabetes, substantially exceeding semaglutide’s 15% in the comparable STEP 1 trial.

For a detailed trial-level comparison of SURPASS and SURMOUNT against the semaglutide programme, see the semaglutide vs tirzepatide trials deep-dive.

Triple agonism: retatrutide and the glucagon-receptor dimension

Retatrutide extends the dual-agonist concept to triple agonism: GLP-1R + GIP-R + glucagon-R. The glucagon-receptor component is the novelty and requires explanation because glucagon-receptor activation seems, naively, to work against weight loss (glucagon elevates glucose, and stimulates gluconeogenesis).

The insight is that glucagon-receptor activation also increases energy expenditure. Acute glucagon elevation in hepatic tissue and brown adipose tissue drives thermogenesis and lipid oxidation. If glucagon-receptor activation is combined with GLP-1R and GIP-R agonism, the appetite-reducing and insulin-sensitising effects of the incretin agonism offset the glucose-elevating effect of glucagon agonism, while the energy-expenditure component adds a separate axis of weight reduction beyond what incretin agonism alone produces.

Jastreboff and colleagues’ 2023 Phase 2 retatrutide trial in NEJM documented dose-dependent body-weight reduction up to approximately 24% at 48 weeks ¹⁰. The weight-loss curve had not fully plateaued at trial end, meaning the final steady-state effect could be larger with longer dosing.

Retatrutide is still Phase 2-3 transition in 2025. Long-term safety remains under investigation; some mechanistic concerns about chronic glucagon-receptor activation (hepatic effects, glycaemic control in specific populations) are being addressed in the ongoing clinical programme.

The class narrative in summary

Three decades of research, in roughly five phases:

1. Discovery phase (1980s–1990s): GLP-1 identified, receptor characterised, physiological role established (Baggio/Drucker framework)
2. Engineering phase (1990s–2010s): DPP-IV-resistant analogs developed with progressively extended half-lives (exenatide → liraglutide → semaglutide)
3. Clinical translation phase (2010s): large cardiovascular-outcome trials (LEADER, SUSTAIN-6) establish CV benefit in T2DM
4. Obesity expansion phase (2019–2023): STEP and SURMOUNT programmes establish weight-loss endpoints in non-diabetes populations; SELECT confirms CV benefit in obesity without diabetes
5. Multi-receptor agonism phase (2018–present): dual (tirzepatide) and triple (retatrutide) agonism produce larger effects than GLP-1 monotherapy

The research programme is active across all five phases simultaneously. Single-agonist GLP-1R research continues alongside dual and triple agonism. Formulation research (oral semaglutide, implantable delivery, smaller molecules) is parallel.

What remains open in the class

Long-term effects beyond 2–3 years. The longest published trial follow-ups are approximately 2–3 years. Longer-term effects on weight maintenance, diabetes prevention, cancer risk, gastrointestinal complications, and cognitive function are being characterised through ongoing post-marketing surveillance and extension studies.

Weight maintenance after discontinuation. Short-term data suggest that weight regain is substantial after GLP-1R agonist discontinuation, similar to the Tesamorelin rebound pattern observed in the GH-axis class. Whether intermittent dosing, lower-dose maintenance, or transitioning to other modalities preserves weight loss is an active research question.

Who responds, who doesn’t. A meaningful fraction of patients in STEP and SURMOUNT achieve much less weight loss than the trial averages. Predictors of response are imperfectly characterised. Some genetic variation in GLP-1R and downstream signalling is known but doesn’t fully explain inter-individual variability.

Mechanism of cardiovascular benefit. The SUSTAIN-6 and SELECT cardiovascular-outcome reductions have multiple proposed mechanisms: weight loss, blood pressure reduction, direct vascular effects, inflammatory marker changes. Partitioning the contribution of each is an active research question.

Combination pharmacology. Whether GLP-1R agonists combine productively with other metabolic classes (SGLT2 inhibitors, amylin analogs, melanocortin agonists) at a system-pharmacology level is being studied.

Where to order

All three compounds discussed are supplied by Thailand Peptides through the Bangkok research desk:

• Buy Semaglutide: research-grade, ≥98% HPLC purity
• Buy Tirzepatide: research-grade, ≥98% HPLC purity
• Buy Retatrutide: research-grade (confirm availability in chat), ≥98% HPLC purity

Same-week Thailand delivery, supplier COA on file, WhatsApp ordering. For the trial-level walkthrough of semaglutide and tirzepatide data, see the semaglutide vs tirzepatide trials deep-dive. For a use-case-level comparison including AOD-9604 and 5-Amino-1MQ, see best peptides for fat loss.