Dual and Triple Gut Peptide Agonists on the Horizon for the Treatment of Type 2 Diabetes and Obesity. An Overview of Preclinical and Clinical Data

After consuming a meal, the L cells of the intestine produce and release the 30 amino acid peptide GLP- 1, which in turn promotes insulin secretion [6]. GLP- 1-based therapy exerts its pharmacological effects by activating the GLP- 1 receptor, a class of G protein-coupled receptor consisting of 463 amino acids. The receptor is a glycoprotein that, like all G protein-coupled receptors, has a seven-transmembrane helix domain and an N-terminal extracellular signal peptide [7]. When GLP- 1 receptors are activated, adenosine triphosphate (ATP) is converted to cyclic adenosine monophosphate (cAMP). Increased signaling through exchange proteins directly activated by cAMP (EPACs), specifically EPAC2, and activation of protein kinase A (PKA) follow the rise in cAMP. An increase in PKA activity also causes ATP-sensitive potassium channels to close, thereby depolarizing the cell membrane and opening voltage-gated calcium channels. This absorbs calcium and stimulates exocytosis of secretory granules, which in turn causes the release of insulin. Furthermore, the endoplasmic reticulum releases calcium in response to EPAC2 activation, increasing intracellular calcium levels and promoting exocytosis. Activation of the GLP- 1 receptor also prevents beta cell apoptosis. Additional consequences of GLP- 1 receptor activation include appetite suppression, cardioprotective effects, and inhibition of glucagon secretion associated with inhibition of gastric emptying [6]. Whether GLP- 1 increases energy expenditure has also been studied. Although there are data in animal models supporting that this is true, particularly in diet-induced obese mice, in which brown adipose tissue activity is regulated [8], there is no convincing evidence that this occurs in humans [6]. As a result, GLP- 1's primary effects include decreasing body weight, protecting against cardiovascular disease, and lowering fasting and postprandial blood glucose levels—all of which are therapeutic targets in T2DM. The short half-life of native GLP- 1-based therapy in the circulation (approximately two minutes after intravenous administration and one to five hours after subcutaneous administration) posed a problem during its early development [9]. For this reason, the first experiments of the glucose-lowering effect of GLP- 1 used continuous intravenous administration or intramuscular injections [10]. The short duration of action needed to be addressed to exploit the promising effects of GLP- 1 for the development of a clinically beneficial drug.

During fasting, K cells (found in the duodenum and jejunum) constitutively secrete the 42 amino acid protein GIP, which exhibits a marked increase in secretion after meals [11 –13]. GIP was initially discovered because it inhibited the secretion of gastric acid. Since then, numerous investigations have revealed the effects of GIP on multiple organs [14]. The GIP receptor, a class of B G-protein-coupled receptors (GPCRs) that is widely expressed in a variety of tissues, is the binding site that mediates the systemic effects. The stomach, adipose tissue, bone, and pancreas all express GIP receptors, and these have also been found in the cerebral cortex, hippocampus, and olfactory bulb, among other parts of the brain [15, 16]. GIP is an incretin hormone that indirectly regulates glucagon secretion and increases glucose-stimulated insulin secretion [17]. GIP and GLP- 1 have opposing effects on glucagon release, despite their similarities. Studies conducted in isolated rat islets have shown that GIP does, in fact, increase intracellular cAMP levels, which in turn stimulate glucagon secretion [18]. The clinical application of GIP agonists for the treatment of diabetes is hampered by this increase in glucagon secretion, which has also been verified in both healthy and subjects with T2DM [19, 20]. In addition to its effects on insulin, GIP plays a crucial role in fat accumulation by promoting the activity of adipocyte-expressed lipoprotein lipase and promoting bone formation by preventing osteoclast apoptosis, as evidenced by the thinner bone trabeculae seen in mice lacking GIP receptors [20 –22]. Better memory was also demonstrated by GIP-transgenic mice, which is most likely connected to increased neurogenesis [23]. In support to this theory, it has been demonstrated that GIP infusion increases the proliferation of neuronal progenitors in the dentate gyrus, while mice lacking GIP displayed deterioration in memorial tasks as a result of reduced neurogenesis [24].

The first description of the hyperglycemic effects of glucagon was made more than a century ago [25, 26]. By attaching itself to its receptors, this hormone works in tandem with insulin to control blood sugar levels [14]. Glucagon's capacity to regulate hepatic glucose metabolism is primarily responsible for its hyperglycemic effects [27]. In order to maintain a steady supply of glucose, glucagon strongly stimulates glycogenolysis and suppresses glycogenesis in the liver [28, 29]. Many studies have revealed numerous additional functions of glucagon in the brain, liver, heart, and adipose tissue in addition to its well-known function in regulating blood sugar levels [30 –32]. Nearly 40 years ago, it was discovered that the rat brain contained glucagon receptors, indicating that the hormone may play a part in controlling brain activity [33]. Accordingly, glucagon has been shown to reduce appetite, food intake, and promote weight loss in both humans and rodents [34, 35]. Despite the fact that the relationship between glucagon and body weight is well established, some research found no changes in food intake following glucagon administration, which suggests that appetite-independent mechanisms may play a role in the glucagon-mediated regulation of body weight [14]. Rats that receive a single subcutaneous dose of glucagon exhibit a sharp rise in metabolic rate, which is consistent with this theory [36]. By promoting oxygen consumption in brown adipose tissue (BAT), glucagon increases metabolic rate. This has been demonstrated by elevated BAT temperature in rats [37, 38]. By preventing lipogenesis and promoting lipolysis, glucagon also has lipolytic effects on white adipose tissue [14]. Hormone-sensitive lipase (HSL) in adipocytes is activated to produce the lipolytic effect, which is then enhanced by indirect processes such as secretion of growth hormone, cortisol, and adrenaline [39]. Adipose tissue is not the only area where glucagon affects lipid metabolism; the liver also experiences an increase in ketogenesis [40]. In fact, by continuously depriving the liver of esterified fatty acids and inhibiting the hepatic glycolytic pathway, glucagon promotes the production of ketone bodies [41 –43] Hepatic ketogenesis is boosted as a result, and mitochondrial fatty acid oxidation is improved [14, 43]. By attaching itself to its receptors and activating adenylate cyclase (AC), which raises cAMP levels in the myocardium, glucagon also increases cardiac output. Glucagon has very rapid chronotropic and inotropic effects, peaking five minutes after administration and lasting for twenty minutes [14, 32].

GLP- 1 receptor, GIP receptor, and glucagon receptor are mainly expressed on beta, alpha, and delta cells in the islets of Langerhans, where they influence hormone secretion and regulate the survival and proliferation of endocrine cells. The search for GLP- 1 receptor agonists to treat T2DM was sparked by studies showing the insulinotropic and glucose-lowering effects of GLP- 1 in beta cells, as well as the hyperglycemic effects of GLP- 1 receptor antagonism [44]. Importantly, individuals with T2DM continue to benefit from GLP- 1's glucose-lowering effects. Insulin gene transcription and translation, as well as the enhancement of glucose-stimulated insulin secretion (insulinotropic effects), are caused by GLP- 1 receptor activation in the beta cell. GLP- 1 receptor activation has longer-term consequences, such as increased b cell proliferation and neogenesis along with cytoprotective (non-insulinotropic) effects [45 –51].

The heterotrimeric G proteins are known to mediate signaling through the canonical GLP- 1 receptor. These G proteins have both a Gb/c dimer subunit and an independent Ga subunit. GPCRs that are agonist-activated promote the synthesis of guanine triphosphate (GTP), which when bound to the G-protein causes the Ga and Gbc subunits to separate, which in turn can activate signaling proteins downstream [52]. It was initially discovered that GLP- 1 activates adenylate cyclase in tissue from the central nervous system (CNS) [53]. It has been demonstrated that ligand-activated GLP- 1 receptor interacts with the Gas subunit and activates adenylate cyclase to generate cAMP. Both the EPAC2 and PKA are activated in response to elevated cAMP [54]. By increasing Ca²⁺ influx and consequently insulin granule exocytosis, both pathways work together to promote insulin secretion. PKA directly phosphorylates sulfonylurea receptor 1 (SUR1), a regulatory subunit of voltage-dependent K⁺ channels and KATP channels, increasing membrane depolarization and voltage-gated Ca²⁺ channel activation. Through several mechanisms, including direct interaction with SUR1 and membrane depolarization, intracellular calcium mobilization, and insulin granule priming modification, EPAC2 activation may promote the exocytosis of insulin granules [55, 56].

There have also been reports of non-insulinotropic effects of GLP- 1 receptor activation. In fact, the GLP- 1 receptor-cAMP-PKA axis stimulates the growth of beta cells by activating the transducer of regulated cAMP response element-binding protein (CREB) (TORC2) and the expression of the CREB and insulin receptor substrate 2 genes [57] Additionally, by boosting bcl- 2 activity and blocking the proapoptotic Bax, activated CREB also supports beta cell survival [58]. GLP- 1's anti-apoptotic and proliferative effects are also mediated by the PI3 K/Akt axis through the transactivation of the epidermal growth factor receptor (EGFR), which is connected to GLP- 1 receptor through c-Src activation and the generation of endogenous EGF like ligands [59]. GLP- 1 receptor agonism has not been thoroughly studied, but it has been demonstrated in certain cell lines to activate additional G-protein subunits and alternative downstream signaling pathways [60 –62]

The mature 42 amino acid GIP attaches itself to the b cell surface and activates its cognate receptor, the GIP receptor. Ligand binding to the GIP receptor activates Gas, which in turn activates adenylate cyclase, resulting in the production of cAMP, in accordance with GLP- 1 receptor activation. PKA and EPAC are activated by elevated cAMP. Additionally, cAMP and PKA phosphorylate ERK1/2, which controls genes involved in proliferative and anti-apoptotic processes, and activate several proteins, such as the mitogen activated protein kinase (MAPK) cascades [63]. By the same mechanisms (potassium channel closure-mediated membrane depolarization) as the GLP- 1 receptor, the GIP receptor activation of PKA results in the secretion of insulin. Additionally, non-insulinotropic effects like regulating the survival and proliferation of pancreatic beta cells can be facilitated by GIP receptor activation [64, 65]. Furthermore, PKA activation is known to inhibit AMPK in GIP receptor and GLP- 1 receptor signaling, which results in the transfer of TORC2 into the nucleus [66]. The anti-apoptotic gene bcl- 2 is a promoter of transcription in the nucleus by a complex between CREB and TORC2. Additionally, the nuclear transcription factor (Foxo1) is phosphorylated upon activation of phosphoinositide- 3-kinase–protein kinase B/Akt (PI3 K-PKB/Akt), which in turn deactivates molecules to restrict the activity of proapoptotic pathways [66].

The signaling pathways in beta cells that are triggered by ligand binding to the GIP receptor and GLP- 1 receptor share a lot of similarities. The PKA and EPAC2 pathways are triggered by both receptors'activation of adenylate cyclase, which raises intracellular cAMP and causes insulin synthesis and release, cellular proliferation, and antiapoptotic effects. Recruitment of beta arrestins desensitizes both receptors, and this is followed by internalization, recycling, and inactivation of Gas. There are also several distinctions. First, beta arrestins and Gaq can both internalize the GLP- 1 receptor, and multiple ligands can affect internalization. On the other hand, GIP receptor internalization depends solely on arrestins and is not easily impacted by new ligands [67]. By inhibiting the Gas proteins and facilitating receptor trafficking through internalization and recycling, these beta arrestins are essential for GIP receptor desensitization. Second, it has been demonstrated that GIP receptor activation in beta cells leads to MAPK-induced signaling pathways, whereas GLP- 1 receptor activation does not. Third, GIP receptor activation has not been shown to cause proliferation and anti-apoptosis; instead, GLP- 1 receptor activation stimulates EGF receptor signaling [20].

In a randomized controlled trial, scientists reported the discovery and translational therapeutic effectiveness of a peptide with strong, balanced coagonism at both GLP- 1 and GIP receptors [78]. This unimolecular dual incretin, which was derived from a mixed sequence of GLP- 1 and GIP, showed improved antihyperglycemic and insulinotropic activity compared to selective GLP- 1 agonists. It is noteworthy that this superior effectiveness extended to primates (humans and cynomolgus monkeys) and rodent models of obesity and diabetes, such as db/db (diabetic) mice and Zucker diabetic fatty (ZDF) rat. In addition, a selective GIP agonist showed very low effectiveness in reducing weight, while this co-agonist showed synergism in reducing fat mass in obese rodents. The unimolecular dual incretins were more successful than selective monoagonists and tackled the two basic causes of diabetes: pancreatic insulin insufficiency and obesity-induced insulin resistance. To support less frequent administration, site-specific lipidation or PEGylation was performed to prolong the duration of action of the unimolecular dual incretins. These peptides offer pharmacology comparable to native peptides and improved effectiveness compared to similarly modified selective GLP- 1 agonists. The improvement in pharmacokinetics helped prevent the negative gastrointestinal side effects typical of GLP- 1-based selective agonists by reducing maximum drug exposure and reducing reliance on GLP- 1-mediated pharmacology. The identification and confirmation of a highly effective, balanced dual incretin agonist enables a more physiological approach to treating conditions associated with impaired glucose tolerance [78].

The combination of GLP- 1 and GIP is proposed as a promising therapeutic strategy for the treatment of obesity and T2DM. However, the neural mechanisms have not been fully elucidated. In a study, scientists investigated the role of GLP- 1 and GIP on central cells in regulating body weight homeostasis [79]. The effect of GLP- 1 together with GIP on food intake, body weight, and locomotor activity was determined following intracerebroventricular (ICV) administration of GLP- 1 and/or GIP in mice. ICV administration of low-dose GLP- 1 (0.3 nmol) and GIP (1 and 3 nmol) did not alter dietary intake. However, ICV administration of higher doses of GLP- 1 (1 and 3 nmol) and GIP (6 nmol) significantly reduced food intake and body weight. To investigate the synergistic effect of ICV-GLP- 1 and GIP, the subeffective dose GLP- 1 (0.3 nmol) and the subeffective dose GIP (1 nmol) were selected for further studies for concurrent administration. Concomitant ICV administration of GLP- 1 and GIP significantly reduced food intake, body weight, and alcohol consumption. Concurrent ICV administration of GLP- 1 and GIP significantly increased neuronal activation and pro-opiomelanocortin expression in the arcuate nucleus of the hypothalamus. Neural activation and POMC expressions were observed in two different neuronal populations. These results provide clues to neural mechanisms that support the development of GLP- 1 and GIP combination therapeutics for the treatment of obesity and DM [79].

Individuals with T2DM taking tirzepatide, a dual GIP and GLP- 1 receptor agonist, experienced better weight loss and blood sugar control than patients taking GLP- 1 receptor agonism. However, it is unclear how tirzepatide increases potency and what role GIP receptor agonism plays [80]. By increasing insulin sensitivity in obese mice more than GLP- 1 receptor agonism, researchers show that tirzepatide is an effective insulin sensitizer. The effect of tirzepatide was compared to wild-type obese and GLP- 1 receptor-null mice to determine whether GIP receptor agonism plays a role. By improving glucose removal in white adipose tissue, tirzepatide restored insulin sensitivity even when there was no GLP- 1 receptor -induced weight loss. The finding that a long-acting GIP receptor agonist (LAGIPRA) increases insulin sensitivity by promoting glucose capture in WAT confirms this. Interestingly, lower levels of branched-chain amino acids (BCAAs) and keto acids in the blood have been linked to the effect of tirzepatide and LAGIPRA on insulin sensitivity. The upregulation of genes associated with the breakdown of glucose, lipid, and BCAAs in brown adipose tissue has been linked to insulin sensitization. Overall, her research suggests that tirzepatide increased insulin sensitivity both independently and as a function of weight. These results shed light on the role that GIP receptor agonism plays in the therapeutic profile of dual receptor agonism and provide mechanistic explanations for the clinical effectiveness of tirzepatide [80].

In a study, researchers used a GIP receptor antagonist antibody (GIPRAb) and a GLP- 1 receptor agonist to target both pathways [81]. By creating GIP receptor -Ab/GLP- 1 bispecific molecules, they reduced body weight and improved numerous metabolic parameters in mice and monkeys, which is a strategy for treating obesity and its comorbidities. When GIPR-Ab/GLP- 1 is used instead of GIPR-Ab or a control antibody conjugate, the body weight loss is higher, indicating synergistic effects. Additionally, in diet-induced obesity (DIO) mice, GIP receptor-Ab/GLP- 1 lowers the respiratory exchange rate. Recombinant cells expressing both receptors produce more endosomal cAMP when GIPR-Ab/GLP- 1 binds to both receptors simultaneously and internalizes them quickly. This could be the reason why the bispecific molecules work so well. In general, GIPR-Ab/GLP- 1 molecules encourage body weight loss and have potential applications in the treatment of obesity [81].

In another study, researchers discovered that tirzepatide treatment, when combined with a decrease in circulating levels of branched-chain amino (BCAA) and keto (BCKA) acid metabolites linked to the development of systemic insulin resistance (IR) and T2DM improves insulin sensitivity in obese mice and humans with T2DM [82]. Crucially, it was discovered that these systemic effects were linked to elevated expression of BCAA catabolic genes in mice's thermogenic BAT. Tirzepatide may decrease circulating BCAAs/BCKAs by encouraging their catabolism in BAT, according to their hypothesis based on these findings. They used a mouse model of diet-induced obesity and combined metabolomic analyses in BAT and other tissues with stable-isotope tracer studies to answer this question. Through increased labeling of BCKA-derived metabolites and increases in levels of BCAA breakdown, such as glutamate, alanine, and 3-hydroxyisobutyric acid, they discovered that tirzepatide treatment stimulated catabolism of BCAAs/BCKAs in BAT. Additionally, tirzepatide administration over an extended period raised BAT levels of several amino acids that have been demonstrated to be elevated in response to cold exposure. Finally, long-term tirzepatide treatment significantly raised BAT levels of several TCA cycle intermediates, including fumarate, malate, and α-ketoglutarate. Lastly, they propose that tirzepatide causes a thermogenic-like amino acid profile in BAT, which could explain why obese IR mice have lower systemic BCAA levels [82].

In a study scientists evaluate the effects of glucagon, GIP, and GLP- 1 receptors in one compound (i.e. e. triagonism) on energy expenditure, glucose regulation, food intake, and weight loss in a well-established DIO mouse model in comparison to clinically relevant GLP1 receptor agonists [83]. Compared to GLP- 1 receptor mono-agonists and GLP- 1 receptor/GIP receptor co-agonists, optimized triagonists improve energy expenditure and restore body weight in DIO mice. These preclinical findings further implicate glucagon activation as the distinguishing factor between incretin receptor mono-, dual-, and triagonists, and they point to unimolecular polypharmacology as an efficient way to target several mechanisms causing obesity.

GLP- 1 increases insulin secretion through direct interaction with pancreatic beta cells. In addition, GLP- 1 reduces glucose production in the liver by preventing α-cells from secreting glucagon. It also indirectly increases insulin sensitivity in skeletal muscle by promoting microvessel recruitment, which may enhance local insulin action [69]. Numerous studies have also shown how important the portal glucose sensor is in maintaining glucose homeostasis and how GLP- 1 receptors are related to portal glucose sensing, with GLP- 1 being required for portal glucose to trigger the insulin response.

When GLP- 1 receptor agonists are used to treat T2DM, there is also a noticeable reduction in post-meal glycemic fluctuations due to their short-term, transient effect on slowing gastric emptying. The CNS GLP- 1 receptors are not necessary for the physiological regulation of blood glucose levels by endogenous GLP- 1 or the pharmacological control of glucose homeostasis that depends on GLP- 1 receptors, although further research is needed to investigate the possible role of GLP- 1 receptors expressed outside the beta cell, including within the enteric and peripheral nervous systems, in improving beta cell function through enteral glucose mediation [70, 84].

The incretin effect is reduced in people with T2DM, mainly because GIP has less of an impact [85]. Recent studies show that GLP- 1 function is impaired in people with T2DM, although GLP- 1 secretion is preserved. This can lead to reduced expression of GLP- 1 receptors and the development of GLP- 1 resistance. Other manifestations include decreased insulin secretion, increased insulin resistance and increased blood sugar levels [86].

Lastly, GLP- 1 receptor agonists'impact on weight may help improve blood glucose regulation and insulin sensitivity simultaneously.

Tirzepatide is the first dual GLP- 1/GIP receptor co-agonist [98]. Glycemic control appears to be more effective when both receptors are activated [99, 100]. Greater effectiveness in blood glucose control appears to result from activation of both receptors. Compared to placebo, it has been shown to significantly lower HbA1c by up to 2.07% when administered as a once-weekly subcutaneous injection at a maximum dose of 15 mg. Compared to other single agonists, tirzepatide provides better glycemic control [99, 100].

A long-acting amylin analog called cagrilintide is being developed in conjunction with semaglutide 2.4 mg (CagriSema) to treat T2DM [101]. The injection is given subcutaneously once a week. In a phase 2 clinical trial, CagriSema reduced HbA1c by − 2.2% (p = 0.075) in comparison with semaglutide 2.4 mg alone [101].

A subcutaneous injection of mazdutide is given once a week. In a Chinese phase II clinical trial, mazdutide decreased HbA1c by − 1.41% to − 1.67% in a dose-dependent manner, while a placebo decreased HbA1c by + 0.03% [102]. Survodutide was found to significantly lower HbA1c in comparison with a placebo in a phase II trial where it was administered subcutaneously once and twice a week [103].

Orforglipron is an oral, non-peptide, small molecule glucagon-like peptide- 1 receptor agonist [104]. Compared to peptide GLP- 1 receptor agonists, orforglipron, exhibits less receptor desensitization by increasing cyclic AMP signaling [105]. Oral orfoglipron, as opposed to peptide-based oral semaglutide, can be taken without regard to food timing. Orforglipron significantly reduced HbA1c to − 2.1% in a phase II clinical trial assessing its use in T2DM, while dulaglutide achieved − 1.1% and a placebo achieved − 0.43% [106]. In a phase II study, twice-daily danuglipron resulted in a dose-dependent reduction in HbA1c of − 0.49% to − 1.18%, while a placebo caused a decrease of − 0.02% [107].

The GLP- 1 receptor agonists have been shown to improve blood glucose control when used in conjunction with insulin, particularly basal insulin, and may be particularly useful in the treatment of advanced T2DM [108]. For example, the addition of semaglutide at a dose of 0.5 mg or 1 mg per week in patients receiving basal insulin resulted in a − 1% and − 1.8% decrease in HbA1c, respectively, in SUSTAIN- 5. The risk of weight gain and hypoglycemia can also be reduced with this combination therapy. Nevertheless, there is a higher risk of hypoglycemia than with single GLP- 1 treatment. The receptor agonist alone is less effective than insulin alone [108].

The function of GLP- 1 in controlling feeding behavior and energy balance was investigated in rats. According to these studies, satiated rats eat more when exendin (9–39), a GLP- 1 receptor antagonist, is injected directly into their brains. Rats'appetites increase and their weight increases when the proglucagon gene, which encodes GLP- 1, is less expressed in the nucleus tractus solitarius [110, 111]. Administration of GLP- 1 receptor agonists reduces appetite, decreases energy intake, and increases satiety. According to functional magnetic resonance imaging, anticipatory food reward – the anticipated pleasure of eating certain meals – is reduced when GLP- 1 R is activated [110, 111].

Maridebart/cafraglutide, or AMG 133, is a bispecific molecule that was created by conjugating two GLP- 1 analogue agonist peptides with a fully human monoclonal anti-human GIP receptor antagonist using amino acid linkers [124]. The ability of AMG 133 to lower body weight and improve metabolic markers in male obese mice and cynomolgus monkeys is reported by researchers, who also confirm the GIP receptor antagonist and GLP- 1 receptor agonist activities in cell-based systems. AMG 133 demonstrated significant dose-dependent weight loss in a phase 1 randomized, double-blind, placebo-controlled clinical trial involving participants with obesity, along with an acceptable safety and tolerability profile [124].

The side effects reported at the end of a clinical trial may be partially distorted due to gastrointestinal problems if the study is stopped too early [127]. The absence of validated assessment tools (e.g. a stable 13 C-labeled fatty acid breath test) and reliance on self-reported symptoms rather than validated symptom indices for gastrointestinal disorders are also limitations. In addition, differences in the initial situation of gastric emptying, which is a measure of the effectiveness of GLP- 1- receptor co-agonists, are likely to have a different effect on patients with obesity than on people in whom preoperative intragastric food retention after taking GLP- 1 receptor co-agonists is not impaired, and in people with diabetes [127]. Although the American Society of Anesthesiologists Working Group on Preoperative Fasting is based primarily on anecdotal reports and case reports, it has issued guidelines for taking these medications before non-urgent or urgent procedures that require general anesthesia due to concerns about the risk of regurgitation and lung aspiration due to delayed gastric emptying [128]. The risk of biliary disease has been examined in fewer studies, which may be because gallbladder emptying is impaired. Nevertheless, tirzepatide use seemed to be associated with a higher risk (RR 1.97, 95% CI 1.14–3.42, p = 0.558) [129]. Both liraglutide and exenatide showed significant decreases in liver fat, liver enzymes, and fibrosis index associated with metabolically associated steatotic liver disease (MASLD) [130]. In contrast, compared to placebo, semaglutide resulted in a 59% improvement in MASLD and an improvement in liver fibrosis, which slowed the progression of the disease [130]. According to other recent analyses, tirzepatide, retatrutide, and survodutide all significantly reduce liver fat without showing signs of worsening liver fibrosis markers [131].

The goal of treating obesity is to reduce weight over time while improving health and quality of life [132, 133]. Although lifestyle changes are the first line of treatment for obesity, even the most rigorous lifestyle changes typically only produce a 5–10% weight loss and maintaining weight over time is difficult due to compensatory physiological processes that increase appetite and decrease energy expenditure [134]. To improve or achieve remission of some obesity-related complications and lower risk for cardiovascular events, individuals with a BMI of ≥ 35 kg/m² may need to lose more weight (10–25%). 5–10% weight loss is considered clinically meaningful with pharmacotherapy. Bariatric surgery for qualified individuals is the next option if lifestyle changes and pharmacotherapy are unable to meet individualized weight loss goals [132]. Although bariatric surgery can lead to weight loss of at least 25% and long-term weight maintenance, it is not scalable at the population level, and many people are still afraid to have surgery because of the possibility of complications, [135, 136].

A better understanding of the complex physiology of weight regulation and the role of intestinal hormones in regulating food intake, appetite, and glucose homeostasis led to the discovery of GLP- 1 receptor analogs as safe and effective treatments, initially for T2DM and more recently in higher doses to treat obesity (liraglutide 3 mg and semaglutide 2.4 mg). Semaglutide 2.4 mg weekly, a novel GLP- 1 receptor agonist, was the first approved treatment for obesity. In 2021, it resulted in a mean weight loss of roughly 15% and maintenance through decreased appetite. Semaglutide 2.4 mg once weekly may not work well for certain populations, and the weight loss from bariatric surgery varies greatly (32% of patients with T2DM achieve ≤ 5% weight loss with semaglutide 2.4 mg) [112]. Furthermore, some patients cannot tolerate injectable treatments, and GLP- 1 receptor agonist side effects typically depend on it. Despite the lack of extra support for lifestyle modification as part of the SURPASS program, 40–69% of patients achieved ≥ 10% weight loss with the highest doses of tirzepatide (10–15 mg), which ranged from 9.5 kg to 12.9 kg [100].

Other combination pharmacotherapies based on gut hormones have shown positive effects in preclinical models. Associating GLP- 1 or glucagon receptor agonists with core hormones like estrogens, thyroid hormones, and dexamethasone was the strategy employed (reviewed in [137, 138]). For example, a GLP- 1/oestradiol conjugate reduced feeding and body weight in several rodent models of the metabolic syndrome [139]. GLP- 1/oestradiol conjugate was more effective than GLP- 1 alone and triggered the insulin signaling pathway in beta cells, which led to remission of DM and restoration of beta cell function [140]. Thyroid hormones were delivered to the liver specifically by conjugating T3 with glucagon; in multiple mouse models of obesity, this led to a decrease in body weight and the correction of dyslipidemia [141]. Finally, obese mice lost weight more effectively when dexamethasone and GLP- 1 were conjugated than when GLP- 1 was used alone [142]. Combination pharmacotherapy, which involves not only gut hormones but also other molecules, appears to be a viable treatment option for metabolic syndrome. Due to their preclinical testing and lack of comparison with other dual receptor agonists such as tirzepatide, further studies are needed to determine whether the effects of these unimolecular compounds are superior to those of clinically tested drugs.

The effectiveness of GLP- 1 and dual GLP- 1/GIP receptor agonists in the treatment of obesity and overweight has been thoroughly established [143]. As with any medicine, there is a possibility of unwanted effects. Most side effects of GLP- 1 and GLP- 1/GIP receptor agonists are mild to moderate and resolve over time. These are usually gastrointestinal complaints. Concerns have been raised about serious side effects (e.g., gastrointestinal paralysis). The concerns regarding the possibility of medullary thyroid cancer, pancreatic cancer, suicidal ideation have been allayed by recent data. What is critical is that the side effect profiles of these medications are generally very acceptable. Obesity is one of the most common diseases worldwide. A significant risk of cardiovascular disease, cardiovascular death, and cardiometabolic risk factors is also associated with obesity. The same applies to cancers such as breast, ovarian, uterine, pancreatic, and esophageal cancer, among others. The benefits of these medications outweigh the risks for the majority of adults with overweight or obesity, as cancer and cardiovascular disease are the two leading causes of death for adults worldwide [143].

For some people, the benefits of these medications do not outweigh the risks, as with any other pharmacologic intervention (e.g., adults who have previously had medullary thyroid cancer, in which they are contraindicated). A comprehensive physical examination and detailed medical history should be performed to identify these individuals. The most important thing is that these medications are taken under the guidance of a doctor who is trained and experienced in their use. This allows patients to receive comprehensive information about the effectiveness of these medications, as well as advice about possible side effects. Shared decision-making should take place, considering their current health status. The aim is to offer efficient treatment to adults suffering from obesity, considering their individual characteristics, preferences and living conditions.

The demand for the significant weight loss that GLP- 1 receptor agonists and dual and triple anti-obesity agonists produce, the high cost of production, as well as their novelty, are serious factors which increase their price. Semaglutide was the most cost-effective treatment compared to exenatide, dulaglutide, liraglutide, or no treatment in short-term analyses (approximately one and a half years) that assessed quality-adjusted life years and incremental cost-effectiveness ratios [135 –144]. However, compared to endoscopic sleeve gastroplasty, semaglutide is less cost-effective over 5 years with the same outcome parameters and a current annual price of approximately $13,618 in the US [144, 145].

The exact mechanisms underlying the weight loss are still not clear, especially in light of the enhanced effects seen with multi-agonism, despite the large number of incretin agonists presently undergoing clinical trials and their remarkable effectiveness in managing obesity and T2DM [5, 14, 146, 147]. Each incretin receptor agonist effects on body weight and energy metabolism may be slightly different and, in the case of dual and triple agonists, frequently complementary.

Incretin agonist clinical trials most frequently reveal nausea as an adverse effect. Future generations of incretin agonists may be able to maintain the reduction in blood glucose and body weight while minimizing these off-target effects. In preclinical models, GIP receptor agonism attenuates emesis while increasing the amount of body weight reduction in comparison to GLP- 1 agonism alone [148]. Although dual and triagonist medications have so far exhibited side effects comparable to those of GLP- 1 receptor monoagonists in human trials, these preclinical findings imply that GIP receptor agonism may be the secret to obtaining the weight loss advantages of incretin agonism with less severe side effects. Thus, comprehending the mechanisms underlying GIP receptor agonism is of particular interest. To date, we have learned that tirzepatide mainly stimulates insulin secretion through GIP receptor in human islets [149]. Also, the hypothalamus's GIP receptor signaling controls body weight and appetite [150]. Due to its CNS action, the dual GLP- 1/GIP dual agonist exhibits these superior benefits [23].

The main effect of endogenous GIP regarding weight is primarily on adipose tissue, where it promotes positive energy balance. It is probably broken down before it can reach CNS GIP receptors. The GIP-induced peripheral promotion of positive energy balance is likely to be far less effective than the GLP- 1/GIP dual agonist's direct action on the central nervous system, which results in satiety and weight loss. What parts of the brain and what kinds of cells are involved is another important question that has not been fully addressed.

Numerous cell types, including endothelial cells, tanycytes, and neurons in the hypothalamus and hindbrain express GLP- 1, GIP receptors and glucagon receptor, and. In a recent study, Liskiewicz and colleagues discovered that long-acting GIP receptor agonists activate GABAergic neurons that express the GIP receptor in the area postrema, and that GIP receptor signaling in GABAergic neurons is necessary for the effects of GIP to lower body weight [151].