Genetic Differences May Explain Why Some Patients Don’t Respond to Popular Diabetes and Weight Loss Drugs

New research emerging from Stanford Medicine and a global consortium of international collaborators is shedding light on a phenomenon that has long puzzled clinicians: the variable effectiveness of glucagon-like peptide-1 (GLP-1) receptor agonists, a class of medications increasingly prescribed for Type 2 diabetes and obesity. The groundbreaking study suggests that certain genetic variations may be responsible for a significant subset of patients experiencing "GLP-1 resistance," where these powerful drugs prove less effective in regulating blood sugar. This discovery holds the potential to usher in a new era of personalized medicine, enabling physicians to better predict drug response and tailor treatments for improved patient outcomes.

The implications of this research are substantial, given the widespread adoption of GLP-1 receptor agonists. More than one in four individuals diagnosed with Type 2 diabetes currently utilize these medications, and their use for weight management has surged in recent years with blockbuster drugs like Ozempic and Wegovy. However, the current study, published on March 29 in the prestigious journal Genome Medicine, indicates that approximately 10% of the population may carry genetic variants that predispose them to a diminished response to these drugs. This finding could redefine treatment strategies and improve the efficiency of therapeutic interventions for millions worldwide.

Unraveling the Mystery of GLP-1 Resistance

For years, medical professionals have observed a wide spectrum of responses to GLP-1 receptor agonists. While many patients experience significant improvements in blood sugar control and substantial weight loss, others see only marginal benefits, leading to frustration and the need for frequent regimen adjustments. This new research provides a compelling biological explanation for this variability, focusing on a phenomenon termed "GLP-1 resistance."

At the core of this resistance is the hormone GLP-1 itself, a naturally occurring peptide produced in the gut that plays a crucial role in glucose homeostasis. GLP-1 is secreted in response to food intake and acts by stimulating insulin secretion from the pancreas, slowing gastric emptying, and suppressing glucagon release. These actions collectively contribute to lower blood sugar levels after meals and also promote satiety, aiding in weight management. GLP-1 receptor agonists are synthetic compounds designed to mimic the actions of this hormone, thereby enhancing its therapeutic effects.

The Stanford-led study identified a specific link between genetic variations in the enzyme peptidyl-glycine alpha-amidating monooxygenase (PAM) and this observed resistance. PAM is a critical enzyme involved in the post-translational modification and activation of a wide range of peptide hormones, including GLP-1. The researchers hypothesized that defects in PAM processing could impact GLP-1’s efficacy.

A Decade of Rigorous Investigation

The findings represent the culmination of approximately ten years of dedicated research, encompassing a multifaceted approach that included experiments with human participants, studies in mouse models, and a comprehensive analysis of existing clinical trial data. This extensive investigation was crucial in validating the initial surprising observations.

"In some of the trials, we saw that individuals who had those variants were unable to lower their blood glucose levels as effectively after six months of treatment," explained Anna Gloyn, DPhil, professor of pediatrics and of genetics at Stanford Medicine and one of the study’s senior authors. "At that point, a doctor would likely change the patient’s drug regimen. Knowing ahead of time who is likely to respond would help patients get on the right drugs faster — a step toward precision medicine."

Markus Stoffel, MD, PhD, professor of metabolic diseases at the Institute of Molecular Health Sciences, ETH Zurich in Switzerland, served as the other senior author. The lead authors of the study are Mahesh Umapathysivam, MBBS, DPhil, an endocrinologist and clinical researcher at Adelaide University in Australia, and Elisa Araldi, PhD, associate professor of medicine and surgery at the University of Parma in Italy. Their collective expertise and collaborative efforts were instrumental in dissecting the complex biological mechanisms involved.

"When I treat patients in the diabetes clinic, I see a huge variation in response to these GLP-1-based medications and it is difficult to predict this response clinically," Umapathysivam commented. "This is the first step in being able to use someone’s genetic make-up to help us improve that decision-making process."

The PAM Gene: A Key Player in Hormone Activation

The research specifically zeroed in on two genetic variants within the PAM gene, designated p.S539W and p.D563G. The PAM enzyme is unique in its ability to perform amidation, a chemical process that significantly enhances the stability and biological potency of peptide hormones. "PAM is a truly fascinating enzyme because it’s the only enzyme we have that’s capable of a chemical process called amidation, which increases the half-life or the potency of biologically active peptides," Gloyn elaborated. "We thought, if you have a problem with this enzyme, there’s going to be multiple aspects of your biology that are not working properly."

Previous studies had already established a correlation between PAM variants and diabetes, noting their potential to impair insulin release from the pancreas. The current team sought to determine if these variants also interfered with the function of GLP-1, a hormone crucial for post-meal blood sugar regulation.

Unexpected Findings in Human Studies

To investigate the impact of PAM variants on GLP-1 activity, researchers conducted studies involving adults who did not have diabetes. This choice was made to minimize confounding factors inherent to the disease itself. Participants were given a sugary solution, and their blood glucose and hormone levels were meticulously monitored every five minutes for four hours.

Contrary to their initial expectations, which predicted lower GLP-1 levels in individuals with PAM variants due to potential processing issues, the researchers observed the opposite. Individuals carrying the p.S539W PAM variant exhibited higher circulating levels of GLP-1. This finding was perplexing, as elevated hormone levels would typically be expected to yield a more pronounced biological effect.

"What we actually saw was they had increased levels of GLP-1," Gloyn stated. "This was the opposite of what we imagined we would find."

The critical insight came when the team assessed the biological activity of the elevated GLP-1. Despite the higher hormone concentrations, individuals with the PAM variant showed no enhanced ability to lower their blood sugar levels. This indicated a clear disconnect between hormone levels and physiological response, defining the state of GLP-1 resistance. "More GLP-1 was needed to have the same biological effect, meaning they were resistant to GLP-1," Gloyn summarized.

Cross-Species Validation and Mechanistic Clues

The unexpected nature of these findings prompted the researchers to rigorously verify their results through a variety of experimental approaches. This included collaborating with scientists in Zurich who were studying mice genetically engineered to lack the PAM gene. These PAM-deficient mice mirrored the human findings, exhibiting elevated GLP-1 levels but failing to achieve adequate blood sugar control.

One of GLP-1’s key functions is to slow down gastric emptying, a process that aids in blood sugar regulation and contributes to weight loss. In the PAM-deficient mice, food passed through the stomach at an accelerated rate, and GLP-1 medications were unable to normalize this process. Further investigations in these mice revealed reduced responsiveness to GLP-1 in both the pancreas and the gut. Intriguingly, the number of GLP-1 receptors in these tissues remained unaffected, suggesting that the issue was not with the receptors themselves but rather with downstream signaling pathways.

Additional experiments conducted with collaborators in Copenhagen provided further mechanistic insights. These studies demonstrated that the PAM defect did not impair GLP-1’s ability to bind to its receptor or interfere with the initial signal transduction process. This pointed towards a disruption occurring further along the biological cascade initiated by GLP-1 receptor activation.

Clinical Trial Data Confirm Reduced Drug Efficacy

To ascertain the real-world impact of GLP-1 resistance on treatment outcomes, the researchers delved into data from several clinical trials involving patients with diabetes. A combined analysis of three trials, encompassing 1,119 participants, revealed a significant disparity in response to GLP-1 drugs based on the presence of PAM variants.

Individuals carrying the p.S539W or p.D563G PAM variants were less likely to achieve target HbA1c levels, a key indicator of long-term blood sugar control. After six months of treatment, approximately 25% of participants without the variants met the recommended HbA1c target, a figure that dropped to 11.5% for those with the p.S539W variant and 18.5% for those with the p.D563G variant.

A crucial aspect of this analysis was the observation that these genetic variants did not influence the response to other common diabetes medications, such as sulfonylureas, metformin, and DPP-4 inhibitors. "What was really striking was that we saw no effect from whether you have a variant on your response to other types of diabetes medications," Gloyn emphasized. "We can see very clearly that this is specific to medications that are working through GLP-1 receptor pharmacology."

However, the findings were not uniform across all studies. Two additional clinical trials, funded by pharmaceutical companies and utilizing longer-acting GLP-1 drugs, did not show a significant difference in response between carriers and non-carriers of the variants. This suggests that the extended duration of action of these newer formulations might help to overcome or mitigate the effects of GLP-1 resistance.

The Unfolding Picture: Implications for Precision Medicine

The discovery of GLP-1 resistance and its genetic underpinnings opens up exciting possibilities for the advancement of precision medicine in diabetes and obesity management. The ability to predict which patients are less likely to benefit from standard GLP-1 therapies could lead to more efficient and effective treatment strategies.

"It’s very common for pharmaceutical companies to collect genetic data on their participants," Gloyn noted. "For the newer GLP-1 medications, it would be useful to look at whether there are genetic variants, like the variants in PAM, that explain poor responders to their medications."

While the exact biological mechanisms driving GLP-1 resistance remain an active area of investigation, the current research represents a significant leap forward. The complexity of the issue is underscored by the fact that the biological cause of GLP-1 resistance is still not fully understood, much like the decades-long pursuit to fully elucidate insulin resistance.

"That is the million-dollar question," Gloyn admitted. "We have ticked off this enormous list of all the ways in which we thought GLP-1 resistance might come about. No matter what we’ve done, we’ve not been able to nail precisely why they are resistant."

Despite the remaining unknowns, the development of effective treatments for insulin resistance, such as insulin sensitizers, offers a hopeful precedent. Researchers are exploring the potential for similar therapeutic strategies for GLP-1 resistance, perhaps through the development of medications that enhance sensitivity to GLP-1 or by optimizing GLP-1 formulations, such as the longer-acting versions already showing promise.

Future Directions and Collaborative Efforts

The research team acknowledged that the current data on weight loss outcomes related to GLP-1 resistance is limited and not definitive. Future studies will need to incorporate weight measurements and explore the potential impact of these genetic variants on weight management efficacy. The accessibility of genetic data from ongoing and future clinical trials will be crucial for advancing this research.

This comprehensive study involved a broad international collaboration, with significant contributions from researchers at the University of Oxford, University of Dundee, University of Copenhagen, University of British Columbia, Churchill Hospital, Newcastle University, the University of Bath, and the University of Exeter. Funding for this extensive research was provided by a range of esteemed organizations, including Wellcome, the Medical Research Council, the European Union Horizon 2020 Programme, the National Institutes of Health (grants U01-DK105535, U01-DK085545, and UM-1DK126185), the National Institute for Health Research Oxford Biomedical Research Centre, the Canadian Institutes of Health Research, the Novo Nordisk Foundation, Boehringer Ingelheim, and Diabetes Australia.

As the scientific community continues to unravel the intricacies of GLP-1 resistance, this research lays a critical foundation for personalized treatment approaches, aiming to ensure that patients receive the most effective medications for their individual genetic profiles, ultimately leading to better health outcomes.