Demand for new weight loss drugs has surged over the past few years.

Led by the antiobesity drugs semaglutide (Wegovy) and tirzepatide (Zepbound), these popular medications — more commonly known as glucagon-like peptide 1 (GLP-1) agonists — have become game changers for shedding excess pounds.

Aside from obesity indications, both drugs have been approved to treat type 2 diabetes under different brand names and have a growing list of other potential benefits, such as reducing inflammation and depression.

These antiobesity drugs could even have a place in cancer care.

While there’s limited data to support the use of GLP-1 agonists for weight loss in cancer, some oncologists have begun carefully integrating the antiobesity agents into care and studying their effects in this patient population.

The reason: Research suggests that obesity can reduce the effectiveness of cancer therapies, especially in patients with breast cancer, and can increase the risk for treatment-related side effects.

The idea is that managing patients’ weight will improve their cancer outcomes, explained Lajos Pusztai, MD, PhD, a breast cancer specialist and professor of medicine at Yale School of Medicine in New Haven, Connecticut.

Although Dr. Pusztai and his oncology peers at Yale don’t yet use GPL-1 agonists, Neil Iyengar, MD, and colleagues have begun doing so to help some patients with breast cancer manage their weight. Dr. Iyengar estimates that a few hundred — almost 40% — of his patients are on the antiobesity drugs.

“For a patient who has really tried to reduce their weight and who is in the obese range, that’s where I think the use of these medications can be considered,” said Dr. Iyengar, a breast cancer oncologist at Memorial Sloan Kettering Cancer Center in New York City.

Why GLP-1s in Cancer?

GLP-1 is a hormone that the small intestine releases after eating. GLP-1 agonists work by mimicking GLP-1 to trigger the release of insulin and reduce the production of glucagon — two processes that help regulate blood sugar.

These agents, such as Wegovy (or Ozempic when prescribed for diabetes), also slow gastric emptying and can make people feel fuller longer.

Zebound (or Mounjaro for type 2 diabetes) is considered a dual GLP-1 and glucose-dependent insulinotropic polypeptide agonist, which may enhance its weight loss benefits.

In practice, however, these drugs can increase nausea and vomiting from chemotherapy, so Dr. Iyengar typically has patients use them afterwards, during maintenance treatment.

Oncologists don’t prescribe the drugs themselves but instead refer patients to endocrinologists or weight management centers that then write the prescriptions. Taking these drugs involves weekly subcutaneous injections patients can administer themselves.

Endocrinologist Emily Gallagher, MD, PhD, of Mount Sinai Hospital in New York City, estimates she has prescribed the antiobesity drugs to a few hundred patients with cancer and, like Dr. Iyengar, uses the drugs during maintenance treatment with hormone therapy for breast cancer. She also has used these agents in patients with prostate and endometrial cancers and has found the drugs can help counter steroid weight gain in multiple myeloma.

But, to date, the evidence for using GPL-1 agonists in cancer remains limited and the practice has not yet become widespread.

Research largely comes down to a few small retrospective studies in patients with breast cancer receiving aromatase inhibitors. Although no safety issues have emerged so far, these initial reports suggest that the drugs lead to significantly less weight loss in patients with cancer compared to the general population.

Dr. Iyengar led one recent study, presented at the 2024 annual meeting of the American Society of Clinical Oncology, in which he and his team assessed outcomes in 75 women with breast cancer who received a GLP-1 agonist. Almost 80% of patients had diabetes, and 60% received hormone therapy, most commonly an aromatase inhibitor. Patients’ median body mass index (BMI) at baseline was 34 kg/m² (range, 23-50 kg/m²).

From baseline, patients lost 6.2 kg, on average, or about 5% of their total body weight, 12 months after initiating GLP-1 therapy.

In contrast, phase 3 trials show much higher mean weight loss — about two times — in patients without cancer.

Another recent study also reported modest weight loss results in patients with breast cancer undergoing endocrine therapy. The researchers reported that, at 12 months, Wegovy led to 4.34% reduction in BMI, compared with a 14% change reported in the general population. Zebound, however, was associated with a 2.31% BMI increase overall — though some patients did experience a decrease — compared with a 15% reduction in the general population.

“These findings indicate a substantially reduced weight loss efficacy in breast cancer patients on endocrine therapy compared to the general population,” the authors concluded.

It’s unclear why the drugs appear to not work as well in patients with cancer. It’s possible that hormone therapy or metabolic changes interfere with their effectiveness, given that some cancer therapies lead to weight gain. Steroids and hormone therapies, for instance, often increase appetite, and some treatments can slow patients’ metabolism or lead to fatigue, which can make it harder to exercise.

Patients with cancer may need a higher dose of GLP-1 agonists to achieve similar weight loss to the general population, Dr. Iyengar noted.

However, Dr. Gallagher said, in her own experience, she hasn’t found the drugs to be less effective in patients with cancer, especially the newer agents, like Wegovy and Zepbound.

As for safety, Wegovy and Zepbound both carry a black box warning for thyroid C-cell tumors, including medullary thyroid carcinoma. (Recent research, however, has found that GLP-1 agonists do not increase thyroid cancer risk).

These antiobesity agents are also contraindicated in patients with a personal or family history of medullary thyroid carcinoma and in patients who have multiple endocrine neoplasia syndrome type 2, which is associated with medullary thyroid carcinoma.

Dr. Gallagher hasn’t seen any secondary tumors — thyroid or otherwise — in her patients with cancer, but she follows the labeling contraindications. Dr. Iyengar also noted that more recent and larger data sets have shown no impact on this risk, which may not actually exist, he said

Dr. Gallagher remains cautious about using GPL-1 agonists in patients who have had bariatric surgery because these agents can compound the slower gastric emptying and intestinal transit from surgery, potentially leading to gastrointestinal obstructions.

Looking ahead, GPL-1 manufacturers are interested in adding cancer indications to the drug labeling. Both Dr. Iyengar and Dr. Gallagher said their institutions are in talks with companies to participate in large, multicenter, global phase 3 trials.

Dr. Iyengar welcomes the efforts, not only to test the effectiveness of GPL-1 agonists in oncology but also to “nail down” their safety in cancer.

“I don’t think that there’s mechanistically anything that’s particularly worrisome,” and current observations suggest that these drugs are likely to be safe, Dr. Iyengar said. Even so, “GLP-1 agonists do a lot of things that we don’t fully understand yet.”

The bigger challenge, Dr. Iyengar noted, is that companies will have to show a sizable benefit to using these drugs in patients with cancer to get the Food and Drug Administration’s approval. And to move the needle on cancer-specific outcomes, these antiobesity drugs will need to demonstrate significant, durable weight loss in patients with cancer.

But if these drugs can do that, “I think it’s going to be one of the biggest advances in medicine and oncology given the obesity and cancer epidemic,” Dr. Iyengar said.

Dr. Iyengar has adviser and/or researcher ties with companies that make or are developing GPL-1 agonists, including AstraZeneca, Novartis, Gilead, and Pfizer. Dr. Gallagher is a consultant for Novartis, Flare Therapeutics, Reactive Biosciences, and Seagen.

A version of this article first appeared on Medscape.com.

Popular Weight Loss Drugs Now for Patients With Cancer?