The competition to develop the next blockbuster weight loss drug is heating up, and Pfizer is looking to make a big splash with its own entry.
On Tuesday, Pfizer presented the latest trial data on its ultra-long-acting GLP-1 experimental drug, code-named PF’3944. People taking PF’3944 lost significantly more weight than those on a placebo, the trial showed, even after switching to a monthly schedule. The company will push forward several Phase III trials of the drug this year.
A potential leg up
Pfizer acquired PF’3944 as part of its larger buyout of the biotech company Metsera, completed last November.
Similar to existing medications on the market like semaglutide (Ozempic and Wegovy), PF’3944 mimics GLP-1, a hormone important to regulating our hunger and metabolism. But it’s designed to stay in the body longer than current GLP-1s, ideally allowing for a less cumbersome dosing schedule.
The Phase 2b VESPER-3 study tested how people on PF’3944 would fare after moving to a less frequent dose. The volunteers were obese and overweight individuals who did not have type 2 diabetes. They were randomized to receive a weekly dose of the drug or placebo for the first 12 weeks. From weeks 13 to 28, they transitioned into monthly maintenance shots.
As with other GLP-1 drugs, people taking PF’3944 lost substantially more weight than the placebo group. By week 28, users had lost up to 12.3% of their body weight compared to placebo. There appeared to be no plateau in the rate of people’s weight loss by week 28, suggesting the monthly switch did not hamper the drug’s effectiveness and that its peak could still be higher. (For other GLP-1s, max weight loss is usually reached by year one). PF’3944 also seemed to be generally well-tolerated, with the most common adverse events being gastrointestinal, as is typical for this drug class.
“These topline results from the Phase 2b VESPER-3 study reinforce the potential of PF’3944 as a monthly treatment with competitive efficacy,” said Jim List, chief internal medicine officer at Pfizer, in a statement from the company.
What’s next
The VESPER-3 study is still ongoing and will continue until participants have been on the drug for 64 weeks. But given the promising results so far from this and other studies, Pfizer is already moving ahead with further development.
In 2026, the company will advance 10 Phase 3 trials of PF’3944, which will test it out as both a weekly and monthly injection. Based on earlier study data, the company has also decided to evaluate the safety and effectiveness of a larger monthly maintenance dose in some of these trials.
Pfizer isn’t the only biotech trying to gain an advantage in the market by developing more conveniently timed GLP-1 drugs. Amgen’s MariTide—which combines a GLP-1 agonist with a GIP antagonist—is also being tested as a monthly injection, with several Phase III trials in the works.
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