On February 3, 2026, Pfizer released topline data from its Phase 2b VESPER-3 study for PF-08653944, the ultra-long-acting obesity injection acquired through its $10 billion acquisition of Metsera. While the drug met its primary endpoints, its efficacy compared to market incumbents led to a slight decline in the company’s stock price.
Phase 2b Clinical Findings
The study evaluated several dosing regimens in 268 participants with obesity or overweight. The protocol involved a 12-week weekly titration followed by once-monthly maintenance dosing through week 28.
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Weight Loss Performance: At 28 weeks, the medium-dose cohort achieved a placebo-adjusted weight loss of 12.3%. The low-dose cohort saw a 10% reduction.
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Continuous Progression: Researchers observed no plateau in weight loss at the 28-week mark, suggesting that body weight may continue to decline as the trial progresses toward its 64-week endpoint.
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Tolerability Profile: Side effects were predominantly mild to moderate gastrointestinal events. The discontinuation rate due to adverse events was approximately 10%, slightly higher than the 6% observed in rival high-dose trials.
Market Context and Strategic Roadmap
Pfizer’s stock fluctuated, dropping up to 5% as analysts noted the results appeared “slightly inferior” to Eli Lilly’s Zepbound at similar timepoints. Despite the competitive pressure, Pfizer is moving forward with an expansive late-stage development plan:
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Higher Dosing in Phase 3: To achieve “best-in-class” results, Pfizer plans to include a 9.6 mg monthly dose in upcoming pivotal trials—double the highest dose used in Phase 2.
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Differentiated Format: The primary advantage lies in its monthly dosing schedule, offering patients significant convenience over existing weekly injectables.
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Pipeline Anchor: Pfizer intends to use PF-08653944 as the foundation for future combination therapies, including co-formulations with amylin analogs.