Petosemtamab

–  63% response rate observed among 43 evaluable patients –  79% overall survival rate at 12-months; 9 months median progression-free survival UTRECHT, The Netherlands and CAMBRIDGE, MA, USA I May 22, 2025 I Merus N.V. (Nasdaq: MRUS) (Merus, the Company, we, or our), an oncology company developing innovative, full-length multispecific antibodies and antibody drug conjugates (Biclonics ® , Triclonics ® and ADClonics ® ), today announced interim clinical data as of a February 27, 2025 data cutoff from the ongoing phase 2 trial of the bispecific antibody petosemtamab in combination with pembrolizumab. These data will be presented by Dr. Carla M. L. van Herpen M.D. Ph.D., Radboud University Medical Center, Nijmegen, Netherlands at the 2025 American Society of Clinical Oncology ® (ASCO ® ) Annual Meeting on Monday, June 2 at 9 a.m. – 12:00 p.m. CT. “By essentially every metric, we believe these interim data are significantly better than pembrolizumab monotherapy, the control arm of our ongoing phase 3 trial, and underscores the opportunity petosemtamab holds to become a new standard of care, if approved, in head and neck cancer,” said Bill Lundberg, M.D., President, Chief Executive Officer of Merus. “Additionally, we believe our execution is outstanding with rapid site initiation. We are looking forward to potentially sharing top line interim readout of one or both of our phase 3 trials in 2026.” “Head and neck squamous cell carcinoma is associated with a poor prognosis and high mortality rate, and there remains a need for new treatment options for patients,” added Dr. van Herpen. “In my clinic, I have witnessed firsthand profound tumor shrinkage with petosemtamab administration, and the efficacy results petosemtamab has shown thus far in combination with the current standard of care, pembrolizumab. I’m excited by the impressive ORR and durability of those responses, and what these results, if replicated more broadly could mean for the future of our practice in head and neck cancer.” Petosemtamab (MCLA-158: EGFR x LGR5 Biclonics ® ): Solid Tumors Presentation title: Petosemtamab (MCLA-158) with pembrolizumab as first-line (1L) treatment of PD-L1+ recurrent/metastatic (r/m) head and neck squamous cell carcinoma (HNSCC): Phase 2 trial Observations in the presentation include as of a February 27, 2025 data cutoff date: Abstract #: 6024 Poster Board: 432 Session Title: Head and Neck Cancer Session Date and Time: June 2, 2025, 9:00-12:00 CT As full presentations become available at the 2025 ASCO ® Annual Meeting, they will contemporaneously be available on the Merus website . An analysis of the confirmed responses observed from administration of petosemtamab across the first line combination (as of the February 27, 2025 data cutoff date) and second-line plus monotherapy phase 2 cohorts (as of the July 5, 2024 data cutoff date), demonstrated that two-thirds of these responses with petosemtamab in HPV-associated p16+ oropharyngeal cancer occurred in never-smokers. About Merus N.V. Merus is an oncology company developing innovative full-length human bispecific and trispecific antibody therapeutics, referred to as Multiclonics ® . Multiclonics ® are manufactured using industry standard processes and have been observed in preclinical and clinical studies to have several of the same features of conventional human monoclonal antibodies, such as long half-life and low immunogenicity. For additional information, please visit Merus’ website , LinkedIn and Bluesky . SOURCE: Merus

iStock, beast01 Analysts were effusive about Merus’ new HNSCC data, writing that petosemtamab could “become the standard of care” in the first-line setting for this indication. Merus’ investigational bispecific antibody petosemtamab, when combined with Merck’s mega-blockbuster Keytruda, led to a nearly 80% overall survival rate when used as a frontline treatment option for certain patients with head and neck cancer, according to a Phase II readout posted Thursday. Writing to investors early on Friday, analysts at William Blair said Merus’ latest data drop continues to “support the best-in-disease pro petosemtamab” ahead of Phase III data expected next year. “With the company’s potential blockbuster market opportunity in HNSCC [head and neck squamous cell carcinoma], we continue to see significant upside to potential share prices.” Leerink Partners was similarly optimistic about petosemtamab, saying in a note late Thursday that these mid-stage data highlight the potential of petosemtamab and Keytruda to “become the standard of care across the full spectrum of HPV+ and HPV-negative 1L [first-line] HNSCC.” In particular, the Leerink analysts pointed to petosemtamab’s overall survival (OS) benefit, which they called “landmark” and “unprecedented in frontline HNSCC.” Thursday’s data come from 43 patients who received at least one dose of the petosemtamab-based regimen and had undergone at least one post-baseline tumor assessment. In these patients, Merus recorded a confirmed overall response rate of 63%, including six patients who showed complete responses and 21 who achieved partial responses. At 12 months, OS hit 79%, and median OS at the time of the readout had not yet been reached. Median progression-free survival was 9 months. “This impressive landmark OS provides confidence that the regimen will beat on survival versus [Keytruda] monotherapy,” according to the Leerink analysts, who wrote that landmark OS for Keytruda alone in this indication is “50-59%.” Merus is scheduled to present these findings on June 2 at the upcoming 2025 annual meeting of the American Society for Clinical Oncology, taking place in Chicago and online. Petosemtamab is a humanized bispecific antibody that binds to both EGFR and LGR5, boosting the immune system’s anti-cancer activity. Merus is running two Phase III trials for petosemtamab, testing it in various lines of treatment for recurrent or metastatic HNSCC. “We are looking forward to potentially sharing top line interim readout of one or both of our phase 3 trials in 2026,” CEO Bill Lundberg said in Thursday’s announcement. But BMO Capital Markets set a higher goal forpetosemtamab’s regulatory prospects. In a Thursday evening note, analysts wrote that Merus’ latest readout could open a “path for potential accelerated approval in the near term.”