– 37% overall response rate (ORR) observed in 43 evaluable patients
– 6 months median duration of response as of Feb. 1, 2023 data cutoff date
– End-of-phase meeting with U.S. Food & Drug Administration provides clarity to potential registration path in HNSCC

UTRECHT, The Netherlands and CAMBRIDGE, MA, USA I April 17, 2023 I Merus N.V. (Nasdaq: MRUS) (“Merus”, “the Company”, “we”, or “our”), a clinical-stage oncology company developing innovative, full-length multispecific antibodies (Biclonics® and Triclonics®), today announced interim clinical data as of a February 1, 2023 data cutoff, from the ongoing phase 1/2 trial of the bispecific antibody petosemtamab in previously treated head and neck squamous cell carcinoma (HNSCC). The Plenary Session presentation by Dr. Ezra EW Cohen, Moores Cancer Center, UC San Diego Health, will occur today at the American Association of Cancer Research (AACR) Annual Meeting 2023, taking place in Orlando, Florida.

Petosemtamab, or MCLA-158, is a human IgG1 Biclonics® designed to bind to cancer cells expressing epidermal growth factor receptor (EGFR) and leucine-rich repeat-containing G protein-coupled receptor 5 (LGR5).

“I am excited by this interim dataset that demonstrates the consistent and clinically meaningful activity of petosemtamab in patients with previously treated head and neck squamous cell carcinoma,” said Dr. Andrew Joe, Chief Medical Officer at Merus. “This is clinical validation of a first of its kind EGFR and LGR5 targeting agent.”

“There is significant unmet medical need in head and neck squamous cell carcinoma,” added Dr. Cohen. “Petosemtamab has the potential to be a meaningful medicine and new standard of care for patients with head and neck cancer.”

Updated information and observations from plenary presentation of the ongoing phase 1/2 trial include:

  • As of the February 1, 2023 data cutoff date, 49 previously treated HNSCC patients (pts) were treated with petosemtamab at the recommended phase 2 dose of 1500 mg intravenous every two weeks
  • Patient population:
    • Median age was 63 (range of 31-77); 78% were male
    • Median prior lines of systemic therapy was 2 (range 1-4); including PD-(L)1 inhibitor in 96% of pts, chemotherapy in 94% and platinum-based chemotherapy in 92% of pts; 2 pts received prior cetuximab
    • Most frequent primary tumor locations were oropharynx (35%), oral cavity (31%), and larynx (16%)
  • 43 pts were evaluable for efficacy, receiving ≥2 treatment cycles (≥8 weeks) with ≥1 post-baseline tumor assessment or experiencing early progressive disease:
    • Antitumor activity among 43 pts:
      • Overall responses rate (ORR) was 37.2% (16/43; 95% CI 23%-53.3%) by RECIST 1.1. per investigator assessment, including 15 confirmed partial responses (PR) and 1 confirmed complete response (CR) (ongoing after 20 months)
      • Disease control rate (CR + PR + stable disease) was 72.1% (31/43; 95% CI 56.3%-84.7%)
      • Median time to response was 1.8 months (range 0.8-3.5)
      • Median duration of response was 6.0 months (95% CI 3.7-NC), with 10 of 16 (62.5%) responders ongoing, and 12 of 43 (27.9%) patients overall ongoing at the time of the data cutoff
      • Median progression free survival was 5.3 months (95% CI 3.7-6.8); with 29 of 43 pts progressing and 14 of 43 pts censored
      • Median overall survival was 11.5 months (95% CI 7.2-20.6); with 29 of 49 pts still alive at the data cutoff date
  • Petosemtamab continued to demonstrate a manageable safety profile:
    • 80 pts were treated with 1500 mg petosemtamab every two weeks across dose escalation and expansion cohorts of the study
    • Gastrointestinal and skin toxicities were mostly mild to moderate
    • No treatment-related Grade 5 AEs:
      • Most frequent related AEs were signs and symptoms of infusion-related reactions (IRRs)
      • 74% Grade 1-4, 21% Grade 3-4 (as grouped term)
      • Mainly occurred during first infusion
      • 6 of 80 pts discontinued on Day 1 due to a Grade 3-4 IRR
      • For all patients rechallenged after an IRR, rechallenge was successful
      • IRRs were manageable with prophylaxis/ prolonged infusion (necessary on Day 1 only)

The presentation is now available on the Merus website.

About Petosemtamab
Petosemtamab, or MCLA-158, is a bispecific Biclonics® low-fucose human full-length IgG1 antibody targeting the epidermal growth factor receptor (EGFR) and the leucine-rich repeat containing G-protein-coupled receptor 5 (LGR5). Petosemtamab is designed to exhibit three independent mechanisms of action including inhibition of EGFR-dependent signaling, LGR5 binding leading to EGFR internalization and degradation in cancer cells, and enhanced antibody-dependent cell-mediated cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP) activity.

About Merus N.V.
Merus is a clinical-stage 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, https://www.merus.nl and https://twitter.com/MerusNV.