Merus N.V. Announces Publication of an Abstract on Petosemtamab in Advanced Gastric/Esophageal Adenocarcinoma for Presentation at the AACR

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 the publication of an abstract for a poster presentation of early clinical data on the bispecific antibody petosemtamab in advanced gastric/esophageal adenocarcinoma (GEA) at the American Association for Cancer Research (AACR) Annual Meeting 2023 taking place in Orlando, Florida April 14-19, 2023.

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).

Petosemtamab is in clinical development in the expansion part of a phase 1/2 open-label, multicenter trial in advanced solid tumors, including advanced GEA.

Although petosemtamab has demonstrated promising clinical activity among pretreated gastric/esophageal adenocarcinoma (GEA) patients having EGFR gene amplification and/or overexpression, the Company has decided to pause further clinical exploration of the GEA cancer cohort at this time. The Company plans to prioritize investigating petosemtamab in head and neck squamous cell carcinoma, in view of the strong clinical activity observed in this cohort.

Information and observations from the cohort of GEA patients treated in the phase 1/2 trial include:

• As of an October 24, 2022 data cutoff date, 14 previously treated GEA patients (pts) were treated with petosemtamab 1500 mg (IV) every two weeks
• Patient Population:
  • Median age was 63 (range of 40-80); 79% were male
  • Median prior lines of systemic therapy was 3 (range 1-4); including platinum-based chemotherapy (36% of pts) and checkpoint inhibitors (14%)
• 14 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 the 14 pts:
    • 1 pt with tumor EGFR protein overexpression and gene copy number amplification (CNA) showed a confirmed sustained partial response (67% tumor reduction; response ongoing after 24 cycles);
    • 3 pts had stable disease (1 with EGFR overexpression and gene CNA; 2 not evaluable for IHC), with tumor reductions of 2%, 17%, and 40%.
• Petosemtamab continues to demonstrate a manageable safety profile:
  • Of 78 pts treated at the recommended phase 2 dose of 1500 mg every two weeks (escalation and all expansion cohorts), the most frequent AEs regardless of causality (all grades/G3-4) were rash (33%/0%), hypotension (26%/6%), dyspnea (26%/4%), nausea (26%/1%), dermatitis acneiform (24%/1%), blood magnesium decreased (19%/5%), erythema (19%/0%), diarrhea (19%/0%); IRRs (composite term) were reported in 74%/21% of pts, mostly at the first infusion, and all resolved. 5 pts (6%) discontinued treatment due to IRRs on Day 1.

Presentation Details:
Title: MCLA-158 (petosemtamab), an IgG1 bispecific antibody targeting EGFR and LGR5, in advanced gastric/esophageal adenocarcinoma (GEA)
Session: Phase II Clinical Trials 1
Date: Monday, April 17, 2023
Time: 1:30 – 5:30 p.m. ET
Poster #: 18
Abstract #: CT156

The abstract can be found on the conference 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.