Merus N.V. presented clinical data on MCLA-158, including clinical responses observed in advanced head and neck squamous cell carcinoma (HNSCC) and preclinical data on zenocutuzumab (Zeno) at the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics. MCLA-158 (petosemtamab): the reported data are from the ongoing phase 1 dose expansion cohort that is investigating the safety, tolerability, and anti-tumor activity of MCLA-158 monotherapy in advanced HNSCC. Observations in the presentation include: enrollment of 10 patients with advanced HNSCC, as of the safety and efficacy data cutoff date of August 9, 2021, with median age of 65 (range 50-77) years, and who were treated with a median of 2 lines of prior therapy. Seven patients were evaluable for an interim efficacy analysis by investigator assessment (three patients were enrolled <8 weeks from the cutoff date). Three of seven patients achieved partial responses, with one achieving complete response after the data cutoff date. Tumor reduction was observed in all seven patients. The safety profile of MCLA-158 was based on 29 patients with advanced solid tumors who were treated at 1500 mg every two weeks across the phase 1 trial. The most frequent adverse events (AEs) were infusion related reactions; 72% any grade, 7% grade = 3. Mild to moderate skin toxicity (3% grade =3). The company is planning its next update on the MCLA-158 trial in 2022. Zeno: observations in the preclinical presentation include: The bispecific HER2/HER3 antibody Zeno blocked cell growth 100 fold more potently than the bivalent HER3 antibody derived from Zeno, in an NRG1 driven growth assay. Zeno potently blocked NRG1-fusion mediated downstream signaling and growth in vitro and in vivo. Zeno induced both antibody-dependent cellular cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP) mediated killing of cancer cells in a dose-dependent manner. As of September 1, 2021 more than 80 patients with NRG1 fusion cancers have been treated with Zeno monotherapy in the company's phase 1/2 eNRGy trial and Early Access Program.