Sponsor: Merus N.V. (industry)
Phase: 3
Start date: Sept. 25, 2024
Planned enrollment: 500
Petosemtamab (MCLA-158) is an investigational, ADCC‑enhanced, full‑length bispecific IgG1 antibody that targets EGFR and LGR5. It is being developed primarily for recurrent/metastatic head and neck squamous cell carcinoma (r/m HNSCC), with ongoing phase 2 and phase 3 development; the FDA has granted Breakthrough Therapy designation for post–PD‑1/platinium r/m HNSCC (May 2024) and for the first‑line combination with pembrolizumab in PD‑L1–positive disease (February 2025). (aacrjournals.org)
Note: Phase 3 trials in first‑line PD‑L1+ and in post‑PD‑1 settings have been initiated/planned by the sponsor based on these data. For current recruitment/status, see the study record for NCT03526835 and sponsor updates. (fdaaa.trialstracker.net)
Notes: Efficacy and safety data above are from interim analyses in early‑phase trials and may evolve with additional follow‑up or phase 3 readouts.
Last updated: Oct 2025
Goal: To determine whether adding petosemtamab to pembrolizumab improves outcomes versus pembrolizumab alone as first-line therapy in patients with recurrent or metastatic PD-L1–positive head and neck squamous cell carcinoma (HNSCC).
Patients: Adults with histologically confirmed recurrent or metastatic HNSCC of the oropharynx, oral cavity, hypopharynx, or larynx that is not amenable to curative local therapy, PD-L1 CPS ≥1, ECOG 0–1, measurable disease by RECIST v1.1, and adequate organ function. No prior systemic therapy in the incurable recurrent/metastatic setting is allowed; prior multimodal therapy for locally advanced disease is permitted if progression occurred ≥6 months after last platinum dose. Prior anti–PD-(L)1 or anti-EGFR therapy is excluded, with a narrow exception for cetuximab given with radiotherapy if progression was >1 year after the last dose. Key exclusions include active/symptomatic CNS disease, uncontrolled comorbidities, significant cardiac disease, active autoimmune disease requiring immunosuppression, and nasopharyngeal or sinonasal primaries.
Design: Phase 3, randomized, open-label, parallel-group trial enrolling approximately 500 participants. Patients are randomized to petosemtamab plus pembrolizumab or pembrolizumab monotherapy as first-line treatment. Stratification factors are not listed here but responses will be assessed centrally by BICR using RECIST v1.1 for key efficacy endpoints.
Treatments: Experimental arm: petosemtamab plus pembrolizumab. Petosemtamab (MCLA-158) is a human IgG1 bispecific antibody targeting EGFR and LGR5. It blocks EGFR ligand binding and promotes EGFR internalization and degradation selectively in LGR5-positive tumor cells, with enhanced Fc-mediated effector function (ADCC/ADCP). Early-phase data in first-line PD-L1–positive r/m HNSCC combined with pembrolizumab have shown promising activity, with interim reports describing ORR around 63% and manageable toxicity, including infusion-related reactions mainly on first infusion. Monotherapy petosemtamab in pretreated r/m HNSCC demonstrated ORR around 36–40% in interim datasets, supporting further evaluation. Control arm: pembrolizumab monotherapy, standard first-line immunotherapy for PD-L1–positive r/m HNSCC.
Outcomes: Co-primary endpoints are overall survival and objective response rate by BICR per RECIST v1.1. Key secondary endpoints include progression-free survival, duration of response, clinical benefit rate, and corresponding investigator-assessed RECIST endpoints; safety and tolerability (TEAEs, serious TEAEs, dose modifications, and discontinuations); health-related quality of life; pharmacokinetics; and anti-drug antibody incidence. Time frames range up to approximately 2 years for efficacy and QoL, up to 3 years for overall survival, and up to 30 days post last dose for safety and immunogenicity.
Burden on patient: Moderate. Requirements include baseline and on-treatment imaging per RECIST, a mandatory recent tumor biopsy if adequate archival tissue is not available, regular clinic visits for intravenous infusions in both arms, and safety labs. The experimental arm may involve additional pharmacokinetic sampling during the first six cycles and monitoring for infusion-related reactions, particularly at the first dose, which can lengthen infusion visits. Overall visit frequency and imaging are similar to standard-of-care pembrolizumab-based management in this setting, but added PK draws and infusion observation increase time on site, leading to a moderate, not high, burden.
Last updated: Oct 2025
Inclusion Criteria:
1. Signed ICF before initiation of any study procedures
2. Age ≥ 18 years at signing of ICF
3. Histologically confirmed HNSCC with evidence of metastatic or locally recurrent disease not amenable to local therapy with curative intent.
4. The eligible HNSCC primary tumor locations are oropharynx, oral cavity, hypopharynx, and larynx.
5. HNSCC patients eligible to receive pembrolizumab as 1L monotherapy with tumors expressing PD-L1, CPS ≥1.
6. HNSCC patients should not have had previous systemic therapy administered in the incurable recurrent or metastatic setting
7. A new tumor biopsy, unless the patient has an available archival tumor sample with sufficient material
8. Measurable disease per Investigator assessment as defined by RECIST v1.1 by radiologic methods
9. ECOG Performance Status (PS) of 0-1
10. Life expectancy ≥ 12 weeks, as per investigator assessment.
11. Left ventricular ejection fraction (LVEF) ≥50% or ≥ institutional normal limit, whichever is higher, by echocardiogram (ECHO) or multigated acquisition (MUGA) scan
12. Adequate organ function as defined per protocol.
13. HIV-positive patients are eligible only if the cluster of differentiation 4 (CD4+) count is ≥ 300/µl, viral load is undetectable, and the patient is currently receiving highly active antiretroviral therapy
Exclusion Criteria:
1. Central nervous system metastases that are untreated or already treated but symptomatic, or require radiation, surgery, or continued steroid therapy to control symptoms within 21 days prior to randomization
2. Known leptomeningeal involvement
3. Any systemic anticancer therapy or investigational drug within 4 weeks or 5 half-lives, whichever is shorter, before randomization
4. Requirement for immunosuppressive medication
5. Major surgery or radiotherapy within 3 weeks of randomization
6. Clinically significant toxicities related to prior anticancer therapies that have not returned to ≤ Grade 1 or baseline except for Grade ≤2- myalgia, neuropathy, alopecia, and any prior therapy related endocrinopathies
7. History of hypersensitivity reaction to any of the excipients of petosemtomab or pembrolizumab.
8. Unstable angina; history of congestive heart failure of Class II-IV New York Heart Association (NYHA) criteria, or serious cardiac arrhythmia requiring treatment; or history of myocardial infarction within 6 months prior to randomization
9. History of prior malignancies within the last 5 years, with the exception of excised local cancer
10. Current dyspnea at rest of any origin, or other diseases requiring continuous oxygen therapy
11. Current serious illness or medical conditions including, but not limited to, uncontrolled active infection, clinically significant pulmonary, metabolic or psychiatric disorders
12. Patients with known infectious diseases as per protocol.
13. Pregnant or breastfeeding patients.
14. The patient has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy of prednisone \>10 mg/day or equivalent, or any other form of immunosuppressive therapy
15. The patient has an active autoimmune disease that has required systemic immune suppressive treatment in the past 2 years; replacement therapy is not considered immune suppressive treatment
16. The patient has had an allogeneic tissue/solid organ transplant.
17. Patient has a primary tumor site of nasopharynx, or sinonasal carcinoma (any histology)
Other protocol defined inclusion/exclusion criteria may apply.
CABA, C1118AAT, Argentina
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Würzburg, 97080, Germany
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Pátrai, Achaia, 265 04, Greece
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Athens, Attica, 151 23, Greece
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Heraklion, Crete, 715 00, Greece
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Chaïdári, 12462, Greece
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Haifa, 3109601, Israel
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Ramat Gan, 5266202, Israel
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Chuo Ku, Tokyo, 104-0045, Japan
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Badalona, 08916, Spain
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Marbella, 29660, Spain
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Kaohsiung City, 80756, Taiwan
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