An experimental antibody added to the immune drug Keytruda helped people with advanced head and neck cancer live longer than Keytruda alone in a large trial, according to reports on October 28, 2026. The combination of petosemtamab plus pembrolizumab (Keytruda’s generic name) met the trial’s two main goals, overall survival and the share of patients whose tumors shrank, and cut the risk of death by 50%1. The full numbers, including side effects, have not yet been made public.
What happened
The trial, called LiGeR-HN1, is a Phase 3 study, the last big test before a drug can be considered for approval. It compared petosemtamab plus pembrolizumab against pembrolizumab alone as the first treatment for head and neck cancer that has come back or spread2. A same-day news summary said the combination “hits” both main goals and cut the risk of death by 50% in this group of patients1.
A 50% lower risk of death is a relative comparison: it describes how often deaths happened in one group versus the other over the time studied. The sources available to Biohack do not give how long patients lived on average in each group, how long they were followed, or how many had died at the time of the analysis.
The timing is earlier than the registry’s planned schedule. ClinicalTrials.gov lists the main data collection as finishing around January 20282. In May 2025, the chief executive of Merus, the company listed as the trial’s sponsor, said the company was looking forward to “potentially sharing top line interim readout of one or both of our phase 3 trials in 2026”3.
How the trial was set up
The trial enrolled 699 people2. Patients were assigned at random to one of the two treatments, and the study was open-label, meaning both patients and doctors knew which treatment each person got2. It started on September 25, 20242, with sites in countries including Argentina, Australia and Belgium4.
To join, adults needed squamous cell head and neck cancer, the common type that starts in the lining of the mouth and throat, in the mouth, the middle of the throat, the lower throat or the voice box4. Their cancer had to have come back or spread in a way that could not be cured with surgery or radiation, and they could not have had drug treatment for that stage yet4. People previously treated with drugs that block PD-1 or PD-L1, or with drugs that block EGFR, were excluded4.
Patients’ tumors also had to score at least 1 on a lab test called PD-L1 CPS. That is the group already eligible to get pembrolizumab on its own as a first treatment4, which is why Keytruda alone was the comparison.
The two main goals were overall survival, measured over up to about 3 years, and the share of patients whose tumors shrank, judged by reviewers who did not know which treatment each patient got, over up to about 2 years2.
What petosemtamab is
Petosemtamab is an antibody designed to attach to two proteins at once, called EGFR and LGR5, according to a Swiss trial registry. The registry says these interactions “may help the immune system fight cancer”5. Drugs that grab two targets like this are often called bispecific antibodies; the drug’s earlier lab code was MCLA-1583.
The evidence before this result
The new trial built on a smaller, earlier study in which every patient got the combination, with no comparison group. In early data presented in 2024, tumors shrank in 67% of 24 patients who could be assessed6.
A later update, with data up to February 27, 2025, covered 45 treated patients3. Among 43 who could be assessed:
- Tumors shrank in 63% (27 people), including 6 whose tumors disappeared on scans3.
- Half of patients went about 9 months before their cancer grew again3.
- 79% were alive after one year3.
Side effects were common in that study. Every patient had at least one side effect during treatment, and 60% had serious ones (grade 3 or higher), including 44% whose serious side effects were judged related to treatment3. Reactions during the drip, called infusion reactions, happened in 38% of patients, mostly during the first infusion3. An earlier data cut reported serious side effects in 40% of patients, with an acne-like skin rash in 44%6. The difference reflects different dates of data collection.
In February 2025, the US Food and Drug Administration gave the combination a “breakthrough therapy designation,” a status meant to speed development of drugs that may be a large improvement over existing options, based on that earlier study6. That designation is not an approval.
What this does not show yet
- The full numbers. Beyond the 50% figure, the sources do not give survival times, the share of patients whose tumors shrank, or how many patients the analysis included1.
- Side effects in the large trial. No safety results from LiGeR-HN1 appear in the sources. The earlier study showed frequent skin problems and serious side effects36, and whether that pattern holds in a larger group is unknown.
- Peer review. The registry shows no results posted yet2, and no journal paper is available.
- Other patients. The trial only included people whose tumors scored at least 1 on the PD-L1 test and who had not had prior drug treatment for advanced disease4. It says nothing about other groups.
Who is behind it
ClinicalTrials.gov lists Merus B.V. as the sponsor of LiGeR-HN12. A Swiss listing for a sister trial of the drug names Genmab A/S of Denmark as sponsor, with a Genmab contact5. The earlier positive statements about the drug came from the company: Merus chief executive Bill Lundberg said in 2025 that the company believed the earlier data were “significantly better than pembrolizumab monotherapy”3, and its chief medical officer said the FDA designation pointed to “potential for substantial improvement over available therapies”6. The earlier study results were presented by Carla van Herpen of Radboud University Medical Center in the Netherlands3.
What to watch next
The first thing to watch is the full data: survival times, how many patients’ tumors shrank, and side effects, presumably at a medical meeting or in a journal. Then, whether the company files for approval.
A second large trial, LiGeR-HN2, is testing petosemtamab on its own against a drug chosen by the patient’s doctor in 621 people whose cancer grew after both immune therapy and platinum chemotherapy7. Its main goal is overall survival, and the registry lists its main data collection finishing around February 20287. No results from that trial appear in the sources yet.
