Researchers described the findings as among the strongest seen in patients with few remaining treatment options.
The treatment, called amivantamab, was tested in people with recurrent or metastatic head and neck cancer, whose disease continued to progress despite standard therapies.
Among 102 patients involved in the international trial, tumours shrank in 43 people. In 15 cases, they disappeared completely.
One of those benefiting from the treatment was Carl Walsh, 56, from Birmingham, who was diagnosed with tongue cancer in 2024.
After chemotherapy and immunotherapy proved unsuccessful, he joined the OrigAMI-4 trial at The Royal Marsden NHS Foundation Trust.
“I now feel able to live a normal life,” Walsh said in a statement released by the Institute of Cancer Research in London.
“Before starting the trial, I struggled to speak properly and found eating difficult because of the swelling and pain.
“Since beginning treatment, the swelling has reduced significantly, and my pain levels have improved considerably. I’m also no longer experiencing the same life-impacting side effects that I had during chemotherapy.”
Researchers said patients typically began responding to treatment within about six weeks.
Dr Kevin Harrington, a biological cancer therapies specialist at ICR and consultant oncologist at The Royal Marsden, said the findings were particularly encouraging because participants had already exhausted standard treatment options.
“These are unprecedentedly strong responses in patients whose disease has become resistant to both chemotherapy and immunotherapy,” he said.
Head and neck cancer is the sixth most common cancer worldwide. Once it recurs or spreads after standard treatment, outcomes are often poor.
The drug, developed by Johnson & Johnson, belongs to a newer generation of targeted cancer therapies known as bispecific antibodies.
It works by blocking two pathways that help tumours grow and evade treatment, while also helping the immune system recognise and attack cancer cells.
Patients receiving amivantamab lived for a median of 12.5 months after starting treatment, while the cancer took an average of more than six months to begin growing again.
Unlike many cancer drugs that require intravenous infusions, amivantamab is delivered through a small injection under the skin every three weeks, making it quicker and more convenient to administer.
Side effects were generally mild to moderate, with fewer than 10 patients discontinuing treatment because of them.
The study involved participants from 55 hospitals across 11 countries and focused on patients with head and neck cancers not caused by the human papillomavirus (HPV), as these cancers tend to have poorer outcomes and respond less favourably to treatment compared with HPV-related cases.
Amivantamab is already approved for certain types of lung cancer and is being studied for use in several other cancers.
The findings were presented at the American Society of Clinical Oncology meeting in Chicago on Sunday.