A major international clinical trial has achieved a historic milestone by demonstrating the first successful use of immunotherapy in prostate cancer, offering new hope for men with advanced disease who have exhausted all existing treatment options. The study, led by researchers at The Institute of Cancer Research, London, and The Royal Marsden NHS Foundation Trust, showed that the checkpoint inhibitor pembrolizumab provided significant survival benefits for a specific subset of patients.
Trial Results Show Sustained Benefits
The trial enrolled 258 men with advanced prostate cancer and revealed encouraging outcomes. After one year of treatment, 38% of participants remained alive, while 11% continued receiving pembrolizumab without experiencing cancer progression. This represents a substantial improvement for patients who previously had no viable treatment alternatives.
"In the last few years immunotherapy has changed the way we treat many advanced cancers – but up to now no one had demonstrated a benefit in men with prostate cancer," said Professor Johann de Bono, Director of the Drug Development Unit at the ICR and The Royal Marsden NHS Foundation Trust.
DNA Repair Mutations Emerge as Key Biomarker
While only 5% of men in the trial experienced tumor shrinkage or complete disappearance, the research revealed crucial insights about patient selection. The proportion of responders was notably higher among men whose tumors harbored mutations in genes involved in DNA repair mechanisms.
The researchers hypothesize that tumors with DNA repair defects accumulate more genetic mutations as they multiply, making these ultra-mutant cancer cells particularly recognizable to the immune system. This finding aligns with data from other cancer types, such as bowel cancer, where tumors with DNA repair defects demonstrate increased susceptibility to immunotherapy.
Biomarker Testing Challenges and Opportunities
The study compared pembrolizumab effectiveness in men whose tumors expressed PD-L1 protein on cancer cell surfaces versus those without this marker. However, researchers found that PD-L1 testing alone was insufficient to predict treatment response. Preliminary evidence suggested that testing for another protein, PD-L2, might serve as a better response predictor, though this requires validation in future clinical trials.
Future Research Directions
Based on these findings, the research team is planning a new clinical trial specifically targeting men with prostate cancer whose tumors contain DNA repair gene mutations. This precision medicine approach aims to validate whether immunotherapy can become a standard treatment component for this patient subset.
"We are planning a new clinical trial, specifically in men with prostate cancer whose tumours have mutations in DNA repair genes, to see if immunotherapy can become a standard part of their treatment," de Bono explained. "It's exciting that immunotherapy could offer some men more time with their loved ones where they have such advanced disease that they have run out of existing treatment options."
Addressing Immunotherapy's Broader Challenges
Professor Paul Workman, Chief Executive of the ICR, emphasized the broader implications of this research for immunotherapy development. "Immunotherapy has proven to be a smarter, kinder treatment for many types of cancer – but it still only works for a minority of patients. The challenges we now face are how to predict in advance who will benefit, and how to make immunotherapy work for more people."
The study, funded by Merck Sharp & Dohme, was presented at the American Society of Clinical Oncology annual meeting in Chicago. This research represents a significant step forward in expanding immunotherapy's reach to prostate cancer, potentially adding this cancer type to the growing list of malignancies treatable with immune checkpoint inhibitors.
