Scientists have reported encouraging early results from a clinical trial testing a new immunotherapy designed to help the body’s immune system recognize and attack prostate cancer. The experimental drug, known as VIR-5500, demonstrated strong responses in men with advanced disease who had stopped responding to existing treatments.
Prostate cancer remains one of the most commonly diagnosed cancers among men worldwide. Each year, about 1.5 million men receive the diagnosis. The disease also presents significant disparities among different populations. In the United States, for example, Black men experience the highest incidence rate, at roughly 194 cases per 100,000 people, compared with 119 for white men, 90 for Hispanic men, 82 for Native American men, and 65 for Asian and Pacific Islander men.
While immunotherapy has transformed treatment for several cancers, including lung and skin cancers, prostate cancer has largely resisted these advances. Tumors often remain “invisible” to the immune system, preventing the body’s natural defenses from mounting an effective attack.
Researchers believe VIR-5500 may help overcome this challenge.
Early Trial Shows Promising Results
The findings come from an early-stage clinical trial involving 58 patients at eight sites worldwide. All participants had advanced prostate cancer that no longer responded to standard therapies.
VIR-5500 belongs to a class of treatments known as T-cell engagers. These therapies are designed to bring immune cells directly into contact with cancer cells. Specifically, the drug links the body’s killer T cells—immune cells responsible for destroying harmful or abnormal cells—to proteins found on the surface of prostate cancer cells.
By physically connecting the immune cells to the tumor, the drug effectively forces the immune system to attack.
“T-cell engagers empower the body’s own immune system to give cancer cells the kiss of death,” said Professor Johann de Bono of the Institute of Cancer Research and the Royal Marsden NHS Foundation Trust, who led the study.
A “Trojan Horse” Approach to Reduce Side Effects
Previous attempts to use T-cell engagers against prostate cancer have often triggered severe inflammatory reactions. These therapies can activate the immune system too broadly, causing widespread side effects that many patients cannot tolerate.
VIR-5500 was designed to avoid this problem.
The drug contains a molecular “cloaking” mechanism that keeps it inactive while circulating in the body. It becomes active only after reaching the tumor environment, acting like a molecular Trojan horse that releases its immune-activating function precisely where it is needed.
This targeted design appears to significantly reduce unwanted reactions. According to the trial data, 88% of participants experienced only mild side effects.
“It’s very positive to see that very few patients have experienced major side effects, as this has been a key challenge in treating prostate cancers with immunotherapies in the past,” De Bono said.
Significant Drops in Cancer Biomarkers
Researchers evaluated the treatment’s effectiveness by measuring prostate-specific antigen (PSA) levels in the blood, a common marker used to monitor prostate cancer activity.
Among 17 patients who received the highest dose, the results were particularly notable:
- 82% experienced a PSA reduction of at least 50%
- 53% saw PSA levels drop by 90%
- 29% recorded reductions of at least 99%
De Bono described the results as unprecedented for a cancer type traditionally resistant to immunotherapy.
“We do need more data, but the results are stunning,” he said.
Tumor Shrinkage in Some Patients
- Beyond biomarker improvements, researchers also observed tumor shrinkage in imaging scans.
- Among 11 high-dose patients with measurable tumors, 45% showed visible tumor reduction.
Several individual cases highlighted the potential impact of the therapy:
- A 63-year-old patient with cancer that had spread to his liver saw 14 metastatic lesions completely disappear after six treatment cycles.
- A 70-year-old participant experienced full regression of small tumors outside the prostate while maintaining an excellent quality of life.
- A 77-year-old patient saw his PSA levels fall to undetectable levels after 17 treatment cycles.
Experts Urge Caution and Further Research
Cancer specialists say the results are highly encouraging but emphasize that larger trials are needed to confirm the therapy’s effectiveness and safety.
“Immunotherapy has transformed the outcomes for many people with cancer, but for those with prostate cancer its benefits have often remained out of reach,” said Professor Kristian Helin, chief executive of the Institute of Cancer Research in London. “It’s encouraging to see this innovative approach showing promising effects in early clinical studies.”
Researchers also stress the importance of including patients from diverse backgrounds in future trials, given the disease’s uneven impact across ethnic groups.
Professor Charlotte Bevan, a cancer biologist at Imperial College London who was not involved in the study, said the development could represent a significant step forward but highlighted the need for broader representation in research.
Hope for Patients With Limited Options
Patient advocacy organizations have welcomed the findings, noting the limited treatment choices currently available for men with advanced prostate cancer.
“These early results are extremely promising, with a number of men on the study responding positively to the treatment with minimal side effects,” said Simon Grieveson, assistant director of research at Prostate Cancer UK.
As VIR-5500 progresses to larger clinical trials, researchers hope the therapy could eventually offer new options for patients whose cancers have stopped responding to existing treatments.
“I look forward to seeing this now tested in larger trials,” Grieveson said, “with the hope that this treatment will offer men more valuable time with their loved ones.”
