A new experimental immunotherapy drug has shown encouraging results in treating advanced prostate cancer, according to findings from a Phase I clinical trial involving patients whose disease no longer responds to standard treatments.
Researchers reported that nearly half of the evaluable patients experienced tumour shrinkage after receiving the drug. In addition, most participants recorded a significant drop in prostate cancer markers. The treatment also appeared to be well tolerated, with most patients experiencing only mild side effects.
The trial was conducted at eight sites worldwide and led by Professor Johann de Bono of The Institute of Cancer Research, London and The Royal Marsden NHS Foundation Trust. The research findings were presented at the American Society of Clinical Oncology Genitourinary Cancers Symposium.
The study evaluated a new drug called VIR-5500 in 58 patients with advanced prostate cancer. The main goal of the Phase I trial was to assess the safety of the treatment and determine the optimal dosage for patients.
VIR-5500 works as a “T-cell engager.” This type of drug links the body’s immune T-cells to cancer cells. It specifically targets prostate-specific membrane antigen (PSMA), a protein commonly found on the surface of prostate cancer cells.
By connecting immune cells directly to cancer cells, the therapy helps the immune system recognize and attack the tumour more effectively.
The drug also uses a built-in cloaking mechanism that keeps it inactive while circulating in the bloodstream. It only becomes active when it reaches the tumour site. Researchers say this design reduces the risk of widespread immune activation, which has been a major problem with earlier T-cell engager therapies.
Previous versions of similar treatments often triggered severe inflammatory responses throughout the body. In this study, however, 88 percent of patients did not experience the common treatment-related side effects typically associated with these drugs.
The cloaking design may also allow the medicine to remain in the bloodstream longer, potentially reducing how often patients need treatment.
Prostate-specific antigen (PSA) is a key marker used to track prostate cancer activity. Among 17 patients who received the highest dose of VIR-5500 and could be evaluated, 82 percent saw their PSA levels reduced by half. More than half of those patients experienced an even greater response, with PSA levels falling by about 90 percent.
Imaging scans also showed measurable tumour shrinkage. Of the 11 patients treated at the highest dose who could be evaluated, five patients — or 45 percent — experienced a reduction in tumour size. Tumours shrank in several areas of the body, including internal organs.
Researchers estimate that more than three-quarters of the roughly 10,000 men diagnosed each year with advanced prostate cancer have tumours with high PSMA levels, meaning many could potentially benefit from treatments like VIR-5500.
Professor de Bono said T-cell engagers help the immune system attack cancer directly.
“These drugs enable the body’s immune system to deliver what we call the ‘kiss of death’ to cancer cells,” he said. “It is remarkable to see early indications that this therapy can produce a strong anti-cancer effect in patients with advanced prostate cancer who urgently need new treatment options.”
He added that the low rate of severe side effects is particularly encouraging, as safety has long been a challenge for immunotherapy treatments targeting prostate cancer.
The clinical trial is still ongoing. Researchers plan to move the drug into larger studies to examine its long-term effectiveness and safety.
Professor Kristian Helin, chief executive of the Institute of Cancer Research, said immunotherapy has transformed treatment for many cancers but has not yet delivered the same benefits for prostate cancer patients.
“It is encouraging to see this innovative approach showing promising results in early studies,” Helin said. “We hope this therapy could eventually provide a much-needed new option for patients with advanced prostate cancer.”
If future trials confirm the early findings, scientists believe treatments like VIR-5500 could improve survival and potentially increase the chances of curing advanced prostate cancer in the years ahead.
