A Phase 1b study evaluating an investigational immunotherapy known as OST-504for biochemically recurrent prostate cancer has completed its last patient visit, with results expected later in 2025. The open-label trial enrolled seven men whose disease recurred after radical prostatectomy or radiation therapy and who were not receiving androgen deprivation therapy at enrollment. Treatment was administered intravenously every four weeks for six doses, followed by four maintenance infusions every 12 weeks, with safety, PSA kinetics, and immunologic activity among the prespecified assessments.

OST-504, previously referred to as ADXS-504, is a live attenuated Listeria monocytogenes-based construct that delivers a truncated listeriolysin O fusion comprising 24 prostate cancer-associated antigens, including commonly expressed targets and selected mutation “hotspots” intended to broaden patient coverage.

The platform is designed to elicit antigen-specific T cell responses and to modulate the tumor microenvironment by reducing regulatory T cells and myeloid-derived suppressor cells, an approach supported by preclinical data and early human experience with related Listeria vectors in prostate cancer.

In a nutshell: when  this therapy is introduced into the body, the immune system recognizes the bacterial vector and the attached cancer proteins, which trains immune cells called T cells to identify and attack prostate cancer cells specifically. This process also helps reduce some immune cells that normally protect the tumor, allowing the immune attack to be stronger. As cancer cells are destroyed, more targets are revealed, which further activates the immune system in a cycle to help control the cancer over time.

The current trial targets men with biochemical recurrence, a population for whom the optimal timing and intensity of systemic therapy remain under study, particularly given the toxicities associated with androgen deprivation therapy. Investigators have reported strong site interest in expanded access following treatment, but no efficacy conclusions have been released pending the formal readout. Data from the completed cohort will be reported in the fourth quarter of 2025, which may clarify safety, immunogenicity, PSA dynamics, and time-to-progression measures relevant to subsequent development.

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