Driving Prostate Cancer Innovation Through Immunotherapy With OS Therapies' Dr. Robert Petit

By Erin Harris, Editor-In-Chief, Cell & Gene
Follow Me On Twitter @ErinHarris_1

Prostate cancer remains one of the most widespread and deadly cancers affecting men worldwide, despite decades of progress in screening and treatment. With more than 3.3 million men in the United States living with a history of the disease, innovations in immunotherapy and targeted approaches could represent the next major breakthroughs. To discuss this evolving landscape and the role of OS Therapies’ experimental platform, OST-504, I caught up with Dr. Robert Petit, Chief Medical and Scientific Officer of OS Therapies.

The Immune System’s Role in Prostate Cancer

“The link between prostate cancer’s development, surveillance, and treatment with immunity and immunotherapy is indeed interesting,” Dr. Petit said. While prostate cancer is primarily driven by androgens, the immune system still plays a central role in restraining its progression. “Shortly after patients begin androgen deprivation therapy (ADT), there is a significant influx of immune cells into the prostate tissue,” he said. “Presumably the immune cells are there to recognize, eliminate, and resist the spread of the cancerous cells. In many patients, ADT takes away the growth stimulus and enables the immune system to control prostate cancer progression.”

This immune surveillance explains why the only FDA-approved cancer vaccine, sipuleucel-T, approved in 2010, was developed for prostate cancer. “Sipuleucel-T gathers a patient’s own immune cells and trains them to better recognize prostate cancer, which can read out as prolonged survival. The activity of this treatment reinforces the fact that immune activity is necessary to resist the progression of prostate cancer,” Dr. Petit stated.

Yet checkpoint inhibitors, the mainstay of immunotherapy in many solid tumors, have not shown the same success in prostate cancer. “Checkpoint inhibitors can only amplify cells that have been previously trained against the cancer,” Dr. Petit said. “If there are not already some immune cells that target prostate cancer, checkpoint inhibitors are not likely to help.”

Instead, Dr. Petit pointed to the emergence of radioligand therapy as a powerful development. “Perhaps the most promising recent development has been the activity of Pluvicto,” he shared. “Radiation causes damage to prostate cancer cells, which in turn attracts the attention of the immune system. The common thread between sipuleucel-T and Pluvicto is generating immunity, preferably T cell immunity, against prostate cancer.”

Advancing Immunotherapy with OST-504

Against this backdrop, OS Therapies is developing OST-504, which has been designed to prime the immune system against multiple known common prostate cancer targets as well as some commonly shared mutations. The investigational therapy uses a live attenuated bacterial vector to engage the immune system, generating T cell responses that identify and attack micrometastases, which are tiny cancer “seeds” that hide throughout the body.

“For patients undergoing biochemical progression of prostate cancer, the T cells generated by OST-504 are intended to find the hiding micrometastases and control them immunologically, eliminating or slowing the progression of what could become a recurrence of metastatic prostate cancer.”

Proactive Options for Patients

One reason investigators and patients have shown strong enthusiasm for OST-504 is the lack of proactive options for men with biochemical progression where biomarkers rise but tumors are undetectable. “This situation is a clinical challenge,” Dr. Petit explained. “Patients are told to just ‘watch and wait’ until it progresses to the point where it merits a more severe type of treatment like chemotherapy. For many patients who want to be proactive, this is very frustrating. OST-504 was well tolerated and offered a treatment option that was more proactive than ‘watch and wait’.”

He also emphasized that early trial enrollment challenges left some patients without access to the therapy, which has fueled demand for expanded access programs.

Platform Potential and Regulatory Acceleration

Looking further ahead, OST-504 may benefit from a regulatory innovation: the FDA’s new Platform Designation. If another OS Therapies asset, OST-HER2, receives approval, the company may be able to accelerate OST-504 development through cross-referenced filings. “The manufacturing process for OST agents is quite modular and uses a lot of redundant processes,” he said. “A platform designation would allow us to cross-reference our products in regulatory filings to expedite the drug development and review process, streamlining communication with FDA and ultimately accelerating availability.”

The Road Ahead for OST-504

As OS Therapies prepares for a Q4 2025 data readout, the team is already envisioning broader applications for OST-504. “Building on the initial success of our trial in biochemical progression would be the next logical step,” Dr. Petit said. “This could be everywhere from combining ADT and/or radiation therapy in newly diagnosed locally advanced prostate cancer all the way to later-stage metastatic castrate-resistant disease in combination with chemotherapy, Pluvicto, or checkpoint inhibitors.”

Harnessing the immune system to provide new and accessible options for patients battling prostate cancer remains mission-critical for Dr. Petit and his team. “Wherever patients may benefit from having a better immune response against prostate cancer is subject for further development.”