AT SITC 2016
National Harbor, MD. (FRONTLINE MEDICAL NEWS) – To date, checkpoint inhibitors have shown little clinical activity as single agents against metastatic, castration-resistant prostate cancer, but a combination of a DNA vaccine and a programmed-death 1 inhibitor shows promise for enhancing anti-tumor immune responses, report investigators in a phase I trial.
“If you vaccinate animals, PD-1 expression transiently goes up, and if you block it at that point you get a better anti-tumor response, and that was in models where anti PD-1 therapy alone didn’t do anything. So we thought this could be a good approach for prostate cancer,” Douglas G. McNeel, MD, PhD, of the University of Wisconsin, Madison, said in an interview at the annual meeting of the Society for Immunotherapy of Cancer.
Dr. McNeel and colleagues are exploring the therapeutic potential of combining the PD-1 inhibitor pembrolizumab (Keytruda) with an investigational DNA vaccine targeted against prostatic acid phosphatase (PAP), the same antigen targeted by sipuleucel-T (Provenge).
They presented data in a scientific poster from a pilot study of the combination in patients with metastatic, castration-resistant prostate cancer (mCRPC).
Vaccine ramps up PD-1 expression
The investigators had previously shown that patients immunized with a DNA vaccine encoding PAP (pTVG-HP; currently in phase II clinical trials) developed PD-1-regulated, PAP-specific T cells and had increased PD-L1 expression in circulating tumor cells. They also demonstrated in preclinical studies with mouse models that increased PD-1 expression on vaccine-induced CD8-positive T cells led to inferior anti-tumor immune responses, and that blocking PD-1 at the time of T-cell activation with vaccine improved anti-tumor responses.
“The current pilot trial was designed to evaluate whether delivery of PD-1 blockade after immunization (when PD-1-regulated T cells are elicited and when PD-L1 expression is induced with vaccination on tumor cells) or with immunization (when PD-1 expression increases on antigen-specific CD8+ T cells with activation) results in anti-tumor responses in patients with advanced, metastatic, castration-resistant prostate cancer,” Dr. McNeel and his colleagues wrote.
They reported preliminary impressions of the efficacy and safety of the combination in 12 patients with mCRPC. The patients all had evidence of progressive disease and were at least 6 months out from chemotherapy. Patients could have previously been treated with enzalutamide (Xtandi) or abiraterone (Zytiga), but were excluded if they had received sipuleucel-T vaccine.
Patients in the ongoing trial are randomized to receive six doses of pTVG-HP over 10 weeks with either four doses of concurrent pembrolizumab, or four doses of pembrolizumab delivered every 3 weeks beginning 2 weeks after the last vaccine dose.
The authors reported that treatment with the combination evokes changes in serum prostate-specific antigen (PSA) that are associated with objective radiographic changes, and “elicits robust and persistent PAP-specific Th1-based immunity.”
They also found evidence to suggest that concurrent administration of the vaccine and the PD-1 inhibitor may be more effective than sequential administration, based on differences in serum PSA and PAP.
Adverse events were generally mild and similar to those seen in other studies of pembrolizumab and DNA vaccine, with only three grade 3 events and no grade 4 events.
The investigators hope to expand the pilot study beyond the 12 weeks originally scheduled, and plan to explore potential biomarkers for vaccine response based on T-cell proliferation in tumors and in regional lymph nodes as seen on imaging modalities.
The study is funded by a 2014 Movember Prostate Cancer Foundation Challenge Award and Madison Vaccines Inc. Dr. McNeel reports an ownership interest and funding support from Madison Vaccines. All other coauthors reported no conflicts of interest.
