NATIONAL HARBOR, MD. – Combining programmed death (PD)-1 blockade with tumor-targeted T-cell activation by a novel DNA vaccine safely enhanced antitumor immune responses in metastatic castration-resistant prostate cancer (mCRPC) patients in a randomized clinical study.

Of 26 patients with mCRPC who were evaluable for response, 13 received treatment with an investigational DNA vaccine (pTVG-HP) that encodes prostatic acid phosphatase (PAP) and concurrent PD-1 blockade, and 13 received sequential vaccination and PD-1 blockade. No difference was seen between the groups with respect to progression-free survival at 6 months, but of eight patients in the concurrent therapy arm who had measurable disease, one experienced a partial response and two experienced a reduction in tumor volume, Douglas G. McNeel, MD, PhD, reported at the annual meeting of the Society for Immunotherapy of Cancer.

“We did not see objective responses in [six patients with measurable disease in the sequential treatment arm], said Dr. McNeel, a professor at the University of Wisconsin, Madison.

Prostate specific antigen (PSA) responses, which may be a more sensitive marker, were also more common in the concurrent treatment group; 8 of 13 patients in that group had a PSA decline from baseline, and 4 of those had a decline of greater than 25% from baseline, whereas only 1 of the 13 patients in the sequential treatment arm experienced any decline in PSA vs. baseline, Dr. McNeel said.

Responses to the vaccine’s target antigen, prostatic acid phosphatase, were seen in both arms, but only those who received the combined treatment and who had evidence of immune response experienced PSA decline, he added.

Pre- and postvaccination biopsies of metastatic sites showed that concurrent treatment, compared with sequential treatment, elicited tumor-infiltrating CD8+ T cells, PD-L1 expression in tumors, and changes associated with CD8+ T-cell activation, he said, adding that immunization with concurrent PD-1 blockade also elicits changes in proliferation detected by (¹⁸F) fluorothymidine PET/CT.

“We’ve been interested in vaccines for cancer, because we know that having the right kind of T cells in the tumor microenvironment is associated with better long-term outcomes,” Dr. McNeel said, noting that the ability of vaccines to activate T cells and augment cytolytic T cells, in particular, should have anticancer activity.

However, the clinical activity of single-agent tumor vaccines has been underwhelming, he noted.

PAP has been a focus in vaccine development, because it is essentially restricted to prostate tissue in humans. A nearly identical prostate-specific rat homologue was used in early studies, and PAP permits evaluation of serum PSA as an independent assessment of response in human trials, he explained.

“It’s the same target as the sipuleucel-T vaccine,” he said, referring to a Food and Drug Administration–approved vaccine for prostate cancer(Provenge).

Two prior phase 1/2 trials looking at DNA vaccine encoding PAP in patients with early biochemically recurrent prostate cancer showed that PAP-specific T-cell immune responses were elicited and that no significant adverse events occurred.

In both trials, the development of persistent PAP-specific, interferon-gamma–secreting T cells was associated with favorable change in PSA doubling time (suggesting a possible impact on the disease), and with PD-L1 expression in circulating tumor cells (suggesting a potential mechanism of resistance), he said.

Laboratory studies helped identify mechanisms of immune resistance following DNA immunization, he said, explaining that immunization elicits T cells secreting interferon-gamma, which leads to an increase in PD-L1 expression on tumor cells.

Encoding epitopes with increased major histocompatibility complex class 1 affinity elicited CD+ t cells with increased and persistent PD-1 expression, and blockade of PD-1 or PD-L1 with vaccination led to improved antitumor responses, he said.

The findings led to the new model focused on timing of PD-1 blockade with vaccine T-cell activation studied in the current trial.

It was hypothesized that PD-1 blockade at the time of T-cell activation with vaccination would be more effective than was blockade of PD-1-regulated T cells previously elicited with vaccination.

Study subjects had mCRPC and evidence of disease progression. Previous treatment with abiraterone(Zytiga), enzalutamide(Xtandi), or chemotherapy, was allowed, but patients with prior sipuleucel-T vaccine exposure were excluded.

Patients in the concurrent treatment arm received both the vaccine and PD-1 blockade with pembrolizumab (Keytruda)over 12 weeks, and those in the sequential therapy arm received vaccination first followed by PD-1 blockade, each for 12 weeks.

Both approaches were well tolerated.

“Essentially, we saw nothing that was unexpected,” Dr. McNeel said.

Adverse events greater than grade 2 included fatigue in one patient, diarrhea in one patient, and autoimmune hepatitis in one patient. No patients discontinued treatment from toxicity, he noted.

One death occurred during follow-up in a patient who had evidence of progression and refused further follow-up, therefore it could not be determined if the death was related to treatment.

The current findings, which are notable in part because PD-1 pathway inhibitors have demonstrated little clinical activity when used as single agents for prostate cancer and which expand upon data presented in a scientific poster at SITC 2016, demonstrate that combining this blockade with tumor-targeted T-cell activation by a DNA vaccine is safe and can augment tumor-specific T cells – as detectable within the peripheral blood and by imaging – and can result in objective antitumor changes.

“To summarize, plasmid DNA vaccines can elicit antigen-specific CD8+ T cells; immunization can increase PD-L1 expression on tumor cells – and we’ve demonstrated, in mice anyway, that this is mediated by the T cells elicited with immunization; PD-1 expression increases on CD8+ T cells following vaccination; and we think this is an opportunity to use checkpoint blockade at the point of vaccination to improve antitumor responses,” Dr. McNeel said.

“So we can look at this as PD-1 blockade to improve the effect of vaccination, but we can also look at it the other way around, and that is that anti-tumor vaccines can elicit the tumor-specific CD8+ T cells needed to enable PD-1 blockade to work,” he said. “ I think this has implications for the choice of vaccine approach, the antigen, and the timing of PD-1 blockade.”

Based on these results, an expansion arm has been opened to evaluate the safety and clinical efficacy of combination treatment beyond 12 weeks, and future studies will look at the combination of two different vaccines to improve antitumor response, he said.

Dr. McNeel disclosed financial relationships (intellectual property rights/patent holder, consultant, ownership interest) with Madison Vaccines Inc. The study is funded by a 2014 Movember PCF Challenge Award and Madison Vaccines.

sworcester@frontlinemedcom.com

SOURCE: McNeel D et al., J Immunother Cancer. 2017 5(Suppl 2):86 Abstract O11.

DNA vaccine + PD-1 blockade shows promise in mCRPC