Concurrent, but not sequential, PD-1 blockade with a DNA vaccine elicits anti-tumor responses in patients with metastatic, castration-resistant prostate cancer

Abstract

T-cell checkpoint inhibitors have demonstrated dramatic clinical activity against multiple cancer types, however little activity in patients with prostate cancer. Conversely, an anti-tumor vaccine was approved for the treatment of prostate cancer, having demonstrated an improvement in overall survival, despite few objective disease responses. In murine studies, we found that PD-1 expression on CD8+ T cells increased following anti-tumor vaccination, and that PD-1/PD-L1 blockade at the time of immunization elicited greater anti-tumor responses. Based on these data we initiated a pilot trial evaluating the immunological and clinical efficacy of a DNA encoding prostatic acid phosphatase (PAP) when delivered in combination with pembrolizumab. 26 patients were treated for 12 weeks with vaccine and received pembrolizumab either during this time or during the subsequent 12 weeks. Adverse events included grade 2 and 3 fatigue, diarrhea, thyroid dysfunction, and hepatitis. Median time to radiographic progression was not different between study arms. 8/13 (62%) of patients treated concurrently, and 1/12 (8%, p=0.01) of patients treated sequentially, experienced PSA declines from baseline. Of these, two were over 50% and one was a complete PSA response. No confirmed CR or PR were observed, however 4/5 patients treated concurrently had measurable decreases in tumor volume at 12 weeks. PSA declines were associated with the development of PAP-specific Th1-biased T cell immunity and CD8+ T cell infiltration in metastatic tumor biopsy specimens. These data are the first report of a clinical trial demonstrating that the efficacy of an anti-tumor vaccine can be augmented by concurrent PD-1 blockade.

Keywords: DNA vaccine; PD-1; pembrolizumab; prostate cancer; prostatic acid phosphatase.

Conflict of interest statement

CONFLICTS OF INTEREST DGM has ownership interest, has received research support, and serves as consultant to Madison Vaccines, Inc which has licensed material described in this manuscript and supported this trial. None of the other authors have relevant potential conflicts of interest.

Figures

Figure 1. Schema

Shown is the treatment schema.

Figure 2. Immunological response - IFNγ and Granzyme B ELISPOT

Peripheral blood mononuclear cells were collected from all subjects at baseline, 6 weeks, 12 weeks, and 24 weeks and evaluated for antigen-specific IFNγ or granzyme B secretion by ELISPOT. (A) Shown are immune responses to PAP protein or peptide library, PSA peptide library (non-specific control), and tetanus (positive control) for patients grouped by study arm. A positive antigen-specific response was defined as a statistically significant response (compared with media control) that was at least 3-fold over the baseline value and with a frequency of at least 1/100,000 cells. Red squares indicate positive responses, black indicates no response, and white indicates no data. (B) Shown are individual ELISPOT data for the three individuals (two from Arm 1 – 10004 and 10006, and one from Arm 2 – 20006) who exhibited PAP-specific immunity at all post-treatment time points in panel A.

Figure 3. Clinical effects

(A) Kaplan-Meier plot of time to radiographic progression by study arm. Time to radiographic regression was defined as the time from randomization to the date of documented radiographic progression or last available follow-up date. (B) Serum PSA values were collected from all individuals prior to treatment and over the course of treatment. Percent changes in serum PSA values were evaluated from day 1 of study. Blue lines show individual patients treated in Arm 1, and red lines show individual patients treated in Arm 2. (C) Shown are CT images collected at baseline and 24 weeks (top panel) or 12 weeks (bottom panel) post-treatment for two individuals treated in Arm 1. Arrows point to lymph node metastases. (D) Percent changes in serum PSA values were evaluated from day of beginning pembrolizumab treatment (day 1 for patients in Arm 1, and week 12 for patients in Arm 2). (E) Best % change in serum PSA from day 1 of study. Asterisks indicate those patients who had evidence of PAP-specific Th1 immunity (significant IFNγ and/or granzyme B response detected at least twice post-treatment, Figure 2).

Figure 4. Tissue and correlative studies

(A) Metastatic tissue biopsies obtained pre-treatment and at 12 weeks were evaluated by immunohistochemistry for CD8+ T cells and PD-L1 expression. Shown are representative sections from four individuals, two from each study treatment arm. 25μm ruler is shown in bottom right panel. (B) CD8+ T cell numbers in pre-treatment and post-treatment biopsy specimens were quantified by investigator blinded to treatment. Shown are the mean and standard deviation of CD8+ cell counts per mm2 from five regions per tumor section.