The current study demonstrated that sequential vaccinations with PROSTVAC-V followed by PROSTVAC-F, including gene sequences for PSA and the three co-stimulatory molecules termed TRICOM, could be administered safely to men with androgen-independent prostate cancer. All patients demonstrated a rise in anti-vaccinia virus antibody titers without a detectable increase in anti-PSA titers. Four patients had stable post-vaccination PSA values at 8 week follow-up, with minimal decrease in two patients.
As a means to induce an immune response, the poxviruses are among the most commonly studied vectors for gene delivery . Advantages of the poxviral vectors include the large size of the genomes, which have allowed as many as seven transgenes to be expressed in a single vaccinia virus vector, and greater immunogenicity of expressed proteins compared to native protein, attributed to the inflammatory response triggered against highly immunogenic viral proteins. PSA has also been commonly studied as an immunological target in prostate cancer [11, 12]. Phase I clinical trials have evaluated the safety and biological effects of a vaccinia virus expressing human PSA in patients with prostate cancer without TRICOM [2–4]. Our prior study using fowlpox and vaccinia PSA-based vaccines without TRICOM in sequence demonstrated forty-one of sixty-four (64.1%) evaluable patients were free of PSA or clinical progression at 6 months with a trend favoring the arm with vaccinia followed by fowlpox vaccines. Although PSA declines of >50% were not noted, in one treatment arm, PSA doubling time changed from 4.5 months to 30.9 months supporting the hypothesis that the vaccine was slowing disease progression . In the current study, patients were not assessed prior to therapy to determine changes in PSA velocity, since the main endpoint was to demonstrate safety of the PSA vaccine prime and boost approach with TRICOM for future studies.
A number of additional co-stimulatory molecules on antigen presenting cells have been identified including ICAM-1, B7.1, and LFA-3. Constructs using poxviral vectors (fowlpox and vaccinia) have been generated that contain this triad of co-stimulatory molecule transgenes, and have been given the designation TRICOM. Preclinical studies using TRICOM constructs have suggested that they are superior to those constructs that contain only one or two of the co-stimulatory molecules [7, 8]. Studies have been completed combining CEA pox vaccines with TRICOM demonstrating safety of this approach. A Phase I clinical study has recently been completed employing vaccinia-CEA/TRICOM and fowlpox-CEA/TRICOM in a diversified prime and boost regimen, with most patients enrolled with GI malignancies . The current study demonstrates that the PSA-based vaccine TRICOM has minimal toxicity. In contrast to previous trials using vaccinia virus or heterologous prime-boost vaccination with vaccinia followed by fowlpox viruses expressing PSA without TRICOM, which demonstrated effects on PSA velocity, this trial did not utilize repeated booster immunizations [4, 6]. The current trial was designed to test the safety and feasibility of co-expressing PSA and TRICOM in patients with androgen-independent prostate cancer. A larger trial would be needed to determine clinical endpoints and should include on-going booster vaccinations in responding patients to determine long-term toxicity, biochemical response, and disease-free survival.
The current study also demonstrated increased anti-vaccinia antibody without any increase in anti-PSA antibody. In prior trials, an interferon-γ ELISPOT assay was used to monitor T cell responses against PSA in patients who were HLA-*A0201 positive, since there is a well defined HLA-A2-restricted peptide for in vitro targeting. Patients enrolled in the current trial were not HLA restricted and a validated T cell assay was not available. Therefore, we chose to measure the anti-PSA antibody response. Similar to previous trials, we failed to demonstrate an increase in anti-PSA antibody. In the randomized Eastern Cooperative Oncology Group Phase II study, there were no significant increases in anti-PSA antibody titers detected despite 46% of patients demonstrating an increase in PSA-reactive T-cells, and 45% biochemical progression-free survival at 19 months . The reason for the absence of anti-PSA antibody response may relate to the sensitivity of the assay or to the presence of antigen-antibody complexes in the serum of men with elevated levels of PSA. Future studies with these vaccines may need to focus on the assessment of PSA-reactive T-cells and the determination of circulating immune complexes.
In summary, these data demonstrated the feasibility and safety of the PSA vaccine using a prime with vaccinia (PROSTVAC-V) followed by a boost with fowlpox (PROSTVAC-F) in which both vectors contain the gene sequences for the three co-stimulatory molecules, B7-1, ICAM-1, and LFA-3 (TRICOM) in men with androgen-independent prostate cancer. These data support further assessment of this vaccine approach in phase II and phase III clinical studies.