Abstract

Prostate stem cell antigen (PSCA) and prostate-specific antigen (PSA) are overexpressed in most prostate cancers. PSCA- and PSA-derived, HLA-A2 binding peptides are specific targets for T cell responses in vitro. A phase I/II trial was performed to demonstrate feasibility, safety, and induction of antigen-specific immunity by vaccination with dendritic cells (DC) presenting PSCA and PSA peptides in patients with hormone- and chemotherapy-refractory prostate cancer. Patients received four vaccinations with median of 2.7x10e7 peptide-loaded mature DC s.c. in biweekly intervals. Twelve patients completed vaccination without relevant toxicities. Six patients had stable disease (SD) after four vaccinations. One patient had a complete disappearance of lymphadenopathy by ultrasound despite rising PSA. Regarding PSA kinetics, in one patient, the log PSA slope became negative after treatment, and three patients had significant decreases in the log PSA slopes. However, duration of response was short, with a median time to progression of 65 days (12–223). Four patients with SD and one progressor developed a positive DTH after the 4th vaccination. With a median survival of all patients of 13.4 months, DTH-positivity was associated with significantly superior survival (22 vs. 8 months, p=0.003). HLA tetramer analysis detected high frequencies of peptide-specific T cells after two vaccinations in one patient who was also the sole responder to concomitant hepatitis B vaccination as an indicator of immune competence and survived 27 months after start of vaccination. Taken together, vaccination with PSA/PSCA peptide-loaded, autologous DCs may induce cellular responses primarily in immunocompetent patients which were associated with superior outcome. The retardation of PSA kinetics upon vaccination suggest a causal influence of the treatment on the natural course of the disease. Testing of DC-based vaccination is warranted for patients at earlier stages of prostate cancer.

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