A phase I trial of DNA vaccination with a plasmid expressing prostate-specific antigen in patients with hormone-refractory prostate cancer

Abstract

Prostate-specific antigen (PSA) is a serine protease secreted at low levels by normal luminal epithelial cells of the prostate and in significantly higher levels by prostate cancer cells. Therefore, PSA is a potential target for various immunotherapeutical approaches against prostate cancer. DNA vaccination has been investigated as immunotherapy for infectious diseases in patients and for specific treatment of cancer in certain animal models. In animal studies, we have demonstrated that vaccination with plasmid vector pVAX/PSA results in PSA-specific cellular response and protection against tumour challenge. The purpose of the trial was to evaluate the safety, feasibility and biological efficacy of pVAX/PSA vaccine in the clinic. A phase I trial of pVAX/PSA, together with cytokine granulocyte/macrophage-colony stimulating factor (GM-CSF) (Molgramostim) and IL-2 (Aldesleukin) as vaccine adjuvants, was carried out in patients with hormone-refractory prostate cancer. To evaluate the biologically active dose, the vaccine was administered during five cycles in doses of 100, 300 and 900 microg, with three patients in each cohort. Eight patients were evaluable. A PSA-specific cellular immune response, measured by IFN-gamma production against recombinant PSA protein, and a rise in anti-PSA IgG were detected in two of three patients after vaccination in the highest dose cohort. A decrease in the slope of PSA was observed in the two patients exhibiting IFN-gamma production to PSA. No adverse effects (WHO grade >2) were observed in any dose cohort. We demonstrate that DNA vaccination with a PSA-coding plasmid vector, given with GM-CSF and IL-2 to patients with prostate cancer, is safe and in doses of 900 microg the vaccine can induce cellular and humoral immune responses against PSA protein.

Figure 1

Flow chart of clinical study. Vaccine (pVAX/PSA) was administered in doses 100, 300 and 900 μg (90% i.m. and 10% i.d.). Recombinant GM-CSF and IL-2 were administered s.c.

Figure 2

Induction of T-cell response to recombinant PSA, prior to and postvaccination, measured as IFN-γ production by ELISA after restimulation with DC loaded with PSA protein. Nonspecific reactivity against a control protein (HSA) has been subtracted from each individual sample. Patient groups vaccinated per cycle with 100 μg (Pat#1–3), 300 μg (Pat#4–6) and 900 μg (Pat#7–9) of pVAX/PSA are shown in panels (A, B and C), respectively. Prior and post study values are presented if otherwise not stated. Bars represent mean values of two individual experiments with standard deviations.

Figure 3

Induction of IgG antibody against recombinant PSA in serum measured at 1 : 100 dilution by ELISA prior to and post five cycles of vaccination. Patient groups vaccinated per cycle with 100 μg (Pat#1–3), 300 μg (Pat#4–6) and 900 μg (Pat#7–9) of pVAX/PSA are shown in panels (A, B and C), respectively. Bars represent mean of duplicates with standard deviations.

Figure 4

Prostate-specific antigen levels in patient serum measured by standard fluoroimmunoassay. Patient groups vaccinated per cycle with 100 μg (Pat#1–3), 300 μg (Pat#4–6) and 900 μg (Pat#7–9) of pVAX/PSA are shown in panels (A, B and C), respectively. Arrows (↓) depict time of individual DNA vaccine administration.