Delivery of DNA vaccines against cancer

DNA vaccines against cancer have shown promising results in animal models. Efficient antigen-specific T cell responses and tumor protection have been demonstrated after vaccination with DNA encoding tumor antigens in mice. Phase I clinical trials have furthermore demonstrated that repetitive DNA vaccinations in humans are well tolerated, and that tumor antigen-specific immune responses can be induced. However, it stands clear that new adjuvants and/or delivery systems need to be explored to enhance the anti-tumor immune responses activated by DNA vaccines in humans. The aim of the studies in this thesis was to investigate different adjuvants and delivery methods for a DNA vaccine against prostate cancer (PC). PC is the most commonly diagnosed cancer in Swedish men (~ 9000 new cases/year), and the fourth leading cause of cancer-related deaths in the developed countries worldwide. Current treatments are debilitating and only available for localized disease. For men with hormone-refractory PC there is no curative treatment and the prognosis is extremely poor. Therefore, there is an urgent need for new treatment strategies. The development of immunotherapeutic vaccination protocols against PC, based on the induction of autoimmunity to prostate tissues, is attractive since the prostate is not a vital organ beyond the reproductive years. The DNA vaccine used in these studies encodes prostate-specific antigen (PSA), a protein expressed almost exclusively by normal prostate epithelial cells and PC cells. The first study of our PSA/DNA vaccine demonstrated that PSA-specific cytotoxic T lymphocytes could be induced in mice. When two cytokine adjuvants, granulocyte macrophage-colony stimulating factor (GM-CSF) and interleukin-2 (IL-2), were co-delivered with the DNA vaccine, 80 % of the mice were protected against a syngenic challenge with PSA-expressing tumor cells. Next the safety, feasibility, and biological efficacy of the PSA/DNA vaccine was evaluated in a phase I clinical trial in patients with hormonerefractory PC. The DNA vaccine was delivered in doses of 100, 300 or 900 μg together with the cytokines GM-CSF and IL-2, during five cycles with monthly intervals. No adverse effects (WHO grade >2) were observed in any patients. PSA-specific cellular responses and an increase in anti-PSA antibodies were detected in two of three patients after vaccination with the highest vaccine dose. This proved an important proof of principle, namely that tolerance to PSA in PC patients could be overcome by vaccination with PSA/DNA. The clinical study furthermore made clear that DNA vaccination must induce more effective immune responses to have an impact on survival of PC patients. To further improve PSA-specific immune responses induced by the PSA/DNA vaccine we investigated a new delivery method, in vivo electroporation (EP). When injecting the DNA vaccine intradermally and applying a short sequence (~ 3 sec) of optimized electrical pulses, the number of induced PSA-specific CD8 T cells was greatly increased. The amount of DNA necessary to induce PSA-specific T cells could be reduced by 80 % when the DNA was delivered intradermally with EP, compared to non-electroporative delivery in muscle. In preparation for a second clinical trial, where a xenogenic DNA vaccine encoding rhesus PSA will be delivered intradermally in combination with EP, an amino acid substitution in rhesus PSA creating a naturally processed and presented epitope with high HLA affinity was identified. As PSA/DNA encoding this modified epitope induces PSAspecific T cells in all human and murine T cell cultures tested, we believe that this epitope will facilitate the monitoring of vaccine efficacy in PSA/DNA vaccinated cancer patients.