Abstract 2836: Metronomic cyclophosphamide enhances antitumor immune response induced by a DepoVaxTM-based peptide vaccine.
Authors: Weir Genevieve M. , Stanford Marianne M. , Berinstein Neil L. , Karkada Mohan , Liwski Robert S. , Mansour Marc 
Published: August 2014

Abstract
Advanced cancers utilize several mechanisms to escape immune-mediated detection and destruction thus reducing the effectiveness of cancer therapeutics on multiple levels. Low dose cyclophosphamide (CPA) has been investigated as an immune modulator to potentiate active immune response towards tumor cells, but a single bolus injection of CPA has been ineffective at boosting response to cancer vaccines in most clinical trials. We sought to determine if a metronomic regimen of low dose CPA (mCPA) could enhance the efficacy of a peptide-based vaccine formulated in DepoVaxTM (DPX). DPX is a novel vaccine platform comprised of liposomes in oil that is formulated with an adjuvant, universal T-helper peptide and specific tumor peptide antigens. Using a murine tumor model (C3) we tested the benefit of combining mCPA and vaccination for controlling well established subcutaneous solid tumors in vivo. Mice were treated starting 5 days after C3 tumor implantation with mCPA administered in drinking water in a one week on/ off schedule and vaccinated every 3 weeks. In this model, neither mCPA nor vaccine treatments alone were sufficient to eradicate advanced tumors. However, combining mCPA with vaccination provided effective tumor control. Ex vivo analysis of immune response after a single treatment of mCPA combined with vaccine showed a significant increase in antigen-specific IFN-γ release by ELISPOT, which became more pronounced after multiple rounds of treatment in tumor bearing mice. This immune response was accompanied by a corresponding increase in antigen-specific cytotoxic T cell activity as measured by in vivo CTL assay. Dextramer analysis of the antigen-specific CD8+ T cells in the vaccine-draining lymph node revealed that while the total number of CD8+ T cells decreased with mCPA treatment, the number of antigen-specific CD8+ T cells was increased by vaccination and not reduced by mCPA. Flow cytometric analysis of splenocyte cell populations did not show any significant reduction in CD4+CD25hiFoxP3+Treg populations due to mCPA treatment, as previously reported by others, but we did find an increase in myeloid-derived suppressor cells due to mCPA treatment that was decreased in mice treated with both mCPA & vaccination. Finally, adoptive transfer of T cells from tumor bearing mice treated with mCPA & vaccination significantly reduced tumor growth in tumor challenged recipients. Overall, our data indicate that mCPA combines with a peptide based DPX vaccine to enhance the T cell response to antigen that ultimately results in an improved immunotherapy for cancer. The immune-enhancing effects of mCPA in this model are multi-faceted and augment the T cell responses independent of direct cytotoxic effects on the tumor. The results of this pre-clinical study have assisted in the designing of a phase II clinical trial for DPX-Survivac vaccine in combination with mCPA in ovarian cancer patients.
Citation Format: Genevieve M. Weir, Marianne M. Stanford, Neil L. Berinstein, Mohan Karkada, Robert S. Liwski, Marc Mansour. Metronomic cyclophosphamide enhances antitumor immune response induced by a DepoVaxTM-based peptide vaccine. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2836. doi:10.1158/1538-7445.AM2013-2836