Abstract 5699: Mesenchymal stem cells (MSCs) as a selective delivery vehicle for a PSA-activated protoxin for advanced prostate cancer
Authors: Chen Shuangling , Brennen Nathaniel , Brodsky Robert A. , Cheng Linzhao , Denmeade Samuel R. , Isaacs John T. 
Published: August 2012

Circulating bone marrow-derived Mesenchymal Stem Cells (MSCs) can differentiate into cells of the mesoderm lineage and have an innate tropism for tumor tissue in response to the inflammatory microenvironment present in malignant lesions. MSCs have been detected in the perivascular space of many tumors, including prostate. MSCs are inherently non-immunogenic, which prevents allogeneic MSCs from being rejected by host defense mechanisms. This immune-privileged status, together with their oncotropic properties, makes possible the infusion of allogeneic MSCs into patients for therapeutic purposes. PRX302 is a PSA-activated aerolysin-based protoxin that forms membrane pores and leads to necrosis by a proliferation-independent mechanism at low picomolar concentrations. Active PSA is bound to inhibitors, such as A2M, in the plasma, and therefore, is only present in an active form at sites of local and metastatic prostate cancer. Consequently, PRX302 will only be converted to a toxic form in the tumor microenvironment and thereby, limit off-target toxicity. Based upon this rationale we hypothesize that MSCs can be used as a cell-based targeting vehicle to selectively deliver therapeutic agents, such as PRX302, to primary and metastatic sites of prostate cancer, and thus spare host toxicity. We have demonstrated that fluorescently-labeled MSCs “home” to sites of prostate cancer in nude mice bearing castration-resistant CWR-22RH xenografts and a significant fraction are localized to the perivascular space. This co-localization with the tumor vasculature suggests that a MSC-delivered cytotoxin would have profound anti-tumor efficacy through effects on both cancer and endothelial cells. Furthermore, we have confirmed that MSCs and stromal cells derived from human prostate cancers (PrCSCs) in men older than 50 (&gt;10 tested), but not those isolated from normal prostates in men under 30 (&gt;7 tested), can differentiate into adipocytes, osteoblasts, and chondrocytes under the appropriate conditions. Furthermore, these PrCSCs are positive for CD90, CD73, CD105, and FAP, but do not express CD34, CD45, CD11b, CD19, or HLA-DR, which is consistent with the hMSC phenotype and suggests that MSCs migrate to cancerous prostates in the human as well. Importantly, PRX302 binds with low nanomolar affinity to GPI-anchor proteins, which are highly expressed on the surface of all mammalian cells. Using PIG-A-targeted zinc finger nucleases (ZFNs) to knockout GPI-anchor synthesis, we have demonstrated that GPI-anchor-deficient MSCs retain their tumor homing properties. Therefore, MSCs can be genetically modified to endogenously express PRX302 from the PIG-A locus as a ‘safe harbor’ and prevent self-sterilization due to protoxin secretion. The therapeutic efficacy and host toxicity of these PRX302-expressing MSCs will be evaluated against a series of human prostate cancer xenograft models.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5699. doi:1538-7445.AM2012-5699