Abstract B21: Mesenchymal stem cells (MSCs) as a selective delivery vehicle for a PSA-activated protoxin for advanced prostate cancer
Authors: Brennen W. Nathaniel , Denmeade Samuel R. , Isaacs John T. 
Published: May 2013

Abstract
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. We have demonstrated that endogenous MSCs migrate to sites of prostate cancer with many of these cells located in the perivascular space similar to observations in multiple other tumor types. MSCs are inherently non-immunogenic, which prevents rejection of allogeneic MSCs 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. Currently, MSCs are being tested in numerous FDA-approved clinical trials for a variety of human diseases and have been safely administered to thousands of patients. PRX302 is a PSA-activated aerolysin-based protoxin that forms membrane pores and leads to necrosis by a proliferationindependent mechanism at low picomolar concentrations. Active PSA is bound to inhibitors in the plasma, and therefore, is only active 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 allogeneic MSCs can be used as a cell-based targeting vehicle to selectively deliver therapeutic agents, such as PRX302, to sites of prostate cancer, and thus spare host toxicity. To achieve optimal delivery of therapeutic concentrations, we are genetically engineering MSCs to endogenously express and secrete the protoxin from a defined genomic locus, or “safe harbor”, using zinc finger nucleases (ZFNs). ZFNs are hybrid proteins generated by fusing the sequence-specific DNA-binding domain of zinc-finger proteins to the non-specific endonuclease domain of the Fok1 restriction enzyme. The nuclease is fused to either the sense or antisense sequence-specific DNA-binding domain to create an obligate heterodimer at the safe harbor gene, which results in a double strand break. When combined with a third vector containing a gene of interest (PRX302) flanked by linkers encoding a complementary sequence to the “safe harbor” gene, homologous recombination occurs.
Importantly, PRX302 binds with low nanomolar affinity to GPI-anchor proteins, which are highly expressed on the surface of all mammalian cells. PIG-A is the first enzyme in GPI-anchor biosynthesis. Using PIGA-targeted ZFNs, 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 are initially being evaluated in preclinical proof-of-principle studies against a series of human prostate cancer xenograft models.
Translation of gene therapy strategies into the clinic has been slow due to legitimate concerns surrounding their safety, largely as a result of random vector insertion, which can be alleviated by the targeted insertion of genes into “safe harbors” using ZFNs. Additionally, clinical translation of this approach will be further expedited due to the fact that PRX302 has already been given safely to more than 120 patients and has undergone extensive toxicology profiling during its clinical development as a local therapy for symptomatic BPH, for which it is currently entering phase III trials. Therefore, we propose that a systemic delivery platform for PRX302 can be created through the use of allogeneic MSCs as a vehicle for the selective delivery of PRX302 to sites of local and metastatic prostate cancer.
Citation Format: W. Nathaniel Brennen, Samuel R. Denmeade, John T. Isaacs. Mesenchymal stem cells (MSCs) as a selective delivery vehicle for a PSA-activated protoxin for advanced prostate cancer [abstract]. In: Proceedings of the AACR Special Conference on Advances in Prostate Cancer Research; 2012 Feb 6-9; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(4 Suppl):Abstract nr B21.