Year 1

The goal of this project is to develop clinically translatable methods for engineering human mesenchymal stem cells (hMSC) to serve as tumor-homing cellular carriers that will deliver a replication-competent retrovirus (RCR) vector throughout primary brain tumors (gliomas). RCR vectors expressing a prodrug activator (also known as a “suicide gene”), which converts a non-toxic “pro-drug” compound into a potent chemotherapy drug directly generated within the infected tumor cells, have recently initiated testing in Phase I/II clinical trials for suicide gene therapy of recurrent high-grade gliomas. We are examining whether MSCs can serve as producer cells for this RCR vector, and whether the tumor transduction efficiency and therapeutic efficacy of this vector can be significantly enhanced, without compromising its safety profile, hMSC-based RCR producer cells (MSC-RCR) are used as a tumor-homing mobile carrier system that releases the virus as the cells migrate toward and within tumor masses in the brain. In particular, we are comparing this MSC-RCR cell-based carrier method against conventional delivery methods by direct intratumoral injection of ‘naked’ virus, in subcutaneous and intracranial brain tumor models.

To date, we have accomplished our milestone tasks for Year 1, by:
– successfully developing efficient methods to transduce hMSCs with RCR vectors and thereby convert them into vector producer cells
– developing and comparing in vitro and in vivo assays to evaluate the tumor-homing migratory activity of hMSCs
– applying these assays to screen and evaluate commercially available hMSC isolates
– demonstrating that the MSC-RCR delivery system can achieve significantly more efficient transduction of subcutaneous glioma models as compared to virus by itself
– confirming that enhanced transduction efficiency by MSC-RCR achieves more rapid tumor growth inhibition, as compared to ‘naked’ RCR alone, when applied to suicide gene therapy in subcutaneous tumor models of human glioma
– confirming that hMSC-mediated RCR delivery does not increase vector biodistribution to normal tissues, nor incur any increased risk of secondary leukemogenesis

Interestingly, through these studies we have found considerable variability in tumor-homing migration activity and intratumoral migration activity between hMSC isolates from different sources, a finding that may have significant implications for the development of hMSC-based clinical products. We are continuing to characterize additional hMSC isolates from various tissue sources, and are preparing a manuscript to publish these results.

Furthermore, based on our favorable results as described above, indicating the enhanced efficiency of tumor transduction and growth inhibitory effects when suicide gene therapy is delivered by MSC-RCR, as compared to RCR alone, we have fulfilled the success criteria for each of our milestone tasks in Year 1, and are currently proceeding with Year 2 studies.