Session Speaker
Preclinical Development And Clinical Translation Of Tumor-Selectively Replicating Retrovirus Vectors
Nori Kasahara
USA

The emerging field of oncolytic virotherapy holds tremendous promise for improving the outcome of gene therapy, by harnessing the replicative capabilities of viruses to be used as vectors against rapidly proliferating cancer cells. Among the different tumor-selectively replicating virus technologies now being tested as oncolytic agents, replication-competent retrovirus (RCR) vectors based on murine leukemia virus (MLV) exhibit uniquely advantageous characteristics.

First, as its capsid contains no nuclear localization signals, MLV exhibits an absolute selectivity for actively dividing cells, and MLV-based RCR vectors can achieve highly efficient and tumor-selective gene transfer at efficiencies of up to >99% as viral replication proceeds throughout entire solid tumor masses, even after initial inoculation at MOIs as low as 0.01, in a wide variety of cancer models.

Furthermore, while not intrinsically cytolytic, RCR vectors engineered with suicide genes can mediate efficient and synchronized cell killing upon prodrug administration, resulting in significant therapeutic benefit in both orthotopic xenograft and syngeneic cancer models.

Due to stable integration of the vector, residual infected cancer cells serve as a reservoir for long-term viral persistence even as they migrate to new sites, enabling multiple cycles of prodrug administration to achieve prolonged survival benefit. In addition to its intrinsic selectivity for actively dividing cells and its restriction by systemic immunity, suicide gene activation also acts as a self-terminating mechanism to limit virus spread.

Replication-defective retroviral vectors have long been a mainstay of gene therapy research, but the use of MLV-based RCR vectors has rarely been contemplated due to potential risks associated with uncontrolled virus spread. Notably, however, after intratumoral or intravenous injection in immunocompetent animals, systemic spread of RCR has been undetectable by sensitive qPCR assays in all normal tissues examined.

To further enhance the safety, efficiency, and long-term efficacy of RCR-mediated cancer gene therapy, we are pursuing the following strategies: 1) incorporation of transcriptional and post-transcriptional regulatory elements to more stringently target virus replication to tumor cells, 2) development of targetable adenovirus-RCR hybrid virus vectors to improve initial titer and tumor localization, 3) conversion of tumor-infiltrating cytotoxic T lymphocytes into 'carrier' cells for more efficient intratunoral dissemination of virus and to combine gene therapy and immunotherapy strategies.

Further testing in different tumor models, development and comparison of new vector designs, and development of GMP manufacturing, are currently being pursued through a multi-national consortium and with a newly formed biotech venture, Tocagen Inc. As of October 2009, an Investigational New Drug (IND) application has been filed with the FDA seeking approval for initiation of clinical trials in early 2010 to test the safety and efficacy of RCR-mediated suicide gene therapy in patients with recurrent glioblastoma.