Employing Tumor Hypoxia for Oncolytic Therapy in Breast Cancer

Hypoxia is a common tumor condition associated with metastases, therapeutic resistance, and poor patient survival. Forty percent of breast cancers are hypoxic, with a median oxygen concentration of 3.9%, and a third of tumors have regions less than 0.3%. Normal breast tissue is reported to have oxygen concentrations greater than 9%. This tumor hypoxia in breast cancer confers resistance to conventional radiation therapy and chemotherapy, as well as making estrogen-receptor-positive tumors less sensitive to hormonal therapy. Novel treatment modalities are needed to target hypoxic tumor cells. Lower tumor oxygen levels compared with surrounding normal tissues may be utilized to target and enhance herpes oncolytic viral therapy in breast cancer. Attenuated oncolytic herpes simplex viruses offer a unique cancer treatment by specifically infecting, replicating within, and lysing tumor cells. They carry genetically engineered mutations to reduce their virulence and attenuate their ability to infect normal tissues. Studies have shown the safety and efficacy of oncolytic herpes simplex viruses in treating breast cancer both in humans and in preclinical models. The placement of essential viral genes under the control of a hypoxia-responsive enhancer, which is upregulated selectively in hypoxic tissue, represents a promising strategy to target oncolytic viruses precisely to hypoxic cancer cells. In this review we describe strategies to harness hypoxia as a trigger for oncolytic viral gene expression in breast cancer, thereby increasing the specificity of viral infection, replication, and cytotoxicity to hypoxic areas of tumor. Such a targeted approach will increase efficacy in the therapy of hypoxic tumors while achieving a reduction in total dose of viral therapy.Supported in part by AACR-Astra Zeneca Cancer Research and Prevention Foundation Fellowship (P.S.A), grants RO1 CA 75416 and RO1 CA/DK80982 (Y.F.) from the National Institutes of Health, grant MBC-99366 (Y.F.) from the American Cancer Society, grant BC024118 from the US Army (Y.F.), grant IMG0402501 from the Susan G. Komen Breast Cancer Foundation (Y.F. and P.S.A.) and grant 032047 from Flight Attendant Medical Research Institute (Y.F. and P.S.A.)