Biological purging of breast cancer cells using an attenuated replication-competent herpes simplex virus in human hematopoietic stem cell transplantation

Autologous hematopoietic stem cell transplantation after myelosuppressive chemotherapy is used for the treatment of high-risk breast cancer and other solid tumors. However, contamination of the autologous graft with tumor cells may adversely affect outcomes. Human hematopoietic bone marrow cells are resistant to herpes simplex virus type 1 (HSV-1) replication, whereas human breast cancer cells are sensitive to HSV-1 cytotoxicity. Therefore, we examined the utility of G207, a safe replication-competent multimutated HSV-1 vector, as a biological purging agent for breast cancer in the setting of stem cell transplantation. G207 infection of human bone marrow cells had no effect on the proportion or clonogenic capacity of CD34+ cells but did enhance the proliferation of bone marrow cells in culture and the proportion of CD14+ and CD38+ cells. On the other hand, G207 at a multiplicity of infection of 0.1 was able to purge bone marrow of contaminating human breast cancer cells. Because G207 also stimulates the proliferation of human hematopoietic cells, it overcomes a limitation of other purging methods that result in delayed reconstitution of hematopoiesis. The efficient infection of human bone marrow cells in the absence of detected toxicity suggests that HSV vectors may also prove useful for gene therapy to hematopoietic progenitor cells.