Activation of lytic Epstein-Barr virus (EBV) infection by radiation and sodium butyrate in vitro and in vivo : A potential method for treating EBV-positive malignancies

The consistent presence of the EBV genome in certain tumors offers the potential for novel EBV-directed therapies. Switching the latent form of EBV infection present in most EBV-positive tumor cells into the cytolytic form may be clinically useful because lytic EBV infection leads to host cell destruction, and very few normal cells contain the EBV genome. It would also be therapeutically advantageous to induce expression of EBV-encoded lytic proteins that convert the nucleoside analogues ganciclovir (GCV) and 3'-azido-3'deoxythymidine (AZT) into their active, cytotoxic forms. In this report, we have explored two different approaches for activating the lytic form of EBV infection in tumors. We show that gamma-irradiation at clinically relevant doses induces lytic EBV infection in lymphoblastoid cell lines in vitro as well as in EBV-positive B-cell tumors in SCID mice. In addition, sodium butyrate (given as a single i.p. dose) is effective for activating lytic viral infection in some EBV tumor types in SCID mice. We also examined whether low-dose gamma-irradiation treatment of EBV-positive lymphoblastoid cells in vitro promotes GCV or AZT susceptibility. The combination of radiation with either GCV or AZT induced significantly more cell killing in vitro than either radiation or prodrug treatment alone. Most importantly, we found that the combination of gamma-irradiation and GCV was much more effective in treating EBV-positive lymphoblastoid tumors in SCID mice than either agent alone. Thus, GCV or AZT treatment could potentially enhance the therapeutic efficacy of radiation therapy for EBV-positive lymphomas in patients.