The nitroreductase/CB1954 combination in Epstein-Barr virus-positive B-cell lines: Induction of bystander killing in vitro and in vivo

Epstein-Barr virus (EBV)-based gene delivery vectors that preferentially ex press toxic genes in EBV-infected cells could be used to target EBV-positiv e tumors for destruction. We have shown previously that the cytosine deamin ase (CD) enzyme, which converts the prodrug 5-fluorocytosine (5-FC) into th e toxic compound 5-fluorouracil efficiently kills EBV-positive cells in the presence of 5-FC, with a substantial bystander killing effect in vitro and in vivo. To identify the optimal enzyme/prodrug combination for treating E BV-positive lymphomas, we have compared the effectiveness of the CD/5-FC co mbination with the nitroreductase (NTR)/CB1954 combination for killing EBV- positive B-cell lines. NTR metabolizes CB1954 Into an alkylating agent that cross-links DNA. When the CD gene or the NTR gene were transfected into tw o different EBV-positive B-cell lines in vitro, similar to 90% of cells wer e killed in a prodrug-dependent manner, although the transfection efficienc y was (5%, However, severe combined immunodeficient mouse tumors containing either 30% or 100% of NTR-expressing Burkitt lymphoma (Jijoye) cells were growth inhibited, but not cured, by treatment with intraperitoneal CB1954 ( 20 mg/kg/day) for 10 days. These results suggest that the NTR/CB1954 combin ation induces efficient bystander killing of EBV-positive B-cell lines in v itro but may not be as effective as the CD/5-FC combination for treating B- cell lymphomas in vivo.