KILLING EPSTEIN-BARR VIRUS-POSITIVE B-LYMPHOCYTES BY GENE-THERAPY - COMPARING THE EFFICACY OF CYTOSINE DEAMINASE AND HERPES-SIMPLEX VIRUS THYMIDINE KINASE

Epstein-Barr virus (EBV)-positive lymphomas are frequent among inmunos uppressed patients, We have examined the feasibility of killing EBV-im mortalized B lymphocytes by gene transfer involving the use of ''suici de'' genes whose expression in target cells renders them susceptible t o killing by a prodrug, We examined two gene/prodrug pairs: the Escher ichia coli cytosine deaminase (CD) gene with the prodrug 5-fluorocytos ine (5-FC), and the herpes simplex virus thymidine kinase (HSV-TK) gen e with the prodrug ganciclovir, Retroviral vectors and drug selection were used to obtain CD or HSV-TK expression in cells, Both the CD/5-FC and the HSV-TK/ganciclovir combinations yielded substantial killing o f EBV-immortalized B lymphocytes in vitro, although the CD/5-FC regime n had a significantly greater therapeutic margin than the HSV-TK/ganci clovir combination, The CD/5-FC pair, but not the HSV-TK/ganciclovir p air, was shown to have a ''bystander killing effect'' in vitro, When o nly 30% of the cells expressed the suicide gene, scid mouse tumors reg ressed in both the CD/5-FC regimen and the HSV-TK/ganciclovir regimen, documenting an in vivo bystander effect with both regimens, However, a greater percentage of tumors completely regressed with the CD/5-FC r egimen, Overall, the sum of our data indicates that the CD/5-FC combin ation is the more promising regimen for treatment of EBV-associated ly mphomas in vivo.