An episomally maintained MDR1 gene for gene therapy

Potential applications of the MDR1 multidrug transporter in gene therapy in clude protecting sensitive bone marrow cells against cytotoxic drugs during cancer chemotherapy and serving as a dominant selectable marker when coexp ressed with a corrective passenger gene. To address safety concerns associa ted with integrating viral systems, such as retroviruses, we tested the fea sibility of maintaining a nonvirally delivered MDR1 gene (pEpiHaMA) episoma lly. An MDR1 vector containing the Epstein-Barr virus (EBV) origin of repli cation (OriP) and its nuclear retention protein (EBNA-1) was transfected in to human (KB-3-1) cells. MDR1 was expressed at a higher level in cells carr ying the episomal vector, pEpiHaMA, compared with the vector lacking sequen ces needed for episomal maintenance (pHaMA). Furthermore, more drug-resista nt KB-3-1 colonies were obtained on selection after transfection with pEpiH aMA. These observations correlated with longer maintenance of episomes in c ells transfected with pEpiHaMA, In addition, episomes could still be recove red for more than 1 month from tumor explants in nude mice that were inject ed with pEpiHaMA-liposome complexes after drug selection, suggesting that t hese constructs can be maintained extrachromosomally in vivo.