LEONARD MEUSE , ' BRIAN WINTHER , ' and MARK A

Recombinant retroviral vectors represent an attractive means of transferring genes into the liver because they integrate in the host cell genome and result in permanent gene expression. However, efficient gene transfer is limited by the requirement of active cell division for integration. Surgical partial hepatectomy has been the traditional method of inducing hepatocellular proliferation, but this invasive approach would be difllcult to justify in clinical gene therapy. As an alternative, w e used a recombinant adenovirus expressing a nonsecreted form of urokinase plaminogen activator ( A d . P G K m u P A ) , which results in liver regeneration over a period of 8 days. W h e n a high-titer retroviral vector was continuously infused into the portal vein of mice during this period of hepatocyte proliferation, 33.5% of hepatocytes were stably transduced. In addition, high-level expression of a secreted transgene reporter was sustained for at least 48 weeks (length of experiment). W e investigated the influence of vector titer on the in vivo transduction efficiency in our system, and found that the total number of infectious retroviral particles delivered per target cell is a critical factor. The results presented here demonstrate the ability to obtain a high gene transfer efficiency and long-term gene expression in hepatocytes in vivo without the need for surgical hepatectomy. The two-vector system described here m a y be of clinical relevance, as the level of hepatic gene transfer achieved has potential to be curative for a large number of genetic liver diseases.