The present invention relates to methods for gene therapy, especially to adenovirus-based gene therapy, and related cell lines and compositions. In particular, novel nucleic acid constructs and packaging cell lines are disclosed, for use in facilitating the development of high-capacity and targeted vectors. The invention also discloses a variety of high-capacity adenovirus vectors and related compositions and kits including the disclosed cell lines and vectors. Finally, the invention discloses methods of preparing and using the disclosed vectors, cell lines and kits.