Chairman's Summary

Abstract

The airways are exposed to a torrent of alien DNA in inspired air ranging from human nasal and bronchial secretions to pollens, house dust mite feces, animal danders, and flying insects. It is no surprise that the airways are well defended against the risk that alien DNA may reach the nuclei of airway cells and induce them to express the proteins that it encodes. These defense mechanisms are obstacles that must be overcome in inhalational gene therapy for lung disease. The barriers are many and include: (1) delivery to a distal lung site in a congested and obstructed airway—a topic covered in more depth under particulates; (2) access to the target cells, some of which lie in mucosal glands, not on the surface; (3) entry into the cytoplasm of a large and insoluble piece of DNA; (4) passage from the cytoplasm into the nucleus; and (5) stable, and ideally controllable, expression of the encoded protein. At a symposium I attended 10 years ago a leading molecular biologist declared, roundly, that we were “nowhere” in achieving any of these objectives. It is in some ways disappointing how little progress has been made in the interval—but there has been some. It has proved possible to transfect the nasal mucosa of mice with cystic fibrosis DNA using liposomes as a delivery vehicle, but the efficiency is so low that most discussants at the TAC doubted that this was a viable approach. Mechanical, electrical, or chemical techniques that cause a transitory increase in permeability can increase efficiency, but their practicality for use deep in the human lung is doubtful. Viral vectors including adenoviruses, adeno-associated viruses, and paramyxoviruses can achieve higher efficiency because they carry the biochemical machinery to deliver a relatively large payload across both the cell membrane and the nuclear membrane. But viruses have serious problems. The human immune response makes repeated administration problematic—although the extent of this problem varies with the virus. Virus-transfected cells may have reduced viability or become the target of killer T cells. Barring a tangential discovery—never to be discounted in science—a step-by-step approach will be necessary, probably using viral mechanisms for crossing the cell and nuclear membranes, but not a complete viable virus. Another suggestion was that ligands to internalized G protein–coupled receptors could be used, although there was doubt about the size of the possible payload. One discussant suggested moving the gene through the membrane in pieces and reassembling it in the nucleus—akin to the building of the space station in orbit and probably fraught with comparable levels of difficulty. Another discussant commented that we need something like a hundredfold improvement in transfection efficiency before we shall have an effective therapy.