Rationale: Ongoing efforts to improve pulmonary gene transfer thereby enabling gene therapy for the treatment of lung diseases, such as cystic fibrosis (CF), has led to the assessment of a lentiviral vector (simian immunodeficiency virus [SIV]) pseudotyped with the Sendai virus envelope proteins F and HN.

Objectives: To place this vector onto a translational pathway to the clinic by addressing some key milestones that have to be achieved.

Methods: F/HN-SIV transduction efficiency, duration of expression, and toxicity were assessed in mice. In addition, F/HN-SIV was assessed in differentiated human air–liquid interface cultures, primary human nasal epithelial cells, and human and sheep lung slices.

Measurements and Main Results: A single dose produces lung expression for the lifetime of the mouse (∼2 yr). Only brief contact time is needed to achieve transduction. Repeated daily administration leads to a dose-related increase in gene expression. Repeated monthly administration to mouse lower airways is feasible without loss of gene expression. There is no evidence of chronic toxicity during a 2-year study period. F/HN-SIV leads to persistent gene expression in human differentiated airway cultures and human lung slices and transduces freshly obtained primary human airway epithelial cells.

Conclusions: The data support F/HN-pseudotyped SIV as a promising vector for pulmonary gene therapy for several diseases including CF. We are now undertaking the necessary refinements to progress this vector into clinical trials.