The design of safe and efficient gene transfer vectors remains one of the key challenges in gene therapy. Despite their remarkable transfection efficiency, viral vectors suffer from known safety issues. Consequently, significant research activity has been undertaken to develop nonviral approaches to gene transfer during the last decade. Numerous academic and industrial research groups are investigating synthetic cationic vectors, such as cationic amphiphiles, with the objective of increasing the gene transfection activity. Within this area, the development of functional synthetic vectors that respond to local environmental effects have met with success. These synthetic vectors are based on mechanistic principles and represent a significant departure from earlier systems. Many of these systems for gene delivery in vitro and in vivo are discussed in this article.
