To achieve continuous, site-specific delivery of therapeutic molecules to the central nervous system (CNS), a new therapeutic approach was developed combining in vitro gene transfer with a new delivery device. Xenogenic cells genetically modified to secrete specific bioactive substances were encapsulated into polymer-based fibers. A semipermeable membrane allows for passage of nutrients and cell-released therapeutic agents, but restricts inward diffusion of larger molecules and cells of the host's immune system, thus facilitating xenograft survival. This novel technique was successfully tested for the in vitro and in vivo delivery of various therapeutic agents, including neurotrophic factors, neurotransmitters and hormones. Phase I clinical trials were reported for the treatment of amyotrophic lateral sclerosis (ALS) and chronic cancer pain.