Bringing Neural Cell Therapies to the Clinic: Past and Future Strategies.

Cell replacement therapy in the nervous system has a rich history, with ∼40 years of research and ∼30 years of clinical experience. There is compelling evidence that appropriate cells can integrate and function in the dysfunctioning human nervous system, but the clinical results are mixed in practice. A number of factors conspire to vary patient outcome: the indication, cell source, patient selection, and team performing transplantation are all variables that can affect efficacy. Most early clinical trials have used fetal cells, a limited cell source that resists scale and standardization. Direct fetal cell transplantation creates significant challenges to commercialization that is the ultimate goal of an effective cell therapy. One approach to help scale and standardize fetal cell preparations is the expansion of neural cells in vitro. Expansion is achieved by transformation or through the application of mitogens before cryopreservation. Recently, neural cells derived from pluripotent stem cells have provided a scalable alternative. Pluripotent stem cells are desirable for manufacturing but present alternative concerns and manufacturing obstacles. All cell sources require robust and reproducible manufacturing to make nervous system cell replacement therapy an option for patients. Here, we discuss the challenges and opportunities for cell replacement in the nervous system. In this review, we give an overview of completed and ongoing neural cell transplantation clinical trials, and we discuss the challenges and opportunities for future cell replacement trials with a particular focus on pluripotent stem cell-derived therapies.