Risk of tumorigenesis in first-in-human trials of embryonic stem cell neural derivatives: Ethics in the face of long-term uncertainty.

While the successful differentiation of human embryonic stem cells into high purity populations of specialized cells is viewed as a major step towards the safety of prospective human transplantation trials, concerns about the possible risks of tumor formation remain. The potential tumorigenicity of transplants derived from embryonic stem cells raises distinctive ethical challenges regarding trial design, evaluation of their risks to potential benefits ratio, population selection, and post-trial subject monitoring. These challenges are examined within the context of foreseen trials for disorders of the central nervous system.