Neurotrophic factor therapy--prospects and problems.

Over the past 15 years neurotrophic factors have generated considerable excitement for their potential as therapy for a wide variety of degenerative neurological disorders, for which there is currently no treatment. The first part of this period was marked by the discovery, characterization, and cloning of many new growth factors, and by successful testing of these factors in animal models of neurological disease. In recent years the biotechnology industry and pharmaceutical industry have attempted to replicate the success of the animal studies in clinical trials. Although some studies have demonstrated moderate efficacy, for the most part the clinical trials have been less successful at demonstrating the therapeutic efficacy of this new class of drugs. For example, nerve growth factor appeared to be efficacious in two phase II clinical trials for peripheral neuropathy, but failed in a large scale phase III trial. Ciliary neurotrophic factor, brain derived neurotrophic factor and insulin like growth factor-1 have all been tested in clinical trials for the treatment of amyotrophic lateral sclerosis, with at best, variable indications of efficacy. Nevertheless, there are still many reasons to be optimistic that some of these agents may be useful clinically. Many technical and pharmacological issues remain to be adequately addressed, before neurotrophic factors can live up to their potential. Our collective experience with them has re-adjusted previously wild expectations, so that they are now much more realistic. This is necessary and beneficial for the maturation of this field of study.