1. Pfizer Global Research and Development, Ann Arbor, Michigan 48105, Pfizer Global Research and Development, Cambridge, Massachusetts 02139, ActivX Biosciences, 11025 North Torrey Pines Road, La Jolla, California 92037, and Departments of Chemical Physiology, The Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037