1. Department of Chemistry, Developmental Therapeutics Program, Barbara Ann Karmanos Cancer Institute, Wayne State University School of Medicine, Detroit, Michigan 48201-1379, Department of Biologic and Materials Science, School of Dentistry, University of Michigan, Ann Arbor, Michigan 48019-1078, Department of Pediatrics, The University of Alabama School of Medicine, Birmingham, Alabama 35233, and Department of Pharmacology, Yale University School of Medicine, New Haven, Connecticut 06510-8066