1. Division of Medicinal Chemistry, Graduate School of Pharmaceutical Sciences, Duquesne University, Pittsburgh, Pennsylvania 15282, Grace Cancer Drug Center, Roswell Park Cancer Institute, Elm and Carlton Streets, Buffalo, New York 14263, Department of Biochemistry, Tufts University School of Medicine, Boston, Massachusetts 02111, and Department of Pharmacology and Toxicology, School of Medicine, Indiana University, 635 Barnhill Drive, Indianapolis, Indiana 46202