1. Drug Discovery Program, Department of Medicinal Chemistry and Pharmacognosy, University of Illinois at Chicago, 833 South Wood Street, Chicago, Illinois 60612, Division of Experimental Therapeutics, Walter Reed Army Institute of Research, 503 Robert Grant Avenue, Silver Springs, Maryland 20910, and Department of Immunology, Division of Oncology Research, Mayo Clinic College of Medicine, 13-42 Guggenheim, 200 First Street SW, Rochester, Minnesota 55905