1. Laboratory of Medicinal Chemistry, Building 37, Room 5C06, Division of Basic Sciences, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, Laboratory of Clinical Physiology, Diabetes Unit, Gerontology Research Center, National Institute on Aging, National Institutes of Health, Baltimore, Maryland 21224, and Joslin Diabetes Center & Department of Medicine, Harvard Medical School, Boston, Massachusetts 02215