1. Medicinal Chemistry Research Laboratories, Ono Pharmaceutical Co., Ltd., Osaka 618-8585, Japan, Exploratory Research Laboratories, Ono Pharmaceutical Co., Ltd., Ibaraki 300-4247, Japan, Department of Internal Medicine II, Kumamoto University School of Medicine, Kumamoto 860-0811, Japan, and Experimental Retrovirology Section, HIV & AIDS Malignancy Branch, NCI, National Institutes of Health, Bethesda, Maryland 20892