1. Exploratory Chemistry Research Laboratories, Biological Research Laboratories, and Analytical and Metabolic Research Laboratories, Sankyo Company, Ltd., 1-2-58 Hiromachi, Shinagawa-ku, Tokyo 140-8710, Japan, and The Department of Infectious Diseases, The Institute of Medical Science, The University of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108, Japan