1. Graduate School of Pharmaceutical Sciences, Chiba University, 1-8-1 Inohana, Chuo-ku, Chiba 260-8675, Japan, Gunma University, School of Medicine, 3-39-22 Showa-cho, Maebashi, 371-8511, Japan, and Japan Atomic Energy Agency, Tokai-mura, Ibaraki 319-1195, Japan