1. Departments of Medicinal Chemistry, Molecular Pharmacology, and Molecular Structure, Amgen Inc., One Kendall Square, Bldg. 1000, Cambridge, Massachusetts 02139; Departments of Molecular Pharmacology, Oncology Research, Pharmaceutics, and Pharmacokinetics and Drug Metabolism, Amgen, Inc., One Amgen Center Drive, Thousand Oaks, California 91320