1. College of Pharmacy, University of Nebraska Medical Center, 986025 Nebraska Medical Center, Omaha, Nebraska 68198-6025; Pharma Division, Preclinical Research, F. Hoffmann-La Roche Ltd., CH-4070 Basel, Switzerland; Swiss Tropical Institute, CH-4002 Basel, Switzerland; and Division of Experimental Therapeutics, Walter Reed Army Institute of Research, Washington, D. C. 20307-5100