Following a baseline PET scan (PET 1) subjects will be randomized to one of two treatment groups (Group 1 or Group 2), and receive blinded treatment for a total of 4 weeks. The 4-week treatment period will have two phases, Phase 1 and Phase 2. Group 1 will receive darusentan 100 mg for 2 weeks during Phase 1, then placebo for 2 weeks during Phase 2. Group 2 will receive placebo for 2 weeks during Phase 1, then darusentan 100 mg for 2 weeks during Phase 2. Following 4 weeks of treatment with blinded study drug, subjects in both treatment groups will be withdrawn from study drug for an additional 2 weeks. Maximum darusentan exposure in this study will be 2 weeks, and maximum placebo exposure in this study will be 2 weeks. Adjustments to the number or dosage of concomitant medications required for study entry will not be permitted at any time during the study.