This is a randomized, double-blind, placebo- and active-controlled study of a new experimental drug called darusentan. Darusentan is not currently approved by the United States Food and Drug Administration (FDA), which means that a doctor cannot prescribe this drug. The purpose of this study is to determine if darusentan is effective in reducing systolic and diastolic hypertension despite treatment with full doses of three or more antihypertensive drugs, including a diuretic. Subjects will be randomized to darusentan (optimized dose), an active comparator, or placebo, administered orally. The treatment period for this trial is 14 weeks.