BACKGROUND Clazosentan, an endothelin-1 receptor antagonist, has been shown to prevent the development of large vessel angiographic vasospasm after aneurysmal subarachnoid hemorrhage (aSAH). It has been hypothesized that clazosentan can also reverse established angiographic vasospasm. METHODS The REVERSE (resynchronization reverses remodeling in systolic left ventricular dysfunction) study was a prospective, multicenter, open-label, 2-stage pilot study of adult patients with aSAH who had received intravenous clazosentan (15 mg/hour) after developing moderate-to-severe angiographic vasospasm. The primary efficacy endpoint was the reversal of global cerebral vasospasm in large cerebral artery segments 3 hours after clazosentan initiation. The secondary endpoints included large artery vasospasm reversal at 24 hours and the maximum change in the angiographic cerebral circulation time. The change in vasospasm severity in the proximal and distal segments was investigated in an exploratory analysis. RESULTS The primary efficacy endpoint was met in 3 of 11 evaluable patients (27.3%; 95% confidence interval, 6.0-61.0). However, recruitment was stopped after stage 1 in accordance with the predefined interim analysis criteria. In the exploratory analysis, 50.0% and 77.8% of the patients showed a significant reversal of vasospasm or improvement to the admission state in ≥2 distal segments at 3 and 24 hours and 28.6% and 77.8% in ≥2 proximal segments, respectively. CONCLUSIONS Although the main analysis showed a reversal of large vessel vasospasm 3 hours after clazosentan initiation in a few patients, the exploratory analysis indicated a clear pharmacodynamic dilating effect on vasospastic cerebral vessels at 24 hours in most patients, in particular, in the distal arterial beds. This observation supported the inclusion of patients with established vasospasm in the ongoing REACT (prevention and treatment of vasospasm with clazosentan) trial.