The purpose of this study is to evaluate whether MST-188 can reduce the duration of vaso-occlusive crisis (VOC) in subjects with sickle cell disease. The study will also evaluate whether MST-188 can reduce the frequency of rehospitalization of subjects due to a recurrence of VOC. Additionally, this study will compare the development of acute chest syndrome during VOC in subjects who receive MST-188 to those who do not receive MST-188.