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.

Inclusion Criteria:

• Age 4 through 65 years
• Subject has a confirmed diagnosis of HbSS, HbSC, HbSβ+thal, or HbSβ0thal
• Subject is experiencing acute pain typical of vaso-occlusive crisis requiring treatment with parenteral analgesia
• Subject requires hospitalization

Exclusion Criteria:

• Subject has acute chest syndrome
• Subject's laboratory results indicate inadequate organ function
• Subject is pregnant or nursing an infant
• Subject had a painful crisis requiring hospitalization within the preceding 14 days or has experienced > 5 hospitalizations for VOC in the prior 6 months
• Subject has been transfused within the past 14 days
• Subject is hospitalized for a condition other than VOC
• Subject has complications related to SCD