- Subjects exhibiting any of the following: a marked baseline prolongation of QT/QTc interval (repeated demonstration of a calculated QTc interval > 450), a history of additional risk factors for torsades de pointes (TdP) (e.g., heart failure, hypokalemia, family history of long QT syndrome), and subjects unable or unwilling to refrain from using medications that are known to prolong the QT/QTc ratio during the course of the study.