A Phase 2, Multicenter, Randomized, Placebo-Controlled, Double-blind Study in Patients With Acquired Thrombotic Thrombocytopenic Purpura (aTTP) to Evaluate the Pharmacokinetics,Safety and Efficacy of rADAMTS-13 (SHP655) Administered in Addition to Standard Of Care (SoC) Treatment - ICH GCP - Clinical Trials Registry