The overall reason for the amendment 1 was implementation of additional safety monitoring measures after first dose of macitentan, as requested by the Food and Drug Administration (FDA).

The overall reason for the amendment 2 was addition of medications mainly transported by breast-cancer resistant protein to the list of forbidden medications, as requested by the FDA.

The overall reason for the amendment 3 was added description of transition to the SERENADE Open-label (OL) extension study, revision of the eligibility and run-in failure criteria, and introduction of a Clinical Event Committee (CEC). Changes made to the statistical methods to allow N-terminal pro-brain natriuretic peptide values measured at screening to be used for stratification at time of treatment assignment and to the definitions of the placebo run-in and macitentan analysis sets.

The overall reason for the amendment 4 was Addition of the 6-minute walk distance (6MWD) substudy to assess the change in exercise capacity from baseline and removal of the 8-hour safety monitoring period after first dose of macitentan at the start of macitentan run-in. Addition of 2 telephone calls to ensure adequate safety follow-up is established during the run-in phase. Reordered testing hierarchy of the key secondary efficacy endpoints: Kansas City Cardiomyopathy Questionnaire was moved to the first secondary endpoint and accelerometry moved to the second position. Added new hierarchical composite exploratory efficacy endpoint which combined hard (death, hospitalizations) and soft (functional capacity, quality of life) endpoints to allow for a more complete and broader assessment of clinical benefit.

Overall reason for amendment 5 was early termination of enrollment. Subject recruitment targets not met and completion of study within reasonable timeline not realistic. Updated sample size to reflect early termination of recruitment. Reduced length of double-blind treatment (DBT) period to 24 weeks. Week 24 was pre-defined timepoint to assess primary as well as key secondary endpoints. Secondary endpoint of time to worsening HF event however was planned to be assessed up to Week 52 to gather meaningful information for preparation of pivotal clinical trial development program. Due to reduced sample size, number of worsening HF events was expected to be too low for meaningful analysis of time to worsening HF. DBT period was to be stopped at Week 24, and eligible subjects were to be transitioned to SERENADE OL at that timepoint. Subjects who completed Week 24 visit, were scheduled to come back for an EoT visit within 60 days and enroll in OL study, if eligible. Not all sites participated in OL study. Removed CEC which was appointed to review and adjudicate in blinded fashion worsening HF events, reasons for hospitalization, and causes of death, did not affect safety monitoring and therefore decision was also endorsed by IDMC. DBT period reduced from 52 to 24 weeks, low occurrence of worsening HF events which would not allow for meaningful conclusions to be drawn. Investigator assessment of worsening HF events continued. Re-scheduled accelerometry to be performed 9 consecutive days prior to Week 24 for subjects completing Week 24 to ensure assessment performed on DBT. Stopped sub study assessments (6MWD and Borg Dyspnea Index), due to low count of subjects participating in sub study to allow for meaningful interpretation of results. Planned analysis of sub study data amended to reflect above amendment to protocol. Removed new hierarchical composite exploratory efficacy endpoint that was added in prior amendment due to reduced sample size and stopping of 6MWT sub study.

Overall reason for amendment 6 was updated the concomitant therapy sections pertaining to new information regarding a drug-drug-interaction of macitentan with moderate dual cytochrome P450 (CYP)3A4 and CYP2C9 inhibitors or co-administration of a combination of moderate CYP3A4 inhibitors and moderate CYP2C9 inhibitors. Data from clinical trials with macitentan 10 mg were reviewed, identifying cases where macitentan 10 mg was administered concomitantly with dual CYP3A4/CYP2C9 inhibitors, such as fluconazole and amiodarone. The review indicated that co-administration of fluconazole or amiodarone with macitentan was not common (between 1% to 3% of subjects). No safety concerns were identified with concurrent administration of fluconazole or amiodarone and macitentan 10 mg.