Amendment 1: • Update to study design to include adaptive design as Part 2 of the study and corresponding updates to sample size and addition of the new AP30663 doses relevant for Part 2. • Addition of criteria for interim analyses. • Updates to the study objectives to reflect that the assessments were pertaining to 1 or more dose levels of AP30663. • Addition of details for the DMC activities during the Part 2 of the study. • Update to the statistical analysis method for the primary endpoint. • Additional clarification of study procedures, including updates to: 1. time points for collection of plasma samples 2. prior and concomitant medications 3. role of investigators in process of treatment discontinuation 4. study drug administration 5. ECG reading and extraction windows 6. instructions for assessing local infusion site reactions.

Amendment 2: • Update to the study design for Part I of the study: Changed the number of subjects for the first interim analysis to be “up to” 36 randomised subjects, rather than 36 randomised subjects. Added explanation that interim analysis could be planned on a reduced cohort if preliminary blinded safety and efficacy data were sufficient for the DMC to provide a recommendation for Part 2. • Clarification that “up to” 108 subjects were to be enrolled in Parts 1 and 2. • Update to the exclusion criteria to exclude subjects with any malignant cancer within 3 years of signing ICF and to exclude subjects who used any antiarrhythmic drug class I and/or III within 6 months before randomisation. • Update to the schedule of assessments and associated footnotes for clarification regarding testing TSH levels at screening visit, haematology and clinical chemistry testing at Day 1 visit, 12-lead 24-hour Holter (Day 2 stop continued assessment after the last PK sampling at 24 hours). • Update to the patient discontinuation/withdrawal section for clarification which subjects were to be replaced and to remove reference to low potassium and magnesium levels at screening. • Removal of retest of magnesium before randomisation. • Addition of a washout period of 6 months before randomisation for antiarrhythmic class I and III drugs. • Further administrative updates, minor clarifications and typographical and formatting changes that did not affect content.

Amendment 3: • Updates to the exclusion criteria 4, 13 18, 20, 21, 22, 26. • Update to the introduction and study rationale with results of the completed phase 1 studies. • Addition of a 2 hour-window for efficacy endpoint proportion of subjects in SR at 24 hours. • Change from 24 hour-Holter ECG and 24 hour-telemetry to 8 hours post-infusion for both. • Updates and clarifications regarding study procedures, assessment parameters and collection windows in schedule of assessment and associated footnotes as well as in procedure sections. • Adjustment of the quality management and risk evaluation section. • Addition of preinfusion ECG QTcF above 450 ms as withdrawal criterion. • Updates to list of prohibited medications and associated washout periods. • Addition of 1 coagulation parameter (INR or APTT) at screening. • For AE collection, addition of reference to the Investigator’s Brochure (IB) section on guidance for the investigator.