A study to compare how safe and well tolerated study drug Abelacimab (MAA868) is compared to another drug called Rivaroxaban in patients with heart condition that causes an irregular and abnormally fast heart rate

Prior treatment

• Use of other investigational drugs within 5 half-lives prior to enrollment or until the expected pharmacodynamic effect has returned to baseline, whichever is longer.
• Any stroke within 14 days before randomization or TIA within 3 days before randomization.
• Women of child-bearing potential (WOCBP), defined as all women physiologically capable of becoming pregnant, unless they agree to use highly effective methods of contraception during their participation in the trial and for at least 10 weeks after the last dose of abelacimab for women randomized to abelacimab. Highly effective contraception methods include: • Total abstinence (when this is in line with the preferred and usual lifestyle of the patient). Periodic abstinence (e.g., calendar, ovulation, symptothermal, post-ovulation methods) and withdrawal are not acceptable methods of contraception • Female sterilization (have had surgical bilateral oophorectomy with or without hysterectomy) total hysterectomy or tubal ligation at least six weeks before taking investigational drug. In case of oophorectomy alone, only when the reproductive status of the woman has been confirmed by follow up hormone level assessment • Male sterilization of sexual partner (at least 6 months prior to screening). For female patients in the study, the vasectomized male partner should be the sole partner for that patient • Use of oral (estrogen and progesterone), injected or implanted hormonal methods of contraception or placement of an intrauterine device (IUD) or intrauterine system (IUS) or other forms of hormonal contraception that have comparable efficacy (failure rate < 1%), for example hormone vaginal ring or transdermal hormone contraception. Hormonal contraceptive methods should not be used or encouraged if considered to be contraindicated. In case of use of oral contraception women should have been stable on the same pill for a minimum of 3 months before taking investigational drug. Women are considered post-menopausal and not of child-bearing potential if they have had 12 months of natural (spontaneous) amenorrhea with an appropriate clinical profile (e.g., age appropriate, history of vasomotor symptoms) or have had surgical bilateral oophorectomy (with or without hysterectomy), total hysterectomy or tubal ligation at least six weeks ago. In the case of reported menopausal status or oophorectomy alone, only when the reproductive status of the woman has been confirmed by follow up hormone level assessment with follicle stimulating hormone (FSH) is she considered not of child-bearing potential.
• Significant illness which has not resolved within two (2) weeks prior to the start of the study drug.
• Extension period exclusion criteria: Patients with an intracranial or intraocular bleed within the 3 months prior to EoT.
• Patients with an intracranial or intraocular bleed within the 3 months prior to screening.

Other illness

• Planned invasive procedure with potential for uncontrolled bleeding (e.g. major surgery).
• History of drug addiction or alcohol abuse in the past 2 years, as judged by the Investigator.
• History of hypersensitivity to any of the study drugs (including rivaroxaban) or its excipients, to drugs of similar chemical classes, or any contraindication listed in the label for rivaroxaban.
• Any medical or psychiatric condition which in the judgment of the Investigator may preclude patients of complying with study requirements for the duration of the study.
• Extension period exclusion criteria: History of hypersensitivity to abelacimab.
• Extension period exclusion criteria: Clinically significant mitral stenosis (valve area <1.5 cm2) Mechanical heart valve or other indication for anticoagulation therapy other than atrial fibrillation (e.g., venous thromboembolism).
• Extension period exclusion criteria: History of left atrial appendage closure or removal.
• Mechanical heart valve or other indication for anticoagulation therapy other than atrial fibrillation (e.g., venous thromboembolism).
• History of left atrial appendage closure or removal.

Lab values

• Platelet count ≤70,000/mm3 at the Screening Visit.
• Hemoglobin <8 g/dL at the Screening Visit.
• aPTT or PT >1.5x the upper limit of normal (ULN) at the Screening Visit, if the patient is anticoagulant-naïve.

Other

• A CrCl <15 mL/min or on dialysis at the time of Screening.
• Sexually active males with female partners who are WOCBP must agree to use a condomor use other reliable birth control methods during their time in the study and should notfather a child or donate sperm during the study period.
• Extension period exclusion criteria: Known presence of an atrial myxoma or left ventricular thrombus.
• Clinically significant mitral stenosis (valve area <1.5 cm2).
• Known presence of an atrial myxoma or left ventricular thrombus.
• Active endocarditis.
• Systolic BP >180 mm Hg or diastolic BP >100 mm Hg on repeated measurements at screening.

• Able to provide written informed consent before the first study assessment is performed.
• Male and female patients ≥ 55 years old.
• History of AF or atrial flutter with planned indefinite anticoagulation. Patients with newly diagnosed AF are eligible.
• A CHA2DS2-VASc of ≥4 OR a CHA2DS2-VASc of ≥3 with at least 1 of the following: • Planned concomitant use of antiplatelet medication (e.g. aspirin and/or P2Y12 inhibitor) for the duration of the trial. • CrCl ≤50 ml/min by the Cockcroft-Gault equation.
• Extension period inclusion criteria: Ongoing study treatment for the randomized part of the trial at the EoT visit.
• Extension period inclusion criteria: Able to provide written informed consent to enter the extension period.