Postoperative atrial fibrillation (POAF) is associated with increased morbidity and mortality. Epicardial injection of botulinum toxin may suppress POAF. This study sought to assess the safety and efficacy of AGN-151607 for the prevention of POAF after cardiac surgery. This phase 2, randomized, placebo-controlled trial assessed the safety and efficacy of AGN-151607, 125 U and 250 U vs placebo (1:1:1), for the prevention of POAF after cardiac surgery. Randomization was stratified by age (<65,≥65 years) and type of surgery (nonvalvular/valve surgery). The primary endpoint was the occurrence of continuous AF≥30 seconds. Among 312 modified intention-to-treat participants (placebo, n=102; 125 U, n=104; and 250 U, n=106), the mean age was 66.9 ± 6.8 years; 17% were female; and 64% had coronary artery bypass graft (CABG) only, 12% had CABG+ valve, and 24% had valve surgery. The primary endpoint occurred in 46.1% of the placebo group, 36.5% of the 125-U group (relative risk [RR] vs placebo: 0.80; 95%CI: 0.58-1.10; P=0.16), and 47.2% of the 250-U group (RR vs placebo: 1.04; 95%CI: 0.79-1.37; P=0.78). The primary endpoint was reduced in the 125-U group in those≥65 years (RR: 0.64; 95%CI: 0.43-0.94; P=0.02) with a greater reduction in CABG-only participants≥65 years (RR: 0.49; 95%CI: 0.27-0.87; P=0.01). Rehospitalization and rates of adverse events were similar across the 3 groups. There were no significant differences in the rate of POAF with either dose compared with placebo; however, there was a lower rate of POAF in participants≥65 years undergoing CABG only and receiving 125 U of AGN-151607. These hypothesis-generating findings require investigation in a larger, adequately powered randomized clinical trial. (Botulinum Toxin Type A [AGN-151607] for the Prevention of Post-operative Atrial Fibrillation in Adult Participants Undergoing Open-chest Cardiac Surgery [NOVA]; NCT03779841); A Phase 2, Multi-Center, Randomized, Double-Blind, Placebo-Controlled, Dose Ranging Study to Evaluate the Efficacy and Safety of Botulinum Toxin Type A [AGN 151607] Injections into the Epicardial Fat Pads to Prevent Post-Operative Atrial Fibrillation in Patients Undergoing Open-Chest Cardiac Surgery; 2017-004399-68).