ABSTRACT Atrial fibrillation (AF) is the most common arrhythmia and its prevalence is rising alarmingly. It is particularly challenging to treat persistent AF and to restore normal sinus rhythm with currently available antiarrhythmics. Therefore, novel ion channel blocking modalities are needed for the development of the next generation of antiarrhythmic pharmacotherapies in persistent AF. A hallmark of remodeling in the chronically fibrillating atria is the presence of a constitutively active, parasympathetic stimulation independent acetylcholine sensitive inward rectifier potassium current (IKACh). Constitutively active IKACh acts as a background inward rectifier conductance and can contribute to the shortening of the atrial effective refractory period, and consequently to the perpetuation of AF. We demonstrated earlier that IKACh blockade with the peptidotoxin tertiapinQ, a 21-amino acid synthetic peptide blocker of IKACh originally isolated from the European honeybee venom, terminates persistent AF. We thus bioengineered, produced and characterized a novel, and potent IKACh blocking peptibody. Peptibodies are chimeras generated as fusion proteins of the fragment crystallizable (Fc) domain of the human immunoglobulin G (IgG1) with a bioactive “warhead” peptide. Peptibodies combine the biologic/therapeutic activity of a given peptide, with the stability of monoclonal antibodies and are stable and safe molecules that are emerging as viable clinical therapies. Our IKACh blocking peptibody was constructed as a fusion protein between the Fc fragment of human IgG1 and tertiapinQ linked together by an octaglycine spacer. In this application, we propose to test the hypothesis that bioengineered peptibodies designed as potent and bioactive blockers of IKACh are antiarrhythmic in persistent AF. Our goals are: 1- to delineate the structural determinants for the peptibody’s block of IKACh; 2- to determine the atrial specificity, electrophysiological safety and therapeutic potential of anti-IKACh peptibodies in a pig model of persistent AF; and 3- to bioengineer next generation peptibodies that have increased IKACh potency and specificity. Successful accomplishment of our proposal should be a major step forward in the deliberate, innovative, and rational development of much needed and effective antifibrillatory agents based on bioengineering approaches that target ion channels important in the mechanism of a major cardiac disease.