BERLIN — September 9, 2026

Bayer announced the initiation of a global Phase II clinical trial of BAY 3670549, an investigational highly selective G-protein-coupled inwardly rectifying potassium channel 4 (GIRK4) inhibitor being developed as a potential new treatment approach for atrial fibrillation (AFib). The randomized, double-blind, placebo-controlled study will evaluate the efficacy, safety, tolerability, pharmacodynamics and pharmacokinetics of BAY 3670549 in adults with AFib who require electrical cardioversion. The program follows a completed Phase I first-in-human single-ascending-dose study, in which BAY 3670549 demonstrated a favorable safety and tolerability profile in healthy participants. The advancement represents an important milestone for Bayer’s cardiovascular pipeline and reflects the company’s strategy of developing potentially fast-acting therapies for patients with significant unmet needs in cardiac rhythm management.

BAY 3670549 Targets Electrical Activity in the Heart

BAY 3670549 is designed to inhibit GIRK4 channels involved in regulating the electrical activity of cardiomyocytes, offering a differentiated mechanism for controlling abnormal atrial rhythms. GIRK4 channels facilitate potassium ion flow in response to acetylcholine and play an important role in regulating the atrial effective refractory period. Increased GIRK4 activity can shorten this refractory period, reducing the recovery time between electrical impulses and potentially contributing to the initiation and persistence of AFib. By selectively inhibiting GIRK4, BAY 3670549 is being investigated as a potential means of restoring normal electrical activity and supporting rapid cardioversion to normal sinus rhythm. Bayer’s development strategy is focused on determining whether this mechanism can translate into a fast-acting and well-tolerated pharmacological alternative for patients experiencing AFib episodes.

Phase II Study Evaluates Potential Alternative to Cardioversion

The Phase II study is designed to evaluate BAY 3670549 in patients with AFib who need electrical cardioversion, the current standard approach for rapidly restoring normal heart rhythm. Electrical cardioversion can be effective but is resource-intensive and typically requires sedation or anesthesia, external defibrillation and hospital-based care. Bayer is therefore evaluating whether GIRK4 inhibition could offer a potential pharmacological approach that reduces procedural burden for appropriate patients. The trial will use a randomized, placebo-controlled, parallel-group, double-blind, multi-cohort design to assess clinical efficacy alongside safety, tolerability, pharmacodynamic and pharmacokinetic measures. Successful development could position BAY 3670549 as a differentiated treatment option in acute AFib management, particularly if its potential rapid onset and tolerability profile are confirmed in patients.

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Bayer Strengthens Cardiovascular Development Portfolio

The initiation of Phase II development expands Bayer’s cardiovascular research portfolio through a novel ion-channel targeting strategy that originated from the company’s strategic research alliance with the Broad Institute of MIT and Harvard. AFib affects more than 60 million people worldwide and is associated with increased risks of stroke and heart failure, highlighting the need for effective treatment options. Existing anti-arrhythmic medicines can have important safety limitations and are not suitable for broad patient populations, while electrical cardioversion can require significant clinical resources. BAY 3670549 provides Bayer with an opportunity to explore a different approach to acute rhythm control through selective GIRK4 inhibition. With Phase I safety findings supporting continued development and the global Phase II study now underway, Bayer is advancing BAY 3670549 toward clinical proof of concept as a potential new therapeutic strategy for atrial fibrillation..

Source: Bayer, press release