This study tests two new drugs, REGN7508 and REGN9933, for people with atrial fibrillation (AF). AF is when your heart beats too fast and unevenly, causing blood to slow down and form clots. These drugs aim to prevent ischemic stroke (a stroke from a blocked blood vessel) or systemic embolism (a blood clot traveling to other parts of the body) in people who can't take blood thinners.

Key Points:

• Study Duration: This study may require regular visits over a period to monitor drug effects.
• Eligibility: Participants must have atrial fibrillation and be at moderate to high risk for stroke.
• Risks and Benefits: The study will check for side effects and how well the drugs prevent strokes compared to a placebo.

Participants must not have a mechanical heart valve or recent ischemic stroke, among other criteria. The study also measures drug levels in the blood and checks if the body develops antibodies that could affect the drugs' performance. This research will help determine if these drugs are a safe alternative for those who can't use standard blood thinners.