Polycythemia Vera (PV) is a blood disorder where the body makes too many red blood cells. This study is testing a new medicine called sapablursen to see if it is safe and helps patients with PV who are already on standard treatments. The study is divided into three parts: first, a blinded treatment (where neither the doctor nor the patient knows if they're getting the real medicine or not), then an open-label treatment (everyone knows they're getting the real medicine), and finally, a long-term extension to see how the medicine works over time. Participants can be in the study for up to 3 years (156 weeks).

• Participants will have regular check-ups and tests over the study period.
• The study is for people who need regular blood draws (phlebotomy) to manage their PV.
• No prior treatment with certain PV medicines is allowed.