Additional Study Information: The purpose of this study is to learn about whether the study drug (PF-07209326) is safe and effective in people with sickle cell disease and learn about what the right dose is. We will look at drug levels in the blood, see how fast drug levels rise and fall, and what effects the drug has, all of which are known as the pharmacokinetics (PK) and pharmacodynamics (PD) of a drug. In addition, this study will learn if this drug is able to prevent the painful episodes (vaso-occlusive crises) that occur. The study drug (PF-07209326) is an investigational drug because it is not approved for use in this country.