Study Overview: This study is looking at how safe and effective a new drug, called bezuclastinib, is for people with a condition called systemic mastocytosis. Systemic mastocytosis is a disease where a type of blood cell called a mast cell builds up in the body. The study will compare bezuclastinib to a placebo, which looks like the medicine but doesn't have any active drug in it. Participants will get either bezuclastinib or the placebo, along with their usual care, for a period of time. The study is divided into three parts, and some people may continue into Part 3 to receive bezuclastinib.

Eligibility: To join, you must have a type of systemic mastocytosis called indolent or smoldering, and your symptoms must be moderate to severe. You must also be able to maintain a stable treatment routine for at least 14 days before joining. Some medical conditions and treatments within a certain time period before joining might disqualify you.

• Duration: The study is in multiple parts, which means it can last a while depending on participation.
• Compensation: Check with the study coordinators for details.
• Risks: As with any study, there are potential risks, so discuss with your doctor if it's right for you.