This study is testing a new medicine called **bezuclastinib (CGT9486)** for people with a condition called systemic mastocytosis (SM). This condition is where certain immune cells, called mast cells, build up in the body. The study compares bezuclastinib with a placebo (a pill with no medicine) to see which works better. It's for people whose symptoms are not controlled well by their current care. The study has three parts. In the first parts, patients will get either bezuclastinib or a placebo. In the last part, everyone can try bezuclastinib. To join, you need to have a specific type of SM, like indolent or smoldering SM, and have moderate to severe symptoms. People with advanced types of SM or other serious conditions can't join. Also, if you've had certain treatments recently, you might not be eligible.

• **Length**: The study has multiple parts, and participation may extend over several weeks or months.
• **Visits**: Participants will need to visit study locations for assessments and treatments.
• **Treatment**: You may receive bezuclastinib or a placebo, depending on the study part.