This study is for participants that have been diagnosed with Intermediate or High-risk Primary or Secondary Myelofibrosis. This study is testing an investigational drug called Nuvisertib. "Investigational" means it has not been approved by the United States Food and Drug Administration (FDA). The main purpose of this study is to evaluate the efficacy and safety of Nuvisertib. Nuvisertib is an oral PIM1 selective inhibitor. A PIM1 selective inhibitor is a drug that specifically targets and blocks the activity of the PIM-1 kinase, an enzyme implicated in cancer cell growth. This drug is given to participants by mouth. Participants in this study can expect to be in the treatment phase of this study for 19 months and the long term follow up phase for 3 years.

This study is for participants with symptoms of mast cell activation (SMAC). The primary purpose of this study is to learn about clonal mast cell diseases. Clonal mast cell diseases are hard to diagnose because symptoms are not specific, and they can overlap with other diseases. The tools currently used by doctors to look for clonal mast cell diseases in the blood may not identify all patients. This study is being done to develop an investigational blood test that looks for a change in a gene called KIT. "Investigational" means it has not been approved by the United States Food and Drug Administration (FDA). Two types of blood tests will be compared against each other. Participants in this study can expect to be in this study for about 6 months.