Primary Objectives:~• To identify biomarkers associated with response and non-response to nirogacestat in participants with desmoid tumors (DT).~Secondary Objectives:~To assess the 12-month progression-free survival (PFS of participants with DT who receive nirogacestat at 150 mg or 100 mg BID.~To assess MRI volumetric and functional parameters associated with response and non-response to nirogacestat in participants with DT.~To evaluate the histopathological changes in DT biopsy specimens.~To evaluate the safety and tolerability of nirogacestat in participants with DT.~Exploratory Objectives:~To assess clinical benefit and tumor response by additional MRI-based measurements including MRI-modified Choi criteria and WHO.~To evaluate MRI parameters such as intensity histogram analysis from T2-weighted image (T2-WI), short inversion time inversion-recovery (T2-STIR), diffusion-weighted imaging (DWI) with apparent diffusion coefficient (ADC) mapping, post-contrast Water Dixon, perfusion-weighted imaging, and susceptibility-weighted imaging (SWI) with contrast and correlate with therapeutic response and clinical outcome.~To assess the dynamics and concordance of circulating tumor cells (CTCs) in the blood compartment and tumor burden during nirogacestat treatment.~To identify cell-free DNA (cfDNA) biomarkers associated with response and non-response to nirogacestat in patients with DT.~To understand the impact of dose on the incidence of ovarian dysfunction in women of childbearing potential (WOCBP).
To learn about the safety and effects of an investigational drug called nirogacestat when given to participants with a desmoid tumor/aggressive fibromatosis