PAS-004 in Adults Who Have Neurofibromatosis Type 1 With Plexiform Neurofibromas

The main purpose of this clinical trial is to test PAS-004 in people with at least one symptomatic plexiform neurofibroma due to Neurofibromatosis Type 1 (NF1). The main questions it aims to answer are: * How well participants are able tolerate different doses of PAS-004, and * What side effects PAS-004 might have. This study will have two parts, Part A and Part B. The main goal of Part A of this study is to learn more about how participants tolerate different doses of PAS-004, and what side effects PAS-004 might have. What we learn from Part A of the study will help decide what doses of the study drug (PAS-004) should be used in Part B of the study, and if it is safe. In Part B, two different doses from Part A will be tested. The main goal of this part of the study is to keep studying any side effects of PAS-004 at those two dose levels, and to learn more about if the doses picked for this part of the study might have an effect on plexiform neurofibromas. Participants in Part A of the study who were taking doses selected for Part B may be able to continue on to Part B and keep taking the same dose of PAS-004 for 6 more months. Study participants in both parts will have regular visits to the study doctor and be asked to have tests and exams done to check on their health and safety, including blood draws and MRIs. Everyone participating in the study will take PAS-004 by mouth once a day during the study, in 28-day cycles. Participants will be asked to keep a diary to record their daily dose of study drug. Participants will continue on daily PAS-004 for up to 6 months, or until: * They decide to withdraw from the study, or * They experience unacceptable side effects, or * Their disease progresses, or another illness interferes with taking the study drug, or * The sponsor selects a dose level to study further in the next part of the study, or * The sponsor stops the study.