The purpose of this study is to determine the effectiveness of the study drug at improving a plexiform neurofibroma (PN) that is causing significant issues in people with Neurofibromatosis type 1 (NF1).

Participants in the ReNeu study must meet the following requirements*:
- Ages 2 to 17
- Has NF1
- Has a PN that is causing significant issues
*Other requirements will apply.
Participation in the ReNeu study will last up to 26 months (a little over 2 years).
During this time, participants can expect the following:
- Complete the Screening assessments to confirm they can participate
- Take the study drug by mouth in 4-week cycles (twice per day for 3 weeks, then stop for 1 week, and repeat)
- Record each dose of the study drug in an electronic diary called an ePRO device
- Attend study visits and complete health assessments once per month for 7 months, then every other month
- Participate in a monthly wellness check by phone or email (this check may be completed during study visits)
- Complete a follow-up visit 30 days after the last dose of the study drug
Participants may also have the option to continue taking the study drug for additional cycles as part of a Long-term Follow-up Phase.