Key Inclusion Criteria:  Adults ≥ 18 years at enrollment with diagnosis of NF1 with symptomatic, inoperable PN At least one inoperable target PN measurable by volumetric MRI analysis Chronic target PN pain score documented for minimum period during screening period Stable chronic PN pain medication use at enrollment Adequate organ and marrow function  Key Exclusion Criteria:  Confirmed or suspected malignant glioma or MPNST (optic glioma not requiring chemotherapy or radiation therapy are exempt from this exclusion) History of malignancy except for malignancy treated with curative intent with no known active disease ≥ 5 years before the first dose of study intervention and of low potential risk for recurrence Clinically significant cardiovascular disease, including inherited coronary disease, acute coronary syndrome within 6 months prior to enrollment, uncontrolled angina, symptomatic heart failure, cardiomyopathy, severe valvular heart disease, abnormal LVEF and uncontrolled hypertension Ophthalmological findings/conditions including intraocular pressure > 21 mmHg, RPED/CSR or RVO Prior exposure to MEK inhibitors