MEK Inhibitor Mirdametinib (PD-0325901) in Patients With Neurofibromatosis Type 1 Associated Plexiform Neurofibromas

Key Inclusion Criteria: * Participant has documented NF1 mutation or a diagnosis of neurofibromatosis type 1 (NF1) using National Institute of Health (NIH) Consensus Conference criteria inclusive of the presence of a plexiform neurofibroma (PN). * Participant has a PN that is causing significant morbidity. * Participant has a PN that cannot be completely surgically removed. * Participant has a target tumor that is amenable to volumetric MRI analysis. * Participant is willing to undergo a tumor biopsy pre and post treatment if ≥ 18 years of age. * Participant has adequate organ and bone marrow function. Key Exclusion Criteria: * Participant has abnormal liver function or history of liver disease. * Participant has lymphoma, leukemia or any malignancy within the past 5 years (except for resected basal/squamous skin carcinomas without metastases within 3 years). * Participant has breast cancer within 10 years. * Participant has active optic glioma or other low-grade glioma requiring treatment. * Participant has abnormal QT interval corrected or other heart disease within 6 months. * Participant has a history of retinal pathology, risk factors for retinal vein occlusion or has a history of glaucoma. * Participant has known malabsorption syndrome or gastrointestinal conditions that would impair absorption of mirdametinib (PD-0325901). * Participant has received NF1 PN-targeted therapy within 45 days. * Participant previously received or is currently receiving therapy with mirdametinib (PD-0325901) or any other MEK1/2 inhibitor. * Participant has received radiation therapy within 6 months or has received radiation to the orbit at any time. * Participant is unable to undergo or tolerate MRI. * Participant has active bacterial, fungal or viral infection. * Participant has experienced other severe acute or chronic medical or psychiatric conditions within 1 year.