Key Inclusion Criteria:

          -  Three years of age or older, and less than or equal to 18 years of age at the time of
             obtaining informed consent. BSA greater than or equal to 0.55 m2, and able to swallow
             the whole study drug (capsules) without entire contents unpacked from the capsules.

          -  NF1 and inoperable and symptomatic PN who have PN-related morbidities (symptom and/or
             complications), as judged by the investigator.

          -  Inoperable PN is defined as PN that cannot be surgically completely removed without
             risk for substantial morbidity due to encasement of, or close proximity to, vital
             structures, invasiveness, or high vascularity of the PN.

          -  A PN is defined as a neurofibroma that has grown along the length of a nerve and may
             involve multiple fascicles and branches. A spinal PN involves two or more levels with
             connection between the levels or extending laterally along the nerve.

          -  In addition to PN, subjects must have at least 1 other diagnostic criterion for NF1 as
             follows:

               1. Six or more café-au-lait macules >5 mm in greatest diameter in pre-pubertal
                  individuals and >15 mm in greatest diameter in post-pubertal individuals.

               2. Freckling in the axillary or inguinal regions.

               3. Optic glioma.

               4. Two or more Lisch nodules (iris hamartomas).

               5. A distinctive osseous lesion such as sphenoid dysplasia or tibial pseudarthrosis.

               6. A first-degree relative with NF1.

          -  At least one measurable typical or nodular PN in principle, defined as a lesion of at
             least 3 cm measured in one dimension.

          -  Adequate organ/haematological function

        Key Exclusion Criteria:

          -  Evidence of malignant peripheral nerve sheath tumour.

          -  Prior malignancy (except for adequately treated basal cell or squamous cell skin
             cancer, in situ cervical cancer, low grade optic pathway gliomas associated with NF1
             which does not require systemic treatment or other cancer from which the subject had
             been disease free for ≥2 years or which would not have limited survival to <2 years)
             or other cancer requiring treatment with chemotherapy or radiation therapy.

          -  Clinically significant cardiovascular disease

          -  Known history of human immunodeficiency virus, serologic status reflecting active
             hepatitis B virus (HBV) or hepatitis C virus (HCV) infection, or any uncontrolled
             active systemic infection

          -  Subjects with clinically significant ophthalmological findings/conditions

          -  Inability to undergo MRI and/or contraindication for MRI (i.e. prosthesis or
             orthopaedic or dental braces that would interfere with volumetric analysis of target
             PN on MRI).

          -  Have refractory nausea and vomiting, chronic gastrointestinal diseases (e.g.
             inflammatory bowel disease), or significant bowel resection that would adversely
             affect the absorption/bioavailability of the orally administered study medication.

          -  Receiving supplementation with vitamin E greater than 100% of the daily recommended
             dose.

          -  Receiving herbal supplements or medications known to be strong inhibitors or inducers
             of the cytochrome P450 (CYP) 3A4 enzymes unless such products can be safely
             discontinued at least 14 days before the first dose of study medication.