Pharmacokinetics, Safety and Efficacy of the Selumetinib Granule Formulation in Children aged ≥ 1 to < 7 Years with Neurofibromatosis Type 1 (NF1) Related Symptomatic, Inoperable Plexiform Neurofibromas (PN) (SPRINKLE)

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• Participants with confirmed or suspected malignant glioma or MPNST. Participants with low grade glioma (including optic glioma) not requiring systemic therapy are permitted. History of malignancy except for treatment with curative intent with no known active disease ≥ 2 years before the first dose of study intervention and low risk of recurrence. Refractory nausea and vomiting, chronic gastrointestinal disease, inability to swallow the formulated product, or previous significant bowel resection that would preclude adequate absorption, distribution, metabolism, or excretion of selumetinib. A life-threatening illness, medical condition, organ system dysfunction or laboratory finding which, in the Investigator's opinion, could compromise the participant's safety, interfere with the absorption or metabolism of selumetinib, or put the study outcomes at undue risk. Participants with clinically significant cardiovascular disease. As judged by the Investigator, any evidence of disease, including any participant known to have hepatitis B, hepatitis C, or HIV which, in the Investigator's opinion, makes it undesirable for the participant to take part in the study. Total bilirubin > 1.5 × the ULN for age except for those with Gilbert syndrome (≥ 3 × ULN) or AST/ALT > 2 × ULN. Creatinine clearance or radioisotope glomerular filtration rate < 60 mL/min/1.73 m2 or Serum creatinine > 0.8 mg/dL (for participants aged ≥ 1 to < 4 years) or > 1.0 mg/dL (for participants aged ≥ 4 years). An absolute neutrophil count < 1500/μL or Haemoglobin < 9g/dL or Platelets < 100,000/μL or had a transfusion (of red cells or other blood derived products) within the 28 days prior to study entry (date of ICF signature). Participants with ophthalmological findings/condition. Unresolved chronic toxicity with CTCAE Grade ≥ 2 which are associated with previous therapy for NF1-PN (except hair changes such as alopecia or hair lightening). Participants who have previously been treated with a MEKi (including selumetinib) and had disease progression, or due to toxicity either discontinued treatment and/or required a dose reduction. Had major surgery within 4 weeks of the first dose of study intervention, with the exception of surgical placement for vascular access. Have planned major surgery during the treatment period. Received or are receiving an IMP or other systemic NF1-PN target treatment (including MEKi) within 4 weeks of first dose of study intervention, or within a period during which the IMP or systemic PN target treatment has not been cleared from the body (e.g., a period of 5 'half-lives'), whichever is longer. Receiving supplements or medications known to be strong or moderate inhibitors or inducers of CYP3A4 or strong or moderate inhibitors of CYP2C19 enzymes unless such products can be safely discontinued at least 14 days or 5 half-lives (whichever is longer) before the first dose of study medication. Inability to undergo MRI and/or contraindication for MRI examinations. Prosthesis or orthopaedic or dental braces that would interfere with volumetric analysis of target PN on MRI. Received radiotherapy within 6 weeks of start of study intervention or any prior radiotherapy directed at the target or non-target PN. Received growth factors within 1 week of date of ICF signature. Participation in another clinical study with an investigational product administered within 30 days of first dose of study intervention. Known severe hypersensitivity or history of allergic reactions to selumetinib or compounds of similar chemical or biologic composition.