Phase I Study to Assess the Effect of Food on the PK and Gastrointestinal Tolerability of Selumetinib in Adolescent Children With Neurofibromatosis Type 1 Related Plexiform Neurofibromas

Inclusion Criteria: * Male and female participants aged ≥ 12 to \< 18 years at the time of signing the informed consent. * All study participants must be diagnosed with (i) NF1 per NIH Consensus Development Conference Statement and (ii) inoperable PN. In addition to PN, participants must have at least 1 other diagnostic criterion for NF1 as defined in protocol. * Participants must require treatment for NF1 and inoperable PN due to actual symptoms or because of the potential to develop significant clinical complications, as judged by the Investigator, as defined in the protocol. * Participants who have had prior treatment with any MEKi (including selumetinib) may be considered for inclusion in this study. * Participants must have a BSA ≥ 1.3 and ≤ 2.5 m2 Exclusion Criteria: * Evidence or suspicion of optic glioma, malignant glioma, MPNST, or other cancer requiring treatment with chemotherapy or radiation therapy * Prior malignancy requiring active treatment (except for adequately treated basal cell or squamous cell skin cancer, in situ cervical cancer, or other cancer from which the participant had been disease free for ≥ 2 years or which would not have limited survival to \< 2 years). * A life-threatening illness, medical condition, organ system dysfunction of 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 …