AZD6244 Hydrogen Sulfate for Children With Nervous System Tumors

Background: \- Plexiform neurofibromas are tumors that grow in and around nerves. The only way to treat them is with surgery. Some of these tumors cannot be completely removed. The tumors may be too large, too numerous, or in a bad location for surgery. An experimental drug called AZD6244 hydrogen sulfate may be able to prevent the tumors from growing, slow down their growth, or shrink them. This drug has been tested in adults with cancer and in children with some types of brain cancer. This study will test how well this drug works with these types of tumors. Objectives: \- To study the safety and effectiveness of AZD6244 hydrogen sulfate in children and young adults with plexiform neurofibromas that cannot be completely removed by surgery. Eligibility: \- Children and young adults between 12 and 18 years of age who have plexiform neurofibromas that cannot be completely removed by surgery. Design: * Patients will be screened with a physical exam, medical history, blood tests, and imaging studies. * They will take the study drug twice a day with 8 ounces of water, every day for 28-day cycles of treatment. During study visits, participants will have blood and urine tests and physical exams. They will also have imaging studies to examine the tumor sizes and locations. They will answer questions about their health. They may have other tests as needed. * Participants will continue to receive the study drug as long as they have no severe side effects and the disease is not getting worse.

-INCLUSION CRITERIA: 1. Age Phase I: \>=3 years and \=2 and \= 0.55 m\^2, and able to swallow whole capsules. 2. Diagnosis: Patients with NF1 and inoperable PN, 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. The PN has to cause (stratum 1) or have the potential to cause (stratum 2) significant morbidity, such as (but not limited to) head and neck lesions that could compromise the airway or great vessels, paraspinal lesions that can cause myelopathy brachial or lumbar plexus lesions that could cause nerve compression and loss of function, lesions that could result in major deformity (e.g., orbital lesions) or are significantly disfiguring, lesions of the extremity that cause limb hypertrophy or loss of function, and painful lesions. Patients will be enrolled into stratum 1 or 2 based on PN related morbidity. Histologic confirmation of tumor is not necessary in the presence of consistent clinical and radiographic findings, but should be considered if malignant degeneration of a PN is clinically suspected. 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, all study subjects must have either positive genetic testing for NF1 or have at least one other diagnostic criterion for NF1 listed below: (NIH Consensus conference): * Six or more cafe-au-lait macules (\>=0.5cm in prepubertal subjects or \>=1.5 cm in post pubertal subjects) * Freckling in axilla or groin * Optic glioma * Two or more Lisch nodules * A distinctive bony lesion (dysplasia of the sphenoid bone or dysplasia or thinning of long bone cortex) * A first-degree relative with NF1 3. Measurable disease: Patients must have at least one measurable PN, defined as a lesion of at least 3 cm measured in one dimension. Patients who underwent surgery for resection of a PN are eligible provided the PN was incompletely resected and is measurable. Phase II: Measurability and suitability for volumetric MRI analysis of the target PN must be confirmed with the NCI POB prior to enrolling a patient. The target PN will be defined as the clinically most relevant PN, which has to be amenable to volumetric MRI analysis. PN will be classified as "typical PN" versus "nodular PN" versus "solitary nodular PN" prior to enrollment 4. Prior Therapy: Patients with NF1 will only be eligible if complete tumor resection is not considered to be feasible without substantial risk or morbidity. * Since there is no standard effective chemotherapy for patients with NF1 and PN, patients may be treated on this trial without having received prior medical therapy directed at their PN. * Since selumetinib is not expected to cause substantial myelosuppression, there will be no limit to number of prior myelosuppressive regimen for PN or other tumor manifestations associated with NF1 such as optic glioma. * Patients who have received previous investigational agents or biologic therapy, such as tipifarnib, pirfenidone, Peg-Intron, sorafenib, imatinib or other targeted therapies are eligible for enrollment. At least 4 weeks must have elapsed since receiving medical therapy directed at the PN. Patients who received prior medical therapy for their PN must have recovered from the acute toxic effects of all prior therapy to \= 16 years of age must have a Karnofsky performance level of \>=70%, and children \=70%. Patients who are wheelchair bound because of paralysis secondary to a plexiform neurofibroma should be considered ambulatory when they are up in their wheelchair. Similarly, patients with limited mobility secondary to need for mechanical support (such as an airway PN requiring tracheostomy or CPAP) will also be considered ambulatory for the purpose of the study. 6. Hematologic Function: Patients must have an absolute neutrophil count \>=1500/microliter, hemoglobin \>=9g/dl, and platelets \>=100,000/microliter. 7. Hepatic Function: Patients must have bilirubin within 1.5 x the upper limit of normal for age, with the exception of those with Gilbert syndrome, and AST/ALT within \=60ml/min/1.73 m\^2 or a normal serum creatinine based on age, described below. Age (years) / Maximum Serum Creatinine(mg/dL): Age \5 to \10 to \15 / Maximum Serum Creatinine 1.5 mg/dL 9. Cardiac Function: Normal ejection fraction (ECHO or cardiac MRI) \>= 53% (or the institutional normal; if a range is given then the upper value of the range will be used); QTC or QTcF \