occlusion, retinal pigment epithelial detachment and impaired vision. Selumetinib can also cause increased creatinine phosphokinase (CPK). CPK is an enzyme found in the heart, brain and skeletal muscles. When muscle tissue is damaged, CPK leaks into a person's blood, which can be a sign of rhabdomyolysis (breakdown of skeletal muscle due to direct or indirect muscle injury). Further, selumetinib capsules contain vitamin E, and users are at an increased risk of bleeding if their daily intake of vitamin E exceeds the recommended or safe limits. Interactions As selumetinib is thought to be metabolized by the liver enzymes CYP3A4 and CYP2C19, use of moderate to strong CYP3A4 inhibitors (such as grapefruit juice) and of the CYP2C19 inhibitor fluconazole is discouraged for people taking selumetinib. Pharmacology Mechanism of action Selumetinib is a kinase inhibitor, more specifically a selective inhibitor of the enzyme mitogen-activated protein kinase kinase (MAPK kinase or MEK) subtypes 1 and 2. These enzymes are part of the MAPK/ERK pathway, which regulates cell proliferation (i.e., growth and division) and is overly active in many types of cancer. History Selumetinib was discovered by Array BioPharma and was licensed to AstraZeneca. It has been investigated for the treatment of various types of cancer, such as non-small cell lung cancer (NSCLC) and thyroid cancer. The US Food and Drug Administration (FDA) granted the application for selumetinib priority review, breakthrough therapy, and orphan drug designations. It was granted a rare pediatric disease designation for the treatment of pediatric NF-1 along with a rare pediatric disease priority review voucher. In April 2020, selumetinib was approved by the FDA for the treatment of children with NF-1. It is the first drug approved in the US to treat this rare disease. The approval was based on a clinical trial of children who had NF-1 and inoperable plexiform neurofibromas (defined as a PN that could not be completely removed without risk for substantial morbidity to the child), conducted by the National Cancer Institute. The efficacy results were from 50 of the children who received the recommended dose and had routine evaluations of changes in tumor size and tumor-related morbidities during the trial. The children received selumetinib 25 mg/m2 orally twice a day until disease progression or until they experienced unacceptable adverse reactions. The clinical trial measured the overall response rate (ORR), defined as the percentage of subjects with a complete response and those who experienced more than a