Koselugo 25mg Capsules Koselugo contains selumetinib, an oral, selective inhibitor of mitogen-activated protein kinase kinase 1 and 2 (MEK1/2). MEK1/2 proteins are critical upstream regulators of the extracellular signal-related kinase (ERK) pathway. In neurofibromatosis type 1 (NF1), loss-of-function mutations in the NF1 gene lead to dysregulated RAS/MAPK signaling and tumor cell proliferation. Selumetinib blocks MEK kinase activity, preventing phosphorylation of ERK and downstream signal transduction, thereby inhibiting cell proliferation and reducing tumor volume in plexiform neurofibromas. Koselugo is indicated for the treatment of pediatric patients (2 years of age and older) with neurofibromatosis type 1 (NF1) who have symptomatic, inoperable plexiform neurofibromas (PN).