I. To identify the maximally tolerated dose of trametinib to be used in combination with whole brain radiation therapy in patients with brain metastases. (Cohort A) II. To quantify trametinib in resected brain metastatic lesions utilizing high performance liquid chromatography/tandem mass spectrometry (LC/MS/MS) and compare to quantification of adjacent tissues: brain margin, arachnoid, and cerebrospinal fluid (CSF). (Cohort B)