Factors influencing the CNS distribution of a novel MEK-1/2 inhibitor: Implications for combination therapy for melanoma brain metastases

Brain metastases are a major cause of mortality in patients with advanced melanoma. Adequate brain distribution of targeted agents for melanoma will be critical for treatment success. Recently, improvement in overall survival led to US Food and Drug Administration (FDA) approval of the v-raf murine sarcoma viral oncogene homolog B (BRAF) inhibitors, vemurafenib and dabrafenib, and the mitogen-activated protein kinase kinase-1 (MEK)-1/2 inhibitor, trametinib. However, brain metastases and emergence of resistance remain a significant problem. MEK-1/2 is downstream of BRAF in the mitogen-activated protein kinase (MAPK) signaling pathway, making it an attractive target to combat resistance. The recently approved combination of dabrafenib and trametinib has shown improvement in progression-free survival; however, adequate brain distribution of both compounds is required to effectively treat brainmetastases. In previous studies, we found limited brain distribution of dabrafenib. The purpose of the current study was to investigate factors influencing the brain distribution of trametinib. In vitro studies indicated that trametinib is a substrate for both P-glycoprotein (P-gp) and Bcrp, efflux transporters found at the blood-brain barrier. In vivo studies in transgenic mouse models confirmed that P-gp plays an important role in restricting brain distribution of trametinib. The brain-to-plasma partition coefficient (AUC_brain/AUC_plasma) was approximately 5-fold higher in Mdr1a/b^(-/-) (P-gp knockout) and Mdr1a/b^(-/-)Bcrp1^(-/-)(triple knockout) mice when compared with wild-type and Bcrp1^(-/-)(Bcrp knockout) mice. The brain distribution of trametinib was similar between the wild-type and Bcrp knockout mice. These results show that P-gp plays an important role in limiting brain distribution of trametinib and may have important implications for use of trametinib as single agent or in combination therapy for treatment of melanoma brain metastases.