Abstract 3714: Trametinib (Mekinist TM) has limited brain distribution in the mouse model

The BBB can critically limit the delivery of several molecularly-targeted agents into the brain. The discovery of focal BRAF mutations in melanoma has stimulated the development of specific BRAF inhibitors. MEK is downstream of BRAF in the MAPK pathway, and has been targeted for combination therapy with BRAF inhibition. Trametinib is a MEK1/2 inhibitor that has improved survival in combination with dabrafenib in clinical trials for metastatic melanoma. We have recently shown that BRAF inhibitors (vemurafenib and dabrafenib) have limited brain distribution due to active efflux at the blood-brain barrier (BBB). Melanoma metastasizes to the brain and effective treatments require adequate CNS exposure of all drugs used in combination therapy. The study purpose was to investigate mechanisms that influence the CNS distribution of trametinib, particularly focusing on the active efflux transport proteins P-gp and Bcrp in the BBB. Intracellular accumulation and bidirectional flux studies were conducted in polarized MDCKII cells that overexpress human P-gp (MDR1) or murine Bcrp (Bcrp1). The brain and plasma AUCs of trametinib were determined after an intravenous dose of 5 mg/kg in WT, Mdr1a/b (-/-), Bcrp1 (-/-) and Mdr1a/b (-/-) Bcrp1 (-/-) (TKO) mice. The steady-state brain distribution of trametinib was determined using osmotic pumps in WT and Mdr1a/b (-/-) (P-gp KO), Bcrp1 (-/-) (Bcrp KO), and Mdr1a/b (-/-) Bcrp1 (-/-) (TKO) mice. The steady-state brain distribution of a combination of trametinib and dabrafenib were determined in WT and TKO mice. IC50s were determined in patient-derived melanoma cell lines (WT and V600E mutant) using the MTT assay. Intracellular accumulation of trametinib was 45% lower in the MDCKII-MDR1 cells and 81% lower in the MDCKII-Bcrp1 cells as compared to WT. Trametinib permeability was ∼ 2.5 and 3.7-fold greater in the basolateral-to-apical direction as compared to apical-to-basolateral direction in the MDR1 and Bcrp1 cells, respectively. Following a single intravenous dose, the brain-to-plasma AUC ratios were approximately 5-fold greater in P-gp KO and TKO mice as compared to the WT. Interestingly, the brain distribution of trametinib in Bcrp KO mice was no different from WT mice. The steady-state brain to plasma (B/P) ratio of trametinib was 5-fold greater in the TKO mice when compared to the wild-type mice. The B/P ratio of both dabrafenib and trametinib were significantly greater in the TKO mice as compared to WT when dosed simultan