Brain distribution and active efflux of three panRAF inhibitors: considerations in the treatment of melanoma brain metastases.

Targeted inhibition of RAF and MEK by molecularly-targeted agents has been employed as a strategy to block aberrant MAPK signaling in melanoma. While the use of BRAF and MEK inhibitors, either as a single agent or in combination, improved efficacy in BRAF-mutant melanoma, initial responses are often followed by relapse due to acquired resistance. Also, BRAF inhibitors are associated with a paradoxical activation of MAPK pathway, causing the development of secondary malignancies. The use of panRAF inhibitors, i.e., those that target all isoforms of RAF, may overcome paradoxical activation and resistance. The purpose of this study was to perform a quantitative assessment and evaluation of the influence of efflux mechanisms at the BBB (P-glycoprotein and breast cancer resistance protein) on the brain distribution of 3 novel panRAF inhibitors: CCT196969, LY3009120 and MLN2480. In vitro studies using transfected MDCKII cells indicate that only LY3009120 and MLN2480 are substrates of Bcrp, and none of the 3 inhibitors are substrates of P-gp. The 3 panRAF inhibitors show high non-specific binding in brain and plasma. In vivo studies in mice show that the brain distribution of CCT196969, LY3009120 and MLN2480 is limited, and is enhanced in transgenic mice lacking P-gp and Bcrp. While MLN2480 has a higher brain distribution, LY3009120 exhibits superior in vitro efficacy in patient-derived melanoma cell lines. The delivery of a drug to the site of action residing behind a functionally intact BBB along with drug potency against the target, collectively play a critical role in determining in vivo efficacy outcomes.