Improving the Delivery of Molecularly-Targeted Agents to Effectively Treat Melanoma Brain Metastases

Abstract

The FDA approval of molecularly-targeted drugs that specifically targeted aberrant signaling proteins has brought about new hope for the treatment of advanced melanoma. Historically, metastatic melanoma has been an untreatable devastating disease. Two BRAF inhibitors (vemurafenib and dabrafenib), a MEK inhibitor (trametinib), and a combination of dabrafenib and trametinib are currently in use and several other drugs are in clinical development. Melanoma is known to metastasize to distant organs such as the lung, liver and brain. A critical challenge in the successful treatment of metastatic melanoma is the effective treatment of brain metastases. A significant proportion of melanoma patients have brain metastases at autopsy. It is also known that once patients develop clinical signs of CNS disease, they have an abysmally poor survival (less than 6 months). This brings about an important question about the efficacy of current drugs in treating brain metastases. The blood-brain barrier is comprised of a tight network of endothelial cells that are sealed together by tight-junction (TJ) protein complexes. The BBB also expresses several efflux transport proteins that utilize ATP to pump drug molecules against a concentration gradient. Together, the TJ proteins and ATP-dependent efflux transport proteins are known to effectively limit the permeability of several chemotherapeutics across the blood-brain barrier. Of particular interest are two efflux transporters, P-glycoprotein (P-gp) and breast-cancer resistance protein (BCRP) that are known to be highly expressed at the BBB. One of the aims of this thesis project was to understand the factors that potentially limit the efficacy of molecularly-targeted drugs in treating deadly melanoma brain metastases. Through this work, we have shown that several molecularly-targeted agents are substrates for active efflux by P-gp and BCRP. Through a series of carefully planned in vitro experiments and elegant pharmacokinetic studies in mice we conclude that the limited brain distribution of vemurafenib, dabrafenib, trametinib, and GSK2126458 (a Pi3K/mTOR inhibitor) is due to their interaction with P-gp and BCRP. We also investigated potential differences in pharmacokinetics and pharmacodynamics of vemurafenib when administered as pharmacy grade Zelboraf; versus non-pharmacy grade vemurafenib. We observed that formulation differences that affect the solubility of a drug are extremely critical to designing and interpreting meaningful pre-clinical studies. Currently, we are conducting studies in a novel melanoma mouse model in order to understand the efficacy of molecularly- targeted drugs in treating brain metastases (single agent or in-combination). The findings of this thesis provide significant insight into the selection of rational drug combinations and are highly relevant to improving the treatment of melanoma brain metastases