Browsing by Subject "Efflux transporters"
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Item Active efflux transport and CNS distribution of the novel antifolate pemetrexed.(2010-03) Li, LiPemetrexed (PMX, Alimta®) is a novel multi-targeted antifolate approved for the treatment of malignant pleural mesothelioma and non-small cell lung cancer (NSCLC). Given the high incidence of brain metastasis in NSCLC patients and wide use of the classic antifolate, methotrexate (MTX), in primary CNS lymphoma, the clinical use of PMX may eventually include the treatment of CNS tumors. However, previous studies in our laboratory indicated that, like MTX, PMX has difficulty in penetrating across the blood-brain barrier (BBB). Factors limiting the CNS distribution of PMX remain unidentified. One important determinant of CNS distribution is efflux transport by BBB transporters. The overall objective of this study was to characterize the brain-to-blood efflux transport of PMX and to examine role of BBB efflux transporters such as BCRP, MRP2 and other organic anion transporters in brain distribution of PMX. The interaction of PMX with BCRP was examined in vitro and in vivo. In vitro results revealed that PMX is a substrate for BCRP-mediated transport. In vivo examination indicated that deletion of Bcrpl has little influence on brain penetration of PMX. Using the brain efflux index method, the mechanism responsible for the brain efflux of PMX and MTX was investigated. The results revealed that brain elimination half-life of PMX and MTX were 48 and 32 minutes, respectively and both PMX and MTX undergo saturable efflux transport across the BBB. MRP2 does not play a role in the brain distribution of either antifolate. However, BCRP makes a significant contribution to brain elimination of MTX, but not PMX. In addition, it was observed that brain-to-blood transport of PMX and MTX was markedly inhibited by probenecid and benzylpenicillin, suggesting the involvement of organic anion transporters, possibly OAT3. Given the knowledge that solid tumors and some physiological barriers have an acidic extracellular environment, the effect of pH on transport activity of BCRP for PMX was examined. In addition, the molecular basis of the observed pH-dependency in BCRP transport activity was explored based on the recent homology model of BCRP. This study has important implications in the handling of PMX and other chemotherapy drug molecules in the acidic environment of tumors and in the distribution and elimination of the drug molecules. Studies presented in this dissertation provide useful information about the specific mechanisms involved in limiting the brain penetration of PMX and MTX. This new knowledge will help in formulating strategies to improve CNS delivery of these antifolates and maylead to more successful treatment of primary and secondary CNS tumors.Item Improving the Delivery of Molecularly-Targeted Agents to Effectively Treat Melanoma Brain Metastases(2015-03) Vaidhyanathan, ShruthiThe 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 metastasesItem Permeability, binding and distributional kinetics of Ponatinib, a multi-kinase inhibitor: implications for the treatment of brain tumors(2018-01) Laramy, JaniceGlioblastoma (GBM) is the most common malignant brain tumor and one of the unmet medical needs. Among over 1,000 GBM clinical trials testing molecularly-targeted agents, no single agent has demonstrated drastic improvement in patient survival, in part due to manifold drug delivery challenges to the brain tumor. Advances in genomic and proteomic technologies have identified numerous oncogenic targets, such as mutated or amplified receptor tyrosine kinase pathways, which have enabled proteomic-guided drug selection for the treatment of GBM. Despite these advances, drug discovery and development for the treatment of GBM is still complexed by the challenges that are unique to the brain tumor that resides behind the blood-brain barrier (BBB), a formidable barrier for reaching therapeutic drug concentration in the brain tumor. Many molecularly-targeted drugs that have been examined for the treatment of GBM are a substrate of two highly expressed BBB efflux transporters, P-glycoprotein (P-gp) and Breast cancer resistance protein (Bcrp). This dissertation examined the multiple drug delivery challenges, including central nervous system (CNS) penetration, binding, and distributional kinetics and the implications on drug efficacy and/or toxicity, for a tyrosine kinase inhibitor (ponatinib) that can serve as a case example.