Browsing by Subject "P-glycoprotein"

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    Improving delivery of molecularly targeted agents to glioma.
    (2011-06) Agarwal, Sagar Suresh
    Treatment of glioblastoma multiforme is at a crossroads. Promising new molecularly-targeted agents have failed to show any significant clinical benefit. Treatment is particularly challenging since the tumor resides in a tissue that is considered to be a pharmacological and immunological sanctuary due to the presence of the blood-brain barrier. Protective mechanisms at the blood-brain barrier (BBB), such as the endothelial tight junctions and drug efflux transporters, restrict the passage of most large and small molecules into the brain. Limited drug delivery to the tumor is a plausible explanation for the failure of molecularly-targeted therapy in glioma. If therapeutic agents do not reach their target, regardless of their potency, they cannot be effective. The objective of this work was to show that active efflux transporters at BBB restrict delivery of potent molecularly-targeted agents to their targets. More importantly, the aim was to demonstrate that the targets in question are in invasive tumor cells that are left behind after surgery and remain shielded behind an intact blood-brain barrier. The ultimate goal of this endeavor is to improve delivery of molecularly-targeted therapy to the tumor and show that this can translate to enhanced efficacy against this lethal disease. We show that brain distribution of the tyrosine kinase inhibitors, gefitinib, erlotinib and sorafenib, is restricted due to active efflux mediated by p-glycoprotein (P-gp) and the breast cancer resistance protein (BCRP). We further demonstrate that delivery of these drugs to the brain increases dramatically when the two transporters are genetically absent or pharmacologically inhibited. Using a rat xenograft model and a spontaneous mouse model of glioma, we show that the BBB is heterogeneously disrupted in the brain. The blood-brain barrier is disrupted in the tumor core resulting in high tumoral concentrations of erlotinib and dasatinib. However, it is intact in areas immediately adjacent to the tumor, and therefore restricts drug delivery to these sites. Thus, clinical assessment of drug delivery when using drug concentrations in tumor core (the resected tissue) as a guide for the adequacy of drug delivery can be misleading. Furthermore, we show that increasing drug delivery to these areas, by genetic deletion or pharmacological inhibition of P-gp and BCRP, results in a remarkable enhancement in efficacy of the tyrosine kinase inhibitor, dasatinib. Finally, we show that efficacy of dasatinib increases dramatically in tumor bearing transgenic mice, that are deficient in P-gp and BCRP, and consequently, these mice survive for a significantly longer time compared to the wild-type mice. These observations underline that restricted delivery of molecularly-targeted agents to their targets can be a significant determinant of drug efficacy against glioma. In an invasive tumor, such as glioblastoma, it is important to realize that the target resides within the invasive glioma cells, that remain shielded by an intact blood-brain barrier, and evade chemotherapy. Overall, this work highlights the need to develop strategies to improve drug delivery to the invasive tumor in glioma and translate these strategies to the clinic.
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    Improving the Delivery of Molecularly-Targeted Agents to Effectively Treat Melanoma Brain Metastases
    (2015-03) Vaidhyanathan, Shruthi
    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