@phdthesis{10336/8992,
author = {Pinzon-Daza, Martha L.},
year = {2014},
url = {http://repository.urosario.edu.co/handle/10336/8992},
abstract = {Malignant gliomas represent one of the most aggressive forms of Central Nervous System
(CNS) tumors. According to the WHO classification of brain tumors, astrocytomas have been
categorized into four grades, determined by the underlying pathology. Malignant (or highgrade)
gliomas include anaplastic glioma (WHO grade III) as well as glioblastoma multiforme
(GBM; WHO grade IV). These are the most aggressive brain tumors with the worst prognosis
(1). The therapeutic management of CNS tumors is based on surgery, radiotherapy and
chemotherapy, depending on the characteristics of the tumor, the clinical stage and age (2),
(3), however none of the standard treatments is completely safe and compatible with an
acceptable quality of life (3), (4). Chemotherapy is the first choice in disseminated tumors,
like invasive glioblastoma, high-risk medulloblastoma or multiple metastasis, but the
prognosis in these patients is very poor (2),(3). New targeted therapies (2), anti-angiogenic
therapies (3), (4) or gene therapies show a real benefit only in limited groups of patients with
known specific molecular defects (4). Thereby, the development of new pharmacological
therapies for brain tumors is mandatory. Malignant gliomas are frequently chemoresistant
and this resistance seems to depend on at least two mechanisms: first, the poor penetration
of many anticancer drugs across the blood-brain barrier (BBB), the blood-cerebrospinal fluid
barrier (BCSFB) and the blood-tumor barrier (BTB), due to their interaction with several
ATP-binding cassette (ABC) drug efflux transporters that are overexpressed by the
endothelial or epithelial cells of these barriers. Second, ABC drug efflux transporters in tumor
cells confer multidrug resistance (MDR) on several other solid tumors; they are present on
CNS tumors too and their role in gliomas is under investigation (5). Drug delivery across the
blood-brain barrier (BBB) is one of the vital problems in targeted therapy treatments. Recent
studies have shown that some small molecules used in these therapies are substrates of Pglycoprotein
(Pgp), as well as other efflux pumps like multidrug resistance-related proteins
(MRPs) and breast-cancer resistance related protein (BCRP), which extrude several
anticancer drugs and will not allow drugs to reach the tumor (1).
DOXOrubicin (DOXO), a drug widely used in anti-cancer therapy, is a substrate of Pgp and
BCRP, and it is very effective to attack the vitro brain tumor cells, but has a limited clinical use
for its low delivery across BBB and the resistance of tumors. On the other hand BBB cells and
brain tumor cells also have surface proteins, such as Low Density Lipoprotein Receptor
(LDLR), which could be used as a therapeutic target. The importance of this study is based on
the generation of therapeutical strategies to promote the passage of drugs through the BBB
and the intratumoral accumulation, and at the same time, on the analysis of cellular
mechanisms that induce increased expression of ABC transporters, to be used as therapeutic
targets.
In this work we demonstrated that the use of a new strategy based on the "Trojan horse",
which combines DOXOrubicin introduced into a liposome, could safeguards the drug to
prevent its recognition by the ABC transporters in both the BBB and the tumor cells. The
construction of liposome allowed using the LDLR receptor cells as docking receptor, ensuring
the entrance through the BBB and into the tumor cells through a process of endocytosis. This
mechanism was associated with the use of statins, anti-cholesterol drugs which favoured the
expression of LDLR and decreased the activity of ABC transporters, increasing the efficiency
of our Trojan horse. Accordingly, I demonstrated that the use of a new DOXOrubicin
liposomal formulation mimicking LDLs, called ApolipoDOXO, further favors drug delivery through the BBB, overcoming the resistance of tumor and reducing the side effects of
DOXOrubicin dose. In addition this strategy can be considered as a new strategy to increase
the effectiveness of different drugs in several brain tumors and ensures high efficiency even
in a hypoxic environment, characteristic of cancer cells, where the expression of Pgp
transporter was increased.
Taking advantage of another signaling pathway recognized as a modulator of Pgp activity this
study presents not only the strategy of the Trojan horse, but also a second therapeutic
proposal related to the use of Temozolomide plus DOXOrubicin. This strategy showed that
temozolomide (TMZ) penetrated the BBB in a way involved the Wnt/GSK3/β-catenin
signaling pathway, which modulates the expression of Pgp transporter. It was demonstrated
that the TMZ reduces Wnt3 protein and mRNA allowing raising the hypothesis that this drug
decreases Wnt3 gene transcription in BBB cells, decreasing β-catenin pathway activation by
its phosphorylation, reducing β-catenin nuclear translocation and binding to the promoter of
the mdr1 gene. Taking together the results of this study allowed the recognition of three basic
mechanisms related to the down-regulation of Pgp and associated strategies: the first was the
use of statins, which led to the transporter nitration decreasing its activity by NFκB pathway;
the second one was based on the use of temozolomide, which by methylating Wnt3 gene
reduces the activity of the β-catenin signaling pathway, decreasing the expression of Pgp
transporter; the third one consisted on the cross-talk between the Wnt/GSK3/β-catenin axis
and the Wnt/RhoA/RhoA kinase as a modulator of mdr1 transcription: we demonstrated that
RhoA protein kinase promoted the activation of the protein PTB1, which by phosphorylating
GSK3 induced phosphorylation of β-catenin, priming it for destruction by the proteasome,
avoiding the binding to the promoter of the mdr1 gene and therefore reducing Pgp
expression. In conclusion, the therapeutic startegies proposed in this work increased the
cytotoxicity of tumour cells by increasing permeability not only in the BBB, but also in tumor
barrier. Also, the "Trojan horse" strategy could be useful for the therapy of other diseases
associated with the central nervous system. On the other hand, these studies indicate that
recognition of mechanisms associated with the expression of ABC transporters could be a key
tool in the development of new anti-cancer therapies.},
keywords = {Glioblastoma},
keywords = {transportadores ABC},
keywords = {Doxorubicina},
keywords = {Temozolomide},
keywords = {Hipoxia},
keywords = {Pgp},
title = {A "Trojan horse" strategy to reverse drug-resistance in brain tumors},
publisher = {Universidad del Rosario},
keywords = {Glioblastoma},
keywords = {ABC transporters},
keywords = {Hypoxia},
keywords = {Doxorubicin},
keywords = {Temozolomide},
keywords = {Pgp},
}