Part 2- Do the efflux transporters protect the glioma cells?

One of the longstanding questions for DIPG has been."Why doesn't chemo work?"
There have been dozens of trials using all kinds of agents and all of them have virtually the identical Kaplan Meier Curve.  The think we don't know is why that is.   Often times one hears about something in the blood brain barrier that limits chemotherapy effectiveness.   Other times one wonders if it is something in the glioma cells themselves that make them more resistant.   The Netherlands paper tried to answer one part of this in better defining ABC transporters in pediatric glioma.

ABC transporters essentially function as little door ways in the cell's membrane either usher substances into the cell (importers) or out of the cells (exporters).   The exporters are tone ones that are a problem withe drug resistance.   The transporters pump drugs and toxins out of the cells.

There have been 3 different ABC transporters found that escort drugs out of cells.   These include P-glcyoprotein (P-gp, ABCB1), breast cancer resistance protein (BCRP, ABCG2) and multidrug resistance associated proteins (MRPs, ABCC1).

This Netherlands study looked at the above 3 ABC transporters in each of the glioma cell lines.   All cell lines were negative for P-gp.  Only one supratentorial pediatric glioma cell line had BRCP1.  High levels of MRP1 was found in 4 of the 9 glioma cell lines.  That included 2 of 3 of the DIPG cell lines.

 The researchers then looked at actual tumor sample sections to try to determine the amount of ABC transporters in the glioma cells versus the blood vessels. They found that P-gp was not present in most of the glioma cells but was presesnt moderately in the tumor's blood vessels.   BCRP1 was not present in the glioma cells but was highly pressent in the blood vessels. Only MRP1 was seen in both the glioma cells and the blood vessels.   A chart on the cell lines tested (including 2 of the 3 DIPG samples) is available (click here to see chart).

So, what does this mean for DIPG now?

That is hard to say.   So far tying to inhibit the ABC transporters with different agents have been problematic because of significant toxic side effects.   These ABC transporters are meant to protect normal cells as well.   However, since  most of the ABC transporters seem to be in the blood vessels  a way around this blood-brain barrier could be direct tumor delivery (i.e., convenction enhanced delivery).  The researchers also suggest developing drugs that are not a substract for these transporters could also increase chemotherapy effectiveness.

It does tell us that in trying to find a cure for DIPG that we can not just focus on finding targets within the cancer biology but also need to work on finding ways for new agents to be able reach these targets.

Note:  The ATP-binding cassette transporters issue was featured in a recent blogspot regarding a new abstract out of Oren Becher's lab in Duke.

This work was funded by KiKa "Stichting Kinderen Kankervrij"- Dutch Children Cancer-free Foundation.

References:
In vitro drug response and efflux transporters associated with drug resistance in pediatric high grade glioma and diffuse intrinsic pontine glioma
PLoS One. 2013 Apr 29;8(4):e61512. doi: 10.1371/journal.pone.0061512. Print 2013.
Free Full Text-  http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3639279/

KiKa Stichting Kinderen Kankervrij- Dutch Children Cancer-free Foundation
http://www.kika.nl/

ABC transporters limit dansantinib efficacy in mice
http://dipg.blogspot.com/2013/04/abc-transported-limit-dasatinib.html

ABC transporters limit dasatinib efficacy in mice

Ever since the first whole genome DIPG profile paper from Sick Kids in Canada came out in 2010, there has been interest in platelet derived growth factor receptor alpha.  This is because all the samples (9 post-mortem and two pretreated surgical samples) showed PDGRF-alpha expression.  The hope was that PDGFR-alpha could be a target to make a difference in DIPG.   The search was on for an agent to try.

An available PDGFR-alpha antagionist was found, dasantinib.  This Bristol-Myers-Squibb drug is marketed under the name Sprycel for the indication chronic myelogenous leukemia (CML).  When looking in clinicaltrials.gov, there are already 219 trials up for this drug.  It has been already been tried on adults and in children.   The availability and prior use seemed to make this drug easily translatable to trials for DIPG kids.  St. Jude has completed one trials using dasatinib and vandetanib and another open trial using dasatinib and crizotinib.

The thing about dasatinib is that it seems that it can be pumped out a a cell because by ABC (ATP-binding cassette) transporters.   This is a "superfamily" of  proteins that transport things (such as sugars, lipids, toxins and drugs) across the cell membranes.  Three of these ABC transporters are known to cause chemoresistance.  These are MDR, ABDG2 and ABCB1.  Datinib is known to be a substrate for MDR and ABCG2.

An abstract from Oren Becher's lab at Duke is going to be presented at the SNO/PBTF Pediatric Neuro-Oncology Basic and Translational Research Conference next month in Fort Lauderdale looking at whether altering these transport proteins can alter dasatinib activity.   They did this by making DIPG mouse models that were deficient in the transporters ABCG2 and MDR.  At the first signs of tumor, these mice were given dasantinib.  The mice were sacrified a day later and the brains examined for specific things (caspase-3 and phospho-histone H3).

There was no difference in phosoph-histone H3 whether the transporter was present or not in the mice. However,  mice without the transporter had a 20 fold increase in caspace C levels as compared to those mice with the transporter.

These researchers concluded that these ABC transporters impact dasatinib's efficacy.  Further study is need to see if transporter-deficient mice can live longer when treated with dasatinib and if a transporter inhibitor (such as elacridar) can increase efficacy hopefully increasing survival.

Reference:
ABC transporters, ABCG2 and MDR, limit the efficacy of dasatinib in a diffuse intrinsic pontine glioma (DIPG) mouse model
https://soc-neuro-onc.conference-services.net/reports/template/onetextabstract.xml?xsl=template/onetextabstract.xsl&conferenceID=3467&abstractID=740371

Brain distribution and bioavailability of elacridar after different routes of administration in the mouse
http://www.ncbi.nlm.nih.gov/pubmed/22611067

Oren Becher Lab at Duke
http://pediatrics.duke.edu/faculty/details/0541183