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

Results of St Jude Phase 1 Trial Published

The results of the St Jude Phase 1 trial using a combination of small molecule inhibitors, vandetanib and dasatinib,  have just been published electronically (ahead of print) in the journal Clinical Cancer Research.

In a look back at DIPG history, this is a really a new kind of trial.   In the past many trials have been radiation and a single agent.  I think we can say with some certainty that DIPG tumors will need more than a single agent for cure- especially for targeted therapies.  These tumors are complex and have so many pathways that it is likely that the tumors will be able to get around a single agent.

In addition, there seems to be rational basis to chose these agents specifically for DIPG.  Vandetanib has a combination of effects on different targets but is a potent VEGF inhibitor.   This is known to be a player in GBMs (and as been said here before when studied histologically most DIPGs have been found to be GBMs).   Dasatinib has its effects at PDGFRA (platelet derived growth factor receptor alpha).  Several recent publications have highlighted the PDGF pathway as being key in the development of DIPGs.

In this study, 25 newly diagnosed children were started on oral dasatinib daily at the start of radiation.   Eight days later oral vandetanib twice a day was added.     Overall the treatment was tolerated well.

• The M:F ratio was 12:13.
• The age range was 2.3 years to 17.2 with a median age of 5.8 years.
• The median treatment time was 184 days.   
• The most significant toxicity was diarrhea.
• Myelosuppresion (drop in blood counts) was seen in three children.
• Two of the children had tumor biospies.  
• There were 12 post mortem tumor donations.  
• All patients progressed on treatment.   
• The one year overall survival was 52% + 10%.
• The 2 year survival was 9% + 6%.  
• The two patient who were on treatment for more than 2 years had typical clinical and radiologic appearance of DIPG

An interesting part buried in the article text was the approach to radiation.  The radiation field encompassed the tumor and a 2cm margin in the transverse (across) and caudal (down) direction and a 3cm margin in the superior direction.  The increased from 2cm to 3cm in the superior direction was based on preliminary results on progression patterns of DIPG patients.   Of note, one patient had extensive bilateral thalamic extension requiring whole brain radiation.

Three of the young children developed symptomatic radiation necrosis in uninvolved brain areas.  Although there was no institutional knowledge of other patients treated with more than standard radiation fields developing radiation necrosis in uninvolved areas, it was felt that the larger margins may have lead to symptomatic radiation necrosis.   Thus the authors do not recommend to use of enlarged radiation fields.

Another interesting finding was that the researchers showed "for the first time that CSF exposure of vandetanib and dasatinib in humans is modest".   They point out that CSF levels is often used as a surrogate marker for brain penetration but that this assumption is not necessarily valid particularly in places where there is a blood brain barrier disruption.  They also point out that although the dasatinib CSF level was low that it did reach sustained levels "similar to those causing significant inhibition of other drug targets.

Additionally, the researchers looked at plasma angiogenic factors (seeing what factors are in the blood). Surprisingly, increased VEGF in this study was associated with longer overall survival whereas in a prior study it was associated with shorter survival.  The hypothesis is that this might be because of an improved inhibition of alternative pathways because of the combination.  Regardless of the reason, it seems to me that we don't understand plasma factor levels yet.

Despite the continued poor results for DIPG clinical trials, there were some interesting findings in this trial.   There is hope the drug testing (such as what we should see with the pre-clinical consortium) might find new promising targets or drug therapies.

Note-

This work was supported by the U.S. National Institutes of Health Cancer Center

Support (CORE) Grant P30 CA21765, by the Cure Starts Now Foundation, by

AstraZeneca, and by the American Lebanese Syrian Associated Charities (ASLAC).

References:

Phase 1 Trial, Pharmacokinetics and Pharmacodynamics of Vandertanib and Dasatinib in Children with Newly Diagnosed Intrinsic Pontine Gliomas

Clin Cancer Res. 2013 Mar 27. [Epub ahead of print]

http://www.ncbi.nlm.nih.gov/pubmed/23536435

Clinical Trials Entry   

http://clinicaltrials.gov/ct2/show/NCT00996723?term=dipg&rank=2