They have discovered that a protein called Mrp4 blocks the access of a widely used cancer drug - GlaxoSmithKline's Hycamtin (topotecan) - into the brain by actively transporting it back into the bloodstream. If this process can be inhibited, the activity of the drug in brain cancer could be significantly enhanced, they suggest.
Other attempts to overcome drug resistance in cancer cells have tended to focus on a cellular factor called P glycoprotein, which is also thought to work by pumping drugs out of cancer cells before they can exert their effects. P glycoprotein acts as a gatekeeper for the cell, protecting it from toxins, and is present in large quantities in the gut, live and kidney.
But last year, Canada's QTL was forced to discontinue trials of one P glycoprotein blocking drug - tariquidar - after disappointing results in Phase III trials. QTL had in-licensed the drug from UK biotech Xenova.
Other companies, including Novartis (with valspodar), Vertex Pharmaceuticals (with biricodar) and Eli Lilly (zusoquidar) have also explored the use of P glycoprotein inhibitors to improve cancer therapy, but to date no drug based on this mechanism of action has made it through to market.
Both valspodar and biricodar have been discontinued, and Novartis and Vertex have stopped working in this area. The problem? It was discovered that these drugs interfered with the metabolism of cytotoxic compounds, slowing their clearance and increasing their toxicity to healthy tissues.
Although P glycoprotein was the first cellular pump to be targeted by pharmacological interventions, latterly a number of others, including Mrp1, Mrp2 and Mrp3, have been described. But Mrp 4 has properties that distinguish it from its peers, according to the researchers, not least that its action seems to be very localised to the central nervous system.
Other pumps, such as Mrp1, are widely encountered in the body, raising the risk of side effects if they are inhibited.
The investigators, from St Jude Children's Research Hospital, used a mouse model lacking the Mrp4 protein - a type of molecule known as an ABC-dependent transporter - to test whether the therapeutic efficacy of drugs targeting central nervous system tumours might be improved by inhibiting this protein.
Their study showed that Mrp4 works at two levels. By binding to topotecan and transporting it away from the brain Mrp4 restricts the drug's penetration into the brain from the bloodstream. And it also protects brain cells from accumulating toxic levels of topotecan molecules that do escape the bloodstream.
"The ability of Mrp4 to protect the brain from toxins can be a liability in people with brain cancer when this protein also blocks therapeutic drugs from reaching CNS tumours," said John Scheutz, an associate member of the St. Jude Department of Pharmaceutical Sciences. Scheutz is senior author of an article on the discovery, published in the journal Molecular and Cellular Biology (vol 24, issue 17).
The investigators discovered that when topotecan was injected into the veins of specially bred mice that lack Mrp4, the drug accumulated to greater than normal levels in the brain tissue and the cerebrospinal fluid that surrounds the brain.
The finding suggests that like P glycoprotein, the natural role of Mrp4 is to block the passage of certain toxic molecules, which chemically resemble topotecan, from leaving the bloodstream and entering the brain.
But using antibodies against Mrp4 the investigators found that this protein is located in the brain's capillaries as well as in membranes of the choroid plexus - the folds within the brain ventricles that make and release CSF.
"This dual location for Mrp4 is unusual for this type of transporter," Schuetz said. "It suggests that Mrp4 blocks specific molecules from leaving the capillaries. And if such molecules slip out of the blood into the choroid plexus, Mrp4 shuttles them back out of the brain and into the blood before they can cause damage."
The investigators also showed that isolated cells that were modified to over-express Mrp4 did not accumulate as much topotecan as cells lacking this protein. This is strong evidence that over-expression of Mrp4 in tumors contributes to topotecan resistance in patients. Prior research has also suggested that Mrp4 plays a role in HIV resistance.
Discussing the significance of the finding, Schuetz noted that there is an expanding array of topotecan-like drugs being developed, and unless there is a way to block Mrp4 when giving them, their effectiveness.