Multidrug resistance transporters in biology and medicine
Our work centers on finding new approaches to combat multidrug resistances in cancer chemotherapy treatments

Recent press links:


Supercomputers enabling-cancer-drug-discovery

Dynamic models of drug resistance proteins

Video popularizations of our work:
compounding hope video link
Video link to innner workings computer disoveries of MDR inhibitors

What we do and why we do it:

Cancers occur when cells in our bodies lose control of the reproduction of new offspring cells. This results in the uncontrolled proliferation of cells which can manifest as a solid tumor or as a liquid cancer such as in leukemias. Genetic changes in cancer cells that generally make the cancer more aggressive and harder to treat are associated with the growth and aging of the cancer itself. This is the reason that early detection of cancers is so important: It is often true that younger cancers are much less aggressive than older cancers.

One of the changes that cancers undergo during the aging or maturation process is to adapt to the cancer chemotherapeutics that are administered to kill them. These “resistances” can occur in many different ways, from very specific inactivations of individual drugs to more general methods that simply limit the exposure of the cancer cell to the drugs. These latter general methods are very dangerous, since whole categories of chemotherapeutics can be made ineffective by one or two changes in the cancer cell. The most dangerous of these adaptations is the “over-expression” of members of a class of protein called “efflux transporters”.

Over-expression” means that the cells produce more than the normal functional amount of a protein as compared to when it is found in a normal cell. “Efflux transporters” are machines made of protein that are found within our cells that take chemicals from the inside of a cell and push them to the outside. The efflux process is generally reserved for chemicals that can harm the cell. It turns out that many cells, cancers included, treat medicines as toxins and actively pump the medicines out of the cell, where they then lose any positive therapeutic effect.

Some efflux pumps have evolved the ability to push many different substances across the cell membrane and these are especially dangerous when over expressed in cancer, since they then confer resistances to a wide variety of drugs. Of the nearly 100 cancer chemotherapeutics that have been approved by the FDA, nearly all of them can be removed from a cancer cell that overexpresses as few as one of these more generic efflux pumps. These cancers are called multidrug resistant (MDR).

Multidrug resistant cancers develop with relatively high frequency especially in recurring cancers. They are a very significant health problem as they render therapeutic approaches ineffective. One main cause for MDR, both in cancer and in the chronic treatment of infectious diseases like AIDS, is the overexpression of multidrug resistance membrane transporters like breast cancer resistance protein (BCRP), P-glycoprotein (P-gp), and the multidrug resistance associated protein (MRP1). Multidrug resistances develop with high frequency (up to 40%) after chemotherapies and are associated with very poor patient prognoses.

Our lab is dedicated to finding inhibitors for these multidrug resistance pumps and in understanding how these transporters work on the molecular scale.

Human P-glycoprotein

The human P-glycoprotein (P-gp) is a member of a large family of evolutionarily conserved membrane transporters called the ABC-transporters.

These proteins are used to move many different types of substances across cell membranes.

Human P-gp has caused many problems in the medical treatment of cancers and viral infections like HIV-AIDS because this transporter is able to "pump" chemotherapeutic drugs out of the cell.

P-gp is also able to bind many different drugs, which compounds the problem. Cells expressing P-gp or over-expressing P-gp become resistant to the effects of many different drugs because of its ability to "pump them overboard".

See below for an introduction to our current studies on this fascinating enzyme.

Important remaining questions:

What is its structure and how does it work?

In order to understand how the transporter works, we need to know what it looks like.

Before we can attempt to fix the medical problems associated with P-gp, we need to know how it works.

How can we inhibit this Multidrug Resistance transporter?

If we can find a compound that inhibits the pump, we may be able to use it as part of the treatment with the chemotherapeutics.

Knocking out the pump may then destroy the drug resistance so the chemotherapy works again.

Recent progress and publications:

Identification of new inhibitors of P-glycoprotein using computer models

In 2014 we found four inhibitors of P-glycoprotein in ultra-high-throughput computational screening experiments using the SMU High Performance Computing system. These inhibitors were targeted at P-glycoproteins power source to prevent them also from being pumped out of the cell. The four inhibitors were biochemically and biophysically characterized.
Get the paper here.
Title page of Brewer et al. 2014
Molecular pharmacology 86 (6), 716-726 2015
Four identified P-gp inhibitor structuresP-gp with inhibitors bound

Our new inhibitors of P-glycoprotein reverse multidrug resistances in cancer cells

In this 2015 paper, we show that three of the four inhibitors we found in 2014 actually reverse multidrug resistances in prostate cancer cells. Not only that, but they don't seem to have significant toxicities on their own without the co -administered chemotherapeutic.
Get the paper here.
title page for Follit et al 2015
sensitive and multidrug resistant prostate cells
MDR prostate cells with inhibitors
In silico identified P-gp inhibitors potentiate the cytotoxic effects of paclitaxel in the multidrug resistance (MDR) human prostate cancer
cell line DU145TXR. Cells were incubated with the indicated concentrations of the chemotherapeutic paclitaxel. The upper figure shows the sensitive prostate cells (open circles) and the multidrug resistant prostate cancer cells (closed circles). The bottom figure shows the resistant cells in the presence of our inhibitors. One can see resensitization of the multidrug resistant cancer cells to the chemotherapeutic paclitaxel.

Notice how the cells with P-gp inhibitors (red, pink and blue symbols) die when exposed to paclitaxel!

What is the structure of P-glycoprotein and how does it pump drugs through a membrane?

In work reported in 2012 and 2015, we have been able to model the structure of P-glycoprotein as well as show a plausible dynamic mechanism of how it works. In the 2015 paper we were able to show how P-glycoprotein moves drugs through the membrane.
McCormick et al title page
Get the paper 2015.

Wise 2012 title page
Get the paper 2012.

Modeling P-glycoprotein - We were able to build models of the human P-glycoprotein using evolutionary relationships with related proteins of know structure. These homolgy models have been used in drug discovery projects (see above) as well as in mechanistic studies.
P-gp model schematic

Targeted molecular dynamics - This is a computational technique that can be used to get a first look at how a transporter like P-glycoprotein might function. The method using known structural changes in the protein. We arranged four such "conformations" into a catalytic cycle and then used molecular dynamics methods to "push" P-gp from one structure to the next.
The four "targets" we used to simulate P-gp function:
TMD targets
These structures were based on seminal work from several other researchers:

Building the molecular dynamics system for high performace computing use - First we build the protein model, then add it to a phospholipid bilayer system that simulates the cell membrane, and then we add water to hydrate it. After that, we add thermal energy and heat the system to body temperature.
An MD system

Targeted molecular dynamics - Some examples: Note that the water and lipids have been taken out so better views of the drugs are available. All atoms were present in the simulations presented below. The cell membrane is represented with the gold spheres (top layer inside the cell / bottom layer outside the cell).

In these videos notice how the drug moves out of the cell (towards the bottom of the screen).

With an antihypertensive called Verapamil bound
(red spheres).

verapamil pumped by P-gp
Close up  of verapamil
verapamil closeup

With Daunorubicin bound (a cancer chemotherapeutic sometimes called Adriamycin)
Daunorubicin pumped by P-gp

Close up of Daunorubicin being pumped by P-gp
Daunorubicin closeup

Using these techniques and structures, we have been able to track the drug movement through P-glycoprotein and watch it push drugs out of the cell.

We use these models for the computational searches for inhibitors of P-glycoprotein that reverse multidrug resistances in cancer cells shown above.

Current research directions:

We are optimizing the P-glycoprotein inhibitors we have found, are searching for new inhibitors of P-glycoprotein and other MDR tranporters, and are continuing our mechanistic studies of these intriguing ABC transporter drug pumps.

Back to start page