The development of multidrug resistance remains one of the most serious impediments to effective, curative chemotherapy in cancer patients. Resistance develops from a cancer cell's natural response to anticancer drugs. We believe that by understanding these cellular responses, we will learn more about the mechanism of action of specific drugs and about why treatments fail. Ultimately, we hope to contribute to the design of more effective therapeutics and/or treatment protocols and to the advancement of customized therapies based on an individual patient’s likelihood of response. Therefore, the overriding theme in my laboratory is to understand, on a molecular level, the various cellular responses and resulting resistance mechanisms that arise in cancer cells treated with anticancer agents.
Perhaps the best characterized mechanism of drug resistance is that mediated by the Multidrug Resistance-1 (MDR1) gene in humans. MDR1 codes for P-glycoprotein (Pgp), an ATP-dependent plasma membrane protein that acts as a drug pump to prevent intracellular drug accumulation and render cells drug resistant. Pgp can transport and make cancers resistant to a variety of anticancer agents and other xenobiotics, whereas Pgp expression in normal tissues can affect drug pharmacodynamics, pharmacokinetics, and blood-brain distribution in patients. Tissue culture systems have been used extensively to study the functional properties of Pgp and the control of MDR1 expression, but adequate animal models for studying the in vivo regulation and activity of MDR1 have been lacking.
Our early studies focused on using MDR1 as a therapeutic and selectable gene therapy tool to render normal hematopoietic cells resistant to high doses of chemotherapy. We developed a set of gene therapy vectors that allow for delivery of MDR1 and other non-selectable but potentially therapeutic genes to normal cells. These have been used to clarify the stringency of MDR1 functional expression in hematopoietic cells and have exposed a number of key limitations to MDR1 as a potential in vivo selectable and therapeutic tool in humans. More recent studies are focused on developing an animal model system that allows for bioimaging of mouse mdr1 expression in vivo, in real time, and under the influence of various developmental, environmental, and genetic influences. This model is providing new, heretofore unattainable information about the role of mdr1 in drug resistance and normal organ function.
Several projects in the lab have developed from the emergence of novel, mechanism-based anticancer agents that are designed to interfere with signal transduction pathways. As these new agents are tested in the clinic, it will be important to understand potential mechanisms of resistance that will invariably develop against them. As one example, we are investigating resistance mechanisms to trastuzumab (Herceptin®), a monoclonal antibody directed against the Her2/neu receptor that is frequently overexpressed in human breast cancer. Trastuzumab is used clinically for the treatment of Her2-positive breast cancers, but only about 30% of those cancers actually respond to trastuzumab monotherapy. We have isolated trastuzumab-resistant cells in the laboratory and found that they have constitutive activation of the PI-3-kinase/Akt pathway of signal transduction, even in the presence of trastuzumab, which normally turns off this pathway in Her2-dependent cells. One mechanism of sustained PI-3-kinase/Akt signaling appears to be through the over-expression of a protein called t-Darpp. Current studies are focused on understanding the molecular mechanism by which t-Darpp confers resistance to trastuzumab and to other chemotherapeutic agents; elucidating the cellular machinery that controls expression of t-Darpp during the emergence of drug resistance; and determining the role of t-Darpp in clinical outcomes in patients with breast cancer. We expect our work to reveal key details about a cell's response to anticancer drugs such as trastuzumab and, potentially reveal novel cellular targets for future drug development. Moreover, we envision a day when therapy can be customized based on a patient’s likelihood of response as determined by a battery of molecular markers and determinants of that response.