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2012 Program Booklet - MCD Biology - University of Colorado Boulder

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Jeffrey Settleman, PhD<br />

Senior Director, Discovery Oncology<br />

Genentech<br />

Title: The Many Flavors <strong>of</strong> Resistance to<br />

Anti-­‐Cancer Drugs<br />

Saturday, October 13<br />

3:15 – 4:00 pm<br />

JSCBB Butcher Auditorium<br />

Abstract:<br />

The recent clinical success experienced with several “rationally-­‐targeted” anti-­‐cancer drugs, such as<br />

the kinase inhibitors imatinib, erlotinib, crizotinib, and vemurafenib, has defined a paradigm shift in<br />

cancer therapy. However, despite the sometimes impressive clinical activity associated with these<br />

agents, progression during therapy is inevitable due to the acquisition <strong>of</strong> drug resistance. For many<br />

drugs, specific genetic mechanisms <strong>of</strong> resistance have been elucidated, and pre-­‐clinical findings<br />

implicate additional non-­‐genetic mechanisms. Moreover, accumulating evidence implicates<br />

heterogeneity within cancer cell populations in the response to drug treatment, posing an additional<br />

challenge to the development <strong>of</strong> effective cancer therapeutics.<br />

While modeling the acute response to various anti-­‐cancer agents in drug-­‐sensitive tumor cell lines, we<br />

consistently observed a small subpopulation <strong>of</strong> reversibly “drug-­‐tolerant” cells. This drug-­‐tolerant<br />

phenotype, associated with a distinct chromatin state, is transiently acquired and relinquished at low<br />

frequency by individual cells within the population, implicating the dynamic regulation <strong>of</strong> phenotypic<br />

heterogeneity in drug tolerance. The drug-­‐tolerant subpopulation can be selectively ablated by<br />

treatment with chromatin-­‐modifying agents, potentially yielding a therapeutic opportunity. These<br />

findings suggest that cancer cell populations employ an epigenetically-­‐regulated dynamic survival<br />

strategy in which individual cells transiently assume a reversibly drug-­‐tolerant state to protect the<br />

population from eradication by potentially lethal exposures.<br />

Cancer cells typically express multiple receptor tyrosine kinases (RTKs) that mediate signals that<br />

converge on common critical downstream cell survival effectors -­‐ most notably, phosphatidylinositol<br />

3-­‐kinase and mitogen-­‐activated protein kinase. Consequently, increased RTK ligand levels, via<br />

autocrine tumor cell production, paracrine contribution by tumor stroma, or systemic production,<br />

could confer resistance to inhibitors <strong>of</strong> an oncogenic kinase with a similar signaling output. Using a<br />

panel <strong>of</strong> kinase-­‐“addicted” cancer cell lines, we found that most cells can be “rescued” from drug<br />

sensitivity by simply exposing them to one or more RTK ligands. Among the findings with clinical<br />

implications was the observation that hepatocyte growth factor confers resistance to the BRAF<br />

inhibitor vemurafenib in BRAF mutant melanoma cells. These observations highlight the extensive<br />

redundancy <strong>of</strong> RTK-­‐transduced signalling in cancer cells and the potentially broad role <strong>of</strong> widely<br />

expressed RTK ligands in innate and acquired resistance to drugs targeting oncogenic kinases.

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