1. Mechanisms of Acquired Resistance to Epidermal Growth Factor Receptor Tyrosine Kinase Inhibitors (EGFR-TKI) in Non-Small Cell Lung Cancer (NSCLC) Victor Cohen, MDCM, FRCPC Department of Oncology Segal Cancer Center SMBD – Jewish General Hospital McGill University

2. Background EGFR-TKI (gefitinib and erlotinib)developed as therapeutic agents for NSCLC Members of a class of Quinazolium- derived agents Inhibit EGFR pathway by binding (reversible) to ATP pocket domain Antitumor activity in clinical trials but benefits modest

3. Background 10% of (Western) patients treated with EGFR-TKI have dramatic and durable responses Clinical features predicting sensitivity; female, adenocarcinoma, Asian ethnicity and never-smoking history 2004; EGFR mutations as a major determinant underlying dramatic responses following treatment with TKI

4. EGFR Mutations

5. Mutations identified using DNA sequencing methods Considered the “gold standard” Non-sequencing assays offering ease of scoring and high sensitivity have been developed Providing a robust and accessible approach to the rapid identification of EGFR mutations Denaturing High Performance Liquid Chromatography (dHPLC)

6. EGFR Mutations Prospective trials treating therapy- naïve patients with EGFR mutations with EGFR-TKI (200 patients) RR 55-82% and median TTP of 9-13 months (3 to 4-fold greater than observed with chemotherapy) Despite dramatic efficacy, all patients will ultimately develop resistance (acquired) to the agents

7. Acquired Resistance Critical to understand mechanisms of acquired resistance May lead to the development of effective therapies for patients who develop acquired resistance Acquired resistance mechanisms have been studied most extensively in EGFR mutant cancers Remains to be determined if mechanisms are shared with wild-type EGFR cancers

8. EGFR T790M Mutation

9. Detected from tumors of EGFR mutant NSCLC patients who have developed clinical resistance (in vitro EGFR-TKI resistant mutant cell lines) Found in 50% of tumors Analogous position to known resistance mutations to imatinib in other kinases; “gatekeeper mutation” ? Steric hindrance

10. Amplification of MET

11. Redundant activation of Her3 permits cells to transmit same downstream signaling in the presence of EGFR-TKI Concomitant inhibition of both EGFR and MET is required to kill resistant cells 22% of NSCLC with acquired resistance had MET amplification in specimens

12. Acquired Resistance EGFR T790M and MET amplification account for 60-70% of all known causes of acquired resistance to EGFR-TKI Other mechanisms are likely to be discovered TGF- β IL-6 axis mediates selective and adaptive mechanisms of resistance to molecular targeted therapy in lung cancer* * Yao et al. PNAS 35, 15535-40 (2010)

13. Acquired Resistance Existence of subpopulation of cells intrinsically resistant to EGFR-TKI Display features of EMT Activation of TGF- β mediated signaling was sufficient to induce EMT phenotypes Upregulation of TGF- β resulted in increased secretion of IL-6 (cells resisted treatment independently of EGFR pathway)

14. Acquired Resistance Produced during inflammatory response Mouse model used to determine whether inflammation might impair sensitivity Induction of inflammation stimulated IL-6 secretion and was sufficient to decrease tumor response Data provide evidence indicating resistance could arise not only as a consequence of changes within cells but also through activation of tumor microenvironment

15. Challenges Important to continue to study preclinical models (and tumors) that have developed resistance to uncover novel resistance mechanisms Several challenges in translating preclinical studies into effective clinical therapies

16. Challenges Accurately identifying which patients have which mechanisms of resistance No repeated tumor biopsies Critical in that the therapeutic strategy aimed at overcoming resistance may not be effective in all resistant patients E.g. Irreversible inhibitors not effective in resistance mediated by MET amplification

17. Challenges Multiple mechanisms of resistance can occur concurrently in same patient Both EGFR T790M and MET have been detected in same specimens (occur independently in different metastatic sites in the same patient) Therapeutic strategy aimed solely at one mechanism may not be effective or lead only to partial regressions Combination strategies may be more comprehensive and potentially more effective

18. Challenges Biological definition and detection of resistance mechanisms T790M can sometimes be present as minor allele and yet be sufficient to cause resistance but may go undetected Challenges with detection of MET amplification. Definition of what constitutes clinically significant amplification not well defined

19. Conclusion Gefitinib and erlotinib are effective therapies for patients with EGFR mutant NSCLC All patients ultimately develop resistance Important to identify and study mechanisms of resistance as a means of rationally designing the next generation clinical studies Several clinical trials (aimed at inhibiting known resistance mechanisms) are already underway