The Value of Early Depth of Response in Predicting Long-Term Outcome in EGFR-Mutant Lung Cancer

Introduction: Traditionally, marked tumor shrinkage has been assumed to portend better outcome. We investigated whether depth of tumor response was associated with improved survival outcomes in advanced EGFR-mutant NCLC. Methods: Individual patient data from randomized trials (EURTAC, IPASS, ENSURE, LUX-Lung 3, and LUX-Lung 6) were used. The association of depth of response with progression-free survival (PFS) and overall survival was examined using landmark analyses. Depth of response based on radiologic assessments at 6 weeks and 12 weeks was calculated as the relative changes in the sum of the longest diameters of the target lesions from baseline. Results: Of 1081 evaluable patients at 6 weeks with no disease progression, 71.2% achieved Response Evaluation Criteria in Solid Tumors response. Using a landmark analysis, EGFR-TKI was more effective than chemotherapy (PFS hazard ratio = 0.36, p <.0001); and was associated with greater mean tumor shrinkage than chemotherapy (35.1% versus 18.5%, p <.0001). However, there was no significant difference in the relative PFS benefit between treatment groups across the entire spectrum of tumor shrinkage (p =.18 for test of interaction between treatment and continuously measured depth of response). Depth of response at 6 weeks was not associated with PFS when adjusted for treatment effect (hazard ratio = 0.96, p =.78). Similar results were obtained for 12-week landmark analysis and for OS outcome. Conclusions: The PFS advantage of EGFR-TKI over chemotherapy in advanced EGFR mutant NCLC is not explained by depth of response at 6 or 12 weeks. It should not be used as a surrogate of benefit in future trials or routine clinical decision making.