6057 Background: There is a growing body of data indicating that conventional WHO and RECIST criteria do not optimally identify tumor responses associated with increased survival. Methods: We compared several response criteria to assess the efficacy of intratumoral adenoviral p53 gene therapy (Advexin) in a Phase 2 trial of 106 patients with recurrent SCCHN. Results: The percentage of patients with tumor responses defined by reductions in bi-dimensional tumor area by CT scans of 50%, 30%, 25%, 10% or stable disease of > 3 months were 8%, 10%, 11%, 16% and 20% respectively. The median survival for the entire population was 5.9 months while the responder populations defined by tumor decreases of 50%, 30%, 25%, 10% or stable disease > 3 months had median survivals of 40.8, 17.0, 13.7, 12.3 and 11.4 months respectively. There was a statistically significant increase in median survival for each of the responder populations compared to non-responders (p < 0.0009 for each comparison by logrank test). Statistical significance was maintained for the response definitions in landmark analyses excluding patients with survivals less than 3 and 6 months respectively. Locoregional disease control (CR + PR + SD > 3 months) was a stronger predictor of survival by Cox Proportional Hazard analysis (p = 0.0002) than conventional > 50% tumor reduction CR + PR response criteria (p = 0.005). With respect to abnormal p53 detected by immunohistochemistry (≥20% positive cells), 75% of patients (12/16) with p53+ tumors demonstrated locoregional disease control compared to only 18% (2/11) with p53− tumors p=0.0063 Fisher’s Exact Test. In addition, the median survival of patients with abnormal p53 was 11.6 months compared to only 3.5 months in patients with normal p53 (p=0.0007 Logrank Test). Conclusions: Our findings indicate that abnormal p53 is a predictive biomarker that identifies a subset of patients most likely to benefit from p53 gene therapy and that recurrent SCCHN tumor response definitions based upon smaller reductions in tumor size or the absence of progression (stable disease > 3 months) more accurately identified Advexin treated patients with increased survival than conventional response criteria. [Table: see text]