e14075 Background: While FOLFOX +/- Bevacizumab is the most widely used combination in colorectal cancer (CRC), the EGFr antagonists Cetuximab & Panitumumab have not further enhanced activity. On the contrary, the addition of EGFR inhibitors to Irinotecan-based regimens at 1st & 2nd line has improved response and both progression freel & overall survival (Van Cutsem. 2010; Peeters. 2010.) Methods: We applied the ex vivo analysis of programmed cell death (EVA-PCD) (Nagourney, Curr Treat Opt Onc 2006) to examine interactions between EGFr antagonists and Irinotecan or FOLFOX in 534 human tumor primary culture microspheroids isolated from CRC surgical specimens or cytologically (+) fluids. Gefitinib was used as a surrogate for Cetuxima/Panitumumab activity. Tissue samples exposed to Oxaliplatin, 5FU, Irinotecan, and Oxaliplatin & 5FU (FOLFOX) provided dose response curves with and without Gefitinib for formal synergy analysis by median effect (Chou & Talalay). Results: Shown in table below. Conclusions: Results indicate superiority for Irinotecan combination with the EGFr antagonist Gefitinib. The percentage of synergistic interactions for Irinotecan & Gefitinib (41%) is demonstrably higher than that associated with FOLFOX & Gefitinib (4%). This suggests distinct modes of interaction between these classes of agents favoring Irinotecan and could have an impact upon future clinical trial designs. Recognizing the importance of Kras mutation, additional studies are underway to examine the impact of Kras mutation upon synergy. However, as wild-type and mutant samples would be expected to be equally distributed, subset analyses may reveal quantitative (higher % synergy) rather than qualitative differences (relative activity of Irinotecan vs FOLFOX). Results of these analyses will be reported. Supported in part by the Vanguard Cancer Foundation. [Table: see text]