3520 Background: Panitumumab (formerly ABX-EGF) is a fully human monoclonal antibody that binds with high affinity to the epidermal growth factor receptor (EGFr). This ongoing phase 2 study is evaluating the safety and efficacy of panitumumab monotherapy in patients (pts) with mCRC who have failed standard chemotherapy. Here we present mature unaudited data from the ongoing study. Methods: Pts with mCRC who failed therapy with a fluoropyrimidine (± leucovorin) and either irinotecan or oxaliplatin, or both, were eligible for this study. Cohort A included pts with EGFr staining of 2+ or 3+ in ≥ 10% tumor cells; cohort B included pts with EGFr staining of the sum of 1+, 2+ and 3+ in ≥ 10% of tumor cells, but with the sum of 2+ and 3+ in < 10% of tumor cells. Panitumumab was given IV at 2.5 mg/kg once weekly in 8-wk cycles. Pts continued receiving treatment until disease progression or unacceptable toxicity occurred. Endpoints included tumor response (RECIST), time to disease progression, survival, and safety. Results: Enrollment is complete, with treatment ongoing. In total, 148 pts were treated (82 men/66 women; ECOG status of 1 [n = 111] or 0 [n = 37]; mean age 59 yrs [range 21 - 88 yrs]). Efficacy results are shown in the table. The 4 most frequently reported grade (gr) 3 or 4 treatment-related adverse events (AEs) were rash NOS (5 pts), fatigue (4 pts), vomiting NOS (2 pts), nausea (1 pt), and pruritus (1 pt). Skin toxicity-related AEs were reported for 141 pts (95%; 5% gr 3 and no gr 4). One infusion-related gr 3 AE, which did not require dose modification, was reported; no other infusion-related AE has been reported to date. No human anti-human antibodies were observed in 145 pts tested. Conclusions: These mature data confirm previously reported safety and response findings (Hecht et al, 2004) and provide encouraging survival data in pts with mCRC who have failed multiple lines of standard chemotherapy. Author Disclosure Employment or Leadership Consultant or Advisory Role Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration Amgen Amgen, Genentech, Pfizer, Sanofi Amgen Amgen, Bristol-Myers Squibb, Genentech, Novartis, Pfizer, Roche Amgen, Eli Lilly, Pfizer, Roche Amgen