Panitumumab antitumor activity in patients (pts) with metastatic colorectal cancer (mCRC) expressing low (1–9%) or negative (<1%) levels of epidermal growth factor receptor (EGFr)

3547 Background: Panitumumab is a fully human monoclonal antibody directed against EGFr. We investigated the antitumor activity of panitumumab in pts with mCRC who failed prior therapy and had low or negative EGFr tumor expression. Methods: In this multicenter, phase 2 study of 150 planned pts, pts had documentation of disease progression (PD) during or after adequate doses of fluoropyrimidine, irinotecan, and oxaliplatin (centrally confirmed refractory disease CCRD), 2–3 prior regimens, ECOG score 0–2, and low or negative EGFr staining (by IHC) in evaluable tumor cells. Pts received panitumumab at 6 mg/kg Q2W until PD or drug intolerability. Tumor assessments (modified WHO, blinded central review) were taken periodically from wk 8 until PD. Endpoints were objective response (OR) through wk 16 (+ ≥ 4 wk confirmation; primary) and OR throughout study, response duration, progression-free survival (PFS) time, survival time, and safety (secondary). Results: In this interim analysis (6/05), 88 pts were enrolled and had ≥ 1 dose of panitumumab (safety set); 23 pts had ≥ 20 wks before the cutoff and CCRD (efficacy set). The efficacy set consisted of 16M/7W, median age of 65 (range: 46, 85) yrs, 83% white, 100% with ECOG ≤ 1, 74% colon cancer and 26% rectal cancer; all received ≥ 2 prior regimens (equivalent characteristics for safety set). 2/11 (18%) pts with EGFr-negative tumors and 1/12 (8%) with low EGFr staining had a partial response. Duration was up to 16 wks. 7/23 (30%) of all pts had SD. Median (95% CI) PFS was 7.9 (7.0, 23.0) wks. In the safety set, all pts had a treatment-related adverse event; 19% grade (gr) 3; 2% gr 4. Integument and eye toxicities were: 92% skin, 17% eye, 28% nail, 8% hair, and 2% chelitis. 20 (23%) had diarrhea (1 gr 3); 7 (8%) had hypomagnesemia (2 gr 3/4). Three pts had an infusion reaction-1 gr 3 (led to panitumumab discontinuation) and 2 gr 1/2. In 65 pts with both a baseline and post-baseline sample, no human anti-human antibodies to panitumumab were detected. Updated data will be presented. Conclusions: Responses to panitumumab were seen in pts with mCRC with both low and negative EGFr levels. Efficacy appears similar to that in other studies with panitumumab in pts with higher EGFr tumor levels. Table: see text