Tumor bulk to predict clinical outcomes of anti-EGFR therapy in treatment refractory metastatic colorectal cancer independent of sidedness.

Abstract

795 Background: Epidermal growth factor receptor (EGFR) antibodies, cetuximab (C) and panitumumab (P) have shown improvement in clinical outcomes for distinct molecular profiles in metastatic colorectal cancer (mCRC). In the first line setting, EGFR targeting of left-sided tumors has shown favorable clinical outcomes when compared to right-sided tumors. Here we examined the efficacy of EGFR therapy in the treatment refractory setting as well as the impact of tumor bulk on clinical outcomes. Methods: A retrospective cohort of 72 patients (pts) with KRAS wild-type mCRC were identified who received either C or P in the late-line setting. Tumor measurements were performed per RECIST v1.1. Disease bulk was defined as single lesion with longest diameter or lymph node with short axis > 3.5 cm. Right colon primary was defined proximal to splenic flexure. Results: In pts with treatment refractory right-sided disease the response rate (RR) was 16.7%, progression free survival (PFS) was 3.7 months (mo), and overall survival (OS) was 14.0 mo. This was compared to left sided disease with RR of 26.0%, PFS 6.2 mo (p < 0.05), and OS 15.0 mo. In the non-bulky cohort the RR was 32.4%, PFS 7.9 mo, and OS 18.4 mo. In the bulky cohort the RR was 5.3% (p < 0.01), PFS 4.0 mo (p < 0.02), and OS 6.6 mo. In the right-sided non-bulky cohort there was a RR of 33% v. 0% in the right-sided bulky cohort. In the left-sided non-bulky cohort there was a RR of 32.3% v. 7.7% in the bulky cohort (p < 0.05). Conclusions: These data indicate that tumor bulk can predict clinical outcomes for anti-EGFR targeting. In the late line setting, pts with left-sided cancers overall trended towards improvements in clinical outcomes, consistent with prior understanding in first line setting. Despite a limited cohort size, response was observed in pts with non-bulky right-sided disease. There was limited-to-no benefit of anti-EGFR targeting with right-sided bulky disease. Left-sided non-bulky patient received the greatest benefit from anti-EGFR targeting. Future prospective studies of targeted therapeutics should incorporate tumor bulk and sidedness when assessing clinical outcomes and tumor biology.