Impact of Primary Tumor Location on First-line Bevacizumab or Cetuximab in Metastatic Colorectal Cancer

Background: Colorectal cancer is one of the most common malignancies in the United States, with a large proportion of patients presenting with metastatic disease or developing a recurrence. Systemic chemotherapy is the mainstay of therapy in this setting. There is a clear benefit in the addition of bevacizumab or cetuximab (for rat sarcoma [RAS] wild type tumors) to oxaliplatin- and irinotecan-based regimens which can be considered for first-line therapy. However, many significant questions remain as to which agent reflects best practice. Objective: Our review aimed to elucidate the benefit of adding bevacizumab and cetuximab to initial therapy for metastatic colorectal cancer based on primary tumor location and a variety of other disease- and patient-related factors, addressing the paucity of evidence that currently exists in this area and contributing to current literature and clinical practices. Methods: The primary endpoints of the study were first Progression-Free Survival (PFS) and Overall Survival (OS). Secondary endpoints included best response to first- and second-line therapies, Treatment- Related Adverse Events (TRAEs), second PFS, cost of therapy, and an assessment of other patient- and disease-related factors affecting PFS and OS. Results: While there were trends towards improved OS in patients with left-sided primary tumors (n=57) compared to those with right-sided disease (n=23), there were no significant differences between the two groups in either primary endpoint. While no differences were found for patients with left- or right- sided tumors stratified by add-on agent, these analyses were limited by the small number of patients receiving cetuximab with first-line therapy (n=4). However, the bevacizumab cohort (n=76) was sizable enough to provide ample data and produce clinically relevant results. Add-on therapy with bevacizumab in our study achieved impressive survival outcomes in both left-sided (median first PFS = 13 months, 95% CI 11-15 months; median OS = 37 months, 95% CI 21-53 months) and right-sided (median first PFS = 13 months, 95% CI 9-17 months; median OS = 37 months, 95% CI 22-52 months) disease. Conclusion: These results raised questions regarding the true significance of primary tumor location when selecting bevacizumab or cetuximab for first-line therapy, particularly the current thought of using cetuximab for left-sided tumors. While the superiority of bevacizumab over cetuximab in rightsided disease remained evident upon comparison of our analysis with historical controls, survival outcomes with the agent in our analysis appeared to be similar to that of cetuximab in CRYSTAL, FIRE- 3, and CALGB/SWOG 80405 in left-sided disease. Further study is required to determine if bevacizumab truly does produce similar outcomes to cetuximab in left-sided primary tumors. Keywords: Bevacizumab, cetuximab, left-sided tumors, metastatic colorectal cancer, primary tumor location, right-sided tumors, targeted therapy.