A phase Ib/II study of cetuximab and pembrolizumab in RAS-wt mCRC

834 Background: Cetuximab is a chimeric IgG1 anti-EGFR monoclonal antibody approved in RAS-wt metastatic colorectal cancer (mCRC). Pre-clinical data have suggested that cetuximab triggers immunogenic cell death and support an immunologic basis of activity. Small clinical data sets demonstrate that EGFR-based therapy preferentially increases TILs in colorectal hepatic metastases. This Phase Ib/II study was designed to assess the safety and clinical activity of cetuximab and pembrolizumab in metastatic colorectal cancer. Methods: Eligible pts were those with RAS-wt mCRC, ECOG performance status (PS) 0-2, with at least 1 prior line of therapy for advanced stage disease, and no prior treatment with an anti-EGFR antibody. The phase Ib study was designed as a safety lead-in with a de-escalation design. Pembrolizumab was administered at a fixed dose of 200 mg q3week with standard cetuximab dosing (400 mg/m2 loading dose, followed by 250 mg/m2 weekly). DLTs were defined as any ≥ grade 3 clinically significant toxicity which is possibly related and occurs within the first 6 weeks of therapy. For rash and hypomagnesemia only ≥ grade 4 toxicities were considered DLTs. Correlative serial biopsies were performed at baseline and at 9 weeks. Results: 9 patients, ages 33-73, were enrolled onto the Phase Ib study between 8/10/16 and 11/2/16. No DLTs were observed. The most common AEs of any cause observed were xerosis, dermatitis acneiform rash, hypomagnesemia, vomiting, and fatigue. ≥ Grade 3 toxicities were uncommon, but included hypomagnesemia (3), rash (1), urticaria (1), hypocalcemia (1), alk phos elevation (1), and ascites (1). 6/9 patients achieved stable disease lasting ≥ 16 weeks. Conclusions: The combination of cetuximab and pembrolizumab is well-tolerated in this study and can be administered at full dosages. Though hypomagnesemia appears prominent, no additional emergent AEs have been observed during or following the DLT period. The associated ongoing phase II study will enroll an additional 33 patients to assess the true efficacy of this regimen with dual primary endpoints of ORR and 6 month PFS. Clinical trial information: NCT02713373.