Background: Carcinoembryonic antigen (CEA) may reflect response to antitumor treatment in metastatic colorectal cancer (mCRC). The predictive value of CEA has not yet been proven for subsequent maintenance therapy. This analysis aims to evaluate the predictive and prognostic value of pre- and post-induction treatment CEA on maintenance with 5-fluoruracil/leucovorin (FU/FA) plus panitumumab (pmab) [arm A] or FU/FA alone [arm B] in RAS wildtype mCRC patients treated within the PanaMa trial. Methods: Patients with CEA measurements (pre- and post-induction therapy) were grouped as normal (both measurements ≤5 ug/l), stable (between +25% and -25%), decreasing (<-25%), and increasing (>+25%) CEA. Survival parameters (overall survival (OS), progression-free survival (PFS) from initiation of maintenance therapy) were expressed by the Kaplan-Meier method and compared by log-rank testing, and Cox regression. The objective response (OR) to maintenance therapy was analyzed by chi-square testing. Results: Out of 248 patients in the in the full analysis set, 245 patients were eligible for CEA analysis. Normal CEA occurred in 58 (23.7%), stable CEA in 16 (6.5%), decreasing CEA in 161 (65.7%), and increasing CEA in 10 (4.1%) patients. In the subgroup of decreasing CEA, there was a significant difference in the prediction of OR between both treatment arms with a better positive predictive value for the pmab-containing maintenance (44.0% vs. 27.5%, p=0.032). Increasing compared to decreasing CEA was associated with unfavourable survival outcome of maintenance irrespective of treatment arm (Table). Conclusions: CEA kinetics during induction therapy appears to have a predictive value for subsequent maintenance, notably pmab-based. Besides that, CEA levels had a significant impact on survival parameters of maintenance irrespective of the addition of pmab to FU/FA. This analysis is limited by the small number of patients in the subgroup of increasing CEA. Clinical trial information: NCT01991873.

Legend: FU/FA, fluorouracil/folinic acid; pmab, panitumumab; PFS, progression-free survival; OS, overall survival; HR, hazard ratio; CI, confidence interval; ORR, objective response rate. *¹ 1 missing, *² 3 missing, *³ 2 missing, *⁴ 3 missing.