Negative hyperselection for mutations associated with anti-EGFR antibody resistance in RAS wildtype metastatic colorectal cancer (mCRC): Evaluation of the PANAMA trial (AIO-KRK-0212, maintenance therapy with 5-FU, folinic acid (FU/FA) with or without panitumumab).

3536 Background: We evaluated the prognostic and predictive impact of DNA mutations related to anti-EGFR antibody resistance in patients of the PANAMA trial, which compared Panitumumab (Pmab) and FU/FA versus FU/FA maintenance therapy after Pmab-FOLFOX induction therapy in RAS wild-type (wt) mCRC. Methods: Next generation panel sequencing was conducted on 201 of 248 tumors obtained prior to study inclusion from the full analysis set using the Cancer Hotspot Panel v2 on an Illumina MiSeq system. Hyperselection covered mutations of the following genes: KRAS, NRAS, BRAF, HER2, PTEN, AKT1, PIK3CA. Median progression-free (PFS) and overall survival (OS) since start of maintenance were estimated by Kaplan-Meier and Cox-regression (log rank test). Objective response rates (ORR) of maintenance therapy were compared by Chi-square-test. Results: From 201 tumors, 41 (20.4 %) carried at least one mutation: KRAS: 7 (3.5%), BRAF: 23 (11.4%), PTEN: 4 (2.0%), AKT1: 2 (1.0%), PIK3CA: 12 (6.0%), with 6 tumors harboring co-occuring mutations. No mutations were found in NRAS and HER2. Negative hyperselection (wt for all genes) was associated with (numerically) favourable prognosis in terms of PFS (HR 0.79 (95% CI 0.55 – 1.12), p=0.184), OS (HR 0.61 (95% CI 0.40 – 0.95), p=0.028) and ORR (39.4% vs. 29.3%, p=0.279). The benefit of adding Pmab to FU/FA during maintenance was limited to the hyperselection wt subgroup, with significantly longer PFS (9.9 vs. 6.0 months, 0.64 (95% CI 0,46 – 0.90), p = 0.011), numerically longer OS and significantly higher ORR (49.4% vs 26.6%, p=0.009) compared to FU/FA (Table). Conclusions: Mutations related to resistance concerning anti-EGFR antibodies were detected in 41 of 201 (20.4%) of analysed tumors and associated with a worse prognosis compared to hyperselected wt tumors. Negative hyperselection may aid in the identification of patients with relevant benefit from maintenance therapy including Pmab. [Table: see text]