Relationship of RAS and TP53 predictive value for cetuximab (C) benefit: Results of the EXPERT-C trial.

447 Background: We previously reported that TP53 status may predict C benefit in patients with locally advanced rectal cancer treated with neoadjuvant chemotherapy and chemoradiotherapy (CRT) and this effect appeared to be independent of KRAS. Recent studies indicate that NRAS mutations and KRAS mutations beyond exons 2-3 may also preclude benefit from anti-EGFRs. We analyzed whether the predictive value of TP53 in EXPERT-C was independent of RAS. Methods: 164 patients received 4 cycles of CAPOX followed by CRT, surgery, and 4 cycles of adjuvant CAPOX (n=81) or the same regimen plus C (CAPOX-C, n=83) and were analyzed for KRAS (exons 2-3) and NRAS (exon 3). TP53 mutations (exons 4-9) were screened for by CE-SSCA. KRAS (exon 4) and NRAS (exons 2 and 4) mutations were screened for by bi-directional Sanger sequencing. Progression-free survival (PFS) and overall survival (OS) were estimated with Kaplan-Meier methods and log-rank analysis was used to compare the treatment arms. The interaction between treatment and TP53 was adjusted for prognostic variables and RAS in a multivariate model. Results: 75/144 (52%) eligible patients had a TP53 mutation. 81/86 patients with known KRAS (exons 2-3) and NRAS (exon 3) wild-type (WT) status were analyzed for the remaining RAS mutations. Of these, 11 (13%) had tumours with mutation in KRAS exon 4 (11%) or NRAS exons 2/4 (2%). Overall, 75/144 (52%) patients were RAS WT (CAPOX, n=39; CAPOX-C, n=36). After a median follow-up of 65 months, no difference in PFS (HR 1.21, p=0.59) and OS (HR 0.97, p=0.94) was observed between TP53 mutant patients treated with CAPOX or CAPOX-C. In TP53 WT patients, the addition of C was associated with a statistically significant improvement in PFS (HR 0.23, p=0.02) and OS (HR 0.16, p=0.02). A significant interaction between TP53 status and C effect was found (PFS, p=0.029; OS, p=0.036). In multivariate analyses, this interaction remained significant even after adjusting for RAS status (PFS, p=0.026; OS, p=0.033). Conclusions: In EXPERT-C, the value of TP53 as predictive biomarker for C benefit was independent of RAS. The value of monoallelic vs. biallelic TP53 inactivation will be presented at the meeting.