Abstract LB-221: Circulating tumor DNA detects early response to regorafenib in advanced colorectal cancer
Authors: Kehagias Pashalina , Vandeputte Caroline , Ameye Lieveke , Housni Hakim El , Laes Jean-François , Deleporte Amélie , Geboes Karen , Delaunoit Thierry , Demolin Gauthier , Peeters Marc , D'Hondt Lionel , Janssens Jos , Carrasco Javier , Galdon Maria Gomez , Ghanem Ghanem , Heimann Pierre , Paesmans Marianne , Flamen Patrick , Hendlisz Alain 
Published: August 2018

Abstract
Purpose: To demonstrate that an early increase in ctDNA plasmatic levels after 14 days may identify patients (pts) with advanced chemorefractory colorectal cancer (aCRC) unlikely to benefit from regorafenib. Experimental procedures: The RegARd-C prospective multicentric clinical trial (NCT01929616) collected archival tumor tissue and plasma samples at baseline (BL), 14 days (D14) and every 2 months after treatment initiation in aCRC pts (n=141). A targeted gene sequencing was performed on pts' tumor tissue to identify tumor-specific mutations using a panel of 47 genes commonly mutated in CRC. To monitor circulating tumor DNA (ctDNA) levels in plasma samples collected at BL and D14 via droplet digital PCR (Bio-Rad QX200 ddPCR system), we selected tumor-specific mutations based on their allelic frequency.
Results: As foreseen, in 96 pts the most common mutated genes were APC (73%), TP53 (72%), KRAS (66%), and PI3KCA (23%). Among these pts, 24 (25%), 49 (51%), 18 (19%) and 5 (5%) had respectively one, two, three and four tumor-specific mutations followed via ddPCR. On average 2 (range 1-4) tumor-specific mutations were followed per patient. In total, 65 specific paired PrimePCRTM ddPCRTM Mutation Detection Assays were optimized following digital MIQE guidelines to assess ctDNA levels. As previously described1, ctDNA levels were assessed in terms of absolute values of mutated copies per mL of plasma to avoid bias in the mutated/mutated + wild-type copies ratio. We considered an increase in ctDNA level of at least 12% as significant2. Pts were separated according to the presence (n=53) or absence (n=43) of at least one increasing mutation between BL and D14. An increase of at least one tumor-specific mutation as compared to none is associated with a significantly worse clinical outcome with a median PFS of 1.3 months versus 3.0 months (HR 1.88, P=0.002, 95% CI 1.24-2.85) and a median OS of 3.9 months versus 8.5 months (HR 2.04, P&lt;0.001, 95% CI 1.33-3.12).
Conclusion: Early (14 days) change in ctDNA levels under regorafenib therapy describes adequately the outcome of patients and may be used as a reliable tool for treatment personalization. Additionally, we confirmed the importance of using the absolute value of ctDNA instead of the mutated/total copies ratio usually reported in the literature for monitoring the outcome of a toxic treatment such as regorafenib. 1Kehagias P. et al. Oral presentation. LKI symposium, 2017; Leuven 2Whale AS. et al. Anal Chem. 2017
Citation Format: Pashalina Kehagias, Caroline Vandeputte, Lieveke Ameye, Hakim El Housni, Jean-François Laes, Amélie Deleporte, Karen Geboes, Thierry Delaunoit, Gauthier Demolin, Marc Peeters, Lionel D'Hondt, Jos Janssens, Javier Carrasco, Maria Gomez Galdon, Ghanem Ghanem, Pierre Heimann, Marianne Paesmans, Patrick Flamen, Alain Hendlisz. Circulating tumor DNA detects early response to regorafenib in advanced colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-221.