Abstract GS3-07: Circulating tumor DNA (ctDNA) dynamics in patients with hormone receptor positive (HR+)/HER2 negative (HER2-) advanced breast cancer (aBC) treated in first line with ribociclib (R) and letrozole (L) in the BioItaLEE trial
Authors: Bianchini Giampaolo , Malorni Luca , Arpino Grazia , Zambelli Alberto , Puglisi Fabio , Mastro Lucia Del , Colleoni Marco , Montemurro Filippo , Bianchi Giulia , Paris Ida , Allegrini Giacomo , Cazzaniga Marina Elena , Orditura Michele , Zamagni Claudio , Tamberi Stefano , Castelletti Daniela , Benelli Matteo , Callari Maurizio , Santoro Angela , De Laurentiis Michelino 
Published: March 2022

Background: ctDNA analysis is emerging as an attractive non-invasive approach to characterize tumor biology, describe its evolution over time, and predict treatment benefit. Here, we assessed the prognostic and predictive role of baseline and dynamic ctDNA analysis in HR+/HER2- aBC patients (pts) treated with R+L. Methods: 287 postmenopausal pts were enrolled in the BioItaLEE trial (NCT03439046). Liquid biopsies were collected at baseline (D0; n=263), day 15 of cycle 1 (D15; n=238), day 1 of cycle 2 (C2D1; n=241) and at first imaging (FI, at approximately 12 weeks; n=206). ctDNA analysis was carried out using a 533-amplicon Custom AmpliSeq HD Panel, with amplicons covering the coding exons of 39 BC-related genes (limit of detection: 0.1%). Target mutations were defined as single-nucleotide variant (SNV) or Insertion/Deletion detected at D0. When multiple target mutations were detected, the one with the highest variant allele frequency (VAF) was considered. The association between pre-treatment and on-treatment ctDNA dynamics with progression-free survival (PFS) was assessed using Multivariate Cox models. VAF clearance was defined as 100% decrease in a target mutation. Results: Median follow-up was 26.9 months and median PFS was 23.39 (20.8-NE) months. At baseline, target mutations were detected in 113 pts (43.0%), whereas 150 pts were wild-type (wt). Mean (SD) pre-treatment VAF at D0 was 11.3% (14.4). The absence of a target mutation at D0 was associated with good prognosis (HR: 0.41, 95% CI: 0.27–0.61; p&lt;0.0001). Considering early ctDNA dynamics, a significant VAF reduction was observed at D15 and C2D1 with a mean (SD) change of -64.3% (55.9) and -68.6% (52.2), respectively. In pts with a target mutation detected at baseline, early VAF clearance was observed in 47.1% of pts at D15 and in 52.4% of pts at C2D1. Clearance at D15 or C2D1 was associated with improved PFS (D15, HR: 0.51, 95% CI: 0.28-0.91, p=0.0228; C2D1, HR: 0.44, 95% CI: 0.25-0.78, p=0.0052). Pts achieving clearance at D15, which was maintained at C2D1 (39.4%) had the lowest risk of progression compared to those who had no clearance at any or both timepoints (HR: 0.40, 95% CI: 0.20-0.79; p=0.084). Monitoring of the 150 pts without a detectable target mutation at baseline revealed a new, detectable mutation at later timepoints (D15, C2D1 and FI) in 34 pts (22.7%). The absence of new mutations was associated with a lower risk of progression (HR: 0.45, 95% CI: 0.24-0.85; p=0.0143). Considering all time points individually, D15 was the most informative of patient outcome. Indeed, pts without mutation at D15 (42.9%) had an extremely favorable outcome, either because they achieved early treatment-related clearance or maintained baseline absence of a target mutation (HR: 0.32, 95% CI: 0.20-0.51; p&lt;0.0001). Notably, in pts with detectable target mutation at D15, a VAF below the median showed a trend for better prognosis versus high VAF (HR: 0.56, 95% CI: 0.30-1.04; p=0.065). Conclusions: The presence of a detectable mutation in baseline liquid biopsies appears to be a negative prognostic factor. Within this high-risk group, early VAF clearance during the first R+L cycle was informative of treatment benefit and associated with a lower risk of progression. Monitoring of ctDNA in patients without baseline mutations demonstrated that the detection of new mutations by FI assessment was associated with worse outcome. Overall, pre-treatment and early dynamics of ctDNA (assessed by NGS) represent promising prognostic and predictive biomarkers in patients with HR+/HER2- aBC treated with ribociclib/letrozole in the first-line. Further studies are warranted to validate the clinical utility of these biomarkers.

Citation Format: Giampaolo Bianchini, Luca Malorni, Grazia Arpino, Alberto Zambelli, Fabio Puglisi, Lucia Del Mastro, Marco Colleoni, Filippo Montemurro, Giulia Bianchi, Ida Paris, Giacomo Allegrini, Marina Elena Cazzaniga, Michele Orditura, Claudio Zamagni, Stefano Tamberi, Daniela Castelletti, Matteo Benelli, Maurizio Callari, Angela Santoro, Michelino De Laurentiis. Circulating tumor DNA (ctDNA) dynamics in patients with hormone receptor positive (HR+)/HER2 negative (HER2-) advanced breast cancer (aBC) treated in first line with ribociclib (R) and letrozole (L) in the BioItaLEE trial [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr GS3-07.