Role of ctDNA in Predicting the Outcome of Patients with Hormone Receptor–Positive, HER2-Negative Advanced Breast Cancer Treated with First-line Ribociclib and Letrozole: BioItaLEE Trial

Purpose: This phase IIIb study prospectively evaluated the prognostic and predictive value of baseline and dynamic circulating tumor DNA (ctDNA) in postmenopausal patients with hormone receptor–positive (HR+), human epidermal growth factor receptor 2–negative (HER2_) advanced breast cancer (ABC) treated with first-line ribociclib/letrozole. Experimental Design: A total of 287 patients were enrolled, with ctDNA analyzed at baseline (n ¼ 263), day 15 of cycle 1 (C1D15; n ¼ 238), C2D1 (n ¼ 241), and first imaging (n ¼ 206). The primary objective was to identify ctDNA alterations, char-acterize their evolution across treatment time points, and assess their association with progression-free survival (PFS). Results: Median PFS was 23.4 months (95% confidence in-terval, 20.8 to not estimable). At baseline, the most frequently altered genes were PIK3CA (22.1%) and TP53 (15.5%). Alterations in TP53, MYC, and HER_ and cyclin-dependent kinase 4/6_ pathway genes were linked to early progression. Absence of a detectable mutation at baseline (n ¼ 150, 57%) was associated with a better prognosis [hazard ratio (HR) ¼ 0.41]. Among patients with a detectable mutation at baseline (n ¼ 104), early clearance (mutation undetectability) was observed in 47.1% at C1D15 and 52.4% at C2D1 and was associated with improved PFS (C1D15, HR ¼ 0.51; C2D1, HR ¼ 0.44). In patients without a detectable mutation at baseline, 22.7% (n ¼ 34) developed new mutations at C1D15, C2D1, or first imaging. Patients without new mutations had a lower risk of progression (HR ¼ 0.45). Conclusions: Pretreatment and early dynamics of ctDNA represent promising prognostic and predictive biomarkers in patients with HR+/HER2_ ABC treated with ribociclib/letrozole. Early ctDNA dynamics seem to be a promising surrogate bio-marker for treatment. Further studies are warranted to validate their clinical utility.