Role of plasma circulating HER2/EFTUD2 ratio in predicting pathological complete response (pCR) after neoadjuvant dual anti-HER2 therapy in HER2-positive breast cancer (HER2+BC).

e12592 Background: Achieving pCR following neoadjuvant chemo and anti-HER2 therapy is associated with a significantly better survival in HER2+ BC. There is currently a lack of a robust biomarker to predict pCR in HER2+ BC. Using EFTUD2 as chromosome 17 reference probe, circulating HER2/EFTUD2 plasma DNA copy number ratio (H-ratio) detected by droplet digital PCR (ddPCR) was shown to have high concordance with the tumor HER2 status. We aim to evaluate whether the change in H-ratio is associated with response to neoadjuvant dual anti-HER2 therapy. Methods: We prospectively recruited patients with HER2+ BC who received neoadjuvant taxane, trastuzumab and pertuzumab followed by radical surgery from May 2019 to April 2022. Serial plasma samples (n = 49) were collected at pre-treatment (Tpre), at 4th cycle (Tmid) and at completion of neoadjuvant treatment (Tpost). H-ratio was determined in each plasma sample using droplet ddPCR (QX200 ddPCR system, Bio-rad). H-ratio responders were defined as patients having a declining Tpost H-ratio, whereas patients having a rising or persistent Tpost H-ratio were defined as H-ratio non-responders. The relationship between pCR, H-ratio and various clinicopathological characteristics were evaluated by Spearman’s correlation, Fisher’s exact test and Wilcoxon signed-rank test. Results: Eighteen clinical stage II or III HER2+ BC patients with a median age of 56 years old (range 33 - 71) were included. The median H-ratio at Tpre, Tmid and Tpost were 1.27, 1.12 and 1.10, respectively. Higher Tpre H-ratio was significantly associated with larger tumor size (p = 0.004) and higher tumor grade (G1-2 vs G3; median H-ratio 1.15 vs 1.78; p = 0.024). Six patients (33.3%) were H-ratio responders. Ten (55.6%) of 18 patients achieved pCR after completion of neoadjuvant treatment. The pCR rate in H-ratio responders was significantly higher than the H-ratio non-responders (100% vs. 36.4%; p = 0.017). In predicting pCR, H-ratio response outperformed hormonal receptor (HR) negativity (HR- vs HR+; 72.7% vs 28.6%; p = 0.088) or high tumor grade (G1-2 vs G3; 66.7% vs 33.3%; p = 0.201). There was no association between pCR and Tpre/Tmid/Tpost H-ratio. With a median follow up of 22.8 months (range 8.2 - 41.8), there were no relapse or death. Conclusions: A high pre-treatment plasma circulating HER2/EFTUD2 ratio is associated with large tumor size and high tumor grade in HER2+ BC. The decline of HER2/EFTUD2 ratio after neoadjuvant dual anti-HER2 therapy predicts pCR, and may serve as a potential biomarker for treatment de-escalation.