Abstract 5155: Noninvasive detection of HER2 amplification in breast cancer with plasma DNA digital PCR

Abstract Introduction: Human epidermal growth factor receptor 2 (HER2) amplification is commonly detected in breast cancer tissue samples by immunohistochemistry (IHC) and/or fluorescent in situ hybridization (FISH) tests in clinical practice. It has been reported that cell-free DNA (cfDNA) may better capture the heterogeneity of acquired resistance than tumor biopsy. This study aims to develop a noninvasive digital polymerase chain reaction (PCR) assay for the detection of HER2 amplification in the plasma cfDNA of breast cancer patients. We further examine the concordance rate of HER2 amplification detected by blood-based digital PCR with tissue-based IHC/FISH tests. Materials and Methods: Plasma samples from 32 breast cancer patients were prospectively collected at Queen Elizabeth Hospital (Hong Kong SAR, China). According to previous IHC/FISH records, 15 patients were scored as HER2 amplified (IHC score 3 and/or FISH positive) and 17 patients as HER2 non-amplified. The detection of plasma cfDNA was performed by droplet digital PCR (ddPCR). The EFTUD2 gene was used as a reference and HER2:EFTUD2 ratio was assessed by ddPCR on the plasma DNA from cancer samples. Results: A median of 1.47 (range 0.92-3.83) was detected in the HER2 amplified patients and a median of 1.03 (range 0.76-1.23) was detected in the HER2 non-amplified patients by ddPCR. Our results showed that using 1.30 as the cutoff, ddPCR assay could well detect HER2 amplification. Receiver operating characteristic analysis was used to evaluate the diagnostic ability of this ddPCR assay and it returned an area under the curve of 0.898. A diagnostic test was used to evaluate the concordance of this ddPCR assay with the IHC/FISH tests and determine the sensitivity (73.33%), specificity (100%), accuracy (87.5%), positive predictive value (100%), and negative predictive value (81%) of the ddPCR assay. Conclusion: Accurate reporting of HER2 amplification status is a prerequisite for the appropriate choice of targeted therapy. We have obtained a high level of concordance in comparison to tissue-based IHC/FISH when cfDNA ddPCR assay was used to determine HER2 amplification in breast cancer patients. Most importantly, we have established a great accuracy of the ddPCR assay. Increasing studies have reported that HER2 levels may change during targeted therapy, but there are additional risks of patients by the invasive nature of IHC/FISH tests. Our results support the potential application of blood-based ddPCR assay to monitor the changes of HER2 amplification status in breast cancer patients during targeted therapy. Citation Format: William C. Cho, Eunice Y. Lau, Jeffrey C. Chan, Anna Y. Tai, Alex K. Leung, Anthony K. Leung, Michelle O. Szeto, Elizabeth Y. Chuk, Tony Y. Yuen, Molly W. Fung, Roger K. Ngan. Noninvasive detection of HER2 amplification in breast cancer with plasma DNA digital PCR [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5155.