Abstract CT143: Baseline circulating tumor DNA (ctDNA) analysis in Asian women with HR+/HER2- advanced breast cancer (ABC) receiving ribociclib + endocrine therapy in the Phase Ib MONALEESASIA trial

Background: MONALEESASIA (NCT02333370) is an ongoing Phase Ib study in which Asian patients (pts) with hormone receptor-positive (HR+)/human epidermal growth factor receptor 2-negative (HER2-) ABC are treated with ribociclib (a cyclin-dependent kinase 4/6 inhibitor) and either letrozole (all pts), fulvestrant, or tamoxifen (Japanese pts). Tumor biomarker analysis in MONALEESASIA using NanoString technology for gene expression was presented previously (Yap YS, et al. ESMO Asia 2018 [Abstract 400]). Here we report additional biomarker analysis of baseline ctDNA. Methods: Plasma samples were collected at baseline and end of treatment; ctDNA was extracted and used to generate next-generation sequencing libraries. The DNA libraries were then enriched for a specific 2.9 Mb of the human genome containing ≈550 oncogenes and tumor suppressor genes. Captured libraries were then combined in equimolar pools and sequenced, targeting ≥70 million sequencing reads per sample to ensure that unique coverage of the genes was >1000× or approached the maximal complexity of the library. Results: ctDNA was analyzed in baseline plasma samples collected from 77 pts (49 Japanese and 28 non-Japanese). Similar gene alteration frequencies were observed in Japanese and non-Japanese pts except for ESR1 alterations, which were more common in Japanese pts (Table). TP53 alterations were more common in pts with progressive disease, and the frequencies of TP53, ESR1, and FAT1 were slightly lower in pre- vs postmenopausal pts; however, these differences were not statistically significant. Response rates by genetic alteration are shown in the table. Conclusion: There appeared to be a trend toward a higher likelihood of alterations in TP53 and PIK3CA in pts with poorer responses, although the sample size was small. These data are hypothesis generating, and additional analysis is ongoing. Citation Format: Yoon-Sim Yap, Faye Su, Nadia Solovieff, Yoshinori Ito, Norikaza Masuda, Takashi Ishikawa, Tomoyuki Aruga, Seung Jin Kim, Wei He, Mihaela Gazdoiu, Joanne Chiu. Baseline circulating tumor DNA (ctDNA) analysis in Asian women with HR+/HER2- advanced breast cancer (ABC) receiving ribociclib + endocrine therapy in the Phase Ib MONALEESASIA trial [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr CT143.