Abstract P2-11-01: Secondary endocrine resistance and high Ki-67 labeling index (≥20%) in patients with low genomic risk identified by 21-gene recurrence score

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Abstract Introduction: Although breast cancer with high Ki-67 index are likely to have high genomic risk, some of these might have low genomic risk. In ER+HER2- breast cancer patients with low genomic risk identified by the 21-gene assay, we addressed a clinical significance of Ki-67 index in terms of recurrence-free survival (RFS). Methods: We retrospectively identified 2,295 patients with ER+HER2- breast cancer who underwent the 21-gene assay (Oncotype DX®). High Ki-67 LI was defined as 20% or greater, and low genomic risk was defined by recurrence score (RS) from the assay (< 26). Recurrence after 3 year was regarded as secondary endocrine resistance, which is defined as recurrence while on 2 years of adjuvant endocrine treatment. Results: In all patients, Ki67 index assigned 870 (38%) as the high and 1,425 (62%) as the low groups. Thirty percent (263/870) of the high Ki67 index had high genomic risk, whereas 84/1425 (6%) of the low Ki67 did. Among the patients with low genomic risk who did not receive chemotherapy (n=1,807), high Ki67 index was an independent poor risk factor for recurrence. Furthermore, recurrence after 3 years differed significantly according to Ki-67 index (P=0.003), whereas recurrence within 3 years did not differ (P=0.900). Multivariable analysis demonstrated high Ki67 index as a risk factor for recurrence after 3 years (hazard ratio 3.12; 95% confidence interval, 1.28–7.66). Conclusions: In ER+HER2- breast cancer with low genomic risk identified by 21-gene RS, high Ki67 index is associated with secondary endocrine resistance. Novel approach with CDK4/6 inhibitors is demanded to overcome secondary endocrine resistance in tumors with high Ki67/low genomic profile. Citation Format: Janghee Lee, Hakyoung Kim, Soong June Bae, Jong Won Lee, Byung Ho Son, Sei Hyun Ahn, Joon Jeong, Sae Byul Lee, Sung Gwe Ahn. Secondary endocrine resistance and high Ki-67 labeling index (≥20%) in patients with low genomic risk identified by 21-gene recurrence score [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P2-11-01.