Comprehensive clinical characteristics and ctDNA mutational profile analysis of endocrine resistance/sensitivity to adjuvant ET therapy in luminal breast cancer from the GEICAM/2014-03_RegistEM registry.

1011 Background: Patients (pts) with hormone receptor-positive (HR) early breast cancer (BC) may develop resistance to adjuvant endocrine therapies (ET), with relapse occurring months (m) to decades after surgery. Our study explores clinical characteristics, prognosis, and oncogenic drivers in circulating tumor DNA (ctDNA) at relapse in HR+ BC pts, based on their sensitivity to adjuvant ET. Methods: In the REGISTEM study, we identified 805 primary HR-positive BC pts who developed metastatic relapse. These cases were classified based on their sensitivity to adjuvant ET, according to the 5th ESO-ESMO ABC Guidelines: 1ET-resistance (1ET-R, relapse within the first 2 years of adj ET; n=138), 2ET-resistance (2ET-R, relapse after the first 2 years or <12 m after completing adj ET; n=284), and ET-sensitive (ET-S, relapse after >12 m of completing adj ET; n=383). Out of this cohort, 139 individuals had plasma samples obtained prior to the initiation of the first-line treatment for advanced disease, and these samples were analyzed using the AVENIO ctDNA assay. Results: Pts with 1ET-R were significantly more likely to have grade 3, stage III, and highly proliferative tumors (Ki67>20%). The median time to distant relapse was 24 m for 1ET-R, 52.8 m for 2ET-R, and 124 m for ET-S cases. For first-line treatment, the median PFS was 8.1 m for 1ET-R, 13 m for 2ET-R, and 21.2 m for ET-S relapse (p<0.0001). With a median follow-up in the advanced setting of 34 m, median OS was 28 m for 1ET-R, 46 m for 2ET-R, and 55 m for ET-S relapses (p<0.0001). Pts with ET resistance had more mutations (mut) and higher mut allele frequency than ET-sensitive relapse. Pts with ET resistance showed higher incidence of mut in TP53, ESR1, PTEN, andhigh copy number in EGFRand ERBB2 than ET-sensitive, but similar incidence of mut in PIK3CA.Mut in ESR1 and high copy number in EGFR were correlated with poorer PFS, while ESR1 mut were additionally associated with worse OS, although these associations were not independent of the ET relapse groups (Table). Furthermore, mut in PIK3CA were associated to worse PFS only in ET-R pts. Conclusions: In HR-positive BC, both primary and secondary resistance to adjuvant ET represents a highly aggressive disease with enriched actionable genetic alterations detected at the time of diagnosis of metastatic relapse. Addressing these alterations has the potential to mitigate the unfavorable prognosis associated with these cases. Clinical trial information: NCT02819882 .[Table: see text]