Abstract P5-11-12: Clinical value ofESR1mutations from cell free DNA in ethynil estradiol treatment for metastatic breast cancer

Background: There are some cases in which estrogen addition therapy with ethinyl estradiol (EE2) is effective in cases in which aromatase inhibotor (AI) acquired resistance has been achieved after prolonged administration. In such case, ESR1 mutation was reported to be frequently seen. In this study, we examined the relation between histopathological factors, or ESR1 mutations detected by multiplex droplet digital PCR from cell free DNA (cfDNA) in patients’ and the effect of EE2 treatment. Patients and methods: From 2011 to 2016, we conducted a prospective observational study of EE2 therapy (0.5 to 3 mg / day PO) in our department (registration No. UMIN000002831). ER/PgR expression by IHC in a part cases, and serum E2 and FSH levels and blood cell counts in all cases were evaluated before/after treatment. Droplet digital PCR (ddPCR) system by the QX200™ Droplet Digital™ PCR System (Bio-Rad laboratories, Hercules, CA, USA) which makes thousands of droplets and each of them contained one or no copy of the sequence of ESR1 gene reacted with a pair of primers and two TaqMan probes. The PCR data were quantified using QuantaSoft™ software (Bio-Rad laboratories) and data are expressed as a percentage of mutant to total (mutant plus wild type) for each sample. Results: In this series, partial response (PR) was seen in 30% (12/40), long stable disease 12.5% (5/40), clinical benefit rate (CBR) 42.5% (17/40). The median EE2 dosing period in cases with CB was 8.4 months. AI was re-administered to 17 cases for which efficacy was obtained by EE2, and 8 cases had CB. After that, EE2 was re-challenge to 4 cases obtained effective cases. After EE2 treatment, serum E2 level increased and FSH decreased. In addition, the neutrophil-lymphocyte ratio increased, but its biological significance was unknown. Point mutations (Y537S, Y537N, D538G) of the ESR1 gene in cell free DNA in plasma were searched in 21 cases before EE2 administration, and High frequent ESR1 mutations were seen in 10 cases (47%) in any mutative site. In the patients with wild type of ESR1 before treatment, there were tended to be more effective for EE2 treatment, but they were not statistically significant. Additionally, no relationship between PIK3CA mutations (E542x, E545x, H1047x, G1049x) and EE2 effect was seen. Conclusion: The action mechanisms of EE2 treatment were reported as estrogen-induced apoptosis, which may not be related to ESR1 nor PIK3CA mutations. Further research related to EE2 treatment with biologic factors, such as gene alterations of the related factors, must be required. Citation Format: Hirotaka Iwase, Takashi Takeshita, Mutsuko Ibusuki, Aiko Sueta, Mai Tomiguchi, Yoshitaka Fujiki, Yutaka Yamamoto. Clinical value of ESR1 mutations from cell free DNA in ethynil estradiol treatment for metastatic breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P5-11-12.