Abstract P5-05-04: Estrogen receptor 1 (ESR1) mutations in circulating tumor DNA (ctDNA) from patients with ER+/HER2- metastatic breast cancer (mBC) treated with lasofoxifene or fulvestrant in the ELAINE 1 study

Abstract

Abstract Introduction: Acquired ESR1 mutations (mutESR1) after long-term endocrine therapy drive treatment resistance, metastasis, and poor prognosis for patients (pts) with ER+/HER2- metastatic breast cancer (mBC). Lasofoxifene (LAS), a selective estrogen receptor modulator, alone or with a CDK4/6 inhibitor (CDK4/6i) reduced tumor growth better than fulvestrant (Fulv) in mutESR1 BC xenograft models. ELAINE 1 is a randomized trial of LAS vs Fulv in pts with mutESR1 and prior progression on aromatase inhibitor and CDK4/6i. Preliminary results (ESMO 2022) showed that LAS prolonged median progression-free survival (mPFS) compared with Fulv with a favorable safety profile. Here, we report changes in ESR1 ctDNA mutant allele frequency (MAF) from baseline to 8 wks and their associations with clinical benefit (CB) and mPFS. Methods: ELAINE 1 pts were randomized to oral LAS 5 mg daily or IM Fulv 500 mg on days 1, 15, and 29, then every 4 wks, until disease progression or severe toxicity. ctDNA mutESR1 mutations (baseline and 8 wks) were assessed using the Sysmex Inostics OncoBeam or SafeSeq assays—which detect mutESR1 at low allele fractions. MAF changes from baseline to wk 8 were characterized as decreased (decrease in ESR1 MAF or fully cleared), increased (increase in MAF), or equivocal (polyclonal patients [>1 mutESR1] with some increasing and decreasing MAF trends); correlations with PFS and CB were explored. Efficacy measures included objective response rate (ORR), PFS, and CB at 24 wks (CB defined as response or stable disease ≥24 wks). Results: 103 pts received LAS (n=52) or Fulv (n=51). Most common baseline ESR1 variants detected were D538G (56%), Y537S (39%), Y537N (29%), E380Q (22%); 56 (54%) pts were polyclonal. Of the 61 pts with evaluable baseline and wk 8 ctDNA, LAS decreased mutESR1 MAF in 29/35 pts (83% [11 complete clearance]) while Fulv decreased mutESR1 MAF in 16/26 pts (61.5% [6 complete clearance]) (Table). mPFS with LAS was 8 and 4 mos for pts with decreased/cleared MAF and increased MAF, respectively, and with Fulv was 4.5 and 2.8 mos, respectively (Table). LAS decreased the common mutESR1 variants more frequently than Fulv (median relative change -87.1% vs -14.7%). In pts with decreased MAF, CB was observed in 16/29 LAS pts (55%) and 4/16 Fulv pts (25%). The predictiveness of ESR1 MAF clearance for CB was also explored. Of 11 pts with ESR1 MAF clearance taking LAS, 10 achieved CB, yielding a positive predictive value (PPV) of 90.9%. In contrast, 2/6 pts with ESR1 MAF clearance taking Fulv had CB for a PPV of 33.3%. Sensitivity for predicting CB based on direction of ESR1 MAF change was 94% with LAS and 80% with Fulv. In pts with Y537S MAF (n=33), LAS decreased Y537S in 13/15 (87%), with a median relative MAF decrease of 89%. In marked contrast, Fulv increased Y537S MAF in 11/18 pts (61%), corresponding to an MAF relative increase of 82%. LAS and Fulv resulted in complete clearance of Y537S MAF in 33% and 6% of pts, respectively. Conclusion: Our data demonstrate that LAS more effectively decreased or cleared mutESR1 than Fulv. Further, mutESR1 clearance was associated with prolonged PFS and more CB in LAS but not Fulv pts, suggesting that LAS results in robust mutESR1 target engagement. Taken together, our data suggest mutESR1 as a potential liquid biomarker for predicting response to LAS in mutESR1, endocrine-resistant mBC pts. Table. Change from baseline to week 8 in ESR1 MAF and clinical benefit at 24 weeks. CI, confidence interval; MAF, mutant allele fraction; ND, none detected; PFS, progression-free survival. Citation Format: Matthew P. Goetz, Einav Gal-Yam, Daniel Stover, Sarah L. Sammons, Stephanie L. Graff, Grace Wang, Massimo Cristofanilli, Gary Riordan, Hillary S. Sloane, Dominic Carroll, Paul V. Plourde, David J Portman. Estrogen receptor 1 (ESR1) mutations in circulating tumor DNA (ctDNA) from patients with ER+/HER2- metastatic breast cancer (mBC) treated with lasofoxifene or fulvestrant in the ELAINE 1 study [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 P5-05-04.