Giridhar, K. V.

Abstract Background: (Z)-endoxifen (ENDX) is an oral SERM that dually targets ERα (> 5 ng/ml) and PKCβ1 (>500 ng/ml). It has been tested in the endocrine resistant setting at doses of 20-360 mg/day; however, the optimal dose in an endocrine naïve population is not defined. We explored 10 mg/day dose of neoadjuvant ENDX in the Endocrine Optimization Pilot (EOP) of I-SPY2. Methods: EOP eligibility included pts with Stage II/III HR+ HER2- breast cancer, MammaPrint (MP) low. Pts with MP High1 scores were eligible if clinically node-negative. Pts received ENDX 10 mg orally once daily for 6 cycles (each cycle = 28 days). Premenopausal pts started ovarian function suppression on cycle 2, day 1. ENDX was continued until the day prior to surgery. The primary study endpoint was feasibility, defined as >75% of pts completing >75% of ENDX. Baseline (T0), 3-wk (T1) biopsies, and the surgical specimen (T3) were assessed centrally for Ki-67. Rates of modified preoperative endocrine prognostic index (mPEPI) 0 were reported, defined as pT1-2 pN0, Ki67< 2.7%, or pathologic complete response. A genomic index for sensitivity to endocrine therapy (SET) index was done at T0. Breast MRI measurement of functional tumor volume (FTV) was performed at T0, T1, 12 weeks (T2), and pre-operatively (T3). Blood was collected for tumor informed ctDNA at T0, T1, T2, T3 and for pharmacokinetic (PK) steady state concentrations (Css) at T1. Safety data using CTCAE V5 is reported. Results: Between March 2023 and May 2024, 20 pts were enrolled in this arm. The median age was 52.5 years (range 32 -73), 13 (65%) were premenopausal, and 55% had clinically positive lymph nodes. Tumors were MPHigh1 in 3 pts (15%) and SET2,3 index high in 16 pts (80%). The primary endpoint was met with 95% of study participants completing ≥75% of therapy. The T0 median Ki-67 was 10.5% (range 2-60%). At T1 (n=19) and T3 (n=18), median Ki-67 was 5% (range 0-35%) and 5% (range 0-45%), respectively. The 3-week rate of Ki-67 ≤10% at T1 was 61.5% in premenopausal and 71.4% in postmenopausal women. The rate of Ki-67 ≤10% at surgery was 81.8% in premenopausal and 57.1% in postmenopausal women. An mPEPI score of 0 was observed in 1 (5%) of participants. The median T0 FTV was 8.9 ccs (range 0.9 to 251.1). The median T3 FTV was 1.3 ccs (range 0 to 11.2) and the median change in FTV from T0 to T3 was -72.0% (range -88.5% to -61.6%). ctDNA analysis was performed in 17 pts longitudinally across 64 blood samples.10 pts had positive ctDNA at T0, with 7 converting to ctDNA undetectable during treatment with ENDX. 1/7 pts who was ctDNA negative at T0 became positive at T1 and then reverted to undetectable ctDNA. The other 6 pts remained ctDNA negative at all timepoints. The median ENDX T1 Css was 79.3 ng/mL (range 73.6 - 906.4 ng /mL). The most common side effects reported were Grade 1 hot flush (70%), nausea (40%), fatigue (40%), and insomnia (35%). Grade 2 side effects included fatigue (15%) and night sweats (5%). No dose reductions or discontinuations due to toxicity occurred. Conclusions: ENDX at 10 mg was well tolerated and demonstrated reductions in Ki-67, MRI FTV, and ctDNA. The ISPY2 EOP is currently enrolling patients to ENDX 40 mg/day (intended to dually target ERα and PKCβ1) in combination with abemaciclib (with and without OFS). Citation Format: K. V. Giridhar, M. P. Goetz, C. Yau, A. M. Wallace, C. Falkson, J. Boughey, M. Arora, E. Stringer-Reasor, C. Nangia, C. Gallagher, A. Elias, A. Zimmer, M. S. Trivedi, C. Omene, L. van 't Veer, L. Brown Swigart, G. L. Hirst, W. Symmans, A. D. Borowsky, N. Onishi, N. Hylton, C. Vakalvas, M. Wei, J. M. Reid, S. Alhafaji, N. Chen, R. M. Mukhtar, L. Huppert, O. Opopade, L. J. Esserman, A. Chein. Low dose (Z)-endoxifen in the I-SPY2 Endocrine Optimization Pilot [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS3-07-10.

Abstract Background: In MBC, CTC enumeration is highly prognostic for both progression free survival (PFS) and overall survival (OS). Multiparametric technologies enable for expanded phenotypic profiling of CTCs, including ER and HER2 assessment. Limited information is available on the concordance of CTC ER and HER2 levels compared to synchronously obtained tissue. We assessed the prognostic performance of CTC enumeration and phenotypic profiling of ER and HER2 on CTCs. Methods: The prospective, multi-institutional Aurora study enrolled pts with MBC of any subtype either at diagnosis of metastatic disease or at time of progression on any line of systemic therapy. Pts underwent a metastatic site biopsy, consented to archival primary tissue collection and donated blood samples serially every 3-4 months until progression. CTCs, from blood collected at study entry, were identified as nucleated, EpCAM+/ cytokeratin+/ CD45- cells using an automated imaging system (RareCyte, Seattle). ER and HER2 expression levels were measured by mean fluorescence intensity (MFI), with thresholds above 80 (ER) and 160 (HER2) considered positive. Comparisons were made to total RNAseq from paired metastatic tissue biopsies. PFS and OS were calculated from the time of study entry to last follow up or death. Results: Baseline CTC enumeration was performed in 154 pts. The most common clinical subtype was HR+/HER2- (59.7%), followed by HER2-positive (16.8%) and TNBC (15.5%). The median lines of prior therapy were 1 (range 0-10). At least 1 CTC was identified in 68/154 pts (44%), 30 (19%) had ≥ 5 CTCs, and 15 (9.7%) had CTC clusters. The median CTC value was 0 (0-877). The presence of CTCs (≥1 or ≥5) and CTC clusters was adversely associated with PFS and OS (Table 1 Table 1. CTC survival analyses Number Median PFS PFS HR (95% CI) Median OS OS HR (95% CI) Presence of CTC 0 CTC 86 7.37 (0.03 - 43.1) reference 20.72 (0.03 - 50.6) reference ≥ 1 CTC 68 4.7 (0.43 - 19.5) 1.65 (1.16-2.36) 12.04. (0.43-44.1) 2.13 (1.44-3.16) Presence of ≥ 5 CTC <5 CTC 124 7.7 (0.03 - 43.1) reference 19.31 (0.03 - 50.6) reference ≥ 5 CTC 30 2.88 (0.43 - 18.7) 1.77 (1.15 - 2.71) 5.74 (0.43 - 29.3) 3.59 (2.26-5.70) Presence of clusters 0 clusters 139 7.37 (0.03 - 43.1) reference 18.06 (0.03 - 50.6) reference ≥ 1 cluster 15 1.91 (0.43 - 18.7) 2.06 (1.16 - 3.68) 3.42 (0.43 - 18.7) 5.44 (2.94-10.09) ). Across all subtypes, the median CTC ER-MFI across was 36.2 (range 4.2-10,016.6) and the median HER2-MFI was 139.6 (range 4.2-2480.5). We observed high intra-patient heterogeneity of ER-MFI (CV: 0.65 [0.34-1.25]) and HER2-MFI (CV: 0.79 [0.42-1.54]). Clinical metastatic subtype from metastasis at study entry was available in 54/68 pts with CTCs. Pts with clinical ER+ MBC had a higher median ER-MFI compared to clinical TNBC (111.9 vs 68.6, p = 0.02). However, 11 pts (26.8%) with clinical ER+ MBC had all CTCs with an ER-MFI <80. Lower ER expression was observed in CTCs collected after prior exposure to a SERD (median ER-MFI: post-SERD [n=8] 27 vs post-aromatase inhibitor [n=4] 252, p<2.22e-16). No correlations were observed between the median CTC ER-MFI or HER2-MFI and paired tissue ESR1 or ERBB2 RNA expression (n=66). Conclusions: CTCs are strongly predictive of PFS and OS. CTC ER- and HER2-MFI were highly heterogeneous without clear correlation with bulk tissue RNA expression. The clinical impact of discordant clinical receptor status and CTC receptor status warrants further exploration. Citation Format: K. V. Giridhar, W. C. Nenad, H. Ye, G. L. Wheeler, T. Hinoue, A. R. Michmerhuizen, B. M. Felsheim, U. R. Chandran, B. J. Kelly, E. R. Mardis, P. W. Laird, J. M. Balko, A. C. Garrido-Castro, S. Coppens, J. Bowen, R. Nanda, N. U. Lin, C. K. Anders, C. Isaacs, J. J. Tao, J. D. Anampa, N. Jahan, L. Huppert, A. DeMichele, B. H. Park, S. Vinayak, A. L. Delson, M. M. Magbanua, M. Balic, H. A. Parsons, A. V. Lee, L. A. Carey, L. Norton, A. C. Wolff, N. E. Davidson, I. Krop, T. A. King, C. M. Perou, K. A. Hoadley, Aurora US Metastases Network. Characterizing circulating tumor cell (CTC) enumeration and phenotypic profiling in blood with synchronously obtained tissue in metastatic breast cancer (MBC) in TBCRC /AURORA US [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS2-07-17.