Choong, Grace M.

525 Background: Adjuvant endocrine therapy (ET) decreases breast cancer (BC) recurrence and improves overall survival (OS) for patients (pts) with estrogen receptor positive (ER+) BC. Omission of adjuvant ET and nonadherence are major issues contributing to worse clinical outcomes. Recent data suggest that the number of pts who omit adjuvant ET may be increasing in Europe [1]. These data have yet to be evaluated in the U.S. We previously reported data from the National Cancer Database (NCDB) that ET omission was more likely in pts with ER+/HER2+ disease who achieved a pathologic complete response (pCR) after neoadjuvant chemotherapy (NAC) [2]. Herein we report an updated analysis with a larger cohort designed to examine trends in ET omission over time and the impact on OS in a group of pts at high risk of recurrence. Methods: The NCDB was queried from 2010-2022 for pts with stage I-III ER+BC treated with NAC and surgery. pCR was defined as ypT0, ypN0. Use of adjuvant ET and the impact of adjuvant ET omission on OS in pts with and without pCR were assessed separately based on HER2 status. Adjuvant ET initiation by 1 year was summarized using the cumulative incidence estimate to account for the competing risk of death. OS was analyzed with ET as a time-dependent covariate using multivariable Cox proportional hazards regression. Results: We identified 130,438 pts with ER+BC (78,778 ER+/HER2-, 51,660 ER+/HER2+) who received NAC. Median follow-up was 5 years. Overall, pCR was achieved in 10.0% of ER+/HER2- and 35.9% of ER+/HER2+. On multivariable analysis, ET omission was significantly associated with worse OS in pts with residual disease (RD) [ER+/HER2- (HR 1.80 p<0.001) ER+/HER2+ (HR 1.66 p<0.001] and in those with a pCR [ER+/HER2- (HR 1.38 p=0.002) ER+/HER2+ (HR 1.42 p<0.001)]. Between 2010-2017, adjuvant ET omission was stable, with approximately 10.0% of pts not having initiated ET by 1 year after surgery. The proportion of pts omitting ET overall increased thereafter to 12.9% in 2018, rising to 24.4% in 2022. A sharp increase in ET omission was observed 2021-2022 in ER+/HER2- BC who achieved pCR, with 42.4% omitting ET compared to 25.1% for 2018-2020 and 18.8% for 2010-2017 (p<0.001). Adjuvant ET omission showed more modest but significant increases for other subgroups (p<0.001): ER+/HER2- with RD (8.5% 2010-2017, 12.3% 2018-2020, 18.7% 2021-2022), ER+/HER2+ with pCR (15.1% 2010-2017, 14.0% 2018-2020, 24.4% 2021-2022), and ER+/HER2+ with RD (9.5% 2010-2017, 12.5% 2018-2020, 22.2% 2021-2022). Conclusions: In ER+BC treated with NAC, omission of ET is increasing over time and associated with a higher risk of death, regardless of HER2, RD, or pCR status. These findings have substantial public health implications as they may result in a reversal of the 4-decade improvements in breast cancer mortality. Additional intervention is critical to improve adherence to guidelines. [1] Verreck et al. Breast 2025 [2] Choong et al. SABCS 2022.

Abstract Background: Endometrial cancer (EC) remains a major gynecologic malignancy driven in part by aberrant estrogen receptor (ER) signaling. The active tamoxifen metabolite (Z)-endoxifen exhibits dual SERM/SERD-like activity, modulating ER function while exerting additional noncanonical antiproliferative effects. Understanding (Z)-endoxifen’s mechanisms of action and its interaction with cell-cycle regulators is critical to optimizing targeted therapy in EC. This study investigated the molecular and functional effects of (Z)-endoxifen, alone and in combination with the CDK4/6 inhibitor abemaciclib, across diverse EC models to define mechanistic activity and translational potential. Methods: ER+ (Ishikawa, HCI-EC23, patient derived organoid U1561.005) and ER-variable (ARK1, ARK2) EC models were treated with estrogen (0-10 nM), (Z)-endoxifen (125 nM-10 µM) or fulvestrant (0-2.5 μM), and abemaciclib (0-10 μM), as single agents or in combination. Cell viability was assessed using luminescence-based assays. Dose-response analyses and combinatorial synergy were evaluated to determine endocrine sensitivity, ER dependency, and downstream cell-cycle effects. Results: Estrogen induced biphasic proliferative responses in ER+ models, confirming ligand-driven mitogenic signaling. (Z)-Endoxifen monotherapy suppressed viability across all cell lines, demonstrating superior potency to fulvestrant and activity independent of ER expression status. Mechanistically, (Z)-endoxifen abrogated estrogen-induced proliferative peaks, consistent with ER antagonism and potential receptor degradation, while also exerting ER-independent effects suggestive of cell-cycle modulation. Abemaciclib monotherapy inhibited viability in both ER+ and ER− models, and its combination with (Z)-endoxifen yielded strong synergistic activity (Chou-Talalay combination index &lt; 1), including in ER-negative ARK2 and the ER+ U1561.005 organoid. This synergy likely arises from concurrent disruption of ER signaling and CDK4/6-cyclin D regulatory pathways. Conclusions: (Z)-Endoxifen demonstrates dual ER-dependent and ER-independent activity in EC preclinical models. Synergy was observed in combination a CDK 4/6 inhibitor. These findings broaden the current understanding of (Z)-endoxifen’s efficacy and support further exploration of its potential therapeutic role in ER-driven and mixed-phenotype EC, including evaluation of responses across ER expression levels. Citation Format: Grace Choong, Xiaonan Hou, Sandra Suarez Hammer, Scott M. Blackburn, John Weroha, Steven C. Quay. (Z)-endoxifen modulates estrogen receptor and cell-cycle signaling to induce synergistic antiproliferative effects with CDK4/6 inhibition in endometrial cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 405.

PURPOSE Low-grade serous carcinoma (LGSOC) of the ovary, fallopian tube, or peritoneum is a hormonally driven, relatively chemoresistant malignancy with limited treatment options in the recurrent setting. Given frequent estrogen receptor (ER) expression and dysregulation of the cyclin-dependent kinases 4 and 6 (CDK4/6)–p16–Rb pathway, features shared with hormone receptor–positive breast cancer, dual endocrine, and CDK4/6 inhibition is a biologically rational strategy. This phase II trial evaluated ribociclib plus letrozole in recurrent LGSOC. METHODS This open-label, single-arm, multicenter phase II study enrolled women with measurable, recurrent LGSOC. Patients received ribociclib (600 mg orally, once daily, days 1-21 of a 28-day cycle) and letrozole (2.5 mg orally, once daily). The primary end point was investigator-assessed objective response rate (ORR) per RECIST 1.1. Secondary end points included clinical benefit rate (CBR), progression-free survival (PFS), overall survival (OS), and safety. RESULTS Of 74 patients screened, 51 were enrolled and 49 treated. The confirmed ORR was 30.6% (90% CI, 19.9 to 43.2), including one complete and 14 partial responses. Among responders, the median duration of response was 21.2 months. The CBR was 84% (90% CI, 72.5 to 91.6). The median PFS was 14.5 months (90% CI, 10.1 to 28.8), and the median OS was 44.5 months (90% CI, 31.8 to not reached). The most common grade ≥3 adverse event (AE) was neutropenia (47%), managed with dose modifications. Three grade 5 events (6%) occurred but were unrelated to treatment. Treatment discontinuation because of AEs occurred in 4%. No dose-limiting toxicities were observed. CONCLUSION Ribociclib plus letrozole met the primary end point, achieving meaningful response rates and durable disease control in recurrent LGSOC. The safety profile was consistent with prior CDK4/6 inhibitor studies. This combination represents a therapeutic option in this rare and genomically distinct subtype.

PURPOSE The difference in time to serum calcium normalization and rates of subsequent refractory hypocalcemia between bisphosphonates requires further investigation. Our primary objective was to assess the difference in normalization of serum calcium between pamidronate and zoledronic acid during treatment of hypercalcemia of malignancy (HCM) and rates of subsequent, refractory hypocalcemia following therapy. METHODS A retrospective review of patients with cancer who received pamidronate or zoledronic acid inpatient at Mayo Clinic for treatment of HCM between January 2018 and January 2024 was conducted. Logistic regression evaluated the association between treatment groups and outcomes. Multivariable modeling was adjusted for baseline differences, including severity of hypercalcemia. RESULTS We evaluated 856 patients. The median age was 68 (IQR, 59-75) years, and 476 (55.6%) were male. The most common cancer diagnoses were lymphoma (n = 158, 18.5%), followed by multiple myeloma (n = 145, 16.9%) and lung cancer (n = 111, 13%). Overall, 292 (46.8%) patients and 154 (32.8%) patients achieved serum calcium normalization by hospital day 4 and day 7, respectively. The rate of normalization was similar between all bisphosphonates, regardless of pamidronate dose. Significantly higher rates of hypocalcemia were observed on day 4 for patients who received zoledronic acid 4 mg once compared with either dose (60 mg once and 90 mg once) of pamidronate (40.7% v 27.6% v 26.8%, P = .049, respectively). No significant difference in rates of hypocalcemia were observed on day 7 (56.8% v 50% v 42.9%, P = .26, respectively) and day 30 (39.1% v 39.0% v 37.5%, P = .50, respectively). CONCLUSION Bisphosphonates were equally efficacious in normalizing serum calcium in patients with HCM. Pamidronate 90 mg was not more efficacious than 60 mg. Rates of refractory hypocalcemia were similar.

529 Background: Neoadjuvant chemotherapy (NAC) is commonly administered to patients (pts) with estrogen receptor alpha (ER) negative (immunohistochemistry [IHC] 0%) and ER-low (IHC 1-10%) breast cancer (BC). For pts without pathological complete response (pCR), ER expression may differ between baseline and residual BC following NAC. We previously demonstrated adjuvant endocrine therapy (ET) omission in ER-low early-stage BC was associated with significantly worse overall survival (OS); however, this effect was restricted to pts with residual BC following NAC and those with higher baseline ER levels (IHC 6-10%) (Choong et al., ASCO 2024). Here, we assessed how often ER was expressed in residual invasive BC for pts treated with NAC for ER-negative and ER-low BC. Methods: Weidentified pts with pre-treatment (tx) ER-negative and ER-low (regardless of HER2 expression) stage I-III BC treated with NAC who underwent BC surgery at Mayo Clinic Rochester between 2009 and 2023. ER IHC was performed in a CAP/CLIA laboratory. We evaluated ER expression in residual invasive BC for pts without pCR. The percent of pts with a post-tx change in ER status was estimated and reported with 95% Wilson confidence intervals. Results: 955 pts (838 [88%] pre-tx ER-negative; 117 [12%] pre-tx ER-low) met inclusion criteria, of whom 69% had HER2-negative and 31% HER2-positive BC. The median age at diagnosis was 52 (range: 24-86). 496 (52%) had residual BC. Residual BC was more common in HER2-negative versus (vs) positive tumors (56% vs 42%, p < 0.001) but did not differ significantly by ER status (ER-negative 51% vs ER-low 57%, p = 0.22). Of those with residual BC, 277/496 (56%) had ER re-testing. Rates of ER re-testing did not vary for pre-tx ER-negative vs -low (57% vs 51%, p = 0.37) but were significantly lower for HER2-positive vs HER2-negative tumors (39% vs 62%, p < 0.001). Among those with post-tx testing, 31/277 (11%, 95% CI: 8-15%) had an increase in ER expression from pretreatment levels (defined as ER IHC < 1% to either 1-10% or > 10%; or ER IHC 1-10% to > 10%). In these 31 pts, the original NAC was for either TNBC (21/31; 68%) or HER2+ BC (10/31; 32%). In pts with pre-tx ER-negative BC, 27/243 (11%, 95% CI: 8-16%) had ER expression in the residual BC including 14/243 (6%, 95% CI: 3-9%) with ER > 10% and 13/243 (5%, 95% CI: 3-9%) with ER 1-10%. For pts with baseline ER-low BC, 4/34 (12%, 95% CI: 5-27%) had ER > 10% in the residual BC. Among the 31 pts where ER increased following NAC, 21/31 (68%) received adjuvant ET, including 17/18 if ER was > 10% in the residual disease and 4/13 in those with ER 1-10%. Conclusions: In pts treated with NAC for ER-negative or ER-low BC not achieving pCR, we identified higher ER expression in the residual breast cancer in > 10% of pts. Given that omission of ET in ER-low BC with residual cancer following NAC is associated with worse survival, repeat biomarker testing should be considered in those without pCR, and ET individualized according to ER expression.

PURPOSE Adjuvant endocrine therapy (ET) improves overall survival (OS) in estrogen receptor (ER)–positive early-stage breast cancer (BC). However, the benefit of ET for those with ER-low BC (ER 1%-10%) is unclear. METHODS Using the National Cancer Database, we studied patients with high-risk stage I to III, ER-low BC (defined as immunohistochemistry 1%-10%) who received (neo)adjuvant chemotherapy and did or did not initiate ET. OS was analyzed with ET initiation as a time-dependent covariate using Cox proportional hazards regression. RESULTS Of 10,362 patients with stage I to III ER-low BC, 7,018 received chemotherapy and met inclusion criteria. ET omission was 42% at 12 months and more common in patients with tumors that were progesterone receptor–negative, human epidermal growth factor receptor 2–negative, higher-grade (grade 2/3) and higher Ki-67 (≥20%; all P < .001) and those who received neoadjuvant chemotherapy (NAC; P < .001). With a median follow-up of 3 years, 586 deaths were observed. In a multivariable analysis, ET omission was associated with a higher risk of death (hazard ratio [HR], 1.23 [95% CI, 1.04 to 1.46]; P = .02), with a greater impact in those with higher ER levels: ER 1%-5% (HR, 1.15 [95% CI, 0.91 to 1.45]; P = .24) versus ER 6%-10% (HR, 1.42 [95% CI, 1.00 to 2.02]; P = .048). Among patients treated with NAC (n = 4,377, 62%), ET omission was associated with worse OS in those with residual disease (RD; HR, 1.26 [95% CI, 1.00 to 1.57]; P = .046) but not in those who achieved a pathologic complete response (HR, 1.06 [95% CI, 0.62 to 1.80]; P = .84). CONCLUSION In ER-low, early-stage BC, ET omission is associated with significantly worse OS, especially in patients with RD after NAC and those with higher (6%-10%) ER levels. Until prospective data are available, patients with ER-low BC should be counseled regarding the potential benefit of ET.

TPS5630 Background: Platinum-resistant ovarian cancer (PROC) portends a poor prognosis and, although treatments are available, response rates are low. Previous studies have demonstrated that DNA topoisomerase I (TOP1) inhibitors exhibit clinical activity in PROC. Irinotecan also has a 15-25% response rate in some studies of platinum-resistant epithelial ovarian cancer but does not have FDA approval for this indication. Additional studies in preclinical models suggest that TOP1 poisons are more active if administered at low doses on prolonged schedules. PLX038 is a long-acting prodrug of the potent TOP1 inhibitor SN-38. In PLX038, SN-38 is covalently conjugated to circulating polyethylene glycol and is slowly released at a constant rate to provide the active agent with a long half-life, a low Cmax and very high exposure – important facets for optimal safety and efficacy of this drug. Further, in preclinical studies the nanoparticle prodrug accumulates and is retained in tumors, where it slowly releases the SN-38. Methods: This single arm, phase 2 clinical trial is open at Mayo Clinic in Rochester, MN. The primary endpoint is overall response rate and secondary endpoints include progression-free survival and assessment of tolerability of PLX038. Correlative goals include i) measurement of TOP1-DNA covalent complexes in tumor biopsies and circulating tumor cells to assess successful target engagement; ii) assessment of homologous recombination status and SLFN11 expression for correlation with tumor response rate; and iii) pharmacokinetics of SN-38 and SN-38G as well as their association with GI toxicity. Assessment of gut microbiota and association with GI toxicity profile are also being investigated. Eligible patients include women with platinum resistant high grade serous ovarian, primary peritoneal, and fallopian tube cancer who have received fewer than two prior chemotherapy regimens in the platinum-resistant setting. All patients must have an ECOG performance status of 0, 1, or 2 and adequate bone marrow, renal, and hepatic function. As of January 31, 2024, 12 of planned 37 patients have enrolled. Clinical trial information: NCT05465941 .

5562 Background: Platinum-resistant or refractory ovarian cancer remains a hard-to-treat disease given the poor prognosis and survival rates. Immune checkpoint inhibitors (ICI; such as pembrolizumab) have not been approved in this indication. Igrelimogene litadenorepvec (Ad5/3-E2F-d24-hTNF-IRES-hIL2; TILT-123), is an oncolytic adenovirus encoding for IL-2 and TNFa, designed to selectively lyse tumor cells while recruiting and activating T-cells, turning immunologically cold tumors hot. The combination of these therapies is appealing as TILT-123 should allow pembrolizumab to be more active, while pembrolizumab should delay emergence of T-cell exhaustion. Methods: PROTA (NCT05271318) is a multicenter open-label phase 1 clinical trial initiated to investigate the safety of TILT-123 in combination with pembrolizumab, in participants bearing ovarian cancer resistant or refractory to platinum-based therapy. In a 3+3 dose-escalation design, TILT-123 was administered intravenously and intratumorally/intraperitoneally to participants with an injectable tumor or carcinomatosis. Pembrolizumab (200mg/Q3wk) was not altered. Eligibility criteria included adequate hepatic and renal functions, and WHO/ECOG performance status of 0-1. Primary endpoint was safety by day 92 evaluated based on adverse events, laboratory values, vital signs, and electrocardiogram. Secondary endpoints included anti-tumor response (RECIST1.1/iRECIST), CA125 levels, pharmacokinetics, transduction of tumors, immune response towards the virus and tumors. Data cutoff was 22Jan2024. Results: 15 patients were enrolled with a median age of 66 (36-78) years, and 14 were evaluable for treatment response with RECIST 1.1/iRECIST. 8 patients were platinum-resistant, and 7 patients were platinum-refractory. Majority of patients had WHO/ECOG performance status 1. Prevalent cancer histology was epithelial ovarian cancer, with some fallopian tube cancer and primary peritoneal cancer. Dose-limiting toxicities were not observed. Frequent grade 1-4 treatment-related AEs (>30% of patients) were anemia, nausea, lymphocyte count decreased, fever and diarrhea. Stable disease (SD) by RECIST1.1 was achieved in 64.3% of evaluable patients and CA125 decreased below baseline in 50% of the patients at least in one timepoint. Median PFS and OS in all patients were 105 and 280 days, respectively. In patients with SD on day 92 by RECIST1.1, median OS was 293 days and median PFS was 174 days. Tumor biopsies indicated the presence of virus and induction of T-cells in injected and non-injected lesions. Conclusions: Combination of TILT-123 and pembrolizumab is safe and appears to induce disease control in a difficult-to-treat patient population. Investigation of TILT-123, pembrolizumab and pegylated liposomal doxorubicin is ongoing in similar patients. Clinical trial information: NCT05271318 .

11145 Background: T-DXd is increasingly used in mBC. A T-DXd toxicity of special interest is interstitial lung disease/pneumonitis (ILD), occurring in 10-15% of pts in the DESTINY-Breast trials, and with potential lower incidence/severity in earlier line settings [Krop et al, ASCO 2023]. However, in routine practice, T-DXd may be used in pts with more heavily pre-treated mBC and previously exposed to other antibody-drug conjugates (ADC) [Premji et al SABCS 2023]. We evaluated the incidence/severity of T-DXd related ILD in mBC at Mayo Clinic. Methods: We retrospectively identified pts with mBC who received ≥1 dose of T-DXd at Mayo Clinic, Rochester, MN, between July 2022-December 2023. Demographic, mBC, and pulmonary clinical variables were abstracted from the clinical records. Data was summarized using descriptive statistics. Diagnosis of ILD was determined by treating clinicians, and severity to CTCAE V5 based on clinical documentation. Results: 77 pts with mBC received T-DXd during the study period. All were female, the majority (70, 91%) were Caucasian, and median age was 58. 53 (69%) had HER2-low mBC (37 HR+, 16 HR-) and 24 (31%) had HER2+ mBC (17 HR+, 7 HR-). 31 (40%) were active or former smokers. 11 (14%) had pulmonary comorbidities, including 6 (7.8%) with asthma, 2 (2.6%) with prior pneumonitis, and 3 (3.9%) with other pulmonary comorbidities. They had previously received a median of 1 line of endocrine therapy, and 4 lines of chemotherapy, with 20 (26%) having received another ADC. 17 (22%) developed any grade ILD [G1: 3 (4%), G2: 7 (9%), G3: 2 (3%), G4: 1 (1%), G5: 4 (5%)]. Among pts with ILD of any grade, 13 (76.5%) presented with at least 1 symptom (cough/SOB). The median number of T-DXd cycles prior to onset of any grade ILD was 6 (range 2-13), or 18 (range 6-39) weeks. 5 (29%) pts with any grade ILD received a prior ADC. Of pts with ≥G3 ILD (n=7), all were Caucasian, 2 (30%) were prior smokers, none had prior lung disease, and had received a median of 5 prior lines of chemo, with 4 (57%) having received a prior ADC. All grade ≥2 ILDs were treated with systemic steroids, and 1 also received IVIG. CT chest was used for diagnosis of all ILD. 7 pts (41%) required hospitalization, all of whom had bronchoscopy, with 5 (29%) requiring intubation. All pts with G1 ILD (3, 18%) restarted T-DXd without ILD recurrence. Conclusions: In this case series,22% of ps treated with T-DXd experienced ILD (5% G5), higher than the rate observed in the pivotal DESTINY-Breast trials, possibly due to real-world use of T-DXd in more heavily pre-treated pts and in pts with pulmonary comorbidities.