ADCs have revolutionized the therapeutic landscape in oncology. Three ADCs are US FDA-approved in MBC: sacituzumab govitecan (SG), trastuzumab deruxtecan (T-DXd) and trastuzumab emtansine (T-DM1), with many others in development. Despite these advances, ADC resistance mechanisms remain unknown. To discern biomarkers of therapeutic resistance, we evaluated genomic and transcriptomic differences in MBC before and after ADC treatment (tx). We analyzed next-generation sequencing (NGS) data from patients (pts) in the Tempus Database that received SG, T-DXd or T-DM1 pre or post biopsy (bx) (nSG=157, nT-Dxd=120, nT-DM1=84; 32.4% TNBC, 27.1% HR+/HER2-, 20.2% HER2+, 20.2% NOS). Pre-tx groups included pts with bx collected < 1 year before or < 15 days after the ADC start date. Post-tx groups included pts who had bx collected < 3 months (mo) after ADC end date. Acquired resistance (AcqRes) was analyzed by comparison of pre- and post-tx bx for pts treated with ADC for > 3 mo. Primary resistance (PrRes) was defined as pre- and post-tx bx for pts treated with ADC for < 3 mo. We evaluated resistance based on ADC mechanisms of action (MoA), including genes related to antigen expression, processing mechanism, payload effect and efflux pumps. Somatic differences were compared using Pearson’s Chi-squared test or Fisher’s exact test, as appropriate. In this exploratory analysis, all p-values are reported as uncorrected. The median log2(TPM) gene expression was compared between groups using the Wilcoxon rank sum exact test. In the AcqRes cohort, there were no significant differences in mutation frequency or expression of the target antigen for pre/post-SG or T-DXd bx, though in the pre- vs post-T-DM1 bx there was a decrease in ERBB2 alterations (69% vs 40%, p=0.047) and expression (10.9 vs. 8.4, p=0.024). A trend toward higher expression across efflux pump genes was seen in pts with AcqRes for SG (ABCB1: 2.7 vs 3.4, p=0.08; ABCC2: 2.4 vs 3.4, p=0.2) and T-DXd (ABCC1: 6.3 vs. 6.7, p=0.016; ABCB1: 2.77 vs. 3.44, p=0.4), with mixed changes in T-DM1 (ABCB1: 3.3 vs. 2.7, p=0.08; ABCC1: 6.3 vs. 6.8, p=0.05; ABCC2: 3.19 vs. 3.18, p=0.2). A trend of higher efflux pump gene expression was associated with PrRes to SG and T-DXd. Overall, there were no significant differences in expression of genes associated with ADC processing or DNA damage repair, though rare mutations in TOP1 and ATM were associated with PrRes to T-DXd. Genomic and transcriptomic analysis identified potential mechanisms of AcqRes and PrRes to SG and T-DXd, including higher drug efflux pump expression. This might be related to the ADC MoA, particularly the payload release and bystander effect. Additional research is needed to validate these novel findings and the molecular underpinnings mediating resistance to ADCs. Samer Alkassis, Marla Lipsyc-Sharf, Catherine Hegarty-Traverso, Jacob Mercer, Binyam Yilma, Stamatina Fragkogianni, Yuan Yuan, Funda Meric-Bernstam, Hope Rugo, Joyce O’Shaughnessy, Mothaffar Rimawi, Rachel Schiff, David Elashoff, John Glaspy, Martina McDermott, Amy Cummings, Calvin Chao, Aditya Bardia. Genomic and transcriptomic mediators of resistance to antibody-drug conjugates (ADCs) in metastatic breast cancer (MBC): A comprehensive multi-center study [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1711.

Supplemental Figure S1: Expression of CLDN6 by transcript in the GTEX dataset of normal tissue samples.

Supplemental Figure S2: Amino acid sequence alignment of CLDN6 (P56747) and CLDN9 (O95484) with the extracellular loops highlighted

1536 Background: The Supreme Court decision of Dobbs v. Jackson to overturn Roe v. Wade gave states authority to regulate reproductive health. This has led to concerns about access to assisted reproductive technologies used in oncofertility. The impact of this legal climate on oncofertility practices remains unknown. This study aims to understand how academic cancer centers across the United States have experienced changes in oncofertility care in a post- Roe world. Methods: In collaboration with the National Comprehensive Cancer Network (NCCN) Best Practices Committee, a survey was developed to evaluate changes in oncofertility access and utilization in the 2 years since the Dobbs decision. In July 2024, the survey was sent to NCCN Member Institutions, which represent 23 states with varied post- Roe protections for reproductive care. The survey responses were de-identified for analysis. Questions were both multiple choice and free response. Results: The survey was sent to 33 NCCN Member Institutions and yielded 24 responses (72.7%). A majority (83.3%) indicated that reproductive care was a moderate-high priority for their cancer center. Most (62.5%) reported an increase in the number of cancer patients receiving fertility preservation. According to the Center for Reproductive Rights, 13 institutions are in states that have restricted reproductive rights since the Dobbs decision. However, only 4 (16.7%) survey respondents reported that reproductive health laws had become more restrictive. The remaining reported that laws had not changed, were less restrictive, or abstained (83.3%). All 4 respondents who indicated more restrictive laws reported moderate-high priority placed on reproductive care and half reported an increase in cancer patients receiving fertility care. In the states that reported no or neutral change (n=14, 58.3%) or less restrictive laws (n=5, 20.8%), most (68.4%) reported an increase in patients receiving fertility care. One respondent who indicated more restrictive laws reported a decrease in resources for fertility care. Many centers have prioritized oncofertility by developing oncofertility programs, assigning fertility navigators, and creating electronic health record-assisted referral alerts and clinical pathways. Conclusions: Large academic NCCN Member Institutions, most in states with no change or less restrictive reproductive laws since the Dobbs decision, reported an increase in number of patients who accessed fertility care. Additional studies will clarify whether this reflects underlying trends or increased fertility care due to a fear of limited future access. Only a minority of the institutions in restrictive states responded to the survey and most who did reported similar or improved access to oncofertility care. The lack of response from restrictive states needs to be examined further as it may reflect concerns about oncofertility care in the new political landscape.

Antibody-drug conjugates (ADCs) have significantly advanced the treatment of breast cancer by integrating the specificity of monoclonal antibodies with the cytotoxic efficacy of chemotherapy, thereby enabling a targeted therapeutic approach that reduces off-target toxicity to normal tissues. Currently, 4 ADCs-sacituzumab govitecan, trastuzumab deruxtecan, trastuzumab emtansine, and the more-recent datopotamab deruxtecan-are approved for clinical application, with several others in advanced stages of development. Although these agents have demonstrated promising clinical efficacy, challenges such as ADC resistance and associated toxicities have emerged, underscoring the need for continued research. Multiple strategies are under investigation to enhance therapeutic benefit through combination regimens with other classes of medications, as are approaches to mitigate resistance mechanisms. Progress in next-generation ADCs, incorporating novel linkers and more potent cytotoxic payloads, holds promise for further improvement in clinical outcomes. Additionally, biomarker-driven strategies to identify those patients most likely to benefit from ADC therapy will support more personalized approaches to treatment. This review explores the structural and mechanistic features of ADCs in breast cancer, highlighting their therapeutic potential, and discusses ongoing clinical trials exploring new-generation ADCs and combination therapies.

Supplemental Figure S3. A, Efficacy of anti-CLDN6 mouse antibodies in CLDN6 positive OV90 ovarian cancer cell line xenografts. B, Efficacy in CLDN6 positive UMUC4 bladder cancer cell line xenografts. C, No efficacy in CLDN6 negative M202 melanoma cell line xenografts. All antibodies are dosed at 10 mg/kg QW IV in each study. Errors bars represent SEM of 8 replicate animals per group.

Supplemental Figure S5: Induction of apoptosis following 48 hr treatment of CLDN6 positive OVCA429 and CLDN6 negative M202 cells with a range of concentrations (50 μg/ml - 0.3658μg/ml) of CLDN6-23-ADC compared to control IgG ADC.

Supplemental Figure S6: In vivo efficacy of a range of doses of CLDN6-23-ADC in M202 xenograft models. ADC dosing is IV QW as indicated by the arrows

Supplemental Figure S8. A. Layout of tissues and B. Scanned whole-slide image of BN1021, normal human tissue microarray stained for CLDN6 expression showing no staining.

This phase III noninferiority (NI) trial evaluated the efficacy and safety of efbemalenograstim alfa, a novel non-PEGylated, long-acting, human granulocyte-colony stimulating factor for the management of chemotherapy-induced neutropenia (CIN), in patients with breast cancer. Adult patients with stage I-III invasive breast cancer (n = 393) undergoing docetaxel and cyclophosphamide chemotherapy treatment were randomly assigned (1:1) to receive a single fixed dose of efbemalenograstim alfa or pegfilgrastim 24 hours after each chemotherapy administration. The primary objective was to demonstrate NI or potential superiority of efbemalenograstim alfa compared with pegfilgrastim. The primary end point is the duration of severe neutropenia (DSN) in cycle 1 with a NI margin of 0.6 days. Additional end points included incidence of severe neutropenia (ISN), febrile neutropenia, infections, and incidence of treatment-emergent adverse events (TEAEs). Among patients randomly assigned to efbemalenograstim alfa (n = 197), the mean DSN in cycle one was 0.2 ± 0.51 days, compared with 0.2 ± 0.45 days for the pegfilgrastim group (n = 196), with a CI of –0.1 to 0.1 days ( P = .7) for the treatment comparison, confirming NI. There was a lower ISN in those receiving efbemalenograstim alfa at cycle 4 ( P = .050). Lower ISN in later cycles was observed in all three efbemalenograstim alfa phase III trials. Treatment with efbemalenograstim alfa was safe, with low incidences of serious AEs and TEAEs, and an overall safety profile comparable with the pegfilgrastim group. Similar to ISN, fewer grade 4 neutropenias in the efbemalenograstim alfa group were observed compared with pegfilgrastim. Results demonstrated that efbemalenograstim alfa was noninferior to current CIN therapy (pegfilgrastim) with comparable efficacy and safety profiles in cycle 1 and showed evidence of less neutropenia in later cycles.

Background: Estrogen receptor (ER) status is an important factor in risk assessment for predicting response to systemic therapy for breast cancer. Currently, the gold standard for determining ER status in metastatic breast cancer is tissue immunohistochemical (IHC) testing. However, some patients have metastases that can be difficult to visualize using standard imaging techniques, particularly in the setting of invasive lobular carcinoma (ILC). ILC tends to proliferate in a single-file, sheet-line, lowly FDG-avid pattern as opposed to the more discrete, FDG avid masses which are typical of invasive ductal carcinomas (IDC). Additional clinical challenges include evaluating the ER expression of areas that are either difficult to biopsy, or difficult to interpret IHC results such as decalcified bone tissue. 18-FES PET is a functional imaging modality that can help identify lesions expressing ER as well as inter-lesion heterogeneity in expression. In this study, we evaluated the clinical impact of 18-FES PET in diagnosing metastatic disease and the utility in patients who had discordant biopsy results. Methods: We performed a retrospective analysis of 40 patients with ER+ early breast cancer (EBC) who underwent 18-FES PET imaging between March 2022 and March 2024 at UCLA. After IRB approval, electronic medical records were reviewed and abstracted for demographics, disease, and treatment. Results: Of 40 patients included, mean age was 61 years, 21 patients had ILC, 14 had IDC, and the rest had IDC with lobular features. ER percentage was > 70% in all patients. Eight (20%) postmenopausal patients had newly diagnosed metastatic disease by 18-FES PET; 4 (50%) with ILC and 4 (50%) with IDC. Metastatic sites of ILC included bone (2), ureter (1) and chest wall (1) while those of IDC included bone (4). Two of the eight patients were asymptomatic, one had clinical recurrence in the chest wall, three had elevated tumor markers, and two had suspicious findings on prior imaging (CT, FDG PET) that prompted obtaining 18-FES PET. Plasma circulating tumor DNA (ctDNA) testing was obtained along with FES PET in two patients; one had detectable ctDNA, but the other did not. Out of six patients with metastatic site avidity on FES-PET, one did not have feasible site to biopsy, but five patients had confirmatory biopsy. Three of them had ILC with metastases to bone (2) and ureter (1), and two had metastatic IDC to bone. For the other two patients (both with ILC and osseous metastases), negative pathologic ER expression was demonstrated despite positive FES activity; one with mild and one with intense radiotracer uptake. However, endocrine therapy was started despite biopsy findings and both patients had significant responses to endocrine therapy. Of the 32 patients (80%) with negative FES PET, no evidence of metastatic disease was observed at subsequent follow-ups. Conclusion: FES PET was helpful in diagnosing metastatic disease and in assessing ER status in patients with ER+ EBC and suspected recurrence when the location of biopsy was challenging, particularly in setting of bone metastases in which IHC for ER expression is less reliable. FES positivity was associated with response to endocrine therapy when ER expression by IHC was negative in bone tissue. Prospective trials are needed to further characterize the role of FES PET in early diagnosis of metastatic disease and potential impact on survival for patients with ER+ breast cancer. Citation Format: Samer Alkassis, Mahbod Jafarvand, Jeremie Calais, David Elashoff, Marla Lipsyc-Sharf, Nicholas P. McAndrew, Kelly E. McCann, Rena D. Callahan, Ashini K. Master, Julia LaBarbera, Mina S. Sedrak, Nicolaos J. Palaskas, John A. Glaspy, Aditya Bardia. Clinical Impact of 18F–fluoro–17b–fluoroestradiol (18F–FES) PET in Patients with Estrogen Receptor-Positive Breast Cancer [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P4-11-29.

PURPOSE:Goal-concordant care is achieved when treatment is aligned with goals. This study describes patient-reported concordance between care goals and treatment intent in advanced cancer compared to other serious illnesses. METHODS:A post hoc cross-sectional analysis of baseline survey responses was conducted in adult patients enrolled in a multisite trial of advance care planning. Patients reported whether they prefer and whether they are receiving treatment that prioritizes longevity (life-extending care) versus comfort (comfort-focused care). Concordance between care preferences and perceived treatment intent in patients with advanced cancer versus other advanced illnesses was compared. Mortality rates for patients with cancer stratified by perceived care concordance are reported. RESULTS:Among 1099 patients, those with advanced cancer (n = 231) reported similar preference for comfort-focused care (49% vs 48%, p = .47) and had similar 24-month mortality (16% v 13%, p = .25) as patients with other serious illnesses (n = 868). Among patients preferring comfort-focused care, patients with cancer (n = 113) were more likely than patients with other illnesses (n = 413) to report receiving (discordant) life-extending care (37% vs. 19%, p < .001). Among patients with cancer preferring comfort-focused care, there was no statistically significant difference in 24-month mortality between those who reported receiving (discordant) life-extending versus (concordant) comfort-focused care (24% v 15%, p = .31). CONCLUSION:Compared to patients with other serious illnesses, a relatively large portion of patients with advanced cancer reported that their treatment discordantly focused on longevity over comfort despite their goal to prioritize comfort over longevity.

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1041 Background: The three antibody-drug conjugates (ADCs) — sacituzumab govitecan (SG), trastuzumab deruxtecan (T-DXd), and trastuzumab emtansine (T-DM1) — that are FDA-approved for treatment of metastatic breast cancer (MBC) have markedly improved patient outcomes. However, most patients with MBC treated with ADCs ultimately have disease progression via either primary or acquired ADC resistance. Here, we characterized the transcriptomic profile of drug efflux genes in MBC prior to ADC treatment (tx) to elucidate biomarkers of response and resistance to SG, T-Dxd, and T-DM1. Methods: We analyzed the transcriptomic tumor profile of six drug efflux pump genes ( ABCB1 , ABCC1 - 4 , ABCG2 ) generated from pre-tx biopsies collected from patients with MBC (N = 453; 36% TNBC, 26% HR+/HER2-, 20.5% HER2+, 19% NOS) 1 year prior to or up to 15 days post-tx with SG (n = 204), T-DXd (n = 178), or T-DM1 (n = 71). RNA-sequencing data were generated and processed with the Tempus xR assay. The correlation between duration of treatment (DoT) and gene expression was tested for all genes of interest using Pearson’s correlation coefficient. Cox proportional hazards models with risk set adjustment were used to test for associations between pre-tx gene expression and overall survival (OS), where gene expression was modeled as a continuous linear predictor. The proportional hazards assumption for OS was tested, and Cox modeling results were omitted when evidence of non-proportional hazards was detected. Given the exploratory nature of the analyses, all p-values are uncorrected and nominal statistical significance was set at p < 0.05. Results: This diverse cohort had a median age of 52 and a range of races (55% White, 14% Black, 7.1% Other, 24% Unknown). Median DoT across all patients was 130 days. Higher expression of drug efflux pump genes was associated with shorter DoT for T-DXd ( ABCB1 : -0.290, p = 0.017; ABCC1 : -0.274, p = 0.025). Additionally, higher expression of ABCB1 was associated with worse OS for T-DXd (HR: 1.30, 95% CI: 1.10 - 1.53, p = 0.002). In the SG cohort, no significant associations between efflux pump expression and DoT were found, but higher pre-tx ABCC1 and ABCC4 gene expression was associated with worse OS (HR: 1.34, 95% CI: 1.02-1.75, p = 0.034; HR:1.19, 95%CI: 1.00-1.41, p = 0.042). In the T-DM1 cohort, no significant associations were found between efflux pump gene expression and DoT or OS. Conclusions: Multi-modal analysis identified drug efflux pump gene expression as a potential biomarker of resistance, primarily to T-DXd. These findings should be further validated, and combinatorial clinical trial strategies may be explored.

Supplemental Figure S4: Binding of CLDN6-23-ADC (5 μg/ml) in artificial cell lines overexpressing CLDN3, CLDN4, CLDN6 or CLDN9 by flow cytometry.

Supplemental Figure S7. CLDN6 expression in ovarian and endometrial cancer tissue samples.

Supplemental Figure S1. Mean pharmacokinetic profiles for single agent talazoparib (BMN673 in phase I clinical trial PRP-001) versus combination therapy.

12034 Background: Optimal care for patients with advanced cancer requires understanding patients’ goals of care and communicating treatment intent. We explored goals of care, perceived treatment intent, and mortality in patients with advanced cancer compared to other advanced conditions. Methods: We performed a cross-sectional analysis of survey data from a multi-site trial of advance care planning in patients with advanced cancer, end-stage liver disease, end-stage renal disease, chronic obstructive pulmonary disease, congestive heart failure, and advanced age (> 75 years). Patients described their physical health and quality of life. They also reported whether their preferred goals of care and their current treatment focus “on extending your life as much as possible, even if it means having more pain and discomfort,” or “relieving your pain and discomfort as much as possible, even if that means not living as long,” or “not sure.” We compared patients’ preference for life-extending care (LEC) versus comfort-focused care (CFC) with their perception of current treatment intent as LEC versus CFC. We evaluated the relationship between patients’ goals of care and perceived treatment intent with 24-month mortality in advanced cancer versus other advanced conditions. We used the chi-square test for statistical analyses. Results: Among 1099 patients, those with advanced cancer (n = 231) were more likely to report very good or excellent physical health (33% v 22%, p < .001) and very good or excellent quality of life (58% v 41%, p < .001) compared to other advanced conditions (n = 868). Patients with cancer had a similar 24-month mortality (16% v 13%, p = .25) and preferred LEC as often as patients with other conditions (25% v 23%, p = .58). Patients with cancer were more likely to report receiving LEC than those with other conditions (51% v 35%, p < .001). Patients with cancer were more likely to have a discordant goal of CFC while describing their treatment as LEC compared to other conditions (36% v 25%, p = .04). Among 113 patients with cancer who preferred CFC, there was no difference in 24-month mortality between those who reported receiving LEC versus CFC (24% v 15%, p = .31). Conclusions: Compared to patients with other advanced conditions, patients with advanced cancer reported better physical health and quality of life, but had similar mortality and goals of care. However, patients with advanced cancer were more likely to report receiving LEC and also more likely to have a discordant goal of CFC while receiving LEC. Among patients with advanced cancer preferring CFC, those who reported receiving LEC instead of CFC did not have improved survival. Good physical health and quality of life may prompt more aggressive care in patients with advanced cancer compared to other advanced conditions. These findings highlight the importance of oncologists explicitly eliciting patients’ goals of care and ensuring that patients understand the intent of treatment.

Abstract Delta like non-canonical Notch ligand 1 (DLK1) is a transmembrane protein that belongs to the NOTCH non-canonical ligand family. It has been implicated in adipogenesis, the regulation of stem cell pools, tissue differentiation during development, cancer differentiation, and cancer stem-like cell maintenance. DLK1 is highly expressed in adrenocortical, uterine, and testicular cancers as well as in a large portion of pancreatic, sarcoma, liver and squamous lung cancer patient samples. In contrast there is limited normal tissue expression beyond the normal adrenal gland, pituitary, and ovarian tissue samples. This expression profile makes DLK1 an attractive target for development of a therapeutic antibody-drug conjugate (ADC). This study describes the generation and preclinical characterization of TORL-4-500, an ADC consisting of a humanized anti-DLK1 monoclonal antibody coupled to monomethyl auristatin E (MMAE) via a cleavable linker. TORL-4-500 showed strong binding to DLK1 by flow cytometry in DLK1 native and artificial overexpressing cell lines. In contrast no binding was observed in DLK1 non-expressing cell lines. TORL-4-500 exhibits nanomolar binding affinity for both human and cynomolgus monkey DLK1 and is rapidly internalized in DLK1 expressing cells. TORL-4-500 exhibited selective efficacy in cell line xenograft models of DLK1 positive human cancers. Treatment with TORL-4-500 induced significant regressions in four DLK1 expressing human cancer cell line xenograft studies encompassing liver, small cell lung cancer (SCLC) and sarcoma cancer. Furthermore, anti-tumor responses were sustained in each of the DLK1 expressing models for several weeks post-final dose. In the case of the two SCLC cell lines, complete regressions of xenograft tumors were measured out past 100 days in the TORL-4-500 treated animals. No significant impact on xenograft tumor growth was observed in either a DLK1 non-expressing colon cell line xenograft or in a DLK1 non-expressing melanoma xenograft study. Each of the doses tested in this study was well tolerated in mice with no dose-limiting toxicity observed. The nonclinical pharmacokinetics and toxicokinetics of TORL-4-500 were characterized in mice and monkeys and results support dosing in humans. In summary, TORL-4-500 is a novel therapeutic for DLK1 positive cancers and on the basis of these promising preclinical efficacy results, a first in human trial to evaluate safety, tolerability, pharmacokinetics, and antitumor activity of TORL-4-500 has been launched in patients with advanced cancer and is currently ongoing (NCT06005740). Citation Format: Martina S. McDermott, Neil A. O'Brien, Ming Lu, Jun Zhang, KeWei Gong, Benjamin Hoffstrom, Tong Luo, Min Liang, Weiping Jia, Kevin Chau, Leonard Presta, John Glaspy, Dennis J. Slamon. Therapeutic potential of TORL-4-500, an antibody-drug conjugate directed against delta like non-canonical notch ligand 1 (DLK1) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1896.