To fully harness precision medicine and transform cancer care for the better will require a strategic shift to highly personalized interventions that embrace innovation and adaptability.

Triple-negative breast cancer (TNBC) accounts for 15

PURPOSE:Antibody-drug conjugates (ADC) harboring topoisomerase I (TOP1) inhibitor payloads have improved survival for patients with metastatic breast cancer. However, knowledge of ADC resistance mechanisms and potential impact on the sequential use of ADCs is limited. In this study, we report the incidence and characterization of TOP1 mutations arising in the setting of ADC resistance in metastatic breast cancer. EXPERIMENTAL DESIGN:Patients with metastatic breast cancer treated with ADCs with available posttreatment plasma-based genotyping were included. TOP1 mutation incidence, mutant allele frequency, and functional characterization were assessed, and incidence was compared with that in patients with metastatic breast cancer not receiving ADC treatment and in The Cancer Genome Atlas. RESULTS:Plasma-based genotyping identified distinct TOP1 mutations (S57C, R364H, W401C, and G359E) in 12.9% of patients (4/31) at the time of disease progression on ADC, compared with 0.7% (3/420) in non-ADC-treated patients with metastatic breast cancer and 0.5% in The Cancer Genome Atlas. The appearance of mutations was associated with clinical cross-resistance, as median duration on the first ADC was 455 versus 52 days for the second ADC. The functional characterization of three novel TOP1-mutant proteins demonstrated that all exhibited reduced enzymatic activity, attenuated covalent DNA binding, and resistance to TOP1 inhibitor ADC payloads SN38 and deruxtecan. CONCLUSIONS:We describe the recurrent emergence of functionally altered, resistance-associated TOP1 mutations in vivo under selective pressure from ADCs and the potential impact on mediating cross-resistance to sequential ADCs. TOP1 mutation may represent a biomarker of resistance in this setting, and additional work is needed to optimize biomarkers and ADC payload design to improve outcomes for the sequential use of ADCs. See related commentary by Gwin and Hurvitz, p. 1824.

Introduction: Triple negative breast cancer (TNBC) accounts for 15-20% of invasive breast cancer cases. While most patients with early-stage TNBC are treated with curative intent, approximately 50% of patients experience recurrence in the first 5 years. There are no current recommended assays for early signs of recurrent disease, though novel diagnostic tests are currently in development. Circulating tumor DNA (ctDNA) assessment has been investigated as a way to evaluate response to treatment, determine prognosis, and detect minimal residual disease (MRD) after curative-intent treatment. However, while ctDNA detection is known to be strongly associated with poor prognosis, the incorporation of ctDNA evaluation in early breast cancer care has been challenging due to lack of evidence of clinical actionability. No data are currently available regarding the predictive utility of ctDNA detection on local recurrence. Herein, we present a case of a patient with early-stage TNBC who had positive ctDNA testing in the adjuvant setting that identified local recurrence. Case: A 53-year-old thin woman palpated a mass in the right breast. A mammogram and ultrasound revealed an 8 mm lesion, and biopsy showed a grade 3 TNBC. CT scan of the chest, abdomen, and pelvis (CT CAP) revealed no evidence of metastatic disease. MRI of the right breast showed conglomerate area spanning up to 5.2 cm, with possible nipple involvement. She received 4 cycles of neoadjuvant carboplatin and Taxol, due to patient preference, followed by repeat MRI showing the same size of the mass with decreased enhancement. She then had bilateral skin-sparing mastectomy, including nipple removal, with implant reconstruction. Pathology revealed an 8 mm TNBC and involvement of one lymph node. She then completed 4 cycles of adjuvant cyclophosphamide and doxorubicin. One month after completing adjuvant chemotherapy, she had positive plasma ctDNA testing. A subsequent PET/ CT scan showed no evidence of metastatic disease. She had three additional plasma ctDNA tests at 2, 3, and 6 months after initial testing which were positive with increasing variant allele frequency. After additional PET/CT and CT CAP at 3 and 6 months from initial PET/CT scan respectively, an MRI breast was performed showing multiple masses in the skin flap of the reconstructed right breast. She underwent complete right mastectomy and implant removal which confirmed multifocal TNBC in the skin flap. She subsequently completed a course of radiation to the right breast, with negative plasma ctDNA afterwards. Discussion: The detection of ctDNA post-treatment in early stage TNBC is a strong predictor of recurrence risk. During neoadjuvant chemotherapy, improvement in ctDNA detection has been associated with improved distant recurrence-free survival, even in patients with extensive residual cancer. The c-TRAK TN prospective trial in adjuvant TNBC found that ctDNA detection was associated with poor prognosis, with median time from ctDNA detection to recurrence of 4.1 months. However, to our knowledge, MRD testing has not yet been systematically utilized for detecting local recurrence. Fortunately, our patient did not have evidence of distant metastatic disease and dedicated breast imaging was obtained to identify local recurrence. Conclusion: This case highlights the potential of MRD assessment in the adjuvant setting in TNBC to identify local recurrence and, in cases of adjuvant ctDNA detection, guide thorough evaluation with both systemic scans and dedicated primary breast imaging. As demonstrated here, this may impact treatment and surveillance. Further prospective trials are needed to determine the clinical utility of MRD assessment for prognosis and early detection of local and distant recurrences in this setting. Citation Format: Samer Alkassis, Marla Lipsyc-Sharf, Shiliang Zhang, Caterina Gianni, Arielle Medford, Aditya Bardia, Shahryar Ashouri, Nimmi Kapoor. Circulating Tumor DNA (ctDNA) Detection of Local Recurrence in a Patient with Early-Stage Triple Negative 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 P1-12-25.

Importance Black patients with metastatic breast cancer (mBC) have higher mortality rates than White patients despite advances in treatment. Objectives To examine whether Black patients with metastatic breast cancer have different genomic profiles compared with White patients and whether there are inequities in targeted treatment use between these groups. Design, Setting, and Participants This retrospective, population-based cohort study assessed adult patients with mBC who underwent genomic profiling at academic institutions in the US between January 1, 2015, and December 31, 2023. Data analysis was performed between July 2023 and July 2024. A validation cohort was also included. Exposures Targeted treatment use. Main Outcomes and Measures The main outcomes were differences in circulating tumor DNA profiles and use of phosphoinositide 3-kinase (PI3K), mammalian target of rapamycin (mTOR), and cyclin-dependent kinase 4/6 (CDK4/6) inhibitors between Black and White patients with metastatic breast cancer. Results The study sample included 1327 women with mBC (mean [SD] age, 58.0 [12.8] years; 140 Black and 1057 White). Black patients had a significantly higher rate of GATA3 single-nucleotide variants (odds ratio, 2.31; 95% CI, 1.17-4.54; P = .02) and CCND2 copy number variants (odds ratio, 4.63; 95% CI, 1.79-11.97; P = .002) on multivariate analysis. These differences were validated in a population-based evidence cohort of 27 224 patients. Black patients with PIK3CA single-nucleotide variants were significantly less likely to receive PI3K inhibitors than White patients (1 of 17 [5.9%] vs 45 of 156 [28.8%]; P = .04), whereas there was no difference in use of CDK4/6 and mTOR inhibitors, which do not require a targetable alteration. Black patients had a shorter overall survival from the time of circulating tumor DNA testing compared with White patients. Conclusions and Relevance This cohort study of patients with mBC found somatic differences, shorter overall survival, and targeted treatment disparities in PI3K inhibitor use in Black compared with White patients despite equal incidence of PIK3CA alterations. Researchers should consider these differences when designing future research and interventions to address the striking and persistent outcomes gap between Black and White patients with mBC.

Supplemental Figure 2. Interaction between HER2-low status and oncogenic pathways on overall survival.

Background: Detecting minimal residual disease (MRD) in the adjuvant setting can help identify patients (pts) with early breast cancer (EBC) at a higher risk of recurrence. Circulating tumor DNA (ctDNA)-based MRD detection is strongly associated with recurrence. Typically, MRD tests require prior knowledge of the tissue genetic alterations. However, obtaining tissue samples can be challenging. This multi-institution retrospective analysis investigates the impact of a plasma-only genetic and epigenetic ctDNA testing on the care of pts with EBC. Methods: Our retrospective study included 73 pts with stage I-III EBC who had MRD testing after curative-intent treatment between 09/2022 - 06/2024 at Weill Cornell Medicine (New York) and University of California Los Angeles (Los Angeles, CA). ctDNA evaluation was performed using the Guardant Reveal® (GR) tissue-free assay (Guardant Health, Redwood City, CA), a next-generation sequencing panel. ctDNA presence was determined by a custom bioinformatics classifier identifying tumor-derived variants and epigenetic methylation profiles. All tests were ordered in the real-world clinical setting, and data were gathered through a retrospective review of electronic medical records with appropriate IRB approval. A descriptive analysis was performed. Results: By 06/2024, 73 EBC pts had plasma-only ctDNA testing for MRD monitoring post-surgery. At diagnosis 22% were stage I, 53% were stage II and 21% of pts were stage III. Tumor subtypes included HR+/HER2- (44; 60%), HR-/HER2- (11; 15%), HER2+ (17; 24%), 1 case was unknown. Fifty-five pts (75%) had received neo/adjuvant chemotherapy. Among these, 10 pts had pathological complete response. At the time of the analysis, 49 pts were still receiving adjuvant therapy. The median recurrence free survival (RFS) since surgery was 22.5 months (interquartile range [IQR] 13.3-36.8). With a median follow-up of 2.2 years (IQR, 1.6-3.2) since diagnosis, 6 pts had a distant recurrence. Among them, 3 were HR+/HER2-, 2 were HR-/HER2- and 1 was HER2+ EBC. From 09/2022 to 06/2024, 124 GR tests were performed, with a median of one test (IQR 1-2) per patient. Thirty-nine pts (53%) had one GR test, while 47% had ≥2 tests. The median time between subsequent tests was 119 days (IQR, 93-200). 16 tests (13%) were positive, with 10 pts having at least one GR+ result over a median time of 7 months (IQR 2.7-12.1) from first testing to last follow-up. The first GR test was ordered after a median time from surgery of 13.8 months (IQR 7.0-50.5). Five out of 6 relapses were preceded by MRD+ testing, except for one patient that developed a solitary brain lesion. Pts with ctDNA+ had a primary EBC stage II or III (7 and 2, respectively, 1 unknown). In 7/10 pts, after a first ctDNA+ result, an imaging scan was planned with detection of distant asymptomatic disease recurrence in 3 cases. In 3/10 patients, a follow-up test after 3 months led to further scans and detection of 2 additional relapses. At the time of the analysis 5 patients MRD+ were free of detectable metastases (clinically or radiologically). Overall, ctDNA detection was the only prognostic factor for RFS (p <.001). These data estimate a negative predictive value of 98.4% and a positive predictive value of 50% for this GR test, with a related sensitivity and specificity of 83.3% and 92.5%, respectively. Conclusions: Our analysis suggests the utility of a tissue-free ctDNA assay in diagnosing minimal residual disease. This could have clinical implications by enabling earlier diagnosis and possibly intervention to delay or prevent metastatic recurrence. We confirm prior studies showing high specificity for recurrence detection with multi-omic plasma-only MRD testing. Sensitivity limitations may be due to the small sample size and short follow-up. These findings warrant further study in larger cohorts and future clinical trials. Citation Format: Caterina Gianni, Eleonora Nicolo’, Marla Lipsyc-Sharf, Brian DiCarlo, Eleni Andreopoulou, Ashley Schreier, Jeannine Donahue, Letizia Pontolillo, Laura S. Muñoz-Arcos, Mara Serena Serafini, Elisabetta Molteni, Nadia Bayou, Kelly Eng, Amanda Kaylan Strickland, Marko Velimirovic, Lorenzo Gerratana, Andrew A. Davis, Arielle Medford, Olivier Elemento, Aditya Bardia, Ugo De Giorgi, Carolina Reduzzi, Massimo Cristofanilli. Clinical impact of MRD detection via ctDNA tumor-agnostic assay in early-stage breast cancer patients: a real-world experience [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 P4-01-19.

Optimal timing and dosing of adjuvant cyclin-dependent kinase (CDK) 4/6 inhibitor in early breast cancer is controversial. This prospective phase II clinical trial investigated tolerability and safety of two ribociclib dosing schedules. Patients with stage I–III hormone receptor-positive (HR+)/HER2– breast cancer on adjuvant endocrine therapy (ET) were randomized to two ribociclib dosing schedules: 400 mg continuous vs 600 mg intermittent, with initiation in early (prior ET < 2 years) vs delayed (prior ET ≥ 2 years) setting. Primary objective was to evaluate safety and tolerability of continuous vs intermittent schedule. Primary endpoint was proportion of patients who discontinued ribociclib before completion of all 12 cycles (measured at 12 months). Recurrence free survival (RFS) and circulating tumor DNA (ctDNA) detection were also evaluated. 81 patients were enrolled. Only six serious adverse events occurred, with no significant difference between treatment arms and no subject deaths. Twenty-five patients (31%) discontinued ribociclib before completion of 12 months, with no significant difference between treatment arms. Ribociclib discontinuation was higher in early vs delayed initiation (36% vs 21%). At median follow-up of 20 months, two patients in the intermittent arm (600 mg; Arm 2) experienced disease recurrence (2-year RFS 97%, 95%CI 88–99%), vs none in the continuous arm (400 mg; Arm 1) (2-year RFS 100%). ctDNA was only identified in the two subjects with recurrent disease at median of 7.5 months prior to radiological recurrence. Ribociclib is a safe and well-tolerated adjunct to adjuvant ET in early-stage breast cancer. Delayed initiation of ribociclib at 400 mg continuous dosing was feasible, better tolerated and associated with promising outcomes. ctDNA detection preceded clinical evidence of recurrence and may be considered as a surveillance tool in breast cancer.

Background: PI3K/AKT/PTEN signaling pathway alterations are present in approximately half of patients with hormone receptor positive (HR+), HER2 negative breast cancer (BC). The PI3K alpha-selective inhibitor (PI3Ki) alpelisib in combination with fulvestrant is approved for patients with PIK3CA-mutated HR+ advanced breast cancer, and the AKT inhibitor (AKTi) capivasertib in combination with fulvestrant is approved for patients with advanced/metastatic HR+/HER2- breast cancer (MBC) with alterations in PIK3CA, AKT1, or PTEN. Patients with PIK3CA mutant MBC are now candidates for both therapies, but there is minimal data to guide sequencing of these agents. Here we report our single-institution experience. Methods: All patients with HR+/HER2- MBC treated at the Mass General Cancer Center who received AKTi (either capivasertib or an experimental AKTi in a clinical trial/TAKTIC) were included. Patients were divided into three categories: those who received AKTi only (control), those who received PI3Ki first and then AKTi (PI3K > AKT) and those who received AKTi and later PI3Ki (AKT > PI3K). Patients could have intervening therapies between sequential PI3Ki/AKTi. Clinical information was abstracted by chart review. Progression-free survival (PFS) was evaluated as time from start of treatment to disease progression or death from any cause. Time on treatment was defined as the start of treatment to discontinuation. 95% confidence intervals were calculated for all mean and median estimates. Results: 56 patients were identified who received AKTi with/without PI3Ki for advanced/metastatic HR+/HER2- breast cancer from August 2019-June 2024. The median age at start of first PI3Ki/AKTi was 61.5 years. 42 patients (76.4%) experienced disease progression on their first AKTi/PI3Ki. 49 patients (89.1%) had known alterations in PIK3CA/AKT/PTEN pathway, and six patients (10.9%) had no known alterations and were treated on trial. Mutations of PIK3CA were present in 30 patients (54.5%), AKT in 8 patients (14.5%), and PTEN in 8 patients (14.5%). 39 patients (69.6%) received AKTi only, 10 patients (17.9%) were in the PI3K > AKT group, and 4 (7.1%) received AKT > PI3K, and the remaining 3 patients (5.4%) received 3 or more AKTi/PI3Ki. Overall median time on first treatment was 5.8 months (95% CI 4.6-11.0 months) and median time on second treatment was 5.4 months (95% CI 2.0-NR). Median progression-free survival (PFS) for patients who received AKTi alone was 5.5 months (95% CI 3.7-9.2 months). For patients who received PI3Ki before AKTi, the median PFS on second treatment (AKTi) was 4.6 months (95% CI 2.0-NR). An analysis of genomic predictors of response is in process and a multi-institution follow-up analysis is planned. Further updates to be presented at the meeting. Conclusions: In this single-institution study, we report median treatment times for patients treated with PI3Ki prior to AKTi and AKTi alone. Patients may still benefit from receiving second-line AKTi after PI3Ki. Biomarker data is needed to drive personalized selection of sequential use of agents targeting the same pathway. Citation Format: Rachel Abelman, Amanda Jung, Laura M. Spring, Geoffrey Fell, Neelima Vidula, Arielle Medford, Janice Kim, Steven J. Isakoff, Beverly Moy, Leif W. Ellisen, Dejan Juric, Aditya Bardia, Andreas Varkaris, Seth A. Wander. Sequencing PIK3CA and AKT Inhibitors in Hormone Receptor-Positive, HER2-Negative Metastatic Breast Cancer: A Retrospective Analysis [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 P3-10-10.

Background: In the NATALEE trial, RIB + NSAI has demonstrated invasive disease-free survival and DDFS benefits in patients with stage II and III HR+/HER2− EBC. In an analysis with all patients off RIB, RIB + NSAI reduced the risk of distant disease recurrence vs NSAI alone. Understanding the impact of adjuvant treatments on distant disease recurrences across patient subgroups is critical for clinical decision-making. We present DDFS data from the 4-y landmark analysis of NATALEE across clinically relevant subgroups. Methods: In NATALEE, patients were randomized 1:1 to receive either RIB (400 mg/d, 3 wk on/1 wk off for 3 y) + NSAI (anastrozole 1 mg/d or letrozole 2.5 mg/d for 5 y) or NSAI alone, with men and premenopausal women also receiving goserelin. NATALEE included patients with anatomic stage IIA (either node-negative [N0] with additional risk factors or N1 [1-3 axillary lymph nodes]), IIB, or III disease per AJCC (8th edition). DDFS was a secondary endpoint and was defined as the time from randomization to the first event of distant recurrence, second primary non-breast invasive cancer (except for basal and squamous cell skin carcinomas), or death from any cause. DDFS was assessed across patient subgroups, including anatomic stage and nodal status, using Kaplan–Meier analysis and a Cox proportional hazards model. Results: At the data cutoff of April 29, 2024, with a median duration of follow-up for DDFS of 44.2 months and all patients off RIB, RIB + NSAI demonstrated both a DDFS benefit (HR, 0.715 [95% CI, 0.604-0.847]; nominal P value <.0001) and a distant recurrence-free survival benefit (HR, 0.705 [95% CI, 0.589-0.844]; nominal P value <.0001) in the intent-to-treat population. The DDFS benefit was consistent regardless of anatomic stage (stage IIA [n=1001]: HR, 0.396 [95% CI, 0.218-0.720]; stage IIB [n=1045]: HR, 0.806 [95% CI, 0.524-1.238]; stage IIIA [n=1832]: HR, 0.697 [95% CI, 0.524-0.926]; stage IIIB [n=317]: HR, 0.569 [95% CI, 0.326-0.994]; stage IIIC [n=890]: HR, 0.878 [95% CI, 0.649-1.188]). The absolute DDFS benefit with RIB + NSAI vs NSAI alone increased from 3 y to 4 y for all stage subgroups (stage IIA, 2.3% to 5.1%; IIB, 0.8% to 2.6%; IIIA, 3.1% to 4.6%; IIIB, 6.8% to 11.4%; IIIC, 2.4% to 4.5%). Similarly, a consistent improvement in DDFS was observed regardless of nodal status (N0 [n=613]: HR, 0.696 [95% CI, 0.403-1.204]; node-positive [N+] [n=4480]: HR, 0.726 [95% CI, 0.608-0.867]), and the absolute DDFS benefit increased from 3 y to 4 y across nodal subgroups (N0, 2.7% to 4.2%; N+, 2.4% to 4.6%). The DDFS benefit with RIB + NSAI vs NSAI alone, with increasing benefit up to 4 y, was consistent across other clinically relevant subgroups, including those based on menopausal status and Ki-67 status. Conclusions: With all patients off RIB, RIB + NSAI consistently reduced the risk of distant recurrence across clinically relevant subgroups, including in patients with N0 disease. The DDFS benefit was sustained after the 3-y RIB treatment duration, with increasing absolute benefit up to 4 y. These findings further support adding RIB to adjuvant NSAI in a broad population of patients with HR+/HER2− EBC. Citation Format: Sara Hurvitz, Michal Jarzab, Montserrat Munoz Mateu, Erin Cobain, Jin Zhang, Arielle Medford, Alistair Ring, Priyanka Sharma, Christian Schem, Ionut Temciuc, Zheng Li, Murat Akdere, Juan Pablo Zarate, Denise A. Yardley. Distant disease-free survival (DDFS) across key subgroups from the phase 3 NATALEE trial of ribociclib (RIB) plus a nonsteroidal aromatase inhibitor (NSAI) in patients with HR+/HER2− early breast cancer (EBC) [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 P4-09-22.

HER2 subtype and genomic alterations for all patients

The study explored endocrine resistance by leveraging machine learning to establish the prognostic stratification of predicted Circulating tumor cells (CTCs), assessing its integration with circulating tumor DNA (ctDNA) features and contextually evaluate the potential of CTCs-based transcriptomics. 1,118 patients with a diagnosis of luminal-like Metastatic Breast Cancer (MBC) were characterized for ctDNA through NGS before treatment start, predicted CTCs were computed through a K nearest neighbor algorithm. Differences across subgroups were analyzed through chi square or Fisher’s exact test according to sample size and corrected for False Discovery Rate. Differences in survival were tested by log-rank test and uni- and multivariable Cox regression. CTCs transcriptomics was performed through RNAseq after sorting with DEPArray NxT. Univariable and multivariable analysis adjusted for ctDNA alterations revealed a significant impact of CTCs predictive stratification on both progression-free survival (PFS) and overall survival (OS). Alterations in RTK and ER pathways were significantly correlated with predicted-Stage IVaggressive. The combined impact of CTCs stratification and RTK/ER pathway alterations influenced patient outcomes, with predicted-Stage IVaggressive having a negative impact on PFS regardless of the mutational status. The pilot exploratory CTCs transcriptomics analysis showed transcriptional changes linked to cell proliferation such as under expression of MALAT1 and overexpression of GREM1, GPR85 and OCM. Our data underline the potential of an integration between ctDNA and CTCs, both through quantification and transcriptomic analysis, for a deeper understanding of tumor biology and treatment response in HR-positive, HER2-negative MBC.

Triple-negative breast cancer (TNBC) comprises 10–15

IntroductionEstrogen receptor positive (ER+) breast cancer is the most common breast cancer subtype, and therapeutic management relies primarily on inhibiting ER signaling. In the metastatic setting, ER signaling is typically targeted by selective estrogen receptor degraders (SERDs) or aromatase inhibitors (AIs), the latter of which prevent estrogen production. Activating ESR1 mutations are among the most common emergent breast cancer mutations and confer resistance to AIs.Areas CoveredUntil 2023, fulvestrant was the only approved SERD; fulvestrant is administered intramuscularly, and in some cases may also have limited efficacy in the setting of certain ESR1 mutations. In 2023, the first oral SERD, elacestrant, was approved for use in ESR1-mutated, ER+/HER2- advanced breast cancer and represents a new class of therapeutic options. While the initial approval was as monotherapy, ongoing studies are evaluating elacestrant (as well as other oral SERDs) in combination with other therapies including CDK4/6 inhibitors and PI3K inhibitors, which parallels the current combination uses of fulvestrant.Expert OpinionElacestrant's recent approval sheds light on the use of biomarkers such as ESR1 to gauge a tumor's endocrine sensitivity. Ongoing therapeutic and correlative biomarker studies will offer new insight and expanding treatment options for patients with advanced breast cancer.

The time required to conduct clinical trials limits the rate at which we can evaluate and deliver new treatment options to patients with cancer. New approaches to increase trial efficiency while maintaining rigor would benefit patients, especially in oncology, in which adjuvant trials hold promise for intercepting metastatic disease, but typically require large numbers of patients and many years to complete. We envision a standing platform - an infrastructure to support ongoing identification and trial enrolment of patients with cancer with early molecular evidence of disease (MED) after curative-intent therapy for early-stage cancer, based on the presence of circulating tumour DNA. MED strongly predicts subsequent recurrence, with the vast majority of patients showing radiographic evidence of disease within 18 months. Such a platform would allow efficient testing of many treatments, from small exploratory studies to larger pivotal trials. Trials enrolling patients with MED but without radiographic evidence of disease have the potential to advance drug evaluation because they can be smaller (given high probability of recurrence) and faster (given short time to recurrence) than conventional adjuvant trials. Circulating tumour DNA may also provide a valuable early biomarker of treatment effect, which would allow small signal-finding trials. In this Perspective, we discuss how such a platform could be established.

Purpose/Objective(s) Studies examining ctDNA, a blood-based marker of tumor burden, have largely focused on its use as a prognostic and predictive biomarker. While patient reported outcomes are most often associated with treatment-related toxicities, tumor burden may also contribute to these symptoms, including pain. Approximately one third of patients (pts) who receive radiation (RT) to painful bone metastases (mets) do not experience pain relief, but few studies have identified predictors of response to palliative RT. We hypothesize that ctDNA will predict persistent pain in pts with metastatic breast cancer and could be used to predict response to palliative RT. Materials/Methods Plasma was collected before and 3 months (mos) after RT in pts with stage IV HR+, HER2- breast cancer in the context of a multi-institutional Phase II study of palbociclib, hormone therapy (HT), and palliative RT [30 Gy in 10 fractions (fxns) or 20 Gy in 5 fxns] for bone mets. ctDNA was detected using Snapshot, an anchored multiplex assay that detects 28 clinically significant cancer genes. Pts were eligible if treated with HT and palbociclib for at least one cycle and had either painful bone mets or asymptomatic bone mets at risk for a clinical event. Response, the primary endpoint, was prespecified as either a 2-point decrease in maximum pain score on the Brief Pain Inventory in the irradiated site(s) 3 mos post RT or prevention of a clinical event (e.g., cord compression) among pts with asymptomatic disease. The relationship between ctDNA and response to palliative RT was examined. Results 35 pts completed baseline and 3-month post RT assessments. 27 were treated for painful bone mets. With concurrent palbociclib, 61% and 37% of pts received aromatase inhibitors and fulvestrant, respectively. Median age was 60 years (31-83). 25% of pts were non-Hispanic Black. 72% of pts received 5 fxn RT and 69% received RT to one bone region. 29 pts (83%) were responders [95% CI: 66%-93%, p=.003]. Median progression free survival (PFS) was 30.4 mos (1.3 - 44.0). Three-year PFS and overall survival were 45.6% and 67.2%, respectively. 41% and 39% of pts had detectable ctDNA prior to and 3 mos post RT, respectively. 39% of pts had stable or increased ctDNA proportions 3 mos post RT relative to baseline. Six pts, all of whom presented with painful bone mets, did not respond to palliative RT. 100% of pts (n=15) without detectable ctDNA before and after RT responded to palliative RT, while 39% of pts with stable or increased ctDNA 3 mos post RT had no pain relief to RT (p=0.005). Four pts with ctDNA at baseline had no detectable ctDNA after RT, and all responded to RT. Conclusion Our multi-institutional Phase II trial is the first to demonstrate high rates of response to concurrent RT with palbociclib and HT and to evaluate ctDNA as a biomarker of pain and response to palliative RT in pts with metastatic breast cancer. Our novel findings indicate that ctDNA detection correlates with persistent pain, a patient reported symptom, and predicts the efficacy of palliative RT.

570 Background: Antibody-drug conjugates (ADC) selectively deliver chemotherapy to tumor cells expressing a target antigen. Currently approved ADCs in breast cancer target HER2 or Trop-2. Enfortumab vedotin, an ADC targeting Nectin-4, a cell adhesion molecule, is approved in advanced/metastatic urothelial cancer. This study evaluated the expression landscape of Nectin-4 in breast cancer and its association with clinical outcomes. Methods: Processed mRNAseq data was assessed for expression of the gene encoding Nectin-4 in primary breast cancer tissue and adjacent normal breast tissue from The Cancer Genome Atlas (TCGA), and normal tissue from the Genotype-Tissue Expression (GTEx) Project. Low gene expression was determined as <100 transcripts per million (TPM). TCGA samples had been annotated for putative copy-number alterations using GISTIC 2.0; genes with a score of 2 were marked as amplified. Multivariable Cox regression analysis was used to analyze the association of low vs medium/high gene expression with disease-free survival (DFS) and overall survival (OS), adjusting for age, stage, and subtype. Results: RNA-Seq data was available for 1086 patients with primary breast cancer, including known hormone receptor positive (HR+)/HER2- (n=530), HER2+ (n=112), and triple negative breast cancer (TNBC) (n=120). NECTIN4was amplified in 15% of cases (n=165): 15% (n=78) in HR+, 15% (n=17) in HER2+ and 21% (n=25) in TNBC. NECTIN4 expression was medium/high in 356 (33%) patients: 149 (28%) HR+/HER2- (median 77.8 TPM, IQR 50.1-116.5), 32 (29%) HER2+ (median 73.7 TPM, IQR 51.9-107.8), and 59 (49%) TNBC (median 99.7 TPM, IQR 57.8-148.3). Median expression in normal breast tissue was 47 TPM (TCGA) & 16.74 TPM (GTEx), and highest in normal skin at 205 TPM. In multivariable analysis, medium/high NECTIN4 expression was associated with worse OS (HR 1.96, 95% CI 1.32-2.99, p<0.001). As a continuous variable, increasing NECTIN4 expression levels were associated with worse OS. After correcting for age, stage and subtype, for each 10-unit increase in NECTIN4TPM, the risk of death increased by 5% (HR 1.0050, 95% CI 1.0028 - 1.0072, p < 0.001). Conclusions: NECTIN4 expression was higher in TNBC and associated with worse overall survival. The high expression level in normal skin possibly contributes to the cutaneous toxicity associated with enfortumab vedotin approval and requires careful consideration as ADCs targeting Nectin-4 are developed to target breast cancer.