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Background: Hypomethylating agent (HMA) and venetoclax (VEN) therapy is a standard of care frontline treatment of acute myeloid leukemia (AML) amongst patients (pts) ineligible for intensive therapy. Despite its convenient flat dose, oral nature, and static excretion irrespective of body weight (wt), VEN has a large, variable volume of distribution and extensive protein binding (Yang, Molecules, 2022). Body mass index (BMI) is a more accurate, albeit imperfect, indicator of body fat that has not been evaluated as an independent predictor of differential VEN exposures and clinical outcomes. We sought to evaluate this possibility amongst pts treated with frontline HMA+VEN in a multi-center, real-world, retrospective study. Methods: We conducted a multi-center, retrospective study through four US academic centers in the Consortium on Myeloid Malignancies and Neoplastic Diseases (COMMAND). 368 pts treated with frontline HMA+VEN from 6/2016-8/2023were evaluated based on BMI within 30 days of induction. Responses were defined by European LeukemiaNet (ELN) 2022 criteria. Pearson's chi-squared test and Wilcoxon rank-sum test were used for comparison between groups. The Kaplan-Meier method and multivariable Cox regression model were applied to overall survival (OS). Continuous variables were summarized as a median with interquartile range (IQR), and categorical variables were reported as a frequency (percentage). Results: The median age was 75 (IQR: 70-79) years, 58% were male, and 76% were white. 107 (29%) had a normal BMI of 18-25 kg/m2, 141 (38%) were overweight (BMI 25-30 kg/m2), and 120 (33%) were obese (BMI >30 kg/m2). There were no significant differences in baseline pt-, disease-, and treatment-specific factors when comparing pts with normal wt to BMI >25.The cohort was enriched for complex karyotype and TP53-mutated disease (27% and 32%, respectively), with 73% categorized as adverse risk by ELN 2022. There was no difference in 30-, 60-, and 90-day mortality rates (6-8%, 14-18%, 19-29%, respectively) by BMI group. Complete remission (CR/CRi) rate was 50% overall and was not impacted by BMI (41%, normal wt; 55%, overweight; 52%, obese) (p=0.10). Overall response rate (ORR) (CR/CRi/MLFS) was 59% overall and differed by BMI groups (50%, normal wt; 66%, overweight; 58%, obese) (p= 0.047). The median OS was 6.6 months for normal wt, 13.7 months for overweight, and 8.3 months for obese pts, and not significantly different when comparing normal to overweight (p=0.052) or obese pts (p=0.756). In multivariable analysis, the effect of BMI varied by the composite remission rate used. Overweight was borderline significantly associated with higher likelihood of achieving CR compared to normal wt (odds ratio [OR] 1.98, 95% confidence interval [CI] 0.99-4.03; p=.056), whereas obesity was not (OR 1.78, CI 0.86-3.74; p=.13). Other co-variates with a significant association with achieving CR included diagnosis of secondary AML (sAML) (OR 0.37, CI 0.18-0.74; p=.006) and presence of IDH (OR 2.81, CI 1.30-6.15; p=.009), ASXL (OR 0.39, CI 0.15-0.96; p=.046), TET2 (OR 2.43, CI 1.17-5.09; p=.02), and DNMT3A (OR 2.37, CI 1.19-4.78; p=.02) mutations. There was a trend towards higher likelihood of achieving CR/CRi for both overweight (OR 1.74, CI 0.92-3.32; p=.09) and obese pts (OR 1.88, CI 0.96-3.74; p=.07). Diagnosis of sAML and presence of TET2, ASXL, and DNMT3A mutations no longer had a significant association with achieving CR/CRi, while IDH mutations maintained an association (p=.01). None of these variables demonstrated a significant impact on ORR (inclusive of MLFS) except overweight (OR: 1.99, CI 1.06-3.80; p=.03) and sAML vs de novo (OR: 0.53, CI 0.29-0.99; p=.047). In multivariable logistic regression, higher ECOG score was associated with risk of early mortality within 60 (p=.01) and 90 (p=.01) days, but not 30 days. sAML vs de novo associated with higher 90-day mortality, presence of NPM and TP53 mutation associated with higher 30-day mortality, and cytogenetic risk associated with higher 60- and 90-day mortality. Conclusions: This multi-center, real-world study demonstrates that BMI may not have a linear effect on response to frontline HMA+VEN, with some indication of negative impact at both lower and higher BMI categories. However, additional dedicated studies are needed to determine if differences in BMI is associated with altered drug exposure as an explanation for these differences.

Introduction: Acute myeloid leukemia (AML) has been considered by many to be an oncologic emergency often leading to hospital admission for expeditious evaluation and treatment. Previous studies reported that time from diagnosis to treatment (TDT) does not impact outcomes (Sekeres, Blood, 2009), (Bertoli, Blood, 2013), (Rollig, Blood, 2020), however these analyses were restricted to intensively treated and/or non-United States (US) patients (pts). We report on a multi-center, retrospective analysis of TDT in adults with newly diagnosed AML receiving Hypomethylating agent (HMA) + venetoclax (Ven) using real-world data from the COMMAND. Methods: Eight US academic medical centers with AML expertise performed a review of adult pts within their center treated with HMA+Ven for newly diagnosed AML. TDT was defined as days from diagnosis of AML to initiation of treatment with HMA+Ven. Cytoreduction with hydroxyurea or leukapheresis was included to control for the effects of severity of presentation on TDT. The primary outcome was impact of TDT on overall survival (OS) (defined as date of diagnosis to date of death). TDT was treated continuously for time-to-event analyses. Secondary outcomes included measurable residual disease (MRD) by flow cytometric analysis (sensitivity <0.1%), complete response (CR), complete response with incomplete count recovery (CRi), defined by European LeukemiaNet (ELN) 2022 Guidelines (Dohner, Blood,2022), and early mortality (death within 30 and 60 days of initiation of HMA+Ven). Pre-planned subgroup analyses included those based on disease biology (FLT3 ITD, IDH1/2, TP53 mutation, ELN 2022 risk), HMA used (azacitidine vs decitabine) and age (<75 vs ≥75 years). For those analyses where TDT was significant in univariable analysis (P=0.1), multivariable analysis was conducted controlling for the following: ECOG performance status (PS), age, receipt of cytoreduction, ELN 2022 risk, and presence of individual pathogenic variants detected on broad myeloid gene next generation sequencing panels used at each institution. All analyses were conducted by a central statistician using RStudio statistical package. Results: 488 pts were included in the analysis. Median age was 76 (interquartile range: 70-80) and the majority of pts had an ECOG PS of 0 or 1 (56%). Most pts (67.6%) had adverse risk with 9.4% and 18.6% of pts having favorable and intermediate risk disease by ELN 2022, respectfully. The most common mutations identified were TP53 (27.5%), IDH1/2 (19.3%), NPM1 (16.4%), DNMT3A (16.8%), ASXL1 (15.4%), and FLT3-ITD (13.7%). Twenty-eight percent of pts received cytoreduction prior to initiation of HMA+Ven. Azacitidine+Ven was the most common regimen utilized (53.9%). Median number of cycles of HMA+Ven was 3 (IQR: 1-6). MRD (-) CR, MRD (+) CR, and CRi was achieved in 21.5%, 10.5%, and 16.4% of pts after one cycle of HMA+Ven. Median OS for the cohort was 10.4 months (95% CI: 9.4-12.6). Median TDT for the cohort was 9 days (IQR: 5-17). In univariate Cox model, longer TDT was associated with improved OS (HR: 0.989, 95% CI: 0.981 to 0.998; p = 0.013). In multivariable analysis, longer TDT was still associated with improved OS (HR: 0.987; 95% CI: 0.973-0.999; p=0.049). TDT and presence of bZIP in-frame CEBPA mutations were the only factors associated with improved OS, while ECOG PS of 2 or 3-4, when compared to ECOG PS=0 (p=0.035 and 0.0002, respectively) and therapy-related AML (p=0.002), were associated with worse OS. Notably, ELN 2022 risk and IDH1/2 or NPM1 mutation status did not impact OS. Discussion: In this real-world, multi-center, US-based analysis of pts with newly diagnosed AML treated with HMA+Ven we show that longer TDT is associated with improved OS even when controlling for severity of presentation, ELN 2022 risk score, ECOG PS, and age. This finding is consistent with data in pts treated with 7+3. Similar to results published in an electronic medical record-based analysis out of Germany (Baden, Haemotologica, 2024) in pts treated with lower-intensity Ven regimens, our results show that pts with newly diagnosed AML eventually treated with HMA+Ven are best served with a complete appraisal of disease biology and treatment planning before initiation of induction therapy. Given the consistent negative impact of ECOG PS and the findings of our study, future consideration for potential patient optimization strategies to improve ECOG PS prior to therapy initiation are warranted.

Background: Acute promyelocytic leukemia (APL) is one of the most treatable leukemias in the current era, with cure rates upwards of 90%. Early mortality due to coagulopathies often seen at diagnosis is a major driver of adverse outcomes. Prompt initiation of all-trans retinoic acid (ATRA) is crucial for suspected APL, as it is known to reverse these coagulopathies. Chemotherapy-free protocols consisting of arsenic trioxide (ATO) and ATRA have become the standard of care for low-risk APL with high cure rates and reduced hematologic toxicities. For high-risk disease, there are multiple treatment options that have not been compared. Since APL is rare, there is a lack of real-world data for this disease. This retrospective review aimed to assess the outcomes of APL patients (pts) across multiple institutions and to examine the most common induction regimens, characteristics linked to early (30-day) mortality, and supportive care strategies. Methods: This retrospective study involved 9 institutions in the US and was conducted through the Consortium on Myeloid Malignancies and Neoplastic Diseases (COMMAND). Data included pts admitted from 8/5/2006 through 2/3/2024 with a confirmed diagnosis of APL. Each institution received a data-collection form and contributed data on APL pts from presentation to last follow-up. Key data categories included patient demographics, presenting laboratory and molecular data, treatment regimen, supportive care, and patient outcomes. Results: Data for 412 pts were identified with a median age of 51 (range 17-90) and 49% were female. Cardiovascular comorbidities were present in 44% of pts and 16% had a known pulmonary comorbidity. At presentation 109 (27%) had high risk-disease, defined as a white blood cell count (WBC) greater than 10 x 109/L. A FLT3 mutation was seen in 50% of tested pts, WT1 (22%), KRAS (3.6%), and NRAS (6.5%). The median WBC at presentation was 2 x 109/L (range: 0-166 x 109/L) and the median platelet count was 27 x 109/L (range: 2.0-315 109/L). Twenty-six percent presented with grade 2+ thrombosis, bleeding, or both. A higher percentage of pts with high-risk vs. low-risk disease presented with grade 2+ thrombosis, bleeding, or both (43% vs. 20%), DIC (60% vs. 36%), and mutated FLT3 (81% vs. 37%). The majority of pts received a chemotherapy-free induction regimen consisting of ATRA plus ATO. It was determined that 36 (8.7%) pts received induction with intermittent ATO plus ATRA (Burnett AK, Lancet Oncol 2015), 276 (67%) received daily ATO plus ATRA (Ravandi F, J Clin Oncol 2009 and Lo Coco, NEJM 2013), and 70 (17%) received a regimen inclusive of an anthracycline plus ATRA. Eleven pts did not start an induction regimen other than single-agent ATRA due to early death, while 19 had their regimen classified as “other” because they did not follow a conventional induction path. In total, 295/358 (82%) received ATRA within 24 hours of admission. At least 1 dose of gemtuzumab ozogamicin was given to 58 (14%) pts, of which 35 (60%) were considered high risk. The incidence of differentiation syndrome (DS) was 37% (n=154). Among the 243 pts who received steroid prophylaxis, 30% developed DS, compared to 52% of those who did not receive prophylaxis. Among the 360 pts with at least 1 available PCR result after starting induction, 94% achieved PCR-negativity. The 30-day mortality rate was 6.6%. On univariate analysis, presenting variables associated with a higher 30-day mortality include older age, diabetes, cardiovascular comorbidities, thrombosis or bleeding, DIC, eGFR < 60, elevated total bilirubin and D-dimer levels, and high-risk disease. The median overall survival (OS) of the cohort was not reached, and the 5- and 10-year OS was 88% (95% CI: 84-91%) and 79% (95% CI: 72-87%) respectively. The cumulative incidence of relapse at 3 years was 2.7% (95% CI: 1.4 - 5.1%), with no additional relapses observed. The incidence of CNS relapse at 3 years was 0.9% (95% CI: 0.3% - 2.8%), with 2 of the 4 pts classified as high-risk. Conclusion: This is the largest real-world evidence data collection in the US evaluating the outcome of pts with APL in the current era, inclusive of ATO as front-line induction. Outcomes for this disease remain extremely positive, with low 30-day mortality rates noted in this study along with durable OS rates and low incidence of relapse. Additional information on outcomes, safety, and supportive care will be presented at the meeting.

Abstract Background The PALLAS trial investigated the addition of palbociclib to standard adjuvant endocrine therapy reduces breast cancer recurrence. This pre-specified analysis was conducted to determine whether adjuvant palbociclib benefited patients diagnosed with lower risk stage IIA disease compared to those with higher stage disease. Methods PALLAS was an international, multicenter, randomized, open-label, phase III trial, representing a public-private partnership between Pfizer, the Austrian Breast Cancer Study Group and, the U.S. ALLIANCE Foundation. Patients diagnosed with stage II-III, hormone-receptor-positive, HER2/neu negative breast cancer who were within 12 months of diagnosis, and had completed all definitive therapy aside from endocrine therapy, which was started within 6 months prior to study entry. All patients were required to submit a formalin-fixed paraffin-embedded (FFPE) tumor block. Patients were randomly assigned 1:1 to receive standard adjuvant endocrine therapy (of physicians’ choice) for at least 5 years with or without 2 years of Palbociclib, administered orally at a starting dose of 125 mg daily, given for 21 days followed by a 7-day break. Results A total of 5,796 patients with HR+/HER2- early breast cancer (including 1,010 with stage IIA) were enrolled. Median follow-up was 50 months for stage IIA patients and 43.1 months overall. In the stage IIA cohort, four-year iDFS in the palbociclib arm was 92.9% versus 92.1% for ET alone (HR 0.75, 95%CI 0.48–1.19, p = 0.23). There was no differential benefit by histologic grade, chemotherapy receipt, age, or anatomic/clinical risk. Additionally, no benefit to palbociclib was seen in this cohort in invasive breast cancer-free survival (iBCFS), locoregional relapse-free survival (LRFS), distant relapse-free survival (DRFS), or overall survival (OS). For the stage IIB/III patients, 4-year iDFS was 85.3% for palbociclib + ET versus 83.6% for ET alone (HR 0.91, 95% CI 0.77–1.07, p = 0.24). Conclusions and Relevance: While there were substantial differences in outcome for stage IIA versus IIB/III patients at 4 years of follow-up, the addition of 2 years of palbociclib did not improve outcomes for patients, regardless of stage. Trial Registration: The study was registered on Clinicaltrials.gov on July 31, 2015, and was assigned the clinical trial number (NCT02513394).

PURPOSE Long-term outcomes of patients with stage I human epidermal growth factor receptor 2 (HER2)–positive breast cancer receiving adjuvant trastuzumab emtansine (T-DM1) remain undefined, and prognostic predictors represent an unmet need. METHODS In the ATEMPT phase II trial, patients with stage I centrally confirmed HER2-positive breast cancer were randomly assigned 3:1 to adjuvant T-DM1 for 1 year or paclitaxel plus trastuzumab (TH). Coprimary objectives were to compare the incidence of clinically relevant toxicities between arms and to evaluate invasive disease-free survival (iDFS) with T-DM1. Correlative analyses included the HER2DX genomic tool, multiomic evaluations of HER2 heterogeneity, and predictors of thrombocytopenia. RESULTS After a median follow-up of 5.8 years, 11 iDFS events were observed in the T-DM1 arm, consistent with a 5-year iDFS of 97.0% (95% CI, 95.2 to 98.7). At 5 years, the recurrence-free interval (RFI) was 98.3% (95% CI, 97.0 to 99.7), the overall survival was 97.8% (95% CI, 96.3 to 99.3), and the breast cancer-specific survival was 99.4% (95% CI, 98.6 to 100). Comparable iDFS was observed with T-DM1 irrespective of tumor size, hormone receptor status, centrally determined HER2 immunohistochemical score, and receipt of T-DM1 for more or less than 6 months. Although ATEMPT was not powered for this end point, the 5-year iDFS in the TH arm was 91.1%. Among patients with sufficient tissue for HER2DX testing (n = 187), 5-year outcomes significantly differed according to HER2DX risk score, with better RFI (98.1% v 81.8%, hazard ratio [HR], 0.10, P = .01) and iDFS (96.3% v 81.8%, HR, 0.20, P = .047) among patients with HER2DX low-risk versus high-risk tumors, respectively. CONCLUSION Adjuvant T-DM1 for 1 year leads to outstanding long-term outcomes for patients with stage I HER2-positive breast cancer. A high HER2DX risk score predicted a higher risk of recurrence in ATEMPT.

e23171 Background: A011502 is a phase 3 trial that randomized high-risk nonmetastatic breast cancer (BC) patients (pts) to aspirin 300 mg daily vs. placebo. Obesity and Black race have been shown to be associated with worse clinical outcomes. We studied the association of stress, depression, sleep and pain with each other, and with BMI and race. Methods: At baseline, 2735, 2610, 2720, and 2422 pts completed Perceived Stress Scale (PSS), Center for Epidemiologic Studies Depression Scale Revised (CESD-R), Brief Pain Inventory (BPI), and Pittsburgh Sleep Quality Index (PSQI) respectively. Stress was categorized with PSS scores: low (0-13), moderate (14-26) or high (27-40). Depression was categorized with CESD-R scores: no depression (0-15) or depression (≥16). Pain was categorized with BPI scores: none/mild (0-3) or moderate/severe (≥4). Sleep quality was categorized with PSQI scores: good (0-5) or poor ( > 5). Associations of measures by race and BMI were analyzed using chi-square test. Results: There was a statistically significant association among stress, depression, pain, and sleep quality (p < 0.01 for all). Compared to pts with BMI < 25, overweight and obese women reported higher stress (2.9%, 2.3% vs. 1.7%, p = 0.04), depression (13.4%, 18% vs. 9.6%, p < 0.01), moderate/severe pain (16.5%, 20.3% vs. 12%, p < 0.01), and poor sleep (64.9%, 71.5% vs. 60.4%, p < 0.01). Black women reported more moderate/severe pain compared to White women or other races (32.9% vs. 14.2% vs. 27.8%, p < 0.01). There was no association of other symptoms with race (Table 1). Conclusions: Obesity was associated with higher stress, depression, poor sleep and pain among BC pts from A011502. Black women reported higher pain compared to other races. Future studies are warranted that determine causal associations, how symptoms may relate to outcome disparities, and develop approaches to address them. U10CA180821, U10CA180882, UG1CA233196; https://acknowledgments.alliancefound.org . Clinical trial information: NCT02927249 . [Table: see text]

11016 Background: A011502 is a phase 3 randomized double-blind clinical trial that enrolled high-risk nonmetastatic breast cancer (BC) patients (pts) and randomized pts to aspirin 300 mg daily vs. placebo. There was no significant difference in invasive disease-free survival (iDFS) between the two arms. A secondary objective was to determine the association of inflammation-affiliated factors (stress, depression, poor sleep quality and pain) with iDFS and overall survival (OS). We hypothesized that these baseline factors are associated with worse clinical outcomes. Methods: At baseline, 2735, 2720, 2422 and 2610 pts completed Perceived Stress Scale (PSS), Brief Pain Inventory (BPI), Pittsburgh Sleep Quality Index (PSQI) and Center for Epidemiologic Studies Depression Scale Revised (CESD-R) respectively. Stress was categorized with PSS scores: low (0-13), moderate (14-26) or high stress (27-40). Pain was categorized with BPI scores: none/mild (0-3) or moderate/severe (≥4). Sleep quality was categorized with PSQI scores: good (0-5) or poor ( > 5).Depression was categorized with CESD-R scores: no depression (0-15) or depression (≥16). Associations between the measures of interest and outcomes were performed with multivariable Cox models controlling for age, body mass index, time since diagnosis, race, ethnicity, hormone receptor status and treatment arm. Results: Median follow up was 35 months. The associations from multivariable Cox model are shown in Table 1. Pts who reported high PSS had significantly worse iDFS and (non-significant) worse OS. Moderate/severe average (avg) pain was significantly associated with worse iDFS and OS. Poor sleep quality and depression were associated with worse iDFS and OS but not statistically significant. Conclusions: Pts reporting higher stress and pain at baseline had worse outcomes. We acknowledge that both pain and stress may be related to other non-cancer issues (chronic comorbidities). Our study highlights the need for clinical trials to consider including questionnaires to assess pt-reported outcomes. Future studies are warranted to determine if measures to decrease pain and stress would improve BC outcomes. U10CA180821, U10CA180882, UG1CA233196; https://acknowledgments.alliancefound.org . NCT02927249: Clinical trial information: NCT02927249 . [Table: see text]

530 Background: Robust biomarkers are needed to personalize treatment in early stage HER2-positive breast cancer. The TBCRC026 trial (n=83) demonstrated that early metabolic changes on FDG-PET/CT [>40% decline in SULmax at day 15 post treatment initiation (C1D15); C1D15 SULmax] were associated with response to neoadjuvant trastuzumab/pertuzumab (HP), without chemotherapy, in an ER-negative, HER2-positive cohort (NCT01937117). We investigated the association of RNA sequencing (RNAseq)-based gene expression signatures (GES) at baseline and on-treatment (C1D15), with early metabolic change on PET/CT and clinical outcomes. Methods: RNAseq (664 GES) was performed on baseline (n=56) and C1D15 (n=31) tumor biopsies (25 paired samples available). Association with pCR following HP (secondary), PET/CT parameters and survival (exploratory) was studied. Gene expression changes between baseline and C1D15 samples were tested by Wilcoxon signed-rank test. Association between GES and PET/CT parameters and GES with pCR were evaluated using logistic regression models. Cox regressions were performed to study the association between PET/CT SULmax at C1D15 and a B cell signature and relapse-free survival (RFS). The likelihood ratio test was used to compare the prognostic ability of two nested Cox models. Results: Both RNAseq and clinical outcomes were available in 53/83 patients. HER2-enriched (HER2-E) intrinsic subtype comprised 71% (40/56) of cases at baseline and 61% (19/31) at D15; basal subtype for 27% (15/56) and 39% (12/31), respectively. pCR rate was 19% (10/53) with all but one exhibiting HER2-E subtype. One basal-like sample underwent pCR, with downregulation of HER2 amplicon and IgG signatures slightly more prominent. Gene expression changes from baseline to C1D15 were analyzed; ERBB2was downregulated after HP (p = 0.007), B cell signatures, CD4, CD8, natural killer cells and Treg cells were significantly upregulated at C1D15. GES associated with 40% decline in SULmax at C1D15 included ERBB2 amplicon genes ( ERBB2, GRB7) and correlation to the HER2-E subtype, whereas basal-like signatures were significantly associated with lack of response on PET at C1D15. Univariate logistic regression analysis suggested that FOS-JUN, fibroblast and B cell GES at baseline were significantly associated with pCR (p=<0.05). Finally, the addition of a B cell signature improved the prognostic value of C1D15 SULmax on PET/CT for RFS (p < 0.05). Conclusions: In this unique cohort with ER-negative, HER2-positive breast cancer treated with neoadjuvant HP without chemotherapy, HER2-E tumors and those with increased immune signaling were more likely to respond to HER2-directed therapy. Combining C1D15 SULmax on PET/CT and a B cell signature provided improved prognostic information and should be further explored as candidate biomarkers in this setting. Clinical trial information: NCT01937117 .

Abstract Background. Approximately 12,000 women aged 40 years or younger are diagnosed with breast cancer every year in the United States alone. Limited data exist regarding survival in young women with metastatic breast cancer (MBC), particularly when considering modern tumor subtyping and treatment, though previous studies have suggested that patients presenting with MBC at diagnosis (de novo stage IV) have better prognosis than those who develop recurrent MBC after initial diagnosis of stage 0-III disease. We sought to determine survival rates in young patients with MBC stratified by disease presentation (de novo versus recurrent) and subtype. Methods. The Young Women’s Breast Cancer Study (YWS) is a multicenter, prospective cohort which enrolled 1,302 women diagnosed with stage 0-IV breast cancer at age ≤40 from 13 North American academic and community-based sites from 2006-2016. The current median follow-up is 10.1 (range 0.4-16.3) years. Histopathology slides were centrally reviewed for 97% of the participants, and clinical tumor molecular subtypes have been detailed. In the recurrent group, receptors were obtained from the initial breast surgery. Descriptive analyses were used to characterize patients with de novo or recurrent MBC, including demographic and disease characteristics, and survival rates. Overall survival was estimated using the Kaplan-Meier method and compared by log-rank test between those who presented with de novo and relapsed disease overall and within tumor subtypes. Results. Among 246 patients included in this analysis, 74.0% (n=182) had recurrent MBC and 26.0% (n=64) had de novo MBC. Most patients were white (n=208, 84.6%), while 6.9% were Asian (n=17), 4.5% Black (n=11), 0.8% Native American (n=2), 0.8% multiracial (n=2) and 2.4% other/unknown (n=6). A total of 4.5% of patients were Hispanic (n=11). Median age at diagnosis of de novo MBC was 37 years and 40 years for diagnosis of metastasis in the recurrent group. Median disease-free interval was 3.23 (range 0.32 – 13.84) years for the recurrent MBC group. Among all patients with MBC, most (55.7%, n=137) had estrogen receptor (ER) and/or progesterone receptor (PR)-positive, human epidermal growth factor receptor 2 (HER2)-negative disease, whereas 17.9% (n=44) had ER and/or PR-positive HER2-positive, 7.3% (n=18) had ER/PR-negative HER2-positive, and 18.7% (n=46) had triple-negative tumors. A greater proportion of participants (43.8%, n=28) had HER2-positive disease in the de novo group than in the recurrent group (18.7%, n=34), P=&lt; 0.0001. A total of 10.9% (n=7) of de novo patients had a germline BRCA mutation, compared to 13.7% (n=25) in the recurrent group. Median survival was 5.09 (95% CI: 3.74 – 6.38) years in the de novo group and 2.63 (95% CI: 1.98 – 3.21) years in the recurrent MBC group (P=0.0004). When stratifying by subtype, survival among patients with de novo ER and/or PR-positive HER2-positive disease was longer than among patients with recurrent disease (P=0.0149), whereas there were no statistically significant differences in survival between the de novo and recurrent disease groups in other disease subtypes (Table 1). Conclusion. Survival in young women with MBC is highly heterogeneous and varies substantially by tumor subtype and disease presentation. Those with de novo ER and/or PR-positive HER2-positive subtype appear to have better outcomes than those with recurrent MBC with the same subtype. These findings can inform patient care and potential future research to improve outcomes for young patients with MBC. Table 1: Median survival by disease presentation and subtype * 1 patient with recurrent MBC was not included because the initial diagnosis was DCIS MBC: metastatic breast cancer; OS: overall survival; CI: confidence interval; ER: estrogen receptor; PR: progesterone receptor; HER2: human epidermal growth factor receptor 2 Citation Format: Leticia Varella, Yue Zheng, Shoshana Rosenberg, Gregory Kirkner, Craig Snow, Kathryn Ruddy, Rulla Tamimi, Jeffrey Peppercorn, Lidia Schapira, Virginia Borges, Steven Come, Laura Collins, Ellen Warner, Kate Dibble, Eric Winer, Ann Partridge. Survival in Young Women with De Novo or Recurrent Metastatic Breast Cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-05-14.

Abstract BACKGROUND: Anti-HER2 therapies have transformed the trajectory for patients with HER2-positive (HER2+) MBC. A subset of patients with HER2+ MBC do exceptionally well and remain on maintenance anti-HER2 therapy for many years. However, current therapy is non-curative and patients are treated indefinitely. While these therapies often are well-tolerated, they may cause significant toxicities, are expensive, and typically require frequent infusion and provider visits. Case reports and retrospective series demonstrate that some patients may do well after stopping anti-HER2 treatment. We currently lack prospective data on the likelihood of clinical impact of successful treatment discontinuation, outcomes among patients who progress and restart HER2-directed therapy, and tools to predict who may safely stop therapy. In this study, we will enroll pts with an exceptional response to anti-HER2 therapy. Pts may elect to either continue or stop anti-HER2 therapy and will be followed clinically and radiographically on a protocol-specified schedule. Considering the central role of patients in choosing the study intervention arm, and the unique nature of stopping therapy as a treatment intervention, patient advocates have been deeply involved in the study design to ensure patient concerns are being heard and addressed. We will retrospectively assess minimal residual disease (MRD), measured in plasma, as a biomarker of ongoing response, in a step toward shifting the treatment paradigm for pts with HER2+ MBC. METHODS: STOP-HER2 is a prospective, non-randomized, phase II, multicenter study of either continuation (cohort 1) or cessation (cohort 2) of anti-HER2 therapy in exceptional responders with HER2+ MBC. Eligible patients must have biopsy-proven HER2+ MBC by ASCO/CAP guidelines and must have been receiving first-line anti-HER2 therapy for MBC for at least 3 years without evidence of progressive disease (PD) as determined by the treating investigator. Patients with prior brain-only PD or oligo-PD outside the brain are eligible at least 2 years after local treatment to the central nervous system (CNS) and in absence of subsequent PD. Up to fifty-two participants will be enrolled into cohort 2, and an additional thirty participants enrolled into the cohort 1, at 10 US sites within the Translational Breast Cancer Research Consortium. Co-primary endpoints are the 1-year progression-free survival (PFS) in cohorts 2 and 1, separately. Secondary endpoints include the clinical benefit rate upon re-initiation of anti-HER2 therapy for patients in cohort 2 experiencing PD after treatment discontinuation; and 3-year overall survival in both cohorts. Correlative endpoints include MRD status at baseline and the correlation between MRD dynamics and outcomes. Patient-reported outcomes will be collected and analyzed, including quality of life, illness intrusiveness, financial toxicity, anxiety, and decision regret. The study will use a two-stage design to assess the primary objective of estimating the rate of participants who are free of progression at one year after discontinuing anti-HER2 therapy. If 50% or less are free of progression at one year, stopping HER2 therapy would not be considered a viable treatment plan (null hypothesis). The study is designed to have 80% power to reject the null hypothesis at the 10% type I error rate when 68% of exceptional responders are free of progression at one year after stopping HER2 therapy. STOP-HER2 began enrollment in Q2 2023 (NCT05721248). Citation Format: Heather Parsons, Kathryn Ruddy, Stefania Morganti, Karen Smith, Victoria Attaya, Eliza Kallfelz, Michelle DeMeo, Jamie LaScala, Shirley Mertz, Teri Pollastro, Patricia Spears, Adam Brufsky, Chau Dang, Susan Dent, Ahmed Elkhanany, WIlliam Gwin, Ciara O'Sullivan, Kathy Miller, Sara Nunnery, Elaine Walsh, Anna Maria Storniolo, Sara Tolaney, Nabihah Tayob, Antonio Wolff, Eric Winer, Mothaffar Rimawi, Ian Krop, Nancy Lin. The STOP-HER2 Trial: A Phase 2 Study of Stopping Trastuzumab - Outcomes in Patients with HER2+ Metastatic Breast Cancer (TBCRC 062) [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-20-01.