TPS1147 Background: Due to dramatic improvements in neoadjuvant and adjuvant HER2-directed therapy (tx), most patients (pts) with early HER2+ breast cancer are cured. As a result, over half of pts newly diagnosed with HER2+ metastatic breast cancer (MBC) now present with de novo stage IV disease. Anti-HER2 tx has also significantly extended survival for pts with HER2+ MBC, with a subgroup of exceptional responders alive many years (yrs) after diagnosis. However, the paradigm for HER2+ MBC remains non-curative, and pts receive tx indefinitely with significant toxicities and costs. The SAPPHO study is investigating whether an intensification approach of sequential, non-cross resistant anti-HER2 tx followed by tx discontinuation is associated with long-term disease control in pts with HER2+ MBC. Methods: SAPPHO is an open-label, phase II, single-arm trial testing a sequential regimen of non-cross resistant, HER2-targeted tx with curative intent in pts with de novo HER2+ MBC. Eligible pts must have biopsy-proven, de novo MBC with high HER2 expression (3+ by immunohistochemistry). Pts with brain metastases are eligible upon receipt of local tx. Treatment consists of an induction regimen (trastuzumab-pertuzumab-taxane [THP] x 4 cycles, followed by trastuzumab deruxtecan [TDXd] x 6 cycles, followed by trastuzumab emtansine [TDM1]-tucatinib x 4 cycles), followed by a maintenance regimen (HP-tucatinib for 1 yr). Given results from DESTINY-Breast09, the sequence of TDXd-P x 6 cycles followed by THP x 4 followed by TDM1-tucatinib x 4 can be chosen as alternative induction tx per investigator and patient choice. Pts who remain progression-free after completing maintenance will stop all anti-HER2 tx. Endocrine tx will be continued for pts with hormone receptor+/HER2+ tumors. Tumor specimens from breast and a metastatic site are collected at baseline and the end of induction. Serial plasma samples for ctDNA analysis are collected at baseline, during treatment, and during follow-up. The primary endpoint is the probability of being progression-free and off anti-HER2 tx 4 yrs from the start of induction. With a sample size of 72 pts, the study is designed to have 91% power to reject the null hypothesis that the probability of being off anti-HER2 tx and progression-free is less than 24%, with an alternative hypothesis of 40%. Key secondary endpoints are overall survival, overall response rate by modified RECIST 1.1 after induction, and safety. Correlative endpoints include the relationship between ctDNA dynamics and outcomes. Patient-reported outcomes will be analyzed, including quality of life, illness intrusiveness, financial toxicity, anxiety, distress about cancer progression, perception of benefit and risk of progression. SAPPHO began enrollment in Q3 2024, and the study is open at 4 US sites within the Translational Breast Cancer Research Consortium (NCT05721248). Clinical trial information: NCT05721248 .

1056 Background: Resistance to HER2-directed therapies remains a major challenge in HER2-positiveMBC. In preclinical models, HER2+ tumors overexpress poly (ADP-ribose) polymerase (PARP), and PARP inhibition induces apoptosis through NF-κB suppression even in the absence of pathogenic mutations in BRCA 1/2 . We evaluated the safety, efficacy, and translational correlates of niraparib combined with trastuzumab in patients (pts) with refractory HER2+ MBC. Methods: This open-label Phase 1b/2 trial (NCT03368729) enrolled pts with HER2+ MBC (IHC 3+ and/or FISH amplified), measurable disease, LVEF ≥50%, and progression on ≥1 prior HER2-targeted therapy; stable CNS disease was allowed. Pts with germline BRCA1/2 mutations or prior PARP inhibitor exposure were excluded. Niraparib 300 mg orally was administered on Days 1–21 of a 21-day cycle with trastuzumab 6 mg/kg IV every 3 weeks (loading dose 8 mg/kg on cycle 1). The Phase 2 portion used a Gehan/Simon two-stage design with early stopping for futility (P0=24%); ≥1 response among the first 20 patients triggered expansion to 40 patients. ORR was assessed by RECIST v1.1, meeting the primary endpoint and allowing response rate estimation with <7% standard error and a two-sided 95% exact CI width of 0.28. Gene expression profiling of paired tumor tissue and peripheral blood mononuclear cells (PBMCs) was performed using the NanoString nCounter Tumor Signaling 360 panel at pretreatment compared to cycle 2. Results: Forty-six pts were enrolled (9/2019–2/2025); 41 were evaluable for response. Pts were heavily pretreated (median 3 prior regimens), including trastuzumab deruxtecan in 22 pts; 33% had brain metastases. Treatment-related adverse events were predominantly grade 1–2 and manageable. Clinical benefit rate (CBR) was 44% (18/41) including 1 complete response, 2 partial responses, and 15 stable disease. Eleven patients achieved disease control ≥ 6 months; 1 patient remained on treatment at data cutoff. Tumor profiling revealed enrichment of a B-cell–driven tertiary lymphoid structure signature with reduced hypoxia signaling in responders. Matched PBMC analyses demonstrated coordinated downregulation of interferon signaling, DNA damage response, metabolic activation, angiogenesis, and resistance pathways in clinical responders, whereas non-responders retained signatures of chronic stress and immune exhaustion. Conclusions: Niraparib plus trastuzumab demonstrated a favorable safety profile and durable clinical benefit in heavily pretreated HER2+ MBC. Integrated tumor and blood-based analyses identify immune-organized tumor microenvironments and systemic immune normalization as key correlates of prolonged response, supporting further investigation of PARP inhibition strategies. Clinical trial information: NCT03368729 .

Background: Gedatolisib is a pan PI3K/mTOR inhibitor and was evaluated in a multicenter, open-label, phase 1b dose-escalation and dose-expansion trial in combination with palbociclib and endocrine therapy in patients with hormone receptor-positive/HER-2 negative (HR+/HER2-) breast cancer with varying categories of prior therapy (ClinicalTrials.gov NCT02684032). The primary objective of the dose-escalation cohorts was safety while the primary endpoint of the dose-expansion cohorts was investigator-assessed objective response. Median progression-free survival at 12 months was 72.1%, 54.5%, 23.6%, and 53.2% for Arms A, B, C, and D, respectively, as defined in the methods. In the dose-expansion cohorts gedatolisib triplet therapy achieved overall response rates (ORR) of 85%, 77%, 36% and 63% in Arms A, B, C and D respectively. Based on the promising ORR of the phase 1b trial, further follow-up was warranted to analyze survival outcomes. Here we report survival results of patients who received gedatolisib combined with palbociclib plus letrozole or fulvestrant. Methods: Female patients aged at least 18 years from 17 sites across the USA with HR+/HER2- advanced breast cancer and an Eastern Cooperative Oncology Group (ECOG) performance status of 0-1 were enrolled in the phase 1b trial. There were 2 dose-escalation arms that assessed safety of gedatolisib 180 mg/week plus standard doses of palbociclib with endocrine therapy of letrozole or fulvestrant. The dose expansion portion had 4 patient cohorts based on prior treatment history, with Arm A comprising patients with no prior systemic treatment for breast cancer, Arm B with patients with 1-2 prior endocrine therapies but no history of CDK4/6 inhibitor, and Arms C and D with patients with one or more prior lines of therapy, including a CDK4/6 inhibitor. Gedatolisib 180 mg was administered intravenously weekly in 28-day treatment cycles for Arms A-C, and on days 1, 8, and 15 for Arm D. Endocrine therapy with letrozole or fulvestrant and CDK4/6 inhibition with palbociclib were administered at standard dose. Participating sites were asked to collect patient outcome for survival analysis. Results: As of the phase 1b study data lock on June 29, 2022, the median overall survival was not reached in any group. The sponsor has initiated overall survival follow up for all patients that will be presented at the conference which provides at least 2 years of additional follow-up time. Conclusions: Gedatolisib in combination with palbociclib and letrozole demonstrated promising overall response rates and progression-free survival in patients with HR+/HER2- advanced breast cancer that warrants further evaluation of overall survival outcomes./ Citation Format: Rachel Layman, Hope S. Rugo, Robert Wesolowski, Hyo Han, Jennifer M. Specht, E. Claire Dees, Erica M. Stringer-Reasor, Peter Kabos, Vandana Abramson, Anthony Shields, Charlotte Moser, Igor Gorbatchevsky, Pratima Nayak, Samuel Suzuki, Sarah Mutka. Overall survival in patients with HR+/HER2- advanced breast cancer treated in a phase 1b trial evaluating gedatolisib in combination with palbociclib and endocrine therapy [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 P4-08-25.

BackgroundThe oral, selective, and potent small molecule cyclin-dependent kinases (CDK) 4/6 inhibitor (CDK4/6i) abemaciclib has demonstrated efficacy in advanced breast cancer and high-risk early breast cancer. This Phase 1b study evaluated the safety, tolerability, pharmacokinetics, and antitumor activity of abemaciclib in combination with endocrine therapies (Parts A–D), exemestane + everolimus (Part E), or fulvestrant + LY3023414 (a PI3K/mTOR inhibitor; Part G) in patients with hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) metastatic breast cancer (MBC), or trastuzumab (Part F), or trastuzumab + pertuzumab (Part H) in patients with HER2-positive (HER2+) MBC.Patients and methodsThis study enrolled women aged ≥18 years old with either HR+, HER2- (Parts E and G), or HER2+ (Parts F and H) MBC. Additional requirements included measurable disease or non-measurable but evaluable bone disease (Parts E and F), or measurable disease (Parts G and H), an Eastern Cooperative Oncology Group performance status of 0–1, and no prior treatment with CDK4/6i (Parts E, F, and H). Adverse events were graded, and tumor response was assessed.ResultsNineteen patients in Part E received abemaciclib (150 mg, n=15; 200 mg, n=4) with exemestane + everolimus, 24 patients in Part F received abemaciclib (150 mg, n=18; 200 mg, n=6) with trastuzumab, 12 patients in Part G received 150 mg abemaciclib with fulvestrant + LY3023414 (100 mg, n=7; 150 mg, n=5), and four patients in Part H received abemaciclib (100 mg) with trastuzumab + pertuzumab (with prophylactic loperamide). The most common treatment-emergent adverse events (TEAEs) were diarrhea, fatigue, neutropenia, and nausea. Grade ≥3 TEAEs were reported in 16, 18, 10, and 4 patients in Parts E–H, respectively. Abemaciclib had no effect on the pharmacokinetics of the combination study drugs. The objective response rates for patients with measurable disease were 46.2%, 10.0%, 66.7%, and 25.0% in Parts E–H, respectively. A recommended Phase 2 dose was not established for Parts E, G, and H at the dose levels evaluated, and was determined to be 150 mg Q12H in Part F.ConclusionsOverall, our results demonstrate safety profiles consistent with those previously established for abemaciclib and provide preliminary data for these combination therapies in the treatment of HR+, HER2- or HER2+ MBC.

Background: Triple-negative breast Cancer (TNBC) accounts for 15-20% of all breast cancers, among which it has the worst prognosis and limited therapeutic options. B7-H3, a type I transmembrane protein, is expressed in 75% to 85% of breast cancer, with the highest expression found in TNBC. Preclinical studies have demonstrated significant anti-tumor activity of B7-H3 CAR-T cells in cell lines and patient-derived xenograft (PDX) models. Preliminary data from an ongoing phase 1 clinical trial of B7-H3 CAR-T cells for patients with relapsed ovarian cancer (NCT04670068) at our institution has shown a tolerable safety profile. Additionally, the implementation of an inducible Caspase 9 safety switch aims to mitigate the risk of severe complications such as cytokine release syndrome (CRS) and immune effector cell-associated neurotoxicity syndrome (ICANS), enhancing the clinical application potential of B7-H3 targeted CAR-T cell therapy in TNBC. Based on these premises, we will conduct the first phase 1 study of B7-H3 CAR-T cells in patients with previously treated metastatic TNBC to establish the safety of this novel treatment modality and identify a recommended dose for further study in this population of patients with poor prognosis and unmet clinical need. Methods: This is a Phase I, single-center, open-label study designed to evaluate the safety of escalating doses of CAR T cells targeting the B7-H3 antigen, incorporating an inducible caspase 9 safety switch (iC9-CAR.B7-H3 T cells) in patients with previously treated metastatic TNBC. The study utilizes a modified 3+3 dose-escalation design, starting with a dose of 1 × 106 transduced cells/kg. Patients with TNBC who meet eligibility criteria will undergo leukapheresis for the collection of cells to manufacture iC9-CAR.B7-H3 T cells. During the time period necessary to manufacture the iC9-CAR.B7-H3 T cells, patients will be allowed to receive standard-of-care bridging therapy at the discretion of their local physician. Eligible patients will then receive lymphodepletion chemotherapy with cyclophosphamide (300 mg/m2) and fludarabine (30 mg/m2) administered intravenously for three consecutive days, followed by the infusion of iC9-CAR.B7-H3 T cells. The primary objective is to evaluate the safety and tolerability of iC9-CAR.B7-H3 T cell administration. Safety assessments will include monitoring for adverse events, graded according to the Common Terminology Criteria for Adverse Events (CTCAE) version 5.0. Secondary objectives include identifying the recommended Phase II dose (RP2D), determining the objective response rate (ORR), duration of response (DOR), progression-free survival (PFS), and overall survival (OS). Patients will be monitored for 15 years post-treatment completion in accordance with guidelines for gene transfer studies. This extended follow-up will evaluate the long-term safety and persistence of the genetically modified T cells. The trial is currently enrolling patients, NCT06347068. Citation Format: Yara Abdou, Barbara Savoldo, Natalie S. Grover, Gianpietro Dotti, Felicia Cao, Catherine J. Cheng, J. Kaitlin Morrison, Jonathan S. Serody, Lisa A. Carey, E. Claire Dees. Phase I Study of B7-H3 Specific Chimeric Antigen Receptor (CAR) T Cell Therapy in Patients with 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 P1-07-22.

Although endocrine therapies, alone or in combination with CDK4/6 inhibitors, have led to notable improvements in the treatment of estrogen receptor-positive (ER+) breast cancer, progression is inevitable for most patients. We report dose escalation and expansion data from a trial of H3B-6545 (a novel selective ER covalent antagonist that inactivates wild-type and mutant ERα) in women with locally advanced/metastatic ER+, HER2-negative breast cancer (BC). This study was a multicenter, open-label, phase 1/2 trial. Women ≥ 18 years of age with ER+, HER2 − BC whose disease progressed on their most recent therapy were eligible. Prior therapy must have included at least 2 hormonal therapies (HTs), or 1 HT and 1 chemotherapy, or 1 HT and a CDK4/6 inhibitor. In phase 1, H3B-6545 was administered orally once daily at doses of 100–600 mg. In phase 2, the efficacy of the recommended phase 2 dose (RP2D) determined in phase 1 was examined in additional patients, including those with/without ERα mutation. The primary endpoints were RP2D determination (phase 1) and objective response rate (ORR) (phase 2, investigator-assessed per RECIST v1.1). Additional primary endpoints (phase 2) included progression-free survival (PFS) and overall survival (OS), per Kaplan-Meier estimates. 151 Patients were treated across phases. During phase 1, 2 DLTs (drug eruption and fatigue, both grade 3) were observed at the 600 mg dose, and 450 mg was deemed the RP2D. In the total population (phases 1 and 2), all patients experienced ≥ 1 treatment-emergent adverse event (TEAE), and 50.3

Chimeric antigen receptor (CAR) macrophages (CAR-Ms) mediate antitumor immunity via phagocytosis, cytokine release, activation of the tumor microenvironment and antigen presentation. We report results from a non-prespecified interim analysis of a first-in-human, phase 1 clinical trial of CT-0508, an anti-human epidermal growth factor receptor 2 (HER2) CAR-M in patients with advanced HER2-overexpressing tumors. Fourteen patients were treated across two different regimens. Patients with breast cancer and gastroesophageal cancer were primarily enrolled and had to have demonstrated overexpression of HER2 according to the American Society of Clinical Oncology/College of American Pathologists guidelines (HER2 immunohistochemistry 3+ or immunohistochemistry 2+/in situ hybridization-amplified). No lymphodepletion chemotherapy was used before infusion. The primary endpoints were safety and CAR-M manufacturability. Secondary endpoints included cellular kinetics and efficacy using objective response rate, overall survival, progression-free survival and duration of response. No dose-limiting toxicities, severe cytokine release syndrome (>= grade 3) or immune effector cell-associated neurotoxicity syndrome were observed; 44% (n = 4 of 9, 95% confidence interval = 14-79%) of HER2 3+ tumors achieved stable disease as best overall response 8 weeks after treatment. No meaningful activity was observed in the HER2 2+ population (n = 5). Correlative analyses of serial biopsies confirmed that CT-0508 traffics to and remodels the tumor microenvironment, resulting in expansion of CD8+ T cells. These findings demonstrate the preliminary safety, tolerability and manufacturing feasibility of CT-0508 for HER2+ tumors. ClinicalTrials.gov registration: NCT04660929.

Nanostring and flow cytometry immune profiling analysis

PI3K/AKT pathway activation may confer chemotherapy resistance, so combining PI3K-targeted therapy with oral chemotherapy is appealing but can have additive toxicity. This phase I study of alpelisib, an oral alpha-specific class I PI3K inhibitor, combined with capecitabine in patients with HER2-negative metastatic breast cancer found that the combination was safe, tolerable, and demonstrated clinical benefit. Background: Alpelisib is an oral alpha-specific class I PI3K inhibitor approved in combination with fulvestrant for the treatment of PIK3CA -mutated hormone receptor-positive (HR + ), human epidermal growth factor receptor 2 negative (HER2-) metastatic breast cancer. The tolerability of this drug with the oral chemotherapy capecitabine is unknown. Patients and Methods: This phase I trial evaluated the dose-limiting toxicities (DLTs) and maximum tolerated dose (MTD) of alpelisib (250 mg or 300 mg daily for 3-weeks) with capecitabine (1000 mg/m2 twice daily for 2-weeks followed by a 1-week rest period) in patients with metastatic HER2-negative breast cancer, regardless of PIK3CA mutation status. Results: Eighteen patients were treated with alpelisib-capecitabine. Half of the patients had HR + breast cancer, and 16 had prior systemic therapy for metastatic disease. The MTD of alpelisib was 250 mg daily in combination with capecitabine 1000 mg/m2 twice daily. DLTs included hyperglycemia, QTc prolongation, fatigue, and chest pain. The most common grade 3 adverse event (AE) was hyperglycemia (28%). No grade 4 AEs were observed. Three patients discontinued therapy due to an AE. One-third of patients required dose reduction of both alpelisib and capecitabine. Four patients experienced a partial response and 8 patients experienced stable disease. The median progression-free survival was 9.7 months (95% CI 2.8-13.5 months) and median overall survival was 18.2 months (95% CI 7.2-35.2 months). Twelve patients had PIK3CA mutation testing completed, of these 2 had known or likely deleterious PIK3CA mutation. Conclusion: This study provides safety data for an oral combination therapy of alpelisib-capecitabine and defines tolerable doses for further study.

Supplementary Figure S2. Progression-free survival.

In vivo depletion of CD8 and NK cells and tumor endpoint MHC-I expression

BackgroundThe PI3K–mTOR pathway is frequently dysregulated in breast cancer. Combining an inhibitor targeting all class I PI3K isoforms and mTOR complex 1 (mTORC1)–mTOR complex 2 (mTORC2) with endocrine therapy and a CDK4/6 inhibitor might provide more effective tumour control than standard-of-care therapy. To evaluate this hypothesis, gedatolisib, a pan-PI3K–mTOR inhibitor, was assessed in a phase 1b trial combined with palbociclib and endocrine therapy in patients with hormone receptor-positive, HER2-negative, advanced breast cancer. Results from the dose expansion portion of this trial are reported herein.MethodsThis multicentre, open-label, phase 1b study recruited female patients aged at least 18 years from 17 sites across the USA with hormone-receptor-positive, HER2-negative, advanced breast cancer and an Eastern Cooperative Oncology Group performance status of 0–1. Four patient groups were studied in the dose expansion portion of the study: treatment-naive in the advanced setting (first line; group A), progression on 1–2 lines of endocrine therapy but CDK4/6 inhibitor-naive (group B); and one or more previous lines (second-line and higher) of therapy, including a CDK4/6 inhibitor (groups C and D). Gedatolisib 180 mg was administered intravenously weekly in 28-day treatment cycles for groups A–C, and on days 1, 8, and 15 for group D. Letrozole (group A), fulvestrant (groups B–D), and palbociclib (all groups) were administered at standard doses and schedules. The primary endpoint was investigator-assessed objective response rate per RECIST version 1.1 in the evaluable analysis set. This trial is completed and registered with ClinicalTrials.gov, NCT02684032.FindingsBetween Dec 19, 2017, and June 19, 2019, 103 female participants were enrolled in the dose expansion groups A (n=31), B (n=13), C (n=32), and D (n=27). Median follow-up was 16·6 months (IQR 5·7–48·4) for group A, 11·0 months (7·6–16·9) for group B, 3·6 months (1·8–7·5) for group C, and 9·4 months (5·3–16·7) for group D for the primary endpoint. Gedatolisib, palbociclib, and endocrine therapy induced an objective response in 23 (85·2%; 90% CI 69·2–94·8) of 27 evaluable first-line participants (group A). In the second-line and higher setting, an objective response was observed in eight (61·5%; 90% CI 35·5–83·4) of 13 evaluable group B participants, seven (25·0%; 12·4–41·9) of 28 evaluable group C participants, and 15 (55·6%; 38·2–72·0) of 27 evaluable group D participants; this included participants with both wild-type and mutated PIK3CA tumours. The most common grade 3–4 treatment-related adverse events were neutropenia (65 [63%] of 103), stomatitis (28 [27%]), and rash (21 [20%]). Grade 3–4 hyperglycaemia was reported in six (6%) participants. 23 (22%) of 103 participants had a treatment-related serious adverse event, and there were no treatment-related deaths. Nine (9%) participants discontinued treatment because of a treatment-emergent adverse event.InterpretationGedatolisib plus palbociclib and endocrine therapy showed a promising objective response rate compared with the published results for standard-of-care therapies and had an acceptable safety profile.FundingPfizer and Celcuity.

Importance Agents targeting programmed death ligand 1 (PD-L1) have demonstrated efficacy in triple-negative breast cancer (TNBC) when combined with chemotherapy and are now the standard of care in patients with PD-L1–positive metastatic disease. In contrast to microtubule-targeting agents, the effect of combining platinum compounds with programmed cell death 1 (PD-1)/PD-L1 immunotherapy has not been extensively determined. Objective To evaluate the efficacy of atezolizumab with carboplatin in patients with metastatic TNBC. Design, Setting, and Participants This phase 2 randomized clinical trial was conducted in 6 centers from August 2017 to June 2021. Interventions Patients with metastatic TNBC were randomized to receive carboplatin area under the curve (AUC) 6 alone or with atezolizumab, 1200 mg, every 3 weeks until disease progression or unacceptable toxic effects with a 3-year duration of follow-up. Main Outcome and Measures The primary end point was investigator-assessed progression-free survival (PFS). Secondary end points included overall response rate (ORR), clinical benefit rate (CBR), and overall survival (OS). Other objectives included correlation of response with tumor PD-L1 levels, tumor-infiltrating lymphocytes (TILs), tumor DNA- and RNA-sequenced biomarkers, TNBC subtyping, and multiplex analyses of immune markers. Results All 106 patients with metastatic TNBC who were enrolled were female with a mean (range) age of 55 (27-79) years, of which 12 (19%) identified as African American/Black, 1 (1%) as Asian, 73 (69%) as White, and 11 (10%) as unknown. Patients were randomized and received either carboplatin (n = 50) or carboplatin and atezolizumab (n = 56). The combination improved PFS (hazard ratio [HR], 0.66; 95% CI, 0.44-1.01; P = .05) from a median of 2.2 to 4.1 months, increased ORR from 8.0% (95% CI, 3.2%-18.8%) to 30.4% (95% CI, 19.9%-43.3%), increased CBR at 6 months from 18.0% (95% CI, 9.8%-30.1%) to 37.5% (95% CI, 26.0%-50.6%), and improved OS (HR, 0.60; 95% CI, 0.37-0.96; P = .03) from a median of 8.6 to 12.6 months. Subgroup analysis showed PD-L1–positive tumors did not benefit more from adding atezolizumab (HR, 0.62; 95% CI, 0.23-1.65; P = .35). Patients with high TILs (HR, 0.12; 95% CI, 0.30-0.50), high mutation burden (HR, 0.50; 95% CI, 0.23-1.06), and prior chemotherapy (HR, 0.59; 95% CI, 0.36-0.95) received greater benefit on the combination. Patients with obesity and patients with more than 125 mg/dL on-treatment blood glucose levels were associated with better PFS (HR, 0.35; 95% CI, 0.10-1.80) on the combination. TNBC subtypes benefited from adding atezolizumab, except the luminal androgen receptor subtype. Conclusions and Relevance In this randomized clinical trial, the addition of atezolizumab to carboplatin significantly improved survival of patients with metastatic TNBC regardless of PD-L1 status. Further, lower risk of disease progression was associated with increased TILs, higher mutation burden, obesity, and uncontrolled blood glucose levels. Trial Registration ClinicalTrials.gov Identifier: NCT03206203

Abstract Background: In pre-clinical studies, CAR macrophages (CAR-M) phagocytose tumor cells, activate the tumor microenvironment (TME), recruit T cells, and induce anti-tumor T cell immunity. CT-0508 is a first-in-class CAR-M product comprised of autologous monocyte-derived macrophages expressing an anti-HER2 CAR. In pre-clinical models, anti-HER2 CAR-M was able to control the growth of syngeneic metastatic HER2+ breast cancer. Here we present preliminary clinical results and translational data from Group 1 of the CT-0508 Phase 1 FIH study. Methods: This multi-center, open-label study is evaluating CT-0508’s safety, tolerability, and manufacturing feasibility in 18 participants with advanced solid tumors overexpressing HER2 with progression on prior therapies. Monocytes are isolated from mobilized apheresis products, differentiated into macrophages, and engineered with an anti-HER2 CAR. Group 1 participants (n = 9) receive a fractionated dose on days 1, 3, 5 and Group 2 participants (n = 9) receive the full dose on day 1. CT-0508 is administered without preparative chemotherapy. Serial blood samples and biopsies (baseline and 2 post-treatment) are collected to investigate safety, pharmacokinetics, and mechanism of action. AU565 and 4T1-HER2 cell lines were utilized to model human and murine breast cancer, respectively, in vitro and in vivo. Results: Nine participants (6F/3M) have been treated in Group 1, comprising breast (4), esophageal (2), cholangiocarcinoma, ovarian, and parotid gland cancers, with a median age of 58. Participants had received a median of 3 (range, 2-11) prior lines of therapy; 8 had received prior anti-HER2 therapy. CT-0508 was successfully manufactured and well tolerated with no dose-limiting toxicities. Three related SAEs occurred in 2 participants: grade 1 CRS with hospitalization for monitoring and grade 2 infusion reaction that resolved within 1 hour were reported in one participant. Grade 2 CRS with fever and hypoxia occurred in another participant and resolved within ~ 72 hours. Five additional participants experienced Grade 1-2 CRS and/or infusion reactions with rapid resolution. There were no cases of Grade 3 or 4 CRS. There were no major organ toxicities. Post-infusion cytokines were transiently elevated in most participants enrolled in group 1 and were self-limiting. Four of the 7 participants evaluated had stable disease. CT-0508 was transiently detectable in the blood and was detected in the TME of 8/9 participants. CT-0508 modulated the TME, leading to myeloid cell activation, effector T cell infiltration, activation, and proliferation. TCR sequencing demonstrated newly expanding T cell clones in the blood post-treatment that accumulated within the TME, suggesting expansion of tumor-reactive T cells upon CT-0508 infusion. Data from participants enrolled in Group 1 will be presented. Most of these patients were breast cancer patients with HER2 overexpressing metastatic disease. Conclusions: CT-0508 was feasible to manufacture and had acceptable safety and tolerability. Early correlative data demonstrate trafficking, TME modulation, and induction of anti-tumor T cell immunity in participants with HER2 overexpressing solid tumors including metastatic breast cancer. The study is actively enrolling (NCT04660929). Citation Format: Yara Abdou, Joanne Mortimer, Paula Pohlmann, Melissa Johnson, Richard Maziarz, Jennifer M. Specht, Claire Dees, Naoto Ueno, Yuan Yuan, Mathew Angelos, Saar Gill, Olga Shestova, Jonathon Serody, Saul Priceman, Rehman Qureshi, Poonam Sonawane, Stefano Pierini, Maria Cecilia Oliveira-Nunes, Daniel Cushing, Michael Klichinsky, Thomas Condamine, Ramona Swaby, Kim Reiss. Translational insights from a phase 1, first-in-human (FIH) clinical trial of the anti-HER2 CAR macrophage CT-0508 in participants with HER2 positive metastatic breast cancer and other HER2 overexpressing solid tumors [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 PO2-04-08.

Abstract Purpose: This multicenter phase II basket trial investigated the efficacy, safety, and pharmacokinetics of Debio 1347, an investigational, oral, highly selective, ATP-competitive, small molecule inhibitor of FGFR1–3, in patients with solid tumors harboring a functional FGFR1–3 fusion. Patients and Methods: Eligible adults had a previously treated locally advanced (unresectable) or metastatic biliary tract (cohort 1), urothelial (cohort 2), or another histologic cancer type (cohort 3). Debio 1347 was administered at 80 mg once daily, continuously, in 28-day cycles. The primary endpoint was the objective response rate. Secondary endpoints included duration of response, progression-free survival, overall survival, pharmacokinetics, and incidence of adverse events. Results: Between March 22, 2019, and January 8, 2020, 63 patients were enrolled and treated, 30 in cohort 1, 4 in cohort 2, and 29 in cohort 3. An unplanned preliminary statistical review showed that the efficacy of Debio 1347 was lower than predicted, and the trial was terminated. In total, 3 of 58 evaluable patients had partial responses, representing an objective response rate of 5%, with a further 26 (45%) having stable disease (≥6 weeks duration). Grade ≥3 treatment-related adverse events occurred in 22 (35%) of 63 patients, with the most common being hyperphosphatemia (13%) and stomatitis (5%). Two patients (3%) discontinued treatment due to adverse events. Conclusions: Debio 1347 had manageable toxicity; however, the efficacy in patients with tumors harboring FGFR fusions did not support further clinical evaluation in this setting. Our transcriptomic-based analysis characterized in detail the incidence and nature of FGFR fusions across solid tumors. See related commentary by Hage Chehade et al., p. 4549

Abstract Introduction: Triple negative breast cancers (TNBCs) are a poor prognosis subset of breast cancer and preclinical data implicate activation of the RAS/MEK/ERK pathway in TNBC chemotherapy resistance. Preclinical data demonstrate that blockade of both RAS/MEK/ERK and PI3K/AKT pathways synergizes to overcome resistance to single agent MEK inhibitors. The purpose of this study was to determine the response rate for the selective allosteric MEK1/MEK2 inhibitor trametinib alone and in combination with AKT inhibitor uprosertib in patients with metastatic TNBC previously treated with chemotherapy. Patients and Methods: This was an open-label, two-part, phase II, single-arm, multicenter study through the ETCTN. Eligibility criteria included patients with metastatic TNBC with measurable disease who had received between 1-3 prior chemotherapy regimens. All patients underwent a mandatory pretreament biopsy and received trametinib alone (2mg) as Part I of the study. At progression, patients underwent a second biopsy and moved to Part II with trametinib (1.5mg) and uprosertib (50mg). Available biopsies were profiled quantitative targeted absolute proteomics (QTAP) for kinome assay and whole transcriptome profiling via microarray. Blood samples were collected at baseline, at cycle 2 day 1 (C2D1), and at progression for circulating tumor DNA (ctDNA) analysis via ultra-low pass whole genome sequencing. Results: Between October 2013 and January 2017, 37 patients were enrolled at 8 centers and, subsequently, 19 of these patients entered Part II. Adverse event data has been previously presented. On trametinib alone (Part I), 2 patients had a partial response (PR) and 6 patients had stable disease (SD) as best response. Of the 19 evaluable patients on trametinib + uprosertib (Part II), 3 patients had a PR and 3 patients had SD. Integrated analysis of matched samples with QTAP kinome and transcriptome data QTAP kinome assay in demonstrated MEK1/2 inhibition at progression on trametinib alone in 4/4 patients with evaluable tissue confirming target engagement and one patient with biopsy after trametinib+uprosertib demonstrated AKT1 inhibition. ctDNA was evaluable in 34 patients at C1D1 with tumor fraction (TFx) range 0-75.9%. 6/23 patients had ctDNA ‘clearance’ (tumor fraction of 0%) at C2D1. Of these 6, 3 (50%) had PR as best response, compared with only PR in only 1/17 patients without ctDNA clearance at C2D1. Conclusions: Trametinib alone and in combination with uprosertib demonstrates anti-tumor activity in a subset of TNBC patients with kinome evidence of target engagment. ctDNA clearance at C2D1 may be useful as an early biomarker to identify patients most likely to respond. Understanding biomarkers of response and resistance may guide future opportunities for MEK/AKT inhibition in TNBC. Citation Format: Vishnu Prasath, Hinda Boutrid, Ewa Mrozek, Maryam Lustberg, Robert Weslowski, Rachel Layman, Mahmoud Abdel-Rasoul, Cynthia Timmers, Erin MacRae, G. Thomas Budd, Lyndsay Harris, Claudine Isaacs, Roohi Ismail-Khan, Elizabeth Claire Dees, Andrew S. Poklepovic, Helen X. Chen, Miguel Villalona-Calero, William Carson, Gary L. Johnson, Daniel G. Stover, Bhuvaneswari Ramaswamy. Phase II study of trametinib alone and in combination with uprosertib in patients with metastatic triple negative breast cancer previously treated with chemotherapy: OSU 13317 [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 6427.

Abstract Background: Activation of the phosphoinositide 3-kinase (PI3K) pathway is a prominent resistance mechanism to endocrine therapy and CDK4/6 inhibition (CDK4/6i) in ER+, HER2- breast cancer. There is reciprocal crosstalk between PI3K and cell cycle regulatory pathways. Triplet therapy that targets ER, CDK4/6, and PI3K prevents or delays disease progression in preclinical models of ER+, HER2- breast cancer. However, the toxicities of triplet therapy hinder its clinical development. Copanlisib (COP) is an intravenous (IV) PI3Ki with potent activity against α and δ isoforms. Intermittent high-dose PI3Ki is expected to be less toxic and more effective than continuous daily dosing. We set out to determine the recommended phase 2 dose (RP2D) for the COP/Fulvestrant (FUL)/Abemaciclib (ABE) combination. Methods: Eligible patients (pts) included men or women with ER+, HER2- metastatic breast cancer (MBC), with no more than 1 prior chemotherapy in the metastatic setting, and no limits on prior endocrine or targeted therapy. Prior FUL, CDK4/6i and PI3K/mTORi were allowed. Pts with HbA1c >8.5% were excluded. Pts received FUL 500 mg IM standard dosing and COP/ABE at assigned doses. Premenopausal women also received a GnRH agonist. Dose-limiting toxicity (DLT) was assessed during the first 28-day cycle. Adverse events (AEs) were graded according to NCI-CTCAE v5.0. Dose escalation/de-escalation was based on continual reassessment (CRM) and the RP2D was defined as the dose with the highest probability of the DLT rate within the acceptable 25~35%. Results: Between June 2020 and June 2023, 24 pts with ER+, HER2- MBC were enrolled. Table 1 shows the dose levels (DLs) and DLTs. The first 10 pts were enrolled to Part A on two consecutive DLs of COP (DL1: 45 mg IV D1 and D15, n=7; DL2: 45 mg IV D1, D8, and D15, n=3), with ABE fixed at 100 mg PO BID continuously. DL2a exceeded the maximum tolerated dose (MTD). DL1a was tolerable based on Cycle 1 AEs. However, during subsequent cycles, only 1 of the 7 pts in DL1a maintained the intended dose of ABE due to neutropenia, others reduced ABE to 50mg bid (n=5) or discontinued (n=1). This triggered a protocol amendment, adding Part B with ABE administered 5 days on and 2 days off (5-on/2-off) intermittent schedule. 14 pts were enrolled to Part B in 2 consecutive DLs of COP (DL1b: n=7; DL2b: n=7), the same as in Part A. ABE was fixed at 100 mg PO Bid 5-on/2-off. DL2b exceeded the MTD. DL1b was tolerable based on Cycle 1 AEs. In addition, in subsequent cycles, only 1 of 7 pts on DL1b required dose reduction of ABE to 50mg bid. RP2D is therefore defined as DL1b. Grade (G) 4 AEs were rare. The most common all cycle G3 AEs included transient hypertension (45%), rash (30%), anemia (25%), neutropenia (25%), and AST elevation (20%). As of 6/14/2023, 22 pts, with a median of 1 prior metastatic regimen (range 0-5), prior CDK4/6i in 18 (82%), liver mets in 8 (36%), and PIK3CA mutation in 8 (50%) of 16 pts with known mutation status, were evaluable for response. There were 5 partial responses and 3 stable diseases lasting for ≥24 weeks. Clinical benefit rate was 36.4% (8/22, 95%CI: 17.19% ~ 59.34%). Responses were observed in pts regardless of prior CDK4/6i, FUL, or PIK3CA mutation. Conclusion: COP/FUL/intermittent ABE can be safely administered with fair overall tolerability. Preliminary anti-tumor activity was observed, which will be further examined in the randomized phase II trial (NCT 03939897). Table 1 Dose-limiting Toxicities by Dose Level Citation Format: Cynthia Ma, Jingqin Luo, Jessica Moss, Maryann Kwa, Ritesh Parajuli, Katia Khoury, Emily Douglas, Ashley Frith, Caron Rigden, Foluso Ademuyiwa, Shana Thomas, Brittney Haas, Gerburg Wulf, Claire Dees, Rabih Said. A phase I trial of the PI3K inhibitor (PI3Ki) copanlisib and fulvestrant in combination with continuous or intermittent abemaciclib in patients with estrogen receptor-positive (ER+), HER2-negative (HER2-) 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 PS17-06.

Antibody HLA specificity and kurtosis parameter