Abstract Background: In the HER2CLIMB trial, the addition of HER2-directed tyrosine kinase inhibitor tucatinib to trastuzumab and capecitabine improved progression-free survival (PFS) in patients with HER2-positive (HER2+) breast cancer brain metastases, with 1-year PFS of 24.9% on tucatinib versus 0% on placebo. While prior studies demonstrated genomic divergence between brain metastases and primary tumors, the molecular effects of tucatinib in the brain remain poorly characterized, in part due to limited availability of resected brain metastasis tissue. We compared genomic and transcriptomic features of HER2+ breast cancer brain metastases with and without tucatinib exposure to identify candidate molecular adaptations potentially relevant to tucatinib resistance. Methods: Brain metastases specimens were obtained from 17 patients with HER2+ breast cancer undergoing neurosurgical resection. All patients consented to donate tissue on an IRB-approved protocol. After quality control, ribosomal RNA-depleted RNA sequencing was performed on 10 tumors: 5 tumors that progressed after tucatinib treatment and 5 tucatinib-unexposed. Differential gene expression analysis was adjusted for hormone receptor (HR) status of brain metastases. Thirteen tumors were analyzed by Whole Exome Sequencing (WES): 7 tumors that progressed after tucatinib and 6 tucatinib-unexposed. Matched normal tissue was available for all samples for WES analyses. Somatic variants were annotated for predicted deleterious functional impact using SIFT and PolyPhen-2 software, as well as data from NCBI ClinVar and the “impact” metric from the Ensembl Variant Effect Predictor, and a variant allele frequency (VAF) threshold of ≥10 % was applied for global analyses. Results: After adjustment for HR status using the RNAseq data, 127 genes were differentially expressed between tucatinib-exposed and unexposed tumors. Tumors progressing on tucatinib showed enrichment of transcriptional programs related to stemness, epithelial-mesenchymal transition (EMT), and metabolic reprogramming. No transcriptional enrichment of established HER2 resistance pathways, including MAPK or PI3K/AKT signaling, was observed. Genomic profiling identified 17 genes with the highest number of activating mutations at VAF≥10% after tucatinib exposure that were predominantly involved in proliferation, migration, immune evasion and cancer stemness. No ERBB2 gatekeeper mutations (T798M, T798I) or common HER2 driver mutations (V777L) were detected at VAF≥10% after tucatinib exposure. Instead, tucatinib-exposed tumors more frequently harbored potentially deleterious mutations in ERBB family members (ERBB3 and ERBB4), ERBIN, and MET, whereas ERBB2 mutations, when present, occurred at low allelic frequency (<5%). Conclusion: In this exploratory cohort of resected HER2+ breast cancer brain metastases, tucatinib exposure was associated with genomic and transcriptomic patterns consistent with adaptive remodeling rather than classic HER2 resistance mutations. Alterations within the broader ERBB signaling network and activation of stemness / EMT-associated transcriptional programs may contribute to tucatinib resistance in the brain and warrant further investigation in larger longitudinal datasets. Citation Format: Savannah C. Roy, Anna Guarnieri, Pradeep Bhartiya, Thomas Danhorn, Andrew Goodspeed, Hannah Parris, Virginia F. Borges, James Costello, David R. Ormond, Elena Shagisultanova. Genomic and transcriptomic changes associated with tucatinib exposure in HER2-positive breast cancer brain metastases [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(8_Suppl):Abstract nr LB166.

Abstract RNA serves a central role in biology by converting genomic information into effector molecules, either as functional non-coding RNAs or as protein-coding mRNAs. While it has long been appreciated that complex RNA transcript profiles can be produced through alternative splicing, numerous discoveries have highlighted the essential role of alternative splicing in cell and developmental biology. Furthermore, aberrant splicing has recently been linked to diseases like cancer, neurodegeneration, and autoimmunity. Of particular interest, cancer-specific splice isoforms have emerged as a potential source of neo-antigens targetable by novel immune therapeutics. Thus, understanding the expression and function of RNA isoforms has become increasingly important in cancer biology research. A current limitation of long-read RNA sequencing (LR-RNA-seq) is the requirement of large amounts of input RNA, which can be unachievable for samples such as resected tumors or sorted single cells. Here, we describe SMART-Seq® mRNA Long Read kit, a new LR-RNA-seq library preparation technology that enables full-length RNA sequencing from single cells (∼10 pg RNA/cell) up to 100ng total RNA. In high-quality bulk RNA inputs, we demonstrate the ability to reliably sequence at an average read length (N50) of 2 kb and to detect full-length transcripts as long as 8 kb, enabling the discovery and quantification of novel mRNA isoforms in samples from both healthy tissues and cancer cells. Analysis of cancer cell lines with evolved resistance to targeted therapies identifies differential isoform usage associated with the evolution of cancer therapeutic resistance. We further describe an update to this technology, SMART-Seq® mRNA Long Read version 2, which expands the input range to 2µg, improves read-length performance, and enables UMI-based analysis. Comparison studies demonstrate that SMART-Seq mRNA Long Read technology substantially outperforms existing bulk and single-cell LR-RNA-seq methods. With PCR barcoding of up to 96 samples at a time, this technology will accelerate discovery as scientists catalog and study splice isoforms in both routine long read cDNA sequencing workflows and in settings where sample input is limited. Citation Format: Jackson Peterson, Yue Yun, Lisa Welter, Kazuo Tori, Alan Du, Yana Ryan, Ning Ma, Rachana Kumar, Shiyi Yin, Mike Covington, Shuwen Chen, Elena Shagisultanova, Mohammad Fallahi, Bryan Bell, Andrew Farmer. RNA isoform discovery and quantification with SMART-Seq® mRNA Long Read (v1 and v2) kits [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1789.

Background: Activating mutations in the gene of phosphotydylinisotol-3 kinase catalytic subunit alpha (PIK3CA) are present in up to 40% of breast cancers overexpressing human epidermal growth factor receptor 2 (HER2). These mutations are linked to resistance to HER2-targeted agents and poorer patient outcomes. Based on preclinical efficacy, we are conducting a study to evaluate the combination of HER2-inhibitor tucatinib and PI3Kα-inhibitor alpelisib in patients with HER2-positive (HER2+) PIK3CA-mutated metastatic breast cancer (MBC) (NCT05230810). Methods: This phase IB study aims to determine the maximal tolerated doses of tucatinib and alpelisib using the Time-to-Event Bayesian Optimal Interval Design, with dose limiting toxicity (DLT) window of 28 days. Safety is evaluated by CTCAE v.5.0 with standard definition for DLTs. Eligible participants include post-menopausal women or pre-menopausal women undergoing ovarian suppression, diagnosed with HER2+ PIK3CA-mutated MBC, and previously treated with at least two HER2-targeted agents. Patients are allowed 1 prior line of HER2 tyrosine kinase inhibitor for MBC, including prior tucatinib. Treatment consists of twice-daily tucatinib and daily alpelisib at prespecified dose level (DL) with concurrent fulvestrant for hormone receptor-positive cases. Results: As of July 1st, 2024, 8 patients have been treated. The median age was 53 years (range: 45-66), with a median of 2 prior lines of therapy for MBC (range: 1-4). Prior HER2-targeted therapies included trastuzumab and pertuzumab (8 patients), T-DM1 (5 patients), tucatinib (4 patients), T-DXd (4 patients), and margetuximab (1 patient). Six patients had visceral metastases and 4 had CNS metastases. No DLTs were observed in 4 patients treated at DL1 (tucatinib 300 mg BID, alpelisib 250 mg daily), with one additional patient in the DLT window at the time of data cut-off. At DL2 (tucatinib 300 mg BID, alpelisib 300 mg daily), all 3 patients experienced DLT. Two patients had grade 3 (G3) diarrhea despite maximal anti-diarrheal regimen that resolved with treatment hold and dose reduction. One patient developed G3 rash with mucositis, eosinophilia and G4 elevated creatinine and came off study, all symptoms resolved without sequelae with hydration and steroid treatment. Other notable adverse events, primarily G1-2, included hyperglycemia (6 patients), decreased appetite (5 patients), weight loss (5 patients), fatigue (5 patients) and elevated liver enzymes (4 patients). These side effects were managed with supportive medications, nutritionist consultations and avoidance of hepatotoxic agents. No new safety signals were observed from the study drugs. One heavily pretreated patient developed progressive CNS metastases after 6 months on study and died on hospice within 30 days of follow up. At the time of data cutoff, 5 patients were evaluable for tumor response: 2 had stable disease, and 3 had partial response (PR). All patients who experienced PR had PIK3CA H1047R mutation. All PRs occurred after 2 cycles of therapy and included significant reduction of breast tumors, liver and lung metastases and resolution of cancer lymphangitic spread in the lungs. Four out of 8 patients remained on the regimen for more than 6 months. The longest treatment duration is 15 months and ongoing. Conclusions: The combination of tucatinib and alpelisib is tolerable at DL1 and shows remarkable antitumor activity with partial responses in 3 out of 5 evaluable patients (60% overall response rate), including responses in patients treated with prior tucatinib and T-DXd. Enrollment continues at DL1. Updated safety and efficacy findings will be presented at SABCS 2024 conference. Citation Format: Elena Shagisultanova, Chelsea D. Gawryletz, Colleen Dougherty-Gray, Leah Adams, Amelia Hardeman, Michelle Loch, Marina Sharifi, Peter Kabos, Virginia Borges. Safety and Preliminary Efficacy of Tucatinib and Alpelisib in Patients with HER2-positive PIK3CA-Mutated Metastatic 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 P5-03-10.

Background: The majority of ER+ breast cancers (BC) express androgen receptors (AR). In a randomized phase II trial for women with ER+/HER2- primary BC T2 or greater, neoadjuvant fulvestrant (Fulv) alone or with enzalutamide (Combo) was given for 4 months prior to surgery. A total of 59 patients were evaluable: 33 on Combo and 26 on Fulv. The addition of AR blockade to Fulv reduced residual tumor at time of surgery as measured by modified preoperative endocrine predictive index (PEPI) score. Fresh tumor biopsies were required at study entry (baseline), after 4 weeks on therapy (W5), and at surgery. The Combo arm achieved PEPI=0 more frequently (24%: 8/33) than Fulv (8%: 2/26). Interestingly, the odds of response were 4.6-fold (95% CI: 0.9-22) higher for patients with invasive lobular cancer (IDC) versus invasive ductal (IDC). Results: When examining all tumors, gene expression analyses showed significantly decreased estrogen response and cell division gene sets in tumors in both arms; however, only Combo treated tumors exhibited significant enrichment of immune activation genes sets, including interferon gamma, complement, inflammation, antigen processing, and B and T cell activation. AR protein was significantly reduced by time of surgery in the Combo arm only (P<0.05) as measured by immunohistochemistry. AR was also significantly lower at time of surgery in the PEPI=0 as compared to PEPI >0 (P<0.05) and in the Ki67 responders versus non-responders (p<0.02). Because of the 4.6-fold higher odds of PEPI=0 response in ILCs versus IDC, we examined phosphoproteins that changed with treatment in these two histologic groups by reverse phase phosphoprotein assay (RPPA) from frozen tumor sections. Both AR and phosphoS650 AR showed significantly more decrease from BL to W5 in ILC versus IDC. Cyclin D1, S6RP, HIF-1 alpha, ATP citrate lyase, were significantly lower in ILC than IDC, while CHK1 and ALK were higher. As would be expected with the higher odds of response in ILC, cell cycle proteins decreased significantly more with treatment in this histologic subtype, as did growth/survival and metabolism proteins. Polaris Multiplex immunofluorescence, used to study the tumor immune microenvironment (TIME), revealed that the number of tertiary lymphoid structures (TLS) per area surrounding resected tumor at time of surgery was higher in the Combo arm and was significantly higher near tumors that achieved PEPI=0 (P<0.03). Additionally, the average number of TLS/mm2 was higher in the ILC versus IDC (P < 0.059). T regulatory cells were reduced in the Combo arm (p<0.0002) and in Ki67 responsive tumors (p<0.004); however, T regs were not significantly different in ILC versus IDC. Tumor-associated macrophages decreased by time of surgery only in the Combo arm (p<0.0001), only in tumors that achieved PEPI=0 (p<0.0005) and trended towards decreased numbers in ILC vs IDC. Conclusions: AR inhibition in combination with a SERD activates the immune system in ER+ BC, particularly in ILC. Citation Format: Jennifer Richer, Anthony D. Elias, Alyse W. Staley, Monica Fornier, Gregory A. Vidal, Vida Alami, Sharon Sams, Nicole S. Spoelstra, Andrew Goodspeed, Peter Kabos, Jennifer R. Diamond, Elena Shagisultanova, Rosa I Gallagher, Julia Wulfkuhle, Emanuel Petricoin, Kathryn Zolman, Tessa McSpadden, Christian Rickert, Kimberly R. Jordon, Jill E. Slansky, Virginia F. Borges, Dexiang Gao. Differential odds of response in ILC versus IDC correlate with changes in the TIME in a phase II trial of pre-operative fulvestrant with or without enzalutamide [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 PS18-07.

PFS before and after palbociclib dose change

Grade 3 and 4 AEs - palbociclib 125mg vs 75mg

PURPOSE:HER2-targeted tyrosine kinase inhibitors (TKIs) tucatinib, lapatinib and neratinib are standard agents for treatments of HER2-positive (HER2+) metastatic breast cancer. Due to their penetration through the blood-brain barrier, these agents are especially beneficial for patients with brain metastases. To date, limited data exists on the sequential use of different HER2-targeted TKIs, and it is unknown if reusing the same TKI as a part of different treatment regimens provides clinical benefits. METHODS:We performed a retrospective single institution study (IRB 23-2337) and identified N = 105 patients with HER2+ metastatic breast cancer and a TKI regimen of at least 1 month. We then performed a manual abstraction to identify patients who had at least 2 TKI regimens (N = 24) and estimated time on treatment (median and range in months). Best responses to TKI treatments were evaluated by MD review. Patients were censored at the time of the last follow up when the end of treatment data were not available. RESULTS:In our patient cohort, 14 of 24 patients (58%) stayed on the second or third TKI regimen for greater than 6 months, and 10 of 24 patients (42%) were on the subsequent TKI regimen(s) for longer than the first one. Time on a non-TKI regimen between the first and second TKI was not associated with the duration of benefit from the second TKI. CONCLUSION:Our data support using different HER2-targeted TKI regimens in sequence, offering additional effective treatment options for patients with HER2+ metastatic breast cancer, including patients with brain metastases.

Breast cancer is the most common malignancy among women of reproductive age, with many women affected before or soon after childbearing. Treatments frequently increase the risk of ovarian injury and infertility, making early family-building decisions and timely counseling essential. Extended adjuvant endocrine therapy can further complicate pregnancy planning; however, recent evidence—including results from the POSITIVE trial—demonstrates that temporarily interrupting treatment to pursue childbearing and breastfeeding is safe for appropriately selected patients. In addition to fertility loss, many patients experience sexual dysfunction and diminished quality of life due to prolonged estrogen deprivation. Despite the availability of successful fertility preservation technologies, many survivors report inadequate counseling and limited access, particularly among minorities and underserved populations. This manuscript reviews fertility risk stratification, preservation options—including implications of ovarian tissue freezing—sexual health management, contraception, and survivorship care. Emphasis is placed on early, individualized counseling, equitable access, and multidisciplinary collaboration to optimize reproductive and sexual health in cancer care.

Representativeness of the study participants

Women diagnosed under the age 45 and within 5-10 years of a childbirth have increased risk of metastasis. These high-risk cancers, defined as post-partum breast cancer (PPBC), have significantly worse prognosis independent of clinical-pathologic features and demonstrate treatment resistance. We hypothesized that PPBC exhibits high risk molecular profile and sought to define the genomic patterns of PPBC. The prospective, observational FLEX Study (NCT03053193) includes stage I-III early BC patients who received MammaPrint®(MP) and/or BluePrint®(BP) testing and consented to full transcriptome and clinical data collection. We identified 377 HR positive and HER2-negative patients, age ≤50, grouped as PP 0-5 years [n=33]; PP 6-10 years [n=51]. Two control groups PP >10 years [n=175] and nulliparous [n=118] were used for comparison. Median age, clinical characteristics, metabolic factors, and genomic test results (MP and BP) were compared between PP groups and nulliparous. Limma R package was used for differential gene expression analysis in age-matched cohorts. Pathway analysis was conducted using Metascape. PPBC 0-5 had a significantly higher frequency of MP High 2 and lower frequency of MP Low Risk compared to nulliparous women (p=0.02). PP 0-5 had higher frequency of Luminal B and Basal compared to PP >10 years (p=0.01). In gene expression analysis, PP 0-5 cohort had 25 genes with 2-fold higher expression compared to nulliparous including genes like MUCL1, CLEC3A, CXCL9. Pathway analysis demonstrated increased adaptive immune-mediated and humoral- response pathways in PP 0-5 years. In this study, PPBC 0-5 years of childbirth demonstrates significantly higher risk of recurrence (MP High 2) and increased immune gene expression, other unique genes/pathways related to PPBC biology compared to BC occurring >10 years after childbirth or in nulliparous women. These findings are consistent with the poor clinical outcomes previously reported for PPBC. Increased frequency of MP High 2 tumors and immune biology indicate that PPBC might benefit from adding immunotherapy, to be explored in future trials.

Most ER+ breast cancers (BC) express androgen receptors (AR). This randomized phase II trial of 4 months of neoadjuvant fulvestrant (Fulv) alone or with enzalutamide (Combo) assessed whether adding AR blockade to Fulv would limit residual tumor at the time of surgery, as measured by modified preoperative endocrine predictive index (PEPI) score. Eligible patients were women with ER+/HER2- primary BC cT2 or greater. Stratification factors were clinical node and T-stage. Fresh tumor biopsies were required at study entry, after 4 weeks on therapy (W5), and at surgery. Laboratory analyses on tumors included immunochemistry (IHC) for ER/PR/AR/GR and Ki67 protein, evaluation of gene expression, multiplex for myeloid lineage immune cells, reverse-phase protein array, and plasma metabolomic analyses. Of 69 consented patients, 59 were evaluable. Toxicity was as expected with endocrine therapy. Combo achieved PEPI = 0 more frequently (24%: 8/33) than Fulv (8%: 2/26). Ki67 was <= 10% across arms by W5 in 76% of tumors. Activation of mTOR pathway proteins was elevated in tumors with poor Ki67 response. Tumors in both arms showed decreased estrogen-regulated and cell division gene sets, while Combo arm tumors uniquely exhibited enrichment of immune activation gene sets, including interferon gamma, complement, inflammation, antigen processing, and B and T cell activation. Multiplex IHC showed significantly reduced tumor-associated macrophages and CD14+/HLADR-/CD68- MDSCs in Combo tumors at W5. In summary, Combo tumors showed a higher PEPI = 0 response, Ki67 response, and more activated tumor immune microenvironment than Fulv. The odds of response were 4.6-fold higher for patients with ILC versus IDC. (Trial registration: This trial is registered at Clinicaltrials.gov (https://www.clinicaltrials.gov/study/NCT02955394?id=16-1042&rank=1). The trial registration number is NCT02955394. The full trial protocol is available under Study Details at the Clinicaltrials.gov link provided).

Up to 50% of patients (pts) with HER2+ breast cancer will develop brain metastases throughout the course of disease, and approximately 50% of them will die from intracranial disease progression. Tyrosine kinase inhibitors (TKIs) play a significant role in the treatment of HER2+ metastatic breast cancer (MBC) due to their ability to cross the blood brain barrier and prevent and treat brain metastases. In the US, available TKIs include neratinib, lapatinib, and tucatinib, which was approved in 2020 based on the HER2CLIMB trial. There is limited data on the sequential use of these agents. Here we present real-life data on sequential use of TKIs for treatment of HER2+ MBC. We performed a retrospective study (IRB 23-2337) and identified N=109 pts with HER2+ MBC and a TKI regimen with duration of ≥1 month. We then performed a manual abstraction to identify pts who had at least 2 TKI regimens and calculated time on treatment (median and range in months). Pts were censored at the time of the last follow up when data on the end of treatment were missing. For pts on active treatment, data cutoff was February 7, 2024. We identified N=22 pt who had confirmed sequential TKI regimens. Specific HER2 TKIs, order of the regimen use, and time on treatment are summarized below. The data show persistence of benefit for a second TKI use, with 64% of pts staying on a second TKI regimen for >6 months. In this small subset, tucatinib after any TKI or after prior tucatinib showed clinical benefit.Table: 207PN1st TKITime on treatment2nd TKITime on treatment7Tucatinib12 (1 - 24)Tucatinib11 (3 - 29)7Tucatinib8 (4 - 22)Neratinib or Lapatinib4 (1 - 9)8Neratinib or Lapatinib9 (1 - 16)Tucatinib20 (2 - 36)22Any TKI10 (1 - 24)Any TKI12 (1 - 36)N – number of pts; time on treatment is presented as median (range) in months. Open table in a new tab N – number of pts; time on treatment is presented as median (range) in months. These data support using HER2 TKI regimens in sequence and merit further exploration. Ongoing studies of HER2 TKIs that allow for prior TKIs (NCT05230810, NCT05458674, and others) will help gain further insights. Detailed clinical characteristics of our study will be presented at the ESMO Breast 2024.

e13021 Background: Activating mutations in the gene of phosphotydylinisotol-3 kinase catalytic subunit ( PIK3CA ) are present in up to 40% of breast cancers overexpressing human epidermal growth factor receptor 2 (HER2). These mutations are associated with resistance to HER2-targeted agents and adverse patient (pt) outcomes. Based on preclinical efficacy, we are conducting phase IB trial of HER2 inhibitor tucatinib combined with PI3Kα inhibitor alpelisib in pts with HER2-positive (HER2+) PIK3CA -mutated metastatic breast cancer (MBC). Methods: This phase IB study is aimed at finding maximal tolerated doses of tucatinib and alpelisib using Time-to-Event Bayesian Optimal Interval Design, with dose limiting toxicity (DLT) window of 28 days. Safety is assessed by CTCAE v.5.0 with standard definition for DLTs. Post-menopausal women, or pre-menopausal women on ovarian suppression, diagnosed with HER2+ PIK3CA -mutated MBC, and treated with at least 2 HER2-targeted agents are eligible. Pts are allowed 1 prior line of HER2 tyrosine kinase inhibitor for MBC, including tucatinib. Treatment consists of twice-daily tucatinib and daily alpelisib at prespecified dose level (DL) with concurrent fulvestrant for hormone receptor-positive cases. Results: As of January 11, 2024, 6 pts have been treated. The median age was 54 years (range: 47-66), with a median of 2.5 prior lines of therapy for MBC (range: 1-4). Previous HER2-targeted therapies included trastuzumab and pertuzumab (6 pts), T-DM1 (5 pts), tucatinib (4 pts), T-DXd (3 pts) and margetuximab (1 pts). Five pts had visceral metastases and 3 had CNS metastases. No DLTs were observed in 3 pts treated at DL1 (tucatinib 300mg BID, alpelisib 250mg daily). However, at DL2 (tucatinib 300mg BID, alpelisib 300mg daily), all 3 pts experienced DLT. Two pts had grade 3 (G3) diarrhea that resolved with treatment hold; they resumed therapy with dose reduction. One pt developed G3 rash with mucositis, eosinophilia and G4 elevated creatinine and came off study; these symptoms resolved without sequelae following hydration and steroid treatment. Other notable adverse events, primarily G1-2, included hyperglycemia (4 pts), elevated liver enzymes (4 pts), weight loss (3 pts), and infections (3 pts), all manageable with supportive medications. There was no new safety signal from the study drugs, and no death on study. Two pts had a partial response after 2 treatment cycles, showing reduction of liver and lung metastases and resolution of cancer lymphangitic spread in the lungs. The longest treatment duration is currently 9 months and ongoing. Conclusions: Tucatinib and alpelisib combination is tolerable at DL1 and demonstrates promising antitumor activity, including responses in pts treated with prior tucatinib and T-DXd. Enrollment continues at DL1. Updated safety and efficacy findings will be presented at ASCO 2024 conference. Clinical trial information: NCT05230810 .

Background: SERENA-1 (NCT03616587) is a Phase 1, multi-part, open-label study of camizestrant in women with ER+/HER2− advanced breast cancer. Parts A/B and C/D (escalation/expansion) examined camizestrant as monotherapy and in combination with palbociclib respectively and have been presented previously.1,2 Here we present data from parts G/H which examined camizestrant in combination with abemaciclib. Methods: The primary objective was to determine the safety and tolerability of camizestrant 75 mg once daily (QD) in combination with abemaciclib 150 mg twice daily (BID). Secondary objectives included investigation of anti-tumor response and pharmacokinetics (PK). Participants were previously treated women of any menopausal status (pre-menopausal women received this combination alongside ongoing ovarian function suppressors). Prior treatment with ≤2 lines of chemotherapy in the advanced setting was permitted. There was no limit on the number of lines of prior endocrine treatment in the advanced setting; previous treatment with CDK4/6 inhibitors (CDK4/6i) and fulvestrant was permitted. Results: As of 1st June 2022, 24 patients had received camizestrant in combination with abemaciclib with a median 7.7 month follow up. Tolerability of the combination of camizestrant and abemaciclib was consistent with that of each drug individually. No patient required camizestrant dose reduction. All camizestrant-related heart rate decreases were Grade 1 (asymptomatic). PK data for camizestrant in combination with abemaciclib were consistent with camizestrant as monotherapy and published abemaciclib steady-state PK data, indicating no clinically relevant drug-drug interaction. In these heavily pre-treated patients (46% prior chemotherapy, 75% prior CDK4/6i, 54% prior fulvestrant; all in the advanced disease setting) and of whom 67% had visceral metastases, the objective response rate was 5/19 (26.3%), the clinical benefit rate at 24 weeks was 16/24 (66.7%) and the median progression-free survival had not been reached, with 8/24 patients experiencing a progression event. These data support the use of camizestrant 75 mg QD combined with the approved abemaciclib dose. Conclusions: Camizestrant 75 mg QD in combination with abemaciclib 150 mg BID was well tolerated with encouraging clinical activity. The inclusion of this regimen in the ongoing Phase 3, SERENA-6 trial 3, of camizestrant combined with CDK4/6i versus an aromatase inhibitor, will further clarify the role of this combination in the treatment of patients with ER+/HER2− advanced breast cancer with tumors expressing ESR1 mutations. References 1. Baird R, Oliveira M, Ciruelos Gil EM, et al. SABCS 2020 Virtual Meeting. Abstract PS11-05. 2. Oliveira M, Hamilton EP, Incorvati J, et al. J Clin Oncol 40, 2022 (suppl 16; abstr 1032). 3. SERENA-6 trial. Available at https://clinicaltrials.gov/ct2/show/NCT04964934 We acknowledge Helen Heffron, PhD, from InterComm International who provided medical writing support funded by AstraZeneca. Citation Format: Nicholas Turner, Christos Vaklavas, Emiliano Calvo, Javier Garcia-Corbacho, Jason Incorvati, Manuel Ruiz Borrego, Chris Twelves, Anne Armstrong, Begoña Bermejo, Erika Hamilton, Mafalda Oliveira, Eva Ciruelos, Peter Kabos, Manish R Patel, Maria Borrell, Howard Burris, Bruno de Paula, Alejandro Falcon, Cristina Hernando, Irene Moreno, Ciara S. O’Brien, Elena Shagisultanova, Ivan Victoria Ruiz, Judy S. Wang, Mei Wei, Tim Brier, Danielle Carroll, Carmela Ciardullo, Lisa Gibbons, itziar irurzun-Arana, Tony Jack, bistra kirova, Teresa Klinowska, Justin Lindemann, Julie Maidment, Alastair Mathewson, Rhiannon Maudsley, Robert McEwen, Christopher Morrow, Andy Sykes, Richard D. Baird. SERENA-1: Updated analyses from a Phase 1 study of the next generation oral selective estrogen receptor degrader camizestrant (AZD9833) combined with abemaciclib, in women with ER-positive, HER2-negative advanced breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P3-07-28.

Correlation between marker expression intensity and sample collection year. Corr. = correlation coefficient, A.U. = arbitrary unites.

Supplementary Table S1 (Excel file). TCGA datasets; Supplementary Table S2. CART cutoff points for each marker and the number of samples (N) in each category; Supplementary Table S3. Adjusted survival analysis; Supplementary Table S4. TCGA p16 expression data for HPV- and HPV+ HNSCC.