e12562 Background: Tumor-infiltrating lymphocytes (TILs) are a robust prognostic biomarker in triple-negative breast cancer (TNBC). However, conventional whole-slide mean density overlooks spatial heterogeneity and potential signals from localized hot spots. We systematically analyzed TIL distribution across multiple spatial scales to identify the optimal evaluation method for prognostication. Methods: We analyzed 61 node-positive TNBC patients who underwent primary surgery without neoadjuvant therapy, selected from 3,902 breast cancer resection cases (2002-2016). Our AI-based pathological image analysis tool, DeepPathFinder, segmented epithelium and lymphocytes, with stroma defined as non-epithelial regions. Pathologist-annotated tumor bed regions were divided into square tiles of 250, 500, and 1000 μm for multi-scale analysis. For each patient and scale, we calculated stromal lymphocyte density per tile and derived patient-level distribution quantiles (Q25, Q50, Q75, and Q90). The conventional TILs score was defined as the ratio of lymphocyte area to stromal area across the entire WSI. Patients were dichotomized into high and low groups based on the cohort median. Survival curves were compared using the log-rank test, and hazard ratios (HR) were estimated via Cox regression. Results: With 23 DFS and 21 OS events, higher quantiles (Q75, Q90) consistently showed superior stratification across all scales. For DFS, 1000-μm hot spot density (Q90) provided the most robust stratification (HR 0.25, 95% CI 0.10-0.63, p = 0.002), significantly outperforming the conventional TILs score (HR 0.40, p = 0.032). For OS, both methods showed similar value (Q90: HR 0.28, p = 0.006; TILs score: HR 0.29, p = 0.006). Conclusions: Optimizing spatial resolution to capture 1000-μm high-density hot spots provides stronger prognostic signals than whole-slide averages, highlighting spatial immune architecture as a critical survival determinant in TNBC. Multi-scale prognostic analysis of stromal TILs density metrics. Metric 250μm DFS 250μm OS 500μm DFS 500μm OS 1000μm DFS 1000μm OS Q25 -** -** 0.68 (0.30-1.55) 0.3584 0.62 (0.26-1.48) 0.2775 0.41 (0.17-0.96) 0.0345* 0.35 (0.14-0.88) 0.0201* Q50 0.41 (0.17-0.96) 0.0345* 0.35 (0.14-0.88) 0.0201* 0.41 (0.17-0.96) 0.0345* 0.35 (0.14-0.88) 0.0201* 0.43 (0.18-1.01) 0.0462* 0.37 (0.15-0.93) 0.0276* Q75 0.35 (0.14-0.84) 0.0146* 0.37 (0.15-0.93) 0.0276* 0.26 (0.10-0.66) 0.0023* 0.27 (0.10-0.71) 0.0046* 0.33 (0.14-0.81) 0.0104* 0.36 (0.14-0.90) 0.0221* Q90 0.26 (0.10-0.66) 0.0023* 0.28 (0.11-0.73) 0.0054* 0.33 (0.14-0.81) 0.0104* 0.37 (0.15-0.92) 0.0252* 0.25 (0.10-0.63) 0.0015* 0.28 (0.11-0.73) 0.0057* Note: Values shown as HR (95% CI) p-value. *p<0.05. **Excluded due to predominance of zero values (>70%), preventing stratification.

Limited data exist on the prevalence of human epidermal growth factor receptor 2 (ERBB2/HER2) amplification in patients with all types of solid tumors. This retrospective, observational study (UMIN ID: UMIN000057382) analyzed the prevalence of HER2 amplification across all solid tumors using comprehensive genomic profiling (CGP) data from the Center for Cancer Genomics and Advanced Therapeutics database in Japan. We analyzed 89,374 eligible patients with solid tumors: HER2 amplification was detected in 5119 patients (5.7%). The highest rates of HER2 amplification were observed in patients with tumors of the esophagus/stomach (12.9%), followed by tumors of the bladder/urinary tract (10.6%), breast (9.5%), biliary tract (8.4%), and uterus (8.4%). Among the five assay platforms, FoundationOne CDx accounted for 69.7% of all CGP tests and had an HER2 amplification detection rate of 7.4%, compared to the other four platforms (range: 1.9-15.4% of all CGP tests [detection rates: 1.5-2.3%]). Substantial differences were observed in the mutation frequencies of multiple genes between HER2 amplified and HER2 non-amplified tumors. The results highlight that HER2 amplification extends beyond conventional tumor types and enables the identification, via CGP testing, of non-traditional tumor subsets (cancers other than breast and gastric cancer), including rare cancers, that could be candidates for HER2-targeting therapy such as trastuzumab deruxtecan in Japan.

OBJECTIVE:The aim of this study was to evaluate the feasibility of photoimmunotherapy (PIT) using ASP-1929, an anti-epidermal growth factor receptor (EGFR) antibody-dye conjugate, for vulvar, vaginal, and cervical cancers by validating EGFR expression and establishing immunohistochemistry (IHC)-based eligibility criteria for a phase II trial, HICARi study. METHODS:Cetuximab-binding activity was analyzed in gynecologic cancer cell lines (CaSki, HeLa, and Ishikawa) using flow cytometry and compared with positive controls (A431 and BxPC-3) and a low-EGFR control (MCF-7). IHC was performed on cell blocks and clinical specimens of vulvar, vaginal, and cervical cancers (validation cohort, n=27; screening cohort, n=40). RESULTS:CaSki demonstrated comparable or higher EGFR binding than BxPC-3, whereas HeLa and Ishikawa showed weaker binding. IHC staining intensity correlated with flow cytometry findings. Squamous cell carcinomas consistently exhibited moderate-to-strong EGFR expression, whereas non-squamous carcinomas displayed variable expression. EGFR positivity was defined as >30% of tumor cells with at least moderate membranous staining (2+ or 3+). In the validation cohort, EGFR positivity was observed in 71.4% of the vulvar, 80.0% of the vaginal, and 87.5% of the cervical cancers. The screening cohort showed similar rates: 88.9% of vulvar, 100% of vaginal, and 75.0% of cervical cancers. CONCLUSION:We demonstrated correlation between cetuximab-binding and IHC staining intensity, establishing IHC criteria (≥2+) for patient selection. EGFR positivity (71.4%-87.5%) indicates substantial eligibility for ASP-1929 PIT. Therapeutic efficacy will be evaluated in the HICARi study.

Lung cancer continues to be associated with a dismal prognosis, despite the recent development of multiple therapeutic modalities. Consequently, there remains an urgent need for the advancement of novel and more efficacious treatment strategies. Cluster of Differentiation 44 (CD44), a cell surface marker associated with cancer stem cells, has been implicated in tumor progression, metastasis, and resistance to therapy, thereby representing a compelling target for oncologic intervention. Near-infrared photoimmunotherapy (NIR-PIT) is an emerging cancer treatment modality that integrates the high specificity of monoclonal antibodies with cytotoxic effects induced by a photosensitizer upon exposure to NIR light. In the present study, we evaluated the therapeutic efficacy of CD44-targeted NIR-PIT utilizing an orthotopic lung cancer mouse model established via transtracheal tumor cell injection, which more faithfully recapitulates the tumor microenvironment compared to conventional subcutaneous models. An antibody–photoabsorber conjugate was synthesized by conjugating an anti-CD44 monoclonal antibody (IM7) with IRDye700DX (IR700). The cytotoxic effects of CD44-targeted NIR-PIT were assessed in vitro using CD44-expressing lung cancer cell lines. Therapeutic efficacy was subsequently evaluated in an orthotopic lung cancer mouse model established by transtracheal injection of tumor cells. Tumor progression was monitored by bioluminescence imaging, and survival and histopathological changes were analyzed following treatment. In vitro experiments employing CD44-expressing cell lines demonstrated that CD44-targeted NIR-PIT induces cytotoxicity in a light dose-dependent manner. The in vivo therapeutic efficacy was subsequently assessed using the orthotopic lung cancer mouse model. Following CD44-targeted NIR-PIT, bioluminescence imaging revealed a marked reduction in tumor burden, accompanied by an exploratory difference in survival in a small cohort. Histological analysis further demonstrated tumor cell elimination and intratumoral hemorrhage in the treated lesions. CD44-targeted NIR-PIT demonstrated potent antitumor activity in an orthotopic lung cancer model. These findings support the potential clinical translation of CD44-targeted NIR-PIT as a novel therapeutic strategy for lung cancer. Further studies are warranted to optimize clinically feasible light-delivery methods and establish the long-term safety of this approach.

PURPOSE:The tumor-agnostic approach has been increasingly adopted in precision oncology. Immunohistochemistry (IHC) is the standard biomarker testing for human epidermal growth factor receptor 2 (HER2)-targeted therapies, whereas next-generation sequencing (NGS) has been widely incorporated in routine clinical practices. Here, we investigated the concordance between NGS-based assays and IHC in HER2 testing. Interfering factors leading to discordant results between the assays were also studied. MATERIALS AND METHODS:Over 78,000 solid tumors across various types from two independent cohorts in the United States and Japan were investigated for HER2 DNA copy number, mRNA expression, and protein overexpression by whole-exome sequencing (WES), whole-transcriptome sequencing (WTS), and IHC, respectively. RESULTS:In the US cohort (n = 77,267), HER2 DNA amplification, mRNA overexpression, and IHC-positive (IHC-P) were detected in 4.9%, 10.1%, and 4.7% of the tumors, respectively. Positive results in at least one of the three assays were observed in 10.7% of tumors, while 3.9% were positive for all three assays. Using IHC as a comparator, WTS showed better sensitivity than WES but a lower positive predictive value. These results were consistent in the Japanese cohort (n = 1,225). Although the overall HER2 RNA expression level correlated well with IHC score and DNA copy number, the degree of correlation varied among tumor types. Heterogeneous distribution of IHC-P tumor cells was associated with discordant results between NGS-based assays and IHC. CONCLUSION:HER2-positive status in protein, mRNA, and DNA showed concordance in general but varied among tumor types. NGS-based assays, especially WTS, could be a useful predictive tool for HER2 testing in tumor-agnostic settings. Intratumor heterogeneity in HER2 protein expression should be considered when bringing bulk sequencing tests into clinical settings.

BACKGROUND:Photoimmunotherapy using ASP-1929 (ASP1929-PIT) is a novel approach targeting epidermal growth factor receptor (EGFR)-expressing tumors. ASP-1929, an anti-EGFR monoclonal antibody conjugated to IRDye 700DX (IR700), binds to EGFR in tumor cells. Upon exposure to 690-nm red laser light, IR700 is activated, disrupting the cellular membrane. Approved in Japan for head and neck cancer, this therapy is the first of its kind globally. Given its mechanism, ASP-1929 may also treat other EGFR-positive malignancies accessible to red light. Thus, we are conducting a phase II clinical trial of ASP1929-PIT for localized recurrent and persistent vulvar, vaginal, and cervical cancers refractory to radiotherapy (RT). These severely impact quality of life, highlighting an unmet need. METHODS:This single-arm, open-label, phase II investigator-initiated trial evaluated the efficacy and safety of ASP1929-PIT for recurrent or persistent vulvar, vaginal, or cervical cancer post-RT (jRCT2011240034). Participants had EGFR expression confirmed by immunohistochemistry, prior RT (including chemoradiotherapy) for the primary lesion, and at least one measurable vulvar, vaginal, or cervical lesion. Patients with uncontrolled distant disease or suspected mucosal invasion of the rectum or lower urinary tract were excluded. ASP-1929 (640 mg/m²) was administered via a 2-hour intravenous infusion, followed 20-28 hours later by 690-nm laser illumination. Day 1 was designated for ASP-1929 administration, with efficacy assessed between days 29 and 56. Patients could receive up to three additional treatments if a complete response was not achieved. The primary outcome was tumor shrinkage in laser-treated lesions. Sixteen patients were enrolled, with the first 4 forming a safety cohort. TRIAL REGISTRATION:Japan Registry of Clinical Trials Identifier: jRCT2011240034.

Extraskeletal myxoid chondrosarcoma (EMC) is a rare soft tissue sarcoma defined by its characteristic multinodular myxoid architecture and distinctive clinicopathological features. Although EMC typically exhibits a multinodular myxoid architecture, its histologic variability and frequent immunophenotypic overlap with other myxoid tumors can make diagnosis challenging. We report a case of EMC arising around the left knee of a 73-year-old male patient. Histologically, the tumor exhibited abundant myxoid stroma with lace-like and haphazard cellular arrangements and focal epithelioid morphology. Immunohistochemically, the lesion showed diffuse positivity for several myoepithelial markers, including epithelial membrane antigen, α-smooth muscle actin, HHF35, calponin, and p63. However, the absence of cytokeratin and SOX10 expression raised diagnostic uncertainty despite the myoepithelial-like immunoprofile. Because immunohistochemistry remained inconclusive, targeted RNA sequencing was performed on formalin-fixed, paraffin-embedded tissue. A TAF15::NR4A3 fusion transcript was identified, leading to revision of the initial diagnosis to EMC. At 3-year follow-up, the patient remains free of recurrence or metastasis. This case demonstrates the potential for EMC to mimic myoepithelial tumors and supports the use of molecular analysis when histologic or immunohistochemical findings are insufficient for diagnosis.

BACKGROUND/AIM:Biliary tract cancer (BTC) is an aggressive malignancy associated with a poor prognosis, yet effective molecular therapeutic targets remain scarce. Chromosome 16 open reading frame 74 (C16orf74) has been implicated in tumor progression; however, its specific role in BTC remains unclear. This study aimed to investigate the prognostic significance of C16orf74 in extrahepatic cholangiocarcinoma (eCCA) and to evaluate its potential as a therapeutic target. MATERIALS AND METHODS:We evaluated C16orf74 protein expression in 146 resected eCCA specimens using immunohistochemistry. Functional analyses in BTC cell lines included reverse transcription-polymerase chain reaction, cell proliferation assays, and migration and invasion assays, followed by the evaluation of downstream Akt/mTOR signaling. The in vivo antitumor efficacy of a C16orf74-targeting dimer-blocking (DB) peptide was assessed using a murine xenograft model. RESULTS:High C16orf74 expression occurred in 45.2% of the tumors and was associated with worse overall survival (5-year survival rate: 27.2% vs. 52.2%). Although this association only trended toward significance following false discovery rate adjustment in the univariate analysis, high C16orf74 expression remained an independent predictor of worse survival in the multivariate analysis. In vitro, the DB peptide inhibited cellular migration and invasion and induced dose-dependent cytotoxicity in C16orf74-high cell lines; these effects were accompanied by decreased Akt phosphorylation. In vivo, DB peptide treatment suppressed tumor growth in the TFK-1 xenograft model. CONCLUSION:High C16orf74 expression is associated with a poor prognosis in patients with resected eCCA. Furthermore, targeting C16orf74 with a DB peptide demonstrates substantial antitumor activity. Therefore, C16orf74 represents a promising prognostic biomarker and a potential therapeutic target for eCCA.

BACKGROUND:The Lung Cancer Compact Panel (cPANEL) is a recently approved highly sensitive multiplex gene panel in Japan that supports both DNA- and RNA-based next-generation sequencing. Although cytological specimens are acceptable for cPANEL, unfixed cell pellets or dedicated preservation tubes are typically recommended. However, evidence remains limited regarding whether residual liquid-based cytology (LBC) cell suspensions prepared for routine cytological diagnosis can be used directly for cPANEL testing without dedicated molecular preservation or additional preanalytical processing. In this study, we evaluated the feasibility of applying LBC specimens that are widely used in contemporary clinical practice to cPANEL. METHODS:We analyzed DNA and RNA quality in 69 clinical LBC specimens. Among these, 51 specimens containing non-small cell lung cancer cells with previously determined driver alteration status were subjected to cPANEL testing to evaluate assay concordance with clinical companion diagnostic results. RESULTS:DNA integrity was generally well preserved (DNA Integrity Number [DIN]: 6.2 ± 1.5). In contrast, RNA integrity showed greater variability (DV200: 16.4 ± 12.1%). ThinPrep-fixed specimens demonstrated lower DIN and DV200 values compared with CytoRich Red-fixed specimens. Although all samples successfully passed the DNA-based cPANEL assay, six cases (11.8%) failed the RNA-based assay, with RNA yield being a major contributing factor. Among the 46 evaluable specimens, concordance was 95.7% and sensitivity was 92.3%, or 88.9% including RNA module failures as cPANEL-negative. CONCLUSIONS:With appropriate fixative selection and adequate cellularity, cPANEL using clinical LBC specimens may serve as a practical diagnostic platform. We demonstrated that routine LBC specimens can be directly applied to cPANEL without special preanalytical processing.

Background: Combined small cell lung carcinoma (SCLC) is a rare entity and characterized by the coexistence of SCLC and non-small cell lung cancer (NSCLC) components. We report the first known case of combined SCLC containing three distinct NSCLC components: squamous cell carcinoma, pleomorphic carcinoma, and adenocarcinoma (AD). Case Description: A 70-year-old man presented with right shoulder pain and underwent right upper lobectomy with en bloc chest wall resection and lymph node dissection. Histopathological examination of the resected specimen revealed four distinct histological components within the primary tumor. Comprehensive immunohistochemistry and molecular analyses, including RNA sequencing, were performed to characterize the tumor. A TP53 mutation was shared across all components, suggesting a common clonal origin, whereas additional genetic alterations were largely component specific. Clonal phylogenetic analysis indicated divergence from a common ancestral clone, supporting a structured multilineage evolutionary process. Transcriptomic profiling demonstrated lineage-associated expression patterns, and tumor microenvironment analysis revealed increased lymphocytic infiltration in the AD and pleomorphic carcinoma components, whereas proliferative signaling pathways were enriched in the SCLC component. Conclusions: These findings highlight pronounced intratumoral molecular and immunological heterogeneity. Although combined SCLC with three NSCLC components is exceedingly rare, this case provides novel insights into lineage diversification and the clonal evolution and biological diversity of combined SCLC. Further accumulation of similar cases with integrated molecular analyses is warranted to improve understanding of this rare subtype and inform future therapeutic strategies.

Abstract Background: KRAS mutations occur in up to 30% of non-small cell lung cancer (NSCLC), predominantly at codons 12 and 13. KRAS G12C, the most frequent variant, accounts for approximately 40% of KRAS-mutated NSCLC. The KRAS G12C inhibitors sotorasib and adagrasib have been introduced into clinical practice for KRAS G12C-mutated NSCLC, but their clinical benefit remains limited. One important mechanism for this limited efficacy is adaptive resistance, linked to activation of membrane-associated signaling molecules such as receptor tyrosine kinases, which in turn reactivate KRAS downstream signaling and reduce drug durability. Identifying factors driving adaptive resistance is critical for developing effective combination strategies.​​ Objective: To identify molecular targets mediating adaptive resistance to KRAS G12C inhibitors in NSCLC and evaluate novel therapeutic approaches to overcome resistance. Methods and Results: To elucidate the mechanism of adaptive resistance to KRAS G12C inhibitors, we performed RNA sequencing after short-term sotorasib exposure in H1373 cells and found TACSTD2 (TROP2) was a gene with early upregulation of its mRNA expression. To test its functional relevance, we silenced TROP2 by siRNA in H358, H2122, and H1373 cells and found that TROP2 knockdown increased sotorasib-induced apoptosis by flow cytometry and enhanced suppression of PI3K-AKT signaling. Consistent with the in vitro findings, sotorasib increased TROP2 expression in H1373 xenograft tumors. Furthermore, TROP2 suppression by shRNA enhanced the antitumor effect of sotorasib in vivo. In addition, in LU65 xenograft model, the combination of sotorasib with the anti-TROP2 antibody-drug conjugate datopotamab deruxtecan achieved more durable tumor shrinkage compared with sotorasib monotherapy. Finally, we utilized an artificial intelligence (AI) model to analyze TROP2 immunohistochemistry scores in clinical specimens from patients with non-small cell lung cancer (NSCLC) and investigated the association with prognosis. Conclusion: TROP2-targeted therapy represents a promising strategy to overcome adaptive resistance to KRAS G12C inhibitors in NSCLC and warrants further clinical investigation. Citation Format: Masahiro Kashima, Hidenori Kitai, Yukiko Yoshida, Yuriko Ishida, Yuma Sato, Daisuke Morinaga, Kosuke Tsuji, Shotaro Ito, Kanako C. Hatanaka, Takuma Kobayashi, Yoshiki Shinomiya, Teppei Konishi, Yutaka Hatanaka, Jun Sakakibara-Konishi, Satoshi Konno. Identification of TROP2 as an adaptive resistance mechanism in KRAS G12C inhibition and its therapeutic targeting [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 2965.

Background: Biliary tract cancer (BTC) is an aggressive malignancy with limited therapeutic options and a poor prognosis. CD73 is upregulated under hypoxic conditions and promotes tumor progression. However, its clinical role in BTC and interaction with tumor-infiltrating lymphocytes (TILs) remain unclear. This study aimed to elucidate the association between CD73 expression and prognosis in BTC, as well as its impact on the tumor microenvironment (TME) and TILs. Methods: This retrospective study included 100 patients who underwent curative BTC surgery at Hokkaido University Hospital between 2018 and 2023. Formalin-fixed tumor specimens were analyzed using DeepPathFinder™ (biomy Inc., Tokyo, Japan), an AI-based digital pathology platform enabling objective quantification of CD73 expression and lymphocyte infiltration within tumoral (T) and stromal (S) compartments. Immunohistochemistry for CD3, CD8, Foxp3, and CD163 was used to identify T-cell subsets and macrophages. Associations between CD73, TIL subsets, and overall survival (OS) were assessed using the Kaplan-Meier, Cox regression, and Spearman correlation analyses. Results: High T-CD73 expression was associated with shorter OS (hazard ratio [HR] = 1.97, p = 0.041), whereas S-CD73 showed no prognostic relevance. Conversely, high S-TIL density was correlated with improved survival (HR = 0.49, p = 0.032). T-CD73 expression was negatively correlated with stromal CD3+ and CD8+ T-cell densities, indicating selective suppression of stromal cytotoxic T-lymphocyte (CTL) infiltration. No significant correlations were observed between Foxp3+ T cells and CD163+ M2 macrophages. Conclusions: CD73 upregulation in tumor cells impairs stromal CTL and TIL activity, leading to a poor prognosis. Spatial distribution, rather than total TIL number, better reflects effective anti-tumor immunity.

Mucin 1 (MUC1) is a highly O-glycosylated transmembrane glycoprotein. Tumor-associated MUC1, characterized by aberrant O-linked glycans, is overexpressed in cancer cells; however, conventional MUC1 antibodies show limited specificity for tumor-associated glycan structures. Recently, a novel epitope-defined antibody (MUC1-Tn antigen epitope-defined antibody [MUC1-Tn ED Ab]) that specifically recognizes the Tn-MUC1 antigen was developed. In this study, we evaluated the potential of MUC1-Tn ED Abs as diagnostic markers of breast cancer. Tissue microarray sections from 124 patients with invasive breast carcinoma (IBC) and 26 whole tissue sections, including multiple neoplastic lesions-flat epithelial atypia (n = 24), ductal carcinoma in situ (n = 26), and IBC (n = 16)-were analyzed. Immunohistochemical distributions of Tn-MUC1 and MUC1 were assessed using the MUC1-Tn ED Ab (clone SN102) and a conventional antibody (clone Ma552), respectively. In tissue microarray analysis, Tn-MUC1 exhibited minimal immunoreactivity in nonneoplastic areas and high specificity for IBC. In IBC tissues, immunoreactivity with Tn-MUC1 was predominantly cytoplasmic, unlike conventional MUC1 staining observed in both the cytoplasm and membrane. In multilesion analysis, cytoplasmic Tn-MUC1 expression was rarely detected in nonneoplastic areas but progressively increased across flat epithelial atypia, ductal carcinoma in situ, and IBC. Knockdown assays in breast cancer cell lines demonstrated that core 1 β1,3-galactosyltransferase 1 (C1GALT1), an enzyme involved in galactosylation of the Tn antigen, significantly influenced the cellular localization of Tn-MUC1. This study demonstrates that Tn-MUC1, as detected by the MUC1-Tn ED Ab, has high specificity for breast cancer and may act as a novel immunohistochemical marker reflecting tumor progression.

6011 Background: Trastuzumab deruxtecan (T-DXd) is a HER2-directed antibody–drug conjugate with established efficacy across multiple HER2-expressing malignancies. Salivary gland carcinoma (SGC) is a rare disease with limited treatment options, particularly for tumors with HER2-low expression. The MYTHOS trial is a multicenter, investigator-initiated phase II study evaluating T-DXd efficacy in recurrent or metastatic (RM) SGC patients with HER2 overexpression or HER2-low expression. Here, we report the efficacy and safety results of the HER2-low cohort (Cohort 2). Methods: Eligible patients had histologically confirmed RM SGC with HER2-low expression (IHC 1+ or IHC 2+/ISH−), as determined by central assessment according to the ASCO/CAP 2018 breast cancer guidelines, and no indication for curative treatment. Patients received T-DXd at 5.4 mg/kg intravenously every 3 weeks. The primary endpoint was confirmed objective response rate (ORR) assessed by independent central review (ICR) according to RECIST v1.1. Secondary endpoints included disease control rate (DCR), progression-free survival (PFS), overall survival (OS), and safety. The required sample size for Cohort 2 was 33 patients, based on a threshold ORR of 25%, an expected ORR of 50%, 83% power, and a two-sided alpha of 0.05 using an exact binomial test. The primary efficacy decision was based on the prespecified Simon’s two-stage minimax design in the first 33 patients; efficacy in the full analysis set (FAS) was summarized descriptively. Results: A total of 36 patients were included in the FAS, including 30 with salivary duct carcinoma (SDC). HER2-low status comprised IHC 2+/ISH− in 14 and IHC 1+ in 22; 25 had received prior systemic therapy for RM disease. Median follow-up was 25.1 months. The ORR by ICR was 38.9% (14/36; 95% CI, 23.1–56.5%), and the DCR was 94.4% (95% CI, 81.3–99.3%). Median PFS was 8.7 months (95% CI, 6.5–13.3), and median OS was 24.8 months (95% CI, 18.7–NE). In a prespecified subgroup analysis by histology, the ORR by ICR was 46.7% (14/30; 95% CI, 28.3–65.7%) in SDC and 0% (0/6; 95% CI, 0–45.9%) in other SGC subtypes. Common grade ≥3 adverse events (>10%) were neutrophil count decreased (36.1%), lymphocyte count decreased (19.4%), white blood cell count decreased (11.1%), and decreased appetite (11.1%). Drug-related interstitial lung disease (ILD)/pneumonitis occurred in 9 (25.0%; grade 1/2 in 7, grade 3 in 1, and grade 5 in 1). There was one drug-related death due to ILD/pneumonitis. Conclusions: Although the prespecified primary efficacy endpoint was not met, T-DXd demonstrated clinically meaningful antitumor activity in patients with HER2-low RM SGC, particularly in those with SDC. The safety profile was generally consistent with the known profile of T-DXd in the Japanese population, with ILD/pneumonitis remaining an important identified risk requiring careful monitoring. Clinical trial information: jRCT2011210017.

589 Background: It has been reported that there are BRCA1/2 -mutated patients in biliary tract, pancreatic, and other gastrointestinal cancers. Niraparib is a poly(ADP-ribose) polymerase (PARP) inhibitor, and PARP inhibitors exert their cytotoxicity against cancer cells in the context of homologous recombination deficiency, such as BRCA mutations. Methods: Main eligibility criteria are unresectable, advanced or recurrent biliary tract cancers (BTC; cohort A), pancreatic cancers (PC; cohort B), and other gastrointestinal cancers (cohort C) with BRCA1/2 gene mutations identified by germline test or genomic profiling test with either circulating tumor DNA (ctDNA) or tumor tissue, refractory or intolerant to previous treatments, and adequate organ function. Patients with body weight of 77 kg or more and a platelet count of 150,000 /µL or more receive 300 mg of niraparib, and less than 77 kg or having a platelet count less than 150,000 /µL received 200 mg of niraparib, orally once daily, until disease progression or intolerable adverse events occurred. Primary endpoint was the investigator-assessed objective response rate (ORR) in each cohort with a threshold response rate of 10% and an expected response rate of 35%. Key secondary endpoints were progression-free survival (PFS), overall survival (OS), disease control rate (DCR), and safety. Furthermore, pre-treatment ctDNA was collected and analyzed by Guardant360. Results: A total of 62 pts, 26 in cohort A, 26 in cohort B, and 10 in cohort C were enrolled between March 2021 and April 2023. Because one patient in cohort C did not receive protocol treatment, 61 pts were identified for primary efficacy and safety analysis. Median number of prior regimens was 1/2/2 (range, 1–2/1–2/1-6). In cohort A/B/C, the confirmed ORR was 15.4/15.4/0% [95% highest posterior density credible interval, 8.4–27.9/8.4–27.9/0.0–25.9]. The DCR, median (m) PFS, mOS were 57.7/53.8/22.2% [95% confidence interval (CI), 36.9–76.6/33.4–73.4/2.8–60.0], 2.7/1.8/1.4 months (95% CI, 1.5–4.1/1.4–9.3/1.0–2.8), 7.8/9.5/7.4 months (95% CI, 5.9–9.8/3.5–NE/2.9–8.6), respectively. In the pts with BRCA mutations by Guardant360 (cohort A/B/C were 18/21/8), ORR was 11.1/19.0/0% (95% CI, 2.0-30.2/6.5–38.3/0.0–28.3). The most common treatment-emergent adverse events were thrombocytopenia (31.1%), anemia (27.9%), neutropenia (14.8%), nausea (36.1%), fatigue (29.5%), anorexia (24.6%). Conclusions: Although niraparib had signs of clinical activity in pts with BRCA -mutated BTC and PC, the primary endpoint was not achieved statistically. No new safety signal was observed. Alternative approaches, such as evaluation in biomarker-selected patients or in combination with other agents, may demonstrate greater clinical activity of niraparib in this setting. Clinical trial information: jRCT2011200023.