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Background: Lapatinib is a small molecular inhibitor of HER2 and EGFR tyrosine kinases, which is approved for HER2-positive metastatic breast cancer as second line treatment. Significant proportion of patients experience disease progression due to acquired resistance. Activation of DNA damage repair (DDR) is one of the drug-resistance mechanism, however, the impact of DDR on sensitivity to lapatinib is unclear. Thus, lapatinib resistance mechanism was explored from the standpoint of DDR activation. Methods: Acquired lapatinib-resistant (LR) SK-BR-3 cell lines were generated by continuously exposing to lapatinib, starting with 30 nmol/L and incrementally increasing to 10 μmol/L over 7 months. MTT assay was used to confirm lapatinib sensitivity. Cell cycle progression was analyzed using BrdU assay. Replication fork speed and stalled fork were analyzed by DNA fiber assay. Expression of the molecules was examined using Western blotting, immunofluorescence assay and transcriptome data analysis. DNA strand breaks and repair efficacy were evaluated through alkaline comet assay. Results: G1/S phase transition was increased and early/late S phase population was increased in SK-BR-3 LR cells. Replication fork speed was accelerated and replication stress are elevated in SK-BR-3 LR cells. p-Chk1, Rad51, Rad51B, Rad51C and XRCC3 were upregulated in SK-BR-3 LR cells. After irradiation or treatment with ATR inhibitor, γ-H2AX was continuously increased in parental cells. In contrast, γ-H2AX did not change significantly in SK-BR-3 LR cells, and Chk1 was activated after irradiation or treatment with ATR inhibitor. Tail of comet disappeared at early time point in in SK-BR-3 LR cells after induction of DNA damage compared with parental cells. These results demonstrated that DDR was activated and DNA damage repair capacity was enhanced in LR cells. Conclusion: Replication stress is elevated in SK-BR-3 LR cells. Up-regulation of molecules involved in the homologous recombination repair pathway was observed in SK-BR-3 LR cells. Moreover, DNA repair capacity was increased in SK-BR-3 LR cells. These data suggested that activation of DNA damage repair pathway caused by replication stress contributes to lapatinib resistance. Citation Format: So Hyeon Kim, Ahrum Min, Seongyeong Kim, Yu Jin Kim, Sujin Ham, Hae Min Hwang, Minyoung Lee, Changyun Lee, Jinyong Kim, Dae-Won Lee, Kyung-Hun Lee, Seock-Ah Im. Replication stress activates the DNA damage response and contributes to lapatinib resistance in HER2-positive SK-BR-3 cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr LB228.

Introduction: ALK-positive ALCL accounts for 0.8% of Korean lymphoma patients. ALK TKIs are effective and durable, but clinical outcomes after treatment discontinuation are unknown. We analyzed outcomes and minimal residual disease (MRD) in adult ALK-positive ALCL patients who discontinued ALK TKIs after achieving treatment-free remission. Methods: Six patients who achieved responses to crizotinib (N=5) and ceritinib (N=1) were analyzed. RNA extracted from the blood or tissues before TKI treatment were subjected to multiplex PCR-based next generation sequencing (NGS) using ARCHER® RNA Fusionplex® to detect NPM-ALK fusion. MRD was assessed by droplet digital PCR (ddPCR) of peripheral blood mononuclear cells (PBMCs) after the treatment discontinuation. Undetectable MRD was defined as less than one lymphocytic cell per 10,000 leukocytes (10-4) in peripheral blood. Results: Patients received ALK TKIs as second line (N=3) or third line and beyond (N=3). All achieved complete metabolic responses after median 28.5 months (range, 1.7-93.9) of treatments (Table 1). Before TKI, LY1 patient had no morphologic evidence of bone marrow involvement, but NPM1-ALK fusions were detected by NGS and ddPCR in blood, bone marrow, and tissue. LY1 patient remained in remission at 27.8 months after crizotinib discontinuation. In all patients, NPM1-ALK fusion were not detected in the PBMCs after discontinuing ALK TKIs for median 38.2 months (range, 0-61.9). No patient re-initiated anti-lymphoma therapies. Conclusion: Out data demonstrated that treatment-free remission was durable for more than two years in ALK-positive ALCL patients following the cessation of ALK TKIs. MRD-guided discontinuation decision may be considered for ALK-positive ALCL patients treated with ALK inhibitors who had undetectable MRD. Table 1. Summary of patient characteristics and outcomes of ALK TKI discontinuation No. Sex Age at diagnosis Ann Arbor stage ALK TKI Lines of prior therapy Treatment duration of ALK TKI (months) Reason for cessation Duration of ALK TKI cessation (months) Best response to ALK TKI Time of MRD assessment after TKI cessation MRD after TKI cessation 1 M 22 IIB Crizotinib 3 93.9 Fertility 27.8 CR 5.6 Negative 2 F 20 IVB Crizotinib 1 90.9 Fertility 25.9 CR 0.0 Negative 3 M 23 IV Ceritinib 1 42.7 Adverse event 61.9 CR 61.9 Negative 4 F 58 IV Crizotinib 3 1.7 Cost 86.6 CR 72.1 Negative 5 F 64 IVB Crizotinib 1 13.6 Cost 31.0 CR 14.4 Negative 6 M 75 IV Crizotinib 2 11.7 Cost 80.2 CR 62.0 Negative Citation Format: Jinyong Kim, Soyeon Kim, Tae Min Kim, Jeonghwan Youk, Miso Kim, Bhumsuk Keam, Dong-Wan Kim, Dae Seog Heo. Treatment-free remission after discontinuation of ALK tyrosine kinase inhibitors (TKIs) in patients with ALK-positive anaplastic large cell lymphoma (ALCL) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2181.

Background: Cyclin dependent kinase 9 (CDK9) regulates progression of RNA polymerase II through phosphorylation. Inhibition of CDK9 induces transcription-replication conflicts (T-R conflicts), leading to replication stress. AZD4573, a highly selective CDK9 inhibitor, showed anti-tumor effects in acute myeloid leukemia in vitro and in vivo. Recently it is being evaluated in phase 2 clinical trial (NCT04630756). However, the anti-tumor effect of AZD4573 is unknown in breast cancer. Thus, we investigated the anti-tumor effect of AZD4573 in breast cancer cells and explored the underlying mechanism. Methods: To evaluate the antitumor effect in vitro, MTT assays were preformed using increasing concentration of AZD4573 (doses range : 0-100 nM). The IC50 values were calculated using Sigma Plot software. Cell cycle analysis was performed using flow cytometry analysis. The expression of signal transduction molecules was determined using quantitative PCR, western blotting, and immunofluorescence. Apoptotic cell death was verified by annexin-V assay. DNA strand breaks and repair efficacy were evaluated through alkaline comet assay. siRNA knock-down system was used to confirm the action mechanism. Results: SK-BR-3, HCC70, HCC1937 and ZR-75-1 were sensitive to AZD4573 with IC50 ranging from 9.16-25.81 nM, whereas T47D and HCC1428 were less-sensitive (IC50 >100 nM), as assessed by MTT assay. After treatment of AZD4573, sensitive and less-sensitive cells showed increased T-R conflicts in 10 minutes. T-R conflicts were maintained in SK-BR-3, HCC70, HCC1937 and ZR-75-1 for 3 hours, but T-R conflicts disappeared immediately in T47D and HCC1428 cells. Moreover, expression of γ-H2AX, cleaved PARP and cleaved caspase-3 was increased, indicating the induction of apoptosis in sensitive cells. Contrary to these results, γ-H2AX induction and apoptosis were not observed in T47D and HCC1428 cells. siRNA knock-down of CDK9 in HCC1937 resulted in an increase of T-R conflicts, γ-H2AX, with induction of PARP cleavage and caspase-3 cleavage. Conclusion: AZD4573 induced T-R conflicts and subsequent DNA damage. Accumulation of DNA damage caused caspase-3-dependent apoptotic cell death in sensitive breast cancer cells. However, less-sensitive breast cancer cells resolved T-R conflicts immediately and maintained genomic stability. These data suggest that AZD4573 induced significant anti-tumor effects by apoptotic cell death through accumulation of DNA damage, following T-R conflicts. Citation Format: Minyoung Lee, So Hyeon Kim, Sujin Ham, Hae Min Hwang, Youlim Noh, Yu-Jin Kim, Ahrum Min, Jinyong Kim, Dae-Won Lee, Kyung-Hun Lee, Seock-Ah Im. Cyclin dependent kinase 9 inhibitor AZD4573 induces cell death through DNA damage accumulation in breast cancer cells in vitro [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5978.

Background Germline mutations of breast cancer susceptibility gene BRCA1 and BRCA2 (gBRCA1/2) are associated with elevated risk of breast cancer in young women in Asia. BRCA1 and BRCA2 proteins contribute to genomic stability through homologous recombination (HR)-mediated double strand DNA break repair (DDR) in cooperation with other HR-related proteins. In this study, we analyzed the targeted sequencing data of the breast cancer patients with gBRCA1/2 mutations to investigate the landscape of HR-related gene mutations and their clinical implications. Materials and Methods Data of the breast cancer patients with pathogenic gBRCA1/2 mutations and qualified targeted next generation sequencing, SNUH FiRST cancer panel, were analyzed. Single nucleotide polymorphisms, small insertions and deletions were analyzed with functional annotations using ANNOVAR. HR-related genes were defined as ABL1, ATM, ATR, BARD1, BRCA1, BRCA2, CDKN1A, CDKN2A, CHEK1, CHEK2, FANCA, FANCD2, FANCG, FANCI, FANCL, KDR, MUTYH, PALB2, POLE, POLQ, RAD50, RAD51, RAD51D, RAD54L, and TP53. Mismatch-repair genes were MLH1, MSH2, and MSH6. Clinical data were analyzed with cox proportional hazard models and survival analyses. Results Fifty five Korean breast cancer patients with known gBRCA1/2 mutations and qualified targeted NGS data were analyzed. Ethnically distinct mutations in gBRCA1/2 genes were noted, with higher frequencies of Val1833Ser (14.8%), Glu1210Arg (11.1%), and Tyr130Ter (11.1%) in gBRCA1 and Arg2494Ter (25.0%) and Lys467Ter (14.3%) in gBRCA2. Considering subtypes, gBRCA1 mutations were associated with triple-negative breast cancers (TNBC), while gBRCA2 mutations were more likely hormone receptor-positive breast cancers. At least one missense mutation of homologous recombination (HR)-related genes were observed in 44 cases (80.0%). The most frequently co-mutated gene was TP53 (38.1%). In patients with gBRCA1/2 mutations, however, genetic variations of TP53 occurred in locations different from the known hotspots of those with sporadic breast cancers. The patients with both gBRCA1/2 and TP53 mutations were more likely to have TNBC, high Ki-67 values, and increased genetic mutations, especially of HR-related genes. Survival benefit was observed in the TP53 mutants of patients with gBRCA2 mutations, compared to those with TP53 wildtypes. Conclusion Our study showed distinct genetic landscape of breast cancer patients with gBRCA1 and gBRCA2 mutations in the Asian populations. Further studies on precision medicine are needed for tailored treatments of patients with genetic diversity among different ethnic groups. Citation Format: Jinyong Kim, Kyeonghun Jeong, Hyeji Jun, Kwangsoo Kim, Tae-Yong Kim, Dae-Won Lee, Go-un Woo, Songyi Park, Hanbaek Yi, Kyung-Hun Lee, Seock-Ah Im. Mutations of TP53 and genes related to homologous recombination repair in breast cancer with germline BRCA1/2 mutations [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 P5-03-20.