Selective Small Molecule Inhibitor Research Articles

Abstract 4019: Preclinical activity of ELVN-002: A potent, selective, and irreversible HER2 and pan-HER2 mutant small molecule inhibitor for the treatment of HER2 driven malignancies

Abstract Mutation or amplification of the human epidermal growth factor receptor 2 (HER2) gene has been identified in numerous solid tumor types, including non-small cell lung cancer (NSCLC), breast, and colorectal cancer. These genetic alterations are believed to confer elevated HER2 activity resulting in oncogenic transformation. In NSCLC, an estimated 3% of patients harbor activating mutations in the HER2 gene. Enhertu (T-DXd), a HER2-specific antibody-drug conjugate was recently approved as an important therapy for this patient population. However, for patients who are not candidates for T-DXd or have discontinued treatment due to an adverse event or disease progression, there exist few therapeutic options. Accordingly, HER2 inhibitors have the potential to provide meaningful therapeutic benefit to these patients. Due to significant structural homology between EGFR and HER2, most investigational small molecule HER2 kinase inhibitors are dual EGFR/HER2 inhibitors and are substantially dose-limited by EGFR-related toxicities in patients. These toxicities likely limit their clinical activity. Currently, tucatinib is the only HER2-selective small molecule inhibitor approved for metastatic breast cancer in combination with trastuzumab and capecitabine. However, it lacks sufficient potency against several of the key clinically relevant mutations, including HER2 YVMA, the most common exon 20 insertion mutation, prevalent in HER2 mutant NSCLC. ELVN-002 is a potent, irreversible inhibitor of HER2 with a >100-fold selectivity over EGFR. ELVN-002 potently inhibits the phosphorylation of HER2 in cell lines endogenously expressing HER2 or in those engineered to express specific clinically relevant HER2 mutants. This activity readily translates into marked anti-proliferative activity against cell lines dependent upon HER2 for their growth and survival. In addition, ELVN-002 is highly selective for HER2 and HER2 mutants versus wild-type EGFR as demonstrated via assessment of both EGFR-driven proliferation and EGFR phosphorylation in cells. Furthermore, ELVN-002 is highly active in mouse models of HER2-driven cancers, including subcutaneous models driven by wild type or mutant HER2. ELVN-002 is also active in an NCI-N87 wild-type HER2 intracranial model. In all models tested, ELVN-002 treatment is well tolerated and resulted in tumor regressions at exposures believed to be clinically achievable. Due to its selectivity and breadth of activity versus the clinically relevant HER2 mutants, ELVN-002 has the potential to be an effective treatment for HER2 mutant NSCLC as well as other HER2 mutant amplified solid tumors, including metastatic HER2 positive breast cancer. Additionally, ELVN-002 could provide a meaningful therapeutic option for patients with CNS metastases. ELVN-002 has been selected for clinical development. Citation Format: Monette Aujay, Amanda J. Broad, Stefan D. Gross, Li Ren, Joseph P. Lyssikatos, Samuel Kintz, Qi Wang, Helen Collins. Preclinical activity of ELVN-002: A potent, selective, and irreversible HER2 and pan-HER2 mutant small molecule inhibitor for the treatment of HER2 driven malignancies. [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 4019.

Abstract 461: Targeting glucose metabolism in lung cancer

Abstract Positron emission tomographic studies with 2-18F-fluoro-deoxy-glucose (FDG-PET) consistently demonstrate that non-small cell lung cancers (NSCLC) take up significantly more glucose than normal lung. NSCLCs preferentially utilize glucose to support their rapid growth- a phenotype driven by several oncogenic alterations among which mutations of the oncoprotein Ras are frequently observed. Oncogenic Ras is shown to activate downstream effectors (e.g. HIF-1α) that stimulate glycolysis - in part by activation of the 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase family of enzymes (PFKFB1-4). The PFKFBs produce fructose-2,6-bisphosphate (F26BP) which activates a key rate-limiting glycolytic enzyme, 6-phosphofructo-1-kinase (PFK1). In preliminary studies, we found that the PFKFB4 family member was highly expressed relative to other PFKFB isoforms (by multiplex RT-PCR) in a series of patient-derived NSCLCs relative to paired normal lung tissue. We further explored PFKFB4 protein expression by immunohistochemistry in NSCLCs derived from a cohort of patients and found markedly elevated PFKFB4 in the majority of tumors relative to adjacent normal tissue. We then determined that silencing PFKFB4 markedly reduced F26BP, glycolysis and cell survival in NSCLC cell lines and inhibited xenograft lung tumor growth in vivo. In order to target PFKFB4, we conducted a virtual screen of compounds that fit into the active site of PFKFB4 and discovered a first-in-class small molecule selective PFKFB4 inhibitor, termed 5MPN, that selectively inhibits recombinant PFKFB4 activity and decreases F26BP, glycolysis and proliferation in multiple NSCLC cell lines, with significantly higher anti-proliferative effects in Ras-driven cell lines. 5MPN also caused marked suppression of growth in established NSCLC xenograft tumors, importantly without systemic toxicity. We have developed an inducible, pan-tissue homozygous floxed Pfkfb4 knockout mouse and crossed it with a Cre recombinase mouse driven by a tamoxifen-inducible β-actin promoter to produce Cre/Pfkfb4fl/fl mice. We crossed Cre/Pfkfb4fl/fl mice with a Ras-driven lung cancer model, induced Pfkfb4 deletion by tamoxifen administration and found significantly lower total lung tumor burden in these mice accompanied by decreased PFKFB4 expression in tumors and lung tissue. Taken together, these data provide strong rationale for the further evaluation of small molecule inhibitors of PFKFB4 as a potential therapeutic strategy against NSCLC. Citation Format: Sucheta Telang, Simone Chang, Nicole Sanders. Targeting glucose metabolism in lung cancer [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 461.

Abstract 2791: Biomarker strategy for a phase 1 study of ORIC-944, a potent and selective allosteric PRC2 inhibitor, in patients with metastatic prostate cancer

Abstract Background: Polycomb repressive complex 2 (PRC2) tri-methylates histone H3 at lysine 27 (H3K27me3) leading to transcriptionally silenced genes. ORIC-944 is a potent, highly selective allosteric small molecule inhibitor of PRC2’s embryonic ectoderm development (EED) subunit that is undergoing clinical development as an orally bioavailable monotherapy in patients with metastatic prostate cancer. We previously reported strong tumor growth inhibition with ORIC-944 in enzalutamide-resistant 22Rv1 prostate cancer xenografts. Here, to devise and implement a biomarker strategy aimed at ORIC-944 dose selection in the ongoing phase 1 study (NCT05413421), we undertook preclinical pharmacology studies and assay development. Methods: Naïve and 22Rv1-tumor bearing male mice were treated with vehicle, 10, 30 or 100 mg/kg ORIC-944 PO for up to 37 days. H3K27me3 was evaluated in dorsal skin (epidermal layer) and peripheral monocytes by IHC and an alphaLISA test, respectively. Since dying tumor cells release nucleosomes to circulation, cell-free (cf)-nucleosomal H3K27me3 levels normalized to cf-H3.1 were also assessed in plasma using a proprietary magnetic bead-based sandwich immunoassay. Tumors were profiled by RNA-sequencing and H3K27me3 chromatin immunoprecipitation sequencing. Results: In the epidermal skin layer, H3K27me3 was present in vehicle-treated naïve animals but strongly depleted in ORIC-944-treated mice in a dose-dependent manner. Likewise in monocytes, H3K27me3 was reduced to undetectable levels in 100 mg/kg ORIC-944-treated mice. In plasma, normalized cf-nucleosomal H3K27me3 levels significantly decreased in response to ORIC-944 in a dose- and time-dependent manner, selectively in 22Rv1-tumor bearing mice compared to non-tumor bearing mice. Putative PRC2 target genes were identified in 22Rv1 xenografts by focusing on those genes with H3K27me3 binding in the promoter, whose expression was de-repressed by ORIC-944 in a time-dependent manner and then re-expressed after dose suspension. In the ongoing phase 1 study, H3K27me3 levels are being evaluated in the stratum spinosum of skin and in blood-derived monocytes using a proprietary IHC assay and alphaLISA test, respectively. Modulation in the expression of the putative PRC2 target genes is being assessed in blood-derived peripheral blood mononuclear cells by RNA-sequencing. Conclusion: This comprehensive biomarker strategy enables us to establish target engagement and capture exposure-dependent pharmacodynamics in the ongoing phase 1 study evaluating ORIC-944 as a best-in-class PRC2 inhibitor for the treatment of patients with advanced prostate cancer. Citation Format: Anneleen Daemen, Natalie Yuen, Aleksandr Pankov, Eric A. Ariazi, Subhash D. Katewa, Frank L. Duong, Amber Wang, Shravani Barkund, Shelly Kaushik, Jessica D. Sun, Lori S. Friedman, Melissa R. Junttila. Biomarker strategy for a phase 1 study of ORIC-944, a potent and selective allosteric PRC2 inhibitor, in patients with metastatic prostate cancer [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 2791.

Abstract 2025: Phosphoproteomic profiling following MAP3K1 kinase inhibition identifies NPM1 as a novel substrate

Abstract In response to diverse cellular cues, MAP3K1, a mitogen-activated protein kinase, participates in various cancer signaling networks including the NFκB, JNK, ERK, and p38 pathways. Functioning as a signaling kinase in these oncogenic pathways, MAP3K1 contributes to tumor growth and metastasis, thus making it an attractive therapeutic target for cancer. Structure-guided design using MAP3K1 AlphaFold and Schrödinger GLIDE led to the development of 51-106, a selective MAP3K1 small molecule inhibitor. Profiling using the KiNativTM platform in a cellular matrix revealed 51-106 as a selective ATP-competitive MAP3K1 inhibitor. 51-106 blocked TNFα-induced MAP3K1-IKKβ-mediated NFκB activity. Phosphoproteomics analysis following MAP3K1 inhibition by 51-106 showed a dose dependent decrease in NPM1 T199 phosphorylation indicating NPM1 as a novel substrate of MAP3K1. NPM1 plays a critical role in DNA damage repair; consistently we observed a dose dependent S-phase arrest upon MAP3K1 inhibition by 51-106, suggesting a dysfunctional DNA damage response. Higher MAP3K1 transcript level in pancreatic cancer patients is associated with poorer (50% vs. 15%) 5-year survival. Treatment of pancreatic cancer cell lines with the MAP3K1 inhibitor 51-106 inhibited cell growth and migration. In combination studies, 51-106 synergistically inhibited growth with gemcitabine in LSL-KrasG12D/+, LSL-Trp53R172H/+, Pdx1-Cre (KPC) cell lines in vitro and in KPC syngeneic orthotopic implantation mouse model of pancreatic cancer in vivo. Our study is the first to identify NPM1 as a potential substrate of MAP3K1, and these results warrant the investigation of MAP3K1 inhibition as a therapeutic option in cancer. Citation Format: Lidia Boghean, Sarbjit Singh, Kiran Mangalaparthi, Surendra K. Shukla, Smitha Kizhake, Amritha Kizhake, Donn Wishka, Jayapal Reddy Mallareddy, Joel Morris, Paul Grothaus, Pankaj K. Singh, Akhilesh Pandey, Amarnath Natarajan. Phosphoproteomic profiling following MAP3K1 kinase inhibition identifies NPM1 as a novel substrate [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 2025.

Abstract 1645: JAB-2485: A potent, highly selective small-molecule Aurora kinase A inhibitor that targets cell division

Abstract Background: Aurora kinase A (AURKA), a crucial mitotic regulator, is frequently dysregulated in a wide range of cancers and contributes to clinical aggressiveness and poor patient survival. Abnormally activated AURKA promotes tumorigenesis through enhancing cancer cell proliferation, epithelial-mesenchymal transition, and cancer stem cell self-renewal, rendering it an attractive therapeutic target. We have developed JAB-2485, a highly selective small-molecule AURKA inhibitor. Methods: Biochemical and cell-based assays were performed to determine the IC50 of JAB-2485 on Aurora kinase family members. Fluorescence activated cell sorting (FACS) and caspase 3/7 assay were performed to detect JAB-2485-induced cell cycle arrest and apoptosis, respectively. The CellTiter-Glo assay was performed to evaluate the effect of JAB-2485 in multiple cancer cell lines. Finally, the antitumor effect of JAB-2485 either as a single agent or in combinations was investigated in tumor cell line-derived xenografts. Results: JAB-2485 is potent and highly selective on AURKA, with IC50 of 0.33nM and around 1700-fold selectivity over AURKB. Following treatment of JAB-2485, the level of AURKA phosphorylation as a pharmacodynamic marker was significantly decreased in a dose-dependent manner. As a single agent, JAB-2485 inhibited in vitro proliferation of small cell lung cancer (SCLC), triple-negative breast cancer (TNBC) and neuroblastoma cell lines along with induced G2/M cell cycle arrest and apoptosis, and significantly inhibited in vivo tumor growth on SCLC and neuroblastoma xenografts. Furthermore, JAB-2485 showed strong synergy with cisplatin and paclitaxel in SCLC and TNBC animal models, respectively. Conclusions: Taken together, preclinical results provide robust evidence supporting further evaluation of JAB-2485 both as a single agent and in combinations for treatment of various types of cancers. A Phase 1/2a clinical trial of evaluating JAB-2485 in adult patients with advanced solid tumors is enrolling patients in the US (NCT05490472). Citation Format: Guiqun Yang, Yanping Wang, Haijun Li, Mingming Chen, Yiwei Lin, Xin Sun. JAB-2485: A potent, highly selective small-molecule Aurora kinase A inhibitor that targets cell division [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 1645.

Abstract 1558: Targeting CDK9 via the small-molecule inhibitor enitociclib as a therapeutic strategy to treat MYCN-amplified rhabdomyosarcoma and neuroblastoma in children

Abstract Introduction: MYCN amplification is the genetic aberration most consistently associated with poor outcomes in alveolar rhabdomyosarcomas (aRMS) and neuroblastoma (NBL). It regulates key oncogenic processes including tumor cell survival, proliferation and metastasis. In aRMS, MYCN transcription is driven by the PAX3-FOXO1 gene fusion. MYCN and other short half-life mRNA transcripts depend on CDK9 to drive transcription of target genes to initiate and maintain the oncogenic program, making CDK9 blockade an effective therapeutic route against MYC-driven cancers. We have previously shown the activity and tolerability of enitociclib in tumor xenograft models. In this study, we investigated the activity of the selective small-molecule CDK9 inhibitor enitociclib (VIP152/BAY1521152) in preclinical models of pediatric MYCN-amplified aRMS and NBL. Methods: Cell-based viability assays were performed in aRMS (Rh30 and Rh41) and NBL (LAN1, SK-N-AS, SK-N-BE(2) and SK-N-MC) cell lines. Enitociclib antitumor activity was measured by Alamar Blue cell viability assay after 96 h exposure at 9 dose levels (8 nM-2 μM) with DMSO used as control. We performed combination screens of enitociclib with several FDA-approved drugs (n=215). Selected agents with IC50 values <1 μM were analyzed at constant dilution ratios of two inhibitors. Drug synergy was calculated by established methods using SynergyFinder 3.0. Target modulation was determined for RNA polymerase II (RNAPII), MYCN and apoptosis inducers caspase-3 and PARP, in addition to PAX3-FOXO1 and PLK1 protein levels in aRMS cells, by western blotting after continuous exposure with enitociclib for 24 h. Results: Significant cytotoxic activity, with IC50 values ranging from 48-182 nM and 39-123 nM was seen for aRMS cells and NBL cells, respectively. SynergyFinder found synergistic effects of enitociclib with irinotecan, carfilzomib, etoposide, bortezomib, selinexor or topotecan tested at clinically relevant concentrations in the aRMS cell line Rh41. Enitociclib induced apoptosis with cleavage of caspase-3 and PARP by western blotting in a dose- and time-dependent manner in addition to the depletion of phosphorylated RNAPII (Ser2) and MYCN protein levels. In aRMS cells, PAX3-FOXO1 fusion and PLK1 proteins were downregulated. Annexin V/propidium iodide staining confirmed a dose-dependent increase in apoptosis after enitociclib exposure. Conclusions: Our results suggest that CDK9 inhibition is potentially clinically relevant for MYCN-amplified solid tumors such as aRMS and NBL due to its significant antitumor activity and pharmacologic targetability. The data provide essential information on the distinct targets, biomarkers of activity and clinically feasible drug combinations for the development of enitociclib in clinical trials addressing an unmet need in pediatric oncology. Citation Format: Son Tran, Patrick Sipila, Melanie M. Frigault, Amy J. Johnson, Joseph Birkett, Raquel Izumi, Ahmed Hamdy, Beatrix Stelte-Ludwig, David P. Douglass, Anne-Marie Langevin, Norman J. Lacayo, Aru Narendran. Targeting CDK9 via the small-molecule inhibitor enitociclib as a therapeutic strategy to treat MYCN-amplified rhabdomyosarcoma and neuroblastoma in children [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 1558.

Abstract 521: Discovery of novel TNKS inhibitors through structure-based virtual screening

Abstract More than 80% of colorectal cancers (CRCs) have somatic mutations in the adenomatous polyposis coli (APC) gene, which affects abnormal WNT/β-catenin signaling. Since inhibition of tankyrases (TNKS1 and TNKS2) belonging to the poly(ADP-ribose) polymerase family stabilizes AXINs and induces degradation of β-catenin, drug development as a therapeutic target for APC-mutated CRCs is required. To identify selective small molecule inhibitors of TNKS, we performed pharmacophore modeling and structural docking studies targeting nicotinamide, adenosine, and both binding subsites of TNKS. Among the 8 million compounds, 149 of the nicotinamide site binders, 23 of the adenosine site binders, and 15 of the dual-site binders of TNKS were selected through computer-based virtual screening. Using in vitro TNKS1 and TNKS2 enzyme assays, we identified that TI2-N17 and TI3-D11 significantly inhibited TNKS1 and TNKS2 enzyme activities at low nanomolar levels. TI2-N17 and TI3-D11 stabilized AXIN2, reduced active β-catenin, and downregulated β-catenin target genes in COLO320DM, APC-mutated CRC cells. Both compounds exhibited anticancer effects on COLO320DM and SW403 cells. In addition, the combination treatment of 5-Fluorouracil (5-FU), an anticancer agent for CRC patients, and these compounds synergistically inhibited the proliferation of CRC cells compared to single compound treatment. Our findings demonstrated that TI2-N17 and TI3-D11 are therapeutic candidates targeting TNKS for APC-mutated CRCs treatment and suggested that the combined treatment with the compounds and 5-FU chemotherapy may be an effective therapeutic strategy for APC-mutated CRCs. Based on these compounds, further study will develop potent and selective TNKS inhibitors by designing and synthesizing novel derivative compounds. Citation Format: Hwani Ryu, Hye-Ran Seo, Hyo Jeong Kim, Jiyeon Ahn. Discovery of novel TNKS inhibitors through structure-based virtual screening [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 521.

Abstract 4998: Selective PLK4 inhibition demonstrates synthetic lethality in TRIM37 amplified neuroblastoma and breast cancer models while less selective inhibitors do not

Abstract Amplification and copy number gains of the 17q23 amplicon are common in breast cancer and neuroblastoma and have been associated with early relapse and poor prognosis (Ganesan et al 2012; Takita et al. 2017). A synthetic lethal interaction of PLK4 with 17q23 amplicon-driven overexpression of TRIM37 was discovered in these tumor types (Meitinger et al. 2020; Yeow et al. 2020). High levels of TRIM37 prevented acentrosomal spindle assembly and rendered cells mitotically vulnerable to inhibition of polo-like kinase 4 (PLK4), a serine/threonine protein kinase that controls centriole duplication. We have discovered that exquisitely selective small molecule inhibitors of PLK4, which are highly selective against the kinome including against the closely related aurora kinases and PLK1-3, display this synthetic lethal interaction with TRIM37, while less selective inhibitors do not. PLK4 protein levels are regulated through proteasomal degradation induced by PLK4 trans-autophosphorylation of the phosphodegron. PLK4 inhibition results in blocked trans-autophosphorylation leading to stabilization of PLK4, thus directly demonstrating target engagement in cells. Importantly, PLK4 protein stabilization correlated with cell viability for selective PLK4 inhibitors but not for less selective compounds, providing a quantifiable pharmacodynamic (PD) association with antitumor activity. Cell viability assessment in cancer cell lines revealed that highly selective PLK4 inhibitors showed greater potency in TRIM37 high cancer cell lines as compared to TRIM37 low cell lines. In contrast, less selective compounds, including from the clinical literature, did not display differential potency in TRIM37 high versus low cancer cell lines. Additionally, selective PLK4 inhibition induced significantly greater apoptosis in TRIM37 high versus low cancer cell lines as measured with a caspase 3/7 assay. We confirmed that only selective PLK4 inhibitors are synthetic lethal with TRIM37 amplification using an engineered cell line system of PLK4 G95L, in which the Leucine mutation blocks compound binding but allows the PLK4 enzyme to function. In cell viability assays, selective PLK4 inhibitors were potent in the parental G95 cells and lost activity in L95 cells, unlike less selective inhibitors whose potency did not depend on PLK4. Oral dosing of a selective PLK4 inhibitor resulted in tumor growth inhibition in TRIM37 high xenograft tumors with no body weight loss. In summary, we have discovered that highly selective small molecule inhibitors of PLK4 confirm the potential of the synthetic lethal impact in treating tumors with high levels of TRIM37. Citation Format: Siobhan K. McRee, Chelsea Chen, Christophe Colas, Wie Fang, Wayne Kong, Fang Liu, Jason Long, Jared Moore, Alex Pankov, Dan Shore, Joanne Tan, Robert Warne, Rakesh Vekariya, Amy Young, Anneleen Daemen, Anthony Romero, Melissa R. Junttila, Lori S. Friedman, Kyle A. Edgar. Selective PLK4 inhibition demonstrates synthetic lethality in TRIM37 amplified neuroblastoma and breast cancer models while less selective inhibitors do not. [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 4998.

Abstract 3260: Potentiation of Type I interferon signaling leads to in vivo efficacy achieved with combination of chemotherapy and the PARP7 inhibitor RBN-2397

Abstract There are many pathways that activate the Type I interferon (IFN) response to combat tumor progression including the cytosolic nucleic acid sensing pathways. PARP7 is a monoART enzyme that was reported to negatively regulate the Type I IFN response by interacting with TBK1 during viral infection. We and others have demonstrated that PARP7 is a novel negative regulator of cytosolic nucleic acid sensing in tumor cells. RBN-2397 is the first potent and selective small molecule inhibitor of PARP7 catalytic function. We have shown that administration of RBN-2397 to CT26 tumor-bearing mice significantly decreased tumor growth and led to complete and durable regressions in a subset of mice. The antitumor effects correlated with activation of p-STAT1 and increased expression of ISGs including CXCL10, MX1, and CCL5 in tumors. A growing body of work has shown that DNA damaging agents such as chemotherapies activate the Type I IFN response by generating accumulation of nucleic acids, including DNA, in the cytosol or tumor microenvironment. These aberrant cytosolic nucleic acids are recognized by the innate immune response via cGAS leading to activation of the STING/TBK pathway and subsequent production of IFN-β and expression of ISGs (interferon induced genes). Given that PARP7 suppresses the Type I IFN response, we hypothesized that the immune-related mechanism observed with chemotherapy would be inhibited in tumors with high PARP7 expression. Combining chemo with RBN-2397 could potentiate Type I IFN response activation in PARP7 expressing tumors and enhance immune mediated anti-tumor responses. We tested the ability of platinum (cisplatin, oxaliplatin, and carboplatin) and taxane (docetaxel and paclitaxel) chemotherapy agents to activate the Type I IFN response in the CT26 mouse colon cancer cell line in vitro by gene expression analysis of several ISGs (CXCL10, CCL5, MX1, IFIT1, and IFN-β). Single agent chemo led to modest increases in the expression of ISGs. The combination of cisplatin or oxaliplatin with RBN-2397 significantly potentiated the effect of RBN-2397 and led to a synergistic increase of the ISGs in CT26 cells after 24 hours of treatment. Similar effects were obtained with the combination of docetaxel or paclitaxel with RBN-2397. To evaluate if the combinatorial effect observed in vitro translated into increased efficacy in vivo, we first identified doses of single agent cisplatin or docetaxel that increased expression of ISGs and enhanced cytosolic-DNA in CT26 tumors. Next, we verified the tolerability of validated doses in combination with RBN-2397. Finally, we performed an efficacy study in CT26 tumor bearing mice and observed that the combination of cisplatin and RBN-2397 increased tumor growth inhibition and survival relative to single agent activity. This study provides rationale for exploration of DNA damaging agents in combination with RBN-2397. Citation Format: Kaiko Kunii, Jeff J. Song, Danielle Zimmerman, Chang Liu, Kristy G. Kuplast-Barr, Jonathan J. Novak, Sunaina P. Nayak, Joseph M. Gozgit, Melissa M. Vasbinder, Kristen McEachern, Kevin W. Kuntz, Heike Keilhack, Jennifer R. Molina. Potentiation of Type I interferon signaling leads to in vivo efficacy achieved with combination of chemotherapy and the PARP7 inhibitor RBN-2397 [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 3260.

Abstract 1143: Development of a CDK2-selective small molecule inhibitor INCB123667 for the treatment of CCNE1hi breast cancers

Abstract Cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitors have been approved for treatment of metastatic ER+ HER2− breast cancer as monotherapy or in combination with endocrine therapy. However, many patients become resistant to CDK4/6 therapy due to various mechanisms such as loss of tumor suppressor Rb, hyperactivation of CDK4/6, and/or CDK2/Cyclin E1 expression and activity, resulting in a highly unmet clinical need. Triple-negative breast cancers (TNBC), owing to a lack of hormone receptor (HR) expression and extensive heterogeneity, remain notoriously aggressive and unresponsive to current therapies. CCNE1 overexpression or amplification has been shown to be an oncogenic driver associated with poor clinical outcomes in ovarian, gastric, endometrial, and HR+ breast cancers. CCNE1hi cancers are dependent on CDK2 for growth and survival. CDK2 in complex with CCNE1 phosphorylates Rb, resulting in E2F target gene expression and G1 to S-phase cell cycle progression. Using a potent and selective CDK2 inhibitor, INCB123667, we demonstrate the efficacy of CDK2 inhibition in CCNE1hi breast cancer models. Analysis of genome-wide CRISPR screen data identified a subset of CCNE1hi breast cancer models dependent on CDK2 comprised of basal and luminal breast cancer subtypes, including TNBC, ER+ HER2−, and HR− HER2+ models. Two CCNE1hi cell lines, HCC1569 and MDAMB157, were evaluated in cell assays monitoring pRb, S-phase, and growth inhibition, and found to be sensitive to treatment with INCB123667 and insensitive to multiple CDK4/6 inhibitors. Through extensive biochemical studies, INCB123667 was observed to be potently active against CDK2 at subnanomolar range and inactive against other CDKs (CDK1, CDK4, CDK6, CDK7, and CDK9). INCB123667 also selectively induced senescence in CCNE1hi HCC1569 cells. In contrast, CCNE1 nonamplified cell lines, MCF7 and T47D, were inactive in pRb, S-phase, and growth inhibition assays when treated with INCB123667. Significant tumor growth inhibition was observed in CCNE1hi PDX and xenograft models with minimal body weight loss. We also observed dose-dependent inhibition of CDK2 substrates, including pRb S780, both in vitro and in vivo. To further characterize downstream signaling and transcriptional changes with CDK2 inhibition, we identified a discrete gene signature associated with cell cycle regulation, which was notably distinct from CDK4/6 responsive target genes. In conclusion, we have demonstrated CDK2 activity in CCNE1hi breast cancers using INCB123667, a potent and selective small molecule inhibitor of CDK2 currently in clinical development. Treatment resulted in cell growth inhibition and antitumor activity in vitro and in vivo. Our data demonstrate the therapeutic potential of CDK2 inhibition in CCNE1hi HR+ HER2− and TNBC breast cancer models. Acknowledgments: R&D services provided by Pharmaron, Crown Bioscience, and Champions Oncology Citation Format: Saswati Chand, Michael Hansbury, Yvonne Lo, Pat Feldman, Justine Carl, Cynthia Timmers, Joshua Hummel, Susan Wee, Sunkyu Kim. Development of a CDK2-selective small molecule inhibitor INCB123667 for the treatment of CCNE1hi breast cancers [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 1143.

Abstract 3095: Development of a novel ADAR1 inhibitor as a potent immunomodulator for the treatment of solid tumors

Abstract Background: Adenosine deaminase, RNA specific (ADAR1), catalyzes the hydrolytic deamination of adenosine (A) to inosine (I) in double-stranded (ds) RNAs. There are 2 isoforms of ADAR1 (p110 in the nucleus; p150 in cytoplasm) and both modify self dsRNA in coding and non-coding regions​. The ADAR1 p150 isoform is expressed from an interferon (IFN)-response promoter and has a Z-DNA/Z-RNA binding domain at the N-terminus. ADAR1 p150 edits 3′-untranslated region dsRNAs comprising of inverted Alu repeats and thereby suppresses MDA5-MAVS-IFN signaling. ADAR1 is commonly overexpressed in multiple myeloma, breast, lung, liver, skin and esophageal cancer where it promotes cancer progression. Inhibition of ADAR1 has promising anti-tumor efficacy as monotherapy and in combination with checkpoint inhibitors, radiotherapy and chemotherapeutic modalities. Herein, we outline the discovery of a potential first-in-class ADAR1 inhibitor for cancer immunotherapy. Methods: AVA-ADR-001 was identified through a high throughput p110 knockout cell-based assay. The ability of AVA-ADR-001 to induce interferons was confirmed in various cell lines like A549 p110 KO, HCT116 and B16F10. Finally, the anti-tumor efficacy of AVA-ADR-001 was evaluated in B16F10 syngeneic melanoma mice model as monotherapy and in combination with anti-PD-1. Results: We have identified a first-in-class small molecule inhibitor of ADAR1, which shows significant IFN response in vitro in an MDA5 dependent manner. In vitro binding studies have confirmed direct binding of AVA-ADR-001 with the Zα domain of ADAR1 thus confirming its selectivity to the p150 isoform. AVA-ADR-001 demonstrates micromolar EC50 and anti-tumor efficacy against B16F10 melanoma syngeneic mouse model. 100 µg of AVA-ADR-001 treatment resulted in 45% tumor growth inhibition (TGI), 1.5x superior to Anti-PD1 treatment. Combining AVA-ADR-001 with Anti-PD1 demonstrated a synergistic effect 2x superior to Anti-PD1 alone. Additionally, several interferon stimulated genes like IFIH1, IFN-β and CXCL-10 were significantly upregulated in the tumor samples of the AVA-ADR-001 monotherapy and combination groups. Conclusions: To our knowledge no selective small molecule inhibitors of ADAR1 have been reported so far and AVA-ADR-001 is the first disclosure of such an inhibitor. AVA-ADR-001 is a potent and selective first-in-class ADAR1 inhibitor which has shown significant IFN induction in various cancer cell lines and in vivo in the tumor microenvironment resulting in substantial tumor growth inhibition as monotherapy and synergistically in combination with Anti-PD1. Considering the immune-suppressive and pro-metastatic role of ADAR1, AVA-ADR-001 serves as a promising starting point for novel ADAR1 inhibitors as therapeutic modalities in cancer immunotherapy. Keywords: Cancer immunotherapy, small molecule inhibitors, ADAR1, innate immunity Citation Format: Aditya Kulkarni, Avijit Goswami, Sandeep Goyal, Kawaljit Singh, Princy Khurana, Ishani Ghoshal. Development of a novel ADAR1 inhibitor as a potent immunomodulator for the treatment of solid tumors [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 3095.

Abstract 1015: VEGFR-3 expression profiling by histology to classify patient population for the selective VEGFR-3 inhbitor EVT801

Abstract Introduction: EVT801 is a highly selective small molecule inhibitor of VEGFR-3 and acts by inhibiting tumor lymphangiogenesis and angiogenesis. Among the tyrosine kinase inhibitors, EVT801 has shown compelling activity in a wide range of cancer models and favorable preclinical toxicology profile. A phase I clinical study (NCT05114668) is underway to characterize safety, tolerability, pharmacokinetics of EVT801, and explore patient stratification which will be key for further development. Literature reports VEGFR-3 expression in several tumor cells - especially in kidney and liver cancers. However, we did not detect VEGFR-3 in tumor cells in translational in vivo models we explored (i.e., chemo-induced liver cancer and panel of PDX models). To clarify the situation, we validated a highly specific protocol for VEGFR-3 immunohistochemistry labelling, readily transferable to clinical centers. Results: We evaluated many different tumor types, and eventually focused our analysis on kidney cancer and soft tissue sarcomas. Initially, VEGFR-3 expression was assessed in 29 primary kidney cancer samples and 23 metastatic kidney cancer samples with CD34 to stain vessels and D2-40 to track lymphatics. We detected VEGFR-3 expression in the endothelial cells within the kidney tumor but not in the tumor cells. Remarkably, we observed a very distinct delineation between the kidney tumor vasculature, which showed high VEGFR-3 expression, and the normal adjacent tissue, which was devoid of VEGFR-3 expression. All vessels around and in the tumor were stained by CD34, whereas D2-40 staining was limited to the peritumoral regions. In addition, we found that VEGFR-3 expression in metastases from kidney tumors matched the expression pattern of the primary tumor. This held true even in patients treated with sunitinib, positioning EVT801 as a potential treatment option after failure of such multi tyrosine kinase inhibitors. Among the samples from 103 patients with soft tissue sarcomas, VEGFR-3 was generally highly expressed; however, 3 distinct groups emerged: i) tumors with high VEGFR-3 expression in tumor cells, tumor vessels and peritumoral vessels; ii) tumors with high expression in all vessels (tumor and peritumoral) but not in the tumor cells (similar to kidney tumors); iii) tumors with intermediate expression in peritumoral vessels. Conclusion: We were able to quantify VEGFR-3 expression in the kidney and soft tissue sarcoma cohorts plus several others, such as hepatocarcinoma and non-small cell lung cancer. This enables us selecting indications that might benefit from EVT801 as a monotherapy (e.g., clear cell renal cell carcinoma and soft tissue sarcomas) or in combination with standard of care. Accordingly, VEGFR-3 expression is retrospectively quantified during EVT801 clinical trial phase 1 trial and may be used to stratify patients in the future. Citation Format: michael Paillasse, pierre-benoit Ancey, Gaelle Badet, Anne Gomez-Brouchet, Janik Selves, philippe Rochaix, Maxime Battistella, Celeste Lebbe, philippe Cassier, carlos Gomez-Roca, Jean-Pierre Delord, Mark Whittaker, Lise Davenne, John Friend, Michael Fitzgerald, James Garner, Pierre Fons. VEGFR-3 expression profiling by histology to classify patient population for the selective VEGFR-3 inhbitor EVT801 [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 1015.

Abstract 1628: A small-molecule inhibitor of WRN selectively kills MSI-H cancer cells and phenocopies WRN genetic defects

Abstract Werner syndrome protein (WRN) is a RecQ-family helicase involved in the maintenance of genome integrity. Germline mutations in WRN cause premature aging and cancer predisposition. Analysis of systematic RNAi and CRISPR screening data has previously revealed that WRN is essential for the survival of cancer cell lines with high microsatellite instability (MSI-H). We have developed potent and selective small-molecule inhibitors of WRN helicase (WRNi) and showed that pharmacological inhibition of WRN causes lethality and induction of DNA damage markers selectively in MSI-H cancer cell lines compared to microsatellite-stable (MSS) cell lines. Screening of WRNi across a large panel of pooled, barcoded cell lines in the PRISM format revealed selective sensitivity in MSI-H cell lines and showed that pharmacological inhibition of WRN is highly correlated with genetic ablation of WRN across this panel, confirming selectivity for WRN. In vivo evaluation demonstrated robust and MSI-selective tumor regressions. These data provide pharmacological proof-of-concept for the WRN/MSI-H synthetic lethal relationship and support WRN inhibition as a novel therapeutic approach for the treatment of MSI-H cancers. Citation Format: Yanhua Rao, Anjana Srivatsan, Marya Liimatta, Diana Munoz, Jeanne Quirit, Jianxia Shi, An Nguyen, Xin Linghu, Federowicz Federowicz, Brian Jones, Melissa Fleury, Zach Newby, Michael P. Dillon, Paul A. Barsanti, Mark R. Lackner, Michael A. White, Yang Lee, Phil Landis, Yang Peng, Michelle Cicchini, Josh Cottom, Leng Nickels, Ed Brnardic, Michael DeMartino, Josh Taygerly. A small-molecule inhibitor of WRN selectively kills MSI-H cancer cells and phenocopies WRN genetic defects [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 1628.

CTP Synthase 1 Is a Novel Therapeutic Target in Lymphoma.

Lymphoma is the most common hematological malignancy and is among the 10 most prevalent cancers worldwide. Although survival has been improved by modern immunochemotherapeutic regimens, there remains a significant need for novel targeted agents to treat both B-cell and T-cell malignancies. Cytidine triphosphate synthase 1 (CTPS1), which catalyzes the rate-limiting step in pyrimidine synthesis, plays an essential and nonredundant role in B-cell and T-cell proliferation but is complemented by the homologous CTPS2 isoform outside the hemopoietic system. This report describes the identification and characterization of CTPS1 as a novel target in B- and T-cell cancers. A series of small molecules have been developed which show potent and highly selective inhibition of CTPS1. Site-directed mutagenesis studies identified the adenosine triphosphate pocket of CTPS1 as the binding site for this small molecule series. In preclinical studies, a potent and highly selective small molecule inhibitor of CTPS1 blocked the in vitro proliferation of human neoplastic cells, showing the highest potency against lymphoid neoplasms. Importantly, pharmacological CTPS1 inhibition induced cell death by apoptosis in the majority of lymphoid cell lines tested, thus demonstrating a cytotoxic mechanism of action. Selective CTPS1 inhibition also inhibited the growth of neoplastic human B- and T- cells in vivo. These findings identify CTPS1 as a novel therapeutic target in lymphoid malignancy. A compound from this series is in phase 1/2 clinical studies for the treatment of relapsed/refractory B- and T-cell lymphoma (NCT05463263).

Treatment Strategies for KRAS-Mutated Non-Small-Cell Lung Cancer.

Activating mutations in KRAS are highly prevalent in solid tumours and are frequently found in 35% of lung, 45% of colorectal, and up to 90% of pancreatic cancers. Mutated KRAS is a prognostic factor for disease-free survival (DFS) and overall survival (OS) in NSCLC and is associated with a more aggressive clinical phenotype, highlighting the need for KRAS-targeted therapy. Once considered undruggable due to its smooth shallow surface, a breakthrough showed that the activated G12C-mutated KRAS isozyme can be directly inhibited via a newly identified switch II pocket. This discovery led to the development of a new class of selective small-molecule inhibitors against the KRAS G12C isoform. Sotorasib and adagrasib are approved in locally advanced or metastatic NSCLC patients who have received at least one prior systemic therapy. Currently, there are at least twelve KRAS G12C inhibitors being tested in clinical trials, either as a single agent or in combination. In this study, KRAS mutation prevalence, subtypes, rates of occurrence in treatment-resistant invasive mucinous adenocarcinomas (IMAs), and novel drug delivery options are reviewed. Additionally, the current status of KRAS inhibitors, multiple resistance mechanisms that limit efficacy, and their use in combination treatment strategies and novel multitargeted approaches in NSCLC are discussed.

Abstract OT3-22-01: First-in-human global multi-center study of RLY-2608, a pan mutant and isoform selective PI3Kα inhibitor, as a single agent in advanced solid tumor patients and in combination with fulvestrant in patients with advanced breast cancer

Abstract Background: Targeting constitutively active mutant kinases with selective small molecule inhibitors is a key therapeutic pillar of precision oncology. Phosphatidylinositol-4,5bisphosphate-3 kinase, catalytic subunit alpha (PIK3CA) mutations leading to oncogenic activation of PI3Kα represent the largest opportunity for this approach in solid tumors. However, there is no selective inhibitor that targets mutant PI3Kα in the clinic. Toxicity related to non-selective inhibition of WT PI3Kα (hyperglycemia) and other PI3K isoforms limits the tolerability, dosing and efficacy of the orthosteric inhibitor, alpelisib, the only approved solid tumor PI3K inhibitor. RLY-2608, a novel oral allosteric PI3Kα inhibitor, is uniquely designed to overcome these limitations via mutant- and isoform-selective PI3Kα inhibition for greater target coverage, improved tolerability and antitumor activity. We initiated a first-in-human (FIH), study to evaluate the clinical activity of RLY-2608 as a single agent in advanced solid tumor patients (pts) with PI3KCA mutations and in combination with fulvestrant in pts with PIK3CA mutant, HR+, HER2- metastatic breast cancer (MBC). Methods: This is a global, multi-center, dose escalation/expansion study (NCT05216432) of RLY2608 as a single agent in adults who have advanced solid tumors and are refractory, intolerant, or declined standard therapy and RLY-2608 in combination with fulvestrant in previously treated pts with HR+/HER2- MBC. Eligibility criteria include presence of PI3KCA mutation (blood or tumor) per local assessment, ECOG performance status 0-1, measurable or evaluable disease per RECIST 1.1 and no prior PI3K inhibitor (except combination group 2). RLY-2608 is administered on a continuous schedule with 4-week cycles. Adverse events (AEs) per CTCAE v5, PK, biomarkers (mutant ctDNAs and insulin pathway markers) and anti-tumor activity are assessed serially. Dose escalation employs a Bayesian Optimal Interval design to identify MTD and RP2D. Following dose escalation, pts will be treated with RLY-2608 at the MTD/RP2D in a monotherapy dose expansion with 5 groups (N=75, 15 each): 1. Clear cell ovarian carcinoma 2. Head and neck squamous cell carcinoma 3. Cervical cancer 4. Other solid tumors 5. PI3KCA double mutations. In addition, two expansion cohorts will enroll patients with HR+/HER2- MBC treated with RLY-2608 and fulvestrant combination (N = 30, 15 each): 1. No prior PI3K therapy 2. Intolerant to PI3K inhibitors. The primary endpoints are MTD/RP2D and AE profile for single agent and combination; key secondary endpoints are PI3KCA genotype in blood and tumor, PK, biomarkers, and overall response rate. US enrollment began December 2021 and ex-USA startup is under way. Citation Format: Andreas Varkaris, Erika Hamilton, Jason Henry, Alexander I. Spira, Alison M. Schram, Julia E. McGuinness, Gege Tan, Xiaoyan Li, Tamieka Hunter, Ramin Samadani, Alison Timm, Djuro Karanovic, Vivek Subbiah, Cesar A. Perez. First-in-human global multi-center study of RLY-2608, a pan mutant and isoform selective PI3Kα inhibitor, as a single agent in advanced solid tumor patients and in combination with fulvestrant in patients with 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 OT3-22-01.

Downregulation of Polo-like kinase 4 induces cell apoptosis and G2/M arrest in acute myeloid leukemia.

Polo-like kinase 4 (PLK4) is a crucial regulator for centriole replication and is reported to be aberrantly expressed in various cancers, where it participates to tumorigenesis. However, PLK4 effect in acute myeloid leukemia (AML), is still uncertain. This study investigates the function of PLK4 in AML. Quantitative real-time PCR was used to measure the level of PLK4. Centrinone, a selective PLK4 small molecule inhibitor, was used for PLK4 inhibition and explore its effect in AML cells. The cell growth was detected by the CCK8, while the cell cycle and apoptosis were assessed by flow cytometry. The level of proteins associated with apoptosis, cell cycle and endoplasmic reticulum (ER) stress were analyzed by western blotting. PLK4 was overexpressed in AML cells. PLK4 knockdown or its specific inhibition by centrinone induced G2/M phase arrest via suppressing the expression of cyclin B1 and Cdc2 and promoting the level of proapoptotic proteins. Moreover, PLK4 targeting enhanced the level of proteins related to ER stress, such as GRP78, ATF4, ATF6, and CHOP. These findings demonstrated that targeting PLK4 can induce apoptosis, G2/M and ER stress in AML cells.

Reversal of malignant ADAR1 splice isoform switching with Rebecsinib.

Adenosine deaminase acting on RNA1 (ADAR1) preserves genomic integrity by preventing retroviral integration and retrotransposition during stress responses. However, inflammatory-microenvironment-induced ADAR1p110 to p150 splice isoform switching drives cancer stem cell (CSC) generation and therapeutic resistance in 20 malignancies. Previously, predicting and preventing ADAR1p150-mediated malignant RNA editing represented a significant challenge. Thus, we developed lentiviral ADAR1 and splicing reporters for non-invasive detection of splicing-mediated ADAR1 adenosine-to-inosine (A-to-I) RNA editing activation; a quantitative ADAR1p150 intracellular flow cytometric assay; a selective small-molecule inhibitor of splicing-mediated ADAR1 activation, Rebecsinib, which inhibits leukemia stem cell (LSC) self-renewal and prolongs humanized LSC mouse model survival at doses that spare normal hematopoietic stem and progenitor cells (HSPCs); and pre-IND studies showing favorable Rebecsinib toxicokinetic and pharmacodynamic (TK/PD) properties. Together, these results lay the foundation for developing Rebecsinib as a clinical ADAR1p150 antagonist aimed at obviating malignant microenvironment-driven LSC generation.

Abstract P6-10-07: CDK2 inhibition with BLU-222 in combination with ribociclib demonstrates robust antitumor activity in pre-clinical models of CDK4/6 inhibitor-naïve and -resistant HR+/HER2- breast cancer

Abstract Cyclin-dependent kinase 4/6 (CDK4/6) inhibitors are standard of care in combination with endocrine therapy in advanced hormone receptor-positive (HR+), human epidermal growth factor receptor 2 negative (HER2-) breast cancer. Despite improved progression-free and overall survival, almost all patients develop CDK4/6 inhibitor resistance and experience disease progression on treatment. Aberrant activation of CDK2/cyclin E is a key resistance mechanism by which tumors can evade CDK4/6 blockade. Therefore, patients with HR+/HER2- breast cancer could benefit from treatment with a selective CDK2 inhibitor in combination with CDK4/6 inhibitors both in the resistant and first-line (1L) settings. BLU-222 is a novel, potent, and selective small-molecule inhibitor of CDK2 with favorable pharmacokinetic properties when administered orally, currently in early-stage clinical development. In pre-clinical studies using the MCF-7 xenograft model of HR+ CDK4/6-responsive breast cancer, treatment with BLU-222 combined with the CDK4/6 inhibitor ribociclib led to pronounced and durable tumor regression superior to ribociclib alone. In a derived palbociclib resistant MCF-7 xenograft model, ribociclib had no anti-tumor activity while BLU-222 led to a strong and durable anti-tumor response (83% tumor growth inhibition [TGI]) that was further improved when given in combination with ribociclib (110% TGI). To further explore the mechanism of aberrant CDK2 activation in CDK4/6 resistant, HR+ breast cancer, we engineered isogenic T47D cell lines to overexpress cyclin E1 (CCNE1) with or without p16, an endogenous inhibitor of CDK4/6 activity. In in vitro proliferation assays, co-expression of CCNE1 and p16 sensitized T47D cells to BLU-222 by approximately 10-fold compared to the parental control (110 nM vs 1078 nM, respectively). CDK4/6 inhibition with ribociclib had no anti-proliferative effect in the CCNE1 overexpressing cell lines regardless of p16 expression status. In vivo, treatment of the empty vector control T47D xenografts with ribociclib led to tumor stasis, while ribociclib in combination with BLU-222 led to tumor regression. T47D xenografts overexpressing both CCNE1 and p16 were resistant to ribociclib however CDK2 inhibition with BLU-222 single-agent treatment led to tumor regression. Finally, the activity of BLU-222 was evaluated in a patient-derived xenograft (PDX) model of CDK4/6 inhibitor-resistant HR+/HER2- breast cancer where the patient had progressed on 1L palbociclib/fulvestrant and 2L abemaciclib/fulvestrant therapy. In this PDX model, BLU-222 in combination with ribociclib led to tumor stasis, even in the absence of fulvestrant. In conclusion, these data support CDK2/cyclin E activation as a key vulnerability in CDK4/6 resistant, HR+/HER2- breast cancer and provide a rationale for the study of BLU-222 in patients with disease progression following CDK4/6 inhibitor treatment. Additionally, the improved durability of response when BLU-222 is combined with CDK4/6 inhibitors in the CDK4/6-naïve setting supports the combination of these agents as 1L treatment. BLU-222 is currently under investigation in VELA (NCT05252416), a phase 1/2, first-in-human trial for patients with cyclin E aberrant cancers and HR+/HER2- breast cancer. Citation Format: Victoria Brown, Nealia House, Phil Ramsden, Karen Ho, Hsin-Jung Wu, Erik Wilker, Jian Guo, Maxine Chen, Douglas Wilson, Neil Bifulco, Steve Wenglowsky, Yoon Jong Choi, Kerrie Faia. CDK2 inhibition with BLU-222 in combination with ribociclib demonstrates robust antitumor activity in pre-clinical models of CDK4/6 inhibitor-naïve and -resistant HR+/HER2- 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 P6-10-07.

2-Ethynylbenzaldehyde-Based, Lysine-Targeting Irreversible Covalent Inhibitors for Protein Kinases and Nonkinases.

Lysine-targeting irreversible covalent inhibitors have attracted growing interests in recent years, especially in the fields of kinase research. Despite encouraging progress, few chemistries are available to develop inhibitors that are exclusively lysine-targeting, selective, and cell-active. We report herein a 2-ethynylbenzaldehyde (EBA)-based, lysine-targeting strategy to generate potent and selective small-molecule inhibitors of ABL kinase by selectively targeting the conserved catalytic lysine in the enzyme. We showed the resulting compounds were cell-active, capable of covalently engaging endogenous ABL kinase in K562 cells with long-residence time and few off-targets. We further validated the generality of this strategy by developing EBA-based irreversible inhibitors against EGFR (a kinase) and Mcl-1 (a nonkinase) that covalently reacted with the catalytic and noncatalytic lysine within each target.