Polo-like Kinase 1 Inhibition Research Articles

Abstract PO2-04-09: Combination of the PLK1 Inhibitor Onvansertib and the PI3Kα Inhibitor Alpelisib Overcomes Palbociclib Resistance in PIK3CA-mutated HR+ Breast Cancer

Abstract In the 1st line setting of hormone receptor-positive HER2 negative (HR+/HER2-) metastatic breast cancer, cyclin dependent kinase 4 and 6 (CDK4/6) inhibitors (palbociclib, abemaciclib and ribociclib) are typically used in combination with endocrine therapy. Though initially effective, acquired resistance to either CDK4/6 inhibitors or endocrine therapy eventually leads to disease relapse in most patients. Activating mutations in PIK3CA, the gene encoding alpha catalytic subunit of phosphatidylinositol-4,5-bisphosphat 3-kinase (PI3K) are found in approximately 40% of HR+ breast cancer patients and have been implicated in mediating resistance to CDK4/6 inhibitors. The PI3Kα inhibitor alpelisib is currently approved for PI3KCA-mutant HR+ breast cancer after progression on CDK4/6 inhibitors. However, drug-associated resistance mechanisms may limit its clinical benefit. Synergistic combination therapies have the potential to improve efficacy and overcome resistance mechanisms. Polo-like kinase 1 (PLK1) is a serine-threonine-protein kinase, key cell cycle regulator that controls G2/M entry and mitotic progression, and that has been shown to mediate resistance to palbociclib in HR+ breast cancer. Onvansertib is an oral and highly specific PLK1 inhibitor currently on clinical development in solid tumors and hematological malignancies. Here we evaluated the potential of onvansertib to increase the activity of alpelisib in PIK3CA-mutant HR+ breast cancer preclinical models. The combination of onvansertib and alpelisib synergistically inhibited cell viability and/or colony formation in three PI3KCA-mutant HR+ breast cancer cell lines, MCF-7 (PIK3CA E545K), EFM-19 (PIK3CA H1047L), and T-47D (PIK3CA 1047R). A significant increase in apoptosis was observed in cells treated with the combination compared to either agent alone. We next tested the combination of onvansertib and alpelisib in patient-derived xenograft (PDX) models. For this purpose, palbociclib-resistant PIK3CA-mutant HR+ breast cancer PDXs were established from primary breast tumor (PDX HBCx-86, PIK3CA E545K) or from metastatic bone biopsies of patients who had progressed on endocrine therapy plus palbociclib (HBCx-180, PIK3CA H1047R) or on PI3Kα inhibitor (HBCx-134palboR31, PIK3CA H1047R). PDX tumors were grafted subcutaneously in nude mice and mice were treated with vehicle, onvansertib (45 mg/kg, oral, 5 days a week), alpelisib (25mg/kg, oral, 5 days a week) or the combination. The combination was well tolerated and showed superior anti-tumor activity than the single agents in the 3 PDX models. In the HBCx-86 PDX, although none of the single agents showed activity, the combination induced potent tumor growth inhibition. In the metastasis-derived PDX HBCx-134palboR31, the combination treatment induced pronounced tumor regression in 62% of mice (5/8), with complete response in 25% (2/8), while mice treated with the single agents showed tumor progression. Finally, in the HBCx-180 PDX, established from a patient with primary resistance to palbociclib, the efficacy of the combination was greater and more durable, with significant survival advantage, compared to the monotherapies. Collectively, our preclinical findings suggest that co-targeting PLK1 and PI3Kα with onvansertib and alpelisib respectively, may constitute a promising therapeutic combination strategy for patients with PIK3CA-mutant HR+ breast cancer failing to respond to first-line standard of care therapies. Citation Format: Sreeja Sreekumar, Pierre Painsec, Davis Klein, Dalia Gonzalez, Laura Sourd, Elodie Montaudon, Tod Smeal, Elisabetta Marangoni, Maya Ridinger. Combination of the PLK1 Inhibitor Onvansertib and the PI3Kα Inhibitor Alpelisib Overcomes Palbociclib Resistance in PIK3CA-mutated HR+ Breast Cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-04-09.

Abstract PO1-04-14: PLK1 Inhibitor Onvansertib Extends the Response and Overcomes Resistance to Paclitaxel in Palbociclib-resistant HR+ Breast Cancer Patient-derived Xenografts

Abstract Paclitaxel is a first-line chemotherapy for hormone receptor positive (HR+) metastatic breast cancer patients after progression on a combination of endocrine and CDK4/6 inhibitor therapy. Response rate to paclitaxel ranges between 20-40%, and most patients progress due to intrinsic or acquired resistance, leaving them with limited treatment choices. Therapeutic strategies to overcome paclitaxel resistance and extend its clinical benefit are urgently needed. Paclitaxel-resistant tumors have increased expression of polo-like kinase 1 (PLK1), a serine-threonine-protein kinase that regulates mitosis and cell cycle progression. PLK1 has also been shown to mediate resistance to CDK4/6 inhibitor palbociclib in HR+ breast cancer. Onvansertib is an orally bioavailable, highly potent, and selective PLK1 inhibitor in clinical development. Onvansertib exhibited synergy in vitro and showed potent anti-tumor activity in vivo in combination with paclitaxel in ovarian cancer and triple negative breast cancer (TNBC) preclinical models. A phase 1b/2 clinical trial is ongoing to evaluate the safety and efficacy of onvansertib plus paclitaxel in advanced TNBC (NCT05383196). Here we investigated whether the potential of onvansertib and paclitaxel combination could be extended to HR+ breast cancer. Co-treatment with onvansertib and paclitaxel synergistically inhibited the viability of HR+ breast cancer cell lines (MCF-7, T-47D, EFM-19, CAMA-1, ZR-75-1). Compared to the single agents, the combination induced increased apoptosis. The combination was further tested in vivo in patient-derived xenograft (PDX) models. To this end, six HR+ breast cancer PDXs with intrinsic or acquired resistance to palbociclib were established from primary breast tumors (n=1) or metastatic bone biopsies (n=5). The PDX tumors were engrafted subcutaneously in nude mice and the effect of onvansertib (45 mg/kg, oral, 5 days a week) and paclitaxel (15-25mg/kg, IP, once a week) was investigated as monotherapy and in combination. The combination of onvansertib and paclitaxel was tolerated with minimal toxicity and showed enhanced anti-tumor activity compared to either agent alone in all 6 PDX models. Paclitaxel and onvansertib monotherapies had little to no anti-tumor activity in 4 out of 6 PDX models (HBCx124palboR25, HBCx-139 palboR5, HBCx-202 and HBCx-131). Importantly, the combination of onvansertib and paclitaxel induced strong anti-tumor activity in these 4 models. Tumor regression was observed in the 2 PDX models with acquired resistance to palbociclib, HBCx-124palboR25 and HBCx-139palboR5, with 62% (5/8 mice) and 54% (6/11 mice) complete response rates, respectively. The combination showed tumor stasis in HBCx-131 and induced tumor regression in HBCx-202, established from patient resistant to palbociclib. In 2 of the 6 PDXs, paclitaxel exhibited anti-tumor activity, inducing tumor regression in HBCx-137paboR26 and tumor stasis in HBCx-86. In comparison, the combination of onvansertib and paclitaxel had greater activity, inducing tumor regression in both models with a higher rate of complete response than the monotherapy. Complete response rates for paclitaxel versus the combination were 62% and 100% in the HBCx-137palboR26 model and 0% and 67% in the HBCx-86 model. Notably, the antitumor activity of the onvansertib and paclitaxel combination was very durable, showing a robust delay in tumor relapse after stopping therapy. Together, our data strongly support that combining paclitaxel with the PLK1 inhibitor onvansertib extends its benefit and overcomes paclitaxel resistance, and represents a promising therapeutic strategy for HR+ breast cancer patients after progression on a combination of endocrine and CDK4/6 inhibitor therapy. Citation Format: Maya Ridinger, Pierre Painsec, Sreeja Sreekumar, Dalia Gonzalez, Davis Klein, Laura Sourd, Elodie Montaudon, Paul Cottu, Tod Smeal, Elisabetta Marangoni. PLK1 Inhibitor Onvansertib Extends the Response and Overcomes Resistance to Paclitaxel in Palbociclib-resistant HR+ Breast Cancer Patient-derived Xenografts [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-04-14.

Exploring Novel PLK1 Inhibitors based on Computational Studies of 2,4-Diarylaminopyrimidine Derivatives

Background: Polo-like kinase 1 (PLK1) is an important target for anti-cancer drugs. A series of novel 2,4-diarylaminopyrimidine derivatives (DAPDs) as PLK1 inhibitors (PLKIs) with remarkable activities have been reported recently. Methods: A systemically computational study was performed on these DAPDs, including threedimensional quantitative structure-activity relationship (3D-QSAR) modeling, molecular docking, and molecular dynamics (MD) simulation. Results: The constructed 3D-QSAR models exhibited reliable predictability with satisfactory validation parameters. The dockings revealed the binding modes of DAPDs in PLK1 protein, and two key residue, Cys133 and Phe183, could interact with DAPDs by hydrogen bonds and π-π stacking, which might be significant for the activity of these PLKIs. Eight compounds with higher predicted activity than the most active DAPD-compound (16) were designed based on the 3D-QSAR models. These newly designed compounds also exhibited higher docking scores than compound 16 in the binding pocket of PLK1. The ADME predictions and MD simulations further indicated that two hit compounds with reasonable pharmacokinetics properties could stably bind with PLK1 and have the potential to become novel PLKIs. Conclusion: Two newly designed compounds might have the potential to be novel PLKIs. These results might provide important information for the design and development of novel PLKIs.

PLK1-activating IFI16-STING-TBK1 pathway induces apoptosis of intestinal epithelial cells in patients with intestinal Behçet's syndrome.

Inflammatory signals from immunological cells may cause damage to intestinal epithelial cells (IECs), resulting in intestinal inflammation and tissue impairment. Interferon-γ-inducible protein 16 (IFI16) was reported to be involved in the pathogenesis of Behçet's syndrome (BS). This study aimed to investigate how inflammatory cytokines released by immunological cells and IFI16 participate in the pathogenesis of intestinal BS. RNA sequencing and real-time quantitative PCR (qPCR) showed that the positive regulation of tumor necrosis factor-α (TNF-α) production in peripheral blood mononuclear cells (PBMCs) of intestinal BS patients may be related to the upregulation of polo like kinase 1 (PLK1) in PBMCs (P = 0.012). The plasma TNF-α protein level in intestinal BS was significantly higher than in healthy controls (HCs; P = 0.009). PBMCs of intestinal BS patients and HCs were co-cultured with human normal IECs (NCM460) to explore the interaction between immunological cells and IECs. Using IFI16 knockdown, PBMC-NCM460 co-culture, TNF-α neutralizing monoclonal antibody (mAb), stimulator of interferon genes (STING) agonist 2'3'-cGAMP, and the PLK1 inhibitor SBE 13 HCL, we found that PLK1 promotes the secretion of TNF-α from PBMCs of intestinal BS patients, which causes overexpression of IFI16 and induces apoptosis of IECs via the STING-TBK1 pathway. The expressions of IFI16, TNF-α, cleaved caspase 3, phosphorylated STING (pSTING) and phosphorylated tank binding kinase 1 (pTBK1) in the intestinal ulcer tissue of BS patients were significantly higher than that of HCs (all P < 0.05). PLK1 in PBMCs of intestinal BS patients increased TNF-α secretion, inducing IEC apoptosis via activation of the IFI16-STING-TBK1 pathway. PLK1 and the IFI16-STING-TBK1 pathway may be new therapeutic targets for intestinal BS.

Identification of naphthalimide-derivatives as novel PBD-targeted polo-like kinase 1 inhibitors with efficacy in drug-resistant lung cancer cells

Targeting polo-box domain (PBD) small molecule for polo-like kinase 1 (PLK1) inhibition is a viable alternative to target kinase domain (KD), which could avoid pan-selectivity and dose-limiting toxicity of ATP-competitive inhibitors. However, their efficacy in these settings is still low and inaccessible to clinical requirement. Herein, we utilized a structure-based high-throughput virtual screen to find novel chemical scaffold capable of inhibiting PLK1 via targeting PBD and identified an initial hit molecule compound 1a. Based on the lead compound 1a, a structural optimization approach was carried out and several series of derivatives with naphthalimide structural motif were synthesized. Compound 4Bb was identified as a new potent PLK1 inhibitor with a KD value of 0.29 μM. 4Bb could target PLK1 PBD to inhibit PLK1 activity and subsequently suppress the interaction of PLK1 with protein regulator of cytokinesis 1 (PRC1), finally leading to mitotic catastrophe in drug-resistant lung cancer cells. Furthermore, 4Bb could undergo nucleophilic substitution with the thiol group of glutathione (GSH) to disturb the redox homeostasis through exhausting GSH. By regulating cell cycle machinery and increasing cellular oxidative stress, 4Bb exhibited potent cytotoxicity to multiple cancer cells and drug-resistant cancer cells. Subcutaneous and oral administration of 4Bb could effectively inhibit the growth of drug-resistant tumors in vivo, doubling the survival time of tumor bearing mice without side effects in normal tissues. Thus, our study offers an orally-available, structurally-novel PLK1 inhibitor for drug-resistant lung cancer therapy.

Proteasome activation is critical for cell death induced by inhibitors of polo-like kinase 1 (PLK1) in multiple cancers

Inhibitors of polo-like kinase (PLK) are currently being evaluated as anticancer drugs. However, the molecular mechanism of PLK inhibitor-induced cell death is not fully understood. In this study, we found that GW843682X and BI2536, two inhibitors of PLK1, significantly induced cell death in multiple type cells. The induction of cell death was related to the preferring expression of PLK1. However, in human umbilical vascular endothelial cells (HUVEC) and human colorectal carcinoma cells, which expressed higher levels of both PLK1 and PLK2, PLK1 inhibitors induced very low levels of cell death. Clinical analysis reveals PLK1 presence in 26 of 30 NPC tumor tissues. In in vivo NPC lung metastasis nude mouse models, PLK1 inhibitors decreased NPC progress. Mechanistically, the PLK1 inhibitor did not activate p53, and the cell death was not reversed by p53 inhibition. Moreover, PLK1 inhibitor-induced cell death was PARP- and caspase-independent. Although PLK1 inhibitors induced down-regulation of calpain inhibitor calpastatin and calpain was activated by PLK1 inhibition, calpain blocking did not reverse cell death induced by PLK1 inhibitors, suggesting the non-involvement of calpain. Surprisingly, we found that PLK1 inhibitors induced the activation of proteasome, and the treatment of cells with PLK1 inhibitors reduced the levels of ubiquitinated proteins. And proteasome inhibitors reversed cell death induced by PLK1 inhibitors in various cell types in which PLK1 was preferentially expressed. Moreover, PLK1 inhibition reversed the degradation of proteins including p53, caspase 8, PARP and calpastatin. These results suggest that the activation of proteasome is critical for cell death induced by PLK1 inhibition.

Abstract LB225: Regulation of breast cancer stem cell identity by PLK1 and FOXC2

Abstract One of the main drivers for the drastic reduction in survival rates caused by metastatic breast cancer is attributed to Epithelial-to-Mesenchymal Transition (EMT). During embryonic development and wound healing, EMT plays a vital role and during cancer pathogenesis, cancer cells reactivate this program to gain plasticity, migration, and invasion promoting metastasis. In addition, EMT also plays a crucial role in generating a subpopulation of highly resilient tumor cells known as cancer stem cells (CSCs). Moreover, following conventional treatment, residual tumor cells that remain after treatment tend to exhibit CSC properties. EMT-induced cancer stem cells exhibit heightened plasticity and stemness, significantly fueling the metastatic spread of cancer and enhancing the endurance and adaptability throughout the process of metastasis. Thus, understanding the molecular intricacies governing stem cell properties holds immense potential for developing targeted therapies to target metastasis and the development of resistance to treatments. To further elucidate EMT-induced cancer stem cell generation, our lab has demonstrated that the Polo-like kinase 1 (PLK1), an important G2/M regulator, plays an important role in the generation of CSCs through Forkhead box protein C2 (FOXC2). Most importantly, we have shown that the FOXC2 transcription factor, known to be crucial in embryonic development, now has emerging roles in metastatic cancer progression. FOXC2 is positioned at the center of the complex network of signal transduction pathways and functions downstream of most EMT-inducing transcription factors. FOXC2 is necessary for the acquisition and maintenance of stem cell property. In addition, we found that phosphorylation of FOXC2 by PLK1 stabilizes, and inhibition of PLK1 causes a decrease in FOXC2 and stemness in cancer cells. This led us to hypothesize that EMT-induced CSC generation is driven by the PLK1-FOXC2 axis. By expressing phosphomimetic and non-phosphorylatable FOXC2 mutants in breast cancer cell lines, we show that PLK1 phosphorylation is necessary for gaining stemness and an increase in symmetric self-renewal type of stem cell division. Evidence will be presented as to how we are targeting the PLK1-FOXC2 axis to make tumors sensitive to standard-of-care treatments. Citation Format: Joanna Joyce Maddela, Maria Castaneda, Petra den Hollander, Nick Kuburich, Sendurai Mani. Regulation of breast cancer stem cell identity by PLK1 and FOXC2 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB225.

Abstract CT275: A phase 2, randomized, open-label study of onvansertib in combination with standard-of-care (SoC) versus SoC alone for first-line treatment of RAS-mutant metastatic colorectal cancer (mCRC)

Abstract Background: Chemotherapy in combination with bevacizumab (Bev) has been a standard first-line treatment for RAS-mutated mCRC patients for the last 2 decades. The prognosis of these patients remains poor. Onvansertib is an orally available, highly potent, and selective inhibitor of Polo-like kinase 1 (PLK1), currently in clinical development. In a Phase 1b/2 study, onvansertib in combination with FOLFIRI + Bev demonstrated safety and promising efficacy in the second-line treatment of KRAS-mutated mCRC patients (NCT03829410). A subgroup analysis showed that patients not exposed to Bev (Bev-naïve) in the first-line setting had superior clinical benefit compared to Bev-exposed patients. Based on this data, we propose to investigate the combination of onvansertib with SoC (chemo + Bev) in the first-line treatment of RAS-mutated, Bev-naïve mCRC patients. Methods: This exploratory Phase 2, open-label, randomized study is designed to assess the safety, efficacy, pharmacokinetics, and pharmacodynamics of 2 doses of onvansertib (20 mg and 30 mg) in combination with SoC (FOLFIRI + Bev or FOLFOX + Bev) versus SoC alone in patients with RAS-mutated metastatic and unresectable CRC in the first-line setting. Eligible participants must have histologically confirmed mCRC, KRAS or NRAS mutation, no previous systemic therapy in the metastatic setting, ECOG performance status of 0 or 1, measurable disease as defined per RECIST v1.1. Exclusion criteria include patients with BRAF-V600 mutations or microsatellite instability high/deficient mismatch repair, prior treatment with a VEGF inhibitor, previous oxaliplatin treatment within 12 months prior to randomization. The primary endpoint is objective response as determined according to RECIST v1.1 by an independent central review. Key secondary endpoints include progression-free survival and duration of response per independent central review. The study will enroll approximately 90 patients. Each patient will be randomized to be treated with onvansertib (20 mg or 30 mg) + SoC or SoC alone. The enrollment period for this study is approximately 12 months. Patients will be followed for overall survival for up to 1 year after the end of treatment. Trial is open and recruiting as of November 2023. This trial is registered as NCT06106308. Citation Format: Heinz-Josef Lenz, Benjamin Kuritzky, Anu Thummala, Stephan Kendall, Ravindranath Patel, Sreenivasa R. Chandana, Alisha H. Bent, Nancy Sherman, Ramanand Arun Subramanian, Fairooz Kabbinavar, Joel Randolph Hecht. A phase 2, randomized, open-label study of onvansertib in combination with standard-of-care (SoC) versus SoC alone for first-line treatment of RAS-mutant metastatic colorectal cancer (mCRC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr CT275.

Design of novel PLK4 inhibitors as TRIM37-amplified breast cancer drugs using 3D-QSAR, molecular docking, and molecular dynamics simulation methods

ABSTRACT A mitosis-associated protein kinase, serine/threonine protein kinase polo-like kinase 4 (PLK4) is thought to be a promising target for the therapy of tripartite motif-containing 37 (TRIM37)-amplified breast cancer. The study's dataset for 3D-QSAR modelling included 37 derivatives of pyrazolo[3,4-d] pyrimidines, and the outcomes showed strong statistical significance (COMFA: q 2 =  0.744, r 2 =  0.997, r pred 2  = 0.702; CoMSIA: q 2 = 0.652, r 2 = 0.991, r pred 2  = 0.722). Findings from an analysis of isopotential maps using the COMFA and COMSIA models, ten novel small molecule inhibitors with potentially high activities were designed. To investigate the processes of interactions among the inhibitor molecules along with the receptor proteins, molecular docking and simulations of molecular dynamics were utilised. According to the results, the amino acids Cys92, Glu96, Asn94, and Asn103 may form hydrogen bonds with the ligands, which could enhance the small molecules’ inhibitory effect. Furthermore, MMPBSA calculations revealed that the newly created compounds have a higher binding affinity to the receptor proteins. Ultimately, these compounds also demonstrated good AMDET properties in pharmacokinetic parameter tests, indicating their promise as innovative medications for the management of breast cancer. These results offer the theoretical direction required to support the creation of effective PLK4 inhibitors.

Polo-like Kinase 1 Inhibition in KRAS-Mutated Metastatic Colorectal Cancer.

Inhibition of Polo-like kinase 1 (Plk1) is a promising new target and therapeutic strategy in metastatic colorectal cancer, especially those with KRAS mutations. New data support further development of onvansertib, and highlights the role of circulating tumor DNA in phase I clinical trials. See related article by Ahn et al., p. 2039.

A polo-like kinase 1 inhibitor enhances erastin sensitivity in head and neck squamous cell carcinoma cells in vitro.

Polo-like kinase 1 (PLK1) is a critical therapeutic target in the treatment of head and neck squamous cell carcinoma (HNSCC). The objective of this study was to investigate the therapeutic effect of the combination of BI 2536, a PLK1 inhibitor, and erastin, a ferroptosis inducer, in HNSCC. The proliferation, invasion, and migration abilities of Tu177 and FaDu cells upon exposure to BI 2536 and erastin, used in combination or alone, were tested. Fe2+, glutathione (GSH), and malondialdehyde (MDA) detection kits were used to determine whether the addition of BI 2536 enhanced the accumulation of Fe2+ and MDA, along with the depletion of GSH. Quantitative real-time PCR, western blot analyses were performed to investigate whether BI 2536 further altered the mRNA and expression level of ferroptosis genes. Furthermore, si PLK1 was used to investigate whether targeting PLK1 gene promoted erastin-induced ferroptosis. The combination of BI 2536 and erastin exerted a stronger cytotoxicity than treatment with a single agent. Compared with erastin treatment alone, the combination of BI 2536 and erastin lowered the ability of tumor cells to self-clone, invade, and migrate. BI 2536 enhanced the accumulation of Fe2+ and MDA, and the depletion of GSH. BI 2536 increased erastin-induced changes in ferroptosis-related gene mRNA and expression. Importantly, targeting PKL1 enhanced the anti-cancer effect of erastin. BI 2536 enhanced the sensitivity of HNSCC cells to erastin, which provides a new perspective for cancer treatment.

Abstract 5726: The potential anti-cancer activity of dual TTK/PLK1 inhibitor, BAL0891, in bladder cancer

Abstract Background: BAL0891 is a novel dual inhibitor of threonine tyrosine kinase (TTK) and polo-like kinase 1 (PLK1) which affects mitotic spindle assembly checkpoint integrity resulting in aberrant mitotic progression of cancer cells. Since unlike other organs, bladder urothelium is highly regenerative in certain circumstances, there are high chances of cell cycle inhibitors to have anti-cancer activity over bladder cancer. In this study, we investigated the therapeutic potential of BAL0891 in bladder cancer compared to single TTK or PLK1 inhibitor and discovered the best therapeutic combination partner for BAL0891. Methods: We performed meta-analysis of the mRNA expression levels of bladder cancer using Gene Expression Profiling Interactive Analysis 2 (GEPIA2). Representative bladder cancer cell lines (253J, 253J-BV, J82, RT4, T24, J82, HT1197, and HT1376) were treated with the dual protein kinase inhibitors, BAL0891, in comparison with TTK inhibitor (BAY1217389) or PLK1 inhibitor (onvansertib). BAL0891 was combined with CDK4/6 inhibitor, another cell cycle inhibitor working at different point from BAL0891. Cell viability was assessed using CCK-8 assay during 72 hours after treatment. Expression levels of TTK and PLK1 were measured by immunoblotting. Cell cycle analysis and cell apoptosis detection were performed on T24 cell. Results: The mRNA expression levels of both TTK (16.7 times) and PLK1 (10.0 times) were significantly upregulated only in bladder cancers compared to normal bladder tissues (P&amp;lt;0.05). Compared to TTK or PLK1 inhibitor, BAL0891 was sensitive in 75% of bladder cancer cell lines (IC50 &amp;lt;0.1 uM) and significantly reduced proliferation of bladder cancer cell lines depending on the dosage. TTK and PLK1 expression levels of cancer cells were associated with the sensitivity to BAL0891. BAL0891 significantly induced necrosis and increased in polyploidy and G2/M arrest in T24 cell. The anti-cancer activity of BAL0891 was further enhanced in certain cell lines (RT4, and J82) by combination of CDK4/6 inhibitor. Conclusion: This is the first study to suggest the potent anti-cancer activity of BAL0891 in bladder cancer. BAL0891 effectively worked on bladder cancer by two tracks which not only induced G2/M arrest but also increased in polyploidy. Furthermore, BAL0891 in combination with CDK4/6 inhibitor promoted anti-cancer effects. Lastly, this study suggests the use of TTK and PLK1 as the predictive biomarker for BAL0891. These data support further investigation of BAL0891 for the treatment of bladder cancer patients. Citation Format: Jee Soo Park, Ji-eun Lee, Myung Eun Lee, Jongchan Kim, Won Sik Jang, Jae Jung Lee, Jun Seung Lee, Minkyung Kang, Won Sik Ham. The potential anti-cancer activity of dual TTK/PLK1 inhibitor, BAL0891, in bladder cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5726.

Abstract 945: In vivo anti-tumor activity of onvansertib, a PLK1 inhibitor, combined with gemcitabine or carboplatin in platinum- resistant ovarian carcinoma patient-derived xenograft models

Abstract Background: The standard treatment for high grade ovarian carcinoma (HGOC) is cytoreductive surgery followed by a platinum based therapy. Despite an initial high rate of complete remission, most of the patients relapse with a platinum resistant disease. Patients with resistant tumors have few options, including monotherapy with gemcitabine, and new effective therapeutic alternatives are needed. The Polo-like kinase 1 (PLK1) is a master regulator of mitosis and recent evidences suggest its role in interfering with several DNA repair mechanisms. We investigated the effect of onvansertib (ONVA), a highly selective ATP-competitor PLK1 inhibitor, in combination with carboplatin (CARBO) and gemcitabine (GEM) in platinum-resistant HGOC patient-derived xenografts (PDXs). Methods: The combination of ONVA with either CARBO or GEM was investigated in two platinum-resistant HGOC PDXs, one intrinsically resistant to platinum (MNHOC#315), the other made resistant after repeated in vivo treatments with cisplatin (MNHOC#266R). Tumors were subcutaneously (s.c.; MNHOC#315) or orthotopically (i.p.; MNHOC#266R) transplanted in nude mice, mice were treated for 4 weeks with vehicle, single agents or drug combinations and followed for tumor growth (s.c. model) and survival. Results: CARBO- and GEM-ONVA combinations were well tolerated, with a maximum body weight loss of 6.6%. Both combinations showed strong anti-tumor activities in the two tested models. In MNHOC#315 PDX, CARBO- and GEM-ONVA combinations caused a tumor stabilization that extended beyond the dosing period and resulted in a higher tumor growth inhibition as compared to vehicle and single agents treated mice. In i.p. MNHOC#266R PDX, single agents were inactive, while a statistically significant increase in survival was observed in CARBO/ONVA and in GEM/ONVA treated mice as compared to vehicle treated mice, with a 6.6- and 7-fold increase in mean survival time, respectively. Pharmacodynamic studies are ongoing to decipher the mechanisms underlying the synergic effects of both combinations. Conclusions: The combinations of ONVA with either CARBO or GEM showed potent anti-tumor activities in two platinum-resistant HGOC PDXs. Additional studies in other HGOC platinum-resistant models, both in vivo and in vitro, as well pharmacodynamic studies are ongoing to validate these results and to get insights in the mechanisms underlying their efficacy. Citation Format: Michela Chiappa, Federica Guffanti, Ilaria Mengoli, Maya Ridinger, Giovanna Damia. In vivo anti-tumor activity of onvansertib, a PLK1 inhibitor, combined with gemcitabine or carboplatin in platinum- resistant ovarian carcinoma patient-derived xenograft models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 945.

Abstract 5178: Guided by a predictive ex vivo test: Bringing the PLK1 inhibitor volasertib back into the clinic for venetoclax-HMA relapsed/refractory acute myeloid leukemia patients

Abstract INTRODUCTION: Volasertib, a polo-like kinase 1 (PLK1) inhibitor, has a roughly 30% CR/CRi response rate in de novo acute myeloid leukemia (AML) patients when combined with low-dose cytarabine. However, its development was halted due in part to toxicity in a Phase 3 study. We have built a flow cytometry-based, high-throughput predictive precision medicine platform to forecast clinical responses to various treatments. Here, we present our design plan for a Phase 2a clinical trial with volasertib in relapsed/refractory (R/R) AML using a modified protocol to reduce toxicity and the development of a companion diagnostic to predict responders in a follow-up Phase 2b study. METHODS: We will initiate an open-label clinical trial in AML with volasertib in venetoclax-hypomethylating agent (HMA) R/R patients while co-developing a companion diagnostic to selectively enroll predicted responders in a follow-up, Phase 2b, study. Primary and secondary objectives will focus on efficacy/safety of intravenous volasertib and ex vivo responses, respectively. To reduce the toxicity seen in the Phase 3 study, specific strategies like tailored dosing will be implemented. To develop the clinical trial assay, peripheral blood or bone marrow samples were collected from AML patients and treated ex vivo with volasertib for 3 days. Surviving blast cells were enumerated by flow cytometry. Using a dynamic separation model restricted to the 35% most sensitive samples, we estimated optimal blast cutoffs for volasertib sensitivity. RESULTS: In single-agent experiments with 41 primary AML samples (31 de novo, 9 R/R, 1 with disease status unclear), we observed dose-response profiles consistent with volasertib’s role as a cell cycle inhibitor. Irrespective of the disease status, median and mean EC50 values were about 10 nM (1st-3rd quartile from 7-12 nM). To identify an optimal concentration of volasertib for patient stratification, we compared the ex vivo resistant blast fractions at each concentration with the areas under the dose-response curves (AUCs), assuming that in our case, AUCs most accurately reflect clinical responses. At 31.6 and 100 nM, the Pearson correlation coefficients were highest (r &amp;gt; 0.8), suggesting that these are optimal stratifying doses. With these concentrations and corresponding blast cutoff values, 32-33% of the de novo and 25% of the R/R patients would have been predicted to be responders were they given volasertib, assuming a positive predictive value of 100%. CONCLUSION: Volasertib’s EC50 values clustered within a narrow range, suggesting that EC50 may not be a suitable metric to stratify patients into responders and nonresponders, in contrast to AUCs and residual blast fractions at 31.6 and 100 nM of volasertib, which displayed more heterogeneous distributions. Limitations include small sample sizes, especially for the R/R group. Citation Format: Markus Daniel Lacher, Glenn Michelson, Allen Ling, Wade Anderson, Jeaneille Vinas, Christine J. Gu, Chris Leonardi, Joseph Wagner, Gabriel N. Mannis. Guided by a predictive ex vivo test: Bringing the PLK1 inhibitor volasertib back into the clinic for venetoclax-HMA relapsed/refractory acute myeloid leukemia patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5178.

Abstract 594: ORIC-613, a potential first- and best-in-class, orally bioavailable, potent and selective PLK4 inhibitor with synthetic lethality in TRIM37 high cancer models

Abstract A synthetic lethal interaction between polo-like kinase 4 (PLK4) and elevated TRIM37 was discovered in breast cancer and neuroblastoma (Meitinger et al. 2020; Yeow et al. 2020). High TRIM37 renders cells dependent on PLK4, a kinase that controls centriole duplication. Amplification and copy number gains of the 17q23 amplicon, where TRIM37 resides, are common in breast cancer and neuroblastoma and associated with early relapse and poor prognosis, making PLK4 a promising new synthetic lethal target for these patients. The development candidate ORIC-613 is an orally bioavailable, potent and exquisitely selective small molecule inhibitor of PLK4, which is synthetic lethal in TRIM37 high cells. ORIC-613 is highly selective against the kinome, including against the closely related aurora and PLK families. ORIC-613 ADME/PK profile includes high cell permeability and low potential for drug-drug interactions based on CYP profiling. ORIC-613 blocked PLK4 trans-autophosphorylation of phosphodegrons, leading to stabilization of PLK4, thus directly demonstrating target engagement in cells. PLK4 protein stabilization correlated with cell viability for ORIC-613, providing a quantifiable pharmacodynamic (PD) association with antitumor activity. Cell viability assessment in cancer cell lines revealed that ORIC-613 showed greater potency in TRIM37 high versus TRIM37 low cell lines. Importantly, ORIC-613 induced significantly greater apoptosis in TRIM37 high versus low cancer cell lines as measured by caspase 3/7 assay. We confirmed that ORIC-613 activity is on-target using an engineered cell line system of PLK4 G95L, in which the leucine mutation maintains enzymatic function but blocks compound binding. In cell viability assays, ORIC-613 was potent in the parental cells and lost activity in G95L cells. Analysis of genomic data from tumors indicates that TRIM37 copy number amplification and gain is prevalent across numerous cancers. Oral dosing of ORIC-613 resulted in tumor growth inhibition and regressions in TRIM37 high xenograft breast tumors with no body weight loss. In summary, ORIC-613 is a potential first- and best-in-class development candidate, with exquisite selectivity for PLK4, which demonstrates synthetic lethality in TRIM37 high tumors and has the potential to benefit these patients. Citation Format: Kyle A. Edgar, Siobhan McRee, Gina Andretta, Chelsea Chen, Christophe Colas, Wei Fang, Wayne Kong, Jason Long, Fang Liu, Jared Moore, Aleskandr Pankov, Dan Shore, Joanne Tan, Robert Warne, Livia Ulicna, Rakesh Vekariya, Anneleen Daemen, F Romero, Melissa R. Junttila, Lori S. Friedman. ORIC-613, a potential first- and best-in-class, orally bioavailable, potent and selective PLK4 inhibitor with synthetic lethality in TRIM37 high cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 594.

Abstract 706: Advancing radiotherapy strategies for high-risk rhabdomyosarcoma: Unveiling BCL-xL inhibition as a potent sensitizer to radiotherapy

Abstract Pediatric soft-tissue sarcomas are a group of solid tumors that account for about 7% of childhood cancer cases. Rhabdomyosarcoma (RMS), the most common subtype, further stratifies into PAX3 or PAX7 and FOXO1 fusion-negative (FN) or fusion-positive (FP) RMS. FP RMS is the higher risk subtype and is more aggressive, metastatic, and less responsive to treatment. Front-line treatments for RMS FP patients have had limited progress in the recent years and existing treatments such as radiotherapy lacks tumor cell selectivity, resulting in toxic side effects. To address this, we conducted a high-throughput drug screen to identify radiosensitizers to enhance efficacy of RMS radiotherapy. Using three patient-derived FP RMS tumoroids, we screened a comprehensive drug library with and without radiation, comprising 528 drugs at varying stages of pre-clinical and clinical trials. A validation screen of the top 29 drugs was conducted on a larger cohort of samples including eight patient-derived FP RMS tumoroids and three non-cancer models. The screening revealed that drugs targeting the anti-apoptotic protein B-cell lymphoma-extra-large (BCL-xL) exhibited both potent radiosensitization and cytotoxicity selectively for FP RMS. Notably, the sensitivity to BCL-xL inhibitors correlated with elevated BCL2L1 expression and PAX3 fusion status. Additionally, drugs targeting polo-like kinase 1 (PLK1) and other cell cycle-related genes exhibited robust radioprotective effects across both RMS and non-cancer models. Our findings propose the therapeutic potential of BCL-xL inhibition, either as a standalone treatment or in combination with radiotherapy. In parallel, we show the protective potential of PLK1 inhibition to mitigate radiation toxicity. In the long term, we aspire to integrate the identified radiosensitizing and radioprotecting compounds with existing treatment regiments for RMS, maximizing precision and efficacy in RMS radiotherapy. Citation Format: Sammy Se Whee Park, Nona Struyf, Tom Erkers, Sören Lehmann, Michael T. Meister, Jarno Drost, Frank C. Holstege, Nikolas Herold, Päivi Östling, Jakob Stenman, Olli Kallioniemi, Jan-Inge Henter, Brinton Seashore-Ludlow, Kasper Karlsson. Advancing radiotherapy strategies for high-risk rhabdomyosarcoma: Unveiling BCL-xL inhibition as a potent sensitizer to radiotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 706.

Abstract B031: The PLK1 inhibitor Volasertib as a treatment for HSF1 and MYC coamplified ovarian cancer

Abstract Ovarian cancer is a disease that afflicts 1 in 78 women. This disease only has a 5-year relative survival rate of 49% due to the fact that the first line chemotherapy for this disease, platinum-based chemotherapy with taxanes, has an 80% recurrence rate. To improve the treatment for this disease, it would be wise to find biomarkers of sensitivity to other drugs. Using publicly available data from The Cancer Genome Project, our lab has shown that 36% of High Grade Serous Ovarian Cancers (HGSOC) have gene amplifications for both Cellular Myelocytomatosis (MYC) and Heat Shock Factor 1 (HSF1) genes. These genes encode for transcription factors and are known to contribute to malignant disease in multiple cancer types, including ovarian cancer. Evidence indicates that MYC and HSF1 are both essential to HGSOC cells with co-amplification of these genes, that MYC and HSF1 transcriptional activities are highly correlated in patient tumors, and the two transcription factors have a protein-protein interaction, all suggesting these two transcription factors cooperate to drive this subset of HGSOC tumors. Many drugs have been developed to inhibit MYC or HSF1 though they have not shown success in clinical development. We aimed to decrease the activity of these proteins by targeting polo-like kinase 1 (PLK1), a kinase that phosphorylates and regulates both MYC and HSF1 proteins. Active PLK1 levels were highly correlated with MYC and HSF1 protein levels and inhibition of PLK1 with volasertib reduced MYC and HSF1 protein levels and transcriptional activity. Furthermore, volasertib was found to have 155-fold greater potency in HGSOC cells with MYC-HSF1 co-amplification compared to cells without these genes amplified. The increased potency of volasertib in MYC-HSF1 co-amplified cells were also observed in clonogenic growth and spheroid models. Together, these data indicate that MYC and HSF1 cooperate in HGSOC when these genes are amplified and targeting a common kinase between them in PLK1 disrupts this cooperation. Furthermore, MYC and HSF1 co-amplification may serve as a biomarker for volasertib through a personalized medicine approach to treat HGSOC patients. Citation Format: Matthew O'Malley, Imade W. Williams, Bobby Walker, Haimanti Ray, Kenneth P. Nephew, Richard Carpenter. The PLK1 inhibitor Volasertib as a treatment for HSF1 and MYC coamplified ovarian cancer [abstract]. In: Proceedings of the AACR Special Conference on Ovarian Cancer; 2023 Oct 5-7; Boston, Massachusetts. Philadelphia (PA): AACR; Cancer Res 2024;84(5 Suppl_2):Abstract nr B031.

KIF2A Upregulates PI3K/AKT Signaling through Polo-like Kinase 1 (PLK1) to Affect the Proliferation and Apoptosis Levels of Eriocheir sinensis Spermatogenic Cells.

E. sinensis is an animal model for studying the reproduction and development of crustaceans. In this study, we knocked down the Es-Kif2a gene by injecting dsRNA into E. sinensis and inhibited Es-Plk1 gene expression by injecting PLK1 inhibitor BI6727 into E. sinensis. Then, the cell proliferation level, apoptosis level, and PI3K/AKT signaling expression level were detected. Our results showed that the proliferation level of spermatogenic cells decreased, while the apoptosis level increased after Es-Kif2a knockdown or Es-Plk1 inhibition. In order to verify whether these changes are caused by regulating the PI3K/AKT pathway, we detected the expression of PI3K and AKT proteins after Es-Kif2a knockdown or Es-Plk1 inhibition. Western Blot showed that in both the Es-Kif2a knockdown group and the Es-Plk1 inhibition group, the expression of PI3K and AKT proteins decreased. In addition, immunofluorescence showed that Es-KIF2A and Es-PLK1 proteins were co-localized during E. sinensis spermatogenesis. To further explore the upstream and downstream relationship between Es-KIF2A and Es-PLK1, we detected the expression level of Es-PLK1 after Es-Kif2a knockdown as well as the expression level of Es-KIF2A after Es-Plk1 inhibition. Western Blot showed that the expression of Es-PLK1 decreased after Es-Kif2a knockdown, while there was no significant change of Es-KIF2A after Es-Plk1 inhibition, indicating that Es-PLK1 may be a downstream factor of Es-KIF2A. Taken together, these results suggest that Es-KIF2A upregulates the PI3K/AKT signaling pathway through Es-PLK1 during the spermatogenesis of E. sinensis, thereby affecting the proliferation and apoptosis levels of spermatogenic cells.

Targeting Oral Squamous Cell Carcinoma with Combined Polo-Like-Kinase-1 Inhibitors and γ-Radiation Therapy.

Polo-like-kinase-1 (PLK-1) is a serine/threonine kinase that regulates the cell cycle and acts as an oncogene in multiple cancers, including oral squamous cell carcinoma (OSCC). The loss of PLK-1 can inhibit growth and induce apoptosis, making it an attractive therapeutic target in OSCC. We evaluated the efficacy of PLK-1 inhibitors as novel, targeted therapeutics in OSCC. PLK-1 inhibition using BI6727 (volasertib) was found to affect cell death at low nanomolar concentrations in most tested OSCC cell lines, but not in normal oral keratinocytes. In cell lines resistant to volasertib alone, pre-treatment with radiotherapy followed by volasertib reduced cell viability and induced apoptosis. The combinatorial efficacy of volasertib and radiotherapy was replicated in xenograft mouse models. These findings highlight the potential of adding PLK-1 inhibitors to adjuvant therapy regimens in OSCC.

PLK4 as a potential target to enhance radiosensitivity in triple-negative breast cancer

Radioresistance is one of the barriers to developing more effective therapies against the most aggressive, triple-negative, breast cancer (TNBC) subtype. In our previous studies, we showed that inhibition of Polo-like Kinase 4 (PLK4) by a novel drug, CFI-400945 significantly enhances the anticancer effects of radiotherapy (RT) compared to single treatment alone. Here we further investigate the role of PLK4 in enhancing radiation effects in TNBC and explore mechanisms of PLK4 inhibition and radiation combinatorial antiproliferative effects. To assess cellular proliferation in response to treatments, we used colony formation assays in TNBC cell lines and patient-derived organoids (PDOs). Downregulation of PLK4 expression was achieved using siRNA silencing in TNBC cell lines. Immunofluorescence against centrin was used to assess the alteration of centriole amplification in response to treatments. We observed that inhibition of PLK4 by CFI-400945 or Centrinone B or its downregulation by siRNA, when combined with RT, resulted in a significant increase in antiproliferative effect in TNBC cells lines and PDOs compared to untreated or single-treated cells. Anticancer synergy was observed using a response matrix in PDOs treated with CFI-400945 and RT. We show that the overamplification of centrioles might be involved in the combined antiproliferative action of RT and PLK4 inhibition. Our data suggest that PLK4 is a promising target for enhancing the anticancer effects of RT in TNBC that, at least in part, is modulated by the overamplification of centrioles. These results support further mechanistic and translational studies of anti-PLK4 agents and RT as an anticancer combination treatment strategy.