Increased interaction between connexin43 and microtubules is critical for glioblastoma stem-like cell maintenance and tumorigenicity

Glioblastoma (GBM) is the most common primary tumor of the central nervous system. One major challenge in GBM treatment is the resistance to chemotherapy and radiotherapy observed in subpopulations of cancer cells, including GBM stem-like cells (GSCs). These cells hold the ability to self-renew or differentiate following treatment, participating in tumor recurrence. The gap junction protein connexin43 (Cx43) has complex roles in oncogenesis and we have previously demonstrated an association between Cx43 and GBM chemotherapy resistance. Here, we report, for the first time, increased direct interaction between non-junctional Cx43 with microtubules in the cytoplasm of GSCs. We hypothesize that non-junctional Cx43/microtubule complexing is critical for GSC maintenance and survival and sought to specifically disrupt this interaction while maintaining other Cx43 functions, such as gap junction formation. Using a Cx43 mimetic peptide of the carboxyl terminal tubulin-binding domain of Cx43 (JM2), we successfully ablated Cx43 interaction with microtubules in GSCs. Importantly, administration of JM2 significantly decreased GSC survival in vitro , and limited GSC-derived tumor growth in vivo . Together, these results identify JM2 as a novel peptide drug to ablate GSCs in GBM treatment.

A Novel Pan-RAS Inhibitor with a Unique Mechanism of Action Blocks Tumor Growth in Mouse Models of GI Cancer

Here we characterize a novel pan-RAS inhibitor, ADT-007, that potently and selectively inhibited the growth of histologically diverse cancer cell lines with mutant or activated RAS irrespective of the RAS mutation or isozyme. Growth inhibition was dependent on activated RAS and associated with reduced GTP-RAS levels and MAPK/AKT signaling. ADT-007 bound RAS in lysates from sensitive cells with sub-nanomolar EC 50 values but did not bind RAS in lysates from insensitive cells with low activated RAS. Insensitivity to ADT-007 was attributed to metabolic deactivation by UGT-mediated glucuronidation, providing a detoxification mechanism to protect normal cells from pan-RAS inhibition. Molecular modeling and experiments using recombinant RAS revealed that ADT-007 binds RAS in a nucleotide-free conformation to block GTP activation. Local injection of ADT-007 strongly inhibited tumor growth in syngeneic immune competent and xenogeneic immune deficient mouse models of colorectal and pancreatic cancer and activated innate and adaptive immunity in the tumor microenvironment.

SIGNIFICANCE

ADT-007 is a novel pan-RAS inhibitor with a unique mechanism of action having potential to circumvent resistance to mutant-specific KRAS inhibitors and activate antitumor immunity. The findings support further development of ADT-007 analogs and/or prodrugs with oral bioavailability as a generalizable monotherapy or combined with immunotherapy for RAS mutant cancers.

BACKGROUND

It is projected that colorectal cancer (CRC) and pancreatic ductal adenocarcinoma (PDA) will cause 52,580 and 49,830 deaths in the US in 2023, respectively (1). The 5-year survival rates for CRC and PDA are 65% and 12%, respectively (1). Over 50% of CRC and 90% of PDA patients harbor mutations in KRAS genes that are associated with poor prognosis, making the development of novel KRAS inhibitors an urgent unmet medical need (2).

Abstract 1658: ADT-007 binds RAS and inhibits RAS signaling

RAS is a critically important oncogenic protein that is mutated in approximately 1/3 of cancers resulting in aberrant activation of downstream signaling, which drives malignant transformation. Current molecular targeted therapeutics, and several in development, inhibit only specific mutant alleles (G12C, G12D). In addition, compounds which directly inhibit RAS via proteolytic degradation or inhibit RAS activation by SOS1 are in preclinical development and are referred to as Pan-KRAS inhibitors. Previously investigators have reported that the NSAID, sulindac, can selectively inhibit RAS mutant tumorigenesis by a cyclooxygenase (COX)-independent mechanism. Sulindac, and more potent analogs have also previously been reported to inhibit RAS-mediated transformation and directly bind RAS. Here we describe an ultra-potent non-COX inhibitory derivative of sulindac, ADT-007, which binds to and inhibits RAS nucleotide binding and RAS-effector association.

ADT-007 binding to KRAS was evaluated by Micro-Tag cell target engagement and by Cellular Thermal Shift Assay (CETSA) which demonstrated a potency of target engagement (EC50) value in the subnanomolar range. Consistent with molecular docking studies, HSQC NMR spectroscopy using recombinant KRAS revealed that ADT-007 interacted with KRAS after Mg2+ chelation to obtain a nucleotide free (NF) state, resulting in chemical shift changes and signal attenuation of residues in the P-loop and nucleotide binding domain. Similarly, biochemical assays confirmed that ADT-007 prevented MANT-GTP binding to recombinant NF KRAS but did not compete with bound GTP. Functional assays also showed that KRAS binding to RAF-RBD(GST) was inhibited by ADT-007. The compound inhibited constitutive RAS activation (RAF-RBD pulldown) in serum starved MiaPaCa-2 pancreatic cancer cells harboring a KRAS-G12C mutation and demonstrated Pan-RAS inhibition in serum- or EGF-stimulated cells. Further, ADT-007 inhibited AKT phosphorylation and EGF-stimulated downstream ERK1/2 phosphorylation. Finally, ADT-007 demonstrated RAS-selective growth inhibition in isogenic pancreatic and colorectal cancer cell pairs (BxPC-3, HT29). Together, these experiments support further development of ADT-007 and related analogs for treatment of RAS-driven cancers.

Citation Format: Adam B. Keeton, Xi Chen, Jacob Valiyaveettil, Chung-Hui Huang, Tyler E. Mattox, Khalda Fadlalla, Jeremy B. Foote, Donald J. Buchsbaum, Kristy L. Berry, Elmar Nurmemmedov, Ivan Babic, Vadim Gaponenko, Gregory Gorman, Lori Coward, Yulia Y. Maxuitenko, Forrest T. Smith, Gary A. Piazza. ADT-007 binds RAS and inhibits RAS signaling [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 1658.

Abstract 2759: Real-time cellular target engagement and protein quantification for drug discovery

Preclinical assessment of direct drug-target engagement and analysis of binding kinetics within the cellular environment is essential for development of safer and more effective therapeutics. Infusion of such critical data into early drug discovery would significantly reduce the failure rate of new drug candidates, accelerate drug discovery, and validate repurposing of existing drugs. Described here is a novel drug-target engagement technology that can sensitively interrogate direct binding of drugs to cellular, bacterial, or viral protein targets within the physiological environment. Micro-Tag cell target engagement technology is based on complementation of a small 15-amino acid subunit with a large subunit into an active RNA-processing enzyme. The small subunit can be cloned to any drug target for transient or stable expression using existing tools such as CRISPR. Upon complementation, the active enzyme cleaves a FRET-based oligonucleotide substrate resulting in rapid generation of fluorescent signal that can be quantified in real time. The Micro-Tag technology enables in-cell quantitation of drug target levels using qPCR systems. This is the next-generation of target engagement technology that allows for real-time monitoring of drug-target interaction in the cell. We show here data demonstrating direct engagement of several reference compounds and novel small molecules with high-profile cancer targets such as K-RAS, MTH1, EGFR, and UBE2N. Selectivity of these drugs to the targets in the cell is further delineated using their mutant counterparts as well as inactive stereoisomer compounds.

The Micro-Tag cell target engagement technology provides the power of cell target engagement to a large family of target proteins. It can be employed for high-throughput screens directed at initial on-target ranking or med-chem optimization of drug candidates. It is amenable for interrogation of drug candidates across various modalities: small molecules, peptides, antibodies and PROTACs. Importantly, this next-generation target engagement technology seamlessly integrates with qPCR systems, fluorescence microscopy, live-cell microscopy, FACS analysis, and offers multiplexing capability, thus allowing for further mechanistic insight.

Citation Format: Ivan Babic, Nikolas Bryan, Claire Cunningham, Avery Sampson, Daniel Starczynowski, Elmar Nurmemmedov. Real-time cellular target engagement and protein quantification for drug discovery [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 2759.

Abstract 1633: ADT-030, a novel dual-acting RAS and β-catenin inhibitor with robust antitumor activity

BACKGROUND: Previous publications have reported that the antineoplastic activity of sulindac occurs by a cyclooxygenase independent mechanism, possibly involving direct inhibition of RAS. Suppression of β-catenin/TCF transcription has also been reported by inhibiting cyclic guanosine monophosphate (cGMP) phosphodiesterase (PDE) and the activation of cGMP-dependent protein kinase (PKG). Here we characterize a potent and selective non-COX inhibitory sulindac derivative, ADT-030, that inhibits cancer cell growth by blocking RAS and β-catenin signaling.

METHODS AND RESULTS: ADT-030 inhibited the proliferation of lung cancer cells expressing the PDE10A isozyme, while normal airway cells lacking PDE10A displayed reduced sensitivity. ADT-030 inhibited recombinant PDE10 activity at concentrations that inhibit lung cancer cells proliferation. ADT-030 induced cell cycle arrest, apoptosis, and morphological characteristics of autophagy blockage. A cellular thermal stability assay showed that ADT-030 binds both RAS and PDE10 in lysates from cancer cells with nanomolar affinity. Consistent with its PDE10 inhibitory activity, ADT-030 activated PKG and phosphorylated β-catenin on amino acid residues known to induce proteasomal degradation, and reduced levels of the unphosphorylated (oncogenic stabilized) form of β-catenin. Consistent with its ability to directly bind RAS, ADT-030 also inhibited EGF-stimulated MAPK/AKT signaling. Oral administration of ADT-030 inhibited tumor growth in an orthotopic xenograft mouse model of lung cancer, providing a durable survival response with 25% tumor-free mice at dosages having no discernable toxicity. ADT-030 was also highly efficacious in two different chemical-induced mouse models of lung tumorigenesis suppressing the carcinogen-activation of RAS, as well as in multiple mouse subcutaneous tumor models, including highly aggressive models of metastasis.

CONCLUSIONS: These results support further development of ADT-030, a novel inhibitor of RAS and β-catenin signaling for the treatment of lung cancer and other RAS driven cancers, including those with co-occurring mutations in the Wnt/β-catenin pathway.

Supported by NIH grants R01 CA238514, R01 CA197147, R01 CA254197.

Citation Format: Veronica Ramirez-Alcantara, Xi Chen, Michele A. Schuler, Dennis Otali, William Grizzle, Kristy Berry, Antonio Ward, Ivan Babic, Elmar Nurmemmedov, Gang Zhou, Chengguo Xing, Greg Gorman, Lori Coward, Yulia Maxuitenko, Adam B. Keeton, Gary A. Piazza. ADT-030, a novel dual-acting RAS and β-catenin inhibitor with robust antitumor activity [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 1633.

ATF6-Mediated Signaling Contributes to PARP Inhibitor Resistance in Ovarian Cancer

High-grade serous ovarian cancer (HGSOC) is the deadliest ovarian cancer histotype due in-part to the lack of therapeutic options for chemotherapy-resistant disease. PARP inhibitors (PARPi) represent a targeted treatment. However, PARPi resistance is becoming a significant clinical challenge. There is an urgent need to overcome resistance mechanisms to extend disease-free intervals. We established isogeneic PARPi-sensitive and -resistant HGSOC cell lines. In three PARPi-resistant models, there is a significant increase in AP-1 transcriptional activity and DNA repair capacity. Using RNA-sequencing and an shRNA screen, we identified activating transcription factor 6 (ATF6) as a mediator of AP-1 activity, DNA damage response, and PARPi resistance. In publicly available datasets, ATF6 expression is elevated in HGSOC and portends a poorer recurrence-free survival. In a cohort of primary HGSOC tumors, higher ATF6 expression significantly correlated to PARPi resistance. In PARPi-resistant cell lines and a PDX model, inhibition of a known ATF6 regulator, p38, attenuated AP-1 activity and RAD51 foci formation, enhanced DNA damage, significantly inhibited tumor burden, and reduced accumulation of nuclear ATF6.

IMPLICATIONS

This study highlights that a novel p38-ATF6-mediated AP-1 signaling axis contributes to PARPi resistance and provides a clinical rationale for combining PARPi and AP-1 signaling inhibitors.

Abstract 2572: A novel strategy for PLK1 kinase inhibition by allosterically targeting the Polo Box Domain

Polo Like Kinase 1 (PLK1) is only expressed in dividing cells and plays a critical role in several stages of mitosis. PLK1 is overexpressed in many tumor types. PLK1 inhibition selectively kills cancer cells because they are dependent on the mitotic functions of PLK1. PLK1 consists of a highly conserved N-terminal catalytic kinase domain and a unique, functionally essential C-terminal Polo Box Domain (PBD). The PBD is a phospho-peptide binding motif that determines substrate recognition and sub-cellular localization. PLK1 catalytic inhibitors have advanced to clinical trials but not demonstrated convincing efficacy. Targeting the PBD offers an attractive alternative to pursue PLK1 inhibition. An iterative strategy called REPLACE, involving computational and synthetic approaches, was utilized to generate fragment-ligated inhibitory peptides and further application of REPLACE resulted in non-peptidic compounds named abbapolins. In recently published studies, abbapolins were found to specifically bind to the PBD of PLK1 in biochemical and cellular assays. The phosphorylation of TCTP, a specific PLK1 substrate, was measured in abbapolin treated cancer cells. Abbapolins produced a dose dependent reduction in p-TCTP. We also made a novel observation that abbapolins upon binding to PLK1 induced its intracellular loss in a mechanism at least partially dependent on the proteasome. Spurred on by this unique mechanism of action, we initiated studies on the interactions of catalytic inhibitors and PBD-binding abbapolins with PLK1 in vitro and in cellular contexts. Biochemical assays were performed with full length PLK1 in vitro and intracellular PLK1 binding was measured by a Cellular Thermal Shift Assay (CETSA). We report novel findings during mitosis inferred from our collective data, namely that catalytic binding by BI2536 or volasertib unexpectedly decreased soluble PLK1 as determined by CETSA, inferring induction of a conformational change in intracellular PLK1. In contrast, abbapolins produced the expected right shift in the melting curve of PLK1. Intriguingly, these differential effects on PLK1 thermal stability have opposing impacts on the fate of intracellular PLK1. Binding by catalytic inhibitors cause accumulation of PLK1, whereas PBD binding by abbapolins ultimately lead to its loss. Results from quantifying intracellular PLK1 were also supported in vitro by evidence of cooperative binding between catalytic inhibitors and abbapolins in the context of the FL protein and suggest that conformational changes induced by binding to the catalytic site increase affinity of abbapolins for the PDB. Collectively, the results shed further insight into the unique mechanism of action for abbapolins potentially due to their engagement of a cryptic hydrophobic pocket of the PBD. Abbapolins are thus a compelling alternative to catalytic-based inhibitors for the development of novel therapeutics targeting PLK1.

Citation Format: Danda Chapagai, Merissa Baxter, Sandra Craig, Guru Ramamoorthy, Jessy Stafford, Sikirzhytski Vitali, Elmar Nurmemmedov, Campbell McInnes, Michael D. Wyatt. A novel strategy for PLK1 kinase inhibition by allosterically targeting the Polo Box Domain [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2572.

Blocking UBE2N abrogates oncogenic immune signaling in acute myeloid leukemia

Dysregulation of innate immune signaling pathways is implicated in various hematologic malignancies. However, these pathways have not been systematically examined in acute myeloid leukemia (AML). We report that AML hematopoietic stem and progenitor cells (HSPCs) exhibit a high frequency of dysregulated innate immune-related and inflammatory pathways, referred to as oncogenic immune signaling states. Through gene expression analyses and functional studies in human AML cell lines and patient-derived samples, we found that the ubiquitin-conjugating enzyme UBE2N is required for leukemic cell function in vitro and in vivo by maintaining oncogenic immune signaling states. It is known that the enzyme function of UBE2N can be inhibited by interfering with thioester formation between ubiquitin and the active site. We performed in silico structure-based and cellular-based screens and identified two related small-molecule inhibitors UC-764864/65 that targeted UBE2N at its active site. Using these small-molecule inhibitors as chemical probes, we further revealed the therapeutic efficacy of interfering with UBE2N function. This resulted in the blocking of ubiquitination of innate immune- and inflammatory-related substrates in human AML cell lines. Inhibition of UBE2N function disrupted oncogenic immune signaling by promoting cell death of leukemic HSPCs while sparing normal HSPCs in vitro. Moreover, baseline oncogenic immune signaling states in leukemic cells derived from discrete subsets of patients with AML exhibited a selective dependency on UBE2N function in vitro and in vivo. Our study reveals that interfering with UBE2N abrogates leukemic HSPC function and underscores the dependency of AML cells on UBE2N-dependent oncogenic immune signaling states.

Structure-activity and mechanistic studies of non-peptidic inhibitors of the PLK1 polo box domain identified through REPLACE

Polo-like kinase 1 (PLK1) is a serine/threonine-protein kinase involved in cell cycle regulation and mitotic progression. Studies have shown that PLK1 is upregulated in many tumors and high levels are adversely related to a poor prognosis. Knocking down or inhibiting PLK1 results in synthetic lethality in PTEN deficient prostate tumors and Kras mutant colorectal tumors, further validating PLK1 as an oncotarget. Substrate recognition by PLK1 occurs through the Polo-Box Domain (PBD), which is a phospho-peptide binding site also responsible for subcellular localization. Much effort has been directed to target this kinase therapeutically through the ATP-binding site, and a few such inhibitors have advanced to clinical trials however with limited clinical efficacy. Moreover, it has been shown that a point mutation in PLK1 (C67V) confers dramatic cellular resistance to catalytic site inhibitors. An alternative approach to target PLK1 potently and selectively is through the PBD to block its protein-protein interactions. Through the REPLACE strategy, for converting peptide inhibitors into more drug-like non peptidic compounds, a PBD targeting compound series ("ABBAs"), has been identified and the key determinants of potency and selectivity elucidated through structure-activity relationship studies. In cellular experiments, the ABBAs were shown to lead to profound effects on the cell cycle, to inhibit tumor proliferation and overcome resistance of cells expressing the PLK1 C67V mutant to ATP-based inhibitors. These non-ATP competitive inhibitors of PLK1 were also used chemical biology probes to investigate the gene regulatory effects of PLK1, known to act on transcription factors such as p53.

Interleukin-2-inducible T-cell kinase (Itk) signaling regulates potent noncanonical regulatory T cells

Regulatory T cells (Tregs) play an important role in controlling autoimmunity and limiting tissue damage and inflammation. IL2-inducible T cell kinase (Itk) is part of the Tec family of tyrosine kinases and is a critical component of T cell receptor mediated signaling. Here, we showed that either genetic ablation of Itk signaling or inhibition of Itk signaling pathways resulted in increased frequency of "noncanonical" CD4+ CD25- FOXP3+ Tregs (ncTregs), as well as of "canonical" CD4+ CD25+ FOXP3+ Tregs (canTregs). Using in vivo models, we showed that ncTregs can avert the formation of acute graft-versus-host disease (GVHD), in part by reducing conventional T cell proliferation, proinflammatory cytokine production, and tissue damage. This reduction in GVHD occurred without disruption of graft-versus-leukaemia (GVL) effects. RNA sequencing revealed that a number of effector, cell adhesion, and migration molecules were upregulated in Itk-/- ncTregs. Furthermore, disrupting the SLP76: ITK interaction using a specific peptide inhibitor led to enhanced Treg development in both mouse and primary human cells. This peptide inhibitor also significantly reduced inflammatory cytokine production in primary GVHD patient samples and mouse T cells without causing cell death or apoptosis. We provide evidence that specifically targeting Itk signaling could be a therapeutic strategy to treat autoimmune disorders.

Suppression of Colon Tumorigenesis in Mutant Apc Mice by a Novel PDE10 Inhibitor that Reduces Oncogenic β-Catenin

Previous studies have reported that phosphodiesterase 10A (PDE10) is overexpressed in colon epithelium during early stages of colon tumorigenesis and essential for colon cancer cell growth. Here we describe a novel non-COX inhibitory derivative of the anti-inflammatory drug, sulindac, with selective PDE10 inhibitory activity, ADT 061. ADT 061 potently inhibited the growth of colon cancer cells expressing high levels of PDE10, but not normal colonocytes that do not express PDE10. The concentration range by which ADT 061 inhibited colon cancer cell growth was identical to concentrations that inhibit recombinant PDE10. ADT 061 inhibited PDE10 by a competitive mechanism and did not affect the activity of other PDE isozymes at concentrations that inhibit colon cancer cell growth. Treatment of colon cancer cells with ADT 061 activated cGMP/PKG signaling, induced phosphorylation of oncogenic β-catenin, inhibited Wnt-induced nuclear translocation of β-catenin, and suppressed TCF/LEF transcription at concentrations that inhibit cancer cell growth. Oral administration of ADT 061 resulted in high concentrations in the colon mucosa and significantly suppressed the formation of colon adenomas in the Apc^+/min-FCCC mouse model of colorectal cancer without discernable toxicity. These results support the development of ADT 061 for the treatment or prevention of adenomas in individuals at risk of developing colorectal cancer. PREVENTION RELEVANCE: PDE10 is overexpressed in colon tumors whereby inhibition activates cGMP/PKG signaling and suppresses Wnt/β-catenin transcription to selectively induce apoptosis of colon cancer cells. ADT 061 is a novel PDE10 inhibitor that shows promising cancer chemopreventive activity and tolerance in a mouse model of colon cancer.

Nonpeptidic, Polo-Box Domain-Targeted Inhibitors of PLK1 Block Kinase Activity, Induce Its Degradation and Target-Resistant Cells

PLK1, polo-like kinase 1, is a central player regulating mitosis. Inhibition of the subcellular localization and kinase activity of PLK1 through the PBD, polo-box domain, is a viable alternative to ATP-competitive inhibitors, for which the development of resistance and inhibition of related PLK family members are concerns. We describe novel nonpeptidic PBD-binding inhibitors, termed abbapolins, identified through successful application of the REPLACE strategy and demonstrate their potent antiproliferative activity in prostate tumors and other cell lines. Furthermore, abbapolins show PLK1-specific binding and inhibitory activity, as measured by a cellular thermal shift assay and an ability to block phosphorylation of TCTP, a validated target of PLK1-mediated kinase activity. Additional evidence for engagement of PLK1 was obtained through the unique observation that abbapolins induce PLK1 degradation in a manner that closely matches antiproliferative activity. Moreover, abbapolins demonstrate antiproliferative activity in cells that are dramatically resistant to ATP-competitive PLK1 inhibitors.

Abstract 1241: Converting high affinity peptides to non-peptidic inhibitors via REPLACE: Novel and new strategies in targeting PLK1 Polo Box Domain (PBD) for anti-cancer therapy

Polo Like Kinase 1 (PLK1) is only expressed in dividing cells and plays a critical role in several stages of mitosis. PLK1 is highly expressed in tumors of various origins while its expression is largely absent in normal tissues. PLK1 inhibition selectively kills cancer cells because they are dependent on the mitotic functions of PLK1 overexpression. PLK1 consists of a highly conserved N-terminal catalytic kinase domain and a unique, functionally essential C-terminal Polo Box Domain (PBD). The PBD of each PLK is a phospho-peptide binding motif that determines substrate recognition and sub-cellular localization. PLK1 catalytic inhibitors have advanced to clinical trials but they inhibit other family members PLK2 and PLK3. An alternative approach to developing potent and selective PLK1 inhibitors is to allosterically target the PBD. An iterative strategy called REPLACE involving computational and synthetic approaches was utilized to generate fragment-ligated inhibitory peptides (FLIPs) by rationally replacing amino acid residues in two PLK1 substrate peptides with drug-like small molecule fragments to generate FLIPs. Further application of REPLACE resulted in non-peptidic compounds named abbapolins. Confirmation of engagement of cellular PLK1 by abbapolins was assessed using a cellular thermal shift assay (CETSA), which determines drug binding to target proteins by measuring the shift in a protein's solubility and hence its thermal stability. Abbapolins specifically bound to full-length cellular PLK1 over PLK3 as determined by CETSA in a pattern consistent with in vitro binding affinities to the purified PBD of PLK1 and PLK3. We made an additional unique observation that mitotic PLK1 had a lower thermal stability than non-mitotic PLK1, suggesting that mitotic PLK1 might adopt a different conformation that is less soluble in the CETSA assay. Moreover, we observed that ATP-based inhibitors decreased the thermal stability of mitotic PLK1, whereas PBD inhibitors abbapolins increased the thermal stability of mitotic PLK1 as measured by CETSA. Additional evidence for engagement of cellular PLK1 was obtained through the unique mechanistic observation that abbapolins induced a striking dose dependent reduction in PLK1 protein levels as measured by western blotting and immunofluorescence. The loss of PLK1 was partially due to proteasome mediated degradation because the proteasome inhibitor MG132 partially prevented PLK1 degradation. Moreover, abbapolins caused preferential loss of nuclear PLK1 as determined by cellular fractionation. The therapeutic potential of these compounds was indicated through their antiproliferative activity on a cell line expressing a PLK1 point mutant that is dramatically resistant to ATP competitive PLK1 inhibitors. Future studies will examine in vivo activity of selected abbapolins.

Citation Format: Danda Chapagai, Guru Ramamoorthy, Merissa Baxter, Sandra Craig, Elmar Nurmemmedov, McInnes Campbell, Michael Wyatt. Converting high affinity peptides to non-peptidic inhibitors via REPLACE: Novel and new strategies in targeting PLK1 Polo Box Domain (PBD) for anti-cancer therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1241.

Small molecules inhibitors of the heterogeneous ribonuclear protein A18 (hnRNP A18): a regulator of protein translation and an immune checkpoint

We have identified chemical probes that simultaneously inhibit cancer cell progression and an immune checkpoint. Using the computational Site Identification by Ligand Competitive Saturation (SILCS) technology, structural biology and cell-based assays, we identify small molecules that directly and selectively bind to the RNA Recognition Motif (RRM) of hnRNP A18, a regulator of protein translation in cancer cells. hnRNP A18 recognizes a specific RNA signature motif in the 3′UTR of transcripts associated with cancer cell progression (Trx, VEGF, RPA) and, as shown here, a tumor immune checkpoint (CTLA-4). Post-transcriptional regulation of immune checkpoints is a potential therapeutic strategy that remains to be exploited. The probes target hnRNP A18 RRM in vitro and in cells as evaluated by cellular target engagement. As single agents, the probes specifically disrupt hnRNP A18–RNA interactions, downregulate Trx and CTLA-4 protein levels and inhibit proliferation of several cancer cell lines without affecting the viability of normal epithelial cells. These first-in-class chemical probes will greatly facilitate the elucidation of the underexplored biological function of RNA Binding Proteins (RBPs) in cancer cells, including their effects on proliferation and immune checkpoint activation.

The miR-181a-SFRP4 Axis Regulates Wnt Activation to Drive Stemness and Platinum Resistance in Ovarian Cancer

Wnt signaling is a major driver of stemness and chemoresistance in ovarian cancer, yet the genetic drivers that stimulate its expression remain largely unknown. Unlike other cancers, mutations in the Wnt pathway are not reported in high-grade serous ovarian cancer (HGSOC). Hence, a key challenge that must be addressed to develop effective targeted therapies is to identify nonmutational drivers of Wnt activation. Using an miRNA sensor-based approach, we have identified miR-181a as a novel driver of Wnt/β-catenin signaling. miR-181a^high primary HGSOC cells exhibited increased Wnt/β-catenin signaling, which was associated with increased stem-cell frequency and platinum resistance. Consistent with these findings, inhibition of β-catenin decreased stem-like properties in miR-181a^high cell populations and downregulated miR-181a. The Wnt inhibitor SFRP4 was identified as a novel target of miR-181a. Overall, our results demonstrate that miR-181a is a nonmutational activator of Wnt signaling that drives stemness and chemoresistance in HGSOC, suggesting that the miR-181a-SFRP4 axis can be evaluated as a novel biomarker for β-catenin-targeted therapy in this disease. SIGNIFICANCE: These results demonstrate that miR-181a is an activator of Wnt signaling that drives stemness and chemoresistance in HGSOC and may be targeted therapeutically in recurrent disease.

DDRE-25. NOVEL TEMOZOLOMIDE ANALOGS TO IMPROVE ANTI-TUMOR EFFICACY AND OVERCOME RESISTANCE IN GLIOBLASTOMA MULTIFORME

Glioblastoma (GBM) is considered one of the most lethal forms of human cancers, and despite considerable advances in multimodality treatments, it remains an incurable disease with an overall survival of 14 to 16 months after diagnosis. Even in the era of personalized medicine and immunotherapy, temozolomide (TMZ), an oral alkylating agent, remains the standard-of-care for GBM. However, intrinisic or acquired resistance to TMZ due to over expression of O6-methylguanine-DNA methyltransferase (MGMT) results in initial treatment inefficacy or tumor relapse, highlighting the significant need for improved treatment strategies. Recently, much effort has been directed towards creating novel TMZ analogs to address the clinical barriers associated with TMZ. While some reported TMZ analogs showed improved brain permeability and anticancer effects in preclinical models, none of them have progressed to testing in humans. There is therefore significant room to improve the brain permeability and anticancer effect profiles of TMZ by incorporating yet unexplored functional groups into new analogs. We have designed and synthesized a series of novel C8-substituted TMZ analogs and have evaluated their anticancer potency against a panel of GBM cell lines with variable levels of MGMT expression. Encouragingly, our analogs demonstrated promising anti-cancer effect in both MGMT low and high expressing lines. We then evaluated our analogs in a variety of cell based assays to compare their activity with TMZ, and performed in vivo brain permeability and anti-tumor efficacy assays in mouse flank models. Our results demonstrated that several of our analogs clearly display improved anti-cancer effects and increased brain permeability over TMZ. This work points to a new direction for the development of novel TMZ analogs for improved patient survival.

DDRE-11. DEVELOPMENT OF THE FIRST-IN-CLASS INHIBITOR OF CHD4 - SENSITIZING RESISTANT GLIOMAS

Glioblastoma is a lethal brain tumor with high recurrence rate. CHD4 overexpression, which drives resistance to DNA damage, is one of the major sources of recurrence. Since standard GBM treatments like radiation and temozolomide chemotherapy create DNA damage, inhibition of CHD4 offers a new therapeutic option for resensitizing GBM. CHD4 is a ubiquitously expressed ATP-dependent chromatin remodeler, which plays a crucial role in epigenetic regulation of gene expression and in DNA damage repair. Structurally, CHD4 contains an HMG-like domain, PHD domains, two chromodomains, a catalytic ATPase module, two domains of unknown function (DUF) and a C-terminal domain CHDCT2. Currently, no specific inhibitors targeting this chromatin remodeler have been reported yet. We aim to develop the first-in-class inhibitor targeting chromo-domain of CHD4. We have performed in silico screens to identify small molecules binding to the chromo-domains of CHD4. We present our growing in vitro data demonstrating biophysical properties and mechanism-of-action of these novel inhibitors. We expect that the experiments proposed here will result in the development of the first-in-class CHD4 inhibitor which can be used in the future not only to better study the physiological role of CHD4 but also to determine its potential as a novel targeted therapy for GBM.

Targeting breast cancer metabolism with a novel inhibitor of mitochondrial ATP synthesis

Inhibitors of mitochondrial respiration and ATP synthesis may promote the selective killing of respiration-competent cancer cells that are critical for tumor progression. We previously reported that CADD522, a small molecule inhibitor of the RUNX2 transcription factor, has potential for breast cancer treatment. In the current study, we show that CADD522 inhibits mitochondrial oxidative phosphorylation by decreasing the mitochondrial oxygen consumption rate (OCR) and ATP production in human breast cancer cells in a RUNX2-independent manner. The enzyme activity of mitochondrial ATP synthase was inhibited by CADD522 treatment. Importantly, results from cellular thermal shift assays that detect drug-induced protein stabilization revealed that CADD522 interacts with both α and β subunits of the F1-ATP synthase complex. Differential scanning fluorimetry also demonstrated interaction of α subunits of the F1-ATP synthase to CADD522. These results suggest that CADD522 might target the enzymatic F1 subunits in the ATP synthase complex. CADD522 increased the levels of intracellular reactive oxygen species (ROS), which was prevented by MitoQ, a mitochondria-targeted antioxidant, suggesting that cancer cells exposed to CADD522 may elevate ROS from mitochondria. CADD522-increased mitochondrial ROS levels were enhanced by exogenously added pro-oxidants such as hydrogen peroxide or tert-butyl hydroperoxide. Conversely, CADD522-mediated cell growth inhibition was blocked by N-acetyl-l-cysteine, a general ROS scavenger. Therefore, CADD522 may exert its antitumor activity by increasing mitochondrial driven cellular ROS levels. Collectively, our data suggest in vitro proof-of-concept that supports inhibition of mitochondrial ATP synthase and ROS generation as contributors to the effectiveness of CADD522 in suppression of tumor growth.

Transducin β-like protein 1 controls multiple oncogenic networks in diffuse large B-cell lymphoma

Diffuse large B-cell lymphoma (DLBCL) is the most common non- Hodgkin lymphoma and is characterized by a remarkable heterogeneity with diverse variants that can be identified histologically and molecularly. Large-scale gene expression profiling studies have identified the germinal center B-cell (GCB-) and activated B-cell (ABC-) subtypes. Standard chemo-immunotherapy remains standard front-line therapy, curing approximately two thirds of patients. Patients with refractory disease or those who relapse after salvage treatment have an overall poor prognosis highlighting the need for novel therapeutic strategies. Transducin b-like protein 1 (TBL1) is an exchange adaptor protein encoded by the TBL1X gene and known to function as a master regulator of the Wnt signaling pathway by binding to β-CATENIN and promoting its downstream transcriptional program. Here, we show that, unlike normal B cells, DLBCL cells express abundant levels of TBL1 and its overexpression correlates with poor clinical outcome regardless of DLBCL molecular subtype. Genetic deletion of TBL1 and pharmacological approach using tegavivint, a first-in-class small molecule targeting TBL1 (Iterion Therapeutics), promotes DLBCL cell death in vitro and in vivo. Through an integrated genomic, biochemical, and pharmacologic analyses, we characterized a novel, β-CATENIN independent, post-transcriptional oncogenic function of TBL1 in DLBCL where TBL1 modulates the stability of key oncogenic proteins such as PLK1, MYC, and the autophagy regulatory protein BECLIN-1 through its interaction with a SKP1-CUL1-F-box (SCF) protein supercomplex. Collectively, our data provide the rationale for targeting TBL1 as a novel therapeutic strategy in DLBCL.

Targeting the ERG oncogene with splice-switching oligonucleotides as a novel therapeutic strategy in prostate cancer

BACKGROUND

The ERG oncogene, a member of the ETS family of transcription factor encoding genes, is a genetic driver of prostate cancer. It is activated through a fusion with the androgen-responsive TMPRSS2 promoter in 50% of cases. There is therefore significant interest in developing novel therapeutic agents that target ERG. We have taken an antisense approach and designed morpholino-based oligonucleotides that target ERG by inducing skipping of its constitutive exon 4.

METHODS

We designed antisense morpholino oligonucleotides (splice-switching oligonucleotides, SSOs) that target both the 5' and 3' splice sites of ERG's exon 4. We tested their efficacy in terms of inducing exon 4 skipping in two ERG-positive cell lines, VCaP prostate cancer cells and MG63 osteosarcoma cells. We measured their effect on cell proliferation, migration and apoptosis. We also tested their effect on xenograft tumour growth in mice and on ERG protein expression in a human prostate cancer radical prostatectomy sample ex vivo.

RESULTS

In VCaP cells, both SSOs were effective at inducing exon 4 skipping, which resulted in a reduction of overall ERG protein levels up to 96 h following a single transfection. SSO-induced ERG reduction decreased cell proliferation, cell migration and significantly increased apoptosis. We observed a concomitant reduction in protein levels for cyclin D1, c-Myc and the Wnt signalling pathway member β-catenin as well as a marker of activated Wnt signalling, p-LRP6. We tested the 3' splice site SSO in MG63 xenografts in mice and observed a reduction in tumour growth. We also demonstrated that the 3' splice site SSO caused a reduction in ERG expression in a patient-derived prostate tumour tissue cultured ex vivo.

CONCLUSIONS

We have successfully designed and tested morpholino-based SSOs that cause a marked reduction in ERG expression, resulting in decreased cell proliferation, a reduced migratory phenotype and increased apoptosis. Our initial tests on mouse xenografts and a human prostate cancer radical prostatectomy specimen indicate that SSOs can be effective for oncogene targeting in vivo. As such, this study encourages further in vivo therapeutic studies using SSOs targeting the ERG oncogene.

Peptidomimetic Polo-Box-Targeted Inhibitors that Engage PLK1 in Tumor Cells and Are Selective against the PLK3 Tumor Suppressor

The polo-box domain (PBD) of PLK1 determines mitotic substrate recognition and subcellular localization. Compounds that target PLK1 selectively are required due to the tumor-suppressor roles of PLK3. A structure-activity analysis of the PBD phosphopeptide binding motif has identified potent peptides that delineate the determinants required for mimicry by nonpeptidic inhibitors and provide insights into the structural basis for the selectivity of inhibitors for the PLK1 PBD. Fragment-ligated inhibitory peptides (FLIPs) obtained through REPLACE have been optimized to enhance in vitro binding and a systematic analysis of selectivity for PLK1 vs PLK3 has been carried out for peptides and peptidomimetics. Furthermore, these more drug-like non-ATP-competitive inhibitors had on-target engagement in a cellular context, as evidenced by stabilization of PLK1 in a thermal-shift assay and by inhibition of the phosphorylation of TCTP, a target of PLK1. Investigation in cells expressing a mutant PLK1 showed that these cells are sensitive to PBD inhibitors but dramatically resistant to clinically investigated ATP-competitive compounds. These results further validate targeting the PBD binding site in the move towards PLK1 inhibitors that are active against tumors resistant to ATP inhibitors.

Thrombospondin-1 counteracts the p97 inhibitor CB-5083 in colon carcinoma cells

p97 has recently emerged as a therapeutic target for cancer due to its essential functions in protein homeostasis. CB-5083 is a first-in-class, potent and selective ATP-competitive p97 inhibitor that induces proteotoxic stress in cancer cells. Potential mechanisms regulating the sensitivity of cells to p97 inhibition remain poorly studied. Here, we demonstrate that Thrombospondin-1 (THBS1) is a CB-5083-upregulated gene that helps confer resistance of HCT116 cells to CB-5083. Our immunoblotting and immunofluorescence data showed that CB-5083 significantly increases the steady-state abundance of THBS1. Blockade of THBS1 induction sensitized cells to CB-5083-mediated growth inhibition. Suppression of THBS1 caused an increase of CB-5083-induced sub-G1 population and caspase 3/7 activity suggesting that its function is linked to the survival of cancer cells in response to p97 inhibition. Altogether our data provide new evidence that THBS1 is important for the susceptibility of cells to p97 inhibition.

Allosteric inhibitor of β-catenin selectively targets oncogenic Wnt signaling in colon cancer

Abnormal regulation of β-catenin initiates an oncogenic program that serves as a main driver of many cancers. Albeit challenging, β-catenin is an attractive drug target due to its role in maintenance of cancer stem cells and potential to eliminate cancer relapse. We have identified C2, a novel β-catenin inhibitor, which is a small molecule that binds to a novel allosteric site on the surface of β-catenin. C2 selectively inhibits β-catenin, lowers its cellular load and significantly reduces viability of β-catenin-driven cancer cells. Through direct binding to β-catenin, C2 renders the target inactive that eventually activates proteasome system for its removal. Here we report a novel pharmacologic approach for selective inhibition of β-catenin via targeting a cryptic allosteric modulation site. Our findings may provide a new perspective for therapeutic targeting of β-catenin.

Evolution of a 4-Benzyloxy-benzylamino Chemotype to Provide Efficacious, Potent, and Isoform Selective PPARα Agonists as Leads for Retinal Disorders

Peroxisome proliferator-activated receptor alpha (PPARα) is expressed in retinal Müller cells, endothelial cells, and in retinal pigment epithelium; agonism of PPARα with genetic or pharmacological tools ameliorates inflammation, vascular leakage, neurodegeneration, and neovascularization associated with retinal diseases in animal models. As such, PPARα is a promising drug target for diabetic retinopathy and age-related macular degeneration. Herein, we report proof-of-concept in vivo efficacy in an streptozotocin-induced vascular leakage model (rat) and preliminary pharmacokinetic assessment of a first-generation lead 4a (A91). Additionally, we present the design, synthesis, and evaluation of second-generation analogues, which led to the discovery of 4u and related compounds that reach cellular potencies <50 nM and exhibit >2,700-fold selectivity for PPARα over other PPAR isoforms. These studies identify a pipeline of candidates positioned for detailed PK/PD and pre-clinical evaluation.

Activation of Wnt signaling promotes olaparib resistant ovarian cancer

Epithelial ovarian cancer (EOC) has one of the highest death to incidence ratios among all cancers. High grade serous ovarian carcinoma (HGSOC) is the most common and deadliest EOC histotype due to the lack of therapeutic options following debulking surgery and platinum/taxane-based chemotherapies. For recurrent chemosensitive HGSOC, poly(ADP)-ribose polymerase inhibitors (PARPi; olaparib, rucaparib, or niraparib) represent an emerging treatment strategy. While PARPi are most effective in homologous recombination DNA repair-deficient (HRD) HGSOCs, recent studies have observed a significant benefit in non-HRD HGSOCs. However, all HGSOC patients are likely to acquire resistance. Therefore, there is an urgent clinical need to understand PARPi resistance and to introduce novel combinatorial therapies to manage PARPi resistance and extend HGSOC disease-free intervals. In a panel of HGSOC cell lines, we established matched olaparib sensitive and resistant cells. Transcriptome analysis of the matched olaparib-sensitive vs -resistant cells revealed activation of the Wnt signaling pathway and consequently increased TCF transcriptional activity in PARPi-resistant cells. Forced activation of canonical Wnt signaling in several PARPi-sensitive cells via WNT3A reduced olaparib and rucaparib sensitivity. PARPi resistant cells were sensitive to inhibition of Wnt signaling using the FDA-approved compound, pyrvinium pamoate, which has been shown to promote downregulation of β-catenin. In both an HGSOC cell line and a patient-derived xenograft model, we observed that combining pyrvinium pamoate with olaparib resulted in a significant decrease in tumor burden. This study demonstrates that Wnt signaling can mediate PARPi resistance in HGSOC and provides a clinical rationale for combining PARP and Wnt inhibitors.

Antibody drug conjugates: Progress, pitfalls, and promises

Antibody drug conjugates (ADCs) represent a promising and an efficient strategy for targeted cancer therapy. Comprised of a monoclonal antibody, a cytotoxic drug, and a linker, ADCs offer tumor selectively, reduced toxicity, and improved stability in systemic circulation. Recent approvals of two ADCs have led to a resurgence in ADC research, with more than 60 ADCs under various stages of clinical development. The therapeutic success of future ADCs is dependent on adherence to key requirements of their design and careful selection of the target antigen on cancer cells. Here we review the main components in the design of antibody drug conjugates, improvements made, and lessons learned over two decades of research, as well as the future of third generation ADCs.

Pritumumab, the first therapeutic antibody for glioma patients

Immunotherapy is now at the forefront of cancer therapeutic development. Gliomas are a particularly aggressive form of brain cancer for which immunotherapy may hold promise. Pritumumab (also known in the literature as CLNH11, CLN-IgG, and ACA-11) was the first monoclonal antibody tested in cancer patients. Pritumumab is a natural human monoclonal antibody developed from a B lymphocyte isolated from a regional draining lymph node of a patient with cervical carcinoma. The antibody binds ecto-domain vimentin on the surface of cancer cells. Pritumumab was originally tested in clinical trials with brain cancer patients in Japan where it demonstrated therapeutic benefit. It was reported to be a safe and effective therapy for brain cancer patients at doses 5-10 fold less than currently approved antibodies. Phase I dose escalation clinical trials are now being planned with pritumumab for the near future. Here we review data on the development and characterization of pritumumab, and review clinical trails data assessing immunotherapeutic effects of pritumumab for glioma patients.

Abstract 3828: Pritumumab mAb binds cell surface expressed vimentin on pancreatic cancer cells and inhibits tumor growth

Monoclonal antibodies (mAbs) as therapeutics for cancer have shown some success in the clinic. These biologics are useful if targeting a cancer specific epitope. Cancer patients can generate tumor-specific B lymphocytes which can be isolated to develop human mAbs against tumor-associated antigens. The best source of anti-tumor antibodies is from sentinel lymph nodes. Pritumumab (also referred to as CLNH-11, CLN-IgG, or ACA-11) is a classic example of a natural human anti-cancer antibody. It is a natural human IgG1 kappa antibody developed by the human hybridoma technology, using B lymphocytes isolated from a regional draining lymph node of a patient with cervical carcinoma. In the original patient from whom the B lymphocyte was isolated, the bioavailability of the target antigen induced a natural immune response resulting in the generation of the pritumumab IgG. The target antigen recognized by pritumumab is cell surface expressed vimentin, also referred to as ecto-domain vimentin (EDV). Vimentin is an intracellular cytoskeletal protein overexpressed during epithelial-to-mesenchymal transition (EMT), a process integral to cancer cell metastasis. Pancreatic cancer is a deadly disease with poor prognosis. No more than 6% of pancreatic cancer patients will survive beyond five years after diagnosis. There is a clear unmet need to help improve outcomes for patients. (A) Purpose of the study: to examine if pritumumab mAb can target pancreatic cancer cells and inhibit pancreatic tumor growth. (B) Experimental Procedures: Immunohistochemical analysis with horseradish peroxidase (HRP)-conjugated pritumumab on pancreatic cancer patient tissue sections along with immunofluorescence, flow cytometry, and In-Cell ELISA to demonstrate antibody binding. Antibody-dependent cell-mediated cytoxicity (ADCC) was performed to determine mechanism of action. In vivo subcutaneous xenograft mouse model treated with pritumumab to assess efficacy of the antibody for tumor inhibition. (C) Results: In this study we demonstrate pritumumab binds to the cell surface of patient pancreatic cancer tissue and binds the surface of established pancreatic cancer cells in culture. We show antibody binding to these cells induces antibody-dependent cell-mediated cytoxicity (ADCC). Furthermore, pritumumab effectively inhibits pancreatic tumor growth in a xenograft mouse model. (D) Statement of Conclusions: overall, these data provide pre-clinical validation of pritumumab mAb as a therapeutic for pancreatic cancer.

Citation Format: Ivan Babic, Natsuko Nomura, Eric Glassy, Elmar Nurmemmedov, Venkata Yenugonda, Mark Glassy, Santosh Kesari. Pritumumab mAb binds cell surface expressed vimentin on pancreatic cancer cells and inhibits tumor growth [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3828.

A novel small molecule inhibitor of p32 mitochondrial protein overexpressed in glioma

BACKGROUND

The mitochondrial protein p32 is a validated therapeutic target of cancer overexpressed in glioma. Therapeutic targeting of p32 with monoclonal antibody or p32-binding LyP-1 tumor-homing peptide can limit tumor growth. However, these agents do not specifically target mitochondrial-localized p32 and would not readily cross the blood-brain barrier to target p32-overexpressing gliomas. Identifying small molecule inhibitors of p32 overexpressed in cancer is a more rational therapeutic strategy. Thus, in this study we employed a pharmacophore modeling strategy to identify small molecules that could bind and inhibit mitochondrial p32.

METHODS

A pharmacophore model of C1q and LyP-1 peptide association with p32 was used to screen a virtual compound library. A primary screening assay for inhibitors of p32 was developed to identify compounds that could rescue p32-dependent glutamine-addicted glioma cells from glutamine withdrawal. Inhibitors from this screen were analyzed for direct binding to p32 by fluorescence polarization assay and protein thermal shift. Affect of the p32 inhibitor on glioma cell proliferation was assessed by Alamar Blue assay, and affect on metabolism was examined by measuring lactate secretion.

RESULTS

Identification of a hit compound (M36) validates the pharmacophore model. M36 binds directly to p32 and inhibits LyP-1 tumor homing peptide association with p32 in vitro. M36 effectively inhibits the growth of p32 overexpressing glioma cells, and sensitizes the cells to glucose depletion.

CONCLUSIONS

This study demonstrates a novel screening strategy to identify potential inhibitors of mitochondrial p32 protein overexpressed in glioma. High throughput screening employing this strategy has potential to identify highly selective, potent, brain-penetrant small molecules amenable for further drug development.

Multiple spatially related pharmacophores define small molecule inhibitors of OLIG2 in glioblastoma

Transcription factors (TFs) are a major class of protein signaling molecules that play key cellular roles in cancers such as the highly lethal brain cancer—glioblastoma (GBM). However, the development of specific TF inhibitors has proved difficult owing to expansive protein-protein interfaces and the absence of hydrophobic pockets. We uniquely defined the dimerization surface as an expansive parental pharmacophore comprised of several regional daughter pharmacophores. We targeted the OLIG2 TF which is essential for GBM survival and growth, we hypothesized that small molecules able to fit each subpharmacophore would inhibit OLIG2 activation. The most active compound was OLIG2 selective, it entered the brain, and it exhibited potent anti-GBM activity in cell-based assays and in pre-clinical mouse orthotopic models. These data suggest that (1) our multiple pharmacophore approach warrants further investigation, and (2) our most potent compounds merit detailed pharmacodynamic, biophysical, and mechanistic characterization for potential preclinical development as GBM therapeutics.

Pritumumab binding to glioma cells induces ADCC and inhibits tumor growth

e14004

Background: Pritumumab is a natural human IgG1 kappa antibody originally isolated from a regional draining lymph node of a patient with cervical carcinoma. This antibody binds ectodomain vimentin on the surface of tumor cells and has demonstrated some benefit to glioblastoma patients in limited clinical trials. We wanted to determine if pritumumab inhibits glioma growth in vivo and if binding to glioma cells induces cell-mediated immunity. Methods: Pritumumab was used in flow cytometry experiments with several glioma cell lines and patient-derived neurosphere lines. Antibody-dependent cell-mediated cytotoxicity (ADCC) reporter assay was used with glioma target cells. Xenograft studies were performed in mice with and without intact B- and NK- cells. Results: We performed flow cytometry using pritumumab antibody and demonstrate binding of pritumumab to the surface of glioma cells and patient-derived glioma initiating cells. We observed significant induction of ADCC by pritumumab binding to glioma cells. Xenograft studies demonstrated pritumumab was effective in preventing tumor growth in nude mice but not in SCID mice. Intact cell-mediated immunity was necessary for pritumumab’s anti-tumor effect. Analysis of a blood brain barrier model showed significant binding of pritumumab in brain tumor areas and minimal distribution in normal brain tissues suggesting the antibody can cross the blood brain barrier. Conclusions: Our data demonstratepritumumab binds glioma cells in vitro and can induce ADCC. In addition, pritumumab can limit the growth of xenograft glioma tumors in vivo only in the presence of intact cell-mediated immunity. Together these data suggest pritumumab is suitable for development as an anti-tumor therapeutic.

Abstract 2520: Novel inhibitors of β-catenin

β-catenin is a key player of the WNT pathway; its aberrant regulation is associated with the onset and progression of numerous types of cancer. β-catenin is therefore an attractive drug target but direct inhibition of β-catenin is a challenging approach toward suppression of β-catenin-driven tumorigenesis. Ideal drug candidates should specifically target nuclear β-catenin, leaving its cytoplasmic and cell junctional functions intact. We have identified several drug-like candidate β-catenin inhibitors using a robust screening method against four allosteric binding sites of β-catenin (TCF4, BCL9 and two novel sites). We show that these inhibitors significantly reduce proliferation of a panel of human β-catenin-driven cancer cell lines, and repress the expression of β-catenin-induced target genes. They cause a shift in the melting temperature of the purified β-catenin protein. We also show that the candidate compounds reduce levels of β-catenin in the nucleus and block β-catenin-driven oncogenic transformation in cell culture. We consider these novel β-catenin inhibitors promising leads that should be further investigated for their molecular mechanism of action and for their potential development as therapeutic drugs.

Key words: β-catenin, oncogene, therapeutic, target, small molecule, protein-protein interaction

This work was supported by NIH Grant R01 CA078230. This is abstract 26043 of The Scripps Research Institute.

Citation Format: Elmar Nurmemmedov, Anton Cheltsov, Peter K. Vogt. Novel inhibitors of β-catenin. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2520. doi:10.1158/1538-7445.AM2014-2520

Challenging packaging limits and infectivity of phage λ

The terminase motors of bacteriophages have been shown to be among the strongest active machines in the biomolecular world, being able to package several tens of kilobase pairs of viral genome into a capsid within minutes. Yet, these motors are hindered at the end of the packaging process by the progressive buildup of a force-resisting packaging associated with already packaged DNA. In this experimental work, we raise the issue of what sets the upper limit on the length of the genome that can be packaged by the terminase motor of phage λ and still yield infectious virions and the conditions under which this can be efficiently performed. Using a packaging strategy developed in our laboratory of building phage λ from scratch, together with plaque assay monitoring, we have been able to show that the terminase motor of phage λ is able to produce infectious particles with up to 110% of the wild-type λ-DNA length. However, the phage production rate, and thus the infectivity, decreased exponentially with increasing DNA length and was a factor of 10(3) lower for the 110% λ-DNA phage. Interestingly, our in vitro strategy was still efficient in fully packaging phages with DNA lengths as high as 114% of the wild-type length, but these viruses were unable to infect bacterial cells efficiently. Further, we demonstrated that the phage production rate is modulated by the presence of multivalent ionic species. The biological consequences of these findings are discussed.

Kinetic behaviour of WT 1's zinc finger domain in binding to the alpha-actinin-1 mRNA

The zinc finger transcription factor Wilms tumour protein (WT 1) is known for its essential involvement in the development of the genitourinary system as well as of other organs and tissues. WT 1 is capable of selectively binding either DNA or mRNA targets. A KTS insertion due to alternative splicing between the zinc fingers 3 and 4 and an unconventional zinc finger 1 are the unique features that distinguish WT 1 from classical DNA-binding C(2)H(2)-type zinc finger proteins. The DNA binding characteristics of WT 1 are well studied. Due to lack of information about its native RNA targets, no extensive research has been directed at how WT 1 binds RNA. Using surface plasmon resonance, this study attempts to understand the binding behaviour of WT 1 zinc fingers with its recently reported and first putative mRNA target, ACT 34, whose stem-loop structure is believed to be critical for the interactions with WT 1. We have analysed the interactions of five WT 1 zinc finger truncations with wild-type ACT 34 and four variants. Our results indicate that WT 1 zinc fingers bind ACT 34 in a specific manner, and that this occurs as interplay of all four zinc fingers. We also report that a sensitive kinetic balance, which is equilibrated by both zinc finger 1 and KTS, regulates the interaction with ACT 34. The stem-loop and the flanking nucleotides are important elements for specific recognition by WT 1 zinc fingers.

New insights into DNA-binding behavior of Wilms tumor protein (WT1)--a dual study

Wilms Tumor suppressor protein (WT1) is a transcription factor that is involved in a variety of developmental functions during organ development. It is also implicated in the pathology of several different cancer forms. The protein contains four C(2)H(2)-type zinc fingers and it specifically binds GC-rich sequences in the promoter regions of its target genes, which are either up or down regulated. Two properties make WT1 a more unusual transcription factor - an unconventional amino acid composition for zinc finger 1, and the insertion of a tri-peptide KTS in some of the splice isoforms of WT1. Using six WT1 constructs in which zinc fingers are systematically deleted, a dual study based on a bacterial 1-hybrid system and surface plasmon resonance measurements is performed. The experiments show that the effect of zinc finger 1 is not significant in terms of overall DNA-binding kinetics, however it influences both the specificity of target recognition and stability of interaction in presence of KTS. The KTS insertion, however, only mildly retards binding affinity, mainly by affecting the on-rate. We suggest that the insertion disturbs zinc finger 4 from its binding frame, thus weakening the rate of target recognition. Finally, for the construct in which both zinc fingers 1 and 4 were deleted, the two middle fingers 2-3 still could function as a 'minimal DNA-recognition domain' for WT1, however the formation of a stable protein-DNA complex is impaired since the overall affinity was dramatically reduced mainly since the off-rate was severely affected.

Expression, purification, and characterization of the 4 zinc finger region of human tumor suppressor WT1

Wilm's Tumor gene 1 (WT1) encodes a zinc finger protein with four distinct splice isoforms. WT1 has a critical role in genesis of various cancer types both at the DNA/RNA and the protein level. The zinc-finger DNA-binding capacity of the protein is located in the C-terminal domain. Two recombinant proteins, 6HIS-ZN-wt1 and 6HIS-ZN+wt1, corresponding to two alternative splice variants of the C-terminal regions of human WT1 (-KTS) and WT1 (+KTS), respectively, were over-expressed with hexa-histidine fusion tags in inclusion bodies in Escherichia coli for crystallization studies. A combination of Ni2+-NTA affinity and size-exclusion chromatography was applied for purification of the proteins in denaturing conditions. The effects of various buffers, salts and other additives were scrutinized in a systematic screening to establish the optimal conditions for solubility and refolding of the recombinant WT1 proteins. Circular dichroism analysis revealed the expected betabetaalpha content for the refolded proteins, with a notable degradation of the alpha-helical segment in the DNA-free state. Electrophoretic mobility shift assay with double-stranded DNA containing the double Egr1 consensus site 5'-GCG-TGG-GCG-3' confirmed that 6HIS-ZN-wt1 has higher DNA binding affinity than 6HIS-ZN+wt1.