STING agonist delivery by tumour-penetrating PEG-lipid nanodiscs primes robust anticancer immunity.

Activation of the innate immune STimulator of INterferon Genes (STING) pathway potentiates antitumour immunity, but systemic delivery of STING agonists to tumours is challenging. We conjugated STING-activating cyclic dinucleotides (CDNs) to PEGylated lipids (CDN-PEG-lipids; PEG, polyethylene glycol) via a cleavable linker and incorporated them into lipid nanodiscs (LNDs), which are discoid nanoparticles formed by self-assembly. Compared to state-of-the-art liposomes, intravenously administered LNDs carrying CDN-PEG-lipid (LND-CDNs) exhibited more efficient penetration of tumours, exposing the majority of tumour cells to STING agonist. A single dose of LND-CDNs induced rejection of established tumours, coincident with immune memory against tumour rechallenge. Although CDNs were not directly tumoricidal, LND-CDN uptake by cancer cells correlated with robust T-cell activation by promoting CDN and tumour antigen co-localization in dendritic cells. LNDs thus appear promising as a vehicle for robust delivery of compounds throughout solid tumours, which can be exploited for enhanced immunotherapy.

Overcoming mTOR resistance mutations with a new-generation mTOR inhibitor.

Precision medicines exert selective pressure on tumour cells that leads to the preferential growth of resistant subpopulations, necessitating the development of next-generation therapies to treat the evolving cancer. The PIK3CA-AKT-mTOR pathway is one of the most commonly activated pathways in human cancers, which has led to the development of small-molecule inhibitors that target various nodes in the pathway. Among these agents, first-generation mTOR inhibitors (rapalogs) have caused responses in 'N-of-1' cases, and second-generation mTOR kinase inhibitors (TORKi) are currently in clinical trials. Here we sought to delineate the likely resistance mechanisms to existing mTOR inhibitors in human cell lines, as a guide for next-generation therapies. The mechanism of resistance to the TORKi was unusual in that intrinsic kinase activity of mTOR was increased, rather than a direct active-site mutation interfering with drug binding. Indeed, identical drug-resistant mutations have been also identified in drug-naive patients, suggesting that tumours with activating MTOR mutations will be intrinsically resistant to second-generation mTOR inhibitors. We report the development of a new class of mTOR inhibitors that overcomes resistance to existing first- and second-generation inhibitors. The third-generation mTOR inhibitor exploits the unique juxtaposition of two drug-binding pockets to create a bivalent interaction that allows inhibition of these resistant mutants.

Design and synthesis of selective CDK8/19 dual inhibitors: Discovery of 4,5-dihydrothieno[3',4':3,4]benzo[1,2-d]isothiazole derivatives.

To develop a novel series of CDK8/19 dual inhibitors, we employed structure-based drug design using docking models based on a library compound, 4,5-dihydroimidazolo[3',4':3,4]benzo[1,2-d]isothiazole 16 bound to CDK8. We designed various [5,6,5]-fused tricyclic scaffolds bearing a carboxamide group to maintain predicted interactions with the backbone CO and NH of Ala100 in the CDK8 kinase hinge region. We found that 4,5-dihydrothieno[3',4':3,4]benzo[1,2-d]isothiazole derivative 29a showed particularly potent enzymatic inhibitory activity in both CDK8/19 (CDK8 IC50: 0.76nM, CDK19 IC50: 1.7nM). To improve the physicochemical properties and kinase selectivity of this compound, we introduced a substituted 3-pyridyloxy group into the scaffold 8-position. The resulting optimized compound 52h showed excellent in vitro potency (CDK8 IC50: 0.46nM, CDK19 IC50: 0.99nM), physicochemical properties, and kinase selectivity (only 5 kinases showed <35% unbound fraction at 300nM. CDK19: 4.6%, CDK8: 8.3%, HASPIN: 23%, DYRK1B: 27%, HIP1: 32%). Based on a docking model of 52h bound to CDK8, we could explain the highly specific kinase activity profile found for this compound, based on the interaction of the pyridyl group of 52h interacting with Met174 of the CDK8 DMG activation loop. In vitro pharmacological evaluation of 52h revealed potent suppression of phosphorylated STAT1 in various cancer cells. The high oral bioavailability found for this compound enabled in vivo studies, in which we demonstrated a mechanism-based in vivo PD effect as well as tumor growth suppression in an RPMI8226 human hematopoietic and lymphoid xenograft model in mouse [T/C: -1% (2.5mg/kg, qd)].

Design and synthesis of fused bicyclic inhibitors targeting the L5 loop site of centromere-associated protein E.

Centromere-associated protein-E (CENP-E) is a mitotic kinesin which plays roles in cell division, and is regarded as a promising therapeutic target for the next generation of anti-mitotic agents. We designed novel fused bicyclic CENP-E inhibitors starting from previous reported dihydrobenzofuran derivative (S)-(+)-1. Our design concept was to adjust the electron density distribution on the benzene ring of the dihydrobenzofuran moiety to increase the positive charge for targeting the negatively charged L5 loop of CENP-E, using predictions from electrostatic potential map (EPM) analysis. For the efficient synthesis of our 2,3-dihydro-1-benzothiophene 1,1-dioxide derivatives, a new synthetic method was developed. As a result, we discovered 6-cyano-7-trifluoromethyl-2,3-dihydro-1-benzothiophene 1,1-dioxide derivative (+)-5d (Compound A) as a potent CENP-E inhibitor with promising potential for in vivo activity. In this Letter, we discuss the design and synthetic strategy used in the discovery of (+)-5d and structure-activity relationships for its analogs possessing various fused bicyclic L5 binding moieties.

Synthetic studies of centromere-associated protein-E (CENP-E) inhibitors: 1.Exploration of fused bicyclic core scaffolds using electrostatic potential map.

Centromere-associated protein-E (CENP-E), a mitotic kinesin that plays an important role in mitotic progression, is an attractive target for cancer therapeutic drugs. For the purpose of developing novel CENP-E inhibitors as cancer therapeutics, we investigated a fused bicyclic compound identified by high throughput screening, 4-oxo-4,5-dihydrothieno[3,4-c]pyridine-6-carboxamide 1a. Based on this scaffold, we designed inhibitors for efficient binding at the L5 site in CENP-E utilizing homology modeling as well as electrostatic potential map (EPM) analysis to enhance CENP-E inhibitory activity. This resulted in a new lead, 5-bromoimidazo[1,2-a]pyridine 7, which showed potent CENP-E enzyme inhibition (IC50: 50nM) and cellular activity with accumulation of phosphorylated histone H3 in HeLa cells. Our homology model and EPM analysis proved to be useful tools for the rational design of CENP-E inhibitors.

Design and synthesis of novel DFG-out RAF/vascular endothelial growth factor receptor 2 (VEGFR2) inhibitors: 3. Evaluation of 5-amino-linked thiazolo[5,4-d]pyrimidine and thiazolo[5,4-b]pyridine derivatives.

Our aim was to discover RAF/vascular endothelial growth factor receptor 2 (VEGFR2) inhibitors that possess strong activity and sufficient oral absorption, and thus, we selected a 5-amino-linked thiazolo[5,4-d]pyrimidine derivative as the lead compound because of its potential kinase inhibitory activities and its desired solubility. The novel tertiary 1-cyano-1-methylethoxy substituent was designed to occupy the hydrophobic region of 'back pocket' of BRAF on the basis of the X-ray co-crystal structure data of BRAF. In addition, we found that N-methylation of the amine linker could control the twisted molecular conformation leading to improved solubility. These approaches produced N-methyl thiazolo[5,4-b]pyridine-5-amine derivative 5. To maximize the in vivo efficacy, we attempted salt formation of 5. Our result indicated that the besylate monohydrate salt form (5c) showed significant improvement of both solubility and oral absorption. Owing to the improved physicochemical properties, compound 5c demonstrated regressive antitumor efficacy in a HT-29 xenograft model.

Design and synthesis of novel DFG-out RAF/vascular endothelial growth factor receptor 2 (VEGFR2) inhibitors: 2. Synthesis and characterization of a novel imide-type prodrug for improving oral absorption.

As an alternative to the previously reported solid dispersion formulation for enhancing the oral absorption of thiazolo[5,4-b]pyridine 1, we investigated novel N-acyl imide prodrugs of 1 as RAF/vascular endothelial growth factor receptor 2 (VEGFR2) inhibitors. Introducing N-acyl promoieties at the benzanilide position gave chemically stable imides. N-tert-Butoxycarbonyl (Boc) introduced imide 6 was a promising prodrug, which was converted to the active compound 1 after its oral administration in mice. Cocrystals of 6 with AcOH (6b) possessed good physicochemical properties with moderate thermodynamic solubility (19µg/mL). This crystalline prodrug 6b was rapidly and enzymatically converted into 1 after its oral absorption in mice, rats, dogs, and monkeys. Prodrug 6b showed in vivo antitumor regressive efficacy (T/C=-6.4%) in an A375 melanoma xenograft model in rats. Hence, we selected 6b as a promising candidate and are performing further studies. Herein, we report the design, synthesis, and characterization of novel imide-type prodrugs.

TAK-676: A Novel Stimulator of Interferon Genes (STING) Agonist Promoting Durable IFN-dependent Antitumor Immunity in Preclinical Studies.

Oncology therapies targeting the immune system have improved patient outcomes across a wide range of tumor types, but resistance due to an inadequate T-cell response in a suppressive tumor microenvironment (TME) remains a significant problem. New therapies that activate an innate immune response and relieve this suppression may be beneficial to overcome this hurdle. TAK-676 is a synthetic novel stimulator of interferon genes (STING) agonist designed for intravenous administration. Here we demonstrate that TAK-676 dose-dependently triggers activation of the STING signaling pathway and activation of type I interferons. Furthermore, we show that TAK-676 is a highly potent modulator of both the innate and adaptive immune system and that it promotes the activation of dendritic cells, natural killer cells, and T cells in preclinical models. In syngeneic murine tumor models in vivo, TAK-676 induces dose-dependent cytokine responses and increases the activation and proliferation of immune cells within the TME and tumor-associated lymphoid tissue. We also demonstrate that TAK-676 dosing results in significant STING-dependent antitumor activity, including complete regressions and durable memory T-cell immunity. We show that TAK-676 is well tolerated, exhibits dose-proportional pharmacokinetics in plasma, and exhibits higher exposure in tumor. The intravenous administration of TAK-676 provides potential treatment benefit in a broad range of tumor types. Further study of TAK-676 in first-in-human phase I trials is ongoing. Significance: TAK-676 is a novel systemic STING agonist demonstrating robust activation of innate and adaptive immune activity resulting in durable antitumor responses within multiple syngeneic tumor models. Clinical investigation of TAK-676 is ongoing.

Repositioning and Characterization of 1-(Pyridin-4-yl)pyrrolidin-2-one Derivatives as Plasmodium Cytoplasmic Prolyl-tRNA Synthetase Inhibitors.

Prolyl-tRNA synthetase (PRS) is a clinically validated antimalarial target. Screening of a set of PRS ATP-site binders, initially designed for human indications, led to identification of 1-(pyridin-4-yl)pyrrolidin-2-one derivatives representing a novel antimalarial scaffold. Evidence designates cytoplasmic PRS as the drug target. The frontrunner 1 and its active enantiomer 1-S exhibited low-double-digit nanomolar activity against resistant Plasmodium falciparum (Pf) laboratory strains and development of liver schizonts. No cross-resistance with strains resistant to other known antimalarials was noted. In addition, a similar level of growth inhibition was observed against clinical field isolates of Pf and P. vivax. The slow killing profile and the relative high propensity to develop resistance in vitro (minimum inoculum resistance of 8 × 105 parasites at a selection pressure of 3 × IC50) constitute unfavorable features for treatment of malaria. However, potent blood stage and antischizontal activity are compelling for causal prophylaxis which does not require fast onset of action. Achieving sufficient on-target selectivity appears to be particularly challenging and should be the primary focus during the next steps of optimization of this chemical series. Encouraging preliminary off-target profile and oral efficacy in a humanized murine model of Pf malaria allowed us to conclude that 1-(pyridin-4-yl)pyrrolidin-2-one derivatives represent a promising starting point for the identification of novel antimalarial prophylactic agents that selectively target Plasmodium PRS.

New Series of Potent Allosteric Inhibitors of Deoxyhypusine Synthase.

Deoxyhypusine synthase (DHPS) is the primary enzyme responsible for the hypusine modification and, thereby, activation of the eukaryotic translation initiation factor 5A (eIF5A), which is key in regulating the protein translation processes associated with tumor proliferation. Although DHPS inhibitors could be a promising therapeutic option for treating cancer, only a few studies reported druglike compounds with this inhibition property. Thus, in this work, we designed and synthesized a new chemical series possessing fused ring scaffolds designed from high-throughput screening hit compounds, discovering a 5,6-dihydrothieno[2,3-c]pyridine derivative (26d) with potent inhibitory activity; furthermore, the X-ray crystallographic analysis of the DHPS complex with 26d demonstrated a distinct allosteric binding mode compared to a previously reported inhibitor. These findings could be significantly useful in the functional analysis of conformational changes in DHPS as well as the structure-based design of allosteric inhibitors.

Discovery of Novel Allosteric Inhibitors of Deoxyhypusine Synthase.

Deoxyhypusine synthase (DHPS) utilizes spermidine and NAD as cofactors to incorporate a hypusine modification into the eukaryotic translation initiation factor 5A (eIF5A). Hypusine is essential for eIF5A activation, which, in turn, plays a key role in regulating protein translation of selected mRNA that are associated with the synthesis of oncoproteins, thereby enhancing tumor cell proliferation. Therefore, inhibition of DHPS is a promising therapeutic option for the treatment of cancer. To discover novel lead compounds that target DHPS, we conducted synthetic studies with a hit obtained via high-throughput screening. Optimization of the ring structures of the amide compound (2) led to bromobenzothiophene (11g) with potent inhibitory activity against DHPS. X-ray crystallographic analysis of 11g complexed with DHPS revealed a dramatic conformational change in DHPS, which suggests the presence of a novel allosteric site. These findings provide the basis for the development of novel therapy distinct from spermidine mimetic inhibitors.

Discovery and pharmacological characterization of a new class of prolyl-tRNA synthetase inhibitor for anti-fibrosis therapy.

Scleroderma has clinical characteristics including skin and other tissue fibrosis, but there is an unmet need for anti-fibrotic therapy. Halofuginone (HF) is a well-known anti-fibrosis agent in preclinical and clinical studies which exerts its effect via inhibition of TGF-ß/Smad3 signaling pathway. Recently, prolyl-tRNA synthetase (PRS) was elucidated as a target protein for HF that binds to the proline binding site of the catalytic domain of PRS. Here, we characterized a new class of PRS inhibitor (T-3833261) that is carefully designed in a way that binds to the ATP site of the catalytic domain and does not disrupt binding of proline. The anti-fibrotic activity and the mechanism of action for T-3833261 on TGF-ß-induced fibrotic assay were compared with those of HF in primary human skin fibroblast. We evaluated in vivo effect of topical application of T-3833261 and HF on TGF-ß-induced fibrotic genes expression in mice. We found that T-3833261 suppressed TGF-ß-induced α-smooth muscle actin (α-SMA) and type I collagen α1 (COL1A1) expression through the Smad3 axis in a similar fashion to HF. In vivo topical application of T-3833261 reduced the increase of fibrotic genes expression such as α-Sma, Col1a1 and Col1a2 by TGF-ß intradermal injection to the ear of a mouse. We revealed that T-3833261 is more effective than HF under the conditions of high proline concentration, as reported in fibrotic tissues. These results suggest the potential of ATP competitive PRS inhibitors for the treatment of fibrotic diseases such as scleroderma.

A Kinase Inhibitor Targeted to mTORC1 Drives Regression in Glioblastoma.

Although signaling from phosphatidylinositol 3-kinase (PI3K) and AKT to mechanistic target of rapamycin (mTOR) is prominently dysregulated in high-grade glial brain tumors, blockade of PI3K or AKT minimally affects downstream mTOR activity in glioma. Allosteric mTOR inhibitors, such as rapamycin, incompletely block mTORC1 compared with mTOR kinase inhibitors (TORKi). Here, we compared RapaLink-1, a TORKi linked to rapamycin, with earlier-generation mTOR inhibitors. Compared with rapamycin and Rapalink-1, TORKi showed poor durability. RapaLink-1 associated with FKBP12, an abundant mTOR-interacting protein, enabling accumulation of RapaLink-1. RapaLink-1 showed better efficacy than rapamycin or TORKi, potently blocking cancer-derived, activating mutants of mTOR. Our study re-establishes mTOR as a central target in glioma and traces the failure of existing drugs to incomplete/nondurable inhibition of mTORC1.

A Novel Time-Dependent CENP-E Inhibitor with Potent Antitumor Activity.

Centromere-associated protein E (CENP-E) regulates both chromosome congression and the spindle assembly checkpoint (SAC) during mitosis. The loss of CENP-E function causes chromosome misalignment, leading to SAC activation and apoptosis during prolonged mitotic arrest. Here, we describe the biological and antiproliferative activities of a novel small-molecule inhibitor of CENP-E, Compound-A (Cmpd-A). Cmpd-A inhibits the ATPase activity of the CENP-E motor domain, acting as a time-dependent inhibitor with an ATP-competitive-like behavior. Cmpd-A causes chromosome misalignment on the metaphase plate, leading to prolonged mitotic arrest. Treatment with Cmpd-A induces antiproliferation in multiple cancer cell lines. Furthermore, Cmpd-A exhibits antitumor activity in a nude mouse xenograft model, and this antitumor activity is accompanied by the elevation of phosphohistone H3 levels in tumors. These findings demonstrate the potency of the CENP-E inhibitor Cmpd-A and its potential as an anticancer therapeutic agent.

Synthetic Studies on Centromere-Associated Protein-E (CENP-E) Inhibitors: 2. Application of Electrostatic Potential Map (EPM) and Structure-Based Modeling to Imidazo[1,2-a]pyridine Derivatives as Anti-Tumor Agents.

To develop centromere-associated protein-E (CENP-E) inhibitors for use as anticancer therapeutics, we designed novel imidazo[1,2-a]pyridines, utilizing previously discovered 5-bromo derivative 1a. By site-directed mutagenesis analysis, we confirmed the ligand binding site. A docking model revealed the structurally important molecular features for effective interaction with CENP-E and could explain the superiority of the inhibitor (S)-isomer in CENP-E inhibition vs the (R)-isomer based on the ligand conformation in the L5 loop region. Additionally, electrostatic potential map (EPM) analysis was employed as a ligand-based approach to optimize functional groups on the imidazo[1,2-a]pyridine scaffold. These efforts led to the identification of the 5-methoxy imidazo[1,2-a]pyridine derivative (+)-(S)-12, which showed potent CENP-E inhibition (IC50: 3.6 nM), cellular phosphorylated histone H3 (p-HH3) elevation (EC50: 180 nM), and growth inhibition (GI50: 130 nM) in HeLa cells. Furthermore, (+)-(S)-12 demonstrated antitumor activity (T/C: 40%, at 75 mg/kg) in a human colorectal cancer Colo205 xenograft model in mice.

Aneuploidy generates proteotoxic stress and DNA damage concurrently with p53-mediated post-mitotic apoptosis in SAC-impaired cells.

The molecular mechanism responsible that determines cell fate after mitotic slippage is unclear. Here we investigate the post-mitotic effects of different mitotic aberrations--misaligned chromosomes produced by CENP-E inhibition and monopolar spindles resulting from Eg5 inhibition. Eg5 inhibition in cells with an impaired spindle assembly checkpoint (SAC) induces polyploidy through cytokinesis failure without a strong anti-proliferative effect. In contrast, CENP-E inhibition causes p53-mediated post-mitotic apoptosis triggered by chromosome missegregation. Pharmacological studies reveal that aneuploidy caused by the CENP-E inhibitor, Compound-A, in SAC-attenuated cells causes substantial proteotoxic stress and DNA damage. Polyploidy caused by the Eg5 inhibitor does not produce this effect. Furthermore, p53-mediated post-mitotic apoptosis is accompanied by aneuploidy-associated DNA damage response and unfolded protein response activation. Because Compound-A causes p53 accumulation and antitumour activity in an SAC-impaired xenograft model, CENP-E inhibitors could be potential anticancer drugs effective against SAC-impaired tumours.

Antitumor activity of the selective pan-RAF inhibitor TAK-632 in BRAF inhibitor-resistant melanoma.

The mitogen-activated protein kinase (MAPK) pathway is particularly important for the survival and proliferation of melanoma cells. Somatic mutations in BRAF and NRAS are frequently observed in melanoma. Recently, the BRAF inhibitors vemurafenib and dabrafenib have emerged as promising agents for the treatment of melanoma patients with BRAF-activating mutations. However, as BRAF inhibitors induce RAF paradoxical activation via RAF dimerization in BRAF wild-type cells, rapid emergence of acquired resistance and secondary skin tumors as well as presence of few effective treatment options for melanoma bearing wild-type BRAF (including NRAS-mutant melanoma) are clinical concerns. Here, we demonstrate that the selective pan-RAF inhibitor TAK-632 suppresses RAF activity in BRAF wild-type cells with minimal RAF paradoxical activation. Our analysis using RNAi and TAK-632 in preclinical models reveals that the MAPK pathway of NRAS-mutated melanoma cells is highly dependent on RAF. We also show that TAK-632 induces RAF dimerization but inhibits the kinase activity of the RAF dimer, probably because of its slow dissociation from RAF. As a result, TAK-632 demonstrates potent antiproliferative effects both on NRAS-mutated melanoma cells and BRAF-mutated melanoma cells with acquired resistance to BRAF inhibitors through NRAS mutation or BRAF truncation. Furthermore, we demonstrate that the combination of TAK-632 and the MAPK kinase (MEK) inhibitor TAK-733 exhibits synergistic antiproliferative effects on these cells. Our findings characterize the unique features of TAK-632 as a pan-RAF inhibitor and provide rationale for its further investigation in NRAS-mutated melanoma and a subset of BRAF-mutated melanomas refractory to BRAF inhibitors.

Discovery of a selective kinase inhibitor (TAK-632) targeting pan-RAF inhibition: design, synthesis, and biological evaluation of C-7-substituted 1,3-benzothiazole derivatives.

With the aim of discovering a selective kinase inhibitor targeting pan-RAF kinase inhibition, we designed novel 1,3-benzothiazole derivatives based on our thiazolo[5,4-b]pyridine class RAF/VEGFR2 inhibitor 1 and developed a regioselective cyclization methodology for the C-7-substituted 1,3-benzothiazole scaffold utilizing meta-substituted anilines. Eventually, we selected 7-cyano derivative 8B (TAK-632) as a development candidate and confirmed its binding mode by cocrystal structure with BRAF. Accommodation of the 7-cyano group into the BRAF-selectivity pocket and the 3-(trifluoromethyl)phenyl acetamide moiety into the hydrophobic back pocket of BRAF in the DFG-out conformation contributed to enhanced RAF potency and selectivity vs VEGFR2. Reflecting its potent pan-RAF inhibition and slow off-rate profile, 8B demonstrated significant cellular activity against mutated BRAF or mutated NRAS cancer cell lines. Furthermore, in both A375 (BRAF(V600E)) and HMVII (NRAS(Q61K)) xenograft models in rats, 8B demonstrated regressive antitumor efficacy by twice daily, 14-day repetitive administration without significant body weight loss.

Design and synthesis of novel DFG-out RAF/vascular endothelial growth factor receptor 2 (VEGFR2) inhibitors. 1. Exploration of [5,6]-fused bicyclic scaffolds.

To develop RAF/VEGFR2 inhibitors that bind to the inactive DFG-out conformation, we conducted structure-based drug design using the X-ray cocrystal structures of BRAF, starting from an imidazo[1,2-b]pyridazine derivative. We designed various [5,6]-fused bicyclic scaffolds (ring A, 1-6) possessing an anilide group that forms two hydrogen bond interactions with Cys532. Stabilizing the planarity of this anilide and the nitrogen atom on the six-membered ring of the scaffold was critical for enhancing BRAF inhibition. The selected [1,3]thiazolo[5,4-b]pyridine derivative 6d showed potent inhibitory activity in both BRAF and VEGFR2. Solid dispersion formulation of 6d (6d-SD) maximized its oral absorption in rats and showed significant suppression of ERK1/2 phosphorylation in an A375 melanoma xenograft model in rats by single administration. Tumor regression (T/C = -7.0%) in twice-daily repetitive studies at a dose of 50 mg/kg in rats confirmed that 6d is a promising RAF/VEGFR2 inhibitor showing potent anticancer activity.