Design and Synthesis of Novel Macrocyclic Mer Tyrosine Kinase Inhibitors

Light-Activatable Photocaged UNC2025 for Triggering TAM Kinase Inhibition in Bladder Cancer

Photopharmacology is an emerging field that utilizes photo-responsive molecules to enable control over the activity of a drug using light. The aim is to limit the therapeutic action of a drug at the level of diseased tissues and organs. Considering the well-known implications of protein kinases in cancer and the therapeutic issues associated with protein kinase inhibitors, the photopharmacology is seen as an innovative and alternative solution with great potential in oncology. In this context, we developed the first photocaged TAM kinase inhibitors based on UNC2025, a first-in-class small molecule kinase inhibitor. These prodrugs showed good stability in biologically relevant buffer and rapid photorelease of the photoremovable protecting group upon UV-light irradiation (<10 min.). These light-activatable prodrugs led to a 16-fold decrease to a complete loss of kinase inhibition, depending on the protein and the position at which the coumarin-type phototrigger was introduced. The most promising candidate was the N,O-dicaged compound, showing the superiority of having two photolabile protecting groups on UNC2025 for being entirely inactive on TAM kinases. Under UV-light irradiation, the N,O-dicaged compound recovered its inhibitory potency in enzymatic assays and displayed excellent antiproliferative activity in RT112 cell lines.

Discovery of a 1,6-naphthyridin-4-one-based AXL inhibitor with improved pharmacokinetics and enhanced in vivo antitumor efficacy

The receptor tyrosine kinase AXL has emerged as an attractive target in anticancer drug discovery. Herein, we described the discovery of a new series of 1,6-naphthyridin-4-one derivatives as potent AXL inhibitors. Starting from a low in vivo potency compound 9 which was previously reported by our group, we utilized structure-based drug design and scaffold hopping strategies to discover potent AXL inhibitors. The privileged compound 13c was a highly potent and orally bioavailable AXL inhibitor with an IC50 value of 3.2 ± 0.3 nM. Compound 13c exhibited significantly improved in vivo antitumor efficacy in AXL-driven tumor xenograft mice, causing tumor regression at well-tolerated dose, and demonstrated favorable pharmacokinetic properties (MRT = 16.5 h, AUC0-∞ = 59,815 ng h/mL) in Sprague-Dawley rats. These results suggest that 13c is a promising therapeutic candidate for AXL-targeting cancer treatment.

Investigation of Macrocyclic mTOR Modulators of Rapamycin Binding Site via Pharmacoinformatics Approaches

The PI3K/Akt/mTOR is an essential intracellular signaling pathway in which the serine/threonine mTOR kinase portrays a major role in cell growth, proliferation and survival. The mTOR kinase is frequently dysregulated in a broad spectrum of cancers, thus making it a potential target. Rapamycin and its analogs (rapalogs) allosterically inhibit mTOR, thereby dodging the deleterious effects prompted by ATP-competitive mTOR inhibitors. However, the available mTOR allosteric site inhibitors exhibit low oral bioavailability and suboptimal solubility. Bearing in mind this narrow therapeutic window of the current allosteric mTOR inhibitors, an in silico study was designed in search of new macrocyclic inhibitors. The macrocycles from the ChemBridge database (12,677 molecules) were filtered for their drug-likeness properties and the procured compounds were subjected for molecular docking within the binding cleft between FKBP25 and FRB domains of mTOR. The docking analysis resulted with 15 macrocycles displaying higher scores than the selective mTOR allosteric site inhibitor, DL001. The docked complexes were refined by subsequent molecular dynamics simulations for a period of 100 ns. Successive binding free energy computation revealed a total of 7 macrocyclic compounds (HITS) demonstrating better binding affinity than DL001, towards mTOR. The consequent assessment of pharmacokinetic properties resulted in HITS with similar or better properties than the selective inhibitor, DL001. The HITS from this investigation could act as effective mTOR allosteric site inhibitors and serve as macrocyclic scaffolds for developing compounds targeting the dysregulated mTOR.

Identification and Strategies to Mitigate High Total Clearance of Benzylamine-Substituted Biphenyl Ring Systems

For most oral small-molecule projects within drug discovery, the extent and duration of the effect are influenced by the total clearance of the compound; hence, designing compounds with low clearance remains a key focus to help enable sufficient protein target engagement. Comprehensive understanding and accurate prediction of animal clearance and pharmacokinetics provides confidence that the same can be observed for human. During a MERTK inhibitor lead optimization project, a series containing a biphenyl ring system with benzylamine meta-substitution on one phenyl and nitrogen inclusion as the meta atom on the other ring demonstrated multiple routes of compound elimination in rats. Here, we describe the identification of a structural pharmacophore involving two key interactions observed for both the MERTK program and an additional internal project. Four strategies to mitigate these clearance liabilities were identified and systematically investigated. We provide evidence that disruption of at least one of the interactions led to a significant reduction in CL that was subsequently predicted from rat hepatocytes using in vitro/in vivo extrapolation and the well-stirred scaling method. These tactics will likely be of general utility to the medicinal chemistry and DMPK community during compound optimization when similar issues are encountered for biphenyl benzylamines.

UNC5293, a potent, orally available and highly MERTK-selective inhibitor

Inhibition of MER receptor tyrosine kinase (MERTK) causes direct tumor cell killing and stimulation of the innate immune response. Therefore, MERTK has been identified as a therapeutic target in a wide variety of human tumors. Clinical trials targeting MERTK have recently been initiated, however, none of these drugs are MERTK-specific. Herein, we present the discovery of a highly MERTK-selective inhibitor UNC5293 (24). UNC5293 has subnanomolar activity against MERTK with an excellent Ambit selectivity score (S₅₀ (100 nM) = 0.041). It mediated potent and selective inhibition of MERTK in cell-based assays. Furthermore, it has excellent mouse PK properties (7.8 h half-life and 58% oral bioavailability) and was active in bone marrow leukemia cells in a murine model.

MERTK in cancer therapy: Targeting the receptor tyrosine kinase in tumor cells and the immune system

The receptor tyrosine kinase MERTK is aberrantly expressed in numerous human malignancies, and is a novel target in cancer therapeutics. Physiologic roles of MERTK include regulation of tissue homeostasis and repair, innate immune control, and platelet aggregation. However, aberrant expression in a wide range of liquid and solid malignancies promotes neoplasia via growth factor independence, cell cycle progression, proliferation and tumor growth, resistance to apoptosis, and promotion of tumor metastases. Additionally, MERTK signaling contributes to an immunosuppressive tumor microenvironment via induction of an anti-inflammatory cytokine profile and regulation of the PD-1 axis, as well as regulation of macrophage, myeloid-derived suppressor cell, natural killer cell and T cell functions. Various MERTK-directed therapies are in preclinical development, and clinical trials are underway. In this review we discuss MERTK inhibition as an emerging strategy for cancer therapy, focusing on MERTK expression and function in neoplasia and its role in mediating resistance to cytotoxic and targeted therapies as well as in suppressing anti-tumor immunity. Additionally, we review preclinical and clinical pharmacological strategies to target MERTK.

MERTK in cancer therapy: Targeting the receptor tyrosine kinase in tumor cells and the immune system

The receptor tyrosine kinase MERTK is aberrantly expressed in numerous human malignancies, and is a novel target in cancer therapeutics. Physiologic roles of MERTK include regulation of tissue homeostasis and repair, innate immune control, and platelet aggregation. However, aberrant expression in a wide range of liquid and solid malignancies promotes neoplasia via growth factor independence, cell cycle progression, proliferation and tumor growth, resistance to apoptosis, and promotion of tumor metastases. Additionally, MERTK signaling contributes to an immunosuppressive tumor microenvironment via induction of an anti-inflammatory cytokine profile and regulation of the PD-1 axis, as well as regulation of macrophage, myeloid-derived suppressor cell, natural killer cell and T cell functions. Various MERTK-directed therapies are in preclinical development, and clinical trials are underway. In this review we discuss MERTK inhibition as an emerging strategy for cancer therapy, focusing on MERTK expression and function in neoplasia and its role in mediating resistance to cytotoxic and targeted therapies as well as in suppressing anti-tumor immunity. Additionally, we review preclinical and clinical pharmacological strategies to target MERTK.

Synthesis and Characterization of a Click-Assembled 18-Atom Macrocycle That Displays Selective AXL Kinase Inhibitory Activity

A novel macrocyclic construct consisting of a pyrazolopyrimidine scaffold concatenated to a benzene ring through two triazoles has been developed to investigate uncharted chemical space with bioactive potential. The 18-atom macrocycle was assembled via a double copper-catalyzed alkyne-azide cycloaddition (CuAAC) reaction between 1,3-bis(azidomethyl)benzene and a bis-propargylated pyrazolo[3,4-d]pyrimidine core. The resulting macrocycle was functionalized further into a multicyclic analog that displays selective inhibitory activity against the receptor tyrosine kinase AXL.

Research Progress of Axl Inhibitors

Axl, a Receptor Tyrosine Kinase (RTK) belonging to the TAM (Axl, Mer, Tyro3) family, participates in many signal transduction cascades after mostly being stimulated by Growth arrestspecific 6(Gas6). Axl is widely expressed in many organs, such as macrophages, endothelial cells, heart, liver and skeletal muscle. Over-expression and activation of Axl are associated with promoting chemotherapy resistance, cell proliferation, invasion and metastasis in many human cancers, such as breast, lung, and pancreatic cancers. Therefore, the research and development of Axl inhibitors is of great significance to strengthen the means of cancer treatment, especially to solve the problem of drug resistance. Axl inhibitors have attracted more and more researchers' attention in recent years. This review discusses the research progress of Axl inhibitors in recent years.

Phosphatidylserine receptor-targeting therapies for the treatment of cancer

Asymmetric distribution of phospholipids across the plasma membrane is a unique characteristic of eukaryotic cells. Phosphatidylcholine and sphingomyelin are exposed in the outer leaflet, and phosphatidylserine (PS) is predominantly located in the inner leaflet. Redistribution of PS to the cell surface can be observed in several physiological conditions, such as apoptosis and platelet activation, or in pathological conditions, such as the release of microvesicles/exosomes from tumor tissues. PS binding to the phosphatidylserine receptor (PSR) on immune cells initiates immunosuppressive pathways that can lead to immune evasion by cancer cells. Conversely, PSR activation of cancer cells plays an important role in their survival, proliferation and metastasis. Herein, we briefly summarize both recent advances in our understanding of the pathological roles of PS and its receptor in cancer biology, as well as relevant pharmacological approaches.

Macrocyclic Compounds for Drug and Gene Delivery in Immune-Modulating Therapy

For decades, macrocyclic compounds have been widely applied in various fields owing to essential physicochemical properties such as their rigid cyclic structures, geometric dimensions (diameter and height), hydrophobic cavity, and hydrophilic interface. This review is an attempt to summarize various research accomplishments involving macrocyclic compounds for drug and gene delivery in immune-modulating therapies: the structures and benefits of main host molecules, their mechanisms regulating the immune system from cell uptake to activation of dendritic cells and T helper lymphocytes, as well as their potential immunotherapy for different diseases. Macrocyclic compounds including cucurbiturils (CBs), calixarenes, pillararenes, cyclodextrins (CyDs), macrocyclic peptides and metallo-supramolecular compounds, have their own unique physicochemical properties and functional derivatizations that enable to improve the biocompatibility, responsiveness to stimuli, and effectiveness of immune-modulating therapy. Based on abundant clarifications of the biological immunity mechanisms, representative constructions of macrocyclic compounds for immune therapies have been conducted for the investigation of treatment of different diseases including cancer, atherosclerosis, Niemann-Pick type C1 disease (NPC1), diabetes, and inflammations. Although there are critical challenges that remain to be conquered, we believe the future of macrocyclic compounds in the immune-modulating therapy must be bright.

Design and Evaluation of Potent EGFR Inhibitors through the Incorporation of Macrocyclic Polyamine Moieties into the 4-Anilinoquinazoline Scaffold

Adenosine triphosphate (ATP)-competitive inhibitors of the epidermal growth factor receptor (EGFR) have provided a significant improvement in the disease outcome of nonsmall cell lung cancer (NSCLC). Unfortunately, some marketed drugs affect a transient beneficial response in EGFR mutant NSCLC patients. We reported a series of potential EGFR inhibitors through incorporation of macrocyclic polyamine into 4-anilinoquinazoline scaffold. It is expected that anilinoquinazoline part effectively bind to EGFR domain, while ATP molecules are captured by a macrocyclic polyamine moiety. In vitro experiments exhibited that most of tested compounds suppressed tumor cell proliferation more strongly than Gefitinib and Lapatinib (dual inhibitor of EGFR/HER2) as controls. In kinase assays, the compound 1f showed excellent dual inhibition activity toward EGFR^WT (IC₅₀ = 1.4 nM) and HER2 (IC₅₀ = 2.1 nM). In vivo pharmacology evaluation of 1f showed significant antitumor activity (TGI = 44.2%) in A549 xenografts mice. The current work provided a feasible solution to optimize anilinoquinazoline-based inhibitors.

The Emerging Role of TYRO3 as a Therapeutic Target in Cancer

The TAM family (TYRO3, AXL, MERTK) tyrosine kinases play roles in diverse biological processes including immune regulation, clearance of apoptotic cells, platelet aggregation, and cell proliferation, survival, and migration. While AXL and MERTK have been extensively studied, less is known about TYRO3. Recent studies revealed roles for TYRO3 in cancer and suggest TYRO3 as a therapeutic target in this context. TYRO3 is overexpressed in many types of cancer and functions to promote tumor cell survival and/or proliferation, metastasis, and resistance to chemotherapy. In addition, higher levels of TYRO3 expression have been associated with decreased overall survival in patients with colorectal, hepatocellular, and breast cancers. Here we review the physiological roles for TYRO3 and its expression and functions in cancer cells and the tumor microenvironment, with emphasis on the signaling pathways that are regulated downstream of TYRO3 and emerging roles for TYRO3 in the immune system. Translational agents that target TYRO3 are also described.

Highly Selective MERTK Inhibitors Achieved by a Single Methyl Group

Although all kinases share the same ATP binding pocket, subtle differences in the residues that form the pocket differentiate individual kinases' affinity for ATP competitive inhibitors. We have found that by introducing a single methyl group, the selectivity of our MERTK inhibitors over another target, FLT3, was increased up to 1000-fold (compound 31). Compound 19 was identified as an in vivo tool compound with subnanomolar activity against MERTK and 38-fold selectivity over FLT3 in vitro. The potency and selectivity of 19 for MERTK over FLT3 were confirmed in cell-based assays using human cancer cell lines. Compound 19 had favorable pharmacokinetic properties in mice. Phosphorylation of MERTK was decreased by 75% in bone marrow leukemia cells from mice treated with 19 compared to vehicle-treated mice.

Discovery of Macrocyclic Pyrimidines as MerTK-Specific Inhibitors

Macrocycles have attracted significant attention in drug discovery recently. In fact, a few de novo designed macrocyclic kinase inhibitors are currently in clinical trials with good potency and selectivity for their intended target. In this study, we successfully engaged a structure-based drug design approach to discover macrocyclic pyrimidines as potent Mer tyrosine kinase (MerTK)-specific inhibitors. An enzyme-linked immunosorbent assay (ELISA) in 384-well format was employed to evaluate the inhibitory activity of macrocycles in a cell-based assay assessing tyrosine phosphorylation of MerTK. Through structure-activity relationship (SAR) studies, analogue 11 [UNC2541; (S)-7-amino-N-(4-fluorobenzyl)-8-oxo-2,9,16-triaza-1(2,4)-pyrimidinacyclohexadecaphane-1-carboxamide] was identified as a potent and MerTK-specific inhibitor that exhibits sub-micromolar inhibitory activity in the cell-based ELISA. In addition, an X-ray structure of MerTK protein in complex with 11 was resolved to show that these macrocycles bind in the MerTK ATP pocket.