5-Keto-3-cyano-2,4-diaminothiophenes as selective maternal embryonic leucine zipper kinase inhibitors

Maternal embryonic leucine zipper kinase in tumor cell and tumor microenvironment: Emerging player and promising therapeutic opportunities

Maternal embryonic leucine zipper kinase (MELK) is a member of the AMPK (AMP-activated protein kinase) protein family, which is widely and highly expressed in multiple cancer types. Through direct and indirect interactions with other proteins, it mediates various cascades of signal transduction processes and plays an important role in regulating tumor cell survival, growth, invasion and migration and other biological functions. Interestingly, MELK also plays an important role in the regulation of the tumor microenvironment, which can not only predict the responsiveness of immunotherapy, but also affect the function of immune cells to regulate tumor progression. In addition, more and more small molecule inhibitors have been developed for the target of MELK, which exert important anti-tumor effects and have achieved excellent results in a number of clinical trials. In this review, we outline the structural features, molecular biological functions, potential regulatory mechanisms and important roles of MELK in tumors and tumor microenvironment, as well as substances targeting MELK. Although many molecular mechanisms of MELK in the process of tumor regulation are still unknown, it is worth affirming that MELK is a potential tumor molecular therapeutic target, and its unique superiority and important role provide clues and confidence for subsequent basic research and scientific transformation.

Synthesis, Biological, and Molecular Docking Studies on 4,5,6,7-Tetrahydrobenzo[b]thiophene Derivatives and Their Nanoparticles Targeting Colorectal Cancer

Initiation of colorectal carcinogenesis may be induced by chromosomal instability caused by oxidative stress or indirectly by bacterial infections. Moreover, proliferating tumor cells are characterized by reprogrammed glucose metabolism, which is associated with upregulation of PDK1 and LDHA enzymes. In the present study, some 4,5,6,7-tetrahydrobenzo[b]thiophene derivatives in addition to Fe₃O₄ and Fe₃O₄/SiO₂ nanoparticles (NPs) supported with a new Schiff base were synthesized for biological evaluation as PDK1 and LDHA inhibitors as well as antibacterial, antioxidant, and cytotoxic agents on LoVo and HCT-116 cells of colorectal cancer (CRC). The results showed that compound 1b is the most active as PDK1 and LDHA inhibitor with IC₅₀ values (μg/mL) of 57.10 and 64.10 compared to 25.75 and 15.60, which were produced by the standard inhibitors sodium dichloroacetate and sodium oxamate, respectively. NPs12a,b and compound 1b exhibited the strongest antioxidant properties with IC₅₀ values (μg/mL) of 80.0, 95.0, and 110.0 μg/mL, respectively, compared to 54.0 μg/mL, which was produced by butylated hydroxy toluene. Moreover, NPs12a and carbamate derivative 3b exhibited significant cytotoxic activities with IC₅₀ values (μg/mL) of 57.15 and 81.50 (LoVo cells) and 60.35 and 71.00 (HCT-116 cells). Thus, NPs12a and compound 3b would be considered as promising candidates suitable for further optimization to develop new chemopreventive and chemotherapeutic agents against these types of CRC cell lines. Besides, molecular docking in the colchicine binding site of the tubulin (TUB) domain revealed a good binding affinity of 3b to the protein; in addition, the absorption, distribution, metabolism, and excretion (ADME) analyses showed its desirable drug-likeness and oral bioavailability characteristics.

Mass spectrometry-based selectivity profiling identifies a highly selective inhibitor of the kinase MELK that delays mitotic entry in cancer cells

The maternal embryonic leucine zipper kinase (MELK) has been implicated in the regulation of cancer cell proliferation. RNAi-mediated MELK depletion impairs growth and causes G₂/M arrest in numerous cancers, but the mechanisms underlying these effects are poorly understood. Furthermore, the MELK inhibitor OTSSP167 has recently been shown to have poor selectivity for MELK, complicating the use of this inhibitor as a tool compound to investigate MELK function. Here, using a cell-based proteomics technique called multiplexed kinase inhibitor beads/mass spectrometry (MIB/MS), we profiled the selectivity of two additional MELK inhibitors, NVS-MELK8a (8a) and HTH-01-091. Our results revealed that 8a is a highly selective MELK inhibitor, which we further used for functional studies. Resazurin and crystal violet assays indicated that 8a decreases triple-negative breast cancer cell viability, and immunoblotting revealed that impaired growth is due to perturbation of cell cycle progression rather than induction of apoptosis. Using double-thymidine synchronization and immunoblotting, we observed that MELK inhibition delays mitotic entry, which was associated with delayed activation of Aurora A, Aurora B, and cyclin-dependent kinase 1 (CDK1). Following this delay, cells entered and completed mitosis. Using live-cell microscopy of cells harboring fluorescent proliferating cell nuclear antigen, we confirmed that 8a significantly and dose-dependently lengthens G₂ phase. Collectively, our results provide a rationale for using 8a as a tool compound for functional studies of MELK and indicate that MELK inhibition delays mitotic entry, likely via transient G₂/M checkpoint activation.

Design, synthesis, biological evaluation and molecular modeling of novel 1H-pyrrolo[2,3-b]pyridine derivatives as potential anti-tumor agents

A class of 3-substituted 1H-pyrrolo[2,3-b]pyridine derivatives were designed, synthesized and evaluated for their in vitro biological activities against maternal embryonic leucine zipper kinase (MELK). Among these derivatives, the optimized compound 16h exhibited potent enzyme inhibition (IC₅₀ = 32 nM) and excellent anti-proliferative effect with IC₅₀ values from 0.109 μM to 0.245 μM on A549, MDA-MB-231 and MCF-7 cell lines. The results of flow cytometry indicated that 16h promoted apoptosis of A549 cells in a dose-dependent manner and effectively arrested A549 cells in the G0/G1 phase. Further investigation indicated that compound 16h potently suppressed the migration of A549 cells, had moderate stability in rat liver microsomes and showed moderate inhibitory activity against various subtypes of human cytochrome P450. However, compound 16h is a multi-target kinase inhibitor and recently several studies reported MELK expression is not required for cancer growth, suggesting that compound 16h suppressed the proliferation and migration of cancer cells should through an off-target mechanism. Collectively, compound 16h has the potential to serve as a new lead compound for further anticancer drug discovery.