Targeting human MutT homolog 1 (MTH1) for cancer eradication: current progress and perspectives

Since accelerated metabolism produces much higher levels of reactive oxygen species (ROS) in cancer cells compared to ROS levels found in normal cells, human MutT homolog 1 (MTH1), which sanitizes oxidized nucleotide pools, was recently demonstrated to be crucial for the survival of cancer cells, but not required for the proliferation of normal cells. Therefore, dozens of MTH1 inhibitors have been developed with the aim of suppressing cancer growth by accumulating oxidative damage in cancer cells. While several inhibitors were indeed confirmed to be effective, some inhibitors failed to kill cancer cells, complicating MTH1 as a viable target for cancer eradication. In this review, we summarize the current status of developing MTH1 inhibitors as drug candidates, classify the MTH1 inhibitors based on their structures, and offer our perspectives toward the therapeutic potential against cancer through the targeting of MTH1.     

Identification of Nucleoside Analogs as Inducers of Neuronal Differentiation in a Human Reporter Cell Line and Adult Stem Cells

Nucleoside analogs (NSAs) were among the first chemotherapeutic agents and could also be useful for the manipulation of cell fate. To investigate the potential of NSAs for the induction of neuronal differentiation, we developed a novel phenotypic assay based on a human neuron‐committed teratocarcinoma cell line (NT2) as a model for neuronal progenitors and constructed a NT2‐based reporter cell line that expressed eGFP under the control of a neuron‐specific promoter. We tested 38 structurally related NSAs and determined their activity to induce neuronal differentiation by immunocytochemistry of neuronal marker proteins, live cell imaging, fluorometric detection and immunoblot analysis. We identified twelve NSAs, which induced neuronal differentiation to different extents. NSAs with highest activity carried a halogen substituent at their pyrimidine nucleobase and an unmodified or 2′‐O‐methyl substituted 2‐deoxy‐β‐D‐ribofuranosyl residue as glyconic moiety. Cladribine, a purine nucleoside with similar structural features and in use to treat leukemia and multiple sclerosis, induced also differentiation of adult human neural crest‐derived stem cells. Our results suggest that NSAs could be useful for the manipulation of neuronal cell fate in cell replacement therapy or treatment of neurodegenerative disorders. The data on the structure and function relationship will help to design compounds with increased activity and low toxicity.     

Design,Synthesis, and Biological Evaluation of Imidazopyridines as PD-1/PD-L1 Antagonists

The PD-1/PD-L1 axis has proven to be a highly efficacious target for cancer immune checkpoint therapy with several approved antibodies. Also, small molecules based on a biphenyl core can antagonize PD-1/PD-L1, leading to the in vitro formation of PD-L1 dimers. However, their development remains challenging, as we do not yet fully understand their mode of action. In this work, we designed a new scaffold based on our previously solved high-resolution structures of low-molecular-weight inhibitors bound to PD-L1. A small compound library was synthesized using the Groebke–Blackburn–Bienaymé multicomponent reaction (GBB-3CR), resulting in the structure–activity relationship of imidazo[1,2-a]pyridine-based inhibitors. These inhibitors were tested for their biological activity using various biophysical assays giving potent candidates with low-micromolar PD-L1 affinities. An obtained PD-L1 cocrystal structure reveals the binding to PD-L1. Our results open the door to an interesting bioactive scaffold that could lead to a new class of PD-L1 antagonists.     

Targeting NAMPT for Therapeutic Intervention in Cancer and Inflammation: Structure‐Based Drug Design and Biological Screening

Nicotinamide phosphoribosyltransferase (NAMPT) is a rate limiting enzyme that plays an important role in the synthesis of nicotinamide adenine dinucleotide (NAD) via a salvage pathway. Along with a role in bioenergetics, NAMPT regulates the activity of proteins such as SIRT‐1 that utilize NAD as a cofactor. As NAD metabolism is usually high in diseased conditions, it has been hypothesized and proven that NAMPT is over expressed in various cancers and inflammatory disorders. Inhibitors targeting NAMPT could therefore be useful in treating disorders arising from aberrant NAMPT signalling. In this study, inhibitors against NAMPT were designed using an energy‐based pharmacophore strategy and evaluated for efficacy in cellular assays. Besides reducing cellular pools of NAD and NMN, NAMPT inhibitors decreased concentrations of reactive oxygen species as well as mRNA levels of TNFα and IL6, thereby implicating their potential in alleviating the inflammatory process. In addition, reduced NAD levels corroborated with an induction of apoptosis in prostate cancer cell lines.     

Design and Synthesis of Aza‐Flavones as a New Class of Xanthine Oxidase Inhibitors

In an attempt to develop non‐purine‐based xanthine oxidase (XO) inhibitors, keeping in view the complications reported with the use of purine‐based XO inhibitors, the flavone framework (a class possessing XO inhibitory potential) was used as lead structure for further optimization. By means of structure‐based classical bioisosterism, quinolone was used as an isoster for chromone (a bicyclic unit present in flavones), owing to the bioactive potential and drug‐like properties of quinolones. This type of replacement does not alter the shape and structural features required for XO inhibition, and also provides some additional interaction sites, without the loss of hydrogen bonding and hydrophobic and arene–arene interactions. In the present study, a series of 2‐aryl/heteroaryl‐4‐quinolones (aza analogs of flavones) was rationally designed, synthesized and evaluated for in vitro XO inhibitory activity. Some notions about structure–activity relationships are presented indicating the influence of the nature of the 2‐aryl ring on the inhibitory activity. Important interactions of the most active compound 3l (IC₅₀ = 6.24 µM) with the amino acid residues of the active site of XO were figured out by molecular modeling.     

Synthesis and Biological Evaluation of 4‐Arylphthalazones Bearing Benzenesulfonamide as Anti‐Inflammatory and Anti‐Cancer Agents

Nine 4‐arylphthalazones bearing benzenesulfonamide (2a – i ) were synthesized by the condensation of the appropriate 2‐aroylbenzoic acid ( 1a – i ) and 4‐hydrazinobenzenesulfonamide in ethanol. The structures of these compounds were elucidated by elemental analysis, IR, ¹H NMR, ¹³C NMR, and MS spectroscopy. Two compounds, 2b and 2i , showed significant anti‐inflammatory activity comparable to that of the standard drug celecoxib in the carrageenan‐induced rat paw edema model. These compounds ( 2b and 2i ) had selective inhibitory activity towards the COX‐2 enzyme. Compound 2b had a better selectivity ratio (COX‐1/COX‐2) compared to that of celecoxib and can be used as a novel template for the design of selective COX‐2 inhibitors. Compounds 2d and 2i were screened for their antiproliferative activity toward 60 human cancer cell lines by the National Cancer Institute (USA). The compounds 2d and 2i displayed mild activity toward the renal cancer cell line UO‐31.     

Discovery of selective protein arginine methyltransferase 5 inhibitors and biological evaluations

Protein arginine methyltransferase 5 (PRMT5) is an important protein arginine methyltransferase that catalyzes the symmetric dimethylation of arginine resides on histones or non‐histone substrate proteins. It has been thought as a promising target for many diseases, particularly cancer. Despite the potential applications of PRMT5 inhibitors in cancer treatment, very few of PRMT5i have been publicly reported. In this investigation, virtual screening and structure–activity relationship studies were carried out to discover novel PRMT5i, which finally led to the identification of a number of new PRMT5i. The most active compound, P5i‐6 , exhibited a considerable inhibitory potency against PRMT5 with an IC₅₀ value of 0.57 μm , and a high selectivity for PRMT5 against other tested PRMTs. It displayed a very good antiviability activity against two colorectal cancer cell lines, HT‐29 and DLD‐1, and one hepatic cancer cell line, HepG2, in a sensitivity assay against 36 different cancer cell lines. Western blot assays indicated that P5i‐6 selectively inhibited the symmetric dimethylations of H4R3 and H3R8 in DLD‐1 cells. Overall, P5i‐6 could be used as a chemical probe to investigate new functions of PRMT5 in biology and also served as a good lead compound for the development of new PRMT5‐targeting therapeutic agents.     

Characterization of a Pyrazolo[4,3‐d]pyrimidine Inhibitor of Cyclin‐Dependent Kinases 2 and 5 and Aurora A With Pro‐Apoptotic and Anti‐Angiogenic Activity In Vitro

Selective inhibitors of kinases that regulate the cell cycle, such as cyclin‐dependent kinases (CDKs) and aurora kinases, could potentially become powerful tools for the treatment of cancer. We prepared and studied a series of 3,5,7‐trisubstituted pyrazolo[4,3‐d]pyrimidines, a new CDK inhibitor scaffold, to assess their CDK2 inhibitory and antiproliferative activities. A new compound, 2i , which preferentially inhibits CDK2, CDK5, and aurora A was identified. Both biochemical and cellular assays indicated that treatment with compound 2i caused the downregulation of cyclins A and B, the dephosphorylation of histone H3 at Ser10, and the induction of mitochondrial apoptosis in the HCT‐116 colon cancer cell line. It also reduced migration as well as tube and lamellipodia formation in human endothelial cells. The kinase inhibitory profile of compound 2i suggests that its anti‐angiogenic activity is linked to CDK5 inhibition. This dual mode of action involving apoptosis induction in cancer cells and the blocking of angiogenesis‐like activity in endothelial cells offers possible therapeutic potential.     

Synthesis and cytotoxicity of novel pyrido[1,2-e]purines on multidrug resistant human MCF7 cells

The anticancer activity of 4-methylaminopyridol[1,2-e]purine 6a, 4-(piperidin-1-yl)pyrido[1,2-e]purine 7a and their 7-methyl derivatives 6b, 7b was investigated against the human MCF7 cancer cell line in vitro. The sensitive cell line showed a range of sensitivities to 6a, 6b, 7a, 7b (IC50: 1.6 to 7.2 x 10(-4) M) and sensitivity to doxorubicin (IC50: 7.5 x 10(-7) M). A resistant cell line with the multidrug resistant phenotype was sensitive to these derivatives (IC50: 1.8 to 6.7 x 10(-4) M), doxorubicin (IC50: 5 x 10(-5) M) and drug activity seems to be not affected by MDR resistance. Our data show that 6a, 6b, 7a and 7b appear to exert a low cytotoxicity on sensitive and MDR resistant MCF7 human cancer cell lines.     

Synthesis and evaluation of 3-aryloxymethyl-1,2-dimethylindole-4,7-diones as mechanism-based inhibitors of NAD(P)H:quinone oxidoreductase 1 (NQO1) activity

NAD(P)H:quinone oxidoreductase 1 is a proposed target in pancreatic cancer. We describe the synthesis of a series of indolequinones, based on the 5- and 6-methoxy-1,2-dimethylindole-4,7-dione chromophores with a range of phenolic leaving groups at the (indol-3-yl)methyl position. The ability of these indolequinones to function as mechanism-based inhibitors of purified recombinant human NQO1 was evaluated, as was their ability to inhibit both NQO1 and cell growth in human pancreatic MIA PaCa-2 tumor cells. The inhibition of rhNQO1 was related to the pKa of the leaving group: compounds with poorer phenolic leaving groups were poor inhibitors whereas those with more acidic leaving groups were more efficient inhibitors. These inhibition data also correlated with the inhibition NQO1 in MIA PaCa-2 cells. However, the data demonstrate that NQO1 inhibition does not correlate with growth inhibitory activity, at least in the MIA PaCa-2 cell line, suggesting that targets in addition to NQO1 need to be considered to explain the potent growth inhibitory activity of this series of indolequinones in human pancreatic cancer cells.     

Design,synthesis, and molecular docking study of 3H‐imidazole[4,5‐c]pyridine derivatives as CDK2 inhibitors

A novel series of imidazo[4,5‐c]pyridine‐based CDK2 inhibitors were designed from the structure of CYC202 via scaffold hopping strategy. These compounds were synthesized and biologically evaluated for their CDK2 inhibitory and in vitro anti‐proliferation potential against cancer cell lines. Several compounds exhibited potent CDK2 inhibition with IC₅₀ values of less than 1 µM. The most potent compound 5b showed excellent CDK2 inhibitory (IC₅₀ = 21 nM) and in vitro anti‐proliferation activity against three different cell lines (HL60, A549, and HCT116). The molecular docking and dynamic studies portrayed the potential binding mechanism between 5b and CDK2, and several key interactions between them were observed, which would be the reason for its potent CDK2 inhibitory and anti‐proliferation activities. Therefore, the pyridin‐3‐ylmethyl moiety would serve as an excellent pharmacophore for the development of novel CDK2 inhibitors for targeted anti‐cancer therapy.

     

Novel 6-substituted uracil analogs as inhibitors of the angiogenic actions of thymidine phosphorylase.

Thymidine phosphorylase (TP) catalyzes the reversible phosphorolysis of thymidine and other pyrimidine 2'-deoxyribonucleosides. In addition, TP has been shown to possess angiogenic activity in a number of in vitro and in vivo assays, and its angiogenic activity has been linked to its catalytic activity. A series of 5- and 6-substituted uracil derivatives were synthesized and evaluated for their abilities to inhibit TP activity. Among the most active compounds was a 6-amino-substituted uracil analog, 6-(2-aminoethyl)amino-5-chlorouracil (AEAC), which was a competitive inhibitor with a K(i) of 165 nM. The inhibitory activity of AEAC was selective for TP, as it did not inhibit purine nucleoside phosphorylase or uridine phosphorylase at concentrations up to 1 mM. Human recombinant TP induced human umbilical vein endothelial cell (HUVEC) migration in a modified Boyden chamber assay in vitro, and this action could be abrogated by the TP inhibitors. The actions of the inhibitors were specific for TP, as they had no effect on the chemotactic actions of vascular endothelial growth factor (VEGF). HUVEC migration was also induced when TP-transfected human colon and breast carcinoma cells were co-cultured in the Boyden chamber assay in place of the purified angiogenic factors, and a TP inhibitor blocked the tumor cell-mediated migration almost completely. These studies suggest that inhibitors of TP may be useful in pathological conditions that are dependent upon TP-driven angiogenesis.     

Symbiotic prodrugs (SymProDs) dual targeting of NFkappaB and CDK

The release of an active drug from the prodrug generates a pro‐fragment that typically has no biological activity and could result in adverse effects. By combining two drugs, wherein each drug acts as a pro‐fragment of the other drug will eliminate the pro‐fragment in the prodrug. As they are prodrugs of each other and are symbiotic, we termed these as symbiotic prodrugs (SymProDs). To test this idea, we generated SymProDs using NFκB inhibitors that contain the reactive α‐methylene‐γ‐butyrolactone moiety and CDK inhibitors with solvent exposed secondary nitrogen atoms. We show that secondary amine prodrugs of α‐methylene‐γ‐butyrolactone containing NFκB inhibitors undergo slow release over a 72 hr period. Using an alkyne‐tagged secondary amine prodrug of α‐methylene‐γ‐butyrolactone containing NFκB inhibitor, we demonstrate target engagement. The NFκB‐CDK SymProDs were ~20‐ to 200‐fold less active against the corresponding CDK inhibitors in in vitro CDK kinase assays. Growth inhibition studies in a panel of ovarian cancer cell lines revealed potency trends of the SymProDs mirrored those of the single treatments suggesting their dissociation in cells. In conclusion, our results suggest that SymProDs offer a productive path forward for advancing compounds with reactive functionality and can be used as dual targeting agents.     

Olomoucine II and purvalanol A inhibit ABCG2 transporter in vitro and in situ and synergistically potentiate cytostatic effect of mitoxantrone

Inhibition of cyclin-dependent kinases by specific small molecules, purine cyclin-dependent kinase inhibitors (CDKi), has become a promising strategy for cancer treatment. Although pharmacodynamic properties of these compounds have been studied extensively, their pharmacokinetic behavior has not been addressed in detail. In this study, we investigated possible inhibitory effect of five purine CDKi on breast cancer resistance protein (ABCG2) transport activity employing in vitro transport and accumulation methods in MCDKII cells transduced with human ABCG2. Hoechst 33342 and glyburide were used as model ABCG2 substrates for these experiments. In addition, in situ method of dually perfused rat term placenta was utilized to confirm our in vitro results at the organ level. Fumitremorgin C was used as a model inhibitor of ABCG2 for comparison purposes. We demonstrate significant inhibition of ABCG2 by four of the five CDKi tested. Regarding their ABCG2-inhibitory potencies, the investigated compounds can be ranked as follows: purvalanol A>olomoucine II≈fumitremorgin C>roscovitine≈bohemine, with slight differences among substrates, concentrations and methods used. Based on our findings, it is reasonable to expect a substantial impact of the studied CDKi on the pharmacokinetic and pharmacodynamic behavior of concomitantly administered ABCG2 substrates. Moreover, using combination index method of Chou-Talalay, we confirmed that the strongest inhibitors, purvalanol A and olomoucine II, can synergistically potentiate cytostatic effect of mitoxantrone, an ABCG2 substrate, in ABCG2 expressing cell lines.     

Bortezomib targets the caspase-like proteasome activity in cervical cancer cells, triggering apoptosis that can be enhanced by nelfinavir

The occurrence of chemoresistance is a serious problem in the treatment of cancer, urging the need for second and third-line treatment options that rely on different cell death pathways to overcome previously acquired resistance mechanisms. The inhibition of proteasomal activity by specific proteasome inhibitors or cross-reactivity of certain protease inhibitors with proteasomal enzymes recently became of interest because of the anti-tumoral properties of these agents. We tested the proteasome inhibitor bortezomib and the HIV protease inhibitor nelfinavir on human cervical cancer cells. Both drugs induced cell cycle arrest in cervical cancer cells, as reflected by marked changes in the expression of cell cycle-regulatory cyclins and ensuing mitochondrial-independent apoptosis. Upregulation of the molecular chaperone BiP and the cell stress marker ATF3 indicated induction of the unfolded protein response (UPR) as the main cause of apoptosis induced by these drugs in cervical cancer cells. Unlike in leukemia cells, bortezomib mainly inhibited the caspase-like activity of the proteasome in cervical cancer cells. Nelfinavir exhibited no effects on proteasomal activity in cervical cancer cells and leukemia cells. Although both bortezomib and nelfinavir acted on cisplatin-resistant cervical cancer cells (SiHa), neither of the drugs induced a sensitization to cisplatin treatment. Instead, both drugs could effectively be combined with each other, and enhanced the efficacy of an apoptosis-inducing TRAIL receptor antibody. These results suggest that both bortezomib and nelfinavir are effective agents against chemoresistant cervical cancer cells and might be of interest for clinical studies on cervical cancer patients with recurrent or metastatic cancer.     

Synthesis,molecular docking and biological evaluation of 1‐phenylsulphonyl‐2‐(1‐methylindol‐3‐yl)‐benzimidazole derivatives as novel potential tubulin assembling inhibitors

A series of new 1‐phenylsulphonyl‐2‐(1‐methylindol‐3‐yl)‐benzimidazole derivatives were designed, synthesized and evaluated as potential inhibitors of tubulin polymerization and anthropic cancer cell lines. Among them, compound 33 displayed the most potent tubulin polymerization inhibitory activity in vitro (IC₅₀ = 1.41 μM) and strong antiproliferative activities against A549, Hela, HepG2 and MCF‐7 cell lines in vitro with GI₅₀ value of 1.6, 2.7, 2.9 and 4.3 μM, respectively, comparable with the positive control colchicine (GI₅₀ value of 4.1, 7.2, 9.5 and 14.5 μM, respectively) and CA‐4 (GI₅₀ value of 2.2, 4.3, 6.4 and 11.4 μM, respectively). Simultaneously, we evaluated that compound 33 could effectively induce apoptosis of A549 associated with G2/M phase cell cycle arrest. Immunofluorescence microscopy also clearly indicated compound 33 a potent antimicrotubule agent. Docking simulation showed that compound 33 could bind tightly with the colchicine‐binding site and act as a tubulin inhibitor. Three‐dimensional‐QSAR model was also built to provide more pharmacophore understanding that could be used to design new agents with more potent tubulin assembling inhibitory activity in the future.     

Design and synthesis of Atglistatin derivatives as adipose triglyceride lipase inhibitors

Adipose triglyceride lipase (ATGL) is a rate‐limiting enzyme that mobilizes fatty acids from cellular triglyceride stores. Metabolic syndrome, which refers to a group of abnormalities that occur together and increase the risk of coronary artery disease, stroke, type 2 diabetes, and cachexia, can be treated using ATGL‐specific inhibitors. Atglistatin ( 1 ) is the first small‐molecule inhibitor of ATGL. In this study, we designed and synthesized 29 Atglistatin derivatives and evaluated their inhibition of forskolin‐stimulated lipolysis in 3T3‐L1 adipocytes as an indicator of their potential to inhibit ATGL in adipose tissues. Among all the tested Atglistatin analogs, we previously found that the thiourea compound 9e showed potent ATGL inhibitory activity in vitro, which was much stronger than that of Atglistatin, and its inhibitory activity in vivo was similar to that of Atglistatin. This tool compound could be used to study the pathophysiology and druggability of ATGL in animal models of metabolic disease and cachexia.     

Pyrimidine containing epidermal growth factor receptor kinase inhibitors: Synthesis and biological evaluation

Structure‐based design and synthesis of pyrimidine containing reversible epidermal growth factor receptor (EGFR) inhibitors 1a – d are reported. The compounds ( 1a – d ) inhibited the EGFR kinase activity in vitro with IC₅₀ range 740 nm to 3 μm . mRNA expression of EGFR downstream target genes, that is twist, c‐fos and aurora were found to be altered upon treatment with compounds 1a – d . The compounds 1a – d exhibited excellent anticancer activity at low micromolar level (3.2–9 μm ) in lung, colon and breast cancer cell lines. Furthermore, compounds induced the alteration in mitochondrial membrane potential and reactive oxygen species level and. Selected compound 1b was found to increase sub‐G1 population indicative of cell death, the mode of cell death was apoptotic as evident from Annexin V verses propidium iodide assay. Molecular modelling further helped to investigate the binding recognition pattern of the compounds in ATP binding EGFR domain similar to erlotinib and dissimilar to WZ4002.     

Repression of MicroRNA‐372 by Arsenic Sulphide Inhibits Prostate Cancer Cell Proliferation and Migration through Regulation of large tumour suppressor kinase 2

As the main component of realgar, arsenic sulphide (As₄S₄) contains antitumour activity by repressing cancer cell proliferation and migration in many tumours. However, the detailed mechanism of these processes is not clear yet. MicroRNAs (miRNAs) can function as tumour suppressor or oncogene based on their target mRNAs in different tumour tissues. Here, we found that As₄S₄ could repress the overexpression of microRNA‐372 (miR‐372) in two prostate cancer cell lines and its overexpression promoted cell proliferation and migration. Large tumour suppressor kinase 2 (LATS2) was confirmed as a direct target of miR‐372 using luciferase assays in these two prostate cancer cell lines. Down‐regulation of LATS2 could promote prostate cancer cell proliferation and migration just as overexpression of miR‐372 did and overexpression of LATS2 could reverse this effect of miR‐372. The antitumour activity of As₄S₄ and the oncogenic function of miR‐372 were further confirmed using a mouse xenograft model. Altogether, our data showed evidence that repressing the overexpression of miR‐372 by As₄S₄ could inhibit prostate cancer cell proliferation and migration by targeting LATS2. Therefore, miR‐372 may be a possible biomarker for the prediction of prostate cancer and As₄S₄ may have potential therapeutic function for prostate cancer.