Polo-like kinases inhibitors.

Polo-like kinases (PLKs) are a family of serine/threonine kinases that play crucial roles in multiple stages of mitosis. PLK1 is the most studied member of the family. It is overexpressed in a wide spectrum of cancer types and is a promising target in oncology. Most of PLK1 inhibitors are ATP-competitive. Despite the structural similarities among various kinases, several inhibitors are selective. Some areas of the PLK1 active site are important for selectivity against other kinases. These include a small pocket formed by Leu 132 in the hinge region, a bulky phenylalanine and a small cysteine at the bottom and in the roof of the ATP pocket, respectively, and an unusual concentration of positively charged residues in the solvent-exposed region. Many ATP-competitive inhibitors are heterocyclic systems able to interact with the unique features of the PLK1 binding site. Other inhibitors target regions outside the ATP pocket, such as the substrate binding domain or a hydrophobic pocket, formed when the kinase is in the inactive conformation. An alternative approach to obtain specificity and to overcome drug resistance often associated with kinase inhibitors is the inhibition of the polo-box domain (PBD) of PLK1. The PBD is unique for the family of PLKs and is essential for PLK functions; so it is a useful target for the development of selective and potent inhibitors for clinical uses. In this review some PLK inhibitors are reported, focusing on chemical structures, structure-activity-relationships (SAR) and biological activities. The great potential of these compounds could open promising perspectives. Moreover, a combination of polo-like kinases inhibitors with other anticancer drugs might offer new opportunities for cancer therapy.

Irreversible protein kinase inhibitors.

Targeting cancer with small molecule irreversible inhibitors of kinases represents an emerging challenge in drug discovery. Irreversible inhibitors bind to kinase active site in a covalent and irreversible form, most frequently by reacting with a nucleophilic cysteine residue, located near the ATP binding pocket. The most common mechanism is the Michael reaction, that refers to the addition of a nucleophile, such as cysteine, to an α,ß unsaturated carbonyl. The nucleophile reacts at the electrophilic ß-position to form an adduct; as a result the inhibitor irreversibly blocks binding of ATP to the kinase, rendering the kinase inactive. Different cysteine-reactive groups have been evaluated, an acrylamide or a substituted acrylamide moiety are the Michael acceptors of choice. There are some advantages for the irreversible kinase inhibition. These compounds are highly selective because they target a specific cysteine and only a limited number of kinases has a cysteine at the corresponding position. Another advantage is that covalent bond formation can overcome competition with the high endogenous concentration of ATP. A further motivation for designing irreversible inhibitors is their longer duration of action respect to conventional inhibitors. In fact, once bound to enzyme, these compounds do not readily dissociate and the inhibition continues even after the inhibitor leaves the circulation. Moreover, these inhibitors have the potential to overcome and prevent the emergence of acquired resistance conferred by mutations. In this review examples of irreversible inhibitors are reported, focusing on chemical structures, SAR and biological activities. The great potential of these compounds could open new and promising perspectives for a broader application of this approach.

Non-ATP competitive protein kinase inhibitors.

Protein kinases represent an attractive target in oncology drug discovery. Most of kinase inhibitors are ATP-competitive and are called type I inhibitors. The ATP-binding pocket is highly conserved among members of the kinase family and it is difficult to find selective agents. Moreover, the ATP-competitive inhibitors must compete with high intracellular ATP levels leading to a discrepancy between IC50s measured by biochemical versus cellular assays. The non-ATP competitive inhibitors, called type II and type III inhibitors, offer the possibility to overcome these problems. These inhibitors act by inducing a conformational shift in the target enzyme such that the kinase is no longer able to function. In the DFG-out form, the phenylalanine side chain moves to a new position. This movement creates a hydrophobic pocket available for occupation by the inhibitor. Some common features are present in these inhibitors. They contain a heterocyclic system that forms one or two hydrogen bonds with the kinase hinge residue. They also contain a hydrophobic moiety that occupies the pocket formed by the shift of phenylalanine from the DFG motif. Moreover, all the inhibitors bear a hydrogen bond donor-acceptor pair, usually urea or amide, that links the hinge-binding portion to the hydrophobic moiety and interacts with the allosteric site. Examples of non ATP-competitive inhibitors are available for various kinases. In this review small molecules capable of inducing the DFG-out conformation are reported, especially focusing on structural feature, SAR and biological properties.

Synthesis and antiproliferative activity of some thiazolylbenzimidazole-4,7-diones.

Some thiazolylbenzimidazole-4,7-diones were synthesized and tested in vitro on two tumor cell lines. Compounds 2d and 2e show a very good activity on K562 cells, whereas compounds 2a and 2b are active on SW620 cells. The importance of the methoxy group on the quinone moiety is confirmed and the function at 4-position of the thiazole ring plays a determining role for the activity.

Synthesis and antiviral assays of some benzimidazole nucleosides and acyclonucleosides.

Some benzimidazole nucleosides and acyclonucleosides were synthesized and tested in vitro as antiviral agents. None of them showed significant activity. Replacement of the benzenesulphonyl group at N-1 with the ribofuranosyl moiety or with the acyclovir side-chain was deleterious.

Synthesis and antimicrobial activity of new diazoimidazole derivatives containing an N-acylpyrrolidine ring.

A series of 4-diazoimidazole-5-carboxamides bearing in position 2 lipophilic substituents was synthesized and their antimicrobial activity was evaluated in vitro against pathogenic Gram-positive, Gram-negative bacteria and fungi. Some compounds presented antifungal activity, particularly two derivatives (1g and 1h) showed good MIC values (10-50 microg/ml) against both moulds and yeasts.

Synthesis and antiproliferative activity of some benzimidazole-4,7-dione derivatives.

A series of benzimidazole-4,7-diones bearing at the 2-position the thiomethyl group or the 2-pyridyl moiety has been synthesized and tested in vitro on three tumor cell lines. Two of them show a very good antiproliferative effect. Compounds 1 and 2d are more active or equiactive, respectively, than MMC against human lymphoblastic leukemia. Both compounds exhibit high activity on human non-Hodgkin lymphoma. Compound 1 is non toxic at all the concentrations used in the antiproliferative assay and 2d is toxic only at high concentration.

Synthesis and antiviral assays of some 2-substituted benzimidazole-N-carbamates.

Some 2-substituted benzimidazole-N-carbamates were synthesized and tested in vitro for antiviral activity. Two derivatives were active at noncytotoxic concentrations. The results confirmed the importance of the substituents at the 2-position of benzimidazole; an isopropylcarboxamide group led to the best activity.

Synthesis and antiviral activity of some N-benzenesulphonylbenzimidazoles.

Some N-sulphonylated benzimidazoles were synthesized as potential antiviral agents. Compound 16b and, to a lesser extent, 19b showed activity against two RNA viruses at micromolar concentrations.

Synthesis, antiviral and antiproliferative activity of some N-benzenesulphonyl-2(2- or 3-pyridylethyl)-benzimidazoles.

Some N-benzenesulphonyl-2(2- or 3-pyridylethyl)-benzimidazoles were synthesized and tested in vitro for antiproliferative and antiviral activity. Only one compound displayed a degree of antiproliferative activity against chronic myeloid leukaemia cells. However, a number of them exerted an antiviral effect at micromolar concentrations. The antiproliferative activity and the maximum potency of antiviral activity correlate with the presence of both the 2-pyridyl moiety bound at the ethylenic bridge in C-2 of benzimidazole and the nitro group in the benzene ring.

Synthesis, metabolism and structure-mutagenicity relationships of novel 4-nitro-(imidazoles and pyrazoles) in Salmonella typhimurium.

A new series of 4-nitro-(imidazoles and pyrazoles) were synthesized as novel antimycotics and tested for their activation to mutagenic forms using Salmonella typhimurium TA98 and TA100, in the presence and in the absence of metabolic activation. TA100NR, TA100/1,8-DNP6, YG1026 and YG1029 strains were employed to identify a specific metabolic reaction which governs the mutagenic potency. Derivatives in the pyrazole group were generally found to be non mutagenic and active imidazoles were weak-direct-acting mutagens. For most of the compounds the mutagenic responses in TA98 were absent or 12- to 22-fold lower compared to TA100. The presence of a methyl or a benzylic group on the imidazole ring and substituents on the N1 and N3 positions were determinant for mutagenicity. Metabolism by bacterial enzyme systems was important to the expression of genotoxicity. Active compounds showed no mutagenicity toward the strain defective in classical nitroreductase and increased mutagenicity, from 2- to 7-fold depending on the test compound, toward the corresponding overproducing bacteria. On the other hand, compounds displayed reduced mutagenicity to the O-acetyltransferase strain without having increased activity in the corresponding overproducing bacteria, YG1029.

Synthesis and antiproliferative activity of some N-sulphonated-2-substituted benzimidazoles and imidazo[4,5-b]pyridines.

Some N-sulphonated-2-substituted benzimidazoles and imidazo[4,5-b]-pyridines were synthesized and tested in vitro for antiviral and antiproliferative activity. None of the compounds had antiviral properties. However, three of them inhibited the proliferation of leukaemia and lymphoma cell lines at micromolar concentrations. The maximum potency of antiproliferative activity is correlated with the presence of an ethylenic spacer between the two heterocycles.

Synthesis and antimicrobial activity of N, N-dialkyl-2-substituted-5-diazo-imidazole-4-carboxamides.

A series of 2-substituted-5-diazoimidazole-4-carboxamides has been synthesized, and their antimicrobial activity has been tested in vitro. Some of the compounds show antifungal activity related to the presence of small groups on the 4-carbox-amido moiety, while the presence of substituents in position 2 was detrimental.

Synthesis, antiviral and antiproliferative activity of a new class of 5-(alkyl or arylthio)-6-vinyl uracils.

Uracil derivatives bearing substituted or unsubstituted vinyl groups at position C6 and alkyl- or arylthio groups at position C5 were synthesized and tested in vitro for antiviral and antiproliferative activity. None of the compounds were active against HIV-1. However, some of them inhibited the proliferation of leukemia, lymphoma and solid tumor-derived cell lines at micromolar concentrations. The maximum potency of antiproliferative activity correlates with the presence of unsubstituted vinyl groups and alkyl- or arylthio substituents.

Synthesis and antimicrobial assays of 3-diazoindole-2-carboxamides.

Some 3-diazoindole-2-carboxamides have been synthesized and their antimicrobial activity have been tested. Antimicrobial activity was practically lacking.

Some new 3-methoxy-5-methyl-1,4-substituted pyrazoles.

A series of 3-methoxypyrazole derivatives was synthesized and tested as antifungal agents. The substituents were chosen on the base of their lipophylicity and for their presence in well-known antifungal drugs. The compounds displayed no significant activity in vitro.

Some new quinoline-based mono and dicarboxylic acids.

Some quinoline-based mono- and dicarboxylic acids structurally related to kynurenic acid have been synthesized and screened as antagonists of neurotransmission of NMDA, AMPA and KA excitatory amino acid receptors. Higher affinity for NMDA receptor was pointed out in the short series, but all the compounds, even those with key structural features of glutamic acid showed no significant activity.

Synthesis of a series of 5-nitro-(benzimidazoles and indoles) as novel antimycotics and evaluation as genotoxins in the Ames test.

Nitrobenzimidazole and nitroindole derivatives, related to oxiconazole and characterized by an oxyiminic function, have been synthesized as novel antimycotics and their mutagenic activity tested in Salmonella typhimurium strains TA100 and TA98 with and without an exogenous metabolizing system. TA98NR and TA98/1,8-DNP6 strains were employed to identify a specific metabolic reaction which governs the mutagenic potency. Active compounds are weak direct-acting mutagens. Only derivatives bearing a nitro group on the phenyl ring linked to the oxyiminic function and lacking halogenated substituents show mutagenic activity. Metabolism by bacterial enzyme systems is important to the expression of genotoxicity. The reductive activation of nitrobenzimidazoles and nitroindoles carried out by the 'classical' nitroreductase of Salmonella, which is defective in TA98NR, is required of mutagenicity. Similarly, the O-acetyltransferase defective in TA98/1,8-DNP6 is required for the efficient production of the ultimate electrophilic nitrogen species, which react with DNA. The role of bacterial metabolism in mutation induction needs careful consideration to assess the potential risk to humans from nitrobenzimidazole and nitroindole antimycotics.

Synthesis and cardiodepressant activity of dialkyl 1,4-dihydro-2,6-dimethyl-4-(pentatomic-heteroaryl)-3,5-pyridine-dicarbo xylates. 2.

A new series of 1,4-dihydropyridine bearing substituted pyrazole and imidazole at C-4 position was synthesized and characterized as inotropic, chronotropic and calcium antagonist agents, in order to evaluate the effect on pharmacological activity of replacement of the 4-aryl group of nifedipine-like drugs by pyrazole and imidazole moieties. All compounds were tested on myocardial muscle and vascular smooth muscle of guinea-pigs. The new compounds showed both significant selective negative inotropic activity and fairly good negative chronotropic effects. All compounds elicited weak calcium antagonist activity, except compound 9 which is a selective bradycardic agent.