•New inhibitors of the NLRP3 inflammasome are described•NLRP3 inflammasome activation is independent of Ca2+•These new inflammasome inhibitors are effective in vivo•The inhibitors described may lead to the development of new drugsNLRP3 is a receptor important for host responses to infection, yet is also known to contribute to devastating diseases such as Alzheimer's disease, diabetes, atherosclerosis, and others, making inhibitors for NLRP3 sought after. One of the inhibitors currently in use is 2-aminoethoxy diphenylborinate (2APB). Unfortunately, in addition to inhibiting NLRP3, 2APB also displays non-selective effects on cellular Ca2+ homeostasis. Here, we use 2APB as a chemical scaffold to build a series of inhibitors, the NBC series, which inhibit the NLRP3 inflammasome in vitro and in vivo without affecting Ca2+ homeostasis. The core chemical insight of this work is that the oxazaborine ring is a critical feature of the NBC series, and the main biological insight the use of NBC inhibitors led to was that NLRP3 inflammasome activation was independent of Ca2+. The NBC compounds represent useful tools to dissect NLRP3 function, and may lead to oxazaborine ring-containing therapeutics.Download high-res image (177KB)Download full-size image

Inflammasomes are high molecular weight complexes that sense and react to injury and infection. Their activation induces caspase-1 activation and release of interleukin-1β, a pro-inflammatory cytokine involved in both acute and chronic inflammatory responses. There is increasing evidence that inflammasomes, particularly the NLRP3 inflammasome, act as guardians against noninfectious material. Inappropriate activation of the NLRP3 inflammasome contributes to the progression of many noncommunicable diseases such as gout, type II diabetes, and Alzheimer’s disease. Inhibiting the inflammasome may significantly reduce damaging inflammation and is therefore regarded as a therapeutic target. Currently approved inhibitors of interleukin-1β are rilonacept, canakinumab, and anakinra. However, these proteins do not possess ideal pharmacokinetic properties and are unlikely to easily cross the blood–brain barrier. Because inflammation can contribute to neurological disorders, this review focuses on the development of small-molecule inhibitors of the NLRP3 inflammasome.

•A series of non-symmetric furan, pyrrole and N-methylpyrrole-amidines were synthesized.•The binding modes of the amidine compounds to NQO2 were analysed by molecular modelling.•The in vitro NQO2 inhibitory activities of the amidines were evaluated, the most active giving an IC50 of 15 nM.•The Tm values of DNA in the presence of some compounds were measured, the non-symmetric furan amidines showing no chelation to DNA.•Some of the non-symmetric furan amidines inhibited the Plasmodium parasite, the most active having an IC50 of 9.6 nM.NRH:quinone oxidoreductase 2 enzyme (NQO2) is a potential therapeutic target in cancer and neurodegenerative diseases, with roles in either chemoprevention or chemotherapy. Here we report the design, synthesis and evaluation of non-symmetrical furan-amidines and their analogues as novel selective NQO2 inhibitors with reduced adverse off-target effects, such as binding to DNA. A pathway for the synthesis of the non-symmetrical furan-amidines was established from the corresponding 1,4-diketones. The synthesized non-symmetrical furan-amidines and their analogues showed potent NQO2 inhibition activity with nano-molar IC50 values. The most active compounds were non-symmetrical furan-amidines with meta- and para-nitro substitution on the aromatic ring, with IC50 values of 15 nM. In contrast to the symmetric furan-amidines, which showed potent intercalation in the minor grooves of DNA, the synthesized non-symmetrical furan-amidines showed no affinity towards DNA, as demonstrated by DNA melting temperature experiments. In addition, Plasmodium parasites, which possess their own quinone oxidoreductase PfNDH2, were inhibited by the non-symmetrical furan-amidines, the most active possessing a para-fluoro substituent (IC50 9.6 nM). The high NQO2 inhibition activity and nanomolar antimalarial effect of some of these analogues suggest the lead compounds are worthy of further development and optimization as potential drugs for novel anti-cancer and antimalarial strategies.

(S)-4,5-Dihydroxypentane-2,3-dione [(S)-DPD, (1)] is a precursor for AI-2, a quorum sensing signalling molecule for inter- and intra-species bacterial communication. The synthesis of its fluoro-analogue, 4-fluoro-5-hydroxypentane-2,3-dione (2) is reported. An intermediate in this route also enables a new, shorter synthesis of the native (S)-DPD. 4-Fluoro-DPD (2) completely inhibited bioluminescence and bacterial growth of Vibrio harveyi BB170 strain at 12.5 μM and 100 μM, respectively.

Using biomimetic chemical reduction or Clostridium perfringens cell extract containing azoreductase, the dimer-fluorescent probe 2,4-O-bisdansyl-6,7-diazabicyclooct-6-ene, which possesses a conformationally constrained cis-azo bridge, is reduced to the tetra-equatorial 2,4-O-bisdansyl-cyclohexyl-3,5-bisammonium salt which exhibits fluorescence indicative of a dansyl monomer.

Ester prodrugs have the potential to eliminate the gastrotoxicity associated with the carboxylic acid group of indomethacin. 4,6-Bis-O-2′-[1′-(4″-chlorobenzoyl)-5′-methoxy-2′-methyl-1′H-indol-3′-acetyl]-myo-inositol-1,3,5-orthoacetate (2) was synthesised and evaluated as a COX-2 inhibitor. It adopts a conformationally restricted chair with two indomethacin groups in the sterically hindered 1,3-diaxial positions. Acid-induced cleavage of the orthoacetate lock of the prodrug leads to a ring flip of the myo-inositol ring with the two indomethacin groups now in 1,3-diequatorial positions. This increases the susceptibility of hydrolysis of the ester groups to release indomethacin under acidic conditions. The long half-life (152 min) of decomposition of (2) at ∼pH 1–2 suggests that it may bypass the stomach with minimal hydrolysis upon oral administration. Indomethacin ester (2) was completely stable at pH 4.0–8.5 over 24 h at 37 °C and showed comparable activity to indomethacin in a COX-2 assay (pH 8.0).Graphical abstractCompound (2) was synthesised and evaluated as a COX-2 inhibitor. Acid-induced hydrolysis of the orthoacetate lock gives (7eq) in a pentaequatorial conformation, which undergoes hydrolysis to release indomethacin.Highlights► Novel compounds (1) and (2) were synthesised as myo-inositol esters of indomethacin. ► Hydrolysis studies monitoring the release of indomethacin have been reported. ► The indomethacin ester (2), showed comparable activity in a COX-2 assay.

Novel non-nucleobase-derived inhibitors of the angiogenic enzyme, thymidine phosphorylase, have been identified using molecular modelling, synthesis and biological evaluation. These inhibitors are 2,4,5-trioxoimidazolidines bearing N-(substituted)phenylalkyl groups, together with, in most cases, N′-(CH2)n-carboxylic acid, ester or amide side chains. The best compound from this series is 3-(2,4,5-trioxo-3-phenylethyl-imidazolodin-1-yl)propionamide, with an IC50 of 40 μM against Escherichia coli TP. Molecular modelling suggests that this ligand, when complexed with closed-cleft human TP, would have the phenylalkyl group in the active site region normally occupied by a thymine-containing structure.A series of parabanic acid derivatives have been synthesised and evaluated against the angiogenic enzyme, thymidine phosphorylase. 3-(2,4,5-Trioxo-3-phenylethyl-imidazolodin-1-yl)propionamide is identified as the most potent.Research highlights► Angiogenic thymidine phosphorylase (TP) is a target for cancer chemotherapy. ► Virtual screening of NCI/ACD databases with TP identified imidazolidine inhibitors. ► A series of imidazolidines were prepared and their TP activity evaluated. ► The lead compound was 3-(2,4,5-trioxo-3-phenylethyl-imidazolidin-1-yl)propionamide.

The intramolecular heterodimer probe 4-O-dabsyl-2-O-dansyl-myo-inositol-1,3,5-orthoformate undergoes biomimetic reduction of the azo bond in the dabsyl group to give a fluorescence emission band at 560 nm. This is attributed to the loss of energy transfer between the fluorophore (dansyl) and quencher (dabsyl) groups as a result of the formation of 4-O-(4-aminobenzene sulfonyl)-2-O-dansyl-myo-inositol-1,3,5-orthoformate.

myo-Inositol phosphates possessing the 1,2,3-trisphosphate motif share the remarkable ability to completely inhibit iron-catalysed hydroxyl radical formation. The simplest derivative, myo-inositol 1,2,3-trisphosphate [Ins(1,2,3)P3], has been proposed as an intracellular iron chelator involved in iron transport. The binding conformation of Ins(1,2,3)P3 is considered to be important to complex Fe3+ in a ‘safe’ manner. Here, a pyrene-based fluorescent probe, 4,6-bispyrenoyl-myo-inositol 1,2,3,5-tetrakisphosphate [4,6-bispyrenoyl Ins(1,2,3,5)P4], has been synthesised and used to monitor the conformation of the 1,2,3-trisphosphate motif using excimer fluorescence emission. Ring-flip of the cyclohexane chair to the penta-axial conformation occurs upon association with Fe3+, evident from excimer fluorescence induced by π–π stacking of the pyrene reporter groups, accompanied by excimer formation by excitation at 351 nm. This effect is unique amongst biologically relevant metal cations, except for Ca2+ cations exceeding a 1:1 molar ratio. In addition, the thermodynamic constants for the interaction of the fluorescent probe with Fe3+ have been determined. The complexes formed between Fe3+ and 4,6-bispyrenoyl Ins(1,2,3,5)P4 display similar stability to those formed with Ins(1,2,3)P3, indicating that the fluorescent probe acts as a good model for the 1,2,3-trisphosphate motif. This is further supported by the antioxidant properties of 4,6-bispyrenoyl Ins(1,2,3,5)P4, which closely resemble those obtained for Ins(1,2,3)P3. The data presented confirms that Fe3+ binds tightly to the unstable penta-axial conformation of myo-inositol phosphates possessing the 1,2,3-trisphosphate motif.

Natural myo-inositol phosphate antioxidants containing the 1,2,3-trisphosphate motif bind Fe3+ in the unstable penta-axial conformation.

2-O-tert-Butyldimethylsilyl-4,6-bis-O-pyrenoyl-myo-inositol-1,3,5-orthoformate (6) and 2-O-tert-butyldimethylsilyl-4-O-[4-(dimethylamino)benzoyl]-6-O-pyrenoyl-myo-inositol-1,3,5-orthoacetate (10) adopt conformationally restricted unstable chairs with five axial substituents. In the symmetrical diester 6, the two π-stacked pyrenoyl groups are electron acceptor–donor partners, giving a strong intramolecular excimer emission. In the mixed ester 10, the pyrenoyl group is the electron acceptor and the 4-(dimethylamino)benzoyl ester is the electron donor, giving a strong intramolecular exciplex emission. The conformation of the mixed ester 10 was assessed using 1H NMR spectroscopy (1H-NOESY) and computational studies. which showed the minimum inter-centroid distance between the two aromatic systems to be ∼3.9 Å. Upon addition of acid, the orthoformate/orthoacetate trigger in 6and 10 was cleaved, which caused a switch of the conformation of the myo-inositol ring to the more stable penta-equatorial chair, leading to separation of the aromatic ester groups and loss of excimer and exciplex fluorescence, respectively. This study provides proof of principle for the development of novel fluorescent molecular probes.

(S)-4,5-Dihydroxypentane-2,3-dione [(S)-DPD, (1)] is a precursor for AI-2, a quorum sensing signalling molecule for inter- and intra-species bacterial communication. The synthesis of its fluoro-analogue, 4-fluoro-5-hydroxypentane-2,3-dione (2) is reported. An intermediate in this route also enables a new, shorter synthesis of the native (S)-DPD. 4-Fluoro-DPD (2) completely inhibited bioluminescence and bacterial growth of Vibrio harveyi BB170 strain at 12.5 μM and 100 μM, respectively.

Natural myo-inositol phosphate antioxidants containing the 1,2,3-trisphosphate motif bind Fe3+ in the unstable penta-axial conformation.

2-O-tert-Butyldimethylsilyl-4,6-bis-O-pyrenoyl-myo-inositol-1,3,5-orthoformate (6) and 2-O-tert-butyldimethylsilyl-4-O-[4-(dimethylamino)benzoyl]-6-O-pyrenoyl-myo-inositol-1,3,5-orthoacetate (10) adopt conformationally restricted unstable chairs with five axial substituents. In the symmetrical diester 6, the two π-stacked pyrenoyl groups are electron acceptor–donor partners, giving a strong intramolecular excimer emission. In the mixed ester 10, the pyrenoyl group is the electron acceptor and the 4-(dimethylamino)benzoyl ester is the electron donor, giving a strong intramolecular exciplex emission. The conformation of the mixed ester 10 was assessed using 1H NMR spectroscopy (1H-NOESY) and computational studies. which showed the minimum inter-centroid distance between the two aromatic systems to be ∼3.9 Å. Upon addition of acid, the orthoformate/orthoacetate trigger in 6and 10 was cleaved, which caused a switch of the conformation of the myo-inositol ring to the more stable penta-equatorial chair, leading to separation of the aromatic ester groups and loss of excimer and exciplex fluorescence, respectively. This study provides proof of principle for the development of novel fluorescent molecular probes.

myo-Inositol phosphates possessing the 1,2,3-trisphosphate motif share the remarkable ability to completely inhibit iron-catalysed hydroxyl radical formation. The simplest derivative, myo-inositol 1,2,3-trisphosphate [Ins(1,2,3)P3], has been proposed as an intracellular iron chelator involved in iron transport. The binding conformation of Ins(1,2,3)P3 is considered to be important to complex Fe3+ in a ‘safe’ manner. Here, a pyrene-based fluorescent probe, 4,6-bispyrenoyl-myo-inositol 1,2,3,5-tetrakisphosphate [4,6-bispyrenoyl Ins(1,2,3,5)P4], has been synthesised and used to monitor the conformation of the 1,2,3-trisphosphate motif using excimer fluorescence emission. Ring-flip of the cyclohexane chair to the penta-axial conformation occurs upon association with Fe3+, evident from excimer fluorescence induced by π–π stacking of the pyrene reporter groups, accompanied by excimer formation by excitation at 351 nm. This effect is unique amongst biologically relevant metal cations, except for Ca2+ cations exceeding a 1:1 molar ratio. In addition, the thermodynamic constants for the interaction of the fluorescent probe with Fe3+ have been determined. The complexes formed between Fe3+ and 4,6-bispyrenoyl Ins(1,2,3,5)P4 display similar stability to those formed with Ins(1,2,3)P3, indicating that the fluorescent probe acts as a good model for the 1,2,3-trisphosphate motif. This is further supported by the antioxidant properties of 4,6-bispyrenoyl Ins(1,2,3,5)P4, which closely resemble those obtained for Ins(1,2,3)P3. The data presented confirms that Fe3+ binds tightly to the unstable penta-axial conformation of myo-inositol phosphates possessing the 1,2,3-trisphosphate motif.

Using biomimetic chemical reduction or Clostridium perfringens cell extract containing azoreductase, the dimer-fluorescent probe 2,4-O-bisdansyl-6,7-diazabicyclooct-6-ene, which possesses a conformationally constrained cis-azo bridge, is reduced to the tetra-equatorial 2,4-O-bisdansyl-cyclohexyl-3,5-bisammonium salt which exhibits fluorescence indicative of a dansyl monomer.