Synthetic studies towards isomeric pyrazolopyrimidines as potential ATP synthesis inhibitors of Mycobacterium tuberculosis. Structural correction of reported N-(6-(2-(dimethylamino)ethoxy)-5-fluoropyridin-3-yl)-2-(4-fluorophenyl)-5-(trifluoromethyl)pyrazolo[1,5-α]pyrimidin-7-amine.

During our studies into preparing analogues of pyrazolopyrimidine as ATP synthesis inhibitors of Mycobacterium tuberculosis, a regiospecific condensation reaction between ethyl 4,4,4-trifluoroacetoacetate and 3-(4-fluorophenyl)-1H-pyrazol-5-amine was observed which was dependent on the specific reaction conditions employed. This work identifies optimized reaction conditions to access either the pyrazolo[3,4-ß]pyridine or the pyrazolo[1,5-α]pyrimidine scaffold. This has led to the structural confirmation of the previously reported pyrazolopyrimidine 17b which was reported as pyrazolo[1,5-α]pyrimidine structure 2 which was corrected to pyrazolo[3,4-ß]-pyrimidine 19.

Parallel discovery of selective and dual inhibitors of tryptophan dioxygenases IDO1 and TDO2 with a newly-modified enzymatic assay.

The expression of tryptophan catabolising enzyme indoleamine 2,3-dioxygenase 1 (IDO1) or tryptophan 2,3-dioxygenase 2 (TDO2) in cancers is associated with suppressed immunity and poor patient prognosis. Results from human clinical trials of IDO1 inhibitors have been disappointing. There is now a strong interest in the development of TDO2-selective or dual IDO1/TDO2 inhibitors that may surpass IDO1 inhibitors by providing broader efficacy and blocking constitutively-expressed hepatic TDO2. To expedite the discovery of novel TDO2-specific and dual inhibitors, an assay that enabled the efficient and accurate measurement of the inhibitory activity of compounds against both IDO1 and TDO2 enzymes, concurrently in the same experiment was established to screen 5,682 compounds that included the National Cancer Institute Diversity set 5, for inhibition of IDO1 and TDO2 activity. This screen identified 82 compounds that inhibited either IDO1, TDO2 or both enzymes > 50% at 20 µM. Thirty Pan Assay Interference compounds were removed from the list and the IC50 of the remaining 52 compounds against IDO1 and TDO2 was subsequently determined using the newly-developed concurrent assay. Ten compounds were confirmed as dual IDO1/TDO2 inhibitors having IC50 values under 50 µM against both enzymes and within 2-fold of each other. Six compounds with IC50 values between 1.39 and 8.41 µM were identified as potential TDO2-selective leads. The use of this concurrent protocol is anticipated to expedite the discovery of novel leads for dual and selective inhibitors against IDO1 and or TDO2 and speed the evaluation of novel analogues that will ensue.

Variations in the C-unit of bedaquiline provides analogues with improved biology and pharmacology.

Analogues of the anti-tuberculosis drug bedaquiline, bearing a 3,5-dimethoxy-4-pyridyl C-unit, retain high anti-bacterial potency yet exert less inhibition of the hERG potassium channel, in vitro, than the parent compound. Two of these analogues (TBAJ-587 and TBAJ-876) are now in preclinical development. The present study further explores structure-activity relationships across a range of related 3,5-disubstituted-4-pyridyl C-unit bedaquiline analogues of greatly varying lipophilicity (clogP from 8.16 to 1.89). This broader class shows similar properties to the 3,5-dimethoxy-4-pyridyl series, being substantially more potent in vitro and equally active in an in vivo (mouse) model than bedaquiline, while retaining a lower cardiovascular risk profile through greatly attenuated hERG inhibition.

Synthesis and structure-activity relationships for tetrahydroisoquinoline-based inhibitors of Mycobacterium tuberculosis.

A series of 5,8-disubstituted tetrahydroisoquinolines were shown to be effective inhibitors of M. tb in culture and modest inhibitors of M. tb ATP synthase. There was a broad general trend of improved potency with higher lipophilicity. Large substituents (e.g., Bn) at the tetrahydroquinoline 5-position were well-tolerated, while N-methylpiperazine was the preferred 8-substituent. Structure-activity relationships for 7-linked side chains showed that the nature of the 7-linking group was important; -CO- and -COCH2- linkers were less effective than -CH2- or -CONH- ones. This suggests that the positioning of a terminal aromatic ring is important for target binding. Selected compounds showed much faster rates of microsomal clearance than did the clinical ATP synthase inhibitor bedaquiline, and modest inhibition of mycobacterial ATP synthase.

Synthetic Studies to Help Elucidate the Metabolism of the Preclinical Candidate TBAJ-876-A Less Toxic and More Potent Analogue of Bedaquiline.

Bedaquiline is a novel drug approved in 2012 by the FDA for treatment of drug-resistant tuberculosis (TB). Although it shows high efficacy towards drug-resistant forms of TB, its use has been limited by the potential for significant side effects. In particular, bedaquiline is a very lipophilic compound with an associated long terminal half-life and shows potent inhibition of the cardiac potassium hERG channel, resulting in QTc interval prolongation in humans that may result in cardiac arrhythmia. To address these issues, we carried out a drug discovery programme to develop an improved second generation analogue of bedaquiline. From this medicinal chemistry program, a candidate (TBAJ-876) has been selected to undergo further preclinical evaluation. During this evaluation, three major metabolites arising from TBAJ-876 were observed in several preclinical animal models. We report here our synthetic efforts to unequivocally structurally characterize these three metabolites through their independent directed synthesis.

3,5-Dialkoxypyridine analogues of bedaquiline are potent antituberculosis agents with minimal inhibition of the hERG channel.

Bedaquiline is a new drug of the diarylquinoline class that has proven to be clinically effective against drug-resistant tuberculosis, but has a cardiac liability (prolongation of the QT interval) due to its potent inhibition of the cardiac potassium channel protein hERG. Bedaquiline is highly lipophilic and has an extremely long terminal half-life, so has the potential for more-than-desired accumulation in tissues during the relatively long treatment durations required to cure TB. The present work is part of a program that seeks to identify a diarylquinoline that is as potent as bedaquiline against Mycobacterium tuberculosis, with lower lipophilicity, higher clearance, and lower risk for QT prolongation. Previous work led to the identification of compounds with greatly-reduced lipophilicity compounds that retain good anti-tubercular activity in vitro and in mouse models of TB, but has not addressed the hERG blockade. We now present compounds where the C-unit naphthalene is replaced by a 3,5-dialkoxy-4-pyridyl, demonstrate more potent in vitro and in vivo anti-tubercular activity, with greatly attenuated hERG blockade. Two examples of this series are in preclinical development.

Structure-activity relationships for unit C pyridyl analogues of the tuberculosis drug bedaquiline.

The ATP-synthase inhibitor bedaquiline is effective against drug-resistant tuberculosis but is extremely lipophilic (clogP 7.25) with a very long plasma half-life. Additionally, inhibition of potassium current through the cardiac hERG channel by bedaquiline, is associated with prolongation of the QT interval, necessitating cardiovascular monitoring. Analogues were prepared where the naphthalene C-unit was replaced with substituted pyridines to produce compounds with reduced lipophilicity, anticipating a reduction in half-life. While there was a direct correlation between in vitro inhibitory activity against M. tuberculosis (MIC90) and compound lipophilicity, potency only fell off sharply below a clogP of about 4.0, providing a useful lower bound for analogue design. The bulk of the compounds remained potent inhibitors of the hERG potassium channel, with notable exceptions where IC50 values were at least 5-fold higher than that of bedaquiline. Many of the compounds had desirably higher rates of clearance than bedaquiline, but this was associated with lower plasma exposures in mice, and similar or higher MICs resulted in lower AUC/MIC ratios than bedaquiline for most compounds. The two compounds with lower potency against hERG exhibited similar clearance to bedaquiline and excellent efficacy in vivo, suggesting further exploration of C-ring pyridyls is worthwhile.

PEG-Benzaldehyde-Hydrazone-Lipid Based PEG-Sheddable pH-Sensitive Liposomes: Abilities for Endosomal Escape and Long Circulation.

PURPOSE: To fabricate an acid-cleavable PEG polymer for the development of PEG-cleavable pH-sensitive liposomes (CL-pPSL), and to investigate their ability for endosomal escape and long circulation. METHODS: PEG-benzaldehyde-hydrazone-cholesteryl hemisuccinate (PEGB-Hz-CHEMS) containing hydrazone and ester bonds was synthesised and used to fabricate a dual pH-sensitive CL-pPSL. Non-cleavable PEGylated pH-sensitive liposome (pPSL) was used as a reference and gemcitabine as a model drug. The cell uptake and endosomal escape were investigated in pancreatic cancer Mia PaCa-2 cells and pharmacokinetics were studied in rats. RESULTS: The CL-pPSL showed accelerated drug release at endosomal pH 5.0 compared to pPSL. Compared to pPSL, CL-pPSL released their fluorescent payload to cytosol more efficiently and showed a 1.4-fold increase in intracellular gemcitabine concentration and higher cytotoxicity. In rats, injection of gemcitabine loaded CL-pPSL resulted in a slightly smaller Vd (149 ± 27 ml/kg; 170 ± 30 ml/kg) and shorter terminal T1/2 (5.4 ± 0.3 h; 5.8 ± 0.6 h) (both p > 0.05) but a significantly lower AUC (p < 0.01), than pPSL, due to the lower PEGylation degree (1.7 mol%) which means a 'mushroom' configuration of PEG. A five-time increase in the dose with CL-pPSL resulted in a 11-fold increase in AUC and a longer T1/2 (8.2 ± 0.5 h). CONCLUSION: The PEG-detachment from the CL-pPSL enhanced endosome escape efficiency compared with pPSL, without significantly compromising their stealth abilities.

Structure-activity relationships for analogs of the tuberculosis drug bedaquiline with the naphthalene unit replaced by bicyclic heterocycles.

Replacing the naphthalene C-unit of the anti-tuberculosis drug bedaquiline with a range of bicyclic heterocycles of widely differing lipophilicity gave analogs with a 4.5-fold range in clogP values. The biological results for these compounds indicate on average a lower clogP limit of about 5.0 in this series for retention of potent inhibitory activity (MIC90s) against M.tb in culture. Some of the compounds also showed a significant reduction in inhibition of hERG channel potassium current compared with bedaquiline, but there was no common structural feature that distinguished these.

Synthesis and evaluation of analogues of the tuberculosis drug bedaquiline containing heterocyclic B-ring units.

Analogues of bedaquiline where the phenyl B-unit was replaced with monocyclic heterocycles of widely differing lipophilicity (thiophenes, furans, pyridines) were synthesised and evaluated. While there was an expected broad positive correlation between lipophilicity and anti-TB activity, the 4-pyridyl derivatives appeared to have an additional contribution to antibacterial potency. The majority of the compounds were (desirably) more polar and had higher rates of clearance than bedaquiline, and showed acceptable oral bioavailability, but there was only limited (and unpredictable) improvement in their hERG liability.

6-Nitro-2,3-dihydroimidazo[2,1-b][1,3]thiazoles: Facile synthesis and comparative appraisal against tuberculosis and neglected tropical diseases.

As part of a quest for backups to the antitubercular drug pretomanid (PA-824), we investigated the unexplored 6-nitro-2,3-dihydroimidazo[2,1-b][1,3]-thiazoles and related -oxazoles. The nitroimidazothiazoles were prepared in high yield from 2-bromo-4-nitroimidazole via heating with substituted thiiranes and diisopropylethylamine. Equivalent examples of these two structural classes provided broadly comparable MICs, with 2-methyl substitution and extended aryloxymethyl side chains preferred; albeit, S-oxidised thiazoles were ineffective for tuberculosis. Favourable microsomal stability data for a biaryl thiazole (45) led to its assessment in an acute Mycobacterium tuberculosis mouse model, alongside the corresponding oxazole (48), but the latter proved to be more efficacious. In vitro screening against kinetoplastid diseases revealed that nitroimidazothiazoles were inactive versus leishmaniasis but showed interesting activity, superior to that of the nitroimidazooxazoles, against Chagas disease. Overall, "thio-delamanid" (49) is regarded as the best lead.

Formation of fluorophores from the kynurenine pathway metabolite N-formylkynurenine and cyclic amines involves transamidation and carbon-carbon bond formation at the 2-position of the amine.

BACKGROUND: Tryptophan catabolism along the kynurenine pathway is associated with a number of pathologies including cataract formation and cancer. Whilst the chemical reactions of kynurenine are well studied, less is known about the reactivity of its precursor N-formylkynurenine (NFK). We previously reported the generation of a strong fluorophore in an aqueous reaction of NFK with piperidine, and herein we describe its structure and mechanism of formation. METHODS: Compounds were identified using NMR, mass and UV spectroscopic techniques. The products from the reaction of amines with amino acids were quantified using HPLC-MS. RESULTS: The novel fluorophore was identified as a tetrahydroquinolone adduct (PIP-THQ), where piperidine is N-formylated and attached at its 2-position to the quinolone. NFK is initially deaminated to generate an unsaturated enone, which forms an adduct with piperidine and is subsequently converted into the fluorophore. Testing of a variety of other secondary amines showed that only cyclic amines unsubstituted at both positions adjacent to nitrogen could form fluorophores efficiently. The amino acids tryptophan and kynurenine, which lack the formamide group do not form such fluorophores. CONCLUSIONS: NFK forms fluorophores in a not previously published reaction with cyclic amines. GENERAL SIGNIFICANCE: Our study is the first to provide evidence for concurrent transamidation and substitution at the 2-position of a cyclic amine occurring under moderately-heated aqueous conditions with no added catalysts. The high reactivity of NFK demonstrated here could result in formation of biologically relevant metabolites yet to be characterised.

Biarylmethoxy 2-nitroimidazooxazine antituberculosis agents: Effects of proximal ring substitution and linker reversal on metabolism and efficacy.

Certain biaryl analogues of antitubercular drug PA-824 displayed enhanced in vivo efficacies yet retained some susceptibility towards oxidative metabolism; therefore, two new strategies were explored to address this. Ortho-substitution of the proximal aryl ring with larger electron-withdrawing substituents maintained or improved compound stability but reduced aerobic potency; however, fluoro and cyano were well tolerated. In vivo, only 2'- or 3'-fluoro mono-substitution preserved high efficacy against acute infection, although one example was twofold more effective than delamanid against chronic infection. Reversal of the 6-oxymethylene linkage also permitted high potency and improved stability towards human liver microsomes, albeit, in vivo results were inferior. These novel findings provide further insight into the preferred structural features for lead candidates in this important drug class.

Discovery and characterisation of hydrazines as inhibitors of the immune suppressive enzyme, indoleamine 2,3-dioxygenase 1 (IDO1).

Screening of a fragment library identified 2-hydrazinobenzothiazole as a potent inhibitor of indoleamine 2,3-dioxygenase 1 (IDO1), an enzyme expressed by tumours that suppresses the immune system. Spectroscopic studies indicated that 2-hydrazinobenzothiazole interacted with the IDO1 haem and in silico docking predicted that the interaction was through hydrazine. Subsequent studies of hydrazine derivatives identified phenylhydrazine (IC50=0.25 ± 0.07 µM) to be 32-fold more potent than 2-hydrazinobenzothiazole (IC50=8.0 ± 2.3 µM) in inhibiting rhIDO1 and that it inhibited cellular IDO1 at concentrations that were noncytotoxic to cells. Here, phenylhydrazine is shown to inhibit IDO1 through binding to haem.

Formation of an N-formylkynurenine-derived fluorophore and its use for measuring indoleamine 2,3-dioxygenase 1 activity.

Indoleamine 2,3-dioxygenase 1 (IDO1) is a tryptophan-catabolizing enzyme whose expression by a broad range of clinical tumors is associated with immunosuppression and poor patient outcome. Here we describe a new fluorescence assay for measuring IDO1 activity suitable for high-throughput screening of compound libraries for novel IDO1 inhibitors. This assay is easy to perform, requiring the addition of only one reagent prior to readout. In place of measuring kynurenine, it uses the in situ formation of an N-formylkynurenine-derived fluorophore (NFKPIP) measured at an excitation wavelength of 400 nm and an emission wavelength of 500 nm. The fluorescence intensity of the NFKPIP formed is directly related to the amount of enzyme activity, and the signal is stable over 8 h. This assay has a lower limit of detection, equating to 153 nM N-formylkynurenine, which is over 30-fold lower than the limits of detection of existing assays for IDO1 activity. When we compared the performance of the new assay with that of the published colorimetric absorbance assay in screening the National Cancer Institute Diversity Set III of 1,597 compounds for IDO1 inhibitors, we obtained an identical list of the 25 most active compounds in the two assays. Although 93 compounds (aldehydes, ketones, and aromatic amines) in the library interfered with the absorbance readout, only 18 compounds (conjugated systems and fused cycles) interfered with the readout of the new fluorescence assay. IC(50) values determined using the new assay for three known IDO1 inhibitors-1,4-naphthoquinone, 4-amino-N-(3-chloro-4-fluorophenyl)-N'-hydroxy-1,2,5-oxadiazole-3-carboximidamide and 4-phenyl-1H-imidazole-were consistent with their literature values, further validating the new assay for measuring IDO1 activity.

Evaluation of Novel Inhibitors of Tryptophan Dioxygenases for Enzyme and Species Selectivity Using Engineered Tumour Cell Lines Expressing Either Murine or Human IDO1 or TDO2.

Indoleamine 2, 3-dioxygenase 1 (IDO1) is commonly expressed by cancers as a mechanism for evading the immune system. Preclinical and clinical studies have indicated the potential of combining IDO1 inhibitors with immune therapies for the treatment of cancer, strengthening an interest in the discovery of novel dioxygenase inhibitors for reversing tumour-mediated immune suppression. To facilitate the discovery, development and investigation of novel small molecule inhibitors of IDO1 and its hepatic isozyme tryptophan dioxygenase (TDO2), murine tumour cells were engineered to selectively express either murine or human IDO1 and TDO2 for use as tools to dissect both the species specificity and isoenzyme selectivity of newly discovered inhibitors. Lewis lung carcinoma (LLTC) lines were engineered to express either murine or human IDO1 for use to test species selectivity of the novel inhibitors; in addition, GL261 glioma lines were engineered to express either human IDO1 or human TDO2 and used to test the isoenzyme selectivity of individual inhibitors in cell-based assays. The 20 most potent inhibitors against recombinant human IDO1 enzyme, discovered from a commissioned screening of 40,000 compounds in the Australian WEHI compound library, returned comparable IC50 values against murine or human IDO1 in cell-based assays using the LLTC-mIDO1 and LLTC-hIDO1 line, respectively. To test the in vivo activity of the hits, transfected lines were inoculated into syngeneic C57Bl/6 mice. Individual LLTC-hIDO1 tumours showed variable expression of human IDO1 in contrast to GL261-hIDO1 tumours which were homogenous in their IDO1 expression and were subsequently used for in vivo studies. W-0019482, the most potent IDO1 inhibitor identified from cell-based assays, reduced plasma and intratumoural ratios of kynurenine to tryptophan (K:T) and delayed the growth of subcutaneous GL261-hIDO1 tumours in mice. Synthetic modification of W-0019482 generated analogues with dual IDO1/TDO2 inhibitory activity, as well as inhibitors that were selective for either TDO2 or IDO1. These results demonstrate the versatility of W-0019482 as a lead in generating all three subclasses of tryptophan dioxygenase inhibitors which can be applied for investigating the individual roles and interactions between IDO1 and TDO2 in driving cancer-mediated immune suppression.

Synthesis and structure-activity relationships for a new class of tetrahydronaphthalene amide inhibitors of Mycobacterium tuberculosis.

Drug resistant tuberculsosis (TB) is global health crisis that demands novel treatment strategies. Bacterial ATP synthase inhibitors such as bedaquiline and next-generation analogues (such as TBAJ-876) have shown promising efficacy in patient populations and preclinical studies, respectively, suggesting that selective targeting of this enzyme presents a validated therapeutic strategy for the treatment of TB. In this work, we report tetrahydronaphthalene amides (THNAs) as a new class of ATP synthase inhibitors that are effective in preventing the growth of Mycobacterium tuberculosis (M.tb) in culture. Design, synthesis and comprehensive structure-activity relationship studies for approximately 80 THNA analogues are described, with a small selection of compounds exhibiting potent (in some cases MIC90 <1 µg/mL) in vitro M.tb growth inhibition taken forward to pharmacokinetic and off-target profiling studies. Ultimately, we show that some of these THNAs possess reduced lipophilic properties, decreased hERG liability, faster mouse/human liver microsomal clearance rates and shorter plasma half-lives compared with bedaquiline, potentially addressing of the main concerns of persistence and phospholipidosis associated with bedaquiline.

Synthesis and Structure-Activity Relationships for the Anti-Mycobacterial Activity of 3-Phenyl-N-(Pyridin-2-ylmethyl)Pyrazolo[1,5-a]Pyrimidin-7-Amines.

Pyrazolo[1,5-a]pyrimidines have been reported as potent inhibitors of mycobacterial ATP synthase for the treatment of Mycobacterium tuberculosis (M.tb). In this work, we report the design and synthesis of approximately 70 novel 3,5-diphenyl-N-(pyridin-2-ylmethyl)pyrazolo[1,5-a]pyrimidin-7-amines and their comprehensive structure-activity relationship studies. The most effective pyrazolo[1,5-a]pyrimidin-7-amine analogues contained a 3-(4-fluoro)phenyl group, together with a variety of 5-alkyl, 5-aryl and 5-heteroaryl substituents. A range of substituted 7-(2-pyridylmethylamine) derivatives were also active. Some of these compounds exhibited potent in vitro M.tb growth inhibition, low hERG liability and good mouse/human liver microsomal stabilities, highlighting their potential as inhibitors of M.tb.

Release of nitrite from the antitubercular nitroimidazole drug PA-824 and analogues upon one-electron reduction in protic, non-aqueous solvent.

The one-electron reduction chemistry of the antituberculosis drug PA-824, together with a series of closely related compounds, has been investigated in irradiated anaerobic propan-2-ol solution. The protic solvent, of low dielectric constant, was chosen to mimic the environment of a water-restricting active site of a model protein, which is capable of reducing the compounds. Radiolytic reduction of the compounds containing electron donating substituents in the 2-position of the imidazole ring released nitrite, with compounds that are highly active against Mycobacterium tuberculosis exhibiting high yields of nitrite. The release of cytotoxic reactive nitrogen species through a one-electron pathway, by as yet unidentified proteins, may play a role in the activity of this class of compounds against TB. The described radiolytic quantification of nitrite release may have utility as a preliminary screening test for nitroaromatic candidate drugs against the disease.

Gemcitabine sensitization by checkpoint kinase 1 inhibition correlates with inhibition of a Rad51 DNA damage response in pancreatic cancer cells.

The protein kinase checkpoint kinase 1 (Chk1) has been implicated as a key regulator of cell cycle progression and DNA repair, and inhibitors of Chk1 (e.g., UCN-01 and EXEL-9844) potentiate the cytotoxic actions of chemotherapeutic drugs in tumor cells. We have examined the ability of PD-321852, a small-molecule Chk1 inhibitor, to potentiate gemcitabine-induced clonogenic death in a panel of pancreatic cancer cell lines and evaluated the relationship between endpoints associated with Chk1 inhibition and chemosensitization. Gemcitabine chemosensitization by minimally toxic concentrations of PD-321852 ranged from minimal (<3-fold change in survival) in Panc1 cells to >30-fold in MiaPaCa2 cells. PD-321852 inhibited Chk1 in all cell lines as evidenced by stabilization of Cdc25A; in combination with gemcitabine, a synergistic loss of Chk1 protein was observed in the more sensitized cell lines. Gemcitabine chemosensitization, however, did not correlate with abrogation of the S-M or G2-M checkpoint; PD-321852 did not induce premature mitotic entry in gemcitabine-treated BxPC3 or M-Panc96 cells, which were sensitized to gemcitabine 6.2- and 4.6-fold, respectively. In the more sensitized cells lines, PD-321852 not only inhibited gemcitabine-induced Rad51 focus formation and the recovery from gemcitabine-induced replication stress, as evidenced by persistence of gamma-H2AX, but also depleted these cells of Rad51 protein. Our data suggest the inhibition of this Chk1-mediated Rad51 response to gemcitabine-induced replication stress is an important factor in determining gemcitabine chemosensitization by Chk1 inhibition in pancreatic cancer cells.