Recent Advances in the Research of Heterocyclic Compounds as Antitubercular Agents

Tuberculosis (TB) is a major health problem, with approximately one‐third of the world′s population infected with Mycobacterium tuberculosis, eight million people in the active disease state, and two million dying annually. Furthermore, the prevalence of TB/HIV co‐infection, and the emergence of multidrug‐resistant tuberculosis (MDR‐TB) and extensively drug‐resistant tuberculosis (XDR‐TB) have further aggravated the spread of this disease and thus mortality by it. There is an urgent need for novel antitubercular agents with improved properties, such as lower toxicity, shortened duration of therapy, rapid bactericidal action, and enhanced activity against MDR strains. Fortunately, a number of new potential antitubercular candidate drugs with heterocyclic rings, which are most likely to be effective against resistant strains, have entered clinical trials in recent years. This review highlights recent advances in the research of novel heterocyclic compounds, with particular focus on their antimycobacterial activity, mechanisms of action, toxicity, and structure–activity relationships (SARs).     

Novel 1,4‐Substituted‐1,2,3‐Triazoles as Antitubercular Agents

Tuberculosis (TB) remains a pressing unmet medical need, particularly with the emergence of multidrug‐resistant and extensively drug‐resistant tuberculosis. Here, a series of 1,4‐substituted‐1,2,3‐triazoles have been synthesized and evaluated as potential antitubercular agents. These compounds were assembled via click chemistry in high crude purity and in moderate to high yield. Of the compounds tested, 12 compounds showed promising antitubercular activity with six possessing minimum inhibitory concentration (MIC) values <10 μg mL^?1, and total selectivity for Mycobacterium tuberculosis (Mtb) growth inhibition. A second set of 21 compounds bearing variations on ring C were synthesized and evaluated. This second library gave an additional six compounds displaying MIC values ≤10 μg mL^?1 and total selectivity for Mtb growth inhibition. These compounds serve as an excellent starting point for further development of antitubercular therapies.     

Preliminary Evaluation of Artemisinin–Cholesterol Conjugates as Potential Drugs for the Treatment of Intractable Forms of Malaria and Tuberculosis

To evaluate the feasibility of developing drugs that may be active against both malaria and tuberculosis (TB) by using in part putative cholesterol transporters in the causative pathogens and through enhancement of passive diffusion in granulomatous TB, artemisinin–cholesterol conjugates were synthesized by connecting the component molecules through various linkers. The compounds were screened in vitro against Plasmodium falciparum (Pf) and Mycobacterium tuberculosis (Mtb). Antimalarial activities (IC₅₀) against Pf drug‐sensitive NF54, and drug‐resistant K1 and W2 strains ranged from 0.03–2.6, 0.03–1.9, and 0.02–1.7 μm . Although the compounds are less active than the precursor artemisinin derivatives, the cholesterol moiety renders the compounds relatively insoluble in the culture medium, and variation in solubilities among the different compounds may reflect in the range of efficacies observed. Activities against Mtb H37Rv were assessed using a standardized colony‐forming unit (CFU) assay after 24 h pretreatment of cultures with each of the compounds. Percentage inhibition ranged from 3–38 % and 18–52 % at 10 and 80 μm , respectively. Thus, in contrast to the comparator drug artemether, the conjugates display enhanced activities. The immediate aims include the preparation of conjugates with enhanced aqueous solubilities, assays against malaria and TB in vivo, and for TB, assays using an infected macrophage model and assessment of granuloma influx.     

Synthesis,Biological Evaluation of 1,1‐Diarylethylenes as a Novel Class of Antimitotic Agents

The cytotoxic activities of 23 new isocombretastatin A derivatives with modifications on the B‐ring were investigated. Several compounds exhibited excellent antiproliferative activity at nanomolar concentrations against a panel of human cancer cell lines. Compounds isoFCA‐4 ( 2 e ), isoCA‐4 ( 2 k ) and isoNH₂CA‐4 ( 2 s ) were the most cytotoxic, and strongly inhibited tubulin polymerization with IC₅₀ values of 4, 2 and 1.5 μM , respectively. These derivatives were found to be 10‐fold more active than phenstatin and colchicine with respect to growth inhibition but displayed similar activities as tubulin polymerization inhibitors. In addition, cell cycle arrest in the G₂/M phase and subsequent apoptosis was observed in three cancer cell lines when treated with these compounds. The disruptive effect of 2 e , 2 k and 2 s on the vessel‐like structures formed by human umbilical vein endothelial cells (HUVEC) suggest that these compounds may act as vascular disrupting agents. Both compounds 2 k and 2 s have the potential for further prodrug modification and development as vascular disrupting agents for treatment of solid tumors.     

Synthesis of Tamoxifen-Artemisinin and Estrogen-Artemisinin Hybrids Highly Potent Against Breast and Prostate Cancer

In the search for new and effective treatments of breast and prostate cancer, a series of hybrid compounds based on tamoxifen, estrogens, and artemisinin were successfully synthesized and analyzed for their in vitro activities against human prostate (PC-3) and breast cancer (MCF-7) cell lines. Most of the hybrid compounds exhibit a strong anticancer activity against both cancer cell lines – for example, EC₅₀ (PC-3) down to 1.07 μM, and EC₅₀ (MCF-7) down to 2.08 μM – thus showing higher activities than their parent compounds 4-hydroxytamoxifen (afimoxifene, 7 ; EC₅₀=75.1 (PC-3) and 19.3 μM (MCF-7)), dihydroartemisinin ( 2 ; EC₅₀=263.6 (PC-3) and 49.3 μM (MCF-7)), and artesunic acid ( 3 ; EC₅₀=195.1 (PC-3) and 32.0 μM (MCF-7)). The most potent compounds were the estrogen-artemisinin hybrids 27 and 28 (EC₅₀=1.18 and 1.07 μM, respectively) against prostate cancer, and hybrid 23 (EC₅₀=2.08 μM) against breast cancer. These findings demonstrate the high potential of hybridization of artemisinin and estrogens to further improve their anticancer activities and to produce synergistic effects between linked pharmacophores.     

Structure-Activity Relationship and Mode-Of-Action Studies Highlight 1-(4-Biphenylylmethyl)-1H-imidazole-Derived Small Molecules as Potent CYP121 Inhibitors

CYP121 of Mycobacterium tuberculosis (Mtb) is an essential target for the development of novel potent drugs against tuberculosis (TB). Besides known antifungal azoles, further compounds of the azole class were recently identified as CYP121 inhibitors with antimycobacterial activity. Herein, we report the screening of a similarity-oriented library based on the former hit compound, the evaluation of affinity toward CYP121, and activity against M. bovis BCG. The results enabled a comprehensive SAR study, which was extended through the synthesis of promising compounds and led to the identification of favorable features for affinity and/or activity and hit compounds with 2.7-fold improved potency. Mode of action studies show that the hit compounds inhibit substrate conversion and highlighted CYP121 as the main antimycobacterial target of our compounds. Exemplified complex crystal structures of CYP121 with three inhibitors reveal a common binding site. Engaging in both hydrophobic interactions as well as hydrogen bonding to the sixth iron ligand, our compounds block a solvent channel leading to the active site heme. Additionally, we report the first CYP inhibitors that are able to reduce the intracellular replication of M. bovis BCG in macrophages, emphasizing their potential as future drug candidates against TB.     

Antibacterial activity of silver‐doped manganese dioxide nanoparticles on multidrug‐resistant bacteria

BACKGROUND: As a result of evolution of multiple drug resistance in human pathogens (bacteria) there is increasing demand for novel antibacterial agents, and recently, due to their high antibacterial and catalytic activities, metal nanoparticles have attracted the attention of researchers and medical microbiologists worldwide. RESULTS: Ni‐, Ce‐ and Ag‐doped MnO₂ nanoparticles were synthesized by a co‐precipitation method. Antibacterial activity of these synthesized nanoparticles on methicillin‐resistant Staphylococcus aureus and lead‐resistant Pseudomonas aeruginosa strain 4EA was investigated using a disc diffusion method. Only Ag‐doped MnO₂ nanoparticles showed an antibacterial property against methicillin‐resistant Staphylococcus aureus and lead‐resistant Pseudomonas aeruginosa strain 4EA at low levels of 60 µg/disc and 85 µg/disc, respectively. Scanning electron microscopy and transmission electron microscopy (SEM‐TEM) coupled with energy dispersive X‐ray (EDX) analysis revealed the nano‐size and composition of these synthesized nanoparticles. CONCLUSION: It was confirmed through a disc diffusion method that chemically synthesized silver doped MnO₂ nanoparticles have antibacterial activity against multidrug‐resistant Staphylococcus aureus and lead‐resistant Pseudomonas aeruginosa strain 4EA at low levels therefore these nanoparticles can be employed to fight and prevent infections caused by multidrug‐resistant bacterial pathogens. © 2012 Society of Chemical Industry     

Biological Evaluation of Potent Triclosan‐Derived Inhibitors of the Enoyl–Acyl Carrier Protein Reductase InhA in Drug‐Sensitive and Drug‐Resistant Strains of Mycobacterium tuberculosis

New triclosan (TRC) analogues were evaluated for their activity against the enoyl–acyl carrier protein reductase InhA in Mycobacterium tuberculosis (Mtb). TRC is a well‐known inhibitor of InhA, and specific modifications to its positions 5 and 4′ afforded 27 derivatives; of these compounds, seven derivatives showed improved potency over that of TRC. These analogues were active against both drug‐susceptible and drug‐resistant Mtb strains. The most active compound in this series, 4‐(n‐butyl)‐1,2,3‐triazolyl TRC derivative 3 , had an MIC value of 0.6 μg mL^?1 (1.5 μM ) against wild‐type Mtb. At a concentration equal to its MIC, this compound inhibited purified InhA by 98 %, and showed an IC₅₀ value of 90 nM . Compound 3 and the 5‐methylisoxazole‐modified TRC 14 were able to inhibit the biosynthesis of mycolic acids. Furthermore, mc²4914, an Mtb strain overexpressing inhA, was found to be less susceptible to compounds 3 and 14 , supporting the notion that InhA is the likely molecular target of the TRC derivatives presented herein.     

N‐Cinnamoylation of Antimalarial Classics: Quinacrine Analogues with Decreased Toxicity and Dual‐Stage Activity

Plasmodium falciparum, the causative agent of the most lethal form of malaria, is becoming increasingly resistant to most available drugs. A convenient approach to combat parasite resistance is the development of analogues of classical antimalarial agents, appropriately modified in order to restore their relevance in antimalarial chemotherapy. Following this line of thought, the design, synthesis and in vitro evaluation of N‐cinnamoylated quinacrine surrogates, 9‐(N‐cinnamoylaminobutyl)‐amino‐6‐chloro‐2‐methoxyacridines, is reported. The compounds were found to be highly potent against both blood‐stage P. falciparum, chloroquine‐sensitive 3D7 (IC₅₀=17.0–39.0 nM ) and chloroquine‐resistant W2 and Dd2 strains (IC₅₀=3.2–41.2 and 27.1–131.0 nM , respectively), and liver‐stage P. berghei (IC₅₀=1.6–4.9 μM ) parasites. These findings bring new hope for the possible future “rise of a fallen angel” in antimalarial chemotherapy, with a potential resurgence of quinacrine‐related compounds as dual‐stage antimalarial leads.     

Sigma‐2 Receptor Agonists as Possible Antitumor Agents in Resistant Tumors: Hints for Collateral Sensitivity

With the aim of contributing to the development of novel antitumor agents, high‐affinity σ₂ receptor agonists were developed, with 6,7‐dimethoxy‐2‐[4‐[1‐(4‐fluorophenyl)‐1H‐indol‐3‐yl]butyl]‐1,2,3,4‐tetrahydroisoquinoline ( 15 ) and 9‐[4‐(6,7‐dimethoxy‐1,2,3,4‐tetrahydroisoquinolin‐2‐yl)butyl]‐9H‐carbazole ( 25 ) showing exceptional selectivity for the σ₂ subtype. Most of the compounds displayed notable antiproliferative activity in human MCF7 breast adenocarcinoma cells, with similar activity in the corresponding doxorubicin‐resistant MCF7adr cell line. Surprisingly, a few compounds, including 25 , displayed enhanced activity in MCF7adr cells over parent cells, recalling the phenomenon of collateral sensitivity, which is under study for the treatment of drug‐resistant tumors. All of the compounds showed interaction with P‐glycoprotein (P‐gp), and 15 and 25 , with the greatest activity, were able to revert P‐gp‐mediated resistance and reestablish the antitumor effect of doxorubicin in MCF7adr cells. We therefore identified a series of σ₂ receptor agonists endowed with intriguing antitumor properties; these compounds deserve further investigation for the development of alternate strategies against multidrug‐ resistant cancers.     

Hydrazone-Tethered 5-(Pyridin-4-yl)-4H-1,2,4-triazole-3-thiol Hybrids: Synthesis,Characterisation, in silico ADME Studies,and in vitro Antimycobacterial Evaluation and Cytotoxicity

Compounds containing arylpyrrole-, 1,2,4-triazole- and hydrazone structural frameworks have been widely studied and demonstrated to exhibit a wide range of pharmacological properties. Herein, an exploratory series of new 1,2,4-triazole derivatives designed by amalgamation of arylpyrrole and 1,2,4-triazole structural units via a hydrazone linkage is reported. The synthesised compounds were tested in vitro for their potential activity against Mycobacterium tuberculosis (MTB) H₃₇Rv strain. The most promising compound 13 – the derivative without the benzene ring appended to the pyrrole unit displayed acceptable activity (MIC₉₀=3.99 μM) against MTB H₃₇Rv, while other compounds from the series exhibited modest to weak antimycobacterial activity with MIC₉₀ values in the range between 7.0 and >125 μM. Furthermore, in silico results, predicated using the SwissADME web tool, show that the prepared compounds display desirable ADME profile with parameters within acceptable range.     

Synthesis,Characterization, and Antimicrobial Activity of Silver(I) and Copper(II) Complexes of Phosphate Derivatives of Pyridine And Benzimidazole

Two silver(I) complexes—{[Ag(4‐pmOpe)]NO₃}_n and [Ag(2‐bimOpe)₂]NO₃—and three copper(II) complexes—[Cu₄Cl₆O(2‐bimOpe)₄], [CuCl₂(4‐pmOpe)₂], and [CuCl₂(2‐bis(pm)Ope]—were synthesized by reaction of silver(I) nitrate or copper(II) chloride with phosphate derivatives of pyridine and benzimidazole, namely diethyl (pyridin‐4‐ylmethyl)phosphate (4‐pmOpe), 1H‐benzimidazol‐2‐ylmethyl diethyl phosphate (2‐bimOpe), and ethyl bis(pyridin‐2‐ylmethyl)phosphate (2‐bis(pm)Ope). These compounds were characterized by ¹H, ¹³C, and ³¹P NMR as well as IR spectroscopy, elemental analysis, and ESIMS spectrometry. Additionally, molecular and crystal structures of {[Ag(4‐pmOpe)]NO₃}_n and [Cu₄Cl₆O(2‐bimOpe)₄] were determined by single‐crystal X‐ray diffraction analysis. The antimicrobial profiles of synthesized complexes and free ligands against test organisms from the ATCC and clinical sources were determined. Silver(I) complexes showed good antimicrobial activities against Candida albicans strains (MIC values of ～19 μM ). [Ag(2‐bimOpe)₂]NO₃ was particularly active against Pseudomonas aeruginosa and methicillin‐resistant Staphylococcus epidermidis, with MIC values of ～5 and ～10 μM , respectively. Neither copper(II) complexes nor the free ligands inhibited the growth of test organisms at concentrations below 500 μg mL^?1.     

Optimization of Marine Triterpene Sipholenols as Inhibitors of Breast Cancer Migration and Invasion

Sipholenol A, a sipholane triterpene isolated from the Red Sea sponge Callyspongia siphonella, has the ability to reverse multidrug resistance in cancer cells that overexpress P‐glycoprotein (P‐gp). Here, the antimigratory activity of sipholenol A and analogues are reported against the highly metastatic human breast cancer cell line MDA‐MB‐231 in a wound‐healing assay. Sipholenol A and sipholenone A were semisynthetically optimized using ligand‐based strategies to generate structurally diverse analogues in an attempt to maximize their antimigratory activity. A total of 22 semisynthetic ester, ether, oxime, and carbamate analogues were generated and identified by extensive one‐ and two‐dimensional NMR spectroscopy and high‐resolution mass spectrometry analyses. Sipholenol A 4β‐4‐chlorobenzoate and 19,20‐anhydrosipholenol A 4β‐4‐chlorobenzoate esters were the most potent of all tested analogues in the wound‐healing assay, with IC₅₀ values of 5.3 and 5.9 μM , respectively. Generally, ester derivatives showed better antimigratory activities than the carbamate analogues. A KINOMEscan of 19,20‐anhydrosipholenol A 4β‐benzoate ester against 451 human protein kinases identified protein tyrosine kinase 6 (PTK6) as a potential target. In breast tumor cells, PTK6 promotes growth factor signaling and migration, and as such the semisynthetic sipholanes were evaluated for their ability to inhibit PTK6 phosphorylation in vitro. The two analogues with the highest antimigratory activities, sipholenol A 4β‐4‐chlorobenzoate and 19,20‐anhydrosipholenol A 4β‐4‐chlorobenzoate esters, also exhibited the most potent inhibition of PTK6 phosphorylation inhibition. None of the compounds exhibited cytotoxicity in a normal epithelial breast cell line. These derivatives were evaluated in an in vitro invasion assay, where sipholenol A succinate potently inhibited MDA‐MB‐231 cell invasion at 10 μM . These results highlight sipholane triterpenoids as novel antimigratory marine natural products with potential for further development as agents for the control of metastatic breast malignancies.     

A New Class of 1‐Aryl‐5,6‐dihydropyrrolo[2,1‐a]isoquinoline Derivatives as Reversers of P‐Glycoprotein‐Mediated Multidrug Resistance in Tumor Cells

A number of aza‐heterocyclic compounds, which share the 5,6‐dihydropyrrolo[2,1‐a]isoquinoline (DHPIQ) scaffold with members of the lamellarin alkaloid family, were synthesized and evaluated for their ability to reverse in vitro multidrug resistance in cancer cells through inhibition of P‐glycoprotein (P‐gp) and/or multidrug‐resistance‐associated protein 1. Most of the investigated DHPIQ compounds proved to be selective P‐gp modulators, and the most potent modulator, 8,9‐diethoxy‐1‐(3,4‐diethoxyphenyl)‐3‐(furan‐2‐yl)‐5,6‐dihydropyrrolo[2,1‐a]isoquinoline‐2‐carbaldehyde, attained sub‐micromolar inhibitory potency (IC₅₀: 0.19 μm ). Schiff bases prepared by the condensation of some 1‐aryl‐DHPIQ aldehydes with p‐aminophenol also proved to be of some interest, and one of them, 4‐((1‐(4‐fluorophenyl)‐5,6‐dihydro‐8,9‐dimethoxypyrrolo[2,1‐a]isoquinolin‐2‐yl)methyleneamino)phenol, had an IC₅₀ value of 1.01 μm . In drug combination assays in multidrug‐resistant cells, some DHPIQ compounds, at nontoxic concentrations, significantly increased the cytotoxicity of doxorubicin in a concentration‐dependent manner. Studies of structure–activity relationships and investigation of the chemical stability of Schiff bases provided physicochemical information useful for molecular optimization of lamellarin‐like cytotoxic drugs active toward chemoresistant tumors as well as nontoxic reversers of P‐gp‐mediated multidrug resistance in tumor cells.     

Identification of a novel protein synthesis inhibitor active against gram-positive bacteria

In an effort to identify novel antibacterial chemotypes, we performed a whole‐cell screen for inhibitors of Staphylococcus aureus growth and pursued those compounds with previously uncharacterized antibacterial activity. This process resulted in the identification of a benzothiazolium salt, ABTZ‐1, that displayed potent antibacterial activity against Gram‐positive pathogens. Several clinically desirable qualities were demonstrated for ABTZ‐1 including potent activity against multidrug‐resistant clinical isolates of methicillin‐resistant S. aureus (MRSA) and vancomycin‐resistant enterococci (VRE), retention of this activity in human serum, and low hemolytic activity. The antibacterial activity of ABTZ‐1 was attributed to its inhibition of bacterial translation, as this compound prevented the incorporation of [³⁵S]methionine into S. aureus proteins, and ABTZ‐1‐resistant strains were cross‐resistant to known inhibitors of bacterial translation. ABTZ‐1 represents a promising new class of antibacterial agents.     

Synthesis of Long-Chain β-Lactones and Their Antibacterial Activities against Pathogenic Mycobacteria

In the quest for new antibacterial agents, a series of novel long- and medium-chain mono- and disubstituted β-lactones was developed. Their activity against three pathogenic mycobacteria—M. abscessus, M. marinum, and M. tuberculosis—was assessed by the resazurin microtiter assay (REMA). Among the 16 β-lactones synthesized, only 3-hexadecyloxetan-2-one (VM005) exhibited promising activity against M. abscessus, whereas most of the β-lactones showed interesting activities against M. marinum, similar to that of the classical antibiotic, isoniazid. Regarding M. tuberculosis, six compounds were found to be active against this mycobacterium, with β-lactone VM008 [trans-(Z)-3-(hexadec-7-en-1-yl)-4-propyloxetan-2-one] being the best growth inhibitor. The promising antibacterial activities of the best compounds in this series suggest that these molecules may serve as leads for the development of much more efficient antimycobacterial agents.     

Synthesis and Biological Evaluation of N‐Substituted Sophocarpinic Acid Derivatives as Coxsackievirus B3 Inhibitors

A series of novel N‐substituted sophocarpinic acid derivatives was designed, synthesized, and evaluated for their anti‐enteroviral activities against coxsackievirus type B3 (CVB3) and coxsackievirus type B6 (CVB6) in Vero cells. Structure–activity relationship analysis revealed that the introduction of a benzenesulfonyl moiety on the 12‐nitrogen atom in (E)‐β,γ‐sophocarpinic acid might significantly enhance anti‐CVB3 activity. Among the derivatives, (E)‐12‐N‐(m‐cyanobenzenesulfonyl)‐β,γ‐sophocarpinic acid ( 11 m ), possessing a meta‐cyanobenzenesulfonyl group, exhibited potent activity against CVB3 with a selectivity index (SI) of 107. Furthermore, compound 11 m also showed a good oral pharmacokinetic profile, with an AUC value of 7.29 μM h^?1 in rats, and good safety through the oral route in mice, with an LD₅₀ value of >1000 mg kg^?1; these values suggest a druggable characteristic. Therefore, compound 11 m was selected for further investigation as a promising CVB3 inhibitor. We consider (E)‐β,γ‐N‐(benzenesulfonyl)sophocarpinic acids to be a novel class of anti‐CVB3 agents.     

In vitro Characterization of Enzymes Involved in the Synthesis of Nonproteinogenic Residue (2S,3S)‐β‐Methylphenylalanine in Glycopeptide Antibiotic Mannopeptimycin

Mannopeptimycin, a potent drug lead, has superior activity against difficult‐to‐treat multidrug‐resistant Gram‐positive pathogens such as methicillin‐resistant Staphylococcus aureus (MRSA). (2S,3S)‐β‐Methylphenylalanine is a residue in the cyclic hexapeptide core of mannopeptimycin, but the synthesis of this residue is far from clear. We report here on the reaction order and the stereochemical course of reaction in the formation of (2S,3S)‐β‐methylphenylalanine. The reaction is executed by the enzymes MppJ and TyrB, an S‐adenosyl methionine (SAM)‐dependent methyltransferase and an (S)‐aromatic‐amino‐acid aminotransferase, respectively. Phenylpyruvic acid is methylated by MppJ at its benzylic position at the expense of one equivalent of SAM. The resulting β‐methyl phenylpyruvic acid is then converted to (2S,3S)‐β‐methylphenylalanine by TyrB. MppJ was further determined to be regioselective and stereoselective in its catalysis of the formation of (3S)‐β‐methylphenylpyruvic acid. The binding constant (K_D) of MppJ versus SAM is 26 μM . The kinetic constants with respect to k_{cat Ppy} and K_{M Ppy}, and k_{cat SAM} and K_{M SAM} are 0.8 s^?1 and 2.5 mM , and 8.15 s^?1 and 0.014 mM , respectively. These results suggest SAM has higher binding affinity for MppJ than Ppy, and the C? C bond formation in βmPpy might be the rate‐limiting step, as opposed to the C? S bond breakage in SAM.