Practical applications and feasibility of efflux pump inhibitors in the clinic--a vision for applied use

The world of antibiotic drug discovery and development is driven by the necessity to overcome antibiotic resistance in common Gram-positive and Gram-negative pathogens. However, the lack of Gram-negative activity among both recently approved antibiotics and compounds in the developmental pipeline is a general trend despite the fact that the plethora of covered drug targets are well-conserved across the bacterial kingdom. Such intrinsic resistance in Gram-negative bacteria is largely attributed to the activity of multidrug resistance (MDR) efflux pumps. Moreover, these pumps also play a significant role in acquired clinical resistance. Together, these considerations make efflux pumps attractive targets for inhibition in that the resultant efflux pump inhibitor (EPI)/antibiotic combination drug should exhibit increased potency, enhanced spectrum of activity and reduced propensity for acquired resistance. To date, at least one class of broad-spectrum EPI has been extensively characterized. While these efforts indicated a significant potential for developing small molecule inhibitors against efflux pumps, they did not result in a clinically useful compound. Stemming from the continued clinical pressure for novel approaches to combat drug resistant bacterial infections, second-generation programs have been initiated and show early promise to significantly improve the clinical usefulness of currently available and future antibiotics against otherwise recalcitrant Gram-negative infections. It is also apparent that some changes in regulatory decision-making regarding resistance would be very helpful in order to facilitate approval of agents aiming to reverse resistance and prevent its further development.     

Synthetic antimicrobial peptidomimetics with therapeutic potential

A series of synthetic antimicrobial peptidomimetics (SAMPs) have been prepared and found to be highly active against several Gram-negative and Gram-positive bacterial strains. These derivatives comprise the minimal structural requirements for cationic antimicrobial peptides and showed high selectivity for Gram-negative and/or Gram-positive bacteria compared to human red blood cells. We have found that SAMPs share many of the attractive properties of cationic antimicrobial peptides inasmuch that a representative SAMP was found to insert into the bilayers of large unilamellar vesicles, permeabilized both the outer and cytoplasmic membrane of Escherichia coli ML-35p, and displayed an extremely rapid bacterial killing for Staphylococcus aureus. However, while antimicrobial peptides are prone to proteolytic degradation, high in vitro stability in human blood plasma was shown for SAMPs. A combination of high antibacterial activity against methicillin-resistant staphylococci and low toxicity against human erythrocytes makes these molecules promising candidates for novel antibacterial therapeutics.     

Antibacterial activity and chemical modifications of PS-5 at the C-3 side chain

Using PS-5 as starting material, the effects of chemical modification at the C-3 side chain were studied on the antibacterial activity against Gram-positive and Gram-negative bacteria including beta-lactamase-producers. Among 35 side chains tested, 4-pyridylthio showed the highest antibacterial activity against the Gram-positive bacteria, and D-cysteinyl against the Gram-negative microbes. In general, compared with acetamidoethylthio in PS-5, basic side chains showed improved antibacterial activity against the staphylococci and pseudomonads, whereas the antibiotic activity against the Gram-negative bacteria decreased with bulky side chains. The introduction of 6-aminopenicillanate and 7-aminocephalosporanate to the C-3 side chain of carbapenem significantly reduced the antibacterial activity against the beta-lactamase-producing microbes.     

具有手性吡咯烷侧环的噁唑烷酮类化合物的合成及抗菌活性研究

目的 设计合成具有手性吡咯烷侧环的噁唑烷酮类化合物,并考察其体外抗菌活性。方法 以手性脯氨酸和3,4-二氟硝基苯为原料,通过多步反应合成目标化合物;采用微量液体稀释法检测目标化合物的抗菌活性。结果与结论 合成了 8 个新化合物,其结构经 ¹H-NMR、MS 谱确证。体外活性试验表明：由（R）-脯氨酸衍生的化合物7b 对革兰阳性菌[金黄色葡萄球菌、表皮葡萄球菌以及耐甲氧西林金黄色葡萄球菌（MRSA）具有良好的抑制活性,对革兰阴性菌具有较弱的抑制活性。由（S）-脯氨酸衍生的化合物 7a 的抗菌活性明显低于 7b。研究结果表明,吡咯烷侧环的手性因素对抗菌活性具有显著影响。     

Outer membranes and efflux: the path to multidrug resistance in Gram-negative bacteria

Intrinsic and acquired multidrug resistance in Gram-negative bacteria owes much to the synergy between limited outer membrane permeability and energy-dependent multidrug efflux. The importance of the outer membrane vis-a-vis resistance is aptly demonstrated by the impact of mutational changes in outer membrane constituents on drug susceptibility. Changes in lipopolysaccharide (LPS) that correlate with increased drug susceptibility confirm, for example, the significance of this macromolecule in the intrinsic antimicrobial resistance of Gram-negative bacteria. Alterations in LPS and porins correlating with increased resistance to a variety of antimicrobials are also known and highlight the significance of the outer membrane vis-a-vis acquired antimicrobial resistance. Efflux systems accommodating a range of structurally distinct antimicrobials, including antibiotics, detergents, dyes, biocides and aromatic hydrocarbons have been identified in a number of Gram-negative organisms. Mutational studies have confirmed the importance of these systems to intrinsic and acquired antimicrobial resistance in important disease-causing organisms. As such, strategies aimed at thwarting efflux and or the outer membrane barrier are effective at reversing antimicrobial resistance in these organisms.     

铜绿假单胞菌主动外排泵介导的多重抗生素耐药性

铜绿假单胞菌作为一种机会致病菌，对多种临床常用抗生素呈现明显的固有与获得性耐药。这种多重耐药性的形成机制在于该菌具有的多种能量依赖性的药物主动外排泵和低通透性外膜屏障协同作用所致。近10年对铜绿假单胞菌药物外排泵的研究已经颇为深入，本文对有关进展予以讨论。在铜绿假单胞菌已报道了6种属于“耐药-生节-分裂(RND)”类的药物外排泵系统，它们可在天然野生株表达或者由于基因突变而诱导表达，从而介导了对β-内酰胺类(包括β-内酰胺酶抑制剂)、氨基糖苷类、氟喹诺酮类、大环内酯类及四环素类等的耐药性。这些外排泵中尤其以MexAB—OprM系统的作用底物范围最广，在耐药性形成中起主要作用。RND类外排泵系统也普遍存在于其它革兰氏阴性细菌。各外排泵通常由内膜转运体蛋白、内膜融合蛋白及外膜通道蛋白一起形成功能性转运复合体将药物排至胞外。药物外排泵常影响对灭活酶较稳定的抗菌药物。一些新抗菌药物如Linezolid(Oxa—zolidinoes类)、Telithromyein(Ketolides类)及Tigecycline(Glycyclines类)也是MexAB-OprM等RND类外排泵的作用底物，故这些药物抗革兰氏阳性细菌的活性较强。铜绿假单胞菌外排泵表达的调控机制多是在转录水平上受局部阻遏物或激活物的作用。这些局部调节子的基因突变能导致外排泵活性增强。并常见于应用抗生素或其它抗菌消毒制剂后从临床分离的或实验室筛选的多重耐药铜绿假单胞菌。这些耐药菌的形成再次说明合理限制抗菌药物应用的必要性。外排泵系统还与其它耐药机制如细菌的药物作用靶位改变或产生药物灭活酶等一起发挥明显的协同作用，使细菌的耐药程度进一步地增高。现已研制了针对铜绿假单胞菌等细菌的外排泵抑制剂。应用外排泵抑制剂可阻断药物外排泵这一耐药机制，以维护或提高抗菌药物的抗菌活性。如外排泵抑制剂在体外和动物感染模型均被证实明显增强了氟喹诺酮类对铜绿假单胞菌敏感株和耐药株的抗菌活性，并降低了耐药菌的发生率。外排泵抑制剂尚处于临床前的基础实验研制阶段。     

利奈唑胺-黄酮杂合体的合成与抗菌活性研究

目的 针对利奈唑胺日益严重的细菌耐药问题,将具有外排泵抑制作用的黄酮骨架引入利奈唑胺分子中,以提高抗耐药菌活性。方法 通过黄酮的氧烷基化反应,连接臂酯基的选择性水解,TBTU存在下的缩合反应,得到黄酮-利奈唑胺杂合体5;采用MTT法测定其抗菌活性,采用荧光分光光度法测定其外排率,用SYBYL软件研究5与靶点可能的结合模式。结果 化合物5抗金黄色葡萄球菌MIC50为0.39μg/mL,活性是利奈唑胺的近2倍;金黄色葡萄球菌对化合物5的外排率(13.1%)远低于其母体化合物利奈唑胺(49.0%);化合物5和利奈唑胺具有相同的靶点结合模式。结论 化合物5对革兰阳性和阴性菌均具有优良的抗菌活性;对核糖体具有高亲和性;对外排泵具有显著抑制作用。     

Synthesis and antibacterial activity of new tetracyclic quinolone antibacterials.

A series of 8-substituted-9,1-(epoxymethano)-7-fluoro-5-oxo-5H- thiazolo[3,2-a]quinoline-4-carboxylic acids having a novel tetracyclic structure was synthesized and tested for antibacterial activity. The nature of the heteroatom (N, O, or S) substituted at the 8-position had little influence on the antibacterial activity. Among the six pyrrolidinyl derivatives and the five piperazinyl derivatives, the 8-(3-hydroxy-1-pyrrolidinyl) derivative 6h and the hydrochloride of the 8-(4-methyl-1-piperazinyl) derivative 6l showed the most potent activity against both Gram-positive and Gram-negative bacteria. Against nalidixic acid resistant strains, isolated from Escherichia coli KC-14, compound 6h was less potent than 6l. Replacement of the piperazinyl nitrogen atom by a carbon atom, an oxygen atom, or a sulfur atom (corresponding to the piperidino, morpholino, or thiomorpholino group, respectively) enhanced the activity against Gram-positive bacteria, but reduced the activity against Gram-negative bacteria. Compound 6l also showed potent in vivo antibacterial activity against Gram-positive and Gram-negative bacteria, and did not cause convulsions in mice with the concomitant administration of fenbufen. Replacement of the carboxy group by a sulfonic acid group in 6l resulted in a complete loss of antibacterial activity.     

Effect of 13-epi-sclareol on the bacterial respiratory chain

13-epi-sclareol is a labdane-type diterpene isolated from the resinous exudates of the medicinal plant species Pseudognaphalium cheiranthifolium (Lam.) Hilliard et Burtt. and P. heterotrichium (Phil.) A. Anderb. This compound has antibacterial activity only against Gram-positive bacteria, showing a bactericidal and lytic action. The interaction of 13- epi-sclareol with the bacterial respiratory chain was analyzed. The compound inhibited oxygen consumption of intact Gram-positive cells, but not with Gram-negative bacteria. The compound inhibited NADH oxidase and cytochrome c reductase activities, while coenzyme Q reductase and the cytochrome c oxidase activities were not affected. These results suggest that the target site of 13-epi-sclareol is located between coenzyme Q and cytochrome c. Using cytoplasmic membrane fractions, the results of the analysis of the enzyme activities associated with the respiratory chain complexes were the same for both Gram-positive and Gram-negative bacteria, indicating that the compound has no access to the cytoplasmic membrane of intact Gram-negative bacteria. Thus, the Gram-negative envelope may act as a physical barrier that prevents the access of this compound to the site of action.     

Antibacterial peptides (XX). Synthesis and antibacterial activity of alpha-acyltetrapeptides containing basic amino acids (IX)

In order to compare the antibacterial activity with Acyl-Lys-Lys(Thr)-Glu-OH (I), already reported, 3 kinds of Acyl-Lys-Lys(Glu)-Thr-OH (9a - c) and corresponding tetrapeptide (10), which were exchanged Thr for Glu on I, were synthesized by dicyclohexylcarbodiimide method. It was proved the (+)-6-methyloctanoyl-Lys-Lys(Glu)-Thr-OH (9a) shows antibacterial activity against 5 kinds of Gram-negative bacteria and against 2 kinds of Gram-positive bacteria, but that the others (9b), (9c), only have the weak one, and that tetrapeptide has against Gram-positive bacteria; and that those compounds have almost the same one as Acyl-Lys-Lys(Thr)-Glu-OH (I) has.     

Antimicrobial activity of N-phthaloylamino acid hydroxamates

Antibacterial and antifungal activity of N-phthaloylamino acid hydroxamates [C6H4(CO)2N-X-CONHOH, X=amino acid residues of glycine, beta-alanine or D-phenylglycine], was examined against 44 strains of Gram-positive and Gram-negative bacteria, and 10 species of yeasts. The level of antimicrobial activity was established using the in vitro agar assay and the standard broth dilution susceptibility test. N-phthaloyl-D-phenylglycine-hydroxa- mic acid , the substance with the highest lipophilicity (log P), showed the best antibacterial activity, especially against Gram-negative bacteria. Minimum inhibitory concentration of was 0.008 mg mL-1 in the activity against Yersinia enterocolitica O3, confirmed by a large inhibition zone (30 mm) by the diffusion test. Hydroxamates inhibit growth by chelation of the PDF enzyme metal in both Gram-positive and Gram-negative bacteria, and LpxC enzyme in Gram-negative enzyme. Phthalimides appear to contribute to inhibition by destabilizing m-RNA. Antifungal activity of substances is not very expressed.     

Synthesis and antibacterial activity of pyridazino[4,3-b]indole-4-carboxylic acids carrying different substituents at N-2

The synthesis and the in vitro evaluation of antibacterial activity of new pyridazino[4,3-b]indole-4-carboxylic acids 2-4, 6 against some selected representative of Gram-positive and Gram-negative bacteria are reported. The role of the lipophilicity in the modulation of the antibacterial activity of the tested compounds is discussed. All the synthesized compounds appear quite weak against Gram-positive bacteria, whereas have no significant activity against Gram-negative bacteria. Only derivative 2g possesses an interesting activity against Gram-positive bacteria.     

Synthesis and antibacterial activity of isomeric 15-membered azalides

A series of 3-keto and 3-O-acyl derivatives of both 6-O-alkyl-8a-aza-8a-homoerythromycin A and 6-O-alkyl-9a-aza-9a-homo-erythromycin A were synthesised and tested against Gram-positive and Gram-negative bacteria. Derivatives of 8a-aza-8a-homoerythromycin A have potent antibacterial activity against not only azithromycin-susceptible strains, but also efflux (M) and inducible macrolide-lincosamide-streptogramin (iMLSB) resistant Gram-positive pathogens, while the corresponding 9a-isomers were less active. Introduction of an additional ring such as 11,12-cyclic carbonate reduced antibacterial activity of both series. 3-Keto and 3-O-(4-nitrophenyl)-acetyl derivatives of 6-O-methyl-8a-aza-8a-homo-erythromycin A show typical macrolide pharmacokinetics in preliminary in vivo studies in mice, and their in vivo efficacy is demonstrated.     

Synthesis and in vitro antibacterial activity of some N-(5-aryl-1,3,4-thiadiazole-2-yl)piperazinyl quinolone derivatives

A series of N-[5-(1-methyl-5-nitro-2-imidazolyl)-1,3,4-thiadiazole-2-yl] and N-[5-(nitrophenyl)-1,3,4-thiadiazole-2-yl] piperazinyl quinolone derivatives (5a-c and 5d-l) were synthesized and evaluated for in vitro antibacterial activity against some Gram-positive and Gram-negative bacteria. The antibacterial data revealed that all nitroimidazole derivatives (5a-c) showed interesting activity against tested Gram-positive bacteria (minimum inhibitory concentration, MIC=0.008-0.03 microg/ml) while they did not show good activity against Gram-negative organisms. Despite the significant activity of nitroimidazole series, all nitrophenyl analogues (5d-l) were inactive against both Gram-positive and Gram-negative bacteria. Among all of the tested compounds, 5a (ciprofloxacin derivative in nitroimidazole series) exhibited excellent activity against Staphylococcus aureus and Staphylococcus epidermidis (MIC=0.008 microg/ml).     

Identification of a Novel Cathelicidin from the Deinagkistrodon acutus Genome with Antibacterial Activity by Multiple Mechanisms

The abuse of antibiotics and the consequent increase of drug-resistant bacteria constitute a serious threat to human health, and new antibiotics are urgently needed. Research shows that antimicrobial peptides produced by natural organisms are potential substitutes for antibiotics. Based on Deinagkistrodon acutus (known as five-pacer viper) genome bioinformatics analysis, we discovered a new cathelicidin antibacterial peptide which was called FP-CATH. Circular dichromatic analysis showed a typical helical structure. FP-CATH showed broad-spectrum antibacterial activity. It has antibacterial activity to Gram-negative bacteria and Gram-positive bacteria including methicillin-resistant Staphylococcus aureus (MRSA). The results of transmission electron microscopy (TEM) and scanning electron microscopy (SEM) showed that FP-CATH could cause the change of bacterial cell integrity, having a destructive effect on Gram-negative bacteria and inducing Gram-positive bacterial surface formation of vesicular structure. FP-CATH could bind to LPS and showed strong binding ability to bacterial DNA. In vivo, FP-CATH can improve the survival rate of nematodes in bacterial invasion experiments, and has a certain protective effect on nematodes. To sum up, FP-CATH is likely to play a role in multiple mechanisms of antibacterial action by impacting bacterial cell integrity and binding to bacterial biomolecules. It is hoped that the study of FP-CATH antibacterial mechanisms will prove useful for development of novel antibiotics.     

Mode of action of BO-1341: transport pathway through the outer membrane of Escherichia coli

BO-1341, a new semisynthetic cephalosporin having a 6,7-dihydroxyisoquinolinium moiety at the C-3 methylene, possesses potent antibacterial activity against Gram-negative bacteria including glucose-nonfermentative bacteria, especially Pseudomonas aeruginosa. In order to elucidate the mechanisms of action, the transport pathway of BO-1341 through the outer membrane of Escherichia coli was studied. The antibacterial activity of BO-1341 was not affected by the deficiency in OmpF and OmpC porin channels compared with its wild-type strain, but was affected by the iron concentration in the medium. Susceptibility testing with the mutants involved in the iron transport system indicated that the tonB mutant was resistant to BO-1341. Analysis of the spontaneous mutants resistant to BO-1341 revealed a mutation in the tonB gene. The strong activity of BO-1341 and the lack of cross-resistance to other cephalosporins may be attributable to the unique transport system through the outer membrane of Gram-negative bacteria.     

In vitro study on efficacy of combination use of aspoxicillin and beta-lactam preparations (ceftazidime, cefmetazole and aztreonam) against bacteria isolated from abdominal infections]

An in vitro study was done to evaluate combination use of aspoxicillin (ASPC) with each of 3 beta-lactam preparations, ceftazidime (CAZ), cefmetazole (CMZ) and aztreonam (AZT). The results obtained are summarized as follows: 1. ASPC has strong activity against Gram-positive bacteria and anaerobic bacteria, while CMZ and CAZ have strong activity against Gram-negative bacteria. 2. Rates of beta-lactamase producing strains among the isolated bacteria (a total of 383 isolates) were 4.4% among Gram-positive bacteria, 71.6% among Gram-negative bacteria and 89.3% among anaerobic bacteria. The overall rate of beta-lactamase secreting strains among all isolates was 46.5%. 3. Efficacies of combination uses were studied using the FIC index. Combination of ASPC and CAZ was effective against 95.0% of the isolates, ASPC and AZT against 85.7%, and ASPC and CMZ against 83.5%. Combination of ASPC and CMZ showed antagonism in 12.8% of the isolates. In conclusion, combination use of ASPC with any one of CMZ, AZT or CAZ proved to be highly effective. In particular, combination of ASPC and CAZ appeared to be the best in view of complementing antibacterial spectra.     

Screening of Ethanol, Petroleum Ether and Chloroform Extracts of Medicinal Plants, Lawsonia inermis L. and Mimosa pudica L. for Antibacterial Activity

Organic extracts (ethanol, petroleum ether and chloroform) of two medicinal plants Lawsonia inermis L. and Mimosa pudica L. were proven for antibacterial properties against 15 Gram-positive and Gram-negative human pathogenic bacteria. Among the three types of extracts tested, ethanol extract was found to possess maximum antibacterial activity. The diameter of the zone of inhibition of bacterial growth showed that Gram-negative bacteria are more sensitive than Gram-positive bacteria to plant extracts. Between the two plants species studied, Lawsonia inermis extract showed more antibacterial activity compared to Mimosa pudica extract.     

Are outer-membrane targets the solution for MDR Gram-negative bacteria?

The outer membrane (OM) of Gram-negative bacteria confers a significant barrier to many antibacterial agents targeting periplasmic and cytosolic functions. ‘Synergist’ approaches to disrupt the OM have been hampered by poor specificity and accompanying toxicities. The OM contains proteins required for optimal growth and pathogenesis, including lipopolysaccharide (LPS) and capsular polysaccharide (CPS) transport, porins for uptake of macromolecules, and transporters for essential elements (such as iron). Does the external proximity of these proteins offer an enhanced potential to identify effective therapies? Here, we review recent experiences in exploiting Gram-negative OM proteins (OMPs) to address the calamity of exploding antimicrobial resistance.Teaser: Multidrug-resistant (MDR) Gram-negative bacteria are a growing crisis. Few new antimicrobial chemotypes or targets have been identified after decades of screening. Are OMP targets a solution to MDR Gram-negative bacteria?     

Bacterial resistance to antibiotics: active efflux and reduced uptake

Antibiotic resistance of bacterial pathogens is a fast emerging global crisis and an understanding of the underlying resistance mechanisms is paramount for design and development of new therapeutic strategies. Permeability barriers for and active efflux of drug molecules are two resistance mechanisms that have been implicated in various infectious outbreaks of antibiotic-resistant pathogens, suggesting that these mechanisms may be good targets for new drugs. The synergism of reduced uptake and efflux is most evident in the multiplicative action of the outer membrane permeability barrier and active efflux, which results in high-level intrinsic and/or acquired resistance in many clinically important Gram-negative bacteria. This review summarizes the current knowledge of these two important resistance mechanisms and potential strategies to overcome them. Recent advances in understanding the physical structures, function and regulation of efflux systems will facilitate exploitation of pumps as new drug targets.