Discovery of Azetidinyl Ketolides for the Treatment of Susceptible and Multidrug Resistant Community-Acquired Respiratory Tract Infections

Evaluation of Icotinib as a Potent and Selective Inhibitor of Aldehyde Oxidase for Reaction Phenotyping in Human Hepatocytes

Aldehyde oxidase (AO) is a molybdenum cofactor-containing cytosolic enzyme that has gained prominence due to its involvement in the developmental failure of several drug candidates in first-in-human trials. Unlike cytochrome P450s (P450) and glucuronosyltransferase, AO substrates have been plagued by poor in vitro to in vivo extrapolation, leading to low systemic exposures and underprediction of human dose. However, apart from measuring a drug's AO clearance rates, it is also important to determine the relative contribution to metabolism by this enzyme (fm,AO). Although hydralazine is the most well-studied time-dependent inhibitor (TDI) of AO and is frequently employed for AO reaction phenotyping in human hepatocytes to derive fm,AO, multiple studies have expressed concerns pertaining to its utility in providing accurate estimates of fm,AO values due to its propensity to significantly inhibit P450s at the concentrations typically used for reaction phenotyping. In this study, we characterized icotinib, a cyclized analog of erlotinib, as a potent TDI of AO-inactivating human liver cytosolic zoniporide 2-oxidation equipotently with erlotinib with a maximal inactivate rate/inactivator concentration at half maximal inactivation rate (K I) ratio of 463 and 501 minute-1mM-1 , respectively. Moreover, icotinib also exhibits selectivity against P450 and elicits significantly weaker inhibition against human liver microsomal UGT1A1/3 as compared with erlotinib. Finally, we evaluated icotinib as an inhibitor of AO for reaction phenotyping in cryopreserved human hepatocytes and demonstrated that it can yield more accurate prediction of fm,AO compared with hydralazine and induce sustained suppression of AO activity at higher cell densities, which will be important for reaction phenotyping endeavors of low clearance drugs SIGNIFICANCE STATEMENT: In this study, we characterized icotinib as a potent time-dependent inhibitor of AO with ample selectivity margins against the P450s and UGT1A1/3 and demonstrated its utility for reaction phenotyping in human hepatocytes to obtain accurate estimates of fm,AO for victim DDI risk predictions. We envisage the adoption of icotinib in place of hydralazine in AO reaction phenotyping.

Development of In Silico Models for Predicting Potential Time-Dependent Inhibitors of Cytochrome P450 3A4

Cytochrome P450 3A4 (CYP3A4) is one of the major drug metabolizing enzymes in the human body and metabolizes ∼30-50% of clinically used drugs. Inhibition of CYP3A4 must always be considered in the development of new drugs. Time-dependent inhibition (TDI) is an important P450 inhibition type that could cause undesired drug-drug interactions. Therefore, identification of CYP3A4 TDI by a rapid convenient way is of great importance to any new drug discovery effort. Here, we report the development of in silico classification models for prediction of potential CYP3A4 time-dependent inhibitors. On the basis of the CYP3A4 TDI data set that we manually collected from literature and databases, both conventional machine learning and deep learning models were constructed. The comparisons of different sampling strategies, molecular representations, and machine-learning algorithms showed the benefits of a balanced data set and the deep-learning model featured by GraphConv. The generalization ability of the best model was tested by screening an external data set, and the prediction results were validated by biological experiments. In addition, several structural alerts that are relevant to CYP3A4 time-dependent inhibitors were identified via information gain and frequency analysis. We anticipate that our effort would be useful for identification of potential CYP3A4 time-dependent inhibitors in drug discovery and design.

Put a ring on it: application of small aliphatic rings in medicinal chemistry

Aliphatic three- and four-membered rings including cyclopropanes, cyclobutanes, oxetanes, azetidines and bicyclo[1.1.1]pentanes have been increasingly exploited in medicinal chemistry for their beneficial physicochemical properties and applications as functional group bioisosteres. This review provides a historical perspective and comparative up to date overview of commonly applied small rings, exemplifying key principles with recent literature examples. In addition to describing the merits and advantages of each ring system, potential hazards and liabilities are also illustrated and explained, including any significant chemical or metabolic stability and toxicity risks.

Synthetic Strategies, Reactivity and Applications of 1,5-Naphthyridines

This review covers the synthesis and reactivity of 1,5-naphthyridine derivatives published in the last 18 years. These heterocycles present a significant importance in the field of medicinal chemistry because many of them exhibit a great variety of biological activities. First, the published strategies related to the synthesis of 1,5-naphthyridines are presented followed by the reactivity of these compounds with electrophilic or nucleophilic reagents, in oxidations, reductions, cross-coupling reactions, modification of side chains or formation of metal complexes. Finally, some properties and applications of these heterocycles studied during this period are examined.

Effective Application of Metabolite Profiling in Drug Design and Discovery

At one time, biotransformation was a descriptive activity in pharmaceutical development, viewed simply as structural elucidation of drug metabolites, completed only once compounds entered clinical development. Herein, we present our strategic approach using structural elucidation to enable chemistry design/SAR development. The approach considers four questions that often present themselves to medicinal chemists optimizing their compounds for candidate selection: (1) What are the important clearance mechanisms that mediate the disposition of my molecule? (2) Can metabolic liabilities be modulated in a favorable way? (3) Does my compound undergo bioactivation to a reactive metabolite? (4) Do any of the metabolites possess activity, either on- or off-target? An additional question necessary to support compound development relates to metabolites in safety testing (MIST) and our approach also addresses this question. The value in structural elucidation is derived from its application to better design molecules, guide their clinical development, and underwrite patient safety.

Metabolism by Aldehyde Oxidase: Drug Design and Complementary Approaches to Challenges in Drug Discovery

Aldehyde oxidase (AO) catalyzes oxidations of azaheterocycles and aldehydes, amide hydrolysis, and diverse reductions. AO substrates are rare among marketed drugs, and many candidates failed due to poor pharmacokinetics, interspecies differences, and adverse effects. As most issues arise from complex and poorly understood AO biology, an effective solution is to stop or decrease AO metabolism. This perspective focuses on rational drug design approaches to modulate AO-mediated metabolism in drug discovery. AO biological aspects are also covered, as they are complementary to chemical design and important when selecting the experimental system for risk assessment. The authors' recommendation is an early consideration of AO-mediated metabolism supported by computational and in vitro experimental methods but not an automatic avoidance of AO structural flags, many of which are versatile and valuable building blocks. Preferably, consideration of AO-mediated metabolism should be part of the multiparametric drug optimization process, with the goal to improve overall drug-like properties.

Aldehyde oxidase and its role as a drug metabolizing enzyme

Aldehyde oxidase (AO) is a cytosolic enzyme that belongs to the family of structurally related molybdoflavoproteins like xanthine oxidase (XO). The enzyme is characterized by broad substrate specificity and marked species differences. It catalyzes the oxidation of aromatic and aliphatic aldehydes and various heteroaromatic rings as well as reduction of several functional groups. The references to AO and its role in metabolism date back to the 1950s, but the importance of this enzyme in the metabolism of drugs has emerged in the past fifteen years. Several reviews on the role of AO in drug metabolism have been published in the past decade indicative of the growing interest in the enzyme and its influence in drug metabolism. Here, we present a comprehensive monograph of AO as a drug metabolizing enzyme with emphasis on marketed drugs as well as other xenobiotics, as substrates and inhibitors. Although the number of drugs that are primarily metabolized by AO are few, the impact of AO on drug development has been extensive. We also discuss the effect of AO on the systemic exposure and clearance these clinical candidates. The review provides a comprehensive analysis of drug discovery compounds involving AO with the focus on developmental candidates that were reported in the past five years with regards to pharmacokinetics and toxicity. While there is only one known report of AO-mediated clinically relevant drug-drug interaction (DDI), a detailed description of inhibitors and inducers of AO known to date has been presented here and the potential risks associated with DDI. The increasing recognition of the importance of AO has led to significant progress in predicting the site of AO-mediated metabolism using computational methods. Additionally, marked species difference in expression of AO makes it is difficult to predict human clearance with high confidence. The progress made towards developing in vivo, in vitro and in silico approaches for predicting AO metabolism and estimating human clearance of compounds that are metabolized by AO have also been discussed.

Multicomponent synthesis of α-acylamino and α-acyloxy amide derivatives of desmycosin and their activity against gram-negative bacteria

Bacterial resistance to the existing drugs requires constant development of new antibiotics. Developing compounds active against gram-negative bacteria thereby is one of the more challenging tasks. Among the many approaches to develop successful antibacterials, medicinal chemistry driven evolution of existing successful antibiotics is considered to be the most effective one. Towards this end, the C-20 aldehyde moiety of desmycosin was modified into α-acylamino and α-acyloxy amide functionalities using isonitrile-based Ugi and Passerini reactions, aiming for enhanced antibacterial and physicochemical properties. The desired compounds were obtained in 45-93% yield under mild conditions. The antibacterial activity of the resulting conjugates was tested against gram-negative Aliivibrio fischeri. The antibiotic strength is mostly governed by the amine component introduced. Thus, methylamine derived desmycosin bis-amide 4 displayed an enhanced inhibition rate vs. desmycosin (99% vs. 83% at 1 µM). Derivatives with long acyclic or bulky amine and isocyanide Ugi components reduced potency, whereas carboxylic acid reagents with longer chain length afforded increased bioactivity. In Passerini 3-component products, the butyric ester amide 22 displayed a higher activity (90% at 1 µM) than the parent compound desmycosin (2).

Role of Molybdenum-Containing Enzymes in the Biotransformation of the Novel Ghrelin Receptor Inverse Agonist PF-5190457: A Reverse Translational Bed-to-Bench Approach

(R)-2-(2-methylimidazo[2,1-b]thiazol-6-yl)-1-(2-(5-(6-methylpyrimidin-4-yl)-2,3-dihydro-1H-inden-1-yl)-2,7-diazaspiro[3.5]nonan-7-yl)ethan-1-one (PF-5190457) was identified as a potent and selective inverse agonist of the ghrelin receptor [growth hormone secretagogue receptor 1a (GHS-R1a)]. The present translational bed-to-bench work characterizes the biotransformation of this compound in vivo and then further explores in vitro metabolism in fractions of human liver and primary hepatocytes. Following oral administration of PF-5190457 in a phase 1b clinical study, hydroxyl metabolites of the compound were observed, including one that had not been observed in previously performed human liver microsomal incubations. PF-6870961 was biosynthesized using liver cytosol, and the site of hydroxylation was shown to be on the pyrimidine using nuclear magnetic resonance spectroscopy. The aldehyde oxidase (AO) inhibitor raloxifene and the xanthine oxidase inhibitor febuxostat inhibited the formation of PF-6870961 in human liver cytosol, suggesting both enzymes were involved in the metabolism of the drug. However, greater inhibition was observed with raloxifene, indicating AO is a dominant enzyme in the biotransformation. The intrinsic clearance of the drug in human liver cytosol was estimated to be 0.002 ml/min per milligram protein. This study provides important novel information at three levels: 1) it provides additional new information on the recently developed novel compound PF-5190457, the first GHS-R1a blocker that has moved to development in humans; 2) it provides an example of a reverse translational approach where a discovery in humans was brought back, validated, and further investigated at the bench level; and 3) it demonstrates the importance of considering the molybdenum-containing oxidases during the development of new drug entities. SIGNIFICANCE STATEMENT: PF-5190457 is a novel ghrelin receptor inverse agonist that is currently undergoing clinical development for treatment of alcohol use disorder. PF-6870961, a major hydroxyl metabolite of the compound, was observed in human plasma, but was absent in human liver microsomal incubations. PF-6870961 was biosynthesized using liver cytosol, and the site of hydroxylation on the pyrimidine ring was characterized. Inhibitors of aldehyde oxidase and xanthine oxidase inhibited the formation of PF-6870961 in human liver cytosol, suggesting both enzymes were involved in the metabolism of the drug. This information is important for patient selection in subsequent clinical studies.

Design, synthesis and structure-activity relationships of novel macrolones: Hybrids of 2-fluoro 9-oxime ketolides and carbamoyl quinolones with highly improved activity against resistant pathogens

Constitutively erythromycin-resistant apathogens are more difficult to address than inducibly resistant and efflux-resistant strains. Three series of the 4th generation 2-fluoro 9-oxime erythromycin ketolides were synthesized and evaluated. Incorporation of substituted heteroaryl groups (a - m), in contrast to previously reported the unsubstituted heteroaryl groups, proved to the beneficial for enhancement of the activities of the 9-propgargyl ketolide 8 series and the 9-allyl ketolide 14 series. But these aryl groups (a - m) cannot supply the resulting compounds 8 and 14, unlike corresponding the 6-allyl ketolide 20 series, with activity against constitutively resistant Streptococcus pneumoniae. However, hybrids of macrolides and quinolones (8, 14 and 20, Ar = n - t) exhibited not only high activities against susceptible, inducibly erm-mediated resistant, and efflux-mediated resistant strains, but also significantly improved potencies against constitutively resistant Streptococcus pneumoniae and Streptococcus pyogenes. The capacity was highlighted by introduction of newly designed carbamoyl quinolones (q, r, s and t) rather than commonly seen carboxy quinolones (o and p) as the pharmacophores. Structure-activity relationships and molecular modelling indicated that 8r, 14r and 20q may have different binding sites compared to current erythromycins. Moreover, 8r, 14r and 20q have 2.5-3.6 times prolonged half-life and 2.3- to 2.6-fold longer mean residence time in vivo over telithromycin. These findings pave the way for rational design of novel non-telithromycin macrolides that target new binding sites within bacterial ribosomes.

Nature nurtures the design of new semi-synthetic macrolide antibiotics

Erythromycin and its analogs are used to treat respiratory tract and other infections. The broad use of these antibiotics during the last 5 decades has led to resistance that can range from 20% to over 70% in certain parts of the world. Efforts to find macrolides that were active against macrolide-resistant strains led to the development of erythromycin analogs with alkyl-aryl side chains that mimicked the sugar side chain of 16-membered macrolides, such as tylosin. Further modifications were made to improve the potency of these molecules by removal of the cladinose sugar to obtain a smaller molecule, a modification that was learned from an older macrolide, pikromycin. A keto group was introduced after removal of the cladinose sugar to make the new ketolide subclass. Only one ketolide, telithromycin, received marketing authorization but because of severe adverse events, it is no longer widely used. Failure to identify the structure-relationship responsible for this clinical toxicity led to discontinuation of many ketolides that were in development. One that did complete clinical development, cethromycin, did not meet clinical efficacy criteria and therefore did not receive marketing approval. Work on developing new macrolides was re-initiated after showing that inhibition of nicotinic acetylcholine receptors by the imidazolyl-pyridine moiety on the side chain of telithromycin was likely responsible for the severe adverse events. Solithromycin is a fourth-generation macrolide that has a fluorine at the 2-position, and an alkyl-aryl side chain that is different from telithromycin. Solithromycin interacts at three sites on the bacterial ribosome, has activity against strains resistant to older macrolides (including telithromycin), and is mostly bactericidal. Pharmaceutical scientists involved in the development of macrolide antibiotics have learned from the teachings of Professor Satoshi Omura and progress in this field was not possible without his endeavors.

Synthesis and antibacterial evaluation of novel 4″-glycyl linked quinolyl-azithromycins with potent activity against macrolide-resistant pathogens

A new azithromycin-based series of antibacterial macrolones is reported, which features the use of a 4″-ester linked glycin for tethering the quinolone side chain to the macrolide scaffold. Among the analogs prepared, compounds 9e and 22f with a quinolon-6-yl moiety were found to have potent and well-balanced activity against clinically important respiratory tract pathogens, including erythromycin-susceptible and MLSB resistant strains of Streptococcus pneumoniae, Streptococcus pyogenes, and Haemophilus influenzae. In addition, potential lead compounds 9e and 22f demonstrated outstanding levels of activity against Moraxella catarrhalis and inducibly MLSB resistant Staphylococcus aureus. The best member of this series 22f rivals or exceeds, in potency, some of the most active ketolide antibacterial agents known today, such as telithromycin and cethromycin.

The global challenge of new classes of antibacterial agents: an industry perspective

With rising antibiotic resistance and the fear of returning to the pre-penicillin era, incentives are being provided for developing novel antibiotics. The hurdles faced by antibiotic developers include the difficulty in discovering novel chemicals that have selectivity and the increased regulatory scrutiny for safety and efficacy. Furthermore, the demonstration of superiority is essential in order to rationalize pricing and to assure a return on investment. Suggestions are provided to overcome each of these hurdles in order to prevent the antibiotic pipeline from running dry.

Introduction of a nitrogen-containing side chain appended on C-10 of cethromycin leads to reduced CYP3A4 inhibition (WO2014049356A1)

INTRODUCTION

Infections caused by antibiotic-resistant bacteria pose an increasing risk for clinical treatment. Macrolide-lincosamide-streptogramin B is becoming increasingly ineffective due to the methylation at the binding site of bacteria. Despite great efforts on the natural product, erythromycin, only one derivative, that is, telithromycin, capable of fighting against resistant bacteria has so far been marketed. However, the 3'-dimethylamino group is readily metabolized to a nitroso group, which would inhibit CYP3A4, a very important metabolic enzyme responsible for nearly half of all marketed drugs.

AREAS COVERED

Modifications at C-10 of erythromycin were seldom reported. This invention disclosed novel ketolides that had a side chain comprising additional nitrogen atoms in place of the original 10-methyl group. Surprisingly, introduction of the side chain at C-10 led to reduced cytochrome inhibition and increased metabolic stability. As a result, the limited ability to inhibit CYP3A4 would relieve the drug-drug interaction and improve the safety of drug co-administration.

EXPERT OPINION

This invention opens a new avenue for future modifications to the erythromycin family. It remains unclear how the side chain effected on reduction of CYP inhibition. To fully identify structure-activity relationships, the MIC data of the derivatives on gram-negative bacteria is desirable.

Synthesis and Structure-Activity Relationships of α-Amino-γ-lactone Ketolides: A Novel Class of Macrolide Antibiotics

An efficient synthesis of α-amino-γ-lactone ketolide (3) was developed, which provided a versatile intermediate for the incorporation of a variety of aryl and heteroaryl groups onto the C-21 position of clarithromycin via HBTU-mediated amidation. The biological data for this important new class of macrolides revealed significantly potent activity against erythromycin-susceptible strains as well as efflux-resistant and erythromycin MLSB-resistant strains of S. pneumoniae and S. pyogenes. In addition, ketolide 11o showed excellent in vitro antibacterial activity against H. influenzae strain as compared to telithromycin. These results indicate that C-21 substituted γ-lactone ketolides have potential as a next generation macrolide antibiotics.

A simple litmus test for aldehyde oxidase metabolism of heteroarenes

The bioavailability of aromatic azaheterocyclic drugs can be affected by the activity of aldehyde oxidase (AO). Susceptibility to AO metabolism is difficult to predict computationally and can be complicated in vivo by differences between species. Here we report the use of bis(((difluoromethyl)sulfinyl)oxy)zinc (DFMS) as a source of CF₂H radical for a rapid and inexpensive chemical “litmus test” for the early identification of heteroaromatic drug candidates that have a high probability of metabolism by AO.

Effect of structural variation on aldehyde oxidase-catalyzed oxidation of zoniporide

Current studies explored the effect of structural changes on the aldehyde oxidase (AO)-mediated metabolism of zoniporide (1). Zoniporide analogs with modifications of the acylguanidine moiety, the cyclopropyl group on the pyrazole ring, and the quinoline ring were studied for their AO-catalyzed metabolism using the human S9 fraction. Analysis of the half-lives suggested that subtle changes in the structure of 1 influenced its metabolism and that the guanidine and the quinoline moieties were prerequisites for AO-catalyzed oxidation to 2-oxozoniporide (M1). In contrast, replacement of the cyclopropyl group with other alkyl groups was tolerated. The effect of structural variation on AO properties was rationalized by docking 1 and its analogs into the human AO homology model. These studies indicated the importance of electrostatic, π-π stacking and hydrophobic interactions of the three motifs with residues in the active site. Differences in substrate properties were also rationalized by comparing their half-lives with cLogD, electrophilicity parameters [electrostatic potential (ESP) charges and energy of lowest unoccupied molecular orbitals (E(LUMO))], and the energies of formation of tetrahedral intermediates (J Med Chem 50:4642-4647, 2007). Whereas the success of energetics in predicting the AO substrate properties of analogs was 87%, the predictive ability of other descriptors was none (cLogD) to 60% (ESP charges and E(LUMO)). Overall, the structure-metabolism relationship could be rationalized using a combination of both the energy calculations and docking studies. This combination method can be incorporated into a strategy for mitigating AO liabilities observed in the lead candidate or studying structure-metabolism relationships of other AO substrates.

Hydralazine as a selective probe inactivator of aldehyde oxidase in human hepatocytes: estimation of the contribution of aldehyde oxidase to metabolic clearance

Aldehyde oxidase (AO) metabolism could lead to significant underestimation of clearance in prediction of human pharmacokinetics as well as unanticipated exposure to AO-generated metabolites, if not accounted for early in drug research. We report a method using cryopreserved human hepatocytes and the time-dependent AO inhibitor hydralazine (K(I) = 83 ± 27 μM, k(inact) = 0.063 ± 0.007 min(-1)), which estimates the contribution of AO metabolism relative to total hepatic clearance. Using zaleplon as a probe substrate and simultaneously monitoring the AO-catalyzed formation of oxozaleplon and the CYP3A-catalyzed formation of desethyzaleplon in the presence of a range of hydralazine concentrations, it was determined that >90% inhibition of the AO activity with minimal effect on the CYP3A activity could be achieved with 25 to 50 μM hydralazine. This method was used to estimate the fraction metabolized due to AO [f(m(AO))] for six compounds with clearance attributed to AO along with four other drugs not metabolized by AO. The f(m(AO)) values for the AO substrates ranged between 0.49 and 0.83. Differences in estimated f(m(AO)) between two batches of pooled human hepatocytes suggest that sensitivity to hydralazine varies slightly with hepatocyte preparations. Substrates with a CYP2D6 contribution to clearance were affected by hydralazine to a minor extent, because of weak inhibition of this enzyme. Overall, these findings demonstrate that hydralazine, at a concentration of 25 to 50 μM, can be used in human hepatocyte incubations to estimate the contribution of AO to the hepatic clearance of drugs and other compounds.

Deuterium isotope effects on drug pharmacokinetics. I. System-dependent effects of specific deuteration with aldehyde oxidase cleared drugs

The pharmacokinetic properties of drugs may be altered by kinetic deuterium isotope effects. With specifically deuterated model substrates and drugs metabolized by aldehyde oxidase, we demonstrate how knowledge of the enzyme's reaction mechanism, species differences in the role played by other enzymes in a drug's metabolic clearance, and differences in systemic clearance mechanisms are critically important for the pharmacokinetic application of deuterium isotope effects. Ex vivo methods to project the in vivo outcome using deuterated carbazeran and zoniporide with hepatic systems demonstrate the importance of establishing the extent to which other metabolic enzymes contribute to the metabolic clearance mechanism. Differences in pharmacokinetic outcomes in guinea pig and rat, with the same metabolic clearance mechanism, show how species differences in the systemic clearance mechanism can affect the in vivo outcome. Overall, to gain from the application of deuteration as a strategy to alter drug pharmacokinetics, these studies demonstrate the importance of understanding the systemic clearance mechanism and knowing the identity of the metabolic enzymes involved, the extent to which they contribute to metabolic clearance, and the extent to which metabolism contributes to the systemic clearance.

The chemistry of peptidyltransferase center-targeted antibiotics: enzymatic resistance and approaches to countering resistance

The continued ability to treat bacterial infections requires effective antibiotics. The development of new therapeutics is guided by knowledge of the mechanisms of action of and resistance to these antibiotics. Continued efforts to understand and counteract antibiotic resistance mechanisms at a molecular level have the potential to direct development of new therapeutic strategies in addition to providing insight into the underlying biochemical functions impacted by antibiotics. The interaction of antibiotics with the peptidyltransferase center and adjacent exit tunnel within the bacterial ribosome is the predominant mechanism by which antibiotics impede translation, thus stalling growth. Resistance enzymes catalyze the chemical modification of the RNA that composes these functional regions, leading to diminished binding of antibiotics. This review discusses recent advances in the elucidation of chemical mechanisms underlying resistance and driving the development of new antibiotics.

Discovery of 4''-ether linked azithromycin-quinolone hybrid series: influence of the central linker on the antibacterial activity

A series of novel C-4''-substituted azithromycins was synthesized and evaluated for in vitro antibacterial activity against a panel of representative erythromycin-susceptible and macrolide-lincosamide-streptogramin (MLS) resistant pathogens. In summary, azithromycin and quinolone substructures merged in a mutually SAR-compatible design gave rise to a new class of antimicrobials with an improved spectrum and potency over azithromycin. Prototypical analogues 7f and 8f display an improved potency versus azithromycin against Gram-positive and fastidious Gram-negative pathogens. In particular, these new leads maintain activity against MLS-resistant strains of Streptococcus pneumoniae and Streptococcus pyogenes. In addition, they represent an improvement over telithromycin (1) and cethromycin (2) against the fastidious Gram-negative pathogen Haemophilus influenzae.

New macrolide, lincosaminide and streptogramin B antibiotics

IMPORTANCE OF THE FIELD

New antibiotics are needed to overcome microbial resistance and to improve on the therapeutic index and clinical effectiveness of existing agents.

AREA COVERED IN THIS REVIEW

This review covers the journal and patent literature published from about the mid-2000s to 2010 to provide an overview of the large diversity of new chemical entities in the macrolide, lincosaminide and streptogramin B (MLS(B)) class.

WHAT THE READER WILL GAIN

The review identifies areas of the greatest effort and recent results in pursuing structure-activity relationships among MLS(B) antibiotics and highlights preclinical and clinical candidates that have arisen from these diverse discovery programs.

TAKE HOME MESSAGE

Research on the MLS(B) class appears promising for the eventual registration and commercialization of several new antibiotics that improve the clinical effectiveness of existing agents and combat antibiotic-resistant pathogens.

Synthesis and structure-activity relationships of novel 8a-aza-8a-homoerythromycin A ketolides

A series of novel 6-O-substituted 8a-aza-8a-homoerythromycin A ketolides was synthesized and evaluated for in vitro antibacterial activity. Key strategic elements of the synthesis include the base-induced E-Z isomerization of 3-O-descladinosyl-6-O-allylerythromycin A 9(E)-oxime followed by ring-expanding reaction of the resulting 9(Z)-oxime via Beckmann rearrangement. The ketolides showed potent activity against a variety of erythromycin-susceptible and macrolide-lincosamide-streptogramin B (MLS(B)) resistant Gram-positive and fastidious Gram-negative pathogens. The best compounds in this series overcome all types of resistance in relevant clinical Gram-positive pathogens and display in vitro activity comparable to telithromycin and cethromycin.