Novel hybrids of 15-membered 8a- and 9a-azahomoerythromycin A ketolides and quinolones as potent antibacterials

Macrolones target bacterial ribosomes and DNA gyrase and can evade resistance mechanisms

Growing resistance toward ribosome-targeting macrolide antibiotics has limited their clinical utility and urged the search for superior compounds. Macrolones are synthetic macrolide derivatives with a quinolone side chain, structurally similar to DNA topoisomerase-targeting fluoroquinolones. While macrolones show enhanced activity, their modes of action have remained unknown. Here, we present the first structures of ribosome-bound macrolones, showing that the macrolide part occupies the macrolide-binding site in the ribosomal exit tunnel, whereas the quinolone moiety establishes new interactions with the tunnel. Macrolones efficiently inhibit both the ribosome and DNA topoisomerase in vitro. However, in the cell, they target either the ribosome or DNA gyrase or concurrently both of them. In contrast to macrolide or fluoroquinolone antibiotics alone, dual-targeting macrolones are less prone to select resistant bacteria carrying target-site mutations or to activate inducible macrolide resistance genes. Furthermore, because some macrolones engage Erm-modified ribosomes, they retain activity even against strains with constitutive erm resistance genes.

Synthetic macrolides overcoming MLSBK-resistant pathogens

Conventional macrolide-lincosamide-streptogramin B-ketolide (MLSBK) antibiotics are unable to counter the growing challenge of antibiotic resistance that is conferred by the constitutive methylation of rRNA base A2058 or its G2058 mutation, while the presence of unmodified A2058 is crucial for high selectivity of traditional MLSBK in targeting pathogens over human cells. The absence of effective modes of action reinforces the prevailing belief that constitutively antibiotic-resistant Staphylococcus aureus remains impervious to existing macrolides including telithromycin. Here, we report the design and synthesis of a novel series of macrolides, featuring the strategic fusion of ketolide and quinolone moieties. Our effort led to the discovery of two potent compounds, MCX-219 and MCX-190, demonstrating enhanced antibacterial efficacy against a broad spectrum of formidable pathogens, including A2058-methylated Staphylococcus aureus, Streptococcus pneumoniae, Streptococcus pyogenes, and notably, the clinical Mycoplasma pneumoniae isolates harboring A2058G mutations which are implicated in the recent pneumonia outbreak in China. Mechanistic studies reveal that the modified quinolone moiety of MCX-190 establishes a distinctive secondary binding site within the nascent peptide exit tunnel. Structure-activity relationship analysis underscores the importance of this secondary binding, maintained by a sandwich-like π-π stacking interaction and a water-magnesium bridge, for effective engagement with A2058-methylated ribosomes rather than topoisomerases targeted by quinolone antibiotics. Our findings not only highlight MCX-219 and MCX-190 as promising candidates for next-generation MLSBK antibiotics to combat antibiotic resistance, but also pave the way for the future rational design of the class of MLSBK antibiotics, offering a strategic framework to overcome the challenges posed by escalating antibiotic resistance.

Design and synthesis of novel macrolones bridged with linkers from 11,12-positions of macrolides

Resistance to telithromycin and off-target effects associated with the metabolic instability present serious and challenging problems for the development of novel macrolides. Herein, studies of hybrids of macrolides and quinolones (termed macrolones) bridged with linkers from 11,12-cyclic carbamate of macrolides revealed different structure-activity relationships from the previously reported macrolones bridged with linkers derived from 6-, 9- and 4''-positions of macrolides. The optimized macrolone 34 g with a longer and rigid sidechain than telithromycin had improved metabolic stability compared to telithromycin (t_1/2: 110 vs 32 min), whose future has been heavily clouded by metabolic issues. Moreover, 34 g was 38-fold more potent than telithromycin against A2058/2059-mutated Mycoplasma pneumoniae (8 vs 315 μM), which may be attributed to a novel mode of action between the carboxylic acid of quinolone moiety and the bacterial ribosome. This work increases the prospect for discovery of novel and safe antibacterial agents to combat serious human infectious diseases.

Dissecting erm(41)-Mediated Macrolide-Inducible Resistance in Mycobacterium abscessus

Macrolides are the cornerstone of Mycobacterium abscessus multidrug therapy, despite that most patients respond poorly to this class of antibiotics due to the inducible resistance phenotype that occurs during drug treatment. This mechanism is driven by the macrolide-inducible ribosomal methylase encoded by erm(41), whose expression is activated by the transcriptional regulator WhiB7. However, it has been debated whether clarithromycin and azithromycin differ in the extent to which they induce erm(41)-mediated macrolide resistance. Herein, we show that macrolide resistance is induced more rapidly in various M. abscessus isolates upon exposure to azithromycin than to clarithromycin, based on MIC determination. Macrolide-induced expression of erm(41) was assessed in vivo using a strain carrying tdTomato placed under the control of the erm(41) promoter. Visualization of fluorescent bacilli in infected zebrafish demonstrates that azithromycin and clarithromycin activate erm(41) expression in vivo That azithromycin induces a more rapid expression of erm(41) was confirmed by measuring the β-galactosidase activity of a reporter strain in which lacZ was placed under the control of the erm(41) promoter. Shortening the promoter region in the lacZ reporter plasmid identified DNA elements involved in the regulation of erm(41) expression, particularly an AT-rich motif sharing partial conservation with the WhiB7-binding site. Mutation of this motif abrogated the macrolide-induced and WhiB7-dependent expression of erm(41). This study provides new mechanistic information on the adaptive response to macrolide treatment in M. abscessus.

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.

Ciprofloxacin derivatives and their antibacterial activities

Bacterial infections represent a significant health threat globally, and are responsible for the majority of hospital-acquired infections, leading to extensive mortality and burden on global healthcare systems. The second generation fluoroquinolone ciprofloxacin which exhibits excellent antimicrobial activity and pharmacokinetic properties as well as few side effects is introduced into clinical practice for the treatment of various bacterial infections for around 3 decades. The emergency and widely spread of drug-resistant pathogens making ciprofloxacin more and more ineffective, so it's imperative to develop novel antibacterials. Numerous of ciprofloxacin derivatives have been synthesized for seeking for new antibacterials, and some of them exhibited promising potency. This review aims to summarize the recent advances made towards the discovery of ciprofloxacin derivatives as antibacterial agents and the structure-activity relationship of these derivatives was also discussed.

4-Quinolone derivatives and their activities against Gram positive pathogens

Gram-positive bacteria are responsible for a broad range of infectious diseases, and the emergency and wide spread of drug-resistant Gram-positive pathogens including MRSA and MRSE has caused great concern throughout the world. 4-Quinolones which are exemplified by fluoroquinolones are mainstays of chemotherapy against various bacterial infections including Gram-positive pathogen infections, and their value and role in the treatment of bacterial infections continues to expand. However, the resistance of Gram-positive organisms to 4-quinolones develops rapidly and spreads widely, making them more and more ineffective. To overcome the resistance and reduce the toxicity, numerous of 4-quinolone derivatives were synthesized and screened for their in vitro and in vivo activities against Gram-positive pathogens, and some of them exhibited excellent potency. This review aims to outlines the recent advances made towards the discovery of 4-quinolone-based derivatives as anti-Gram-positive pathogens agents and the critical aspects of design as well as the structure-activity relationship of these derivatives. The enriched SAR paves the way to the further rational development of 4-quinolones with a unique mechanism of action different from that of the currently used drugs to overcome the resistance, well-tolerated and low toxic profiles.

Synthesis and antibacterial evaluation of novel 11-O-carbamoyl clarithromycin ketolides

A series of novel 11-O-carbamoyl clarithromycin ketolides were designed, synthesized and evaluated for their in vitro antibacterial activity. The results showed that the majority of the target compounds displayed improved activity compared with references against erythromycin-resistant S. pneumoniae A22072 expressing the mef gene, S. pneumoniae B1 expressing the erm gene and S. pneumoniae AB11 expressing the mef and erm genes. In particular, compounds 9, 18, 19 and 22 showed the most potent activity against erythromycin-resistant S. pneumoniae A22072 with the MIC values of 0.5μg/mL. Furthermore, compounds 11, 18, 19, 24 and 29 were also found to exhibit favorable antibacterial activity against erythromycin-susceptible S. pyogenes with the MIC values of 0.125-1μg/mL, and moderate activity against erythromycin-susceptible S. aureus ATCC25923 and B. subtilis ATCC9372.

Synthesis and antibacterial activity of novel 3-O-descladinosylazithromycin derivatives

Novel series of novel 3-O-arylalkylcarbamoyl descladinosylazithromycin derivatives with the 2'-O-acetyl and 11,12-cyclic carbonate groups, the 11,12-cyclic carbonate group and the 11-O-arylalkylcarbamoyl side chain, and 2'-O-arylalkylcarbamoyl descladinosylazithromycin with the 11,12-cyclic carbonate group were designed, synthesized and evaluated for their antibacterial activity using broth microdilution method. The results showed that the majority of the target compounds showed moderate to favorable activity against six kinds of susceptible strains and almost all of them displayed significantly improved activity compared with references against three erythromycin-resistant strains of S. pneumoniae B1 expressing the ermB gene, S. pneumoniae AB11 expressing the ermB and mefA genes, and S. pyogenes R1. In particular, compound 6h exhibited the most potent activity against susceptible B. subtilis ATCC9372 (0.5 μg/mL), penicillin-resistant S. epidermidis (0.125 μg/mL), and erythromycin-resistant S. pneumoniae B1 (1 μg/mL) and S. pneumoniae AB11 (1 μg/mL), which were 2-, 2-, 256-, 256-fold better activity than azithromycin, respectively. Additionally, compounds 6f (0.5 μg/mL) and 6g (0.25 μg/mL) were the most active against S. pneumoniae A22072, which were 8- and 16-fold better activity than azithromycin (4 μg/mL). As for erythromycin-resistant S. pyogenes R1, compound 5a presented the most excellent activity (8 μg/mL), showing 32- and 32-fold higher activity than azithromycin (256 μg/mL) and clarithromycin (256 μg/mL).

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.

The wobbly status of ketolides: where do we stand?

Ketolides are erythromycin A derivatives with a keto group replacing the cladinose sugar and an aryl-alkyl group attached to the lactone macrocycle. The aryl-alkyl extension broadens its antibacterial spectrum to include all pathogens responsible for community-acquired pneumonia (CAP): Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis as well as atypical pathogens (Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella pneumophila). Ketolides have extensive tissue distribution, favorable pharmacokinetics (oral, once-a-day) and useful anti-inflammatory/immunomodulatory properties. Hence, they were considered attractive additions to established oral antibacterials (quinolones, β-lactams, second-generation macrolides) for mild-to-moderate CAP. The first ketolide to be approved, Sanofi-Aventis' telithromycin (RU 66647, HMR 3647, Ketek®), had tainted clinical development, controversial FDA approval and subsequent restrictions due to rare, irreversible hepatotoxicity that included deaths. Three additional ketolides progressed to non-inferiority clinical trials vis-à-vis clarithromycin for CAP. Abbott's cethromycin (ABT-773), acquired by Polymedix and subsequently by Advanced Life Sciences, completed Phase III trials, but its New Drug Application was denied by the FDA in 2009. Enanta's modithromycin (EDP-420), originally codeveloped with Shionogi (S-013420) and subsequently by Shionogi alone, is currently in Phase II in Japan. Optimer's solithromycin (OP-1068), acquired by Cempra (CEM-101), is currently in Phase III. Until this hepatotoxicity issue is resolved, ketolides are unlikely to replace established antibacterials for CAP, or lipoglycopeptides and oxazolidinones for gram-positive infections.

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.