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

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.

Design, synthesis and structure-activity relationships of novel non-ketolides: 9-Oxime clarithromycin featured with seven-to thirteen-atom-length diamine linkers at 3-OH

We report here on the structure-activity relationships of hybrids combining 3-descladinosyl clarithromycin with quinolones linked by extended diamine connectors. Several hybrids, exemplified by 23Bc, 23Be, 23Bf, 26Be, and 30Bc, not only restored potency against inducibly resistant pathogens but also exhibited significantly enhanced activities against constitutively resistant strains of Staphylococcus pneumoniae and Staphylococcus pyogenes, which express high-level resistance independent of clarithromycin or erythromycin induction. Additionally, the novel hybrids showed susceptibility against Gram-negative Haemophilus influenzae. Notably, hybrid 23Be demonstrated dual modes of action by inhibiting both protein synthesis and DNA replication in vitro and in vivo. Given these promising characteristics, 23Be emerges as a potential candidate for the treatment of community-acquired bacterial pneumonia.

Synthesis and antibacterial medicinal evaluation of carbothioamido hydrazonyl thiazolylquinolone with multitargeting antimicrobial potential to combat increasingly global resistance

The global microbial resistance is a serious threat to human health, and multitargeting compounds are considered to be promising to combat microbial resistance. In this work, a series of new thiazolylquinolones with multitargeting antimicrobial potential were developed through multi-step reactions using triethoxymethane and substituted anilines as start materials. Their structures were confirmed by 1H NMR, 13C NMR and HRMS spectra. Antimicrobial evaluation revealed that some of the target compounds could effectively inhibit microbial growth. Especially, carbothioamido hydrazonyl aminothiazolyl quinolone 8a showed strong inhibitory activity toward drug-resistant Staphylococcus aureus with MIC value of 0.0047 mM, which was 5-fold more active than that of norfloxacin. The highly active compound 8a exhibited negligible hemolysis, no significant toxicity in vitro and in vivo, low drug resistance, as well as rapidly bactericidal effects, which suggested its favorable druggability. Furthermore, compound 8a was able to effectively disrupt the integrity of the bacterial membrane, intercalate into DNA and inhibit the activity of topoisomerase IV, suggesting multitargeting mechanism of action. Compound 8a could form hydrogen bonds and hydrophobic interactions with DNA-topoisomerase IV complex, indicating the insertion of aminothiazolyl moiety was beneficial to improve antibacterial efficiency. These findings indicated that the active carbothioamido hydrazonyl aminothiazolyl quinolone 8a as a chemical therapeutic candidate demonstrated immense potential to tackle drug-resistant bacterial infections.

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.

Quinoline derivatives volunteering against antimicrobial resistance: rational approaches, design strategies, structure activity relationship and mechanistic insights

Emergence of antimicrobial resistance has become a great threat to human species as there is shortage of development of new antimicrobial agents. So, its mandatary to combat AMR by initiating research and developing new novel antimicrobial agents. Among phytoconstituents, Quinoline (nitrogen containing heterocyclic) have played a wide role in providing new bioactive molecules. So, this review provides rational approaches, design strategies, structure activity relationship and mechanistic insights of newly developed quinoline derivatives as antimicrobial agents.

Current scenario of quinolone hybrids with potential antibacterial activity against ESKAPE pathogens

The ESKAPE (Escherichia coli/E. coli, Staphylococcus aureus/S. aureus, Klebsiella pneumonia/K. pneumoniae, Acinetobacter Baumannii/A. baumannii, Pseudomonas aeroginosa/P. aeroginosa and Enterobacter spp.) pathogens, which could escape or evade common therapies through diverse antimicrobial resistance mechanisms and biofilm formation, are deemed as highly virulent bacteria responsible for life-threatening diseases, calling for novel chemotherapeutics. Quinolones including 2-quinolones and 4-quinolones have occupied a propitious place in drug design and development due to their excellent pharmacological profiles. Quinolones especially fluoroquinolones could inhibit the synthesis of nucleic acid of ESKAPE pathogens, leading to the rupture of bacterial chromosome. However, the resistance of ESKAPE pathogens to quinolones develops rapidly and spreads widely. Accordingly, it has become increasingly urgent to enhance the potency of quinolones against both drug-susceptible and drug-resistant ESKAPE pathogens. Quinolone hybrids can bind with different drug targets simultaneously and have been considered as useful prototypes to circumvent drug resistance. The purpose of this review is to summarize the current scenario (2018-present) of quinolone hybrids with potential antibacterial activity against ESKAPE pathogens, together with the structure-activity relationships and mechanisms of action to facilitate further rational design of more effective candidates.

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.

The search for novel treatment strategies for Streptococcus pneumoniae infections

This review provides an overview of the most important novel treatment strategies against Streptococcus pneumoniae infections published over the past 10 years. The pneumococcus causes the majority of community-acquired bacterial pneumonia cases, and it is one of the prime pathogens in bacterial meningitis. Over the last 10 years, extensive research has been conducted to prevent severe pneumococcal infections, with a major focus on (i) boosting the host immune system and (ii) discovering novel antibacterials. Boosting the immune system can be done in two ways, either by actively modulating host immunity, mostly through administration of selective antibodies, or by interfering with pneumococcal virulence factors, thereby supporting the host immune system to effectively overcome an infection. While several of such experimental therapies are promising, few have evolved to clinical trials. The discovery of novel antibacterials is hampered by the high research and development costs versus the relatively low revenues for the pharmaceutical industry. Nevertheless, novel enzymatic assays and target-based drug design, allow the identification of targets and the development of novel molecules to effectively treat this life-threatening pathogen.

Actinomycete-Derived Polyketides as a Source of Antibiotics and Lead Structures for the Development of New Antimicrobial Drugs

Actinomycetes are remarkable producers of compounds essential for human and veterinary medicine as well as for agriculture. The genomes of those microorganisms possess several sets of genes (biosynthetic gene cluster (BGC)) encoding pathways for the production of the valuable secondary metabolites. A significant proportion of the identified BGCs in actinomycetes encode pathways for the biosynthesis of polyketide compounds, nonribosomal peptides, or hybrid products resulting from the combination of both polyketide synthases (PKSs) and nonribosomal peptide synthetases (NRPSs). The potency of these molecules, in terms of bioactivity, was recognized in the 1940s, and started the "Golden Age" of antimicrobial drug discovery. Since then, several valuable polyketide drugs, such as erythromycin A, tylosin, monensin A, rifamycin, tetracyclines, amphotericin B, and many others were isolated from actinomycetes. This review covers the most relevant actinomycetes-derived polyketide drugs with antimicrobial activity, including anti-fungal agents. We provide an overview of the source of the compounds, structure of the molecules, the biosynthetic principle, bioactivity and mechanisms of action, and the current stage of development. This review emphasizes the importance of actinomycetes-derived antimicrobial polyketides and should serve as a "lexicon", not only to scientists from the Natural Products field, but also to clinicians and others interested in this topic.

Recent advances in the synthetic and medicinal perspective of quinolones: A review

In the modern scenario, the quinolone scaffold has emerged as a very potent motif considering its clinical significance. Quinolones possess wide range of pharmacological activities such as anticancer, antibacterial, antifungal, antiprotozoal, antiviral, anti-inflammatory, carbonic anhydrase inhibitory and diuretic activity etc. The versatile synthetic approaches have been successfully applied and several of the resulted synthesized compounds exhibit fascinating biological activities in numerous fields. This has prompted to discover quinolone-based analogues among the researchers due to its great diversity in biological activities. In the past few years, various new, efficient and convenient synthetic approaches (including green chemistry and microwave-assisted synthesis) have been designed and developed to synthesize diverse quinolone-based scaffolds which represent a growing area of interest in academic and industry as well as to explore their biological activities. In this review, an attempt has been made by the authors to summarize (1) One of the most comprehensive listings of quinolone-based drugs or agents in the market or under various stages of clinical development; (2) Recent advances in the synthetic strategies for quinolone derivatives as well as their biological implications including insight of mechanistic studies. (3) Further, the biological data is correlated with structure-activity relationship studies to provide an insight into the rational design of more active agents.