Polyene antifungal amphotericin B (AmB) has been used for over 60 years, and remains a valuable clinical treatment for systemic mycoses, due to its broad antifungal activity and low rate of emerging resistance. There is no consensus on how exactly it kills fungal cells but it is certain that AmB and the closely-related nystatin (Nys) can form pores in membranes and have a higher affinity towards ergosterol than cholesterol. Notably, the high nephro- and hemolytic toxicity of polyenes and their low solubility in water have led to efforts to improve their properties. We present the synthesis of new amphotericin and nystatin amides and a comparative study of the effects of identical modifications of AmB and Nys on the relationship between their structure and properties. Generally, increases in the activity/toxicity ratio were in good agreement with increasing ratios of selective permeabilization of ergosterol- vs. cholesterol-containing membranes. We also show that the introduced modifications had an effect on the sensitivity of mutant yeast strains with alterations in ergosterol biosynthesis to the studied polyenes, suggesting a varying affinity towards intermediate ergosterol precursors. Three new water-soluble nystatin derivatives showed a prominent improvement in safety and were selected as promising candidates for drug development.

The World Health Organization (WHO) reports that tuberculosis (TB) is one of the top 10 leading causes of global mortality. The increasing incidence of multidrug-resistant TB highlights the urgent need for an intensified quest to discover innovative anti-TB medications In this study, we investigated four new derivatives from the quinoxaline-2-carboxylic acid 1,4-dioxide class. New 3-methylquinoxaline 1,4-dioxides with a variation in substituents at positions 2 and 6(7) were synthesized via nucleophilic aromatic substitution with amines and assessed against a Mycobacteria spp. Compound 4 showed high antimycobacterial activity (1.25 μg/mL against M. tuberculosis) and low toxicity in vivo in mice. Selection and whole-genomic sequencing of spontaneous drug-resistant M. smegmatis mutants revealed a high number of single-nucleotide polymorphisms, confirming the predicted mode of action of the quinoxaline-2-carboxylic acid 1,4-dioxide 4 as a DNA-damaging agent. Subsequent reverse genetics methods confirmed that mutations in the genes MSMEG_4646, MSMEG_5122, and MSMEG_1380 mediate resistance to these compounds. Overall, the derivatives of quinoxaline-2-carboxylic acid 1,4-dioxide present a promising scaffold for the development of innovative antimycobacterial drugs.

Objectives: Aminoglycosides are one of the first classes of natural antibiotics which have not lost relevance due to their broad spectrum of action against Gram-positive, Gram-negative bacteria and mycobacteria. The high growth rate of antimicrobial resistance (AMR) together with the severe side effects of aminoglycosides increase the importance of developing improved semisynthetic derivatives. Methods: In this work, we proposed a synthetic route to new tobramycin derivatives modified at the 6″-position with aminoalkylamine or guanidinoalkylamine residues. Results: The antibacterial activity of the new compounds against reference strains of microorganisms was comparable to the parental tobramycin. In striking contrast to tobramycin (resistance index, >256), its 6″-modified derivatives were significantly more potent against resistant clinical isolates of P. aeruginosa strains (resistance index = 4–16) and they demonstrated a promising AMR circumvention in E. coli strains associated with mutations in the fusA gene encoding elongation factor G. All the obtained tobramycin derivatives exhibited reduced cytotoxicity for the eukaryotic HEK293T cells compared to the tobramycin and thereby they potentially may have improved therapeutic index. The proposed modification of the 6″-position of tobramycin does not change the mechanism of aminoglycoside’s antibacterial activity: new compounds induced translation errors which resulted in the inhibition of protein synthesis in bacterial cells. Conclusions: Taken together, we can suggest that further modifications of the 6″-position of tobramycin may be beneficial for circumvention of AMR to aminoglycosides or used for conjugation with other molecules of interest.

The sustained rise of antimicrobial resistance (AMR) causes a strong need to develop new antibacterial agents. One of the methods for addressing the problem of antibiotic resistance is through the design of hybrid antibiotics. In this work, we proposed a synthetic route for the conjugation of an azithromycin derivative with chloramphenicol and metronidazole hemisuccinates and synthesized two series of new hybrid molecules 4a–g and 5a–g. While a conjugation did not result in tangible synergy for wild-type bacterial strains, new compounds were able to overcome AMR associated with the inducible expression of the ermC gene on a model E. coli strain resistant to macrolide antibiotics. The newly developed hybrids demonstrated a tendency to induce premature ribosome stalling, which might be crucial since they will not induce a macrolide-resistant phenotype in a number of pathogenic bacterial strains. In summary, the designed structures are considered as a promising direction for the further development of hybrid molecules that can effectively circumvent AMR mechanisms to macrolide antibiotics.