Synthesis of Human Milk Oligosaccharides-Fluoroquinolone Conjugates and Investigation of Their Antimicrobial Activity

The copper-catalyzed azide-alkyne cycloaddition reaction was applied to conjugate four human milk oligosaccharides (HMOs) with antimicrobial fluoroquinolones (FQs) (ciprofloxacin and sparfloxacin) to obtain a total of 12 per-O-acetylated and unprotected HMO-FQ conjugates. These compounds underwent antimicrobial testing against Gram-positive (Streptococcus agalactiae, Enterococcus faecalis, and Staphylococcus aureus) and Gram-negative (Escherichia coli and Pseudomonas aeruginosa) bacteria. The HMO-FQ conjugates exhibited the most significant antimicrobial activity against E. coli and S. aureus reaching minimum inhibitory concentration as low as 1.56 μM and 3.13 μM, respectively. A general trend was that per-O-acetylated HMO-FQ conjugates exhibited higher antimicrobial activity than their unprotected counterparts. The antimicrobial activity of the tested HMO-FQ conjugates is expected to arise from the ciprofloxacin and sparfloxacin moieties because per-O-acetylated and unprotected HMOs did not exhibit any antimicrobial activity. Additionally, the HMO-FQ conjugates displayed no cytotoxicity as evident from test on two cell lines (HepG2 and MRC5).