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

Abstract 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).