Defining a lipophilicity window for the antimicrobial activity of 7-Alkoxy-3-Amino coumarin amphiphiles

The continued rise of antimicrobial resistance (AMR) demands the research and development of novel antimicrobial agents that act via mechanisms less prone to resistance. Cationic antimicrobial peptides (CAMPs) are naturally occurring molecules that exhibit potent antimicrobial activity but have not encountered significant AMR. Synthetic mimetics of CAMPs offer a cost-effective route to new antimicrobials. We have previously reported a series of amphiphilic coumarin derivatives that elicit potent activity against several pathogenic bacterial strains, including planktonic methicillin-resistant Staphylococcus aureus (MRSA) and MRSA biofilms. Although compound lipophilicity plays a significant role in antimicrobial activity, this relationship needs to be accurately defined for each class of compound to facilitate the design of improved therapeutic agents. In this study, six cationic coumarin amphiphiles were designed with varying lipophilic character. These compounds were synthesised, characterised, and evaluated against a panel of clinically relevant pathogenic bacteria (Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and MRSA) and fungi (Candida albicans and Cryptococcus neoformans), to establish a lipophilic range that correlates to potent activity for this family of compounds. Our results suggest that when these compounds exist in their cationic state, a cLogP range of 0.75-1.82 correlates with best activity, with minimum inhibitory concentration (MIC) values as low as 0.02 µg/mL against C. neoformans and 1 µg/mL against MRSA obtained for derivatives that fall within this lipophilicity window.