The synthesis of niclosamide derivative as a novel colistin adjuvant for reversing colistin resistance and its antibacterial activity with colistin in vitro.

Colistin (COL) serves as a last-line therapeutic agent against multidrug-resistant gram-negative bacteria (GNB) infections. Nevertheless, widespread clinical use of COL has led to severe bacterial resistance. The anthelmintic drug niclosamide (NIC) has been shown to enhance the activity of COL in combination therapy. Despite this synergistic advantage, the two compounds possess distinct physicochemical and pharmacokinetic properties. To enable simultaneous delivery of both drugs to the target site and maximize their synergistic antibacterial activity, we propose the synthesis of a prodrug conjugate linking COL and NIC. As a preparatory step toward a future COL-NIC conjugate, we synthesized and characterized the carboxymethylated derivative of NIC (Axilisuan [AXLS]) to reduce cytotoxicity, improve aqueous solubility, and maintain the NIC synergistic antibacterial effect with COL. AXLS was successfully synthesized and characterized by high-resolution mass spectrometry and proton nuclear magnetic resonance spectroscopy. Solubility studies showed that AXLS exhibited approximately 57-fold higher aqueous solubility than NIC. Both checkerboard assays and time-kill curve analyses indicated that AXLS displayed COL-enhancing activity comparable to that of NIC. Importantly, AXLS showed lower cytotoxicity than NIC, both alone and in combination with COL. Collectively, these findings provide crucial insights for the future development of synthetic strategies toward novel COL-NIC conjugates and offer a rational design framework for the chemical modification of salicylanilide-based antibacterial adjuvants against colistin-resistant GNB.IMPORTANCEColistin remains one of the few treatment options for infections caused by multidrug-resistant gram-negative bacteria, but increasing colistin resistance limits its clinical use. Niclosamide can potentiate colistin activity, but its poor aqueous solubility and safety liabilities limit further development. Here, we synthesized a carboxymethylated niclosamide derivative (Axilisuan) and found that it retained colistin-potentiating activity while showing markedly improved aqueous solubility and reduced in vitro toxicity compared with niclosamide. These findings support phenolic hydroxyl modification as a practical strategy for improving salicylanilide-based antibacterial adjuvants and provide a tractable intermediate for future development of colistin-niclosamide conjugates.