Structure–ATPase Activity Relationship of Rhodamine Derivatives as Potent Inhibitors of P‑Glycoprotein CmABCB1

Understanding the transport and inhibition mechanisms of substrates by P-glycoprotein (P-gp) is one of the important approaches in addressing multidrug resistance (MDR). In this study, we evaluated a variety of rhodamine derivatives as potential P-gp inhibitors targeting CmABCB1, a P-gp homologue, with a focus on their ATPase activity. Notably, a Q-rhodamine derivative with an o,o′-dimethoxybenzyl ester moiety (Rh_Q-DMB) demonstrated superior affinity and inhibitory activity, which was further confirmed by a drug susceptibility assay in yeast strains expressing CmABCB1. Results from a tryptophan fluorescence quenching experiment using a CmABCB1 mutant suggested that Rh_Q-DMB effectively enters and binds to the inner chamber of CmABCB1. These findings underscore the promising potential of Rh_Q-DMB as a tool for future studies aimed at elucidating the substrate-bound state of CmABCB1.