Structure-Activity Relationship Studies of 3-epi-Deoxynegamycin Derivatives as Potent Readthrough Drug Candidates

(+)-Negamycin (1), a natural dipeptidic antibiotic bearing a hydrazide structure, exhibits a readthrough activity toward the nonsense mutation of the dystrophin gene and restores dystrophin expression in muscles of Duchenne muscular dystrophy model mdx mice. Herein to develop more potent readthrough compounds, we performed a structure-activity relationship (SAR) study of 3-epi-deoxynegamycin (2), which is also another natural readthrough compound with little antimicrobial activity, focusing on the main carbon chain length. We found that one carbon atom shorter derivative 9b shows a higher readthrough activity than 1 and 2. Further derivatization at the carboxylic acid part of 9b demonstrates that its meta-chlorobenzyl ester derivative 17e, which has a higher ClogP value, exhibits a more potent readthrough activity than 9b. Interestingly, in the cell-free protein expression system, the readthrough activity of 17e drastically decreases compared to that in the cell-based assay. These results suggest that benzyl ester-type derivatives enhance the hydrophobicity and function as prodrugs to produce active compound 9b in living cell systems.