Early Molecular Insights into Thanatin Analogues Binding to LptA.

The cationic antimicrobial ß-hairpin, thanatin, was recently developed into drug-like analogues active against carbapenem-resistant Enterobacteriaceae (CRE). The analogues represent new antibiotics with a novel mode of action targeting LptA in the periplasm and disrupting LPS transport. The compounds lose antimicrobial efficacy when the sequence identity to LptA falls below 70%. We wanted to test the thanatin analogues against LptA of a phylogenetic distant organism and investigate the molecular determinants of inactivity. () is a critical Gram-negative pathogen that has gained increasing attention for its multi-drug resistance and hospital burden. LptA shares 28% sequence identity with LptA and displays an intrinsic resistance to thanatin and thanatin analogues (MIC values > 32 µg/mL) through a mechanism not yet described. We investigated the inactivity further and discovered that these CRE-optimized derivatives can bind to LptA of in vitro, despite the high MIC values. Herein, we present a high-resolution structure of LptAm in complex with a thanatin derivative and binding affinities of selected thanatin derivatives. Together, these data offer structural insights into why thanatin derivatives are inactive against LptA, despite binding events in vitro.