Structure–Activity Relationship and Crystallographic Study of New Monobactams

Monobactams, a subclass of β-lactam antibiotics with a monocyclic scaffold, are uniquely resistant to hydrolysis by metallo-β-lactamases, providing a distinct therapeutic advantage. Here, we report an <i>in silico</i>-based structure-activity relationship (SAR) investigation of aztreonam-related monobactams. A focused library of monobactam derivatives was synthesized and evaluated for inhibition of penicillin-binding proteins (PBPs) and antibacterial activity. Ten compounds, including aztreonam, were crystallized with truncated PBP1b from <i>Streptococcus pneumoniae</i>, used as a model PBP. Potent PBP1b inhibitors were developed, although high enzymatic potency was not always reflected in strong antibacterial activity. Certain derivatives showed activity against <i>Staphylococcus aureus</i>, which is typically resistant to monobactams. 2D similarity search identified potent inhibitors active against <i>Escherichia coli</i>, <i>Klebsiella pneumoniae</i>, and <i>Acinetobacter baumannii</i>. Crystal structures revealed previously unrecognized binding interactions, including a halogen bond with a conserved threonine residue, underscoring the potential of these interactions to support the development of more potent PBP inhibitors.