Structure−Activity Relationships among Inhibitors of <i>Acinetobacter baumannii</i> and <i>Klebsiella pneumoniae</i> 1-Deoxy- <scp>d</scp> -xylulose 5-Phosphate Reductoisomerase (DXR/IspC) − A Promising Target for Antibiotic Development

Antimicrobial resistance (AMR) is a critical global health crisis, responsible for nearly 5 million deaths in 2019 and projected to impose up to $100 trillion in economic costs by 2050. To address this threat, we are developing new antibiotics targeting the methylerythritol phosphate (MEP) pathway in <i>Acinetobacter baumannii</i> and <i>Klebsiella pneumoniae</i>, major nosocomial pathogens. Our efforts focus on 1-deoxy-d-xylulose 5-phosphate reductoisomerase (IspC/DXR), the committed enzyme in MEP-mediated isoprenoid biosynthesis and absent in humans, making it an attractive therapeutic target. Natural products such as fosmidomycin (FOS) and FR900098 (FR) inhibit IspC but exhibit poor bioavailability. To improve drug-like properties, we synthesized FOS analogs and analyzed their structure-activity relationships (SARs) against recombinant Ab- and Kp-IspC, alongside in vitro susceptibility assays. The most potent analogs, α,β-unsaturated compounds <b>4b</b> and <b>16b</b>, inhibited Ab-IspC (AbIspC) with IC₅₀ values of 0.047 μM and 0.029 μM, and Kp-IspC (KpIspC) with 0.194 μM and 0.054 μM, respectively. These showed MICs of 64 μg/mL for <i>A. baumannii</i> and 512 μg/mL for <i>K. pneumoniae</i>. A prodrug series (<b>1d</b>-<b>8d</b>) demonstrated enhanced activity, with compound <b>3d</b> exhibiting MICs from 0.25 to 32 μg/mL against <i>A. baumannii</i>. X-ray crystallography of Ab-IspC with selected analogs (<b>2b</b>, <b>3b</b>, <b>4b</b>) at 2.0 Å resolution provides structural insights to guide future IspC inhibitor optimization.