Zosurabalpin and the Revival of Pathogen-Specific Antibiotics: Targeting Lipopolysaccharide Transport in Carbapenem-Resistant Acinetobacter baumannii

Carbapenem-resistant Acinetobacter baumannii (CRAB) is classified by the World Health Organization among the critical-priority pathogens, reflecting high mortality, near-ubiquitous nosocomial spread, and a depleted therapeutic pipeline. No antibiotic chemical class with a genuinely novel target and activity against A. baumannii reached patients for more than fifty years, and current options—sulbactam–durlobactam, cefiderocol, polymyxins, high-dose ampicillin–sulbactam and tetracyclines—are constrained by toxicity, inconsistent efficacy, or emerging resistance. Against this background, zosurabalpin (RG6006), the first member of the tethered macrocyclic peptide (MCP) class, represents a potentially important conceptual advance. Identified through whole-cell phenotypic screening of nearly 45,000 macrocyclic peptides and optimized into a zwitterionic clinical candidate, zosurabalpin inhibits the LptB2FGC complex, the inner-membrane ATP-binding-cassette transporter that initiates lipopolysaccharide (LPS) export. By trapping LPS within the transporter, the drug causes lethal accumulation of the molecule at the inner membrane. This mechanism is structurally distinct from that of every clinically used antibiotic and, in preclinical studies, is not affected by the major currently recognized CRAB resistance mechanisms. Zosurabalpin shows potent, narrow-spectrum activity essentially restricted to the Acinetobacter baumannii–calcoaceticus complex, retaining activity in vitro against isolates resistant to cefiderocol and last-line agents; its clinical efficacy in patients, however, remains to be established. Here we review the discovery, structural mechanism, microbiological spectrum, and clinical development of zosurabalpin, and we situate it within a broader revival of pathogen-specific (narrow-spectrum) antibiotic development, discussing the diagnostic, stewardship, and economic implications of this paradigm.