Retargeting Gram-Positive-Only Adarotene-Derived Antibacterials to Broad-Spectrum Antibiotics

Background: Bacterial resistance to antibiotics continues to rise globally, posing a significant public health challenge and incurring substantial social and economic burdens. In response, the World Health Organization (WHO) has published a list of priority pathogens for which effective treatment options are critically limited. Several antibiotics are categorized as Gram-positive-only (GPO) agents due to their lack of activity against Gram-negative species. Although these compounds often target conserved bacterial processes, their limited spectrum is largely attributed to poor penetration of the Gram-negative outer membrane (OM). Results: In this study, we designed and synthesized a series of adarotene-derived compounds to evaluate the impact of introducing positively charged groups on their interaction with the Gram-negative OM. One of the newly synthesized derivatives, SPL 207, displayed minimum inhibitory concentration (MIC) values ranging from 8 to 64 µM against a panel of Gram-positive and Gram-negative bacteria. The ability of SPL207 to disrupt outer and inner membrane permeability was evaluated using fluorescence assays and confocal microscopy, revealing that the compound compromises membrane integrity across all tested Gram-negative bacteria. Strong synergistic activity was observed in combination with colistin against three P. aeruginosa colistin-resistant strains. Atomistic details of membrane interference were elucidated by molecular dynamics (MD) simulations, with SPL207 clearly acting as a membrane destabilizer by enhancing Ca²⁺ ions diffusion and lipids destabilization. Conclusions: Although the observed MIC values remain above clinically acceptable thresholds, these findings provide a promising proof of concept. The further structural optimization of adarotene derivatives may yield novel broad-spectrum agents with improved antimicrobial potency against MDR pathogens.