Antimicrobial resistance (AMR) has been declared as being one of the top 10 global public health threats against humanity. By 2050 estimates are that antibiotic infections could claim upwards of 10 million lives annually and impose a cumulative $100 trillion global economic burden. The list of bacteria that are becoming resistant to treatment with current antibiotics continues to increase, and very few new drugs are in the pipeline creating a strong need for new classes of drugs to combat and prevent public health crises.
“We took a creative, double-pronged strategy to develop new molecules that can kill difficult-to-treat infections while enhancing the natural host immune response,” said Farokh Dotiwala, M.B.B.S., Ph.D., assistant professor in the Vaccine & Immunotherapy Center and lead author of the effort to identify a new generation of antimicrobials named dual-acting immuno-antibiotics (DAIAs).
Antibiotics typically target essential bacterial functions which include nucleic acid and protein synthesis, the building of the cell membrane, and metabolic pathways. Unfortunately, bacteria can be crafty and acquire drug resistance by mutating the bacterial target that the antibiotic is directed against, inactivating the drugs, or pumping them out.
“We reasoned that harnessing the immune system to simultaneously attack bacteria on two different fronts makes it hard for them to develop resistance,” said Dotiwala.
The scientists focused on a metabolic pathway essential for most bacteria but absent in humans making it ideal for antibiotic development. This pathway is the methyl-D-erythritol phosphate (MEP) or non-mevalonate pathway, which is responsible for the biosynthesis of isoprenoids — molecules required for cell survival in most pathogenic bacteria. The lab targeted the IspH enzyme, an essential enzyme in isoprenoid biosynthesis, as a way to block this pathway and kill the microbes. Given the broad presence of IspH in the bacterial world, this approach may target a wide range of bacteria.
Using computer modeling the scientists were able to screen several million commercially available compounds for their ability to bind with the enzyme and selected the most potent ones that inhibited IspH function as starting points for drug discovery.
Since previously available IspH inhibitors could not penetrate the bacterial cell wall, Dotiwala collaborated with Wistar’s medicinal chemist Joseph Salvino, Ph.D., professor in The Wistar Institute Cancer Center and a co-senior author on the study, to identify and synthesize novel IspH inhibitor molecules that were able to get inside the bacteria.
The scientists were able to demonstrate that the IspH inhibitors stimulated the immune system with more potent bacterial killing activity and specificity than current best-in-class antibiotics when tested in vitro on clinical isolates of antibiotic-resistant bacteria, including a wide range of pathogenic gram-negative and gram-positive bacteria. In preclinical models of gram-negative bacterial infection, the bactericidal effects of the IspH inhibitors outperformed traditional pan antibiotics. All compounds tested were shown to be non-toxic to human cells.
“Immune activation represents the second line of attack of the DAIA strategy,” said Kumar Singh, Ph.D., Dotiwala lab postdoctoral fellow and first author of the study.
The compounds tested were also shown to be nontoxic on human cells as well as acting specifically on IspH. “Our DAIA prodrugs are bacteria-permeable and are more effective against several species of multidrug-resistant bacteria than the current best-in-class antibiotics,” wrote the scientists.
“Unlike antibiotics derived from natural sources, no IspH inhibitors have been discovered in microorganisms, so it is less likely that resistance mechanisms—such as β-lactamases and macrolide esterases in the case of β-lactam and macrolide antibiotics—have evolved specifically against our prodrugs,” they wrote. “The family of antibiotics and the antimicrobial strategy that we report here synergize direct antibiotic action with rapid immune response … This dual mechanism of action, an inherent feature of these compounds, could delay the emergence of drug resistance.”
“We believe this innovative DAIA strategy may represent a potential landmark in the world’s fight against AMR, creating a synergy between the direct killing ability of antibiotics and the natural power of the immune system,” echoed Dotiwala.
