“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, MBBS, Ph.D., assistant professor in the Vaccine & Immunotherapy Center and lead author of the team’s work, which is reported in Nature, in a paper titled, “IspH inhibitors kill Gram-negative bacteria and mobilize immune clearance.”
The list of bacteria that are becoming resistant to treatment with antibiotics is increasing, and there are few new drugs in the pipeline creating a dire need for new classes of drugs to prevent public health crises. AMR has been declared one of the top 10 global public health threats against all of humanity, and it is estimated that by 2050 antibiotic-resistant infections could claim the lives of 10 million every year and impose a cumulative $100 trillion burden on the global economy.
Currently, antibiotics target essential bacterial functions like nucleic acid, protein synthesis, building cell membranes, and metabolic pathways. But bacteria can acquire drug resistance by mutating the bacteria 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.
“We focus on the methyl-d-erythritol phosphate (MEP) pathway for isoprenoid biosynthesis, which is essential for the survival of most Gram-negative bacteria and apicomplexans (malaria parasites) but is absent in humans and other metazoans,” the authors write.
MEP or non-mevalonate pathway is responsible for the biosynthesis of isoprenoids, molecules that are required for cell survival in most pathogenic bacteria. The IspH enzyme is essential in isoprenoid biosynthesis, it was targeted as a way to block this pathway and kill the microbes, given the broad presence of IspH in bacteria the approach might target a wide range of bacteria.
Computer modeling was used to screen several million commercially available compounds for their ability to bind with the enzyme to select the most potent inhibitors of IspH function as a starting point for drug discovery. Previously available IspH inhibitors were not able to penetrate the bacteria cell wall prompting the team to collaborate with chemist Joseph Salvino, Ph.D., to identify and synthesize novel inhibitor molecules that can get inside the bacteria.
The team was able to demonstrate that the IspH inhibitors stimulated the immune system with more potent bacterial killing activity and specificity than the 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. Additionally, in preclinical m models of gram-negative bacterial infection, the bactericidal effects of the IspH inhibitors outperformed traditional pan antibiotics.
“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 authors.
“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,” said Dotiwala.
