The emergence of strains of harmful microorganisms that are resistant to antibiotic treatment is a major global health concern. This has prompted ongoing drug-development efforts, including the identification of possible biological targets linked to essential microbial processes. However, the clinical development of drugs is slow, and the emergence of resistance to newly developed compounds remains a continuing problem. Finding alternative ways to tackle the emergence of antibiotic-resistant bacteria is of paramount importance. Writing in Nature, Singh et al.1 outline a two-pronged strategy to address this challenge.
The authors’ approach involved the development of a new class of molecule called immunoantibiotics. These target a key pathway that generates a molecule needed by microbes and, in doing so, stimulate a particular population of immune cells. The cells, from the grouping called γδ T cells, then provide a general (innate-like), potent antimicrobial immune response.
Singh and colleagues focused on inhibition of an enzyme termed IspH (Fig. 1). This catalyses a step in a pathway that makes molecules called isoprenoids. Isoprenoids are key building blocks needed for the synthesis of a diverse range of molecules in prokaryotes (organisms lacking a nucleus, such as bacteria) and eukaryotes (organisms that have a cellular nucleus). However, IspH catalyses a reaction in an arm of the isoprenoid-synthesis pathway, called the methylerythritol 4-phosphate (MEP) pathway, that is found in bacteria and certain protozoa (single-celled eukaryotes), but is absent in animals2. Several enzymes in this pathway, which underpins processes such as the synthesis of the bacterial cell wall and energy production, have already attracted interest as potential antimicrobial drug targets3–7. These targets include IspH itself3,6, which is present in a diverse range of disease-causing microorganisms, including Gram-negative bacteria, mycobacteria (a grouping that includes the microbe that causes tuberculosis) and certain protozoa, such as Plasmodium falciparum (the microorganism that causes malaria).
Choosing IspH as a therapeutic target provides a benefit that extends beyond its role in generating the compounds needed for isoprenoid synthesis. This is because the microbial molecule that it breaks down, (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP), is one that the immune system of primates has evolved the ability to detect8. Humans detect HMBPP in the body by using immune cells known as Vγ9Vδ2 T cells, which represent approximately 1–10% of T cells in the blood. After Vγ9Vδ2 T cells recognize9 cells exposed to HMBPP, through a process that relies on the receptors BTN3A1 and BTN2A1 in exposed cells (Fig. 1), they become activated, proliferate and mediate potent defence responses, including the production of molecules capable of killing cells and the release of signalling molecules called cytokines.
Activated Vγ9Vδ2 T cells can also either directly kill human cells exposed to HMBPP (exposed by, for example, intracellular microbial infection) or kill bacterial cells themselves. Singh and colleagues’ strategy of focusing on IspH, therefore, combines rational targeting to inhibit the isoprenoid pathway, and thereby block a source of crucial microbial molecules, with the stimulation of an immune response due to the resulting accumulation of HMBPP, which is a highly potent signal that drives the activation of Vγ9Vδ2 T cells.
The authors took a structure-directed, in silico screening approach to identify possible IspH inhibitors, and tested around ten million compounds. Strikingly, 2 of the 24 most promising compounds inhibited IspH with high potency (at nanomolar concentrations) when tested in vitro. Further optimization of the molecular structures of these compounds improved their affinity for IspH compared with the affinity of IspH for its natural substrate, HMBPP.
However, the physical characteristics of the inhibitors were expected to limit their entry into bacteria. To circumvent this, Singh et al. adopted a strategy previously used to enable drugs to pass through membranes. This method generates what is called a prodrug — an inactive version of the drug (in this case, an ester derivative of the inhibitor) that can be taken up easily by cells and then metabolized into the active version. Crucially, unlike previous work3,6 that described IspH inhibitors, this prodrug approach allowed such inhibitors to successfully enter bacteria. The authors confirmed that the drugs inhibited enzyme breakdown of HMBPP, hindering essential microbial processes, and that this resulted in the killing of a range of different bacteria, including Escherichia coli, without notable signs of drug toxicity to mammalian cells.
In keeping with the ability to inhibit HMBPP breakdown by IspH, prodrug use also led to the in vitro activation and proliferation of HMBPP-responsive Vγ9Vδ2 T cells during bacterial infection of samples of human peripheral blood mononuclear cells. This result indicates the potential of such prodrugs to act as dual-action immunoantibiotics. When tested in vivo in mice, the prodrugs induced direct antimicrobial effects and controlled bacterial infection through a process mediated by γδ T cells.
Singh et al. explored two key aspects of the potential of these new compounds to combat antimicrobial resistance. First, the researchers present in vitro and in vivo data indicating direct bactericidal effects on a variety of clinically isolated harmful bacteria that are resistant to current antibiotics, including multidrug-resistant microbes. The authors observed that the IspH inhibitors had greater ability to kill multidrug-resistant microbes than do the current best-in-class antibiotics. Second, using an in vitro model system, Singh and colleagues showed that bacteria did not acquire resistance to the IspH inhibitors in the presence of γδ T cells. But in the absence of these T cells, drug resistance occurred over a similar timescale to that observed for conventional antibiotics. These results emphasize the potential advantage that immunoantibiotics might have for tackling the emergence of drug resistance.
Singh and colleagues’ study is a highly promising proof-of-concept that a new class of antimicrobial can be developed with a dual mechanism of action. Leveraging Vγ9Vδ2 T cells is appealing because of the therapeutic advantages offered by harnessing this approach. These cells, present in humans from early in life, are capable of highly potent defence functions10, and, unlike many other types of T cell, don’t depend on the recognition of major histocompatibility complex (MHC) molecules, which differ between individuals. Encouragingly, the pathway containing IspH is shared by a diverse range of clinically relevant disease-causing microorganisms, suggesting that such antimicrobial drugs could have broad applicability.
Antibiotic approaches using monotherapy (a single type of drug) have often resulted in the emergence of drug resistance, whereas combination therapies using multiple drugs, operating through different mechanisms of action, have been more fruitful and have met with relatively fewer resistance problems. This ‘two-in-one’ mechanism underpinning Singh and colleagues’ strategy might, therefore, allow the targeting of existing multidrug-resistant microbes, as well as decrease the chances of resistance emerging. Although the subsequent steps on the road to drug development can often be challenging, the progress of this exciting class of compound towards clinical application will undoubtedly be followed with interest.