Inosine 5'-monophosphate dehydrogenase inhibitors as antimicrobial agents: recent progress and future perspectives

Studies on Methylpyrazole-Substituted Benzimidazoles to Target Helicobacter pylori Infection through HpIMPDH Inhibition

The prevalence of Helicobacter pylori infection has been increasing rapidly due to the genetic heterogeneity and antibacterial resistance shown by the bacteria, affecting over 50% of the world population and over 80% of the Indian population, in particular. In this regard, novel drug targets are currently being explored, one of which is the crucial metabolic enzyme inosine-5'-monophosphate dehydrogenase (IMPDH) involved in the de novo nucleotide biosynthesis pathway, in order to combat the infection and devise efficient therapeutic strategies. The present study reports the development of methylpyrazole-substituted benzimidazoles as small molecule inhibitors of H. pylori IMPDH with a nanomolar range of enzyme inhibition. A set of 19 small molecules have been designed, synthesized, and further evaluated for their inhibitory potential against H. pylori IMPDH using in silico, in vitro, biochemical, and biophysical techniques. Compound 7j was found to inhibit H. pylori IMPDH with an IC50 value of 0.095 ± 0.023 μM, which is close to 1.5-fold increase in the inhibitory activity, in comparison to the previously reported benzimidazole-based hit C91. Moreover, kinetic characterization has provided significant insights into the uncompetitive inhibition shown by these small molecules on H. pylori IMPDH, thus providing details about the enzyme inhibition mechanism. In conclusion, methylpyrazole-based small molecules indicate a promising path to develop cheap and bioavailable drugs to efficiently treat H. pylori infection in the coming years, in comparison to the currently available therapy.

A journey into the regulatory secrets of the de novo purine nucleotide biosynthesis

De novo purine nucleotide biosynthesis (DNPNB) consists of sequential reactions that are majorly conserved in living organisms. Several regulation events take place to maintain physiological concentrations of adenylate and guanylate nucleotides in cells and to fine-tune the production of purine nucleotides in response to changing cellular demands. Recent years have seen a renewed interest in the DNPNB enzymes, with some being highlighted as promising targets for therapeutic molecules. Herein, a review of two newly revealed modes of regulation of the DNPNB pathway has been carried out: i) the unprecedent allosteric regulation of one of the limiting enzymes of the pathway named inosine 5'-monophosphate dehydrogenase (IMPDH), and ii) the supramolecular assembly of DNPNB enzymes. Moreover, recent advances that revealed the therapeutic potential of DNPNB enzymes in bacteria could open the road for the pharmacological development of novel antibiotics.

GuaB3, an overlooked enzyme in cyanobacteria's toolbox that sheds light on IMP dehydrogenase evolution

IMP dehydrogenase and GMP reductase are enzymes from the same protein family with analogous structures and catalytic mechanisms that have gained attention because of their essential roles in nucleotide metabolism and as potential drug targets. This study focusses on GuaB3, a less-explored enzyme within this family. Phylogenetic analysis uncovers GuaB3's independent evolution from other members of the family and it predominantly occurs in Cyanobacteria. Within this group, GuaB3 functions as a unique IMP dehydrogenase, while its counterpart in Actinobacteria has a yet unknown function. Synechocystis sp. PCC6803 GuaB3 structures demonstrate differences in the active site compared to canonical IMP dehydrogenases, despite shared catalytic mechanisms. These findings highlight the essential role of GuaB3 in Cyanobacteria, provide insights into the diversity and evolution of the IMP dehydrogenase protein family, and reveal a distinctive characteristic in nucleotide metabolism, potentially aiding in combating harmful cyanobacterial blooms-a growing concern for humans and wildlife.

Fragment-Based Lead Discovery Strategies in Antimicrobial Drug Discovery

Fragment-based lead discovery (FBLD) is a powerful application for developing ligands as modulators of disease targets. This approach strategy involves identification of interactions between low-molecular weight compounds (100-300 Da) and their putative targets, often with low affinity (K_D ~0.1-1 mM) interactions. The focus of this screening methodology is to optimize and streamline identification of fragments with higher ligand efficiency (LE) than typical high-throughput screening. The focus of this review is on the last half decade of fragment-based drug discovery strategies that have been used for antimicrobial drug discovery.

Inhibitors of Nucleotide Biosynthesis as Candidates for a Wide Spectrum of Antiviral Chemotherapy

Emerging and re-emerging viruses have been a challenge in public health in recent decades. Host-targeted antivirals (HTA) directed at cellular molecules or pathways involved in virus multiplication represent an interesting strategy to combat viruses presently lacking effective chemotherapy. HTA could provide a wide range of agents with inhibitory activity against current and future viruses that share similar host requirements and reduce the possible selection of antiviral-resistant variants. Nucleotide metabolism is one of the more exploited host metabolic pathways as a potential antiviral target for several human viruses. This review focuses on the antiviral properties of the inhibitors of pyrimidine and purine nucleotide biosynthesis, with an emphasis on the rate-limiting enzymes dihydroorotate dehydrogenase (DHODH) and inosine monophosphate dehydrogenase (IMPDH) for which there are old and new drugs active against a broad spectrum of pathogenic viruses.

Irigenin, a novel lead from Iris confusa for management of Helicobacter pylori infection with selective COX-2 and HpIMPDH inhibitory potential

The development of new natural drugs for Helicobacter pylori (H. pylori) management has recently received significant attention. Iris confusa (I. confusa) was long used for the treatment of bacterial infections and gastritis. This study aimed at evaluating its effect on management of H. pylori infection and exploring its bioactive metabolites. The inhibitory potential of the polar (PF), non-polar (NPF) fractions and the isolated compounds against H. pylori using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay in addition to their cyclooxygenases (COX-1 and COX-2), and nitric oxide (NO) inhibitory activities were assessed. The most biologically active compound was tested for its selective H. pylori inosine-5′-monophosphate dehydrogenase (HpIMPDH) inhibitory potential. Chromatographic purification of PF and NPF allowed isolation of tectoridin, orientin, irigenin, tectorigenin, isoarborinol and stigmasterol. The PF exhibited significant anti-H. pylori (MIC 62.50 µg/mL), COX-1, COX-2 (IC₅₀ of 112.08 ± 0.60 and 47.90 ± 1.50 µg/mL respectively, selectivity index SI of 2.34), and NO (IC₅₀ 47.80 ± 0.89 µg/mL) inhibitory activities, while irigenin was the most potent isolated compound. Irigenin was found to have a promising activity against HpIMPDH enzyme (IC₅₀ of 2.07 ± 1.90 μM) with low activity against human hIMPDH2 (IC₅₀ > 10 μM) than clarithromycin, assuring its selectivity. Overall, I. confusa and its isolated compounds may serve as a potential source of plant-based drugs for H. pylori control. This study scientifically validated the claimed anti-bacterial activity of I. confusa and revealed irigenin potential as a novel lead exhibiting anti H. pylori activity in a first record.

Immunogenic Proteins of Group B Streptococcus-Potential Antigens in Immunodiagnostic Assay for GBS Detection

Streptococcus agalactiae (Group B Streptococcus, GBS) is an opportunistic pathogen, which asymptomatically colonizes the gastrointestinal and genitourinary tract of up to one third of healthy adults. Nevertheless, GBS carriage in pregnant women may lead to several health issues in newborns causing life threatening infection, such as sepsis, pneumonia or meningitis. Recommended GBS screening in pregnant women significantly reduced morbidity and mortality in infants. Nevertheless, intrapartum antibiotic prophylaxis, recommended following the detection of carriage or in case of lack of a carriage test result for pregnant women who demonstrate certain risk factors, led to the expansion of the adverse phenomenon of bacterial resistance to antibiotics. In our paper, we reviewed some immunogenic GBS proteins, i.e., Alp family proteins, β protein, Lmb, Sip, BibA, FsbA, ScpB, enolase, elongation factor Tu, IMPDH, and GroEL, which possess features characteristic of good candidates for immunodiagnostic assays for GBS carriage detection, such as immunoreactivity and specificity. We assume that they can be used as an alternative diagnostic method to the presently recommended bacteriological cultivation and MALDI.

Development, synthesis, and biological evaluation of sulfonyl-α-l-amino acids as potential anti-Helicobacter pylori and IMPDH inhibitors

Inosine 5'-monophosphate dehydrogenase (IMPDH) catalyzes a crucial step in the biosynthesis of DNA and RNA, and it has been exploited as a promising target for antimicrobial therapy. The present study discusses the development and synthesis of a series of sulfonyl-α-l-amino acids coupled with the anisamide scaffold and evaluates their activities as anti-Helicobacter pylori and IMPDH inhibitors. Twenty derivatives were synthesized and their structures were established by high-resolution mass spectrometry and ¹ H and ¹³ C nuclear magnetic resonance measurements. Four compounds (6, 10, 11, and 21) were found to be the most potent and selective molecules in the series with minimum inhibitory concentration (MIC) values <17 µM, which were selected to test their inhibitory activities against HpIMPDH and human (h)IMPDH2 enzymes. In all tests, amoxicillin and clarithromycin were used as reference drugs. Compounds 6 and 10 were found to have a promising activity against the HpIMPDH enzyme, with IC₅₀  = 2.42 and 2.56 µM, respectively. Moreover, the four compounds were found to be less active and safer against hIMPDH2 than the reference drugs, with IC₅₀  > 17.17 µM, which makes sure that their selectivity is toward HpIMPDH and reverse to that of amoxicillin and clarithromycin. Also, the synergistic antibacterial activity of compounds 6, 10, amoxicillin, and clarithromycin was investigated in vitro. The combination of amoxicillin/compound 6 (2:1 by weight) exhibited a significant antibacterial activity against H. pylori, with MIC = 0.12 µg/ml. The molecular docking study and ADMET analysis of the most active compounds were used to elucidate the mode-of-action mechanism.

Curcumin and Photobiomodulation in Chronic Viral Hepatitis and Hepatocellular Carcinoma

Immune modulation is a very modern medical field for targeting viral infections. In the race to develop the best immune modulator against viruses, curcumin, as a natural product, is inexpensive, without side effects, and can stimulate very well certain areas of the human immune system. As a bright yellow component of turmeric spice, curcumin has been the subject of thousands of scientific and clinical studies in recent decades to prove its powerful antioxidant properties and anticancer effects. Curcumin has been shown to influence inter- and intracellular signaling pathways, with direct effects on gene expression of the antioxidant proteins and those that regulate the immunity. Experimental studies have shown that curcumin modulates several enzyme systems, reduces nitrosative stress, increases the antioxidant capacity, and decreases the lipid peroxidation, protecting against fatty liver pathogenesis and fibrotic changes. Hepatitis B virus (HBV) affects millions of people worldwide, having sometimes a dramatic evolution to chronic aggressive infection, cirrhosis, and hepatocellular carcinoma. All up-to-date treatments are limited, there is still a gap in the scientific knowledge, and a sterilization cure may not yet be possible with the removal of both covalently closed circular DNA (cccDNA) and the embedded HBV DNA. With a maximum light absorption at 420 nm, the cytotoxicity of curcumin as photosensitizer could be expanded by the intravenous blue laser blood irradiation (IVBLBI) or photobiomodulation in patients with chronic hepatitis B infection, Hepatitis B e-antigen (HBeAg)-positive, noncirrhotic, but nonresponsive to classical therapy. Photobiomodulation increases DNA repair by the biosynthesis of complex molecules with antioxidant properties, the outset of repairing enzyme systems and new phospholipids for regenerating the cell membranes. UltraBioavailable Curcumin and blue laser photobiomodulation could suppress the virus and control better the disease by reducing inflammation/fibrosis and stopping the progression of chronic hepatitis, reversing fibrosis, and diminishing the progression of cirrhosis, and decreasing the incidence of hepatocellular carcinoma. Photodynamic therapy with blue light and curcumin opens new avenues for the effective prevention and cure of chronic liver infections and hepatocellular carcinoma. Blue laser light and UltraBioavailable Curcumin could be a new valuable alternative for medical applications in chronic B viral hepatitis and hepatocarcinoma, saving millions of lives.

In cellulo crystallization of Trypanosoma brucei IMP dehydrogenase enables the identification of genuine co-factors

Sleeping sickness is a fatal disease caused by the protozoan parasite Trypanosoma brucei (Tb). Inosine-5’-monophosphate dehydrogenase (IMPDH) has been proposed as a potential drug target, since it maintains the balance between guanylate deoxynucleotide and ribonucleotide levels that is pivotal for the parasite. Here we report the structure of TbIMPDH at room temperature utilizing free-electron laser radiation on crystals grown in living insect cells. The 2.80 Å resolution structure reveals the presence of ATP and GMP at the canonical sites of the Bateman domains, the latter in a so far unknown coordination mode. Consistent with previously reported IMPDH complexes harboring guanosine nucleotides at the second canonical site, TbIMPDH forms a compact oligomer structure, supporting a nucleotide-controlled conformational switch that allosterically modulates the catalytic activity. The oligomeric TbIMPDH structure we present here reveals the potential of in cellulo crystallization to identify genuine allosteric co-factors from a natural reservoir of specific compounds.

Trypanosoma brucei inosine-5′-monophosphate dehydrogenase (IMPDH) is an enzyme in the guanine nucleotide biosynthesis pathway and of interest as a drug target. Here the authors present the 2.8 Å room temperature structure of TbIMPDH determined by utilizing X-ray free-electron laser radiation and crystals that were grown in insect cells and find that ATP and GMP are bound at the canonical sites of the Bateman domains.

Covalent inactivation of Mycobacterium thermoresistibile inosine-5'-monophosphate dehydrogenase (IMPDH)

Inosine-5'-monophosphate dehydrogenase (IMPDH) is a rate-limiting enzyme involved in nucleotide biosynthesis. Because of its critical role in purine biosynthesis, IMPDH is a drug design target for immunosuppressive, anticancer, antiviral and antimicrobial chemotherapy. In this study, we use mass spectrometry and X-ray crystallography to show that the inhibitor 6-Cl-purine ribotide forms a covalent adduct with the Cys-341 residue of Mycobacterium thermoresistibile IMPDH.

Purine-Metabolising Enzymes and Apoptosis in Cancer

The enzymes of both de novo and salvage pathways for purine nucleotide synthesis are regulated to meet the demand of nucleic acid precursors during proliferation. Among them, the salvage pathway enzymes seem to play the key role in replenishing the purine pool in dividing and tumour cells that require a greater amount of nucleotides. An imbalance in the purine pools is fundamental not only for preventing cell proliferation, but also, in many cases, to promote apoptosis. It is known that tumour cells harbour several mutations that might lead to defective apoptosis-inducing pathways, and this is probably at the basis of the initial expansion of the population of neoplastic cells. Therefore, knowledge of the molecular mechanisms that lead to apoptosis of tumoural cells is key to predicting the possible success of a drug treatment and planning more effective and focused therapies. In this review, we describe how the modulation of enzymes involved in purine metabolism in tumour cells may affect the apoptotic programme. The enzymes discussed are: ectosolic and cytosolic 5'-nucleotidases, purine nucleoside phosphorylase, adenosine deaminase, hypoxanthine-guanine phosphoribosyltransferase, and inosine-5'-monophosphate dehydrogenase, as well as recently described enzymes particularly expressed in tumour cells, such as deoxynucleoside triphosphate triphosphohydrolase and 7,8-dihydro-8-oxoguanine triphosphatase.

Inhibitors of inosine 5'-monophosphate dehydrogenase as emerging new generation antimicrobial agents

Inosine 5'-monophosphate dehydrogenase (IMPDH) is a vital enzyme involved in the de novo synthesis of guanine nucleotides. IMPDH catalyzes a crucial step of converting IMP into XMP that is further converted into GMP. Microbial infections rely on the rapid proliferation of bacteria, and this requires the rate-limiting enzyme IMPDH to expand the guanine nucleotide pool and hence, IMPDH has recently received lots of attention as a potential target for treating infections. Owing to the structural and kinetic differences in the host IMPDH and bacterial IMPDH, a selective targeting is possible and is a crucial feature in the development of new potent and selective inhibitors of bacterial IMPDH. Earlier screening of small molecules revealed a structural requirement for the bacterial/protozoal IMPDH. Early optimization of benzimidazole and benzoxazole scaffolds led to the discovery of new potent and selective inhibitors of pathogenic IMPDH. Further research is vastly focused on the development of highly potent and selective inhibitors of various bacterial IMPDHs. Such studies reveal the importance of this excellent target for treating infectious diseases. The current review focuses on the recent developments in the discovery and development of selective inhibitors of bacterial/protozoal IMPDH with emphasis on the inhibition mechanism and structure-activity relationship.

First-in-class allosteric inhibitors of bacterial IMPDHs

Inosine-5'-monophosphate dehydrogenase (IMPDH) is an essential enzyme in many bacterial pathogens and is considered as a potential drug target for the development of new antibacterial agents. Our recent work has revealed the crucial role of one of the two structural domains (i.e. Bateman domain) in the regulation of the quaternary structure and enzymatic activity of bacterial IMPDHs. Thus, we have screened chemical libraries to search for compounds targeting the Bateman domain and identified first in-class allosteric inhibitors of a bacterial IMPDH. These inhibitors were shown to counteract the activation by the natural positive effector, MgATP, and to block the enzyme in its apo conformation (low affinity for IMP). Our structural studies demonstrate the versatility of the Bateman domain to accommodate totally unrelated chemical scaffolds and pave the way for the development of allosteric inhibitors, an avenue little explored until now.

Design, synthesis, and biological evaluation of Helicobacter pylori inosine 5'-monophosphate dehydrogenase (HpIMPDH) inhibitors

Inosine 5'-monophosphate dehydrogenase (IMPDH) catalyzes a crucial step in the biosynthesis of guanine nucleotides. Being a validated target for immunosuppressive, antiviral, and anticancer drug development, lately it has been exploited as a promising target for antimicrobial therapy. Extending our previous work on Mycobacterium tuberculosis IMPDH, GuaB2, inhibitor development, we screened a set of 23 new chemical entities (NCEs) with substituted flavone (Series 1) and 1,2,3-triazole (Series 2) core structures for their in vitro Helicobacter pylori IMPDH (HpIMPDH) and human IMPDH2 (hIMPDH2) inhibitory activities. All the NCEs possessed acceptable molecular, physicochemical, and toxicity property profiles. The ranges for HpIMPDH and hIMPDH2 inhibition were 9-99.9% and 16-57%, respectively, at 10 μM concentration. The most potent HpIMPDH inhibitor, 25c, exhibited IC₅₀ value of 1.27 μM with no hIMPDH2 inhibitory activity. The moderately potent, structurally novel hit molecule, 25c, may serve as a lead for further design and development of highly potent HpIMPDH inhibitors.

Epitopes of Immunoreactive Proteins of Streptococcus Agalactiae: Enolase, Inosine 5'-Monophosphate Dehydrogenase and Molecular Chaperone GroEL

Three Streptococcus agalactiae (group B streptococci, GBS) immunoreactive proteins: enolase (47.4 kDa), inosine 5'-monophosphate dehydrogenase (IMPDH) (53 kDa) and molecular chaperone GroEL (57 kDa) were subjected to investigation. Enolase protein was described in our previous paper, whereas IMPDH and GroEL were presented for the first time. The aim of our paper was to provide mapping of specific epitopes, highly reactive with umbilical cord blood serum. Bioinformatic analyses allowed to select 32 most likely epitopes for enolase, 36 peptides for IMPDH and 41 immunoreactive peptides for molecular chaperone GroEL, which were synthesized by PEPSCAN. Ten peptides: two in enolase, one in IMPDH and seven in molecular chaperone GroEL have been identified as potentially highly selective epitopes that can be used as markers in rapid immunological diagnostic tests or constitute a component of an innovative vaccine against GBS infections.

Discovery of novel human inosine 5'-monophosphate dehydrogenase 2 (hIMPDH2) inhibitors as potential anticancer agents

The enzyme inosine 5'-monophosphate dehydrogenase (IMPDH) catalyzes an essential step in the de novo biosynthesis of guanine nucleotides, and thus regulates the guanine nucleotide pool required for cell proliferation. Of the two isoforms, human IMPDH type 2 (hIMPDH2) is a validated molecular target for potential immunosuppressive, antiviral and anticancer chemotherapy. In search of newer hIMPDH2 inhibitors as potential anticancer agents, three novel series (A: 5-aminoisobenzofuran-1(3H)-one, B: 3,4-dimethoxyaniline and C: benzo[d]-[1,3]dioxol-5-ylmethanamine) were synthesized and evaluated for in vitro and cell-based activities. A total of 37 molecules (29-65) were screened for their in vitro hIMPDH2 inhibition, with particular emphasis on establishing their structure-activity relationship (SAR) trends. Eight compounds (hits, 30, 31, 33-35, 37, 41 and 43) demonstrated significant enzyme inhibition (>70% @ 10 μM); especially the A series molecules were more potent than B series (<70% inhibition @ 10 μM), while C series members were found to be inactive. The hIMPDH2 IC₅₀ values for the hits ranged from 0.36 to 7.38 μM. The hits displaying >80% hIMPDH2 inhibition (30, 33, 35, 41 and 43) were further assessed for their cytotoxic activity against cancer cell lines such as MDA-MB-231 (breast adenocarcinoma), DU145 (prostate carcinoma), U87 MG (glioblastoma astrocytoma) and a normal cell line, NIH-3T3 (mouse embryonic fibroblast) using MTT assay. Most of the compounds exhibited higher cellular potency against cancer cell lines and notably lower toxicity towards NIH-3T3 cells compared to mycophenolic acid (MPA), a prototypical hIMPDH2 inhibitor. Two of the series A hits (30 and 35) were evaluated in human peripheral blood mononuclear cells (hPBMC) assay and found to be better tolerated than MPA. The calculated/predicted molecular and physicochemical properties were satisfactory with reference to drug-likeness. The molecular docking studies clearly demonstrated crucial interactions of the hits with the cofactor-binding site of hIMPDH2, further providing critical information for refining the design strategy. The present study reports the design and discovery of structurally novel hIMPDH2 inhibitors as potential anticancer agents and provides a guide for further research on the development of safe and effective anticancer agents, especially against glioblastoma.

Mycophenolic Acid Derivatives with Immunosuppressive Activity from the Coral-Derived Fungus Penicillium bialowiezense

Mycophenolic acid (MPA) is a potent inosine-5′-monophosphate dehydrogenase (IMPDH) inhibitor for immunosuppressive chemotherapy. Most importantly, as the 2-morpholinoethyl ester prodrug of MPA, mycophenolate mofetil (MMF) is a well-known immunosuppressant used to prevent rejection in organ transplantations. Nevertheless, due to its frequently occurred side effects, searching for new therapeutic agents is ongoing. In our current work, by virtue of efficient bioassay-guided fractionation and purification, eleven mycophenolic acid derivatives, including five previously unreported metabolites (3⁻7) and six known compounds (1, 2, and 8⁻11), were obtained from the coral-derived fungus Penicillium bialowiezense. Their structures were elucidated by means of extensive spectroscopic analyses (including 1D and 2D NMR and HRESIMS data) and comparison of the NMR and other physical data with those reported in the literature in the case of the known compounds. All the isolates 1⁻11 were evaluated for the immunosuppressive activity, and 1⁻3 showed potent IMPDH2 inhibitory potency with IC50 values of 0.84⁻0.95 μM, which were comparable to that of MPA (the positive control), while 4⁻10 showed significant inhibitory potency with IC50 values of 3.27⁻24.68 μM. All the MPA derivatives showed promising immunosuppressive activity, endowing them as potential drug leads for organ transplantations and autoimmune related diseases.

Newer human inosine 5'-monophosphate dehydrogenase 2 (hIMPDH2) inhibitors as potential anticancer agents

Human inosine 5′-monophosphate dehydrogenase 2 (hIMPDH2), being an age-old target, has attracted attention recently for anticancer drug development. Mycophenolic acid (MPA), a well-known immunosuppressant drug, was used a lead structure to design and develop modestly potent and selective analogues. The steep structure–activity relationship (SAR) requirements of the lead molecule left little scope to synthesise newer analogues. Here, newer MPA amides were designed, synthesised and evaluated for hIMPDH2 inhibition and cellular efficacy in breast, prostate and glioblastoma cell lines. Few title compounds exhibited cellular activity profile better than MPA itself. The observed differences in the overall biological profile could be attributed to improved structural and physicochemical properties of the analogues over MPA. This is the first report of the activity of MPA derivatives in glioblastoma, the most aggressive brain cancer.

Synergistic and antagonistic effects of immunomodulatory drugs on the action of antifungals against Candida glabrata and Saccharomyces cerevisiae

Candidemia and other forms of invasive fungal infections caused by Candida glabrata and to a lesser extent Saccharomyces cerevisiae are a serious health problem, especially if their steadily rising resistance to the limited range of antifungal drugs is taken into consideration. Various drug combinations are an attractive solution to the resistance problem, and some drug combinations are already common in the clinical environment due to the nature of diseases or therapies. We tested a few of the common antifungal-immunomodulatory drug combinations and evaluated their effect on selected strains of C. glabrata and S. cerevisiae. The combinations were performed using the checkerboard microdilution assay and interpreted using the Loewe additivity model and a model based on the Bliss independence criterion. A synergistic interaction was confirmed between calcineurin inhibitors (Fk506 and cyclosporine A) and antifungals (fluconazole, itraconazole, and amphotericin B). A new antagonistic interaction between mycophenolic acid (MPA) and azole antifungals was discovered in non-resistant strains. A possible mechanism that explains this is induction of the Cdr1 efflux pump by MPA in C. glabrata ATCC 2001. The Pdr1 regulatory cascade plays a role in overall resistance to fluconazole, but it is not essential for the antagonistic interaction. This was confirmed by the Cgpdr1Δ mutant still displaying the antagonistic interaction between the drugs, although at lower concentrations of fluconazole. This antagonism calls into question the use of simultaneous therapy with MPA and azoles in the clinical environment.

Hit discovery of Mycobacterium tuberculosis inosine 5'-monophosphate dehydrogenase, GuaB2, inhibitors

Tuberculosis remains a global concern. There is an urgent need of newer antitubercular drugs due to the development of resistant forms of Mycobacterium tuberculosis (Mtb). Inosine 5'-monophosphate dehydrogenase (IMPDH), guaB2, of Mtb, required for guanine nucleotide biosynthesis, is an attractive target for drug development. In this study, we screened a focused library of 73 drug-like molecules with desirable calculated/predicted physicochemical properties, for growth inhibitory activity against drug-sensitive MtbH37Rv. The eight hits and mycophenolic acid, a prototype IMPDH inhibitor, were further evaluated for activity on purified Mtb-GuaB2 enzyme, target selectivity using a conditional knockdown mutant of guaB2 in Mtb, followed by cross-resistance to IMPDH inhibitor-resistant SRMV2.6 strain of Mtb, and activity on human IMPDH2 isoform. One of the hits, 13, a 5-amidophthalide derivative, has shown growth inhibitory potential and target specificity against the Mtb-GuaB2 enzyme. The hit, 13, is a promising molecule with potential for further development as an antitubercular agent.

Fragment-Based Approach to Targeting Inosine-5'-monophosphate Dehydrogenase (IMPDH) from Mycobacterium tuberculosis

Tuberculosis (TB) remains a major cause of mortality worldwide, and improved treatments are needed to combat emergence of drug resistance. Inosine 5′-monophosphate dehydrogenase (IMPDH), a crucial enzyme required for de novo synthesis of guanine nucleotides, is an attractive TB drug target. Herein, we describe the identification of potent IMPDH inhibitors using fragment-based screening and structure-based design techniques. Screening of a fragment library for Mycobacterium thermoresistible (Mth) IMPDH ΔCBS inhibitors identified a low affinity phenylimidazole derivative. X-ray crystallography of the Mth IMPDH ΔCBS–IMP–inhibitor complex revealed that two molecules of the fragment were bound in the NAD binding pocket of IMPDH. Linking the two molecules of the fragment afforded compounds with more than 1000-fold improvement in IMPDH affinity over the initial fragment hit.

Nucleoside Derived Antibiotics to Fight Microbial Drug Resistance: New Utilities for an Established Class of Drugs?

Novel antibiotics are urgently needed to combat the rise of infections due to drug-resistant microorganisms. Numerous natural nucleosides and their synthetically modified analogues have been reported to have moderate to good antibiotic activity against different bacterial and fungal strains. Nucleoside-based compounds target several crucial processes of bacterial and fungal cells such as nucleoside metabolism and cell wall, nucleic acid, and protein biosynthesis. Nucleoside analogues have also been shown to target many other bacterial and fungal cellular processes although these are not well characterized and may therefore represent opportunities to discover new drugs with unique mechanisms of action. In this Perspective, we demonstrate that nucleoside analogues, cornerstones of anticancer and antiviral treatments, also have great potential to be repurposed as antibiotics so that an old drug can learn new tricks.