Activity of and development of resistance to corallopyronin A, an inhibitor of RNA polymerase

A laboratory-based predictive pathway for the development of Neisseria gonorrhoeae high-level resistance to corallopyronin A, an inhibitor of bacterial RNA polymerase

The continued emergence of Neisseria gonorrhoeae strains that express resistance to multiple antibiotics, including the last drug for empiric monotherapy (ceftriaxone), necessitates the development of new treatment options to cure gonorrheal infections. Toward this goal, we recently reported that corallopyronin A (CorA), which targets the switch region of the β' subunit (RpoC) of bacterial DNA-dependent RNA polymerase (RNAP), has potent anti-gonococcal activity against a panel of multidrug-resistant clinical strains. Moreover, in that study, CorA could eliminate gonococcal infection of primary human epithelial cells and gonococci in a biofilm state. To determine if N. gonorrhoeae could develop high-level resistance to CorA in a single step, we sought to isolate spontaneous mutants expressing any CorA resistance phenotypes. However, no single-step mutants with high-level CorA resistance were isolated. High-level CorA resistance could only be achieved in this study through a multi-step pathway involving over-expression of the MtrCDE drug efflux pump and single amino acid changes in the β and β' subunits (RpoB and RpoC, respectively) of RNAP. Molecular modeling of RpoB and RpoC interacting with CorA was used to deduce how the amino acid changes in RpoB and RpoC could influence gonococcal resistance to CorA. Bioinformatic analyses of whole genome sequences of clinical gonococcal isolates indicated that the CorA resistance determining mutations in RpoB/C, identified herein, are very rare (≤ 0.0029%), suggesting that the proposed pathway for resistance is predictive of how this phenotype could potentially evolve if CorA is used therapeutically to treat gonorrhea in the future.

IMPORTANCE

The continued emergence of multi-antibiotic-resistant strains of Neisseria gonorrhoeae necessitates the development of new antibiotics that are effective against this human pathogen. We previously described that the RNA polymerase-targeting antibiotic corallopyronin A (CorA) has potent activity against a large collection of clinical strains that express different antibiotic resistance phenotypes including when such gonococci are in a biofilm state. Herein, we tested whether a CorA-sensitive gonococcal strain could develop spontaneous resistance. Our finding that CorA resistance could only be achieved by a multi-step process involving over-expression of the MtrCDE efflux pump and single amino acid changes in RpoB and RpoC suggests that such resistance may be difficult for gonococci to evolve if this antibiotic is used in the future to treat gonorrheal infections that are refractory to cure by other antibiotics.

Addressing Sexually Transmitted Infections Due to Neisseria gonorrhoeae in the Present and Future

It was in the 1800s when the first public publications about the infection and treatment of gonorrhoea were released. However, the first prevention programmes were only published a hundred years later. In the 1940s, the concept of vaccination was introduced into clinical prevention programmes to address early sulphonamide resistance. Since then, tons of publications on Neisseria gonorrhoeae are undisputed, around 30,000 publications today. Currently, the situation seems to be just as it was in the last century, nothing has changed or improved. So, what are we doing wrong? And more importantly, what might we do? The review presented here aims to review the current situation regarding the resistance mechanisms, prevention programmes, treatments, and vaccines, with the challenge of better understanding this special pathogen. The authors have reviewed the last five years of advancements, knowledge, and perspectives for addressing the Neisseria gonorrhoeae issue, focusing on new therapeutic alternatives.

Myxobacteria: biology and bioactive secondary metabolites

Myxobacteria are Gram-negative eubacteria and they thrive in a variety of habitats including soil rich in organic matter, rotting wood, animal dung and marine environment. Myxobacteria are a promising source of new compounds associated with diverse bioactive spectrum and unique mode of action. The genome information of myxobacteria has revealed many orphan biosynthetic pathways indicating that these bacteria can be the source of several novel natural products. In this review, we highlight the biology of myxobacteria with emphasis on their habitat, life cycle, isolation methods and enlist all the bioactive secondary metabolites purified till date and their mode of action.

Pharmacokinetics and Pharmacodynamics (PK/PD) of Corallopyronin A against Methicillin-Resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is a World Health Organization’s high priority pathogen organism, with an estimated > 100,000 deaths worldwide in 2019. Thus, there is an unmet medical need for novel and resistance-breaking anti-infectives. The natural product Co-rallopyronin A (CorA), currently in preclinical development for filariasis, is efficacious against MRSA in vitro. In this study, we evaluated the pharmacokinetics of CorA after dosing in mice. Furthermore, we determined compound concentrations in target compartments, such as lung, kidney and thigh tissue, using LC-MS/MS. Based on the pharmacokinetic results, we evaluated the pharmacodynamic profile of CorA using the standard neutropenic thigh and lung infection models. We demonstrate that CorA is effective in both standard pharmacodynamic models. In addition to reaching effective levels in the lung and muscle, CorA was detected at high levels in the thigh bone. The data presented herein encourage the further exploration of the additional CorA indications treatment of MRSA- and methicillin-sensitive S. aureus- (MSSA) related infections.

Potent In Vitro and Ex Vivo Anti-Gonococcal Activity of the RpoB Inhibitor Corallopyronin A

Gonorrhea remains a major global public health problem because of the high incidence of infection (estimated 82 million cases in 2020) and the emergence and spread of Neisseria gonorrhoeae strains resistant to previous and current antibiotics used to treat infections. Given the dearth of new antibiotics that are likely to enter clinical practice in the near future, there is concern that cases of untreatable gonorrhea might emerge. In response to this crisis, the World Health Organization (WHO), in partnership with the Global Antibiotic Research and Development Partnership (GARDP), has made the search for and development of new antibiotics against N. gonorrhoeae a priority. Ideally, these antibiotics should also be active against other sexually transmitted organisms, such as Chlamydia trachomatis and/or Mycoplasma genitalium, which are often found with N. gonorrhoeae as co-infections. Corallopyronin A is a potent antimicrobial that exhibits activity against Chlamydia spp. and inhibits transcription by binding to the RpoB switch region. Accordingly, we tested the effectiveness of corallopyronin A against N. gonorrhoeae. We also examined the mutation frequency and modes of potential resistance against corallopyronin A. We report that corallopyronin A has potent antimicrobial action against antibiotic-susceptible and antibiotic-resistant N. gonorrhoeae strains and could eradicate gonococcal infection of cultured, primary human cervical epithelial cells. Critically, we found that spontaneous corallopyronin A-resistant mutants of N. gonorrhoeae are exceedingly rare (≤10^-10) when selected at 4× the MIC. Our results support pre-clinical studies aimed at developing corallopyronin A for gonorrheal treatment regimens. IMPORTANCE The high global incidence of gonorrhea, the lack of a protective vaccine, and the emergence of N. gonorrhoeae strains expressing resistance to currently used antibiotics demand that new treatment options be developed. Accordingly, we investigated whether corallopyronin A, an antibiotic which is effective against other pathogens, including C. trachomatis, which together with gonococci frequently cause co-infections in humans, could exert anti-gonococcal action in vitro and ex vivo, and potential resistance emergence. We propose that corallopyronin A be considered a potential future treatment option for gonorrhea because of its potent activity, low resistance development, and recent advances in scalable production.

The RNA Polymerase Inhibitor Corallopyronin A Has a Lower Frequency of Resistance Than Rifampicin in Staphylococcus aureus

Corallopyronin A (CorA) is active against Gram-positive bacteria and targets the switch region of RNA polymerase. Because of the high frequency of mutation (FoM) leading to rifampicin resistance, we determined the CorA FoM in S. aureus using fluctuation analysis at 4 × minimum inhibitory concentration (MIC). Resistant mutants were characterized. S. aureus strains HG001, Mu50, N315, and USA300 had an MIC of 0.25 mg/L. The median FoM for CorA resistance was 1.5 × 10−8, 4.5-fold lower than the median FoM of 6.7 × 10−8 for rifampicin, and was reflected in a 4-fold lower mutation rate for CorA than rifampicin (6 × 10−9 for CorA vs. 2.5 × 10−8 for rifampicin). In CorA-resistant/rifampicin-sensitive strains, the majority of amino acid exchanges were S1127L in RpoB or K334N in RpoC. S. aureus Mu50, a rifampicin-resistant clinical isolate, yielded two further exchanges targeting amino acids L1131 and E1048 of the RpoB subunit. The plating of >1011 cells on agar containing a combination of 4 × MIC of rifampicin and 4 × MIC of CorA did not yield any growth. In conclusion, with proper usage, e.g., in combination therapy and good antibiotic stewardship, CorA is a potential antibiotic for treating S. aureus infections.

Corallopyronin A: antimicrobial discovery to preclinical development

Covering: August 1984 up to January 2022Worldwide, increasing morbidity and mortality due to antibiotic-resistant microbial infections has been observed. Therefore, better prevention and control of infectious diseases, as well as appropriate use of approved antibacterial drugs are crucial. There is also an urgent need for the continuous development and supply of novel antibiotics. Thus, identifying new antibiotics and their further development is once again a priority of natural product research. The antibiotic corallopyronin A was discovered in the 1980s in the culture broth of the Myxobacterium Corallococcus coralloides and serves, in the context of this review, as a show case for the development of a naturally occurring antibiotic compound. The review demonstrates how a hard to obtain, barely water soluble and unstable compound such as corallopyronin A can be developed making use of sophisticated production and formulation approaches. Corallopyronin A is a bacterial DNA-dependent RNA polymerase inhibitor with a new target site and one of the few representatives of this class currently in preclinical development. Efficacy against Gram-positive and Gram-negative pathogens, e.g., Chlamydia trachomatis, Orientia tsutsugamushi, Staphylococcus aureus, and Wolbachia has been demonstrated. Due to its highly effective in vivo depletion of Wolbachia, which are essential endobacteria of most filarial nematode species, and its robust macrofilaricidal efficacy, corallopyronin A was selected as a preclinical candidate for the treatment of human filarial infections. This review highlights the discovery and production optimization approaches for corallopyronin A, as well as, recent preclinical efficacy results demonstrating a robust macrofilaricidal effect of the anti-Wolbachia candidate, and the solid formulation strategy which enhances the stability as well as the bioavailability of corallopyronin A.

Elaborations on Corallopyronin A as a Novel Treatment Strategy Against Genital Chlamydial Infections

Ascending Chlamydia trachomatis infection causes functional damage to the fallopian tubes, which may lead to ectopic pregnancy and infertility in women. Treatment failures using the standard regimens of doxycycline and azithromycin have been observed. We tested the polyketide-derived α-pyrone antibiotic Corallopyronin A (CorA) that inhibits the bacterial DNA dependent RNA polymerase and has strong activity against various extracellular and some intracellular bacteria. Extensive testing in cell culture infection models and in an ex vivo human fallopian tube model under different oxygen concentrations was performed to assess the anti-chlamydial efficacy of CorA at physiological conditions. CorA showed high efficacy against C. trachomatis (MIC_N/H: 0.5 μg/mL for serovar D and L2), C. muridarum (MIC_N/H: 0.5 μg/mL), and C. pneumoniae (MIC_N/H: 1 μg/mL) under normoxic (N) and hypoxic (H) conditions. Recoverable inclusion forming units were significantly lower already at 0.25 μg/mL for all tested chlamydiae. CorA at a concentration of 1 μg/mL was also effective against already established C. trachomatis and C. pneumoniae infections (up to 24 h.p.i.) in epithelial cells, while efficacy against C. muridarum was limited to earlier time points. A preliminary study using a C. muridarum genital infection model revealed corresponding limitations in the efficacy. Importantly, in an ex vivo human fallopian tube model, the growth of C. trachomatis was significantly inhibited by CorA at concentrations of 1–2 μg/mL under normoxic and hypoxic conditions. The overall high efficacies of CorA against C. trachomatis in cell culture and an ex vivo human fallopian tube model under physiological oxygen concentrations qualifies this drug as a candidate that should be further investigated.

Natural products from myxobacteria: novel metabolites and bioactivities

Covering: 2011-July 2016Myxobacteria are a rich source for structurally diverse secondary metabolites with intriguing biological activities. Here we report on new natural products that were isolated from myxobacteria in the period of 2011 to July 2016. Some examples of recent advances on modes-of-action are also summarised along with a more detailed overview on five compound classes currently assessed in preclinical studies.

Insights into Structure-Activity Relationships of Bacterial RNA Polymerase Inhibiting Corallopyronin Derivatives

The new compound precorallopyronin A is a stable precursor in the biosynthesis of the antibiotic corallopyronin A. This natural product was isolated from the producer strain Corallococcus coralloides B035. Together with various semisynthetically obtained corallopyronin A derivatives its antibacterial effects were evaluated. In combination with an X-ray crystallization model limitations of derivatization possibilities were revealed. The antibiotic potential of the novel precorallopyronin A is comparable to that of the structurally more complex corallopyronin A, which highlights that the additional chiral center is not essential for activity.

Structural basis of head to head polyketide fusion by CorB

Corallopyronin A is a polyketide derived from the myxobacterium Corallococcus coralloides with potent antibiotic features.

Corallopyronin A is a polyketide derived from the myxobacterium Corallococcus coralloides with potent antibiotic features. The gene cluster responsible for the biosynthesis of corallopyronin A has been described recently, and it was proposed that CorB acts as a ketosynthase to interconnect two polyketide chains in a rare head-to-head condensation reaction. We determined the structure of CorB, the interconnecting polyketide synthase, to high resolution and found that CorB displays a thiolase fold. Site-directed mutagenesis showed that the catalytic triad consisting of a cysteine, a histidine and an asparagine is crucial for catalysis, and that this triad shares similarities with the triad found in HMG-CoA synthases. We synthesized a substrate mimic to derivatize purified CorB and confirmed substrate attachment by ESI-MS. Structural analysis of the complex yielded an electron density-based model for the polyketide chain and showed that the unusually wide, T-shaped active site is able to accommodate two polyketides simultaneously. Our structural analysis provides a platform for understanding the unusual head-to-head polyketide-interconnecting reaction catalyzed by CorB.

Antibiotics from myxobacteria

Covering: up to the end of 2013. Myxobacteria produce a vast range of structurally diverse natural products with prominent biological activities. Here, we provide a detailed description and judge the potential of all antibiotically active myxobacterial compounds as lead structures, pointing out their particularities and, if known, their mode of action. Thus, the review provides an overview of the potential of specific compounds, suitable for future investigations and possible clinical applications.

Corallopyronin A - a promising antibiotic for treatment of filariasis

Lymphatic filariasis and onchocerciasis are diseases of severe morbidity that affect the poorest of the poor in the world. The diseases are caused by filarial nematodes that are transmitted by mosquitoes or biting blackflies and are endemic to more than 80 countries worldwide, mainly in the tropics and sub-tropics. Current control programs aim to eliminate the diseases by distributing antifilarial drugs. However, the primary effect of the drugs is to kill the microfilariae in the blood or skin, thus preventing uptake by the obligate insect vector. Since the adult worms live 10 years or longer, drug distribution requires many years of treatment, which is a heavy burden on the burgeoning health care systems. Sub-optimal response, possible resistance and inadequate population coverage lessen the chances for successful elimination in all endemic areas. The search for new drugs that could enhance elimination by permanently sterilizing or killing adult worms has identified the Wolbachia intracellular bacteria of filarial nematodes as a target. Depleting the obligate endosymbionts from the worms with doxycycline or rifampicin causes a permanent block in oogenesis, embryogenesis and development, and in slow death of the adult worms. These two antibiotics are suitable for individual drug administration, but caveats exist for their inclusion in broader drug administration programs. Here we review Wolbachia as targets for antifilarial drug discovery and highlight the natural product corallopyronin A as an effective drug that is currently being developed specifically for use against filarial nematodes.

Frequency, spectrum, and nonzero fitness costs of resistance to myxopyronin in Staphylococcus aureus

The antibiotic myxopyronin (Myx) functions by inhibiting bacterial RNA polymerase (RNAP). The binding site on RNAP for Myx-the RNAP "switch region SW1/SW2 subregion"-is different from the binding site on RNAP for the RNAP inhibitor currently used in broad-spectrum antibacterial therapy, rifampin (Rif). Here, we report the frequency, spectrum, and fitness costs of Myx resistance in Staphylococcus aureus. The resistance rate for Myx is 4 × 10(-8) to 7 × 10(-8) per generation, which is equal within error to the resistance rate for Rif (3 × 10(-8) to 10 × 10(-8) per generation). Substitutions conferring Myx resistance were obtained in the RNAP β subunit [six substitutions: V1080(1275)I, V1080(1275)L, E1084(1279)K, D1101(1296)E, S1127(1322)L, and S1127(1322)P] and the RNAP β' subunit [five substitutions: K334(345)N, T925(917)K, T925(917)R, G1172(1354)C, and G1172(1354)D] (residues numbered as in Staphylococcus aureus RNAP and, in parentheses, as in Escherichia coli RNAP). Sites of substitutions conferring Myx resistance map to the RNAP switch region SW1/SW2 subregion and do not overlap the binding site on RNAP for Rif, and, correspondingly, Myx-resistant mutants exhibit no cross-resistance to Rif. All substitutions conferring Myx resistance exhibit significant fitness costs (4 to 15% per generation). In contrast, at least three substitutions conferring Rif resistance exhibit no fitness costs (≤0% per generation). The observation that all Myx-resistant mutants have significant fitness costs whereas at least three Rif-resistant mutants have no fitness costs, together with the previously established inverse correlation between fitness cost and clinical prevalence, suggests that Myx resistance is likely to have lower clinical prevalence than Rif resistance.

Novel rapidly diversifiable antimicrobial RNA polymerase switch region inhibitors with confirmed mode of action in Haemophilus influenzae

A series of inhibitors with a squaramide core was synthesized following its discovery in a high-throughput screen for novel inhibitors of a transcription-coupled translation assay using Escherichia coli S30 extracts. The inhibitors were inactive when the plasmid substrate was replaced with mRNA, suggesting they interfered with transcription. This was confirmed by their inhibition of purified E. coli RNA polymerase. The series had antimicrobial activity against efflux-negative strains of E. coli and Haemophilus influenzae. Like rifampin, the squaramides preferentially inhibited synthesis of RNA and protein over fatty acids, peptidoglycan, and DNA. However, squaramide-resistant mutants were not cross-resistant to rifampin. Nine different mutations were found in parts of rpoB or rpoC that together encode the so-called switch region of RNA polymerase. This is the binding site of the natural antibiotics myxopyronin, corallopyronin, and ripostatin and the drug fidaxomicin. Computational modeling using the X-ray crystal structure of the myxopyronin-bound RNA polymerase of Thermus thermophilus suggests a binding mode of these inhibitors that is consistent with the resistance mutations. The squaramides are the first reported non-natural-product-related, rapidly diversifiable antibacterial inhibitors acting via the switch region of RNA polymerase.

Influence of DNA template choice on transcription and inhibition of Escherichia coli RNA polymerase

In recent decades, quantitative transcription assays using bacterial RNA polymerase (RNAP) have been performed under widely diverse experimental conditions. We demonstrate that the template choice can influence the inhibitory potency of RNAP inhibitors. Furthermore, we illustrate that the sigma factor (σ(70)) surprisingly increases the transcription efficiency of templates with nonphysiological nonprokaryotic promoters. Our results might be a useful guideline in the early stages of using RNAP for drug discovery.

New target for inhibition of bacterial RNA polymerase: 'switch region'

A new drug target - the 'switch region' - has been identified within bacterial RNA polymerase (RNAP), the enzyme that mediates bacterial RNA synthesis. The new target serves as the binding site for compounds that inhibit bacterial RNA synthesis and kill bacteria. Since the new target is present in most bacterial species, compounds that bind to the new target are active against a broad spectrum of bacterial species. Since the new target is different from targets of other antibacterial agents, compounds that bind to the new target are not cross-resistant with other antibacterial agents. Four antibiotics that function through the new target have been identified: myxopyronin, corallopyronin, ripostatin, and lipiarmycin. This review summarizes the switch region, switch-region inhibitors, and implications for antibacterial drug discovery.

Evaluating the activity of the RNA polymerase inhibitor myxopyronin B against Staphylococcus aureus

Myxopyronin B (MyxB) binds to the switch region of RNA polymerase (RNAP) and inhibits transcriptional initiation. To evaluate the potential development of MyxB as a novel class of antibiotic, we characterized the antimicrobial activity of MyxB against Staphylococcus aureus. Spontaneous MyxB resistance in S. aureus occurred at a frequency of 8 × 10(-8) , similar to that of rifampin. The MyxB-resistant mutants were found to be altered in single amino acid residues in the RNAP subunits that form the MyxB-binding site. In the presence of human serum albumin, the MyxB minimum inhibitory concentration against S. aureus increased drastically (≥128-fold) and 99.5% of MyxB was protein bound. Because of the high serum protein binding and resistance rate, we conclude that MyxB is not a viable starting point for antibiotic development.