Opportunities and Challenges for Natural Products as Novel Antituberculosis Agents

Isoflavonoid and Furanochromone Natural Products as Potential DNA Gyrase Inhibitors: Computational, Spectral, and Antimycobacterial Studies

In pursuit of new antitubercular agents, we here report the antimycobacterial (H₃₇Rv) and DNA gyrase inhibitory potential of daidzein and khellin natural products (NPs). We procured a total of 16 NPs based on their pharmacophoric similarities with known antimycobacterial compounds. The H₃₇Rv strain of M. tuberculosis was found to be susceptible to only two out of the 16 NPs procured; specifically, daidzein and khellin each exhibited an MIC of 25 μg/mL. Moreover, daidzein and khellin inhibited the DNA gyrase enzyme with IC₅₀ values of 0.042 and 0.822 μg/mL, respectively, compared to ciprofloxacin with an IC₅₀ value of 0.018 μg/mL. Daidzein and khellin were found to have lower toxicity toward the vero cell line, with IC₅₀ values of 160.81 and 300.23 μg/mL, respectively. Further, molecular docking study and MD simulation of daidzein indicated that it remained stable inside the cavity of DNA GyrB domain for 100 ns.

INSIGHTS INTO THERAPEUTIC LIQUID MIXTURES AND FORMULATIONS TOWARDS TUBERCULOSIS THERAPY

Therapeutic liquid mixtures, as deep eutectic systems, are considered a sustainable strategy that can be useful for the modification and enhancement of the pharmacokinetics and pharmacodynamics of different active ingredients. In this study, we assessed the stability and antibacterial activity of therapeutic liquid formulations prepared with anti-tuberculosis drugs. Tuberculosis therapy presents various pitfalls related, for example, to the administration of prolonged regimens of multiple drugs, different severe adverse effects, low compliance of the patient to treatment and the development of drug resistance. During this study, it was possible to assess the physicochemical stability of the formulations for 6 months, by polarized optical microscopy, ¹H-NMR and FTIR-ATR. Furthermore, the mixtures present an antibacterial effect against a drug-susceptible Mycobacterium tuberculosis strain (H37Rv). This was particularly evident for the mixtures with ethambutol incorporated, making them interesting to pursue with further studies and evaluation of clinical applicability. Upon infection, it was also observed that a single and higher dose appears to be more effective than lower separate doses, which could allow the production of patient-friendly formulations.

Investigation of Copaifera genus as a new source of antimycobaterial agents

AIM

This paper reports on the antimycobacterial activity of the oleoresins and extracts obtained from Copaifera spp.

MATERIALS & METHODS

The minimum inhibitory concentration (MIC) and fractional inhibitory concentration index techniques helped to evaluate the effect of these oleoresins and extracts against six strains of mycobacteria that cause tuberculosis.

RESULTS & CONCLUSION

Among the assayed oleoresins and plant extracts, the Copaifera langsdorffii, Copaifera duckei, Copaifera reticulata and Copaifera trapezifolia oleoresins provided the lowest MIC values against some of the tested strains. The combination of Copaifera spp. samples with isoniazid did not evidence any synergistic action. Some Copaifera spp. oleoresins may represent a future source for the discovery of new antimycobacterial drugs due to their low MIC values.

Evaluation of peptide-rich root extracts of Calliandria portoriscensis (Jacq.) Benth (Mimosaceae) for in vitro antimicrobial activity and brine shrimp lethality

BACKGROUND

Several Host defence peptides (HDPs) are low molecular weight (< 50 amino acids residues) peptides detected in several ethnomedicinal plants and have particularly gained research interest in recent times. Due to their wide range of bioactivity, occurrence, abundance and ability to induce very little resistance, they hold promising potentials in drug development. This study investigated the presence of bioactive peptides in the roots of Calliandra portoricensis (CPr) (Mimosaceae) and evaluated its antimicrobial activity against gram-negative and gram-positive bacteria.

METHODS

The crude peptide extract was obtained and pre-purified on pre-loaded tube of RP-C18 solid phase cartridges (strata giga tube C18-E; 5 g, 20 mL, Phenomenex, Germany). Peptide enriched fraction was chemically analysed for arginine-rich/aromatic amino acid-rich peptides using a modified G-250 analytical stain and ninhydrin on thin layer chromatography (TLC) for a preliminary screening. Furthermore, MALDI TOF/TOF peptidomics was used to detect the presence and masses of the peptides. Extracts from CPr were used to test the ability to inhibit microbial growth using p-INT (Para-iodonitrotetrazolium violet) dye, with 0.1% gentamycin as positive control. The concentration that inhibits the growth of microorganisms by 50% (IC50) were determined. Toxicity of the two extracts was accessed using freshly hatched nauplii of Artemia salina. Data analysis were evaluated using Microsoft excel and GraphPad Prism5.

RESULTS

Low molecular weight (LMW) peptides were detected in CPr using TLC and MALDI-TOF MS. Generally, the extracts exhibited good inhibition (70-95%) against the gram-negative and gram-positive bacteria, except MRSA6 typed strain. Enhanced activity was observed in the pre-purified peptide fraction than in the methanol crude, except on MRSA6. The greatest antimicrobial inhibition by pre-purified peptide fraction was against MRSA22 (IC50 = 0.69 ± 0.33 μg/mL). The crude methanol extract (LC50 = 5.13 μg/mL) was slightly more toxic than the peptide extract (LC50 = 6.12 μg/mL).

CONCLUSIONS

This is the first report on detection of bioactive LMW peptides in Mimosaceae family. These peptides appear to be rich in arginine and aromatic amino acids. The peptide extract, in its pre-purified form showed a lower Brine shrimp cytotoxicity and an enhanced antimicrobial activity against the tested gram-negative and gram-positive bacteria.

Marine Natural Products and Drug Resistance in Latent Tuberculosis

Pyrazinamide (PZA) is the only drug for the elimination of latent Mycobacterium tuberculosis (MTB) isolates. However, due to the increased number of PZA-resistance, the chances of the success of global TB elimination seems to be more prolonged. Recently, marine natural products (MNPs) as an anti-TB agent have received much attention, where some compounds extracted from marine sponge, Haliclona sp. exhibited strong activity under aerobic and hypoxic conditions. In this study, we screened articles from 1994 to 2019 related to marine natural products (MNPs) active against latent MTB isolates. The literature was also mined for the major regulators to map them in the form of a pathway under the dormant stage. Five compounds were found to be more suitable that may be applied as an alternative to PZA for the better management of resistance under latent stage. However, the mechanism of actions behind these compounds is largely unknown. Here, we also applied synthetic biology to analyze the major regulatory pathway under latent TB that might be used for the screening of selective inhibitors among marine natural products (MNPs). We identified key regulators of MTB under latent TB through extensive literature mining and mapped them in the form of regulatory pathway, where SigH is negatively regulated by RshA. PknB, RshA, SigH, and RNA polymerase (RNA-pol) are the major regulators involved in MTB survival under latent stage. Further studies are needed to screen MNPs active against the main regulators of dormant MTB isolates. To reduce the PZA resistance burden, understanding the regulatory pathways may help in selective targets of MNPs from marine natural sources.

Antibacterial Activity and Mechanism of Action of Aspidinol Against Multi-Drug-Resistant Methicillin-Resistant Staphylococcus aureus

This study aimed at investigating the antibacterial activity of aspidinol, an extract from Dryopteris fragrans (L.) Schott, against methicillin-resistant Staphylococcus aureus (MRSA). MRSA isolates were treated with aspidinol to determine the differential expression of genes and associated pathways following the drug treatment. Aspidinol displayed significant anti-MRSA activity, both in vivo (minimum inhibitory concentration = 2 μg/mL) and in vitro, and achieved an antibacterial effect comparable to that of vancomycin. In the lethal septicemic mouse study, a dose of 50 mg/kg of either aspidinol or vancomycin provided significant protection from mortality. In the non-lethal septicemic mouse study, aspidinol and vancomycin produced a significant reduction in mean bacterial load in murine organs, including the spleen, lung, and liver. After treatment with aspidinol, we found through RNA-seq and RT-PCR experiments that the inhibition of the formation of ribosomes was the primary S. aureus cell-killing mechanism, and the inhibition of amino acid synthesis and the reduction of virulence factors might play a secondary role.

Selective Killing of Dormant Mycobacterium tuberculosis by Marine Natural Products

The dormant phenotype acquired by Mycobacterium tuberculosis during infection poses a major challenge in disease treatment, since these bacilli show tolerance to front-line drugs. Therefore, it is imperative to find novel compounds that effectively kill dormant bacteria. By screening 4,400 marine natural product samples against dual-fluorescent M. tuberculosis under both replicating and nonreplicating conditions, we have identified compounds that are selectively active against dormant M. tuberculosis This validates our strategy of screening all compounds in both assays as opposed to using the dormancy model as a secondary screen. Bioassay-guided deconvolution enabled the identification of unique pharmacophores active in each screening model. To confirm the activity of samples against dormant M. tuberculosis, we used a luciferase reporter assay and enumerated CFU. The structures of five purified active compounds were defined by nuclear magnetic resonance (NMR) and mass spectrometry. We identified two lipid compounds with potent activity toward dormant and actively growing M. tuberculosis strains. One of these was commercially obtained and showed similar activity against M. tuberculosis in both screening models. Furthermore, puupehenone-like molecules were purified with potent and selective activity against dormant M. tuberculosis In conclusion, we have identified and characterized antimycobacterial compounds from marine organisms with novel activity profiles which appear to target M. tuberculosis pathways that are conditionally essential for dormancy survival.

New Isoflavonoids from the extract of Rhynchosia precatoria (Humb. & Bonpl. ex Willd.) DC. and their antimycobacterial activity

ETHNOPHARMACOLOGY RELEVANCE

The evaluation of the antimycobacterial activity of extracts of medicinal plants used by Mayos against tuberculosis and respiratory problems, allowed the identification of Rhynchosia precatoria (Humb. & Bonpl. ex Willd.) DC (Fabaceae) as the best candidate to find new antimycobacterial compounds.

AIM OF THE STUDY

To isolate and characterize the compounds of R. precatoria responsible for the inhibitory and bactericidal activity against Mycobacterium tuberculosis H37Rv and Mycobacterium smegmatis ATCC 700084. To determine antimycobacterial synergistic effect of pure compounds and their selectivity index towards Vero cells.

MATERIALS AND METHODS

A total of six flavonoids were purified by silica gel column chromatography. Structural elucidation of the isolated compounds was achieved by using 1D and 2D NMR spectroscopy techniques. The configuration at the C-3 chiral center was established by quantum mechanical calculation of the electronic circular dichroism (ECD) spectrum. In vitro inhibitory and bactericidal activity against M. tuberculosis and M. smegmatis were determined with the redox indicator Alamar Blue (resazurin). Synergy was determined by X/Y quotient. Cytotoxicity was measured by MTT assay.

RESULTS

The isolated compounds were identified as precatorin A (1), precatorin B (2), precatorin C (3), lupinifolin (4), cajanone (5) and lupinifolinol (6). Compounds 1-3 are new. Compounds 1 to 5 inhibited the growth of M. tuberculosis (MIC ≥31.25µg/mL); compounds 1, 2, 4 and 5 killed the bacteria (MBC ≥31.25µg/mL) and also inhibited M. smegmatis (MIC ≥125µg/mL), while 1 and 4 also resulted bactericidal (MBC ≥125µg/mL). Compounds 4 and 5 presented synergistic effect (X/Y quotient value <0.5) at a concentration of 1/2 MIC of each compound in the combination. Cytotoxicity in murine macrophages (RAW 264.7 cells) gave IC₅₀ values of 13.3-46.98µM, for compounds 1-5.

CONCLUSIONS

In this work we isolated two new isoflavanones (1 and 2), and one new isoflavone (3) with a weak antimycobacterial activity. The (3R) absolute configuration was assigned to 1 by computational analysis of its ECD spectrum and to 2 and 5 by similarity of their ECD spectra with that of 1. We are also reporting by first time, activity against virulent strain of M. tuberculosis for compounds 4 and 5 and their antimycobacterial synergistic effect.

Collaborative drug discovery for More Medicines for Tuberculosis (MM4TB)

Neglected disease drug discovery is generally poorly funded compared with major diseases and hence there is an increasing focus on collaboration and precompetitive efforts such as public-private partnerships (PPPs). The More Medicines for Tuberculosis (MM4TB) project is one such collaboration funded by the EU with the goal of discovering new drugs for tuberculosis. Collaborative Drug Discovery has provided a commercial web-based platform called CDD Vault which is a hosted collaborative solution for securely sharing diverse chemistry and biology data. Using CDD Vault alongside other commercial and free cheminformatics tools has enabled support of this and other large collaborative projects, aiding drug discovery efforts and fostering collaboration. We will describe CDD's efforts in assisting with the MM4TB project.