Current Trends and Future Approaches in Small-Molecule Therapeutics for COVID-19.

The novel coronavirus (SARS-CoV-2) pandemic has created a global public health emergency. The pandemic is causing substantial morbidity, mortality and significant economic loss. Currently, no approved treatments for COVID-19 are available, and it is likely to takes at least 12-18 months to develop a new vaccine. Therefore, there is an urgent need to find new therapeutics that can be progressed to clinical development as soon as possible. Repurposing regulatory agency-approved drugs and experimental drugs with known safety profiles can provide important repositories of compounds that can be fast-tracked to clinical development. Globally, over 500 clinical trials involving repurposed drugs have been registered, and over 150 have been initiated, including some backed by the World Health Organisation (WHO). This review is intended as a guide to research into small-molecule therapies to treat COVID-19; it discusses the SARS-CoV-2 infection cycle and identifies promising viral therapeutic targets, reports on a number of promising pre-approved small-molecule drugs with reference to over 150 clinical trials worldwide, and offers a perspective on the future of the field.

New Broad-Spectrum Antibiotics Containing a Pyrrolobenzodiazepine Ring with Activity against Multidrug-Resistant Gram-Negative Bacteria.

It is urgent to find new antibiotic classes with activity against multidrug-resistant (MDR) Gram-negative pathogens as the pipeline of antibiotics is essentially empty. Modified pyrrolobenzodiazepines with a C8-linked aliphatic heterocycle provide a new class of broad-spectrum antibacterial agents with activity against MDR Gram-negative bacteria, including WHO priority pathogens. The structure-activity relationship established that the third ring was particularly important for Gram-negative activity. Minimum inhibitory concentrations for the lead compounds ranged from 0.125 to 2 mg/L for MDR Gram-negative, excluding Pseudomonas aeruginosa, and between 0.03 and 1 mg/L for MDR Gram-positive species. The lead compounds were rapidly bactericidal with >5 log reduction in viable count within 4 h for Acinetobacter baumannii and Klebsiella pneumoniae. The lead compound inhibited DNA gyrase in gel-based assays, with an IC50 of 3.16 ± 1.36 mg/L. This study provides a new chemical scaffold for developing novel broad-spectrum antibiotics which can help replenish the pipeline of antibiotics.

Noncytotoxic Pyrrolobenzodiazepine-Ciprofloxacin Conjugate with Activity against Mycobacterium tuberculosis.

The development of new antitubercular agents for the treatment of infections caused by multidrug-resistant (MDR) Mycobacterium tuberculosis is an urgent priority. Pyrrolobenzodiazepines (PBDs) are a promising class of antibacterial agents that were initially discovered and isolated from a range of Streptomyces species. Recently, C8-linked PBD monomers have been shown to work by inhibiting DNA gyrase and have demonstrated activity against M. tuberculosis. However, both PBD monomers and dimers are toxic to eukaryotic cells, limiting their development as antibacterial agents. To eliminate the toxicity associated with PBDs and explore the effect of C8-modification with a known antibacterial agent with the same mechanism of action (i.e., ciprofloxacin, a gyrase inhibitor), we synthesized a C8-linked PBD-ciprofloxacin (PBD-CIP, 3) hybrid. The hybrid compound displayed minimum inhibitory concentration values of 0.4 or 2.1 µg/mL against drug-sensitive and drug-resistant M. tuberculosis strains, respectively. A molecular modeling study showed good interaction of compound 3 with wild-type M. tuberculosis DNA gyrase, suggesting gyrase inhibition as a possible mechanism of action. Compound 3 is a nontoxic combination hybrid that can be utilized as a new scaffold and further optimized to develop new antitubercular agents.

Effects of Systematic Shortening of Noncovalent C8 Side Chain on the Cytotoxicity and NF-κB Inhibitory Capacity of Pyrrolobenzodiazepines (PBDs).

The systematic shortening of the noncovalent element of a C8-linked pyrrolobenzodiazepine (PBD) conjugate (13) led to the synthesis of a 19-member library of C8-PBD monomers. The critical elements of 13, which were required to render the molecule cytotoxic, were elucidated by an annexin V assay. The effects of shortening the noncovalent element of the molecule on transcription factor inhibitory capacity were also explored through an enzyme-linked immunosorbent assay-based measurement of nuclear NF-κB upon exposure of JJN-3 cells to the synthesized molecules. Although shortening the noncovalent interactive element of 13 had a less than expected effect upon compound cytotoxicity due to reduced DNA interaction, the transcription factor inhibitory capacity of the molecule was notably altered. This study suggests that a relatively short noncovalent side chain at the C8 position of PBD is sufficient to confer cytotoxicity. The shortened PBD monomers provide a new ADC payload scaffold because of their potent cytotoxicity and drug-like properties.

C8-Linked Pyrrolobenzodiazepine Monomers with Inverted Building Blocks Show Selective Activity against Multidrug Resistant Gram-Positive Bacteria.

Antimicrobial resistance has become a major global concern. Development of novel antimicrobial agents for the treatment of infections caused by multidrug resistant (MDR) pathogens is an urgent priority. Pyrrolobenzodiazepines (PBDs) are a promising class of antibacterial agents initially discovered and isolated from natural sources. Recently, C8-linked PBD biaryl conjugates have been shown to be active against some MDR Gram-positive strains. To explore the role of building block orientations on antibacterial activity and obtain structure activity relationship (SAR) information, four novel structures were synthesized in which the building blocks of previously reported compounds were inverted, and their antibacterial activity was studied. The compounds showed minimum inhibitory concentrations (MICs) in the range of 0.125-32 µg/mL against MDR Gram-positive strains with a bactericidal mode of action. The results showed that a single inversion of amide bonds reduces the activity while the double inversion restores the activity against MDR pathogens. All inverted compounds did not stabilize DNA and lacked eukaryotic toxicity. The compounds inhibit DNA gyrase in vitro, and the most potent compound was equally active against both wild-type and mutant DNA gyrase in a biochemical assay. The observed activity of the compounds against methicillin resistant S. aureus (MRSA) strains with equivalent gyrase mutations is consistent with gyrase inhibition being the mechanism of action in vivo, although this has not been definitively confirmed in whole cells. This conclusion is supported by a molecular modeling study showing interaction of the compounds with wild-type and mutant gyrases. This study provides important SAR information about this new class of antibacterial agents.