Simple and Efficient Synthesis of 3-Aryl-2-oxazolidinone Scaffolds Enabling Increased Potency toward Biofilms.

Infectious diseases caused by bacterial pathogens are a leading cause of mortality worldwide. In particular, recalcitrant bacterial communities known as biofilms are implicated in persistent and difficult to treat infections. With a diminishing antibiotic pipeline, new treatments are urgently required to combat biofilm infections. An emerging strategy to develop new treatments is the hybridization of antibiotics. The benefit of this approach is the extension of the useful lifetime of existing antibiotics. The oxazolidinones, which include the last resort antibiotic linezolid, are an attractive target for improving antibiofilm efficacy as they present one of the most recently discovered classes of antibiotics. A key step in the synthesis of new 3-aryl-2-oxazolidinone derivatives is the challenging formation of the oxazolidinone ring. Herein we report a direct synthetic route to the piperazinyl functionalized 3-aryl-2-oxazolidinone 17. We also demonstrate an application of these piperazine molecules by functionalizing them with a nitroxide moiety as a strategy to extend the useful lifetime of oxazolidinones and improve their potency against Methicillin-resistant Staphylococcus aureus (MRSA) biofilms. The antimicrobial susceptibility of the linezolid-nitroxide conjugate 11 and its corresponding methoxyamine derivative 12 (a control for biofilm dispersal) was assessed against planktonic cells and biofilms of MRSA. In comparison to linezolid and our lead compound 10 (a piperazinyl oxazolidinone derivative), the linezolid-nitroxide conjugate 11 displayed a minimum inhibitory concentration that was 4-16-fold higher. The opposite effect was seen in biofilms where the linezolid-nitroxide hybrid 11 was >2-fold more effective (160 µg/mL versus >320 µg/mL) in eradicating MRSA biofilms. The methoxyamine derivative 12 performed on par with linezolid. The drug-likeness of the compounds was also assessed, and all compounds were predicted to have good oral bioavailability. Our piperazinyl oxazolidinone derivative 10 was confirmed to be lead-like and would be a good lead candidate for future functionalized oxazolidinones. The modification of antibiotics with a dispersal agent appears to be a promising approach for eradicating MRSA biofilms and overcoming the antibiotic resistance associated with the biofilm mode of growth.

Comparative Assessment of Docking Programs for Docking and Virtual Screening of Ribosomal Oxazolidinone Antibacterial Agents.

Oxazolidinones are a broad-spectrum class of synthetic antibiotics that bind to the 50S ribosomal subunit of Gram-positive and Gram-negative bacteria. Many crystal structures of the ribosomes with oxazolidinone ligands have been reported in the literature, facilitating structure-based design using methods such as molecular docking. It would be of great interest to know in advance how well docking methods can reproduce the correct ligand binding modes and rank these correctly. We examined the performance of five molecular docking programs (AutoDock 4, AutoDock Vina, DOCK 6, rDock, and RLDock) for their ability to model ribosomal-ligand interactions with oxazolidinones. Eleven ribosomal crystal structures with oxazolidinones as the ligands were docked. The accuracy was evaluated by calculating the docked complexes' root-mean-square deviation (RMSD) and the program's internal scoring function. The rankings for each program based on the median RMSD between the native and predicted were DOCK 6 > AD4 > Vina > RDOCK >> RLDOCK. Results demonstrate that the top-performing program, DOCK 6, could accurately replicate the ligand binding in only four of the eleven ribosomes due to the poor electron density of said ribosomal structures. In this study, we have further benchmarked the performance of the DOCK 6 docking algorithm and scoring in improving virtual screening (VS) enrichment using the dataset of 285 oxazolidinone derivatives against oxazolidinone binding sites in the S. aureus ribosome. However, there was no clear trend between the structure and activity of the oxazolidinones in VS. Overall, the docking performance indicates that the RNA pocket's high flexibility does not allow for accurate docking prediction, highlighting the need to validate VS. protocols for ligand-RNA before future use. Later, we developed a re-scoring method incorporating absolute docking scores and molecular descriptors, and the results indicate that the descriptors greatly improve the correlation of docking scores and pMIC values. Morgan fingerprint analysis was also used, suggesting that DOCK 6 underpredicted molecules with tail modifications with acetamide, n-methylacetamide, or n-ethylacetamide and over-predicted molecule derivatives with methylamino bits. Alternatively, a ligand-based approach similar to a field template was taken, indicating that each derivative's tail groups have strong positive and negative electrostatic potential contributing to microbial activity. These results indicate that one should perform VS. campaigns of ribosomal antibiotics with care and that more comprehensive strategies, including molecular dynamics simulations and relative free energy calculations, might be necessary in conjunction with VS. and docking.

In Vitro Activities of Oxazolidinone Antibiotics Alone and in Combination with C-TEMPO against Methicillin-Resistant Staphylococcus aureus Biofilms.

Infections caused by methicillin-resistant Staphylococcus aureus (MRSA) are a global health concern. The propensity of MRSA to form biofilms is a significant contributor to its pathogenicity. Strategies to treat biofilms often involve small molecules that disperse the biofilm into planktonic cells. Linezolid and, by extension, theoxazolidinones have been developed to treat infections caused by Gram-positive bacteria such as MRSA. However, the clinical development of these antibiotics has mainly assessed the susceptibility of planktonic cells to the drug. Previous studies evaluating the anti-biofilm activity of theoxazolidinones have mainly focused on the biofilm inhibition of Enterococcus faecalis and methicillin-sensitive Staphylococcus aureus, with only a few studies investigating the activity of oxazolidinones for eradicating established biofilms for these species. Very little is known about the ability of oxazolidinones to eradicate MRSA biofilms. In this work, five oxazolidinones were assessed against MRSA biofilms using a minimum biofilm eradication concentration (MBEC) assay. All oxazolidinones had inherent antibiofilm activity. However, only ranbezolid could completely eradicate MRSA biofilms at clinically relevant concentrations. The susceptibility of the MRSA biofilms to ranbezolid was synergistically enhanced by coadministration with the nitroxide biofilm dispersal agent C-TEMPO. We presume that ranbezolid acts as a dual warhead drug, which combines the mechanism of action of the oxazolidinones with a nitric oxide donor or cytotoxic drug.

Strategies to Improve the Potency of Oxazolidinones towards Bacterial Biofilms.

Biofilms are part of the natural lifecycle of bacteria and are known to cause chronic infections that are difficult to treat. Most antibiotics are developed and tested against bacteria in the planktonic state and are ineffective against bacterial biofilms. The oxazolidinones, including the last resort drug linezolid, are one of the main classes of synthetic antibiotics progressed to clinical use in the last 50 years. They have a unique mechanism of action and only develop low levels of resistance in the clinical setting. With the aim of providing insight into strategies to design more potent antibiotic compounds with activity against bacterial biofilms, we review the biofilm activity of clinically approved oxazolidinones and report on structural modifications to oxazolidinones and their delivery systems which lead to enhanced anti-biofilm activity.