In a recent study posted to the bioRxiv* server, researchers in Germany investigated the effectiveness of the antibiotic nitroxoline against the currently circulating mpox viruses, previously called monkeypox virus (MPXV). This antibiotic has been used in Europe for about fifty years and has been proven effective in fighting biofilm infections.
Study: Repurposing of the antibiotic nitroxoline for the treatment of mpox. Image Credit: Dotted Yeti / Shutterstock
Currently, mpox clade IIB is spreading globally through sustained human-to-human transmission outside its endemic region, Southern Africa. As a result, the World Health Organization (WHO) declared the 2022 mpox outbreak a public health emergency in July 2022.
Around 10% of mpox patients seek hospitalization for treatment of pain and bacterial superinfections. The current mpox outbreak has a unique clinical presentation, and there is no evidence of which drugs are clinically effective against it. However, clinicians are using three antiviral drugs, brincidofovir, cidofovir, and tecovirimat, for their treatment. Unfortunately, Brincidofovir and cidofovir produce toxic side effects. Likewise, tecovirimat availability is limited and affected by resistance formation. Indeed, there is an urgent need for additional and more effective drugs for mpox disease treatment.
About the study
In the present study, researchers investigated whether nitroxoline, 8-hydroxyquinoline-derivative, currently used as the first-line drug therapy for urinary tract infections, could effectively combat mpox viruses. Specifically, they studied the effects of nitroxoline on the 12 mpox virus isolates, especially their replication activity, in a skin explant model.
The researchers chose this antibiotic drug as a repurposing candidate because it is known to inhibit two signaling pathways [e.g., mechanistic Target of Rapamycin (mTOR)], which are critically involved in orthopoxvirus replication. Moreover, it displays a favorable safety profile and exceptional activity against Gram-negative bacteria and fungi.
First, the team cultured primary human foreskin fibroblasts (HFF) and primary human foreskin keratinocytes (HFK) infected with 106 median tissue culture infectious dose (TCID50) per ml of MPXV1 per well of 500 µL. Next, they observed these cultures treated with nitroxoline for 48 hours. Finally, the team used immunohistochemical (IHC) staining to detect virus infection in the skin tissue. Furthermore, the researchers examined the effects of nine nitroxoline analogs for anti-mpox virus activity in HFF.
IHC staining showed that nitroxoline inhibited mpox viral isolates in both skin cultures, albeit in a dose-dependent fashion. Accordingly, IC50, the antibiotic concentration that inhibited mpox virus infection by 50%, ranged between 2.4 and 4.6 µM in HFF and 0.5 and 1.5 µM in HFK, respectively.
Interestingly, time-of-addition experiments showed that nitroxoline inhibited mpox virus infection similarly when added two hours post-infection and simultaneously with the mpox virus. It indicated that it interfered with the mpox virus replication cycle post-viral entry. Basically, it only reduced virus titers when added after the virus absorption that occurred in two hours time window. Experiments with split-thickness human skin grafts showed that 10µM of nitroxoline treatment reduced the number of mpox-infected cells substantially.
Of all nine nitroxoline analogs, compounds 1, 7, and 9 displayed a comparable not superior antiviral activity as nitroxoline. These three harbored halogen ions and a hydroxy group at positions 5, 8, and 9, respectively. Notably, compound 9, a clioquinol, is also under clinical investigation as a treatment for skin infections.
Further, nitroxoline showed potent activity against sexually transmitted bacteria, such as Neisseria gonorrhoeae and Escherichia coli (E. coli), frequently co-transmitting with the mpox virus, which could aggravate the current mpox outbreak. It also inhibited infections by two herpes viruses (HPV), HPV type 1 and varicella-zoster virus, frequently detected with mpox virus, but at >10µM concentrations, higher than what is needed to block mpox virus infection.
Furthermore, the researchers established a tecovirimat-resistant mpox virus strain, MPXV1r TECO. It was also sensitive to nitroxoline at therapeutically achievable concentrations. In contrast, they could not develop a nitroxoline-resistant mpox virus strain using the same approach, thus, indicating that drugs directly targeting virus proteins are not associated with reduced resistance formation.
Overall, nitroxoline effectively inhibited the replication of 12 mpox virus isolates from the current mpox outbreak, with IC50s between 0.5 and 4.6µM but well within the range of therapeutic plasma levels. Nitroxoline also exhibited additive antiviral effects with tecovirimat and synergistic effects with brincidofovir. Notably, its antiviral and antibacterial mechanisms of action differed substantially. For instance, it exerted its antibacterial effects by chelating metal ions, such as Fe2+ and Mn2+.
To conclude, nitroxoline is a good repurposing candidate for mpox disease treatment and might also help control and prevent future global outbreaks. Further research could unravel nitroxoline analogs with a higher anti-mpox virus activity than nitroxoline.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
- Repurposing of the antibiotic nitroxoline for the treatment of mpox, Denisa Bojkova, Nadja Zöller, Manuela Tietgen, Katja Steinhorst, Marco Bechtel, Tamara Rothenburger, Joshua D. Kandler, Julia Schneider, Victor M. Corman, Sandra Ciesek, Holger F. Rabenau, Mark N. Wass, Stefan Kippenberger, Stephan Göttig, Martin Michaelis, Jindrich Cinatl jr., bioRxiv pre-print 2022; DOI: https://doi.org/10.1101/2022.12.29.522228, https://www.biorxiv.org/content/10.1101/2022.12.29.522228v1