RR:C19 Evidence Scale rating by reviewer:
Reliable. The main study claims are generally justified by its methods and data. The results and conclusions are likely to be similar to the hypothetical ideal study. There are some minor caveats or limitations, but they would/do not change the major claims of the study. The study provides sufficient strength of evidence on its own that its main claims should be considered actionable, with some room for future revision.
The manuscript titled “Zinc pyrithione is a potent inhibitor of PLPro and cathepsin L enzymes with ex vivo inhibition of SARS-CoV-2 entry and replication” presents interesting results about the biological activity of zinc-bound-compounds, which is a prerequisite for their possible use as therapeutic options for COVID-19.
The manuscript is generally well-written and the topic and purpose of the research are clearly presented. Indeed, zinc has been suggested to play a role in the relationship between the immune system and SARS-CoV-2 infection, although zinc supplementation did not appear to have any real therapeutic application, as correctly pointed out by the authors. In addition, the investigation of potential therapeutic options for ‘new’ diseases, including the repurposing of already approved drugs is valuable. In particular, the study aims at assessing the possible inhibitory effects of several zinc-bound compounds, namely containing the ligand pyrithione and its analogues, in SARS-CoV-2 infection.
The literature provided in the Introduction is relevant and updated and adequately presents the underlying mechanisms supporting the role of zinc in SARS-CoV-2 infection. The authors also acknowledged the limitation of zinc supplementation alone having no real therapeutic application, thus supporting the need to study zinc-bound compounds.
The methods are generally clear and detailed. All study procedures are adequately described for replication of the experiments and supporting citations are provided in case of reference to methods already reported, including details about quality controls. A clear section of data analysis may be added to summarize the overall modality of data handling and presentation, which is currently only partially provided in the text—generally in figure headings.
Results are also clear and provide an adequate description of the two experimental phases. Using in vitro enzymatic assays, the study suggests inhibitory properties of zinc-bound compounds on both cathepsin L and papain-like protease, with differential effects depending on the type and position of the pyrithione-analogue ligand. The study also compares the effects of compounds alone without zinc and of their paired zinc-bound compounds and supports the importance to use zinc complexes due to higher efficacy. The study also used ex vivo assays to test SARS-CoV-2 entry and replication performed on primary human lung tissue. Despite some differential results from previous experiments, the authors selected one zinc-bound compound only for this second experiment. This limited the generalization of the latter study's findings to other complexes. Nonetheless, the study suggests the higher inhibitor effects of zinc-pyrithione complex compared to both ruthenium-pyrithione complex and pyrithione alone.
Finally, the Discussion is generally updated to recent literature on the topic and presents the main study findings in an unbiased manner. Indeed, it provides references supporting the possible biological activity of zinc complexes against SARS-CoV-2 infection, as well as highlighting the still limited evidence for the use of zinc supplementation for COVID-19 provided by clinical trials, concluding one investigated compound is a promising candidate against COVID-19 adding caveat due to study limitations.
Overall, the study can be considered reliable and minor revisions seem necessary to improve the general good quality of the report.