Palladium concave nanocrystals with high-index facets accelerate ascorbate oxidation in cancer treatment

Intravenous pharmacological dose of ascorbate has been proposed as a potential antitumor therapy; however, its therapeutic efficacy is limited due to the slow autoxidation. Here, we report that palladium (Pd) nanocrystals, which possess intrinsic oxidase-like activity, accelerate the autoxidation of ascorbate, leading to the enhancement of its antitumor efficacy. The oxidase-like activity of Pd nanocrystals was facet-dependent, with the concave nanostructure enclosed by high-index facets catalyzing ascorbate autoxidation more efficiently than the planar nanostructure enclosed by low-index facets. Our first-principles calculations provide the underlying molecular mechanisms for the facet-dependent activation of O2 molecule and subsequent ascorbate oxidation. Further in vitro and in vivo assays demonstrate the enhancement of the antitumor efficacy of ascorbate with these Pd concave nanocubes. Our animal experiments also indicate the combined approach with both ascorbate and Pd concave nanocubes displays an even better efficacy than currently available clinical medicines, with no obvious cytotoxicity to normal cells.


Re: Manuscript ID: NCOMMS-18-05843
Dear Dr. Ylenia Lombardo, We sincerely thank you for your prompt response to our submission and also appreciate both referees' very positive and constructive comments, which have further improved our manuscript.
We have identified the oxidized products of ascorbate in the presence of Pd concave nanocubes by mass spectrometry ( Figure  Finally, we would like to express our gratitude to you for overseeing this review process. We hope the revised manuscript is now acceptable for publication in Nature Communications.
Look forward to hearing from you soon.

IBM Research and Columbia University
Reviewer #1 (Remarks to the Author): Major claims of the paper: The paper claims that palladium nanocrystals can be used to oxidize ascorbate, a reaction that can be used in cancer therapy. A number of palladium nanoparticle morphologies are designed and tested and it is claimed that the reaction is specific to the shape of the nanocrystal. Overall this is a well written paper and the data shown is well organized. RESPONSE: Thanks for the very positive comment. We are grateful for the time and effort expended by the reviewer.
1. Despite the good presentation, the results are rather surprising. Pd NPs are well known to produce ROS, and so are many other metallic NPs. So the overall result of having Pd in combination with the ascorbate result in oxidation is expected. What is surprising is that the ROS generated does not affect healthy/normal cells. This is an interesting result though it is not clear how well this is relative to other cells (perhaps not tested in this paper). This is a significant claim and I would like to suggest that other cells are tested. RESPONSE: We thank the reviewer for this concern and giving us the opportunity to explain this result. In our study, the as-prepared Pd NPs showed no obvious cytotoxicity in concentrations up to 100 μg/mL for both cancer and normal cells, as shown in Figure S6. This good biocompatibility may come from the presence of remnants of the PVP stabilizer on the surface of the Pd NPs.
However, in the presence of ascorbate, Pd NPs could selectively kill the tumor cells.
That is because the differential sensitivity between cancer cells and normal cells to the product of ascorbate oxidation, H 2 O 2 . Altered redox-active iron metabolism in cancer cells makes them more sensitive to the change of H 2 O 2 level than normal cells. [1] In addition, in order to further confirm this selective killing effect, other cells,  2. The other issue is trying to determine the molecular mechanism for the reaction of ascorbate oxidation. Products have not been analyzed using molecular structural techniques such as nuclear magnetic resonance spectroscopy or mass spectrometry.
These are important techniques that could shed significant information on the molecular mechanisms by which ascorbate is oxidized. RESPONSE: We thank the reviewer for this valuable suggestion. To confirm the structural transformation of ascorbate in the presence of Pd concave nanocubes, we analyzed its oxidation product using mass spectrometry (MS). As shown in Figure S2, we found that (i) negative ion mode provide the best response for both ascorbic acid 3. I believe the article will inspire thinking in the field, but before that it is important to get a better experimental understanding of the molecular mechanisms involved given the major claims made in this article.
Overall this is interesting work. RESPONSE: We thank the Reviewer for this concern. We have carried out time-dependent bio-distribution experiment to investigate the clearance behaviors of Pd nanoparticles. The results showed that these Pd NPs could be efficiently cleared from the mouse body, with a majority of the Pd NPs eliminated through fecal excretion. In the future work, we will fine tune the size and surface property of nanoparticles to increase the renal excretion of nanoparticles.
5. The initial size of the tumor is extremely small (about 50 mm3). This should more considered as a preventive rather than a curative study.
RESPONSE: Thank you for this good point. The initial tumor volume of mice have been further increased to approximately 80 mm 3 in our renewed in vivo experiments in comparison with oxaliplatin and 5-fluorouracil (5-FU), as mentioned above, which is widely used in mouse model for optimal cancer therapy. As shown in the new Figure 7, the combined treatment of Pd/Vc remarkably suppresses tumor growth, as effective as the current clinical first-line medicines, but with less side effects (weight loss). Therefore, we believe the application of Pd nanocrystals might indeed show promise in the development of novel ascorbate-based antitumor agents.