Could Iron Oxide Nanozymes Treat, Prevent, and Even Diagnose Dental Caries?

By Dr. Mehmood Asghar

Dental caries affect over 2.3 billion people worldwide (Wen et al.). This multi-factor disease has also been regarded as the most common infection condition affecting humankind (Peres et al., 2019). Unfortunately, the current surgical-restorative protocol to treat dental caries has become unsustainable globally and puts immense pressure on the economy and healthcare systems worldwide. Hence, the treatment philosophy is shifting toward a cost-effective, non-invasive caries management model.

Dental Caries and Oral Biofilm

Among the primary factors for caries development and progression is the formation of pathogenic biofilms on the teeth caused by poor oral hygiene and intake of a sugar-rich diet. The currently available methods to prevent or disrupt these pathogenic biofilms – and to avoid caries progression – involves using antibacterial agents with limited efficacy. Currently, caries research focuses on developing preventive agents with potent antibacterial and biofilm disruption efficacy. The use of bioactive nanoparticles has shown great promise in this regard.

Iron Oxide Nanozymes Could be the Next Caries Preventive Agents

Recently, a collaborative research study of the University of Pennsylvania and Indiana University has shown that iron oxide nanoparticles (NPs) – an FDA-approved formulation for treating iron deficiency anemia (IDA) – could potentially treat, prevent, and even diagnose dental caries. Although there is a growing body of evidence linking IDA to dental caries, this study has gone one step further, indicating that iron oxide nanoparticles had a targeted effect on the cariogenic microbes in the dental plaque due to their enzyme-like action. These NPs with combined antibacterial and enzymatic effects are also known as nanozymes.

“We found that this approach is both precise and effective,” says Hyun (Michel) Koo DDS, MS, Ph.D., who is one of the authors and a professor at the University of Pennsylvania School of Dental Medicine. “It disrupts biofilms, particularly those formed by Streptococcus mutans, which cause caries, and it also reduces the extent of enamel decay. This is the first study we know of done in a clinical setting that demonstrates the therapeutic value of nanozymes against infectious disease.” This study (Liu et al., 2021) was published in the scientific journal Nano Letters.

The Methodology

The authors conducted a randomized crossover study in which an iron oxide NP formulation known as Ferumoxytol (FerIONP)/ hydrogen peroxide (H2O2) combination was used. A total of 16 patients were recruited for this study who were randomly allocated into three groups, as shown below:

The study was divided into three 14-day long phases, where the subjects were required to visit ten times during the study. The subjects were provided mandibular dentures containing two bovine teeth used for the analysis. High caries risk conditions were generated by exposing the specimen teeth to 20% sucrose solutions four times daily. Similarly, specimen teeth did not receive any mechanical cleaning – apart from water rinsing – to ensure biofilm and plaque buildup. The other teeth were brushed using the provided non-fluoridated toothpaste.

At the end of each treatment phase, the dentures were removed, and the specimen teeth were tested for biofilm analysis and antibacterial efficacy. In addition, the ability of FerIONP to detect oral pathogenic bacterial species such as Streptococcus oralis, Streptococcus gordonii, Streptococcus sanguinis, and Actinomycetes naeslundi was also investigated.

The Results

1. FerIONP as Antibiofilm and Anticaries Agent: Bacterial biofilm analysis showed the complete elimination of S. mutans from the bacterial biofilm on enamel specimens in Group 2. Interestingly, the treatment of H2O2 alone did not exhibit considerable neutralization of S. mutans colonies. Similar results were also achieved through confocal microscopy, which showed the highest live/dead ratio for Group 1, indicating the efficacy of FerIONPs/H2O2 in disrupting bacterial biofilms and preventing dental caries.

2. Surface Microhardness: At the end of each testing phase, the enamel specimens were removed from the dentures, and the surface microhardness was measured. The highest surface microhardness – an indirect measure of the tooth’s mineral content – was shown by Group 1 specimens, followed by Group 2, and the least for Group 3. These results again revealed that FerIONPs could prevent caries formation and progression, thereby enabling the specimens to maintain their mineral content.

3. Antibacterial Efficacy of FerIONPs Against S. mutans: To evaluate the efficacy of FerIONPs in binding to and killing oral pathogens, 1mg/mL of FerIONPs were incubated individually with common pathogenic bacteria such as Streptococcus oralis, Streptococcus gordonii, Streptococcus sanguinis, and Actinomycetes naeslundi. Interestingly, FerIONPs had the highest binding affinity – and hence neutralization – against S. mutans, while similar results were obtained for A. naeslundi. Again, this proved that the FerIONPs possessed potent antibacterial efficacy, and they could play a pivotal role in caries prevention.

4. FerIONPs Can Detect Pathogenic Bacteria: To assess whether FerIONPs could detect pathogenic bacteria in biofilms, hydroxyapatite-coated discs were used to promote biofilms of S. mutans and S. oralis –individually or as a multi-species biofilm model. The bacteria were grown in high- or low-sugar conditions, thereby generating acidic or non-acidic biofilms, respectively. The discs were coated with the respective bacterial species, followed by a colorimetric assay using 3,3′,5,5′-tetramethylbenzidine (TMB). TMB is oxidized by FerIONPS, forming blue reaction products. This test showed that the biofilms exposed to high-sugar conditions had higher S. mutans counts, as evidenced by the significantly higher absorption of blue color. So, the higher the S. mutans count, the higher will be blue color absorption. This test proved that FerIONPs could be used effectively for caries detection.

The Clinical Impact

The pioneering, multi-pronged results of this study can have long-lasting results. FerIONPs/H2O2 combination can neutralize oral pathogens by releasing reactive oxygen species (ROS). Besides, iron oxide/H2O2 NPs could be used to detect cariogenic bacteria in the future. However, further research is still needed to translate these findings into clinical settings.

Author: Dr. Mehmood Asghar is a dentist by profession and an Assistant Professor of Dental Biomaterials at the National University of Medical Sciences, Pakistan. Dr. Asghar received his undergraduate and postgraduate dental qualifications from the National University of Science and Technology (NUST). He is also currently pursuing a Ph.D. in Restorative Dentistry from Malaysia. Apart from his hectic clinical and research activities, Dr. Asghar likes to write evidence-based, informative articles for dental professionals and patients. Dr. Asghar has published several articles in international, peer-reviewed journals.

References:

Liu, Y., Huang, Y., Kim, D., Ren, Z., Oh, M. J., Cormode, D. P., Hara, A. T., Zero, D. T., & Koo, H. (2021). Ferumoxytol nanoparticles target biofilms causing tooth decay in the human mouth. Nano Letters,21(22), 9442-9449.

Peres, M. A., Macpherson, L. M., Weyant, R. J., Daly, B., Venturelli, R., Mathur, M. R., Listl, S., Celeste, R. K., Guarnizo-Herreño, C. C., & Kearns, C. (2019). Oral diseases: A global public health challenge. The Lancet, 394(10194), 249-260.

Wen, P. Y. F., Chen, M. X., Zhong, Y. J., Dong, Q. Q., & Wong, H. M. Global burden and inequality of dental caries, 1990 to 2019. Journal of Dental Research, 0(0), 00220345211056247.