Gold Nanoparticles
Compared to AgNPs, AuNPs exhibit reduced toxicity on healthy cells, making them more attractive for in vivo and clinical applications.38 Indeed, AuNPs have been successfully tested as inhibitors of viral entry into the host cells. AuNPs interact with hemagglutinin (HA), where Au is able to oxidize the disulfide bond of this glycoprotein causing its inactivation, thus impeding the membrane fusion of the virus with host cells. Targeting HA has emerged as an alternative strategy to the actual therapies (e.g., matrix protein 2 and neuramidase), especially to pandemic viruses that show an accelerated mutation speed of their surface proteins, hence a resistance to conventional treatments increasing their infectivity and mortality.38 This strategy has been applied to influenza (e.g., H1N1, HCV) and herpes viruses.39−44 The activity of AuNPs is proportional to the surface area exposed. As a consequence, the size and the morphology of these metal NPs play a substantial role in their antiviral activity. Recently, Kim et al. have reported that porous AuNPs are able to inhibit influenza A infection more efficiently than nonporous AuNPs.39 This effect has been associated with the higher surface area of the porous material that favors their interaction with capsids and thus increases their antiviral activity (Figure 4).
Figure 4  Schematic illustration of inactivation of influenza A virus (IAV) treated with porous AuNP (PoGNP). PoGNP interacts with IAV surface proteins and cleaves their disulfide bonds. Inactivated viruses exhibit lower infectivity to cells. Reproduced with permission under a Creative Commons CC-BY license from ref (39). Copyright 2020 BioMed Central Ltd., Springer Nature. Besides the per se antiviral activity, AuNP surface modifications have been developed in order to enhance their overall therapeutic benefits. The engineering of tailored AuNPs with selected ligands has allowed the preparation of efficient antiviral nanoagents. The target ligands can be introduced directly during the particle synthesis via ligand exchange reactions or ligand modifications. For instance, direct reduction of gold ions in the presence of gallic acid produced homogeneous AuNPs able to sensibly reduce herpes simplex virus infection in vitro.40 Compared to free ligand NPs, functionalized AuNPs benefit from the multivalency effect and higher circulation times, decreasing the needed therapeutic concentrations.39 Functionalized AuNPs can present organic groups that mimic host cell surfaces or other specific molecular patterns that selectively target the virus. Normally, negative charges are used to mimic cell surfaces and favor the interaction between the particles and the capsid. In particular, sulfonates and organic sulfates have been used for their capacity to attract the virus via capsid protein interaction and block the HA activity.41 AuNPs functionalized with sulfonates showed an increasing inhibition of influenza A compared to the nanoparticles capped with succinic acid.42 This study also demonstrated that there is not a correlation between the negative charge and the antiviral activity, but instead the inhibition depends mainly on the organic groups used. Thiol-capped AuNPs also displayed powerful inactivation of bovine viral diarrhea virus in vitro.43 Multivalency has been exploited in more complex systems using dendrons as capping agents. This strategy allows to generate higher concentrations of the target ligand in close proximity to the AuNPs and to increase the binding efficiency of the nanoparticles to the capsid. The driving force of the antiviral efficiency relies on the concentration of the targeting agent onto the particles. Sulfonated dendrons were grafted to AuNPs via a sulfide bond and tested for HIV inhibition.44 The results showed that the decorated AuNPs exerted a higher affinity to the virus. Additionally, comparing AuNPs functionalized with different generation dendrons, those with a third generation displayed the highest inhibition performance with an IC50 below 0.1 μmol/mL, thus making them attractive for in vivo translation. It is worth noting that the inhibition efficiency is strictly dependent on the available sulfonate groups present on the surface of the NPs, making crucial a thorough characterization of the material.44 The size of the AuNPs clearly plays an important role in the concentration of targeting ligands exposed per particle.45 Indeed, too big NPs have a limited surface area, while too small would not allow an efficient grafting of the dendrons due to steric hindrance. For instance, it has been shown that dendron-functionalized AuNPs showed a size-dependent antiviral activity for influenza virus, where 14 nm particles exhibited a higher efficiency than 2 nm AuNPs. This has been associated with the low functionalization grade of the small nanoparticles and to the inappropriate spatial distribution of the interacting ligand/receptor pairs.
The development of viral proteomics has profoundly transformed the antiviral and disinfection strategies. In particular, small molecules and peptides able to target and block the viral biochemical machinery have been developed. However, despite these efforts into the drug design, many of these molecules suffer from poor biological effect, low concentration in the diseased areas, and undesired side effects. In this context, AuNPs have been coupled to biologically inactive small molecules to create biologically active multivalent AuNP therapeutics. A bright example has been reported by Bowman et al., where the authors functionalized AuNPs with SDC-1721, a small membrane fusion inhibitor of HIV.46 The results demonstrated that, while pure SDC-1721 has low activity, functionalized AuNPs are able to inhibit HIV replication at μM concentrations. Similar results have been reported using targeting peptides. In particular, it was evidenced that the functionalized AuNPs can sensibly reduce the IC50 up to 2 orders of magnitude compared to pure peptides.47 Preliminary results in vivo confirmed the biosafety of the AuNPs.48