Nanoparticle delivery systems offer advantages over free drugs, in that they increase solubility and biocompatibility. Nanoparticles can deliver a high payload of therapeutic molecules while limiting off target side effects. Therefore, delivery of an existing drug with a nanoparticle frequently results in an increased therapeutic index. Whether of synthetic or biologic origin, nanoparticle surface coatings are often required to reduce immune clearance and thereby increase circulation times allowing the carriers to reach their target site. To this end, polyethylene glycol (PEG) has long been used, with several PEGylated products reaching clinical use. Unfortunately, the growing use of PEG in consumer products has led to an increasing prevalence of PEG-specific antibodies in the human population, which in turn has fueled the search for alternative coating strategies. This review highlights alternative bio-inspired nanoparticle shielding strategies, which may be more beneficial moving forward than PEG and other synthetic polymer coatings.

Platform technologies based on plant virus nano-particles (VNPs) and virus-like particles (VLPs) are attracting the attention of researchers and clinicians because the particles are biocompatible, biodegradable, noninfectious in mammals, and can readily be chemically and genetically engineered to carry imaging agents and drugs. When the Physalis mottle virus (PhMV) coat protein is expressed in Escherichia coli, the resulting VLPs are nearly identical to the viruses formed in vivo. Here, we isolated PhMV-derived VLPs from ClearColi cells and carried out external and internal surface modification with fluorophores using reactive lysine-N-hydroxysuccinimide ester and cysteine-maleimide chemistries, respectively. The uptake of dye-labeled particles was tested in a range of cancer cells and monitored by confocal microscopy and flow cytometry. VLPs labeled internally on cysteine residues were taken up with high efficiency by several cancer cell lines and were colocalized with the endolysosomal marker LAMP-1 within 6 h, whereas VLPs labeled externally on lysine residues were taken up with lower efficiency, probably reflecting differences in surface charge and the propensity to bind to the cell surface. The infusion of dye and drug molecules into the cavity of the VLPs revealed that the photosensitizer (PS), Zn-EpPor, and the drugs crystal violet, mitoxantrone (MTX), and doxorubicin (DOX) associated stably with the carrier via noncovalent interactions. We confirmed the cytotoxicity of the PS-PhMV and DOX-PhMV particles against prostate cancer, ovarian and breast cancer cell lines, respectively. Our results show that PhMV-derived VLPs provide a new platform technology for the delivery of imaging agents and drugs, with preferential uptake into cancer cells. These particles could therefore be developed as multifunctional tools for cancer diagnosis and therapy.

Molecular imaging approaches and targeted drug delivery hold promise for earlier detection of diseases and treatment with higher efficacy while reducing side effects, therefore increasing survival rates and quality of life. Virus-based nanoparticles are a promising platform because their scaffold can be manipulated both genetically and chemically to simultaneously display targeting ligands while carrying payloads for diagnosis or therapeutic intervention. Here, we displayed a 12-amino-acid peptide ligand, GE11 (YHWYGYTPQNVI), on nanoscale filaments formed by the plant virus potato virus X (PVX). Bioconjugation was used to produce fluorescently labeled PVX-GE11 filaments targeted toward the epidermal growth factor receptor (EGFR). Cell detection and imaging was demonstrated using human skin epidermoid carcinoma, colorectal adenocarcinoma, and triple negative breast cancer cell lines (A-431, HT-29, MDA-MB-231), all of which upregulate EGFR to various degrees. Nonspecific uptake in ductal breast carcinoma (BT-474) cells was not observed. Furthermore, co-culture experiments with EGFR+ cancer cells and macrophages indicate successful targeting and partitioning toward the cancer cells. This study lays a foundation for the development of EGFR-targeted filaments delivering contrast agents for imaging and diagnosis, and/or toxic payloads for targeted drug delivery.

The molecular imaging of in vivo targets allows non-invasive disease diagnosis. Nanoparticles offer a promising platform for molecular imaging because they can deliver large payloads of imaging reagents to the site of disease. Magnetic resonance imaging (MRI) is often preferred for clinical diagnosis because it uses non-ionizing radiation and offers both high spatial resolution and excellent penetration. We have explored the use of plant viruses as the basis of for MRI contrast reagents, specifically Tobacco mosaic virus (TMV), which can assemble to form either stiff rods or spheres. We loaded TMV particles with paramagnetic Gd ions, increasing the ionic relaxivity compared to free Gd ions. The loaded TMV particles were then coated with silica maintaining high relaxivities. Interestingly, we found that when Gd(DOTA) was loaded into the interior channel of TMV and the exterior was coated with silica, the T1 relaxivities increased by three-fold from 10.9 mM−1 s−1 to 29.7 mM−1s−1 at 60 MHz compared to uncoated Gd-loaded TMV. To test the performance of the contrast agents in a biological setting, we focused on interactions with macrophages because the active or passive targeting of immune cells is a popular strategy to investigate the cellular components involved in disease progression associated with inflammation. In vitro assays and phantom MRI experiments indicate efficient targeting and imaging of macrophages, enhanced contrast-to-noise ratio was observed by shape-engineering (SNP > TMV) and silica-coating (Si-TMV/SNP > TMV/SNP). Because plant viruses are in the food chain, antibodies may be prevalent in the population. Therefore we investigated whether the silica-coating could prevent antibody recognition; indeed our data indicate that mineralization can be used as a stealth coating option to reduce clearance. Therefore, we conclude that the silica-coated protein-based contrast agent may provide an interesting candidate material for further investigation for in vivo delineation of disease through macrophage imaging.