Tiny Things with Enormous Impact: Nanotechnology in the Fight Against Infectious Disease

Abstract

Nanoparticles have exceptional properties that make them outstanding candidates for improving diagnostics and the treatment of infectious disease. Their small size, distinctive intrinsic properties, and ability to be decorated with a variety of complex functionalities make them uniquely capable of detection and targeting of certain diseases. Nanotechnology has the ability to increase the sensitivity of detection methods, the potency and ease of treatment, and the effectiveness of vaccinations. However, major challenges remain to their application in low-resource settings due in large part to the sensitivity of these particles to their local environment, a property that makes them both exceptional for detection and prone to complications or failure during synthesis and utilization. These challenges are likely to be solved only by continued and enhanced communication across scientific disciplines, for example, medical doctors and diagnosticians providing information about what is needed in new technologies. This information will enable materials scientists and engineers to rapidly address the corresponding technical challenges, such as the scalable and reproducible generation of nontoxic and stable, yet responsible nanoparticles.

Detection, treatment, and prevention of infectious disease have been vexing challenges to human kind since our origin. While the last century has brought phenomenal advances in our understanding of and treatment for myriad microbial invaders, huge scientific challenges still remain due in large part to the complexity of these organisms [e.g., parasitic life cycle of Plasmodium, rapid mutation of human immunodeficiency virus (HIV)]. These difficulties are further compounded by the fact that the most rapid spread of disease often occurs in resource-limited settings, making the need for cheap and easy-to-use devices and medications dire. Nanotechnology has begun to play critical roles in the battle against these tiny invaders and offers distinctive attributes to all aspects of the medical pipeline, from prevention to the detection and treatment of disease.

Nanotechnology is the study and application of nanoparticles, which have traditionally been defined as a particle with dimensions of 1–100 nm. However, this definition has been challenged because it restricts inclusion of particles that fall outside of this size range that still possess unique properties not found in the bulk material,¹ such as magnetism, electrochemical behavior, and surface plasmon resonance. Here, we provide examples that have harnessed these distinct capacities (Figure 1) and outline important directions for future development.

Figure 1.

A brief overview of the applications of nanotechnology in detection, treatment, and prevention. Nanotechnology can improve the detection of disease biomarkers by harnessing the optical properties of gold nanoparticles, enhance sensitivity through sequestration, and improve current methods such as ELISA. The unique properties of nanoparticles have also shown promise as adjuvants for vaccinations. Nanoparticles can be decorated with a variety of organic molecules to enable targeted delivery of pharmaceuticals and act as nanomimics for preventative treatment.

DETECTION

Many current diagnostics are lacking in at least one of the critical parameters necessary to be considered successful in developing nations: robust, rapid, sensitive, simple, and low-cost. For example, of the three methods used to detect malaria, microscopic analysis, nucleic acid–based analytical techniques, and immune-based rapid diagnostic tests, the later are recommended by the World Health Organization, yet are not capable of the sensitivity required for early detection to slow the spread of the disease.² Therefore, scientists have sought to use nanotechnology as a platform that fulfills these needs.

Exciting research using gold nanoparticles (AuNPs) as a method for visual diagnosis has harnessed the unique color changing capability of these materials, which is due to shifts in their surface plasmon resonance.³ In these studies, an aptamer specific to a malaria biomarker (PfLDH) controls the aggregation of AuNPs through interaction with a surfactant, benzalkonium chloride. When PfLDH is present in a sample, the aptamer binds to the biomarker leaving the surfactant free to aggregate AuNPs. However, if there is no PfLDH in the sample, the surfactant forms a complex with the aptamer and the AuNPs remain dispersed in solution. Stable AuNPs are red, but when these particles aggregate in response to the free surfactant, the solution turns blue. The gradient of color change between red and blue is proportional to the amount of biomarker and, therefore, the severity of the infection. This diagnostic technique can be used for qualitative measurement by eye or quantitatively with an easy-to-use and common UV/vis spectrophotometer. A similar strategy has been utilized to detect Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), in a paper-based assay.⁴ The authors of this work suggest that the use of a smartphone app for image processing could enable implementation of this method in resource-limited settings.

Nanoparticles can also improve the sensitivity of traditional methods such as the enzyme-linked immunosorbent assay (ELISA). Recently, Hewlett and coworkers took advantage of Eu, which provides a bright, photostable, and minimally toxic source of fluorescence, in the form of SiO₂:Eu.⁵ This particle was utilized to enable fluorescent read-out in a modified ELISA platform called silica nanoparticle immunoassay (SNIA). Streptavidin-conjugated SiO₂:Eu was employed with a biotin-functionalized antibody that recognizes a component of HIV, such as the p24 antigen. This strategy provided a method that required only a minimal amount of blood and yielded a 1000-fold increase in sensitivity over conventional colorimetric ELISA. Nanoparticles have also been used to improve the detection of TB. For example, a commercialized assay is utilized to rapidly, cheaply, and simply detect lipoarabinomannan (LAM), an indicator of TB infection, in urine. However, it is relatively insensitive and enables detection of LAM only in TB patients who are coinfected with HIV. To improve upon this noninvasive strategy, a proof-of-concept study has demonstrated that utilizing a copper-containing bait dye, Reactive Blue 221, to sequester LAM in a hydrogel nanocage yielded assay sensitivity of pg/mL.⁶ In the absence of the nanocage, LAM was not detected.

TREATMENT

The development of treatments for infectious disease is challenging due in large part to the rapid evolution of resistance, often requiring patients to take multiple drugs for extended periods of time. Nanotechnology seeks to improve drug regimens by reducing tablet load and enabling targeted delivery to pathogenic sites or reservoirs of disease. For example, Plasmodium parasites invade red blood cells (RBCs) during infection, making these cells a critical target for such therapeutics. Traditional small molecule drugs will interact with all cells in the blood. However, researchers have designed nanoscale carriers with liposomes, which have long been important drug delivery tools, that are functionalized with half antibodies to recognize proteins that are uniquely found on the surface of Plasmodium-infected RBCs.⁷ The liposomes fuse with the RBC membrane and release their cargo, chloroquine, with 100% specificity to infected RBCs over noninfected cells in 90 min. This is particularly impressive given that RBCs often do not readily perform endocytosis, making delivery of exogenous agents challenging. Because less total drug is likely required and it is delivered directly to the target, such delivery efforts could reduce side effects of treatment. In addition, the nanometer size of the particles may increase bioavailability.

Patient compliance is another huge challenge in the treatment of many diseases given the requirement for frequent administration, high drug load, and long duration (i.e., months). For example, the current therapy for TB necessitates that the patient take multiple drugs over a 6–9 month period. Nanotechnology has the potential to combine these drugs into a single dose and reduce the required frequency of administration. For example, murine models of TB infection were treated with only five oral doses of drug-loaded poly(dl-lactide-co-glycolide) polymer nanoparticles, containing a combination of rifampicin, isoniazid, and pyrazinamide, that eradicated the infection from tissue in 50 days.⁸ Traditional approaches would require more than 9× as many treatments in the same time period. The natural bioadhesive properties of the polymers are made more efficacious in nanoparticle form. This enables increased absorption through the intestinal mucosa, which was not as successful with large particles. These results highlight both the sustained release properties of nanoparticles and that their size can facilitate delivery.

In addition to the ability to deliver organic molecules, nanoparticles can dispense metals. For example, gallium(III) meso-tetraphenylporphyrin particles were used to provide sustained release (over 15 days) of Ga(III) to inhibit the growth of both M. tuberculosis and HIV in macrophages.⁹ Ga(III) replaces Fe(III) in important enzymes, which results in their inactivation as unlike iron, Ga(III) cannot be reduced under biological conditions.

PREVENTION

Vaccination is by far the cheapest and most effective method of combatting disease. However, the diversity of infectious organisms and their unique behaviors makes the development of such interventions a seemingly insurmountable challenge. Nanotechnology can be utilized to not only target specific cell types for vaccine delivery but also mimic the natural target of the disease. A recent study has shown the application of nanoparticles as a physical barrier to prevent disease invasion. Specifically, the potency of a known merozoite invasion inhibitor, heparin, was improved by two orders of magnitude when it was coupled to a poly(dimethylsiloxane) PDMS block copolymer and mixed with the polymersome forming poly(2-methyl-2-oxazoline)-block-poly(dimethylsiloxane)-block-poly(2-methyl-2-oxazoline) (PMOXA-b-PDMS-b-PMOXA) as a nanomimic of RBCs. This heparin–polymersome complex has both drug- and vaccine-like activity against malaria infection as it prevents invasion of the parasite (drug function), leaving it vulnerable to the immune system of the host (vaccine function).¹⁰ Nanotechnology has also been used to address the challenges of vaccine stability and lack of immunogenicity. Fullerenol nanoparticles are excellent adjuvants due to their virus-like self-assembly and size. The nanoadjuvant fullerenol has been used in the development of an HIV-1 vaccine where it functioned as a plasmid DNA carrier and protector of this precious cargo.¹¹ It was also found to boost the immune response mediated by multiple Toll-like receptor signaling pathways.

WHAT’S NEXT?

Nanotechnology can give new life to existing medications as well as abandoned pharmaceuticals.¹² These tiny particles have the power to increase patient compliance as their unique properties may enable scientists to convert treatments into more favorable consumption methods such as tablets or inhalants. In addition, nanotechnology can be used to design particles with favorable drug release profiles and treatments that target specific tissues for delivery, enabling the use of lower doses for shorter periods of time. Nanoparticles can also increase treatment effectiveness by protecting the active agents from degradation or filtration and promoting sustained release. Together, these capabilities could lead to increased compliance due to the simplification of currently complex, multidrug schedules, which may ultimately reduce the rate of resistance evolution in pathogenic organisms. However, much is yet to be learned about how to safely and effectively implement nanoparticles in human health.

In comparison to traditional small molecule drugs and modern biologics, nanoparticles require that a daunting number of factors be considered, ranging from the size and composition of the particles to surface conjugations. Furthermore, because nanoparticle-conjugated drugs are designed to protect their cargo from degradation and filtration and to cross membranes or barriers within the body, the inherent toxicity of the particles or their constitutive parts (e.g., metal ions, ligands) is of critical concern and will likely be a major factor in how rapidly nanoparticle-containing technologies are approved for use in humans. In addition, the very properties that make nanoparticles ideally suited for the development of sensitive and selective diagnostics and drug-delivery systems may also hinder this work. For example, although the inherent properties of nanoparticles make them responsive to minor changes in their local environment, which is critical for disease detection or cargo release, this also lends them susceptible to matrix effects from biological fluids, an especially concerning issue given that this can vary from patient to patient. In addition, the synthesis and storage conditions required for some nanoparticles may not be conducive to applications in low-resource settings. Thus, it is clear that for nanotechnology to become widely integrated into detection and treatment applications, a deeper understanding of how to design particles that respond only to the desired biomolecules and to cheaply and reproducibility synthesize and store them in bulk is essential.

The final major challenge that must be addressed for the widespread implementation of nanotechnology in the detection, treatment, and prevention of infectious disease is communication. To overcome the aforementioned technical hurdles, communication between medicinal and materials chemists, experts in disease biology, engineers, and the individuals who will be applying these technologies must be dramatically enhanced. These interactions will ensure that each group understands both what is needed for widespread implementation of a given diagnostic or therapeutic agent and how these needs might be met by existing nanoparticles or require the development of completely new technologies. It is clear that nanoparticles may revolutionize how we address infectious disease, from increased sensitivity and speed in diagnosis, to dramatically decreasing the length of treatment required for currently recalcitrant diseases. However, the challenges faced both in addressing myriad infectious diseases and the technological hurdles in implementing nanotechnology will be overcome only by a concerted effort across many scientific disciplines.