Could gold nanoprobes be an important tool in cancer diagnostics?

Keywords::

• cancer
• gold nanoparticles
• gold nanoprobes
• nanodiagnostics
• nanotechnology

Cancer nanotechnology

The advent of nanotechnology has radically changed the way we diagnose, image and treat cancer, with novel nanodevices capable of one or more clinically important functions, including detecting cancer at its earliest stages and location, as well as delivering anticancer therapeutics specifically to tumor cells [101]. These strategies have focused on the development of nanoscale devices and platforms that can be used for improved biomarker characterization, such as nucleic acids (DNA or RNA) and protein, compared with conventional techniques. Several advances have been reported on DNA analysis in real time with high resolution and at high throughput. The novel nanotechnology molecular tools for cancer diagnostics ought to be capable of incorporating the vast quantity of information made available by the genome sequencing and analysis projects, including SNP categorization, gene-expression profiles and biomarker characterization.

The nanotechnology approach to cancer has focused on three main vectors: early detection; imaging diagnostics for diagnostics and/or assessment of targeted delivery; and multifunctional therapeutics, where nanodevices have been loaded with multiple functional moieties capable of selective targeting, imaging, signaling and delivery of specific drugs to malignant cells [1–6]. The majority of current diagnostic approaches rely on the identification and quantitation of disease biomarkers, where proteins have a decisive and prominent role, capable of detecting disease after disease onset. Alternatively, technologies based on nucleic acid characterization may allow detection of biomarkers before onset of disease, predict and/or evaluate risk and indicate predisposition to cancer phenotype.

Gold nanoprobes for early detection of cancer

The early detection of cancer (i.e., malignant cells and/or specific tumor biomarkers) is of utmost importance, as it allows the disease to be tackled at its earliest stages, resulting in better prognosis and ease of treatment. A plethora of nanoparticle (NP)-based detection systems have been proposed for the early molecular diagnosis and increased sensitivity for cancer. Among these, gold nanoparticles (AuNPs) have been extensively explored in platforms capable of detection and delivery of drugs, due to their remarkable characteristics, such as optical properties, chemical stability, high surface-to-volume ratio, and ease of synthesis and of multifunctionalization, with a multitude of biomolecules for in vitro diagnostics as well as in vivo imaging. Several highly sensitive detection methods for disease biomarkers (e.g., nucleic acids, proteins and enzymes, among others) have been developed by exploring different physico–chemical properties of AuNPs, such as localized surface plasmon resonance (LSPR), fluorescence enhancement/quenching, surface-enhanced Raman scattering and electrochemical activity, among others [7,8]. Among these detection strategies, colorimetric approaches based on the LSPR of AuNPs have been the most explored, owing to their simplicity and high sensitivity. Typically, colloidal solutions of spherical AuNPs present a red color, with their LSPR band centered at approximately 520 nm. These LSPR bands are usually weakly dependent on the size of the NPs and the refractive index of the surrounding media, but strongly change with interparticle distance – aggregation of NPs leads to a pronounced color change and a concomitant red-shift of the LSPR absorption band peak. These AuNPs are commonly functionalized with biomolecules (e.g., ssDNA) and termed Au nanoprobes, which are then used in detection reactions capable of recognizing molecular events associated with cancer development. Attention should be paid to the molecular target (or biomarker) of these detection protocols, since they usually report DNA sequence analysis and/or circulating proteins, clearly disregarding the information and relevance of assessing the transcriptome (e.g., mRNA and miRNAs that have increasingly been considered excellent biomarkers for the early diagnosis of cancer). Au nanoprobe detections systems, either for DNA or RNA, have enhanced the detection sensitivity down to a femtomolar level of target molecule with single base discrimination resolution and, in most cases, without the need for cumbersome and/or specialized apparatus, making it possible to use at the point of need [9–11]. It is expected that application of these Au nanoprobe systems in the clinics could provide the means to detect abnormal biomolecular events that initiate cancer before an observable lesion has established itself in the cell lineage, thus anticipating the moment of starting therapy with concomitant improvement of disease management.

These Au nanoprobes can also be used for ex vivo and in vivo evaluation of cancer. El-Sayed and coworkers have reported on anti-EGFR antibody-conjugated nanoparticles that specifically bind to the surface of cancer cells with 600% greater affinity than to noncancerous cells. The specific binding gives a relatively sharper surface plasmon resonance absorption band with a red-shifted maximum, compared with that observed when added to noncancerous cells, suggesting that AuNPs labeled with antibodies could be useful for in vivo molecular detection of cancer [12,13]. Since then, several other applications of AuNPs for ex vivo and in vivo diagnostics (and therapy) have been described, supporting the idea that Au nanoprobes could well constitute the future in cancer diagnostics.

Another amazing application of Au nanoprobes for cancer diagnostics has been shown by the biobarcode assay, suitable for high sensitivity use for nucleic acid or protein targets. This utilizes a combination of microarray approaches with magnetic microparticles holding recognition elements (antibodies or DNA) for specific targets and Au nanoprobes harboring a secondary recognition agent and hundreds of ssDNA barcode strands [14]. Through this approach it has been possible to detect the prostate-specific antigen (PSA) in clinical samples with 300-times more sensitivity than commercial PSA immunoassays [15]. In addition, the use of Au nanoprobes allows for improved motorization of PSA levels, thus providing valuable data on disease onset and evolution. Furthermore, with this approach, it is possible to detect a disease marker at much lower concentrations than those currently associated with a disease-positive phenotype.

The use of such sensitive Au nanoprobe detection systems poses an interesting question: how does a given biomarker effectively relate to disease? When quantification is required, where does the threshold for disease lie? The use of Au nanoprobes and other nanotechnology-based platforms has raised the issue of biomarker identification and characterization to a different level.

Gold nanoprobes for cancer diagnostics: what’s next?

The above-mentioned reports on Au nanoprobes for the early detection of cancer are supported by data originating from selected and well-controlled experimental settings (i.e., proof-of-concept demonstrations in research laboratories). Despite the clear advantages in terms of sensitivity and operating costs, not to mention ease of operation and the smaller amount of sample material required for effective molecular characterization, the decisive step of translating these laboratory strategies into clinical practice still remains to be taken. However, one has to be optimistic, since the available examples of effective translation into the clinic have shown that Au nanoprobes have performed well above the existing methodologies, either by its enhanced multiplexing capability or by surpassing current limits of detection. The latter has prompted the need to redefine the clinical significance of a given biomarker (e.g., the biobarcode application on PSA determination). In fact, it could be predicted that in coming years strong emphasis will be put onto assessing the relevance and information provided by currently used cancer biomarkers. What is the clinical relevance of the biomarkers? And what does an increased sensitivity mean in terms of effective diagnostics? Most of these Au nanoprobe-based methodologies still need to make their way into the clinic before enough information can be gathered to correctly assess their impact on cancer diagnostics. Despite the widespread development of in vitro diagnostics systems based on Au nanoprobes, they still require validation and calibration using real clinical samples. The versatility and the possibility of portability may allow the specific molecular diagnostics potential of Au nanoprobes to be used at point of need (e.g., community hospital or small decentralized laboratory), which may lead to reduced time and costs of cancer diagnostics, with obvious advantages to disease management. The next few years will be dedicated to improving and optimizing these Au nanoprobe platforms towards translation into the clinic, and validation by regulatory bodies such as the US FDA and the EMA.

Another field to be explored is the concomitant use of Au nanoprobes for the molecular diagnosis of cancer and for selective delivery of a specific anticancer agent, joining diagnostics and therapy on a single nanodevice – nanotheranostics, which is most definitely revolutionizing the way we manage cancer.

Financial & competing interests disclosure

PV Baptista acknowledges financial support from the Centre for Research in Human Molecular Genetics – Fundação para a Ciência e Tecnologia/Ministério da Ciência e Ensino Superior. The author has no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.