DNA nanotechnologies: ideal tools for theranostics?

Theranostics is an emerging field that combines the power of diagnostics and therapeutic at the same time. The idea is not completely new, Nuclear Medicine has been using for example radioactive iodine to both image and treat certain types of thyroidal diseases (1), but nanotechnologies have the potential to generate highly controllable systems and expand the therapeutic arsenal of clinicians.

The goal of the Theranostics concept is to accelerate the caring of patients and monitor the real-time effect of a therapy, in a non-invasive manner. Among many other nanotechnologies, such as quantum dots or carbon nanotubes, DNA nanoconstructions were quickly exploited in therapeutics for their high functionalisation potential and their versatility. The elegance of the Watson-Crick base pairing model and the maturity of the whole oligonucleotide chemistry field allow one to easily synthetise and imagine plenty different designs for medical applications (2).         

The field has an amazing potential for making tailored therapies and answer the need for personalized medicine, especially in cancers. More and more, scientists try to achieve the “one drug for one patient” and nanotechnologies may be key players to answer that challenge. In this quick article we will try to see a couple of examples of what has already been achieved in the DNA theranostics field.

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An American group led by Peixuan Guo (3) successfully designed a three-arm RNA junction where each branch carries a different function (Figure 1). One arm carries a therapeutic agent, an anti-mRNA able to regulate the expression of an oncogenic gene. The second arm carries an aptamer targeting the epidermal growth factor receptors, to enhance the selectivity of the structure. EGFR are often overexpressed in cancerous cells. Finally, the third arm carries an imaging agent, in that case a fluorophore. The junction showed promising results in a triple negative breast cancer murine model, this type of cancer is known to be very aggressive, and the therapeutic arsenal is still limited to this day. The targeting and delivery of the junction were very efficient, little product was found in the healthy organs and the tumour growth was successfully inhibited in mice.

Figure 1: Adapted from Shu et al. Three-arm RNA junction carrying 1) a fluorophore, researchers used Alexa674 2) an aptamer anti-EGFR increasing the cancerous cells specificity and 3) a therapeutic agent, in that case an anti-mRNA strand that regulates a key gene in oncogenesis.

Nucleic acids are also being explored as a programmable biomaterial, able to react upon the presence of various stimuli. This concept is interesting for therapeutics, were one tries to deliver a drug to a precise cell population and limits the effects that drug could have on healthy cells. In this spirit, Lei et al. (4) successfully designed a DNA nanotriangle carrying an activable aptamer probe and doxorubicin molecules, a highly potent anti-cancer drug. In its ground state the aptamer probe is quenched, but upon the binding to its target cancer cell (in that case the target is the PTK7 receptor), it generates a detectable signal and leads to the internalization of the triangle inside the cell (Figure 2). It then slowly releases the doxorubicin to perform its effect. The system was both effective in vitro and in vivo.

Figure 2: Adapted from Lei et al. DNA nano triangle.

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The spherical nucleic acids and their cousins DNA-functionalized metal nanoparticles are also promising candidates for the theranostic field. Ye et al. (5)  designed a Copper-Gold (Cu-Au) nanoparticle, coated with a fluorophore-labelled aptamer. The Cu-Au alloy is a better thermal conductor than Au alone, making the nanoparticle a better candidate for photothermal therapy, a type of intervention that uses irradiation to kill cancerous cells (Figure 3). The system allows the fluorescence-imaging of the particles thanks to the Cy5-labeled aptamer while improving the targeting on human acute lymphoblastic leukaemia, T cells. The combination of a better targeting and a better thermal conductor led to a better therapeutic effect in vitro and in vivo.

Figure 3: Photothermal therapy combined with Cu-Au nanoparticles.

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Unfortunately, no technology is flawless. DNA nanoconstructions have a great potential but challenges remain. The nanostructures can be quite bulky and require a high concentration of bivalent cations to properly fold. One can imagine that physiological concentrations can hardly vary to meet that requirement, thus chemical modifications are sought after to improve the in vivo stability of the structures. DNA is well tolerated, and 3D structures are more resistance to enzymatic degradation than linear DNA strands (6). Nevertheless, they still need to enter cells to perform their effect, and the intracellular delivery of the structure remains to be fully understood and controlled.

While nanotechnologies are being explored in clinical trials (see here), nanothernanostics haven’t reached patients yet (to this day and in my knowledge, please do correct me if I’m wrong). The field still needs to mature before reaching potential patients, but progress is made and is promising. So far, the data describes theranostics agents as more potent compared to the non-nanotechnological intervention, while exhibiting lower side-effects (at least in mice). If we push the study of the biophysical behaviour of such systems, to better understand their properties and fully optimize them, it is only a matter of time before we assist to the first clinical trials in the theranostics field.

To go further:

Nicolson, F., Ali, A., Kircher, M. F., Pal, S., DNA Nanostructures and DNA-Functionalized Nanoparticles for Cancer Theranostics. Adv. Sci. 2020, 7, 2001669.

Deepika Singh, Fahima Dilnawaz, and Sanjeeb Kumar Sahoo, Challenges of moving theranostic nanomedicine into the clinic. Nanomedicine 2020 15:2, 111-114

Bibliography:

1.         Silberstein EB. Radioiodine: the classic theranostic agent. Semin Nucl Med. 2012 May;42(3):164–70.

2.         Mathur D, Medintz IL. The Growing Development of DNA Nanostructures for Potential Healthcare-Related Applications. Adv Healthc Mater. 2019;8(9):1801546.

3.         Shu D, Li H, Shu Y, Xiong G, Carson WE, Haque F, et al. Systemic Delivery of Anti-miRNA for Suppression of Triple Negative Breast Cancer Utilizing RNA Nanotechnology. ACS Nano. 2015 Oct 27;9(10):9731–40.

4.         Lei Y, Qiao Z, Tang J, He X, Shi H, Ye X, et al. DNA nanotriangle-scaffolded activatable aptamer probe with ultralow background and robust stability for cancer theranostics. Theranostics. 2018;8(15):4062–71.

5.         Ye X, Shi H, He X-X, Yu Y, He D, Tang J, et al. Cu-Au alloy nanostructures coated with aptamers: a simple, stable and highly effective platform for in vivo cancer theranostics. Nanoscale. 2016;

6.         Mei Q, Wei X, Su F, Liu Y, Youngbull C, Johnson R, et al. Stability of DNA Origami Nanoarrays in Cell Lysate. Nano Lett. 2011 Apr 13;11(4):1477–82.