Background

Boron neutron capture therapy (BNCT) is a two-step therapeutic process that utilizes Boron-10 (10B) in combination with low energy neutrons to effectively eliminate targeted cells. Primarily used for difficult to treat head and neck carcinomas; recent advances have expanded this method to cover a broader range of carcinomas. However, it remains an unconventional therapy. One of the key barriers for widespread adoption is the that adequate delivery of 10B rich compounds to target cells, which is necessary for a potent treatment, has not been achieved and continues to be a research priority. Considerable research has been focused on developing carrier systems which are capable of targeting cancer cells for BNCT, including lipid based nanocarriers, inorganic nanoparticles, drug conjugates and more recently, peptide-based carrier systems

Development of BNCT as a first-line clinical treatment option requires: 

1. A carrier compound that allows for the selectively delivery of the isotope 10B isotope to tumour cells. The fact that BNCT is a spatially limited non-invasive therapeutic approach is a major advantage of the technique. However, this implies that the 10B compounds have to be placed either in close spatial vicinity to or, more preferably, inside the tumour cell to maximize dosage to the cell’s nucleus.
2. To ensure effective neutron radiation treatment, delivery amounts of > 20 µg 10B/g tumour are required.
3. Any 10B delivery methods need to meet specific requirements such as high tumour/tissue uptake ratios, low toxicity and rapid clearance from the patient’s body. 

Technology Overview

The development of effective targeting strategies and the sufficient upload of 10B isotopes are urgent problems in the area of BNCT research. In general, nanoparticles offer certain advantages over non-conjugated compounds, such as overcoming drug solubility issues, increased chemical stability, decreased toxicity and improved distribution with potential tissue-specific targeting. 

Researchers at the University of Manitoba have characterized an unconventional nanoparticle drug delivery device, entitled Right-Handed Coiled Coil - Nanotube (RHCC-NT), which was found to be surprisingly suited to upload boron compounds into its large cavities. RHCC-NT was first described as an S-layer protein component of the archaea S. marinus and has been shown to possess highly unusual properties in respect to cargo uptake, targeted drug delivery and robustness. In addition to incorporating boron compounds, these nanotube structures can penetrate eukaryotic cells, which offers possibilities for targeted drug delivery in BNCT. 

The proteinaceous nanotube RHCC-NT provides a novel route for boron drug delivery based on its ability to take up o-carborane, a boron rich molecule. We have demonstrated that RHCC-NT can successfully uptake 10B-rich o-carboranes and deliver them into tumour cells for the use in BNCT. Molecular uptake was confirmed by NMR studies and by structural studies through crystallography, which allowed for the atomic characterization of the protein–ligand complex. In the case of o-carborane, the ligand is stabilized in the center of the cavities through hydrophobic interactions with amino acid side chains which cause the entry of o-carborane to be energetically favourable. The uptake occurs spontaneously due to the hydrophobic effect which seeks to replace the native and energetically unstable water molecules present in the cavities. Chemical shifts in NMR demonstrated that o-carborane shifted from a hydrophilic to a hydrophobic environment in the presence of the protein nanotube. Mechanistically, MD simulations show that interhelical channels to the cavities form thus allowing o-carborane to move into the nanotube. Importantly, these intermediate channels do not seem to disturb the final overall shape and structure of the o-carborane bound nanotube. This same effect has previously been observed for other hydrophobic molecules taken up by RHCC-NT.

Further Detail

Publication ‑ Boron rich nanotube drug carrier system is suited for boron neutron capture therapy. Heide F. et al. Sci Rep. 2021 Jul 30;11(1):15520.

Benefits

The RHCC-NT is a coiled coil motif which can be linked to any kind of pathfinder system. In cancer cell lines, we have shown that RHCC-NT can be internalized by an entire cell population and accumulate in the cytoplasm around the nucleus. For the proposed application in BNCT, cell internalization of RHCC-NT is not necessary as the mere attachment of the drug filled nanotubes to the target cells would allow for effective killing upon neutron exposure. However, efficient uptake of the nanotube is still desirable, as neutron therapy results improve when 10B is closer to the cell’s nucleus as DNA damage from free radicals and ionizing radiation is most effective in initiating cell death. Additionally, release of o-carborane from the nanotube is not necessary since neutron beams would penetrate the protein structure inside of a cell causing it to activate its desired effects. Thus, o-carborane loaded nanotubes in combination with intravenous injections could prove to be a compelling BNCT drug delivery method. 

As these proteins are highly modular, there are additional possibilities for attaching biomolecular tags that would allow the specific delivery of the nanotube to target cells. Certain cell receptors such as vascular endothelial growth factor (VEGFR) or epidermal growth factor receptor (EGFR) which are overexpressed in cancer cells, offer potential targets for nanotube modifications. Previous research has also focused on attaching folate molecules to nanoparticles for cancer therapy development which offers the advantage of targeting overexpressed folate receptors on tumor cells. In the case of RHCC-NT, functionalizing the nanotube is a possibility that warrants further investigation.

Applications

Treatment of difficult to treat head and neck carcinomas; recent advances have expanded the use of BNCT to cover a broader range of carcinomas. 

Opportunity

The University is actively seeking an industry partner, entrepreneur or investor to move this technology to market. 

The University would also welcome a collaborative partner to assist in further validating the utility of this novel nanoparticle as a targeted delivery vehicle for 10B in BNCT therapy.