Preparation and functionalization of boron nitride containing carbon nanohorns for boron neutron capture therapy

Boron neutron capture therapy requires boron carriers that can deliver abundant boron to tumors. To obtain such a carrier, we have prepared boron nitride (BN) containing carbon nanohorn aggregate (CNH) by heating CNHs with ammonia borane at 800 °C. The obtained BN-CNH had a C:B:N mole ratio of 5:2:2, as estimated from X-ray photoemission spectroscopy. The nanohorn tubule walls had graphene–BN double layers or BN–graphene–BN triple layers. The graphite-like sheets constituting the CNHs together with the nanohorn tubules also had BN layers on their surfaces. BN release from BN-CNH was small in cell culture medium, and about 70–80% of the BNs remained on the CNHs. For selective accumulation in tumor cells, the BN-CNH was coated with phospholipid polyethylene glycol having folate (BN-CNH/PLPEG-FA). In primary cell culture experiments, human tumor KB cells overexpressing FA receptors ingested more BN-CNH/PLPEG-FA than those without FA, while normal human FHs 173We cells did not show preferential uptake due to FA. The quantity of boron per cell exceeded the criteria required for boron neutron capture therapy by more than 100 times in the cell experiments. These results suggest that BN-CNH is potentially a good carrier for boron neutron capture therapy.