Biodistribution of nanodiamonds is determined by surface functionalization

Solid-core nanoparticles made of carbon, metals and other inorganic materials are gaining interest as nanocarriers to facilitate nucleic acid therapeutics. Our group designed amino acid-functionalized nanodiamonds (NDs, collectively fNDs) to serve this purpose via covalent conjugation of basic amino acid lysine and di-peptide lysyl-histidine on the ND surface. The purpose of this study is to elucidate the functionalization-dependent biodistribution profiles of fNDs, a key step in the process of developing these novel nano-systems. Here the aim was two-fold i.e., to establish an optimized protocol for radiolabeling of fNDs and to study the functionalization-dependent differences in pharmacokinetic behavior of NDs. Both types of fNDs yielded a surface loading of 1.94 and 14.45 mmol/g of lysine and lysyl-histidine amino acid conjugates respectively. Deferoxamine (DFO) was conjugated directly on the ND surface for the first time to chelate zirconium (89Zr), the radiolabel of interest for in vivo tracking. To achieve an effective balance between radiolabeling efficiency, surface chemistry and colloidal stability of DFO-conjugated fNDs, attachment of DFO was evaluated in the range of 1–8 %. A 3 % loading with DFO was sufficient to chelate 2 MBq 89Zr with good radiochemical yield (≥90 %) while maintaining the physicochemical characteristics of the fNDs, namely particle size ≤300 nm and overall positive zeta potential of higher than 15 mV. Furthermore, 89Zr-labeled fNDs remain stable in phosphate buffered saline and mouse serum for over 96 h maintaining a radiochemical stability of ≥97 %. Following process optimization, we evaluated differential organ distribution and pharmacokinetic profile of 89Zr-labeled fNDs. All NDs showed the highest uptake in the liver, followed by the spleen. However, the degree of uptake and retention time in the liver varied based on the surface functionalization. The lysyl-histidine-NDs showed the highest AUC, followed by the carboxylated NDs and the lysine-NDs. Unlike lysine-NDs, which depleted quickly from the liver (marked decline in ND-associated radioactivity from 5.6 to 2.3 %), the lysyl-histidine-NDs maintained a longer residence for at least 72 h in the liver marked by ND-associated radioactivity of 8.9 % after 6 h that increased further to 11.7 % after 72 hours post injection.