Nanodiamonds have emerged as promising candidates for drug delivery platforms and bioimaging probes in the biomedical field because of their unique physicochemical properties and excellent biocompatibility (3). However, although there have been extensive investigations of nanodiamonds in biomedical applications, the tissue distribution kinetics of nanodiamonds are lacking because of various difficulties, including quantification and controlled synthesis. Therefore, as shown in this study, the distribution kinetics of nanodiamonds via intravenous injection can provide critical information on the efficient and safer-by-design of nanodiamonds in biomedical applications.
In this study, precise control of the sp3/sp2 carbon ratio without significant changes in the size, shape, and surface area was possible because the panel of nanodiamonds originated from the same (19). Thus, the selection of these two types of nanodiamonds is a good model for evaluating the effect of the sp3/sp2 carbon ratio. UV-Raman characterization showed that the diamond peak at 1325 cm− 1 significantly increased and the G peak strongly upshifted from 1590 cm− 1 in highND. In addition, the color difference between lowND and highND distinctively demonstrated that lowND contains more graphite (22, 23). Furthermore, the XRD pattern again confirms that the lowND has more graphite because the broad peak at 2 θ = 26° corresponds to graphite (24). Finally, the HR-TEM images show that lowND has a higher number of graphitic shells covering the diamond cores than the highND.
The quantification of carbon nanomaterials in organs is difficult but possible using various methods, such as thermal-optical analysis and mass spectrometry (25, 26). Furthermore, nanodiamonds among carbon nanomaterials are more challenging to measure concentrations distributed in organs. In this regard, an efficient and straightforward method for quantifying nanodiamonds in organ samples has been developed. Our previous study proposed a new approach to measure the organ burden of low sp3/sp2 nanodiamonds using PK and UV-Vis spectrophotometry (27); however, this study further suggested that this method can be broadly applied to any nanodiamond type. Furthermore, the LOQ of nanodiamonds (10 µg/mL) in this study is sufficient for the organ burden of nanodiamonds, considering the high injection dosage for in vivo injection (27–29).
In this study, the intravenous injection of nanodiamonds exhibited a fast distribution to the liver, spleen, and lungs within 30 min post-injection and persisted for up to 28 d. Accumulation in organs did not differ according to the sp3/sp2 carbon ratio. Although the sp3/sp2 carbon ratio determines the inflammation potential of nanodiamonds because sp2 carbon is the main source of reactive oxygen species (19), the tissue distribution pattern is not related to oxidative stress but to hydrodynamic size. The three accumulating organs of nanodiamonds, the liver, spleen, and lungs, shown in this study, are unique and different from other nanomaterials in previous studies (30–33). For example, intravenously injected gold and silica nanoparticles accumulated in RES organs; however, particles were eliminated via the biliary or urinary routes, which was not observed in this study (30, 34). In addition, single-walled carbon nanotubes (SWCNT) after intravenous injection showed an accumulation propensity in the liver, spleen, and lung; however, there was a decreasing tendency in time-course accumulation in all three organs, which is inconsistent with our findings (35).
Furthermore, it was noted that the injected nanodiamonds were hardly excreted, as the combined levels in the liver, spleen, and lungs were almost consistent throughout the study period (i.e., 28 days). The extreme accumulation pattern of nanodiamonds without any adverse effects can be advantageous for biomedical applications, particularly in theragnostic applications, because they can be used for surveillance and therapy at any time (36, 37). For example, the controlled deposition and persistence of nanodiamonds without any histological alterations in the RES organ can provide excellent properties for cancer therapy from primary to metastatic tumors (38, 39). However, further investigations on the theragnostic approach of nanodiamonds are needed.
In this study, agglomerated nanodiamonds were deposited in the alveolar capillaries of the lungs, which is consistent with previous studies that used gold and SWCNT (40, 41). However, this study suggests that the hydrodynamic size can control the lung deposition pattern. The pulmonary distribution of nanodiamonds is not in the alveoli or interstitium but in the alveolar capillaries. Furthermore, the deposited nanodiamonds in the lung were re-distributed to secondary organs such as the spleen and liver. Size is an important factor affecting the biodistribution of nanoparticles (42). Based on current knowledge, most particles accumulate rapidly in the liver and spleen as the particle size increases. In contrast, most particles accumulate more in the kidney as the particle size decreases (12, 43–45). The size of particles that determines organ distribution and clearance depends on whether they can penetrate biological barriers, such as capillaries and epithelium. Specifically, Blanco et al. suggested that particles > 150 nm are more likely to be entrapped in the liver and spleen (43). Danaei et al. suggested that nanocarriers with 100–150 nm diameter are distributed in the kidney and lung, whereas nanocarriers with 20–100 nm diameter may be distributed to the spleen, liver, and some secondary organs with leaky capillaries (44).
Information regarding the size ranges determining organ distribution would be helpful in understanding the size effect on biodistribution over a broad scope. However, because the biological consequences can vary depending on factors such as the pore size of the fenestrated capillaries and the behavior of nanoparticles at the cellular level, the suggestion of size ranges for each nanoparticle is required to fulfill its function in the field of drug delivery systems. In this way, our findings of the threshold size limit of approximately 300 nm to evade pulmonary deposition can provide information on modulating the biokinetics of nanodiamonds.