Quantitative biokinetics over a 28 day period of freshly generated, pristine, 20 nm silver nanoparticle aerosols in healthy adult rats after a single 1½-hour inhalation exposure
Authors: Kreyling Wolfgang G. , Holzwarth Uwe , Hirn Stephanie , Schleh Carsten , Wenk Alexander , Schäffler Martin , Haberl Nadine , Gibson Neil 
Published: June 2020

Abstract
Background
There is a steadily increasing quantity of silver nanoparticles (AgNP) produced for numerous industrial, medicinal and private purposes, leading to an increased risk of inhalation exposure for both professionals and consumers. Particle inhalation can result in inflammatory and allergic responses, and there are concerns about other negative health effects from either acute or chronic low-dose exposure.

Results
To study the fate of inhaled AgNP, healthy adult rats were exposed to 1½-hour intra-tracheal inhalations of pristine 105Ag-radiolabeled, 20 nm AgNP aerosols (with mean doses across all rats of each exposure group of deposited NP-mass and NP-number being 13.5 ± 3.6 μg, 7.9 ± 3.2•1011, respectively). At five time-points (0.75 h, 4 h, 24 h, 7d, 28d) post-exposure (p.e.), a complete balance of the [105Ag]AgNP fate and its degradation products were quantified in organs, tissues, carcass, lavage and body fluids, including excretions.
Rapid dissolution of [105Ag]Ag-ions from the [105Ag]AgNP surface was apparent together with both fast particulate airway clearance and long-term particulate clearance from the alveolar region to the larynx. The results are compatible with evidence from the literature that the released [105Ag]Ag-ions precipitate rapidly to low-solubility [105Ag]Ag-salts in the ion-rich epithelial lining lung fluid (ELF) and blood. Based on the existing literature, the degradation products rapidly translocate across the air-blood-barrier (ABB) into the blood and are eliminated via the liver and gall-bladder into the small intestine for fecal excretion. The pathway of [105Ag]Ag-salt precipitates was compatible with auxiliary biokinetics studies at 24 h and 7 days after either intravenous injection or intratracheal or oral instillation of [110mAg]AgNO3 solutions in sentinel groups of rats. However, dissolution of [105Ag]Ag-ions appeared not to be complete after a few hours or days but continued over two weeks p.e. This was due to the additional formation of salt layers on the [105Ag]AgNP surface that mediate and prolonge the dissolution process. The concurrent clearance of persistent cores of [105Ag]AgNP and [105Ag]Ag-salt precipitates results in the elimination of a fraction > 0.8 (per ILD) after one week, each particulate Ag-species accounting for about half of this. After 28 days p.e. the cleared fraction rises marginally to 0.94 while 2/3 of the remaining [105Ag]AgNP are retained in the lungs and 1/3 in secondary organs and tissues with an unknown partition of the Ag species involved. However, making use of our previous biokinetics studies of poorly soluble [195Au]AuNP of the same size and under identical experimental and exposure conditions (Kreyling et al., ACS Nano 2018), the kinetics of the ABB-translocation of [105Ag]Ag-salt precipitates was estimated to reach a fractional maximum of 0.12 at day 3 p.e. and became undetectable 16 days p.e. Hence, persistent cores of [105Ag]AgNP were cleared throughout the study period. Urinary [105Ag]Ag excretion is minimal, finally accumulating to 0.016.

Conclusion
The biokinetics of inhaled [105Ag]AgNP is relatively complex since the dissolving [105Ag]Ag-ions (a) form salt layers on the [105Ag]AgNP surface which retard dissolution and (b) the [105Ag]Ag-ions released from the [105Ag]AgNP surface form poorly-soluble precipitates of [105Ag]Ag-salts in ELF. Therefore, hardly any [105Ag]Ag-ion clearance occurs from the lungs but instead [105Ag]AgNP and nano-sized precipitated [105Ag]Ag-salt are cleared via the larynx into GIT and, in addition, via blood, liver, gall bladder into GIT with one common excretional pathway via feces out of the body.