Intravenous (i.v.), subcutaneous (s.c.) and intramuscular (i.m.) injections are the most common routes for medical treatments in the clinics. The first route is chosen when rapid absorption is required and when the two other routes are not suitable, because the injected substances are too irritable for the tissue (Jin et al., 2015). In contrast, intraperitoneal injection (i.p.) is used in the clinics when regional therapy is needed for example for peritonitis or malignancies within the peritoneal cavity. This application route has the advantage to provide locally a longer half-life of the drug, while it takes longer until the drug reaches the circulation. It is thereby superior to i.v. injections under these circumstances.
Nevertheless, i.p. injections are facing various challenges as it can have a negative impact on the target specificity of the drug and/ or the penetration into the target tissue (Bajaj and Yeo, 2010, Pham et al., 2018). However, many
DISCUSSION
124 animal studies involve i.p. injections as it is technically easier when a drug is injected several times daily and/ or for longer periods of time. Furthermore, a study demonstrated that the commercially available anti-fungal drug amphotericin B is better tolerated when given i.p. at high dose rather than when given i.v. (Chang et al., 2010). With regard to the application route of nanoformulations, analysis of AmBisome®, a liposomal fungal drug formulation revealed major differences in the tissue distribution when administrated i.v. or i.p. as it was noted upon administration of nanoparticles with a size of about 20 nm (Chang et al., 2010, Gao, 2017). In addition, also differences in the blood concentration were observed in another study depending on the application
route when injecting iron-oxide nanoparticles labeled with high density-lipoproteins (Jung et al., 2014). Upon i.v. administration, the
concentration of liposomes and iron-oxide NPs in the blood circulation was either highest 30 minutes after injection or had already declined at that time point, and thereafter further decreased due to the uptake into liver, spleen, lung and kidney. The uptake into other tissues such as heart, muscle or femur, however, was neglectable. In contrast after i.p. administration the concentrations within the blood slowly increased and peaked within 2 hours after the injection. This indicates a slow release from the peritoneal cavity into the lymph vessels, for example through the mesentery with its large absorbent surface, and afterwards via the thoracic duct into the blood (Chang et al., 2010, Jung et al., 2014, Gao, 2017). Within 4 hours after injection the level of NPs measured within the peritoneal cavity dropped again to control level. This indicates that the majority of NPs have been distributed within the body, absorbed into the blood stream and the pancreas, the latter by tissue-resident and peritoneal macrophages. NPs become cleared from the systemic circulation through the permeable endothelium and accumulate in liver, spleen, intestine, stomach, lung, kidney and bladder, but not in heart and brain (Pham et al., 2018, Arvizo et al., 2011, Gao, 2017). Collectively, i.v. administration of NPs seems to accumulate quickly and predominantly in liver and spleen. In contrast, i.p. administration leads to the slow accumulation in further organs via the ability to cross the peritoneal barrier, is excreted mainly via the intestine and consequently can be tracked within the feces and also due to a renal excretion
DISCUSSION
125 in the urine. Interestingly, i.p. injection causes a more rapid clearance than i.v.
injection (Gao, 2017).
Our own MRI and ICP-MS studies revealed i.p. injected IOH-NPs follow a similar path as described in the literature. We observed a slow release from the peritoneal cavity during the first two hours after injection until the signal here dropped to control levels. Meanwhile, IOH-NPs were detected within one hour after the injection in the liver, where they constantly accumulated over time.
IOH-NPs have also been found in the stomach two hours after the injection, where the levels increased and remained constant three hours after the injection. Two hours after the injection the IOH-NPs also reached the small intestine and their concentration increased here from now on with fluctuations.
Interestingly, IOH-NPs spared the kidney and the lung. 24 hours upon the injection, the signals in the liver and jejunum decreased again, arguing for an intestinal excretion of IOH-NPs. As we failed to prove the presence of IOH-NPs in the urine by MRI (data not shown) and since no IOH-NPs were found in the kidney, it is highly likely that indeed intestinal clearance dominates and might reach it through the stomach. Noteworthy, the IOH-NPs biodistribution after i.v.
injection has been characterized before. Here, IOH-NPs were detected in the gall bladder, liver, lung and kidney, but not in the stomach. The studies suggested that i.v. injected IOH-NPs are predominantly excreted via the intestinal tract and a small fraction also via the kidney (Poß, 2017).
Consequently, the application route strongly determines the fate of IOH-NPs in vivo after injection and results in a different organ distribution. Most importantly, our own results indicate that i.p. injection is favorable as it results in a slower release, a more even distribution between the organs and a lower risk of inducing nephrotoxicity due to its clearance route.
It should be pointed out that our data concerning the biodistribution of IOH-NPs were obtained by using healthy mice. Since we mostly used IOH-NPs in the treatment of mice suffering from aGvHD in this thesis, it would be interesting whether the pharmacokinetics differs under such conditions from the healthy situation. A recent study in a rat arthritis model for instance indicated that a
DISCUSSION
126 single systemic administration of copolymer-dexamethasone conjugate resulted in a higher influx in inflamed joints than healthy joints. This was achieved by the dynamic uptake by synoviocytes, which are a mixture of macrophage-like, fibroblast-like and dendritic cells, and by the enhanced vascular permeability in inflamed joints (Quan et al., 2010). In another study, polystyrene NPs loaded with a fluorescence dye were used for inhalation in a mouse model of allergic airway inflammation (AAI). Here, AAI lungs displayed a significantly stronger fluorescence signal as healthy lungs (Markus et al., 2015). The accumulation of BMP-NPs within inflamed tissue would also be desirable in aGvHD mice.
4.4 IOH-NPs as a new therapeutic approach for GC therapy of aGvHD