The uptake of nanoparticles after oral administration has been studied in a number of investigations [278-281, 314-319]. A calculation of the total radioactivity excreted within 48 h after oral administration of 14C-labelled poly(methyl methacrylate) nanoparticles via bile and urine amounted to over 10% of the administered dose . Nevertheless, the radioactivity in individual organs was rather low. The highest concentrations were found in the liver, kidneys, muscles, and bone marrow [278, 280]. Up to 10% of the administered dose was found in the gastrointestinal walls. In general, gastrointestinal uptake of nanoparticles can occur by (a) a paracellular pathway, (b) intracellular uptake and transport via the epithelial cells lining the intestinal mucosa, and (c) lymphatic uptake via the M-cells and the Peyer's patches. Evidence exists for all three pathways, and the simultaneous existence of more than one pathway was reported . Both surface properties [280, 318, 319] as well as the particle size  seem to play a role in the uptake pathway and in the amount of particles taken up. Concerning the influence of coadministered surfactants the results are contradictive; some studies suggest a reduced uptake [318, 319] whereas other investigators report a considerably enhanced uptake . Again the different composition of the nanoparticle core between these studies might be responsible for the observed trend. Lymphatic uptake seems to be favored by hydrophobic particles [315-320].
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