Characterization of size-dependent exchange of PEG molecules between the blood and extravascular space in the pig

Different sized polyethylene glycols (PEGs) have been used as probe molecules in studies of size-dependent permeation through the intestinal wall and the glomerular membranes. We have curve-fitted a three-compartment model to the urinary recovery data following intravenous injection of different sized PEG molecules in the pig. The rate constants to and from the extravascular space demonstrate a strong size-dependent selectivity for PEG molecules less than 502 Da, but the rate constants to the urine are almost the same for all PEG molecules. This is discussed in relation to the selectivity in the glomerular filtration, distribution volume, and the use of PEG molecules as tracers in the permeability studies.