Passage of Molecules through the Wall of the Gastrointestinal Tract

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Abstract
The urinary recovery of different-sized (282–1206 dalton) polyethylene glycols (PEGs) after intravenous administration was determined. Mixtures of different-sized PEGs were injected intravenously in anesthetized pigs, and the urinary recovery at different times was determined by means of reversed-phase high-performance liquid chromatography. PEGs in the 722–1206-dalton range were all recovered to the same extent, whereas the recovery of PEGs in the 282–678-dalton range was related to molecular size; that is, the smaller the molecule, the less the urinary recovery. Moreover, less was found of the 370–414-dalton PEGs than of the 678–722-dalton PEGs in the blood after intravenous injection. These findings have a bearing on the measurements of intestinal permeability to different-sized PEGs, in which absorption-molecular weight profiles are assessed by urinary recovery after oral load. Thus, if PEGs in the 282–678-dalton range are used for such measurements, the absorption-molecular weight profile should be corrected for non-intestinal, size-dependent filtration.