Gastrointestinal transit variation

Coupe, A. J., Davis, S. S., and Wilding, I. R. Variation in gastrointestinal transit of pharmaceutical dosage forms in healthy subjects. Pharm. Res. 8 360-364, 1991. 

In most cases, oral drug delivery is the cheapest and most convenient method of dosing. Unfortunately, it is difficult to achieve a precise control of the plasma-concentration-time profile by this route due to marked intra- and intersubject variation in gastrointestinal transit even under the rigidly controlled conditions of the clinical trial. Daily patterns of food intake, activity, and posture are large contributors to this variation. Drugs that are only absorbed from specific areas of the gastrointestinal tract, i.e., have a narrow window of absorption, will be most affected by alterations in transit. The major determinants of this variation will be the amount of food and drink consumed. 

For dmgs that are normally taken orally, administration through the skin can eliminate the vagaries which influence gastrointestinal absorption, such as pH changes and variations in food intake and intestinal transit time. 

The human body presents variations on pH along the gastrointestinal (GI) tract, and also in some specific areas like certain tissues (and tumoral areas) or sub-cellular compartments. The polymer-polymer and the polymer-solvent interactions (the solvent in biomedical applications will usually be water) show an abrupt re-adjustment in small ranges of pH, and this is translated to a chain transition between extended and compacted coil states. 

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