Drug absorption from the gastrointestinal tract

We first consider the physiological situation which might impinge on passive dmg absorption. The delivery of macromolecules, including peptides and proteins, is treated in a separate chapter. 

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Stirred tank models have been widely used in pharmaceutical research. They form the basis of the compartmental models of traditional and physiological pharmacokinetics and have also been used to describe drug bioconversion in the liver [1,2], drug absorption from the gastrointestinal tract [3], and the production of recombinant proteins in continuous flow fermenters [4], In this book, a more detailed development of stirred tank models can be found in Chapter 3, in which pharmacokinetic models are discussed by Dr. James Gallo. The conceptual and mathematical simplicity of stirred tank models ensures their continued use in pharmacokinetics and in other systems of pharmaceutical interest in which spatially uniform concentrations exist or can be assumed. 

Transport of the benzodiazepines into the brain is rapid, the rate of uptake being determined by the physicochemical properties of the drug. Absorption from the gastrointestinal tract, or from an injection site, is the rate-limiting step governing the speed of onset of the therapeutic response. Oral absorption is more rapid when the drug is taken on an empty stomach. 

The rate or extent of drug absorption from the gastrointestinal tract can be influenced in a number of ways. Examples of these various influences follow. 

These approaches place particular emphasis on the spatial aspects of the drug absorption from the gastrointestinal tract. The small intestine is assumed to be a cylindrical tube with fixed dimensions where the drug solution or suspension follows a homogeneous flow. Mass balance relationships under steady-state as-... 

Surfactant Influence on Drug Absorption from the Gastrointestinal Tract... 

The variability associated with drug absorption from the gastrointestinal tract can be overcome by using a parenteral preparation (dosage form). It should preferably be administered either by intravenous infusion or slow intravenous injection to avoid circulatory overload. Intraosseous administration is a useful alternative to intravenous injection of some antimicrobial agents (e.g. sodium ampicillin or amoxycillin, cefotaxime, ceftriaxone, gentamicin or amikacin sulphate) in neonatal foals (Fig. 7.1) (Golenz et al, 1994) and puppies (Lavy et al, 1995). This particularly applies when the neonate is in a state of septic shock and/or dehydration. Total plasma protein concentration is an inaccurate index of hydration status unless monitored (repeatedly measured) and interpreted in conjunction with packed cell volume (PCV). 

Godbillon, J., D. Evard, and N. Vidon. 1985. Investigation of drug absorption from the gastrointestinal tract of man. III. Metoprolol in the colon. Brit. J. Clin. Pharmacol. 19 113s-118s. 

For the majority of drugs, absorption from the gastrointestinal tract occurs via passive processes that cannot distinguish between the enantiomers of a racemate. Additionally, there is no evidence that any of the antiasthma drugs are absorbed by a carrier mediated process thus stereoselectivity in the absorption of antiasthma agents currently used in clinical practice is not expected [115]. 

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