In vivo Methods for Determining Bioavailability

Another refinement, that avoids the necessity of developing suitable fecal extraction and chromatographic methods, is to dose the radiolabeled compound by both the i.v. and p.o. routes in two separate studies. Knowing that, by definition, the whole of the i.v. dose must have been bioavailable a comparison of the proportion of the dose in the urine after the two different routes allows estimation of the percent absorbed. An analogous approach can be used without the use of a radiolabel, when the urine from the two studies is analyzed either for the parent compound or, more usually, for a major common metabolite. Assuming quantitatively identical clearance after both the i.v. and p.o. doses, the ratio of the amounts of analyte in the two experiments gives the absorption. 

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Given the overwhelming influence of the physical properties of skin in determining bioavailabilities via the dermal route, assessment of dermal penetration is one area in metabolism and toxicology where in vitro methods can be effectively used to predict in vivo results and to screen chemicals. Apparatus and equipment exist that one can use to maintain sections of skin (obtained from euthanized animals or from human cadavers or surgical discard) for such experiments (Holland et al., 1984). These apparatus are set up to maintain the metabolic integrity of the skin sample between two reservoirs the one on the stratum comeum side, called the application reservoir and the one on the subcutaneous side, called the receptor reservoir. One simply places radiolabeled test material in the application reservoir and collects samples from the receptor fluid at various time points. 

Oxidative Biotransformation in Microsomes The rapid determination of pharmacokinetic parameters, solubility, permeability, and in vitro stability in plasma or liver tissue can often provide a reasonable explanation of the mechanisms limiting oral bioavailability. An approach that is often used is to extrapolate the in vitro rate of metabolism to estimate the hepatic clearance using in vitro-in vivo correlation methods.82-86 These methods use in vitro kinetic parameters, usually Vmllx/Km or in vitro t ji, to determine the intrinsic clearance, which is then scaled to hepatic clearance using the amount of tissue in the in vitro incubation, the weight of the liver, and the well-stirred model for hepatic clearance. 

Rodriguez et al. (34) evaluated the ability of the two in vitro gastrointestinal methods to estimate bioavailable As in contaminated soil and media. One method incorporated an adsorbent in a permeable membrane to act as a sink for dissolved As to mimic gastrointestinal absorption (IVG-AB). The other method (IVG) did not use an adsorbent and simply relied on dissolution of As from soil. In vitro results were compared with in vivo relative bioavailable As determined from dosing trials using immature swine. Fifteen contaminated soils collected from mining/smelter sites ranging from 401 to 17,500 mg As kg were evaluated. 

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