Bioavailability species differences

Y-L He, S Murby, G Warhurst, L Gifford, D Walker, J Ayrton, R Eastmond, M Rowland. Species differences in size discrimination in the paracellular pathway reflected by oral bioavailability of poly (ethylene glycol) and D-peptides. J Pharm Sci 87 626-633, 1998. 

The total metal concentration in a solution can be easily determined using methods such as atomic absorption spectroscopy (AAS) however, the bioavailability of different metal species likely varies. In addition, much of the original concentration may have speciated into insoluble precipitates. Therefore, the concentration of some bioavailable species may be extremely low, perhaps even within or below the nanomolar range.99 Ion-selective electrodes are useful for measuring the bioavailable concentration of a metal because they measure only the free, ionic species, which is often most prevalent.102... 

The term bioavailability has different meanings in different contexts and disciplines. Numerous definitions of bioavailability exist. Research on the relationship between bioavailability and chemical speciation (forms) originated in the field of soil fertility in the search for a good predictor for the bioavailability of essential plant nutrients (Traina and Laperche 1999). It is well accepted that dissolved nutrients are more labile and bioavailable to plants than solid-phase forms (including sorbed species). The same has been considered to be true for organic contaminants and their availability for microbial degradation. 

Preclinical studies across species noted that bioavailabilities calculated from tissue-derived data were consistently higher than plasma-derived bioavailabilities. This difference varied across tissues, but in general amounted to about a two-fold difference between plasma and tissue bioavailability (Fig. 10.4). While the tissue... 

Pharmacokinetic and bioavailability (absolute and relative) experiments are usually designed and conducted to evaluate dose proportionality over the dose range used, or expected to be used, in toxicology studies and possible species-to-species differences in pharmacokinetic profiles. With the incorporation of one or two I.V. dose levels into the study protocol, the drug candidate s absolute... 

Fiserova-Bergerova V, Hughes HC. 1983. Species differences on bioavailability of inhaled vapor and gases. In Fiserova-Bergerova V, ed. Modeling of inhalation exposure to vapors Uptake distribution and elimination. Boca Raton, FL CRC Press, 2 97-106. 

The physiological features of the neonatal period that influence the bioavailability and disposition of drugs should be considered when selecting the route of administration and the dosage interval. An appreciation of species differences in the rate at which age-related physiological variables mature, and... 

Such factors may change not only the kinetics of an enzyme reaction but also the whole pattern of metabolism, thereby altering the bioavailability, pharmacokinetics, pharmacological activity, or toxicity of a xenobiotic. Species differences in response to xenobiotics must be considered during the extrapolation of pharmacological and toxicological data from experiments in animals to humans. The primary factors in these differences probably are the rate and pattern of drug and xenobiotic metabolism in the various species. 

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