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  • 1
    Electronic Resource
    Electronic Resource
    An Equation for the Systemic Availability of Drugs Undergoing Simultaneous Enterohepatic Cycling, First-Pass Metabolism, and Intestinal Elimination (1987)
    Springer
    Pharmaceutical research 4 (1987), S. 195-199 
    Details
    ISSN: 1573-904X
    Keywords: enterohepatic recirculation ; pharmacokinetics ; bioavailability ; area under the curve ; bile ; hepatic extraction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract A relationship between systemic availability and its determinants has been derived for a physiologically realistic model of drug disposition that includes enterohepatic cycling (EHC), gallbladder emptying (with an arbitrary time course), first-pass metabolism to noncycling metabolites, and fecal excretion. Systemic availability (F) has been shown to be determined by the fraction of the dose initially absorbed (f a*), the fraction of the drug excreted into the GI tract that is reabsorbed with each cycle (f a), the hepatic extraction ratio (E), and the fraction of extracted drug that is transported to the gallbladder for EHC (f g) according to the relationship F = f a*(1 −E/(1 − f a f g E) The implications of the above relationship are that (1) systemic availability is dependent on EHC, (2) values of F calculated to be greater than unity cannot be explained simply by the presence of EHC, (3) calculations of E based on the usual expression F = f a* (1 − E) are erroneous for drugs subject to EHC, and (4) a compound that has a high systemic availability and is subject to EHC is not necessarily inefficiently metabolized. The quantitative interrelationship of systemic availability and its determinants is illustrated using a contour plot. Slices through the surface are used to demonstrate that the presence of EHC changes the sensitivity of F to changes in E.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1016447826075
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Estimation of area under the curve for drugs subject to enterohepatic cycling (1985)
    Springer
    Journal of pharmacokinetics and pharmacodynamics 13 (1985), S. 589-608 
    Details
    ISSN: 1573-8744
    Keywords: enterohepatic recirculation ; pharmacokinetics ; hepatic extraction ; area under the curve ; model ; bile ; simulated concentration-time curves
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract A physiologically realistic model is used to provide insight into the design of sampling protocols for accurate determination of AUC(0– ∞) for drugs subject to enterohepatic cycling. Through simulation of plasma concentration- time curves for such drugs it is found that (1) more than one peak is predicted after oral and intravenous administration of a single dose of drug, (2) the relative magnitude of peaks is dependent on the hepatic extraction ratio for both oral and intravenous drug administration, (3) the percent of the AUC(0– ∞) in later time intervals is also a function of the hepatic extraction ratio, and (4) present methods for the design of sampling protocols may not provide accurate estimates of AUC(0– ∞) (especially for highly extracted drugs), because (a) peaks are only evident at later times after intravenous administration when plasma sampling is less frequent, (b) much of the area occurs at later times, and (c) the amount of drug in the sampling compartment after oral administration is much lower than that after intravenous administration of drug and could be incorrectly interpreted as low bioavailability if sampling is not carried out for a long period of time. The types of oral and intravenous profiles predicted for highly extracted drugs are exemplified by data for naltrexone in the monkey.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01058903
    Library Location Call Number Volume/Issue/Year Availability
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    Paper (German National Licenses)
    Fulltext
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