Post-distribution rate constant

Relationship between the slow disposition (or post-distribution) rate constant (fi) and the elimination rate constant (ICio), the apparent... 

For ordinary pharmacokinetics, the distribution rate constant (a) is greater than the slow disposition, or post-distribution rate constant (/ )... 

Figure 13.10 A plasma concentration (Cp) versus time profile for a drug that obeys a two-compartment model following intravenous bolus administration plotted on semilogarithmic paper, p, slow disposition, or post-distribution, rate constant 6, empirical constant 4, apparent volume of distribution for the central compartment /C21, transfer rate constant Xq, administered dose a, distribution rate constant.

When an administered drug exhibits the characteristics of a two-compartment model, the difference between the distribution rate constant (a) and the slow (post-) distribution rate constant (/ ) plays a critical role. The greater the difference between these, the more conspicuous is the existence of a two-compartment model and, therefore, the greater is the need to apply all the equations for a two-compartment model. Failure to do so will, undoubtedly, result in inaccurate clinical predictions. If, however, the difference between the distribution and the slow post-distribution rate constant is small and will not cause any significant difference in the clinical predictions, regardless of the model chosen to describe the pharmacokinetics of a drug, then it may be prudent to follow the principle of... 

Once the values of distribution rate constant and the post-distribution rate constant, as well as the values of the two empirical constants A andU (the two y-axis intercepts) are obtained by the methods described above, or are taken to be the values reported in the literature, the micro rate constants for elimination and inter-compartmental transfer can be generated using Equation 13.6 ... 

Note that A + B = (Cp)o-The distribution rate constant, the post-distribution rate constant and the empirical constants A and B can be determined from the plasma concentration versus time data. Using these values and Eq. 13.14, the inter-compartmental rate constant (IC21) can be calculated. Note that this is a first-order rate constant associated with the transfer of a dmg from compartment 2 (i.e. peripheral or tissue) to the central compartment (compartment 1). 

Please note that in Eq. 13.31 the last observed plasma concentration is divided by the post-distribution rate constant (/3) because of the presence of a two-compartment model. Compare Eq. 13.31 with Eq. 4.26 (p. 66) and Eq. 7.15 (p. 134), which were employed when the administered drug exhibited the characteristics of a one-compartment model. 

A clear distinction must be made between the elimination rate constant (Kio) and the slow disposition or post-distribution rate constant ip). The constant ICio is the elimination rate constant from the central compartment at any time while the disposition or post-distribution... 

Both the intrinsic rate constant and the effective diffusivity (KD) can be extracted from measurements of the reaction rate with different size fractions of the zeohte crystals. This approach has been demonstrated by Haag et al. [116] for cracking of n-hexane on HZSM5 and by Post et al. [117] for isomerization of 2,2-dimethylbutane over HZSM-5. It is worth commenting that in Haag s analysis the equilibrium constant (or distribution coefficient K) was omitted, leading to erroneously large apparent diffusivity values. 

Determination of the elimination half life (fi/a), the elimination rate constant (K oiKd) and the apparent volume of distribution (V) from post-infusion (i.e. following the cessation of infusion) concentration versus time data and from concentration versus time data obtained during infusion. 

Use of post-infusion plasma concentration data to obtain half life, elimination rate constant and the apparent volume of distribution... 

Determination of post-distribution (or slow disposition, or terminal) half life (ti/z)/ and its corresponding rate constant (/3). 

The distribution ( ) and post-distribution (fi) rate constants are complex constants that serve to define other constants which unequivocally characterize distribution or elimination processes. 

A steady-state model has been set up based on the same kinetics and reactor model as used in the dynamic model. Some assumptions were made regarding the activity of the catalyst and the product distribution. The chain growth probability is supposed to be 0.92 and the olefin content is presumed to be 10%. The activity of the catalyst was taken from Post et al. [6] and is characterized by the rate constant for CO consumption 58 10" mV(m cat s). Stating the feed composition (H2/CO =2) and the conversion level (60%) fixes the feed rates of hydrogen and carbon monoxide. 

In Figure 10.30 the survival rate of the total sedimentary mass for the different Phanerozoic systems is plotted and compared with survival rates for the total carbonate and dolomite mass distribution. The difference between the two latter survival rates for each system is the mass of limestone surviving per unit of time. Equation 10.1 is the log linear relationship for the total sedimentary mass, and implies a 130 million year half-life for the post-Devonian mass, and for a constant sediment mass with a constant probability of destruction, a mean sedimentation rate since post-Devonian time of about 100 x 1014 g y 1. The modem global erosional flux is 200 x 1014 g y-1, of which about 15% is particulate and dissolved carbonate. Although the data are less reliable for the survival rate of Phanerozoic carbonate sediments than for the total sedimentary mass, a best log linear fit to the post-Permian preserved mass of carbonate rocks is... 

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