Pharmacokinetic profile compartmental pharmacokinetics

Refinement and expansion of these steady-state mass balance approaches has led to the development of dynamic models which allow for estimation of the fraction absorbed as a function of time and can therefore be used to predict the rate of dmg absorption [37], These compartmental absorption and transit models (CAT) models have subsequently been used to predict pharmacokinetic profiles of drugs on the basis of in vitro dissolution and permeability characteristics and drug transit times in the intestine [38],... 

There are several approaches to pharmacokinetic modelling. These include empirical, compartmental, clearance-based and physiological models. In the latter full physiological models of blood flow to and from all major organs and tissues in the body are considered. Such models can be used to study concentration-time profiles in the individual organs and e. g. in the plasma [57-60]. Further progress in this area may result in better PK predictions in humans [61]... 

Let s look at pharmacokinetics and the work done with compartmental analysis. We should become involved with the biology of clinical trials in a quantitative way. Can we do more than just say the profile in compartments of the body follows a certain pattern Engineers have to start looking into this other side of regulatory activities. 

For pharmacokinetics in plasma Individual concentrations of XYZ1234 will be tabulated together with descriptive statistics and plotted. Median profiles will be presented graphically by CYP 2C19 metabolizer status and gender. Pharmacokinetic parameters (at least Cmax, tmax, AUC(o-t) [t = 24 h and last > LOQ ], AUCinf, ti/2z, MT, as well as CL/f and Vz/f) will be determined based on plasma concentrations of X YZ1234 using non-compartmental procedures. 

Figure 24.3 Compartmental pharmacokinetic model linking skin absorption determined in an in vitro model to a systemic model to predict plasma concentration time profiles in vivo.

Pharmacokinetics After Oral and Intravenous Administration. For proper characterization of an inhalation drug, information on the systemic pharmacokinetic properties needs to be provided. One of the major challenges for such studies is to provide a suitable formulation for injection, especially because new drug candidates are often very lipophilic. The resulting parameters of such studies (systemic clearance, volume of distribution, half-life, mean residence time) can then easily be extracted from concentration-time profiles after IV administration and subsequent standard pharmacokinetic analysis by noncompartmental approaches. In addition, a detailed compartmental analysis based on concentration-time profiles will be useful in evaluating the systemic distribution processes in sufficient detail. This will be especially important if deconvolution procedures (see later) are included for the assessment of the pulmonary absorption profiles. 

FIGURE 3.6 Compartmental analysis for different terms of volume of distribution. (Adapted from Kwon, Y., Handbook of Essential Pharmacokinetics, Pharmacodynamics and Drug Metabolism for Industrial Scientists, Kluwer Academic/Plenum Publishers, New York, 2001. With permission.) (a) Schematic diagram of two-compartment model for compound disposition. Compound is administrated and eliminated from central compartment (compartment 1) and distributes between central compartment and peripheral compartment (compartment 2). Vj and V2 are the apparent volumes of the central and peripheral compartments, respectively. kI0 is the elimination rate constant, and k12 and k21 are the intercompartmental distribution rate constants, (b) Concentration versus time profiles of plasma (—) and peripheral tissue (—) for two-compartmental disposition after IV bolus injection. C0 is the extrapolated concentration at time zero, used for estimation of V, The time of distributional equilibrium is fss. Ydss is a volume distribution value at fss only. Vj, is the volume of distribution value at and after postdistribution equilibrium, which is influenced by relative rates of distribution and elimination, (c) Time-dependent volume of distribution for the corresponding two-compart-mental disposition. Vt is the starting distribution space and has the smallest value. Volume of distribution increases to Vdss at t,s. Volume of distribution further increases with time to Vp at and after postdistribution equilibrium. Vp is influenced by relative rates of distribution and elimination and is not a pure term for volume of distribution. 

The choice of model should be based on biological, physiological, and pharmacokinetic plausibility. For example, compartmental models may be used because of their basis in theory and plausibility. It is easy to conceptualize that a drug that distributes more slowly into poorly perfused tissues than rapidly perfused tissues will show multi-phasic concentration-time profiles. Alternatively, the Emax equation, one of the most commonly used equations in pharmacodynamic modeling, can be developed based on mass balance principles and receptor binding kinetics. 

Suppose one does a study wherein a single dose of the drug is given both orally and intravenously on two separate occasions and finds that the oral concentrationtime data were best fit using a four-term polyexponential equation, whereas after intravenous administration the concentration-time profile was best fit with a two-term polyexponential equation. In this case there are 27 possible compartmental models to choose from. Whereas most books on pharmacokinetics present the 1- and 2-compartment model, the situation is clearly not that simple (see Wagner s (1993) text for examples). 

Various pharmacokinetic parameters such as CL, Vd, F%, MRT, and im can be determined using noncompartmental methods. These methods are based on the empirical determination of AUC and AUMC described above. Unlike compartmental models (see below), these calculation methods can be applied to any other models, provided that the drug follows linear pharmacokinetics. However, a limitation of the noncompartmental method is that it cannot be used for the simulation n of different plasma concentration-time profiles when there are alterations in dosing regimen or when multiple dosing regimens are used. 

As previously discussed, compartmental models can be effectively used to project plasma concentrations that would be achieved following different dosage regimen and/or multiple dosing. However, for these projections to be accurate, the drug pharmacokinetic profile should follow first-order kinetics where various pharmacokinetic parameters such as CL, V[Pg.114]

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