First-order absorption models assumptions

Applying these results to this simulation, it can be assumed that bioavailability would be 100% and that a first-order absorption model would apply such that the time to maximal concentrations was 1 h (which corresponds to an absorption rate constant of 1.2 per hour). Under these assumptions, the simulated concentrationtime profile after repeated administration of 1 mg/kg every 8 h is shown in Fig. 9.22. Maximal concentrations were attained 1.1 h. after intramuscular administration and were about half the maximal concentration attained after intravenous administration. Trough concentrations at steady-state after intramuscular administration were very near trough concentrations after intravenous administration. Hence, based on this simulation, the physician could either keep the dose as is or increase the dose to more attain the concentrations seen after intravenous administration. 

The standard one-compartment first-order absorption model makes three inherent assumptions about the ADME processes that occur during and after drug delivery. The specific nature and implications of each of these assumptions are described in this section. 

This assumption is the same for all first-order absorption models. See Section 10.9.1.1 for the details regarding this assumption. 

Multicompartment model equations can be written for instantaneous absorption, zero-order absorption, or first-order absorption. For any of these particular absorption situations, the assumptions described previously for the corresponding absorption in one- and two-compartment models remains exactly the same for multicompartment models. 

The Wagner-Nelson method of calculation does not require a model assumption concerning the absorption process. It does require the assumption that (a) the body behaves as a single homogeneous compartment and (b) drug elimination obeys first-order kinetics. The working equations for this calculation are developed next. 

From the above it can be concluded that in many instances the introduction of an artificial radionuclide into the environment provides us with a natural tracer experiment. Indeed, this is the basis for the application of deterministic compartmental models, based on tracer kinetics, to radioecology (Whicker and Schultz, 1982). This approach is largely based on the assumption that radionuclide movements will exhibit first order kinetics although the existence of naturally-occurring tracees (stable isotopes) at relatively high abundance may result in more complex concentration-dependent kinetics. Furthermore, nutrient analogues may exert even more complex effects on processes such as radioion absorption across root plasma membranes this will become evident later in the chapter. 

The enhancement factors are either obtained by fitting experimental results or are derived theoretically on the grounds of simplified model assumptions. They depend on reaction character (reversible or irreversible) and order, as well as on the assumptions of the particular mass transfer model chosen [19, 26]. For very simple cases, analytical solutions are obtained, for example, for a reaction of the first or pseudo-first order or for an instantaneous reaction of the first and second order. Frequently, the enhancement factors are expressed via Hatta-numbers [26, 28]. They can be used in combination with the HTU/NTU-method or with a more advanced mass transfer description method. However, it is generally not possible to derive the enhancement factors properly from binary experiments, and a theoretical description of reversible, parallel or consecutive reactions is based on rough simplifications. Thus, for many reactive absorption processes, this approach appears questionable. 

The advantages of using non-compartmental methods for calculating pharmacokinetic parameters, such as systemic clearance (CZg), volume of distribution (Vd(area))/ systemic availability (F) and mean residence time (MRT), are that they can be applied to any route of administration and do not entail the selection of a compartmental pharmacokinetic model. The important assumption made, however, is that the absorption and disposition processes for the drug being studied obey first-order (linear) pharmacokinetic behaviour. The first-order elimination rate constant (and half-life) of the drug can be calculated by regression analysis of the terminal four to six measured plasma... 

Outline of the Theoreyical Model. The main assumptions for the unsteady state dynamics are as follows l) Only polymer moleciiles which are raised into excited state by absorbing UV light (photon flux, no wavelength, X) near the absorption band charasteristic of polymers can participate in photochemical reactions (efficiency, n molar concentration, C ). (2) Photochemical reactions are i) depolymerization of activated polymer molecules (first order reaction,... 

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