Zero-order absorption models solution

Zero-order absorption occurs when drug enters the systemic circulation at a constant rate. An IV infusion, in which a drug solution is delivered directly into the systemic circulation at a steady flow rate, represents an idealized zero-order absorption case. Because of this, standard zero-order absorption models are typically called IV infusion models and are designed specifically for the IV infusion case. This particular section deals with the one-compartment IV infusion model, so as in the previous one-compartment bolus IV model, the body is modeled as a single compartment with the implication that the distribution process is essentially instantaneous. As with the other standard models, the exact meaning of the assumptions inherent in this model are described next. Model equations then are introduced that allow the prediction of plasma concentrations for drugs with known PK parameters, or the estimation of PK parameters from measured plasma concentrations. Modification of the one-compartment IV infusion (zero-order absorption) model to approximate other types of steady drug delivery are described in Section 10.8.5. 

The buccal permeability of the non-steroidal antiinflammatory drug, diclofenac sodium, has been evaluated in a dog model. The dog was selected because of the similarity of its buccal mucosa to that of man. Analysis of the buccal data indicated that diclofenac sodium permeability followed an essentially zero-order kinetic process with a minimal lag phase. Permeability of the drug was estimated to be 3 mg/cm2.h but significant differences were observed between animals. The absorption rate with the transbuccal delivery device decreased with time whereas the corresponding rate with a saturated solution was constant. This difference was attributed to the time dependency of drug delivery from the device and was modeled on the basis of release from a membrane-dispersed monolith combined with constant buccal permeability. The predictions of the model showed excellent agreement with the experimental data. 

Thus the oxidation of aqueous Na2S03 solutions with C0SO4 as a catalyst proves to be a convenient model reaction for determining interfacial area in gas-liquid reactors. The kinetics of the reaction is not simple many variables influence the reaction rate but, provided the range of cobalt and sulfite concentrations, pH values, and temperatures previously indicated is satisfied, the reaction is zero-order in sulfite, first-order in cobalt, and second-order in O2. The specific rate of absorption is... 

A separate mass balance equation is written in the form of Section 10.6.2 for each compartment in the model. Thus a total of n mass balance equations must be written and solved for an n compartment model. The details of these equations and their solution are not provided in this chapter. However, it will be noted that absorption, distribution, and elimination rates are written in the same form as in the previous one- and two-compartment models. The absorption rate for instantaneous, zero-order, or first-order absorption is identical to the previous forms for one- and two-com-partment models. Distribution and elimination rates are written as first-order linear rate equations using micro rate constants. So the distribution rate from compartment 1 to compartment n is given by kj Aj, the distribution rate from compartment n back to compartment 1 equals k i A , and the elimination rate from any compartment is written k o A schematic diagram for the generalized n compartment model is illustrated in Figure 10.90. 

Both absorption and emission spectra are somewhat blue shifted, which we attributed to the difference between the spectroscopic model (PAUMe) and the compound actually used in the experiments (PAdU). Furthermore, the dispersion interaction was neglected, mainly because the choice of the scaling parameter y Eq. (3-71) for states beyond the first excited state is cumbersome. Reducing the ionization energy of the solute by the excitation energy leads rapidly to a value smaller than zero, and hence to a positive dispersion interaction. In order to avoid this unphysical situation it is better to neglect the dispersion completely. Moreover, it is sometimes assumed that in semi-empirical wave functions electron correlation is accounted for because the parameters come from experiment. (The CIS... 

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