Extravascular administration single dose

Fig. 39.7. (a) Two-compartment catenary model for extravascular (oral or parenteral) administration of a single dose D which is completely absorbed. The transfer constant of absorption is (b) Time courses of the amount in the extravascular compartment Xa, the concentration in the plasma compartment Cp and the content in the elimination pool X. ... 

Serial drug concentrations following single dose extravascular administration of different doses given as a solution (not as a formulated tablet, capsule, etc.) at amounts low enough so as to not precipitate in the gastrointestinal tract. 

Medications that exhibit a linear response or dose-independent response can have the therapy optimized by using a single blood level determination. If the drug is given by a constant-rate infusion or if it is an extravascular administration, the peak-trough fluctuations are minor, and the patient is at steady state, then a simple proportion can be used. 

This approach can be expanded to include other routes of administration such as IV infusion (of duration 7) and extravascular administrations such as oral, intramuscular, subcutaneous, or topical. Schemes and equations for these models are shown in Table 12.1. The integrated equations in Table 12.1 can be used to calculate drug concentrations after a single IV bolus, IV infusion, or oral dose as shown in Eigures 12.2 through 12.4. 

It is also worth mentioning here that the pharmacokinetic parameters obtained following the administration of a single dose of a drug, intra-or extravascularly, may prove to be helpful while tackling some equations in multiple-dosing pharmacokinetics. This includes the intercepts of the plasma concentration versus time data, the systemic clearance and the absolute bioavailability of a drug, when applicable. 

The Dost ratio permits the determination of the amount and/or the plasma concentration of a drug in the body at any time t (range, t=0 to t — T) following the administration of the nth (i.e. second dose, third dose, fourth dose, etc.) dose by intravascular and/or extravascular routes. In other words, this ratio will transform a single dose equation into a multiple-dosing equation. 

It will be very helpful to begin to compare Eq. 11.12 (for an intravenous bolus) and Eq. 12.13 (for extravascularly administered dose) for similarity and differences, if any, and identify the commonality between the two equations. It may he quickly apparent that the information obtained following the administration of a single dose of a drug, either intravenously or... 

The peak time following the administration of a single extravascular dose is obtained by employing the following equation (equivalent to Eq. 6.15) ... 

Fig. 8. The time course of drug-concentration changes in plasma water after multiple extravascular drug administration when the body is considered as a single compartment, c, = drug concentration at time f, x — length of the dosing interval R =r number of doses administered.

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