Drug concentration, in the blood

First order kinetics describes the most common time course of drug elimination. The amount eliminated within a time-interval is proportionate to the drug concentration in the blood. 

The fact that the GIT is so well perfused by the bloodstream permits efficient delivery of absorbed materials to the body. As a result of this rapid blood perfusion, the blood at the site of absorption represents a virtual sink for absorbed material. Under normal conditions, then, there is never a buildup in drug concentration in the blood at the site of absorption. Therefore, the concentration gradient will favor further unidirectional transfer of drug from the gut to the blood. Usually, then, blood flow is not an important consideration in drug absorption. Generally, the properties of the dosage form (especially dissolution rate) or the compound s inherent absorbability will be the limiting factors in absorption. 

Amount of drug in the body Drug concentration in the blood... 

The extent of absorption of a drug can be estimated by comparing the total area under the drug concentration in the blood versus time curve or the total amount of unchanged drug excreted in the urine after administration of drug and compared to the administration of standard (standard may be an intravenous injection, where the bioavailability of a drug reaches 100%). 

Consider a tablet that is taken regularly once a day. We want to find the optimal quantity of the drug (i.e., the only active ingredient) in the tablet in order to keep the drug concentration in the blood within a given therapeutic range [Cj, Cy] as strictly as possible. To predict the drug concentration we use the linear compartmental model shown in Fig. 2.9, one of the most popular models in pharmacokinetics. 

Variation of Drug Concentration in the Blood with Time Key Terms... 

The effect-compartment model relaxes the assumption H3 and it stems from the assumption of prereceptor nonequilibrium between drug concentration in the blood or plasma c (t) and the receptor site y (t). According to this model, an additional compartment is considered, the effect (or biophase) compartment, and... 

Plasma levels for some psychotropic medications, certain correlations have been established between plasma levels (the concentration of a drug in the liquid portion of the blood) and therapeutic benefit. Using this method, the prescribing physician can make adjustments to the doses with the goal of reaching drug concentrations in the blood that are associated with optimal response. In a few instances—for example the antiobsessional clomipramine (trade name Anafranil) and the antide-... 

In this model, the drug concentration in the blood or plasma is proportional to both the concentration in other tissues (e.g. muscle) and to the total amount of drug in the body. When the equilibration of a drug between the central and peripheral compartments occurs less rapidly, relative to elimination, then the disposition kinetics of the drug can be described by assuming that there are two (or sometimes more) distribution compartments. The apparent varies with time after drug administration in these models because of the time required for equilibration between the compartments. Thus, for a two-compartment model, y,. is dose/(A + B), where A and B are the y intercepts (plasma concentrations at time 0) associated with the distribution and elimination phases, respectively. 

Figure 33-1 illustrates the conceptual relationship between pharmacodynamics and pharmacokinetics. The former relates drug concentration at the site of action to the observed magnitude of the effect. Pharmacokinetics, on the other hand, relates dose, dosing interval, and route of administration (regimen) to drug concentration in the blood. For... 

A sample of the results is shown in Figure 6 where the lower curve is for ARA-C the upper curve represents ARA-C plus metabolite, and the points are clinical data. Figure 6 shows that there is a very rapid drop in the drug concentration in the blood followed by a slower exponential phase. This behavior represents a complex interaction of physical knd metabolic processes as discussed in the original source. 

Drug concentrations in the blood and plasma are the most direct methods of determining the systemic availability of a drug. The pharmacokinetic parameters that... 

The two responses could, for example, represent the drug concentration in the blood and some peripheral tissue following a bolus injection. For this case, = -1, k2 =... 

The desired drug concentration in the blood is promptly attained. 

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