Sampling times, drug concentrations

Plasma samples for PK analysis by a one-compartment bolus IV model are typically collected at 5 to 12 time points after drug administration. The time interval between samples generally starts out small for early samples and increases for later samples. The drug concentration in each sample is then measured by some type of analytical chemistry technique. 

As a general rule, in vivo assays are more challenging than in vitro assays because the matrices for the samples are more complex. The most common use for in vivo assays is to measure the concentration of NCE dosed into a laboratory animal by collecting multiple sample time points, one can use the analytical results to plot the PK profile of the NCE and also obtain various PK parameters that help determine a test compound s PK properties. Preclinical PK parameters of a test compound are then used to predict its human PK parameters. Another use of in vivo assays is combining the results with pharmacodynamic (PD) observations to perform PK/PD modeling.77 82 PK/PD modeling is an important aspect of new drug discovery because it can be used to predict the exposures and durations required to determine clinical efficacy of a NCE. 

Milk. The study of passage of a xenobiotic into milk serves to assess the potential risk to breast-fed infants in the absence of human data. The passage into milk can be estimated as the milk plasma ratio of drug concentrations at each sampling time or... 

Pharmacokinetic concentration-time curves for a drug and ifs mefabolifes are used to identify primary exposure metrics such as AUC, or which are not time-dependent unlike the sequential measurements of concentration over time. A peak plasma concentration of a drug is often associated with a PD response, especially with an adverse event. There can be large inter-individual variability in the time-to-peak concentration, and closely spaced sampling times are often critical to determining the peak plasma concentration accurately in individual patients because of differences in demographics, disease states, and food effects, if any. All these elements are clearly spelled out in the protocols written to conduct these studies. 

After sample collection, drugs of abuse concentrations may be determined chemically by means of different methodologies, which target at times common but mostly different groups of substances. A brief description of the methodologies is provided below ... 

Brain delivery of the anticancer drug daunomycin provides an example of the in vivo application of OX26-inununoliposomes [111]. Different formulations of [ H]-daunomycin were i.v. administered to rats either as the free drug or encapsulated in conventional liposomes, sterically-stabilized liposomes, or PEG-conjugated immunohposomes (Table 2.3). Plasma samples were taken at defined time points and after 1 h the animal was killed and drug concentrations in brain tissue were determined. 

To allow adequate time for equilibration of digoxin between serum and tissue, perform sampling of serum concentrations just before the next scheduled dose of the drug. If this is not possible, perform sampling at least 6 to 8 hours after the last dose, regardless of the route of administration or the formulation used. 

Again we will take blood samples at intervals after dosing, measure plasma drug concentrations, and plot the results on a linear graph (Fig. 11). The first and obvious thing to note is that the plasma concentrations rise to a maximum at around 1 h, whereas, of course, the early plasma concentrations taken soon after the intravenous bolus were the highest. The time to reach the peak plasma concentration after an oral dose is often abbreviated to Tmax, and the concentration itself to Cmax - the maximum concentration achieved after that dose. 

The answer is a technique called population kinetics. In this, blood samples are taken on a few occasions, carefully timed in relation to the previous drug dose, in as big a population as can be observed. The blood samples may be obtained at widely different time points after dosing and ah are analyzed for drug concentration. The next step is a statistical treatment of the results which makes the assumption that ah the patients belong to one big, if variable, population. A spread of data points is obtained over the dose interval and one gigantic curve of concentration-time relationships created. If the population is big enough, the mathematics iron out any awkward individuals whose data do not tit the overall pattern and from this derived curve the kinetic parameters we have been discussing can be deduced. 

Monitoring the effluent of a smoke stack, or the concentration of a drug in a patient requires that the sampling rate is as low as possible and that can be predicted with a known probability that between the sampling times no fatal concentration change will occur. If it is expected that the monitored value will exceed a preset value a simple action can prevent that administer some drug, open or close a value, etc. 

Bioavailability refers to the portion of a drug absorbed from the site of administration. The reference site of administration is intravenous, because this route produces 100% absorption. Figure 3-1 illustrates three sample drug concentration curves in plasma as a function of time. The area under the curve (AUC) is the total amount of drug in the systemic circulation available for distribution to the sites of action. The same dose completely absorbed from any of these routes would produce an identical area under the curve (i.e., 100% bioavailability), although the shape would differ. 

The commonsense approach to the interpretation of drug concentrations compares predictions of pharmacokinetic parameters and expected concentrations to measured values. If measured concentrations differ by more than 20% from predicted values, revised estimates of Vd or CL for that patient should be calculated using equation (1) or equation (2). If the change calculated is more than a 100% increase or 50% decrease in either Vd or CL, the assumptions made about the timing of the sample and the dosing history should be critically examined. 

Postpreparative Stability. The stability of processed samples, including the resident time in the autosampler, should be determined. The stability of the drug and the internal standard should be assessed over the anticipated run time for the batch size in validation samples by determining concentrations on the basis of original calibration standards. Reinjection reproducibility should be evaluated to determine if an analytical run could be reanalyzed in the case of instrument failure. 

The limit of quantification is more relevant than the limit of detection in the analysis of drug residues in foods. In these applications, the limit of quantification can be more practically defined as the lowest drug concentration in food samples that can be measured with a desired level of accuracy and precision. It is usually determined by reducing the analyte concentration until a level is reached where the precision of the assay becomes unacceptable. If the required precision of the method at the limit of quantification has been specified, a number of samples with decreasing amounts of the analyte are analyzed 6 times at minimum, and the calculated RSD% of the precision is plotted against the analyte amount the amount that corresponds to the previously defined required precision is equal to the limit of quantification. 

A121a cells were exposed to paclitaxel at concentrations of 0.2, 0.8, 2, and 5 ng/mL. Paclitaxel uptake by cells was able to be quantified at each time point under four treatment conditions. The lowest measured intracellular accumulation was 6.5 pg/106 cells, observed 10 min after the addition of 0.2 ng/mL paclitaxel to cells. The detected concentration in this sample corresponded to approximately 9.6 pg/mL drug in the cell lysate, which is nearly twice the LOQ. Foremost concentrations of paclitaxel, it appeared that the intracellular drug concentrations increased rapidly with exposure time and reached a plateau within 1-3 h. However, for cells exposed to the lowest concentration (0.2 ng/mL), the maximum intracellular drug concentration apparently was not achieved within 6 h, which was longest time interval investigated. In future studies, these data will be expanded to include additional concentrations and exposure times, and analyzed according to cellular pharmacokinetic models such as those published previously [12],... 

Direct evidence that irreversible inhibition is the principle mechanism underlying in vivo drug-drug interactions (DDIs) is often lacking because of the requirement for either direct tissue sampling to reveal inactivated enzyme or in vivo inhibition of activity after drug is essentially eliminated from the body. Nevertheless the steady-state plasma concentrations of several clinically important CYP inhibitors are well below the in vitro estimated competitive inhibition constant, Kv This suggests that competitive inhibition is unlikely to occur in vivo, yet these compounds inhibit CYP activity in a time and concentration-dependant manner when cDNA-expressed CYPs or HLMs are used as an enzyme... 

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