Plasma concentration compounds

Based on the described neuroanatomy, the three key matrices for evaluating compound concentrations to determine the extent and/or rate of CNS penetration are blood, CSF, and brain tissue. Due to the nearly universal analysis of plasma to determine systemic concentrations of small molecules, total plasma compound concentration (Cp) will henceforth be substituted for total blood concentration, which is the product of Cp and compound blood-to-plasma ratio. 

The latter approach is used in the enantioselective determination of a Phase I metabolite of the antihistaminic drug, terfenadine. Terfenadine is metabolized to several Phase I compounds (Fig. 7-13), among which the carboxylic acid MDL 16.455 is an active metabolite for which plasma concentrations must often be determined. Although terfenadine can be separated directly on Chiralpak AD - an amy-lose-based CSP - the adsorption of the metabolite MDL 16.455 is too high to permit adequate resolution. By derivatizing the plasma sample with diazomethane, the carboxylic acid is converted selectively to the methyl ester, which can be separated in the presence of all other plasma compounds on the above-mentioned CSP Chiralpak AD [24] (Fig. 7-14). Recently, MDL 16.455 has been introduced as a new antihistaminic drug, fexofenadine. 

Distribution. Lead in blood partitions between plasma and red blood cells, with the larger fraction (90-99%) associated with red blood cells (Cake et al. 1996 DeSilva 1981 Everson and Patterson 1980 Manton and Cook 1984 Ong and Lee 1980a). Lead in plasma binds to albumin and y -globulins (Ong and Lee 1980a). The fraction that is not bound to protein exists largely as complexes with low molecular weight sulfhydryl compounds these may include cysteine, homocysteine, and cysteamine (Al-Modhefer et al. 1991). Approximately 75% was bound to protein when whole human blood was incubated with 50 ig/dL lead (as lead chloride) approximately 90% of the bound lead was associated with albumin (Ong and Lee 1980a). However, the fraction of lead in plasma bound to protein would be expected to vary with the plasma lead concentration. 

From such rat neuropharmacokinetic studies, in which animals (N = 2/ time point, >2 doses) are euthanized at specific time points postdose for plasma, CSF, and brain collection for compound concentration analysis, composite neuromatrix-specific compound concentration-time curves are generated (Figure 2). 

Figure 2 Mean plasma (Cp), CSF (CCSF), and brain (Cb) compound concentration-time profiles (graph) and matrix-specific neuropharmacokinetic parameters (table) of a compound in rats following subcutaneous administration [42]. Abbreviations Cmax, maximal compound concentration Tmax, time of Cmax tV2, compound half-life.

Sampling Interval To be able to perform valid toxicokinetic analysis, it is not only necessary to properly collect samples of appropriate biological fluids, but also to collect a sufficient number of samples at the current intervals. Both of these variables are determined by the nature of the answers sought. Useful parameters in toxico-kinetic studies are Cmax, which is the peak plasma test compound concentration Tmax, which is the time at which the peak plasma test compound concentration occurs, Cmin, which is the plasma test compound concentration immediately before the next dose is administered AUC, which is the area under the plasma test compound concentration-time curve during a dosage interval, and t which is the half-life for the decline of test compound concentrations in plasma. The samples required to obtain these parameters are shown in Table 18.12. Cmin requires one blood sample immediately before a dose is given and provides information on accumulation. If there is no accumulation in plasma, the test compound may not be detected in this sample. 

Ideal for studying the dose-response relationship for QT interval prolongation taking into account all the pharmacological properties of a compound The dog model is one of the most widely used anesthetized rabbits (especially female rabbits) have also been proposed for high sensitivity It provides complementary information with respect to in vitro tests (activity of metabolites, measurement of plasma drug concentrations, calculation of the volume of distribution) Possibility to induce experimental TdP... 

Fig. 2.12 Receptor occupancy based on PET scanning and in vitro potency combined with free plasma drug concentration. Log D74 values shown below compound names.

An example for stimulus generalization are responses of rats to stress-inducing odors. Laboratoiy rats of the Wistar strain respond to predator odors, specifically mercapto compounds in fox droppings, with stress reactions, for example avoidance behavior such as freezing and increased plasma corticosterone concentrations (Vemet-Mauiy et ah, 1984). The rats were trained to avoid water scented with a mercapto odorant that contained both a keto- and a sulfhydryl group (4-mercapto-4-methyl-2-pentanone). As the animals licked a waterspout, a mild electric shock was applied to their tongue. When different compounds were tested thereafter, the rats avoided compounds with similar... 

The primary endpoint of the toxicokinetic studies is the concentration-time prohle of the substance in plasma/blood and other biological fluids as well as in tissues. The excretion rate over time and the amount of metabolites in urine and bile are further possible primary endpoints of kinetic studies, sometimes providing information on the mass balance of the compound. From the primary data, clearance and half-life can be derived by several methods. From the excretion rate over time and from cumulative urinary excretion data and plasma/blood concentration measured during the sampling period, renal clearance can be calculated. The same is the case for the bUiary excretion. 

Toxicokinetic data can also be used to make informed decisions on testing of chemical substances. In specific circumstances, valid toxicokinetic data may be used to support a decision to omit testing for systemic effects, e.g., in cases where the toxicokinetic data provide sufficient evidence that a substance is not absorbed and therefore not systemically available, i.e., no plasma/blood concentrations were measurable and no parent compound or metabolites could be detected in urine, bile, or exhaled air. For example, in vivo testing for mutagenicity, reproductive toxicity, or carcinogenicity may be omitted if toxicokinetic data or other data indicate a lack of systemic availability. 

The rates of movement of foreign compound into and out of the central compartment are characterized by rate constants kab and kei (Fig. 3.23). When a compound is administered intravenously, the absorption is effectively instantaneous and is not a factor. The situation after a single, intravenous dose, with distribution into one compartment, is the most simple to analyze kinetically, as only distribution and elimination are involved. With a rapidly distributed compound then, this may be simplified further to a consideration of just elimination. When the plasma (blood) concentration is plotted against time, the profile normally encountered is an exponential decline (Fig. 3.24). This is because the rate of removal is proportional to the concentration remaining it is a first-order process, and so a constant fraction of the compound is excreted at any given time. When the plasma concentration is plotted on a logio scale, the profile will be a straight line for this simple, one compartment model (Fig. 3.25). The equation for this line is... 

Plant sterols Commercially available margarines containing hydrogenated plant sterols and sterol esters (predominantly sitostanol esters), when used in place of regular margarine, can reduce LDL plasma cholesterol concentrations. The mechanism by which these compounds lower LDL cholesterol concentrations is to inhibit intestinal absorption of dietary cholesterol and cholesterol secreted into the bile. 

Although the acute vasodilator effects, as shown in in vitro studies (see above), may participate in the antihypertensive effects, the reduced blood pressure persisted even 42-48 h after the last administration of quercetin, when the plasma quercetin concentration and its metabolites fell bellow 25% of the peak post-administration levels [43]. Furthermore, the antihypertensive effects of quercetin did not appear to be related to its antioxidant properties since quercetin did not lower the urinary isoprostane F20 excretion, a prostaglandin-like compound produced in a non enzymatic reaction of arachidonic acid in membrane lipids and superoxide, which is currently used as a reliable marker of oxidative stress. The mechanisms involved in the antihypertensive effects and protection from organ damage... 

Various factors may account for the variability in response to neuroleptics. These include differences in the diagnostic criteria, concurrent administration of drugs which may affect the absorption and metabolism of the neuroleptics (e.g. tricyclic antidepressants), different times of blood sampling, and variations due to the different type of assay method used. In some cases, the failure to obtain consistent relationships between the plasma neuroleptic concentration and the clinical response may be explained by the contribution of active metabolites to the therapeutic effects. Thus chlorpromazine, thioridazine, levomepromazine (methotrime-prazine) and loxapine have active metabolites which reach peak plasma concentrations within the same range as those of the parent compounds. As these metabolites often have pharmacodynamic and pharmacokinetic activities which differ from those of the parent compound, it is essential to determine the plasma concentrations of both the parent compound and its metabolites in order to establish whether or not a relationship exists between the plasma concentration and the therapeutic outcome. 

The contribution of lipophilic antioxidants is small. Escobar et al. (E5) found that the TAC of lipophilic antoxidants in blood plasma was 16.5 1.5 pM and corresponded almost exclusively to a-tocopherol the concentration of this compound in the blood plasma, analyzed independently, was 17.6 0.3 pM. Popov and Lewin (PI9) found TAC of lipid-soluble antioxidants in blood plasma to be 28.0 8.1 /u.M, a value comparable with the concentration of a-tocopherol (20.5 6.6 /U.M). These (and other) results confirm that a-tocopherol is the main lipid-soluble antioxidant of blood plasma (II) and indicates that the contribution of the lipid-soluble antioxidants to TAC of blood plasma is in fact negligible, taking into account that TAC of human blood plasma is of the order of 1 mM (see later). The contribution of ascorbic acid is also low. This situation may differ considerably in other biological fluids and tissue homogenates. In seminal plasma, the concentration ratio of ascorbate to urate is about 1 (G3). Ascorbate and urate contribute 29% of the fast TRAP of human seminal plasma the share of proteins and polyphenolic compounds is 57%, whereas tyrosine contributes 15% of the slow TRAP (R14) (Table 7). Ascorbate and uric acid account for about half of TAC of human tears (K3). TAC of urine is determined mainly by urate and proteins (K5). 

Another important field of application concerns food and beverages, especially wine, juices, and tea (A2, A11, A17, B4, K12, V7, Yl). The antioxidant components of food include vitamin E (a-tocopherol), vitamin A (retinoids), vitamin C (ascorbic acid), and also fi-carotene (provitamin A), other carotenoids (of which more than 600 compounds have been identified), flavonoids, simple phenols, and glucobrasicins (H3). Unfortunately, the TAC value of a food is not informative on the bioavailability of its antioxidants. It has been estimated that polyphenols are normally present in blood plasma at concentrations of 0.2-2 //M (PI). However, it has been demonstrated that feeding rats a quercetin-augmented diet can increase their plasma levels of quercetin and its metabolites up to 10-100 //M (M27), and transient increases in the concentration of plant-derived phenolic compounds can take place after ingestion of food and beverages, which may affect blood plasma TAC (see later). 

Another group of compounds which may nutritionally modify TAC of blood plasma is polyphenols. The possible contribution of polyphenolic components of food and beverages to the TAC of body fluids is a subject of controversy. It has been estimated that polyphenols are present in blood plasma at concentrations of 0.2-2 /xM (PI). However, feeding rats a quercetin-augmented diet can increase the plasma levels of quercetin and its metabolites up to 10-100 /xM (M27). 

The total radioactivity minus the parent compound concentration (determined by the bioanalytical method) in a specimen estimates the amount of metabolites present. If the difference is minimal and does not change over time, the extent of metabolism is low. For plasma or serum specimens, a small difference indicates that metabolites are not present in systemic circulation. For bile or urine specimens, high levels of radioactivity suggest a primary route of elimination for the parent and metabolites. For a drug candidate cleared primarily by metabolism, a preliminary metabolite profile in urine and bile can determine the number of potential metabolites. When the level of a metabolite in a matrix is high, attempts to isolate and identify the metabolite can be undertaken. If sufficient quantities are obtained, the metabolite s pharmacologic and toxicologic... 

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