Section 5 Pharmacokinetics

C. D. Jones, H. Sun, and E. I. Ette, Designing cross-sectional pharmacokinetic studies implications for pediatric and animal studies. Clin Res Regal Affairs 13 133-165 (1996). 

The following points are worthy of note in terms of the placement of data. In the case of studies with multiple objectives, reports should be placed in the section corresponding to their primary purpose. Reports of laboratory studies conducted with human materials to investigate pharmacokinetic effects should be placed in Section 5.3.2 of the clinical module, as opposed to the non-clinical module. A US submission requires that the individual case report forms of all trial subjects that died or were dropped from a study due to adverse events are included in Section 5.3.7. 

What are called physiologically based pharmacokinetic (PBPK) and pharmacodynamic (PBPD) models are more mechanistically complex and often include more compartments, more parameters, and more detailed expressions of rates and fluxes and contain more mechanistic representation. This type of model is reviewed in more detail in Section 22.5. Here, we merely classify such models and note several characteristics. PBPK models have more parameters, are more mechanistic, can exploit a wider range of data, often represent the whole body, and can be used both to describe and interpolate as well as to predict and extrapolate. Complexity of such models ranges from moderate to high. They typically contain 10 or more compartments, and can range to hundreds. The increase in the number of flux relationships between compartments and the related parameters is often more than proportional to compartment count. 

Octanol-water partition (log P) and distribution (log D) coefficients are widely used to make estimates for membrane penetration and permeability, including gastrointestinal absorption [77, 78], BBB crossing [60, 69] and correlations to pharmacokinetic properties [1]. The two major components of lipophilicity are molecular size and H-bonding [57], which each have been discussed above (see Sections 2.5 and 2.6). 

This general approach for solving linear pharmacokinetic problems is referred to as the y-method. It is a generalization of the approach by means of the Laplace transform, which has been applied in the previous Section 39.1.6 to the case of a two-compartment model. 

In the right-hand section, N-ethylation of the aniline nitrogen was optimal, while shifting the methyl to the 4-position increased activity. This, combined with fluorination at the m-position, afforded (23b) as the most potent compound of this series. Since the pharmacokinetic properties of (23b) were not disclosed, it is not clear if manipulation of the hit compound led, apart from an increase in potency, to a corresponding improvement of the pharmacological profile. 

Denileukin diftitox is a combination of the active sections of interleukin 2 and diphtheria toxin. It binds to high-affinity interleukin 2 receptors on the cancer cell (and other cells), and the toxin portion of the molecule inhibits protein synthesis to result in cell death. The pharmacokinetics of denileukin diftitox are best described by a two-compartment model, with an a half-life of 2 to 5 minutes and a terminal half-life of 70 to 80 minutes. Denileukin diftitox is used for the treatment of persistent or recurrent cutaneous T-cell lymphoma whose cells express the CD25 receptor. Side effects include vascular leak syndrome, fevers/chills, hypersensitivity reactions, hypotension, anorexia, diarrhea, and nausea and vomiting. 

Gatz. 1992b. Pharmacokinetics of TBP in rats Section 1 distribution, metabolism, and excretion of 14C-tributyl phosphate. MRI Project No. 9526-F. 

In Sec. VII we dealt with methods of determining the rate (and mechanism) of absorption. In this section we will deal with methods of determining the extent of absorption. In every example, the calculation will involve a comparison between two studies carried out in the same group of volunteers on different occasions. Usually it will be necessary to assume that the volunteers behaved identically on both occasions, especially with regard to their pharmacokinetic parameters. 

The Library/Educational Resources Section of the American Association of Colleges of Pharmacy maintains the AACP Basic Resources for Pharmaceutical Education [53]. The reader is referred to this list of books, periodicals, bibliographies, guides, handbooks, dictionaries, directories, and web sites important to pharmacy. The list contains specific sections for the pharmaceutical sciences, including pharmaceutics, biopharmaceutics and pharmacokinetics, cosmetics and industrial pharmacy. 

Absorbed lead is distributed in various tissue compartments. Several models of lead pharmacokinetics have been proposed to characterize such parameters as intercompartmental lead exchange rates, retention of lead in various pools, and relative rates of distribution among the tissue groups. See Section 2.3.5 for a discussion of the classical compartmental models and physiologically based pharmacokinetic models (PBPK) developed for lead risk assessments. 

While these models simulate the transfer of lead between many of the same physiological compartments, they use different methodologies to quantify lead exposure as well as the kinetics of lead transfer among the compartments. As described earlier, in contrast to PBPK models, classical pharmacokinetic models are calibrated to experimental data using transfer coefficients that may not have any physiological correlates. Examples of lead models that use PBPK and classical pharmacokinetic approaches are discussed in the following section, with a focus on the basis for model parameters, including age-specific blood flow rates and volumes for multiple body compartments, kinetic rate constants, tissue dosimetry,... 

As stated in the Section 6.1, one of the principal purposes of carrying out DMPK studies during the discovery phase is to reduce the failure rate during development. For DMPK this logically means predicting the pharmacokinetics that will be observed and hence the dose that will be required in man when clinical studies are carried out. 

The incorporation of fluorine into a molecule has been widely used to alter the pharmacokinetic properties and overall drug-like properties of compounds. This includes affecting the metabolism, oral absorption, and brain penetration of these molecules [18]. Metabolism can be affected by addition of fluorine directly at or adjacent to the site of metabolism. In addition, substitution with fluorine can increase the lipophilicity of compounds which has been shown to dramatically affect both oral absorption and brain penetration. Finally, the electron-withdrawing characteristic of fluorine has been exploited to lower the P-gp liability of compounds and modulate the pKa of adjacent groups which resulted in increased brain exposure. In the following section, representative examples will highlight the powerful nature of fluorine to modulate overall drug-like properties. 

The use of fluorine to modulate properties including potency, selectivity, pharmacokinetics, and toxicity has have been highlighted. Fluorine has also been suggested as a potential bioisosteric replacement for a number of functional groups, examples of which are presented in the final section. 

The 1,2,4-thiadiazole moiety has been incorporated in (3-lactam antibacterials to modulate pharmacokinetic properties and more recently into a cephalosporin. The cephalosporin 129 displays a good balance of serum stability and in vitro activity. The cephalosporin derivative 48 (see Section 5.08.7.4) also shows good pharmacokinetic properties <2001JAN364>. 

Since the introduction of nalidixic acid in 1963, structural modifications on the quinolones have been performed to improve either the antibacterial efficacy or pharmacokinetic/toxicologic profiles of these compounds. The newest quinolones possess broad-spectrum activity, favorable pharmacokinetic/toxicologic profiles, and potency against bacterial strains that are resistant to older generations of quinolones. This section describes the synthetic procedures for the new generation of quinolones that were studied during the 1995-2005 period. 

Determination in Biological Fluids and Tissues All the advances in pharmacokinetics and drug metabolism described in Sections 7 and 8 would not have been possible without the availability of the proper analytical methods. The following is a tabulation of publications in this field, most of which have already been discussed in Section 5. It should be mentioned that a few publications talk about aspirin blood levels, but really mean salicylate levels. The following tabulation covers only those papers where aspirin was differentiated from other salicylates by chromatography or other means. It seems that the "workhorse" for serum salicylate levels is still the colorimetric (ferric-nitrate) method of Brodie, Udenfriend and Coburn153 published in 1944, or modifications thereof. Simplified versions (cf. 206) may lead to erroneous results under certain conditions.207 The method is also applicable for urinary metabolites after proper hydrolysis (cf. 208). For other methods restricted to salicylic acid, see Section 5.61. 

I would like to express my gratitude to the Archives of Bayer AG, Leverkusen, Germany for historical information, to G. Levy, School of Pharmacy, S.U.N.Y., Buffalo for a critical review of the section on pharmacokinetics, to A. I. Cohen, M. Porubcan and B. Toeplitz for providing spectral information and interpretations, and to M. Bruno for her expert secretarial assistance. 

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