Drugs measuring absorption

This book is written for the practicing pharmaceutical scientist involved in absorption-distribution-metabolism-excretion (ADME) measurements who needs to communicate with medicinal chemists persuasively, so that newly synthesized molecules will be more drug-like. ADME is all about a day in the life of a drug molecule (absorption, distribution, metabolism, and excretion). Specifically, this book attempts to describe the state of the art in measurement of ionization constants (p Ka), oil-water partition coefficients (log PI log D), solubility, and permeability (artificial phospholipid membrane barriers). Permeability is covered in considerable detail, based on a newly developed methodology known as parallel artificial membrane permeability assay (PAMPA). 

Measurements of the dissolution behavior of polymorphic forms of relatively insoluble drugs are a convenient way of measuring thermodynamic parameters which, in turn, provide a rational approach to selection of the more energetic polymorphic forms of these drugs for absorption. Large differences in free energy... 

Systemic bioavailability is the product of fraction of dose absorbed (/a), fraction of dose escaping gut metabolism (/g), and fraction of dose escaping first-pass metabolism (F ). Permeability class is based upon /a, which may be estimated either in vivo or in vitro by direct measurement of mass transfer across human intestinal epithelium. In vivo methods include (i) mass balance studies using unlabeled, stable-isotope labeled, or a radiolabeled drug substance (ii) oral bioavailability using a reference intravenous dose or (iii) intestinal perfusion studies either in humans or an acceptable animal model. Suitable in vitro methods involve the use of either excised human/animal intestinal tissues or cultured epithelial monolayers. All of these methods are deemed appropriate for drugs whose absorption is controlled by passive mechanisms. 

Enormous progress has been made in pharmacokinetics in pediatric patients. Two factors have contributed to this progress (1) the availability of sensitive and specific analytic methods to measure drugs and their metabohtes in small volumes of biologic fluids and (2) awareness of the importance of chnical pharmacokinetics in optimization of drug therapy. Absorption, distribution, metabolism, and elimination of many drugs are different in premature infants, fuU-term infants, and older children, and this topic is discussed in detail in the next few sections. 

Most of the microsomal reactions can be classified as oxidations by what are referred to as mixed-function oxidases utilizing molecular oxygen and cofactors. The key enzyme is an iron-hemecytochrome P-450, a flavoprotein dependent in its reduction and reoxidation on the NADPH to NADP reaction. The 450 notation is based on the 450 nm absorption peak the enzyme exhibits on reaction with carbon monoxide. Thus, drug interactions with this enzyme system can be evaluated by measuring absorption spectra changes. 

Many of these problems in making quantitative drug measurements in vivo are avoided by using ex vivo tissue samples where these can be obtained, e.g. by biopsy. Several techniques have been developed, based on either chemical extraction of the photosensitizer or solubilizing the whole tissue [14], followed by fluorescence or absorption spectroscopy or, in some cases, chromatography or atomic absorption... 

A drug s absorption, as reflected in its bioavailability, is a fairly complex process, and although it is related to the drug structure, it is related in a complex manner. Failure to appreciate and understand these complexities, in an attempt to build models, may provide a prediction of marginal and low confidence. Both fraction absorbed and bioavailability are measures of the extent of absorption. Permeability, on the other hand, is related to the rate of absorption (20) ... 

Passive diffusion or transfer follows first-order kinetics (i.e. the rate of transfer is directly proportional to the concentration of drug at absorption and/or measurement sites). 

A measure of the actual amount of drug in the body can be obtained from the area under the curve of the temporal concentration curve (calculated by integration). Interestingly, the temporal behavior of a drug can be extremely important in therapeutics. For example, consider three preparations of a drug that present identical values for area under the curve (i.e., amount of drug absorbed) but have different kinetics of absorption (Figure 8.23). As shown, preparation B produces a useful profile whereby the concentration exceeds the minimal effective concentration... 

Actifed is a medicinal preparation in which the effective components are the two drugs pseudoephedrine hydrochloride and triprolidine hydrochloride. The absorption spectrum of Actifed tablets dissolved in 0.1M hydrochloric acid is similar to that shown in Fig. 17.14(a) which is clearly of no value for quantitative determinations. A second derivative spectrum however is similar in character to that shown in Fig. 17.14(6) in which peak C corresponds to the pseudoephedrine hydrochloride and D to the triprolidine hydrochloride and from which it is possible to make quantitative measurements. Experience showed that it is advisable to use different response times for the two peaks with the instrument used a response setting of 3 was found to give the best results for pseudoephedrine hydrochloride, whilst a setting of 4 was best for the triprolidine hydrochloride. 

In-vitro models can provide preliminary insights into some pharmacodynamic aspects. For example, cultured Caco 2 cell lines (derived from a human colorectal carcinoma) may be used to simulate intestinal absorption behaviour, while cultured hepatic cell lines are available for metabolic studies. However, a comprehensive understanding of the pharmacokinetic effects vfill require the use of in-vivo animal studies, where the drug levels in various tissues can be measured after different dosages and time intervals. Radioactively labelled drugs (carbon-14) may be used to facilitate detection. Animal model studies of human biopharmaceutical products may be compromised by immune responses that would not be expected when actually treating human subjects. 

AM columns are another means of measuring lipophilic characteristics of drug candidates and other chemicals [99-103]. 1AM columns may better mimic membrane interactions than the isotropic octanol-water or other solvent-solvent partitioning system. These chromatographic indices appear to be a significant predictor of passive absorption through the rat intestine [128]. 

In these instances, the time of the peak in the plasma concentration versus time curve provides a convenient measure of the absorption rate. For example, if three tablets of the same drug are found to be completely absorbed and all give plasma peaks at 1 hour, it can be safely concluded that all three tablets are absorbed at essentially the same rate. (In fact, if all tablets are completely absorbed and all peak at the same time, it would be expected that all three plasma concentration versus time curves would be identical, within experimental error.)... 

As indicated, the ionized form of a drug will be more soluble than the nonionized form in the aqueous fluids of the GIT. The classic studies on the beneficial effects of changing nonionized drugs into salt forms were reported by Nelson for tetracycline [25], and Nelson et al. for tolbutamide [26]. Table 2 combines portions of the data from each study. Urinary excretion of the drug or its metabolite was taken as the in vivo measure of the relative absorption rate for the salt and the nonionized... 

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