Processes of Absorption, Distribution, and Excretion

BY L. F. CHASSEAUD

Much of the organic and inorganic matter ingested by man in his lifetime provides materials for metabolic processes concerned with the production of energy, with the anabolism and catabolism of body tissues and with the general maintenance of the milieu intérieur. Man also ingests, inhales, and absorbs other substances that are not required for these processes but can sometimes seriously upset them. These foreign compounds are generally non-nutrient substances, and include drugs, pesticides, food additives and many other chemicals that affect the environment. They may also be produced within the animal as digestion products of the gut flora.

Metabolic pathways have been evolved which protect the body by converting foreign compounds into more polar derivatives that are more readily excreted. In these metabolic pathways, foreign compounds are chemically transformed by reduction, oxidation, hydrolysis, and conjugation, and undergo one or more of these reactions (see Chapter 5). Foreign compounds seem to be transformed into less toxic or inert products, and for this reason those metabolic pathways have been described as detoxication mechanisms, but sometimes biologically-active metabolites are produced.

Most transformations of foreign compounds are catalysed by enzymes, many of which occur in the liver and to a lesser extent in some other tissues. Those biotransformations depend largely on the structure of the foreign compound, but the effect of the genetic make-up of species, the route of administration, the diet or pretreatment with other substances can regulate metabolism (see Chapter 6).

In the mammalian body, the absorption, distribution, biotransformation, and excretion of foreign compounds are intimately connected processes, themselves subject to many variables. The present chapter discusses the processes of absorption, distribution, and excretion, in some depth, and thereby provides background to the subjects which are developed in the rest of the book, particularly the ones exemplified in Chapter 2.

1 Absorption

Foreign compounds may be taken into the body from the environment by the oral, respiratory, or dermal routes and are circulated in the blood. Drugs, which are generally foreign compounds, are preferably administered orally, intravenously, or intramuscularly.

In a study of its metabolic fate, the foreign compound should be administered by the route by which man, or the species at risk, is likely to be exposed or treated, since the absorption, and therefore the effect of a foreign compound may greatly depend on its route of administration. If rapid metabolism occurs in the liver, many substances are less toxic when administered orally rather than intravenously or intraperitoneally because of the greater concentration first reaching the liver by the hepatic-portal route. Again, if a compound is poorly absorbed, it exerts less effect by oral administration.

Transport Across Membranes. — All foreign compounds absorbed into the body must cross one or more semipermeable membranes, and the effect and distribution of the compound in the body depends on its ability to penetrate these membranes. On both sides of a membrane the effective concentration of the compound is continually decreasing through its localization, bio-transformation, and excretion. Membranes, such as the gastro-intestinal epithelium, the lining of the respiratory tract, and stratum corneum of the skin, delay the passage of the compound into the body, after which membranes enclosing the blood and other body fluids, cell membranes and membranes within cells control the uptake into tissues and sub-cellular components, and ultimately reduce the amount of a compound that reaches its site(s) of action.

These membranes are highly complex, dynamic lipoprotein structures and consist of several layers of cells, such as the skin and the placenta, or of a single layer of cells, such as the intestinal epithelium, or they occur less than one cell in thickness, as in the case of cell membranes. Apart from the thicker, larger biological barriers enclosing intracellular structures, all body membranes are composed of a fundamental structure, that of the cell membrane.

There are several processes by which substances may cross biological membranes (Table 1). Passive transfer processes require that the membrane behaves as an inert solvent–pore boundary through which solutes diffuse by passage through solvent regions or through the pores, or by flowing together with water through the pores. In this case, the solvent is represented by lipoprotein structures of which the aqueous pores comprise a very small part, their size varying with the membrane concerned. These pores should be considered as highly polar regions of the membrane that are solvated with water, not as simple tubes. The equivalent pore radius of the luminal face of rat intestine was shown to be 4 [Angstrom], sufficiently large for molecules such as water or urea to pass through, but too small for mannito1. The equivalent pore size of the membrane of the small intestine may vary, as has been suggested by results obtained with human intestine where the effective pore radius of the jejunum was calculated to be twice that of the ileum which was less than 4 [Angstrom]. However, size is not the only factor that influences the passage of small molecules through these pores. Other factors also restrict the passage of certain small molecules.

Specialized transfer processes exhibit an active nature that shows a high degree of specificity for a particular solute transferred across the membrane, and here structure, conformation, size, and charge are important in determining penetration. Small differences are important in specialized transfer processes, whereas the physical laws apply to the passive transfer of solutes. Specialized transfer processes are largely involved with the transport of nutrients, such as sugars and amino-acids.

Diffusion of Foreign Compounds across Membranes. — Foreign compounds generally cross membranes by passive diffusion down a concentration gradient, which provides the driving force for the movement of these molecules. Some lipid-insoluble foreign compounds of low molecular weight cross membranes by filtration together with water, or by diffusion, through the pores. The driving force for such movement may be provided by electrochemical gradients or by the bulk flow of water. The smaller the compound, the quicker it passes through the pores, but the ionic charge is important for low-molecular-weight ions, for example, certain poorly lipid-soluble organic anions penetrated the erythrocyte membrane much more rapidly than organic cations. Foreign compounds that closely resemble nutrients are transported across membranes by specialized transfer processes developed for those nutrients. Foreign sugars structurally related to glucose and foreign pyrimidines structurally related to uracil exemplify this principle.

Factors Governing Passive Diffusion. — (i) Fick's Law. The rate at which a foreign compound crosses a membrane by passive diffusion is proportional to its concentration gradient across the membrane (C1 – C2), the membrane's thickness (D), the area available for diffusion (A) and the diffusion constant (K) which is related to the molecular structure, lipid solubility, and degree of ionization of the foreign compound. These are parameters expressed by Fick's Law.

Rate of passive diffusion [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

Reports have been made of experimental procedures and of kinetic studies (see Chapter 3) that enable the rate of passage of compounds across membranes to be calculated.

(ii) Lipid Solubility. Lipid-soluble neutral molecules readily pass through membranes by passive diffusion, and compounds diffuse fastest that have high lipid/water partition coefficients (Table 2) and are un-ionized. With increasing ionization of the un-ionized moiety, absorption and rate of passive diffusion decrease (Table 3), because biological membranes hinder the passage of ions unless they are transferred by specialized transport processes or are small enough to pass by diffusion or by filtration through the membrane pores (Table 1). The best correlation between degree of absorption and lipid/water partition coefficients would be expected for a group of structurally related compounds. Also, since partition coefficients are derived from test systems, such as chloroform/water or olive oil/water, they are unlikely to be highly representative of in vivo systems.

(iii) Ionization. An important factor controlling foreign compound absorption, particularly from the gastro-intestinal tract where the pH differs markedly from blood plasma and other body fluids, is ionization. The ionized residue of drugs is usually poorly lipid-soluble and thus does not cross lipoidal membranes readily.

Many drugs are weak acids (e.g. barbiturates) or weak bases (e.g. caffeine), and unlike strong acids and bases, these compounds are often incompletely ionized at physiological pH: the degree of ionization is related to this pH and to the pKa value, which is the negative logarithm of the acid ionization constant (pKa = -log Ka). It is the un-ionized form of these drugs which diffuses across membranes at rates related to its lipid solubility. In a study of the absorption of various steroids from rat intestine, it was shown that the more lipoidal the compound, the greater the absorption rate. The amount of certain organic acids and bases absorbed from rat intestine was correlated to the lipid solubility of their un-ionized forms. Absorption of organic electrolytes from rat small intestine was shown to be roughly related to their pKa values (Table 4), There is rapid absorption of acidic drugs with pKa values greater than 3, and of basic drugs with pKa values less than 8 (Table 4).

The total concentration of a partially ionized compound (ionized plus un-ionized) on both sides of a membrane is a function of the pH on either side of the membrane and the compound's ionization constant (Ka), Where C1 and C2 are the concentrations of the compound on either side of the membrane, the relationship for a weak acid is:

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and for a weak base is:

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At equilibrium, the concentration of an un-ionized drug is the same on both sides of a membrane, but that of an ionized drug may be different because of a difference in pH on the two sides of the membrane. More of a basic drug is absorbed across a membrane when the pH is raised because more is un-ionized. The converse applies for acidic drugs (Table 3). However, changes in pH do not affect the absorption of those drugs for which the degree of ionization remains unaltered.

(iv) Protein Binding. The binding of foreign compounds to proteins and to other macromolecules in vivo provides an additional effective variable that usually operates, since this binding is generally reversible. Such binding, which is often extensive, must be taken into account when the rates and extent of absorption of foreign compounds are measured. The affinity of binding also affects plasma drug concentrations, which are usually measured in a way which gives the total concentration of free plus bound forms.

Gastro-intestinal Absorption. — The main site of absorption of an ionizable substance from the gastro-intestinal tract is related to its pKa value. In man, the gastro-intestinal pH varies from ca. 2 in the stomach to ca. 6·5 in the small intestine and to 8 in the colon.

There is a large pH difference between gastric juice (pH 1) and blood plasma (pH 7·4), and at steady state, weak acids should concentrate on the alkaline side (plasma) of the membrane and weak bases on the acidic side (gastric). This distribution has been observed in dogs for intravenously administered basic drugs, such as antipyrine, and acidic drugs, such as barbitone. The higher the pKa value of the drug, the greater was its gastric juice : plasma concentration ratio and vice versa, but a maximum ratio of 40 was observed for more basic compounds, such as aniline and quinine. This ratio was apparently a limiting value owing to the rate of gastric-mucosal blood flow which limited the amount of drug reaching the gastric mucosa. The limiting value may be altered by factors affecting rates of gastric juice secretion or rates of gastric-mucosal blood flow.

Although the measured pH of the intestinal contents was pH 6•6, results obtained during investigations of the steady-state distribution of drugs between intestinal contents and blood plasma in the rat fitted a value of pH 5•3. pH 5·3 has been taken to be the 'virtual' pH existing at the surface of the intestinal epithelial boundary, and it is to be expected that the pH at the membrane surface is more important in regulating the passive diffusion of ionizable foreign compounds than is the pH of the intestinal contents. However, the presence of this zone of 'virtual' pH of 5•3 has been questioned.

That solutions of mixtures of drugs were absorbed from the gastrointestinal tract of rats at the same rates as solutions of the component drugs were absorbed, provides further evidence that drugs cross the gastro-intestinal epithelium by passive diffusion and not by active processes involving carriers, since mixtures of drugs by those means would compete for transport across the membrane. By contrast, a special protein assists alimentary absorption and possibly transplacental transfer of vitamin B12.

The intestinal absorption of poorly absorbed compounds can sometimes be improved by minor chemical modification of the compound, for example, by acetylation of hydroxy-groups.

The absorbing properties of the gastro-intestinal epithelium can be modified by edta treatment, resulting in increased absorption of some compounds. Such membrane modification has been regarded as a potentially toxic effect since, for example, harmful bacterial toxins, or otherwise unabsorbable products, may be transported across the modified membrane, after this treatment. Furthermore, the absorption of important substances required by the animal may be impaired. On the other hand, Sögnen has found that the presence of such calcium-binding substances as edta, sodium fluoride, and sodium oxalate in the gastro-intestinal tract reduced the effects of certain orally administered drugs. Thus, lethal oral doses of strychnine or of barbiturates, given in combination with one of these calcium-binding substances, did not greatly affect the animals. Decreased blood levels of orally administered barbiturates or sulphonamides compared with controls were observed.

The observed absorption of quaternary ammonium compounds from rat intestine, in some cases to an appreciable extent, is unexpected, since those compounds are completely ionized at the pH of the intestinal lumen, and since the intestinal epithelium appears to be readily permeable only to lipid-soluble un-ionized molecules. It has been suggested that quaternary ammonium compounds may first be complexed in the gut with an endogenous anion, possibly a phosphatidopeptide, and that this neutral complex is absorbed across the intestinal epithelium. The simultaneous oral administration of an anion, trichloroacetic acid, with a quaternary ammonium compound, isopropamide, to form an ion-pair that is lipophilic, increased both the rate and the efficiency of gastro-intestinal absorption of the drug at lower doses, and consequently the pharmacological response to it. In this case, the ion-pair must have been carried together through the strongly acid stomach lumen into the neutral to weakly basic medium of the small intestine.

Although it has been observed that biological membranes are permeable to un-ionized lipid-soluble foreign compounds, this is not true for all substances. After oral administration to rats, dogs, pigs, and man, Ionox 330, [2,4,6-tri-(3',5'-di-t-butyl-4'-hydroxybenzyl) mesitylene], was unabsorbed, and 2,6-bis-(1'-methylheptadecy1)-p-cresol was hardly absorbed in rats. Lack of absorption of both these phenolic antioxidants may partly be due to their molecular size; the molecular weights exceed 600.

Percutaneous Absorption. — Lipid-soluble molecules are more readily absorbed through the skin than ions and lipid-insoluble molecules, because the epidermis is a lipoidal barrier. The barrier properties occur in the dead, top-most horny layer, or stratum corneum, which is keratinous and forms a continuous membane. On the other hand, the underlying dermis is a complex tissue through which most solutes pass readily. The skin is slightly permeable to most molecules, but this only becomes significant for molecules that are small, that are present in high concentration, or that are highly active locally or systemically. Abraded or injured skin, with a damaged stratum corneum, is much more permeable to foreign compounds.