Once a drug has been administered by any route other than IV, it must be absorbed into the bloodstream from the site of administration. The drug then is distributed into various body fluids and tissues to attain an effective, yet safe, concentration for a sufficient period of time at the site of action. Subsequently, the drug is inactivated or eliminated from the body, generally by metabolism (usually lipid-soluble drugs) and excretion (mainly renal and biliary routes). The effectiveness of these processes with respect to time (pharmacokinetics) varies with the particular drug and species of animal. Within species, it is further influenced by physiologic states (eg, age, gender, pregnancy) and disease. Finally, combinations of drugs may lead to drug interactions that can alter both drug response and disposition.

Drug Absorption

Passage of Drugs Across Cellular Membranes

Regardless of the route of administration, a drug usually must cross a number of cell or basement membranes before it reaches its site of action. Membrane barriers may be composed of several layers of cells (eg, skin, vagina, cornea, placenta) or a single layer of cells (eg, enterocytes, renal tubular epithelial cells), or they may consist only of a boundary less than 1 cell in thickness (eg, hepatic sinusoids, mitochondrion, nucleus).

Drugs and other molecules cross cellular membranes by several processes. Methods by which drugs move include bulk flow, passive diffusion, facilitated or active transport, and pinocytosis. Of these, passive diffusion is important for xenobiotics.

The rate at which a drug passively diffuses through membranes is influenced by several factors, the most important of which is the concentration of diffusible (eg, dissolved) drug. Passive diffusion is directly related to the concentration gradient across the diffusible membrane. Drugs must be sufficiently lipid-soluble to pass through the lipid bilayer of the cell membrane but sufficiently water soluble to be dissolved in the aqueous phase on the other side of the barrier. Thus, the ability of a compound to cross a membrane by simple lipid diffusion is a function of its degree of lipid solubility (lipid-to-water partition coefficient). Other factors that influence passive diffusion include the molecular mass of the drug, the thickness of the membrane(s) to be penetrated, the surface area available for diffusion, and the degree of ionization.

Many drugs are weak organic acids or bases. At physiologic pH, they tend to be partially ionized (dissociated) and partially nonionized (undissociated); the ratio of the respective forms depends on the dissociation constant (pK_a) of the drug and the pH of the solution in which the drug is dissolved. The nonionized fraction is able to diffuse through lipid membranes. Distribution across the membrane reflects the degree of ionization on each side of the membrane. However, distribution also depends on the extent to which the drug is bound to proteins or other macromolecules on either side. Despite being present largely in the nonionized state, a drug may be too lipid insoluble to diffuse through the membrane.

Aqueous pores in lipoproteinaceous biologic membranes offer a means of xenobiotic movement through the membrane for predominantly aqueous soluble drugs. Lipid-insoluble (water soluble) compounds pass easily through these pores and to a lesser degree directly through the membrane. A hydrostatic or osmotic pressure difference across a membrane facilitates movement by promoting water flow through the aqueous pores. Bulk fluid movement carries or “drags” solute molecules through the pores as long as the solute molecules are smaller than the aqueous channels.

Several specialized transfer processes account for the passage of certain organic ions and other large lipid-insoluble substances across biologic membranes. Active transport, facilitated diffusion, and exchange diffusion are 3 distinct types of carrier-mediated systems used for moving specific substances across cellular membranes. The highly selective carrier-mediated systems are principally used for transporting nutrients and natural substrates across biologic membranes. Among the mechanisms of active transport are transport proteins that move compounds, including drugs, into or out of cells. Transport proteins are located at portals of entry (eg, enterocytes of the GI tract or sinusoidal hepatic cells), or “sanctuary” tissues (eg, brain, cerebrospinal fluid, placenta, prostate, eyes, or testicles) where they act to make sure that xenobiotics do not enter the protected tissue. As such, transport proteins are able to influence each drug movement (absorption, distribution, metabolism, and excretion). The most well known of the transport proteins is the ATP-binding cassette superfamily of efflux transporters, which includes P-glycoprotein, the multidrug resistance protein. Substrates for P-glycoprotein include both xenobiotics and dietary components. Competition for transport increases oral absorption or distribution of one of the competing molecules.

Pinocytosis is an important transport process in mammalian cells, particularly intestinal epithelial cells and renal tubular cells. Drugs that exist in solution as molecular aggregates, have large molecular masses themselves, or are bound to macromolecules may be transferred across membranes by pinocytosis.

Drug Absorption from the GI Tract

Although the basic principles governing the absorption of drugs from the GI tract are understood, many confounding factors may play a role in modifying the process, and erratic responses may result. Some of the more important factors to be considered include the following: 1) molecular size and shape of the drug and its concentration, 2) degree of ionization at specific pH values (depends on pK_a of the drug), 3) lipid solubility of the neutral or nonionized form of the drug, 4) chemical or physical interactions with coadministered drug preparations or even food constituents, 5) the pharmaceutical preparation and characteristics of the dosage form (especially the disintegration and dissolution rates of solid dosage forms), 6) morphologic and functional differences of the GI tract among the various animal species, 7) gastric motility, secretion, and the rate of gastric emptying, 8) intestinal motility and secretions as well as the intestinal transit time, 9) fluid volume within the GI tract, 10) osmolality of intestinal content, 11) intestinal blood and lymph flow, 12) disruption of the structural and functional integrity of the gastric and intestinal epithelium, and 13) drug biotransformation within the intestinal lumen by microflora, or within the mucosa by host enzyme systems.

Bioavailability

This term is used to define the rate and extent to which a drug administered in a particular dosage form enters the systemic circulation intact. All of the considerations outlined above, as well as the particular product used, can influence bioavailability. Biotransformation by intestinal epithelial cells, and particularly by liver cells, can substantially reduce the amount of unchanged drug that enters the systemic circulation after administration PO. This is known as the “first-pass” effect and is significant for a number of drugs.

Drug Absorption from Topical Administration

Drugs may be absorbed through the skin after topical application; however, the stratum corneum presents an effective barrier to movement of most drugs. The intact skin allows the passage of small lipophilic substances but efficiently retards the diffusion of water-soluble molecules in most cases. Lipid-insoluble drugs generally penetrate the skin slowly in comparison with their rates of absorption through other body membranes. Absorption of drugs through the skin may be enhanced by heat, moisture, or disruption of the stratum corneum. Certain solvents (eg, dimethyl sulfoxide [DMSO]) may facilitate the penetration of drugs through the skin. Damaged, inflamed, or hyperemic skin allows many drugs to penetrate the dermal barrier much more readily. The same principles that govern the absorption of drugs through the skin also apply to the application of topical preparations on epithelial surfaces.

Drug Absorption from Tracheobronchial Surfaces and Alveoli

Because volatile and gaseous anesthetics have relatively high lipid-to-water partition coefficients and generally are rather small molecules, they diffuse practically instantaneously into the blood in the alveolar capillaries. Particles contained in aerosols can be deposited, depending on the size of the droplets, on the mucosal surface of the bronchi or bronchioles, or even in the alveoli. Most drugs are usually absorbed quite rapidly from these sites according to the principles discussed above.

Drug Absorption from Parenteral Delivery Sites

After a drug has penetrated the skin, GI epithelium, or other absorbing surface, or has been deposited by injection into a body tissue, it comes into the immediate vicinity of capillaries. Solutes traverse the capillary wall by a combination of 2 processes: diffusion and filtration. Diffusion is the predominant mode of transfer for lipid-soluble molecules, small lipid-insoluble molecules, and ions. Because most capillaries are fenestrated, all drugs, whether lipid-soluble or not, cross the capillary wall at rates that are extremely rapid compared with their rates across other body membranes. In fact, the movement of most drug molecules in various tissues is limited only by the rate of blood flow rather than by the capillary wall. However, some endothelial cells, such as the blood-brain barrier, have much tighter intercellular junctions than others and, therefore, restrict drug movement more significantly.

Aqueous solutions of drugs are usually absorbed from an IM injection site within 10–30 min, provided blood flow is unimpaired. Faster or slower absorption is possible, depending on the concentration and lipid solubility of the drug, vascularity of the site (there are differences between various muscle groups), the volume of injection, the osmolality of the solution, and other pharmaceutical factors. Substances with molecular weights >20,000 daltons are principally taken up into the lymphatics.

Absorption of drugs from subcutaneous tissues is influenced by the same factors that determine the rate of absorption from IM sites. Some drugs are absorbed as rapidly from subcutaneous tissues as from muscle, although absorption from injection sites in subcutaneous fat is always significantly delayed.

Increasing blood supply to the injection site by heating, massage, or exercise hastens the rate of dissemination and absorption. Spreading and absorption of a large fluid volume that has been injected SC may be facilitated by including hyaluronidase in the solution.

The rate of absorption of an injected drug may be prolonged in a number of ways, including immobilization of the site, local cooling, a tourniquet, incorporation of a vasoconstrictor, an oil base, and implant pellets and insoluble “depot” preparations. Among these depot preparations are drugs that are converted to less soluble salts (eg, procaine and benzathine esters of penicillin or acetate esters of steroids) or less soluble complexes (eg, protamine zinc insulin), or that are administered as insoluble micro-crystalline suspensions (eg, methylprednisolone acetate).

Drug Distribution

After absorption into the bloodstream, drugs are disseminated to all parts of the body. Compounds that permeate freely through cell membranes become distributed, in time, throughout the body water to both extracellular and intracellular fluids. Substances that pass readily through and between capillary endothelial cells, but do not penetrate other cell membranes, are distributed into the extracellular fluid space. Occasionally, the drug molecule may be so large (>65,000 daltons) or so highly bound to plasma proteins that it remains in the intravascular space after IV administration. Drugs may also undergo redistribution in the body after initial high levels are achieved in tissues that have a rich vascular supply, eg, the brain. As the plasma concentration falls, the drug readily diffuses back into the circulation to be quickly redistributed to other tissues with high blood-flow rates, such as the muscles; over time, the drug also becomes deposited in lipid-rich tissues with poor blood supplies, such as the fat depots. Most drugs are not distributed equally throughout the body but tend to accumulate in certain specific tissues or fluids. The general principles that govern the passage and distribution of drugs across cellular membranes (see Pharmacology Introduction: Passage of Drugs Across Cellular Membranes) are applicable. Basic drugs tend to accumulate in tissues and fluids with pH values lower than the pK_a of the drug; conversely, acidic drugs concentrate in regions of higher pH, provided that the free drug is sufficiently lipid soluble to penetrate the membranes that separate the compartments. Even small differences in pH across boundary membranes, such as those that exist between CSF (pH 7.3) and plasma (pH 7.4), milk (pH 6.5–6.8) and plasma, renal tubular fluid (pH 5.0–8.0) and plasma, and inflamed tissue (pH 6.0–7.0) and healthy tissue (pH 7.0–7.4), can lead to unequal distribution of drugs with pK_a values close to those of the pH of the fluid. Only freely diffusible and unbound drug molecules are able to pass from one compartment to another. Binding to macromolecules such as protein components of cells or fluids, dissolution in adipose tissue, formation of nondiffusible complexes in tissues such as bone, incorporation into specific storage granules, or binding to selective sites in tissues all impede movement of drugs in the body and account for differences in the cellular and organ distribution of particular drugs. Therapeutic agents may also be transported by carrier-mediated systems across certain cellular membranes, which leads to higher concentrations on one side than the other. Examples of such non-specific transport mechanisms are found in renal tubular epithelial cells, hepatocytes, and the choroid plexus. Among the transport proteins, genetic differences in P-glycoprotein profoundly impact drug movement, particularly at portals of xenobiotic entry or sanctuaries.

Only the unbound or free fraction of a drug can diffuse out of capillaries into tissues. The most important binding of drugs in circulation is to plasma albumin, although the globulins and, especially, α-1 acid glyco-protein (for bases) may also play a significant role. A drug may become bound to plasma proteins to a greater or lesser degree, depending on a number of factors, eg, plasma pH, concentration of plasma proteins, concentration of the drug, the presence of another agent with a greater affinity for the limited number of binding sites, and the presence of acute-phase proteins during active inflammatory conditions. The degree of plasma-protein binding and the affinity of a drug for the nonspecific protein-binding sites is of great clinical significance in some instances and much less so in others. For example, a potentially toxic compound (such as dicumarol) may be 98% bound, but if for any reason it becomes only 96% bound, then the concentration of the free active drug that becomes available in the plasma is doubled, with potentially harmful consequences. The concentration of a drug administered in overdose may exceed the binding capacity of the plasma protein and lead to an excess of free drug, which can diffuse into various target tissues and produce exaggerated effects. However, more rapid clearance of the drug may mitigate the impact of higher drug concentrations.

Of equal importance is the readiness with which drugs dissociate from plasma proteins. Those that are more tightly bound tend to have much longer elimination half-lives because they are released gradually from the plasma protein reservoir. The long-acting sulfonamides are good examples of this phenomenon. Most unbound drugs distribute easily to extracellular fluid. All membranes are traversed only by the more lipid-soluble drugs. During distribution and elimination from the body, a drug may or may not penetrate certain “physiologic” (eg, blood-brain, placental, and mammary) barriers. A drug may gain access to the CNS by 2 distinct routes—the capillary circulation and the CSF. Drugs penetrate into the cortex more rapidly than into white matter, probably because of the greater delivery rate of drug via the bloodstream to the tissue.

The pharmacologic factors and consequences of the diverse rates of entry of different drugs into the CNS include the following: 1) water-soluble ionized drugs will not enter the CNS; 2) low ionization, low plasma-protein binding, and a fairly high lipid-water partition coefficient confer ready penetration; 3) direct injections into the CSF often produce unexpected effects; and 4) meningoencephalitis can substantially alter the permeability of the blood-brain barrier.

The placental barrier should be considered when selecting an agent to treat a pregnant animal. The potential teratogenicity of any drug needs to be known before its administration; if it is to be used during late gestation, its effects on the fetus and on the process of parturition should be considered. Nutrients such as glucose, amino acids, minerals, and even some vitamins are actively transported across the placenta. The passage of drugs across the placenta is largely by lipid diffusion, and the factors discussed above play a role. The distribution of drugs within the fetus follows essentially the same pattern as in the adult, with some differences with respect to the volumes of drug distribution, plasma-protein binding, blood circulation, and greater permeability of interceding membranous barriers.

The mammary gland epithelium, like other biologic membranes, acts as a lipid barrier, and many drugs readily diffuse from the plasma into milk. The pH of milk varies somewhat, but in goats and cows it is generally 6.5–6.8 if mastitis is not present. Weak bases tend to accumulate in milk because the fraction of ionized, nondiffusible drug is higher. The opposite is true for acidic drugs. Agents delivered by intramammary infusion can diffuse into plasma to a greater or lesser degree by the same processes noted earlier.

Drug Biotransformation

Drugs and foreign chemicals that are lipid soluble are converted by enzymatic processes to compounds of ever-increasing water solubility until they can be excreted via one or several of the routes available. Metabolism or biotransformation and the subsequent excretion of drugs is known as “elimination.” Metabolism generally occurs in 2 phases: Phase I induces a chemical change (most frequently oxidation, but also reduction) that renders the drug more conducive to phase II. Phase II is a conjugative or synthetic addition of a large, polar molecule that renders the drug water soluble and amenable to renal excretion.

There are several possible consequences of the biochemical transformation of drugs: 1) inactivation, during which an active drug is converted to inactive metabolite(s); 2) activation, during which an inactive drug (or pro-drug) is converted to a pharmacologically active primary metabolite; 3) modification of activity after the conversion of an active drug to a metabolite that also has pharmacologic activity; 4) lethal synthesis (or intoxication), in which a drug is incorporated into a normal cellular metabolic pathway that ultimately leads to failure of the reaction sequence because of the presence of spurious substrate (cell death then occurs).

Several aspects of the biotransformation of drugs have direct clinical significance. These include microsomal (cytochrome P450) enzyme induction and inhibition, nutritional state, age, disease conditions, and species differences.

Because drug biotransformation is negligible in early life, neonates are much more sensitive to lipid-soluble drugs than are adults of any species. The postnatal development of drug-metabolizing enzymes in the liver appears to be biphasic, consisting of a rapid and nearly linear increase in activity during the first 3–4 wk, followed by slower development up to the tenth week postpartum; dose or interval for the very young must be reduced accordingly. Hepatic mass, hepatic blood flow, and microsomal enzyme activity may decrease in older animals.

Many disease states impair the normal activity of the hepatic microsomal enzyme system, which in turn prolongs the half-lives of many drugs. Frank hepatotoxicity, acute hepatitis, or other extensive liver lesions invariably depress enzyme activity. Changes in hepatic blood flow, with similar consequences, may be encountered in congestive heart failure, circulatory shock, and cirrhosis. Hypothyroidism tends to reduce microsomal enzyme function, and hyperthyroidism tends to increase activity.

Species variations in the biotransformation patterns of lipid-soluble drugs are common. The last decade has been accompanied by characterization of isoenzymes of the cytochrome P450 families. Pharmacogenomics refers to the science that studies the genetic basis for differential response to drugs. Differences in the duration of action of these drugs in various species frequently can be attributed to differences in their rates of biotransformation. Cytochrome P450 3A4 is responsible for the broadest substrate activity, but over 20 superfamilies of the enzyme have been identified. Variants for certain enzymes have been described in humans as being responsible for potentially lethal differences in drug metabolism. Such variants are currently being described in animals as well. In addition to heterogeneity in cytochrome P450 enzymes, differential handling of enantiomers is increasingly recognized among species. Enantiomers are mirror images that result when groups of atoms rotate around a center or “chiral” carbon. Generally such compounds are sold as racemic mixtures (50:50 of each isomer); however, the body frequently handles each steroisomer differentially, with differences also occurring among species. Many cardiac drugs and NSAID exist as racemic mixtures of enantiomers. This must be remembered when either dosages or withdrawal times are extrapolated from one species to another.

Drug and Metabolite Excretion

The concentration of a drug in the plasma or at its receptor sites may be reduced in 3 ways: 1) distribution or redistribution into various tissue compartments, 2) metabolic inactivation, and 3) excretion from the body. The kidneys are the principal organ of excretion, but the liver, GI tract, and lungs also may play important roles. Milk, saliva, and sweat are usually of less importance, although the presence of an active drug in milk may affect nursing young.

Renal excretion of foreign compounds involves glomerular filtration, passive diffusion into and out of the tubular lumen, and carrier-mediated secretion, mainly in the proximal convoluted tubule. Only unbound molecules <66,000 daltons are readily filtered through the glomerular membranes into the tubular lumen. Acidification or alkalinization of the urine may alter the rate of excretion of some drugs because of ion-trapping in the tubular fluid.

Binding to plasma proteins usually does not hinder tubular excretion of drugs because of the dynamic equilibrium that exists between free and bound drug. As free drug is removed and transported across the tubular epithelium, immediate dissociation of the drug-albumin complex usually occurs. Concurrent administration of either acidic or basic drugs that are substrates for carrier-mediated secretion processes prolongs the elimination of the drug that has the lesser affinity for the carrier sites, thus increasing its duration of action.

Drugs and their metabolites may also be excreted either passively or actively by hepatocytes into the bile canaliculi and, ultimately, into the duodenum in the bile. Drugs may become unconjugated by intestinal microflora. Released drug can be reabsorbed into the systemic circulation. Enterohepatic cycles often account for prolonged half-lives of drugs that are primarily excreted in bile. Impairment of the excretory functions of the hepatocytes or obstruction of bile flow due to any cause interferes with the biliary excretion of drugs. Dose or interval should then be adjusted accordingly. The normal kinetics of a drug's enterohepatic cycle may change in such cases, or may be modified by disruption or elimination of the intestinal flora.

The other routes of excretion are of lesser clinical importance. However, several drugs may diffuse directly into the GI tract and then be eliminated in the feces. The ruminoreticulum can act as a drug reservoir or “sink.” The tracheobronchial tree also may be a potential avenue of excretion. Many drugs that are administered parenterally are found in bronchial secretions. Alveolar elimination is of major significance when inhalant anesthetics are used. The main factors governing elimination by this route are the same as those determining the uptake of inhalant anesthetics—the concentrations in plasma and alveolar air and the blood/gas partition coefficient. The mammary and salivary glands excrete drugs by nonionic passive diffusion. The salivary route of excretion is important in ruminants because they secrete such voluminous amounts of alkaline saliva.

If the excretory functions of the organs concerned with drug elimination are impaired or altered in any way (eg, disease, very young or very old animal), prolonged elimination patterns result. Moreover, several nutritional and pharmacokinetic interactions have the potential to change the rates of drug excretion.

When urinary excretion is an important route of elimination, renal failure results in decreased drug clearance and slower removal of the drug from the body. A usual dosage regimen in such cases tends to lead to accumulation and, ultimately, toxicity. A number of disturbances may occur within failing kidneys, all of which may influence the excretion of drugs: renal ischemia, glomerular involvement, tubular damage, impaired intrarenal perfusion, functional disabilities of the tubular cells, failed homeostatic mechanisms, and obstructive lesions in the tubules or collecting ducts (or even the ureters or urethra). Changes in the pH of the filtrate also alter the excretion rates of drugs with appropriate pK_a values. In addition to the direct effect on renal excretory mechanisms, pathologic changes in the kidneys can influence the disposition and elimination of drugs. In most instances drug toxicity is increased. The binding of many drugs to plasma proteins is decreased in uremic animals. The rate of metabolic reactions may be depressed in renal failure, impairing effective elimination of agents that require biotransformation. Associated clinical signs and pathophysiologic changes, often encountered in renal failure, can also alter pharmacodynamic responses to particular drugs. Derangements of acid-base balance, hyper- and hypokalemia, hyper- and hyponatremia, dehydration, and hyper- and hypotension are examples of systemic conditions that may radically modify a drug's fate or action.

Last full review/revision March 2012 by Dawn Merton Boothe, DVM, PhD, DACVIM, DACVCP; Philip T. Reeves, BVSc, PhD, FACVSc