Pharmacokinetics - Drug absorption and Transport of drugs across biological membranes

Transport of drugs across biological membranes

All pharmacokinetic processes involved transport of the drug across biological membranes.

Biological Membrane

Biological membranes may be viewed as mosaics of functional units composed of lipoprotein complex. Most membranes are composed of a fundamental structure called the unit membrane or plasma membrane. This boundary, which is 80 to 100 A° thick, surrounds single cell and nuclei. More complex barriers such as the intestinal epithelia and the skin are composed of multiples of this functional structure. The plasma membrane consists of a bilayer of amphipathic lipids with their hydrocarbon chains oriented inward to form a continuous hydrophobic phase and their hydrophilic heads oriented outward. Individual lipid molecules in the bilayer can move laterally, endowing the membrane with fluidity, flexibility, high electrical resistance and relative impermeability to highly polar molecules. The known fluidity of the phospholipids in the bilayer explains the mobility of cell surface compounds such as the receptors. This view of membrane structure is known as the fluid mosaic model and is wholly compatible with the known behaviour of drugs at membranes. Membrane protein embedded in the bilayer serves as receptors to elicit electrical or chemical signaling pathways and provide selective targets for drug action. Glycoprotein or glycolipids are formed on the surface by the attachments to polymeric sugars, amino sugars or sialic acids. The proteins are able to freely float through the membrane and some of the intrinsic ones, which extend through the full thickness of the membrane, surround fine aqueous pores. Paracellular spaces or channels also exist between certain epithelial or endothelial cells and other proteins have enzymatic or carrier properties. Biological membranes behave as if they were lipoids punctured by aqueous pores and allow drugs and physiological materials cross by passive or carrier mediated process. Which mechanism operates is determined by the physicochemical properties of drugs and the options available in the membrane in question.

Drugs are transported across the membrane by:

1.   Passive diffusion

2.   Filtration

3.   Facilitated diffusion

4.   Active transport

5.   Pinocytosis & Phagocytosis

Passive Diffusion: It is a random movement of drug molecules from an area of higher concentration to an area of lower concentration. When a drug is injected into the body, it passively diffuses from the injection site to areas of lower concentration, eventually reaching a blood capillary and entering the systemic circulation. In this process no cellular energy is expended and no active transport process is performed by the body to direct the movement of drug molecules. Hence the term passive diffusion is used.

Many drugs pass through the biological membranes such as cell membranes by passive diffusion. For a drug to diffuse from one side to another, the drug must dissolve in the membrane, which is composed of phospholipids, and diffuse down the concentration gradient. Thus, if a drug can dissolve in a cell membrane, it readily passes through the membrane by passive diffusion without any energy expended by the cell to move the drug molecules. Passive diffusion continues until enough molecules have passed from an area of higher concentration to an area of lower concentration to equalize the concentrations. At that point, the concentration gradient is very small and little differences exist in drug concentration in the areas of formerly high concentration and formerly low concentration. Drug molecules continue to move, however, such that an equal number move into and out of both the areas. At that point the drug concentrations are at equilibrium. This is the most important mechanism for majority of drugs. Lipid soluble drugs diffuse by dissolving in the lipoidal matrix of the membrane. The rate of transport being proportional to lipid: water partition coefficient of the drug. A more lipid soluble drug attains a higher concentration in the membrane and diffuses quickly. Also the greater is the difference in the concentration of the drug on the two sides of the membrane, faster is its diffusion. Passive diffusion is mostly dependent upon:

1.   Concentration gradient

2.   Drug molecular size (smaller molecules move more rapidly than bigger molecules)

3.   Lipophilic nature of the molecule

4.   Temperature

5.   Thickness of the membrane (the thicker the membrane the slower the diffusion)

Filtration: Filtration is the passage of drugs through aqueous pores in the membrane or through paracellular spaces. This can be accelerated if hydrodynamic flow of the solvent is occurring under the hydrostatic or osmotic pressure gradient. Lipid insoluble drugs cross biological membranes by filtration if their molecular size is smaller than the diameter of the pores. Majority of the cells have very small pores and drugs with higher molecular weight are not able to penetrate. However capillaries (except those in brains) have large pores and most drugs can filter through these pores. Passage of drugs across capillaries is dependent on the rate of blood flow through them rather than on lipid solvability of the drug or pH of the medium. Neutral or uncharged molecules pass most readily since the pores are believed to be electrically charged. Within the alimentary tract pores are largest and most numerous in jejunal epithelium. Filtration seems to play almost a minor role in drug transfer within the body except for glomerular filtration, removal of drugs from CSF and passage of drugs across hepatic sinusoidal membrane.

Carrier mediated transport: In this transport mechanism the drug combines with a carrier present in the membrane and the complex then translocates from one side of the membrane to the other. The carriers for polar molecules appear to form a hydrophobic coating over the hydrophilic groups and thus facilitate passage through the membranes. Substances permitting transit of ions across membranes are called ionophores. Carrier transport is specific, saturable and competitively inhibited by analogues, which utilize the same carrier. Intestinal absorption sometimes depends on carrier mediated transport. Levodopa is taken up by a carrier that normally transports phenylalanine, flurouracil is transported by the system that carries natural pyrimidines (thymine and uracil), iron is absorbed via a specific carrier on the surface of the mucosal cells in the jejunum and calcium is absorbed by means of a vitamin D dependent carrier system. This carrier mediated transport is of two types, namely facilitated diffusion and active transport.

Facilitated diffusion: Facilitated diffusion is a passive transport mechanism across biological membranes that involve a special "carrier molecule" in the membrane that facilitates the movement of certain drug molecules across the membrane. As in passive diffusion, facilitated diffusion involves no energy expended by the cell to move the drug molecules and the direction of the drug movement is determined by the concentration gradient. In addition, once the equilibrium is attained, the number of drug molecules crossing the membrane is either direction via the carrier remains the same. This proceeds more rapidly than simple diffusion and even translocates non-diffusible substrates, but along their concentration gradient, therefore does not need energy. Vitamin B12 is absorbed from the gut by this process.

Active transport: Like facilitated diffusion, active transport of drug molecule involves a specialized carrier molecule. However, in active transport a drug molecule comes in contact with a specialized carrier molecule in the membrane and the cell expends energy to move the drug molecules across or to reset the carrier molecule after transport so that it may transport again. Unlike diffusion in which the direction of net drug movement is determined by the concentration gradient, active transport can move drug molecules against the concentration gradient from areas of lower concentration to areas of higher concentration within a cell or body compartment. Glucose entry within cell is facilitation diffusion while passage across gastric mucosa and excretion by proximal renal tubular cells is active transport. Drugs related to normal metabolites are actively absorbed from the gut by aromatic amino acid transport processes.

Drugs actively transported may potentially reach such high concentration within cells that they exert a toxic effect, such as that seen with aminoglycoside antibiotics. Because active transport requires cellular energy, anything that disrupts the cell's production of energy such as toxins or certain drugs, prevents active transport of drug molecules across biological membranes. Nonspecific active transport of drugs, their metabolites and some endogenous products occurs in renal tubules and hepatic sinusoids, which have separate mechanisms for organic acids and organic bases. Certain drugs have been found to be actively transported in the brain and choroid plexus also. Examples for active transport include 5-fluorouracil, iron by gut, levodopa crossing the blood brain barrier and secretion of many organic acids and bases by renal tubular cells.

Pincytosis (Cell drinking) and phagocytosis (Cell eating)

Drug molecules may enter a cell by being physically engulfed by the cell. In both pinocytosis and phagocytosis, a portion of the cellular membrane surrounds the drug molecule and takes it within the cell. In these processes transport across the cell membrane is facilitated by formation of vesicles. This is an active process and requires the cell to expend energy. If the engulfed particle is not susceptible to enzyme degradation, it will persist like particles of talc or droplets of liquid paraffin. Pincytosis and phagocytosis are especially important for movement of large drug molecules such as complex proteins or antibodies that would otherwise be unable to enter a cell or pass intact through a membrane barrier. The absorption of immunoglobulins through the gut mucosa of young calves depends on phagocytosis. Pinocytosis and phagocytosis are more of histopathological interest than pharmacological interest.

Effect of transport mechanisms on drug molecule movement

Transport mechanisms determine the direction of drug molecule movement and the rate at which the molecules move from one compartment to other, or through a membrane barrier. In facilitated diffusion and active transport, wherein a carrier is involved, the transport mechanism can only move a limited member of drug molecules at one time. If the transport system becomes overloaded or saturated, some drug molecules waiting to be transported begin accumulating. In contrast to these carrier transport systems, the rate of passive diffusion is not limited because drug molecules simply diffuse through any part of the biological membrane. Pinocytosis and phagocytosis are relatively slow mechanisms for drug transport.

Absorption of drugs from the gastrointestinal tract: Before a drug can be absorbed, it must dissolve in the aqueous contents of the gut. Thus the actual amount of the drug present in a dose is only one of the factors that will affect the amount of drug actually absorbed or available.

Factors affecting absorption include

1. Molecular size and shape of the drug and its concentration.
2. Degree of ionization (ionization depends on the pKa of the drug and pH of the medium. In stomach the pH is 1 - 2, rumen 5.5 - 6.5 and intestine 7 - 8, and hence degree of ionization of drugs varies in different regions and species of the gastrointestinal tract).
3. Lipid solubility of the neutral or nonionized form of the drug.
4. Chemical or physical interaction with co-administered preparations and food constituents.
5. Pharmaceutical preparation and dosage form, especially their disintegration rate and dissolution rate.
6. Morphological and functional difference in the gastrointestinal tract among various species.
7. Gastric motility and secretion as well as gastric emptying.
8. Intestinal motility and secretion as well as intestinal transit time.
9. Fluid volume within the gastrointestinal tract.
10. Osmolality of the intestinal contents.
11.  Intestinal blood and lymph flow.
12.  Disruption of the functional and structural integrity of the gastric and intestinal epithelium.
13.  Drug biotransformation within the intestinal lumen by micro flora or within the mucosa by host enzymes.
14.  Volume and surface area of the absorbing surface. Stomach has a relatively small surface area compared to the duodenum. Hence absorption is more in the duodenum.
15. Presence of food in the stomach. Normally in full stomach there is a delay in absorption because the drug gets diluted in the stomach contents.

Effect of a drug's lipophilic or hydrophilic nature on drug movement

Biologic membranes are largely composed of fats. Therefore to move across a membrane, drug molecules must be in a form that dissolves in fat or oil. Fat loving form is known as lipophilic form. Some drug molecules dissolve readily in water and are called hydrophilic. Drug molecules that are polarized or ionized readily dissolve in an aqueous medium and so are hydrophilic. Non­polarised, non-ionized drugs are less soluble in water and can pass through lipid membranes.

Drugs can be conveniently classified into three groups as follows in the physicochemical sense. They are

1.  Drugs that are charged or uncharged according to the environmental pH (electrolytes),

2. Drugs that are incapable of assuming a charge, whatever is the environmental pH (non-polar) and

3. Drugs that are permanently charged whatever is the environmental pH (polar)

Electrolytes: Many drugs are weak electrolytes i.e., their structural groups ionize to a greater or lesser extent, according to environmental pH. Usually most molecules are present partly in the ionized and partly in the un-ionized state. The degree of ionization influences lipid solubility and in turn their absorption, distribution and elimination. Most of the drugs are either weak acids or bases. The degree to which these drugs are lipid soluble (nonionized, the form in which drugs are able to cross membranes) is determined by their pKa and the pH of the medium containing the drug. pKa of a drug is the pH at which 50% of the drug is ionized and 50% is non-ionized. In monogastric animals with a low stomach pH, weak acids such as aspirin with a pKa of 3.5 tend to be better absorbed from the stomach than the weak bases because of the acidic conditions. Weak bases are poorly absorbed from the acidic environment of the stomach because they exist mostly in the ionized state. Weak bases are better absorbed from the small intestine where the environmental pH is more alkaline.

Henderson Hasselbach equation is used to calculate the percent of a drug that exists in ionized form or to determine the concentration of a drug across the biologic membrane.

concentration of nonionized acid

For weak acid, pKa = pH + log------------------------------------------

concentration of ionized acid

concentration of ionized base

For weak base, pKa = pH + log------------------------------------------

concentration of nonionized base

It can also be expressed as

                                                            100

For a weak acid, % ionized = ---------------------------------

                                                 1 + antilog (pKa – pH)

                                                            100

For a weak base, % ionized = ---------------------------------

                                                1 + antilog (pH – pKa)

Effect of pH gradient on distribution of a weak organic acid (pKa 3.5) between blood plasma and gastric juice

           

                                                Biologic membrane

Plasma (pH 7.4) pKa = pH + log U/I 3.5 = 7.4 + log U/I -3.9 = log U/I Taking antilog on both sides 0.01       = U/I If U = 1 then I = 10,000 10,001 units of the drug on this side of the membrane at equilibrium

Gastric fluid (pH 1.5) pKa = pH + log U/I 3.5 = 1.5 + log U/I 2 = log U/I Taking antilog on both sides 100 = U/I If U = 1then I 0.01 1.01 units of the drug on this side of the membrane at equilibrium

I = Ionized form; U = Unionized form.

At equilibrium

a. the concentration of unionized drug (HA) is the same on both the sides of the membrane

b. there is more total drug (unionized + ionized) on the side of the membrane where the degree of ionization is the greatest (ion trapping).

Because of the relationship between pH and degree of ionization, a relatively small change will produce a large change in the proportion of drug present in non-ionized form, particularly when the pH of the solution is numerically close to the pKa of the weak organic electrolyte.

Acidic urinary reaction of the carnivorous species promotes passive reabsorption of acidic drugs from the distal portion of the nephron. Conversely urinary alkalinisation promotes their excretion.

Weak acids like aspirin and phenylbutazone are well absorbed from the gastrointestinal tract of dogs and cats. Weak organic bases like tetracyclines administered parenterally diffuse into the rumen of cattle and sheep and into the colon of horses. Basic drugs tend to concentrate in fluids that are acidic relative to plasma such as intracellular fluid (pH 7.0)

Non-polar : Eg. Digoxin, chloramphenicol. These drugs have no ionisable group. They are not affected by pH. Lipid solubility favours diffusion of these drugs across tissue boundaries. Drugs with high lipid to water coefficient can diffuse more rapidly through the gastrointestinal epithelium.

Polar : Eg. Negatively charged - Heparin, Positively charged - Tubocurarine. These drugs have limited capacity to cross plasma membrane. They are normally not administered orally.

In general

1. The more lipid soluble (less polar) a drug is, the better it is absorbed from the gut and the more widely it is distributed in the body.
2. Weak acids are absorbed best from the stomach.
3. Weak bases are absorbed best from the intestine.
4. However, it is also true that most drug absorption from the gut occurs in the proximal duodenum.
5. Absorption is a dynamic process; absorbed drug is rapidly carried away by blood or lymph.
6. Most drugs are sufficiently water-soluble to be carried away by blood. This blood enters the protal vein and goes to the liver where some or all the drug may be metabolized.
7. Extremely lipid soluble compounds enter the lymphatics.

First pass effect (Pre systemic metabolism)

All drugs that are absorbed from the intestine enter the portal vein and pass through the liver before they are distributed systemically. Some drugs are almost completely removed from the circulation on their first passage through the liver. The drugs that show poor availability when given orally due to their extensive first pass effect include - meperidine, aspirin, morphine, pentazocine, chlorpromazine, lidocaine, nitroglycerine, isoproterenol and propranolol. However some of these drugs are given orally and the required dose is much greater than that for other routes of administration.

Enterohepatic recycling

A drug that is excreted by the liver and arrives at the duodenum in lipophilic form could be reabsorbed across the intestinal wall and transported by the hepatic portal circulation back to the liver, where it is then excreted again by the liver or reenters the systemic circulation. This movement of drug from the liver to the intestinal tract, and back to the liver is referred to as enterohepatic circulation. Drugs that are reabsorbed intact from the intestinal tract can exert a pharmacologic effect on the body, therefore some hepatically excreted drugs appear to have an extended duration of action in the body. Eg. Phenophthalein.

Absorption of drugs from parenteral administration

Drugs injected subcutaneously or intramuscularly should be in the hydrophilic form so that they may readily diffuse through tissue fluid and reach a capillary to be absorbed. Anything that interferes with diffusion of the drug from the site of administration or alters the blood flow to the injection site can delay absorption of the drug. Aqueous solutions of drugs are usually absorbed from intramuscular injection site within 10-30 minutes provided the blood flow is unimpaired. Faster or slower absorption is possible, depending on the concentration and lipid solubility of the drug, vascularity of the site, volume of injection, the osmolality of the solution and other pharmaceutical factors. Absorption of drugs from subcutaneous tissues is influenced by the same factors that determine the rate of absorption from intramuscular sites. Some drugs are absorbed as rapidly from subcutaneous tissues as from muscles, although absorption from injection sites in subcutaneous fat is always significantly delayed. Increasing the 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 subcutaneously may be facilitated by including hyaluronidase in the solution.

Absorption of drugs from tracheobronchial surfaces and alveoli

The volatile and gaseous anaesthetics are the most important group of drugs administered by inhalation. These substances enter the circulation by diffusion across the alveolar membranes. Since they all have relatively high lipid water partition coefficients and generally are rather small molecules, they equilibrate practically instantaneously with the blood in the alveolar capillaries. Particles contained in aerosols may be deposited, depending on the size of the droplets, on the mucosal surface of the bronchi or bronchioles, or even in the alveoli.

Absorption of drugs from topical sites of application

Drugs may be absorbed through the skin following topical application. 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 the other body membranes. Absorption of drugs through the skin may be enhanced by inunction or more rarely by iontophoresis if the compound is ionized. Certain solvents may facilitate the penetration of drugs through the skin. The best known of these solvents is dimethylsulphoxide. 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 the epithelial surfaces. Many drugs traverse the cornea at rates that are related to their degree of ionization and lipid solubility. Thus organic bases such as atropine, ephedrine and pilocarpine often penetrate quite readily, whereas the highly polar aminoglycoside antibiotics generally penetrate cornea poorly.

Bioavailability The term bioavailability is used to indicate the proportion of drug that passes into systemic circulation after administration taking into account both absorption and local metabolic degradation.