Pharmacokinetics

PHARMACOKINETICS

Pharmacokinetics is the quantitative study of drug movement in, through and out of the body. The overall scheme of pharmacokinetic processes is depicted in Fig. 2.1. The intensity of response is related to concentration of the drug at the site of action, which in turn is dependent on its pharmacokinetic properties. Pharmacokinetic considerations, therefore, determine the route(s) of administration, dose, latency of onset, time of peak action, duration of action and frequency of administration of a drug .

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

Biological membrane

This is a bilayer (about 100 Å thick) of phospholipid and cholesterol molecules, the polar groups (glyceryl phosphate attached to ethanolamine/choline or hydroxyl group of cholesterol) of these are oriented at the two surfaces and the nonpolar hydrocarbon chains are embedded in the matrix to form a continuous sheet. Extrinsic and intrinsic protein molecules are adsorbed on the lipid bilayer.

Glycoproteins or glycolipids are formed on the surface by attachment to polymeric sugars, aminosugars or sialic acids. The specific lipid and protein composition of different membranes differs according to the cell or the organelle type. The proteins are able to freely float through the membrane: associate and organize or vice versa. 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/endothelial cells. Other adsorbed proteins have enzymatic, carrier, receptor or signal transduction properties. Lipid molecules also are capable of lateral movement. Thus, biological membranes are highly dynamic structures.

Drugs are transported across the membranes by:

          Passive diffusion and filtration

          Specialized transport

Passive diffusion

The drug diffuses across the membrane in the direction of its concentration gradient, the membrane playing no active role in the process. This is the most important mechanism for majority of drugs; drugs are foreign substances (xenobiotics), and specialized mechanisms are developed by the body primarily for normal metabolites.

Lipid soluble drugs diffuse by dissolving in the lipoidal matrix of the membrane (Fig. 2.3), the rate of transport being proportional to the lipid : water partition coefficient of the drug. A more lipidsoluble drug attains higher concentration in the membrane and diffuses quickly. Also, greater the difference in the concentration of the drug on the two sides of the membrane, faster is its diffusion.

Influence of pH Most drugs are weak electrolytes, i.e. their ionization is pH dependent (contrast strong electrolytes that are nearly completely ionized at acidic as well as alkaline pH). The ionization of a weak acid HA is given by the equation:

                                               [A¯ ]

                    pH = pKa + log —–—                        ...(1)

                                                [HA]

pKa is the negative logarithm of acidic dissociation constant of the weak electrolyte. If the concentration of ionized drug [A¯ ] is equal to concentration of unionized drug [HA], then

[A¯ ]

—–— = 1

[HA]

Since log 1 is 0, under this condition

                                     pH = pKa                 ...(2)

Thus, pKa is numerically equal to the pH at which the drug is 50% ionized.

If pH is increased by 1 scale, then—log [A¯ ]/[HA] = 1         or      [A¯ ]/[HA] = 10

Similarly, if pH is reduced by 1 scale, then— [A¯ ]/[HA] = 1/10

Thus, weakly acidic drugs, which form salts with cations, e.g. sod. phenobarbitone, sod. sulfadiazine, pot. penicillinV, etc. ionize more at alkaline pH and 1 scale change in pH causes 10 fold change in ionization.

Weakly basic drugs, which form salts with anions, e.g. atropine sulfate, ephedrine HCl, chloroquine phosphate, etc. conversely ionize more at acidic pH. Ions being lipid insoluble, do not diffuse and a pH difference across a membrane can cause differential distribution of weakly acidic and weakly basic drugs on the two sides (Fig. 2.4).

Implications of this consideration are:

Acidic drugs, e.g. aspirin (pKa 3.5) are largely unionized at acid gastric pH and are absorbed from stomach, while bases, e.g. atropine (pKa 10) are largely ionized and are absorbed only when they reach the intestines.

The unionized form of acidic drugs which crosses the surface membrane of gastric mucosal cell, reverts to the ionized form within the cell (pH 7.0) and then only slowly passes to the extracellular fluid. This is called ion trapping, i.e. a weak electrolyte crossing a membrane to encounter a pH from which it is not able to escape easily. This may contribute to gastric mucosal cell damage caused by aspirin.

Basic drugs attain higher concentration intracellularly (pH 7.0 vs 7.4 of plasma).

Acidic drugs are ionized more in alkaline urine—do not back diffuse in the kidney tubules and are excreted faster. Accordingly, basic drugs are excreted faster if urine is acidified.

Lipidsoluble nonelectrolytes (e.g. ethanol, diethylether) readily cross biological membranes and their transport is pH independent.

Filtration

Filtration is 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 hydrostatic or osmotic pressure gradient, e.g. across most capillaries including glomeruli. Lipidinsoluble drugs cross biological membranes by filtration if their molecular size is smaller than the diameter of the pores (Fig. 2.3). Majority of cells (intestinal mucosa, RBC, etc.) have very small pores (4 Å) and drugs with MW > 100 or 200 are not able to penetrate. However, capillaries (except those in brain) have large paracellular spaces (40 Å) and most drugs (even albumin) can filter through these (Fig. 2.8). As such, diffusion of drugs across capillaries is dependent on rate of blood flow through them rather than on lipid solubility of the drug or pH of the medium.

Specialized transport

This can be carrier mediated or by pinocytosis. Carrier transport

All cell membranes express a host of transmembrane proteins which serve as carriers or transporters for physiologically important ions, nutrients, metabolites, transmitters, etc. across the membrane. At some sites, certain transporters also translocate xenobiotics, including drugs and their metabolites. In contrast to channels, which open for a finite time and allow passage of specific ions, transporters combine transiently with their substrate (ion or organic compound)—undergo a conformational change carrying the substrate to the other side of the membrane where the substrate dissociates and the transporter returns back to its original state (Fig. 2.5). Carrier transport is specific for the substrate (or the type of substrate, e.g. an organic anion), saturable, competitively inhibited by analogues which utilize the same transporter, and is much slower than flux through channels. Depending on requirement of energy, carrier transport is of two types:

a. Facilitated diffusion The transporter, belonging to the superfamily of solute carrier (SLC) transporters, operates passively without needing energy and translocates the substrate in the direction of its electrochemical gradient, i.e. from higher to lower concentration (Fig. 2.5A). It mearly facilitates permeation of a poorly diffusible substrate, e.g. the entry of glucose into muscle and fat cells by GLUT 4.

b. Active transport It requires energy, is inhibited by metabolic poisons, and transports the solute against its electrochemical gradient (low to high), resulting in selective accumulation of the substance on one side of the membrane. Drugs related to normal metabolites can utilize the transport processes meant for these, e.g. levodopa and methyl dopa are actively absorbed from the gut by the aromatic amino acid transporter. In addition, the body has developed some relatively nonselective transporters, like Pglycoprotein (Pgp), to deal with xenobiotics. Active transport can be primary or secondary depending on the source of the driving force.

i)  Primary active transport

Energy is obtained directly by the hydrolysis of ATP (Fig. 2.5B). The transporters belong to the superfamily of ATP binding cassettee (ABC) transporters whose intracellular loops have ATPase activity. They mediate only efflux of the solute from the cytoplasm, either to extracellular fluid or into an intracellular organelli (endoplasmic reticulum, mitochondria, etc.)

Encoded by the multidrug resistance 1 (MDR1) gene, Pgp is the most well known primary active transporter expressed in the intestinal mucosa, renal tubules, bile canaliculi, choroidal epithelium, astrocyte foot processes around brain capillaries (the bloodbrain barrier), testicular and placental microvessels, which pumps out many drugs/metabolites and thus limits their intestinal absorption, penetration into brain, testes and foetal tissues as well as promotes biliary and renal elimination. Many xenobiotics which induce or inhibit Pgp also have a similar effect on the drug metabolizing isoenzyme CYP3A4, indicating their synergistic role in detoxification of xenobiotics.

Other primary active transporters of pharmacological significance are multidrug resistance associated protein 2 (MRP 2) and breast cancer resistance protein (BCRP).

ii) Secondary active transport

In this type of active transport effected by another set of SLC transporters, the energy to pump one solute is derived from the downhill movement of another solute (mostly Na+). When the concentration gradients are such that both the solutes move in the same direction (Fig. 2.5C), it is called symport or cotransport, but when they move in opposite directions (Fig. 2.5D), it is termed antiport or exchange transport. Metabolic energy (from hydrolysis of ATP) is spent in maintaining high transmembrane electrochemical gradient of the second solute. The SLC transporters mediate both uptake and efflux of drugs and metabolites.

The organic anion transporting polypeptide (OATP) and organic cation transporter (OCT), highly expressed in liver canaliculi and renal tubules, are secondary active transporters important in the metabolism and excretion of drugs and metabolites (especially glucuronides). The Na+,Cl– dependent neurotransmitter transporters for serotonin and dopamine (SERT and DAT) as well as the vesicular transporter for biogenic amines are active SLC transporters that are targets for action of drugs like tricyclic antidepressants and reserpine, etc. The absorption of glucose in intestines and renal tubules is through secondary active transport by sodiumglucose transporters (SGLT1 and SGLT2).

As indicated earlier, carrier transport (both facilitated diffusion and active transport) is saturable and follows the Michaelis Menten kinetics. The maximal rate of transport is dependent on the density of the transporter in a particular membrane, and its rate constant (Km), i.e. the substrate concentration at which rate of transport is half maximal, is governed by its affinity for the substrate. Genetic polymorphism can alter both the density and affinity of the transporter protein for different substrates and thus affect the pharmacokinetics of drugs. Moreover, tissue specific drug distribution can occur due to the presence of specific transporters in certain cells.

Pinocytosis It is the process of transport across the cell in particulate form by formation of vesicles. This is applicable to proteins and other big molecules, and contributes little to transport of most drugs.