It stands as one of the two bases of Pharmacology. It is what the drugs undergo in our body. It is the quantitative study, the bunch of four processes which constitute to form this term- PHARMACOKINETICS!
The term pharmacokinetics was first introduced by F. H. Dost in 1953. Literally, Pharmacokinetics means the application of kinetics to pharmakon, the Greek word for drugs and poisons. “Pharmacokinetics deals with the study of the fate of drugs once ingested and the variability of drug response in varying patient population.”
It explains the transportation of the drug while from administering through any route to its elimination from the body. The processes for it namely are: Absorption, distribution, metabolism and excretion.
These four pathways control the speed of onset of drug action and the intensity of the drug’s effect. The drug reaches the site of intention to produce the therapeutic effect with avoidance of toxicity.
Pharmacokinetics serves as the quantitative study as well- the kinetics of drug. “Kinetics is that branch of knowledge which involves the change of one or more variables as a function of time.”
Drugs vary in size, shape and nature and so their penetration process as well.
PASSIVE DIFFUSION
Lipid diffusion or passive diffusion constitutes the small, non-polar molecules to pass through the lipid-soluble membrane. Lipid diffusion separates the lipid compartment from the aqueous compartment of the body. Thereby enabling to determine how readily the molecules move between the aqueous and lipid compartments.
The force for passive absorption of a drug is the concentration gradient across the membrane separating the lipid and aqueous body compartments. This process posses a low structural specificity of drug. As the drug molecules are non polar, this process do not need any energy utilization.
Drugs are either weak acids or weak bases such as aspirin, phenobarbitone, sulphonamides etc to move from the aqueous media to lipid media or vice versa vary with the pH of the medium. In case of a weak acid or a weak base, the ratio of lipid-soluble form to water-soluble form can be expressed by the Henderson-Hasselbalch equation:
pH= pKa+log [non protonated species/[protonated species]
For acids, the non protonated species is the anion and protonated form is the acidic drug (the neutral more lipid=soluble form) whereas for basic drugs, the non protonated specie is the uncharged base(neutral form) and protonated specie is the weak basic drug that can permeate through the cell.
Fick’s law of diffusion-
The separation of the two different compartments- lipid and aqueous, the partition coefficient determines the mobitlity of the drug.i.e. how the drug enters from the aqueous to the lipid membrane. The passive flux of molecules from a higher concentration to lower concentration is given by Fick’s law –
Flux(molecules per unit time) = (C1 –C2) x area X permeability coefficient/thickness.
C1= higher concentration and C2= lower concentration. A= area across which diffusion is occurring.
Permeability coefficient= a measure of the drug molecules in the medium of the diffusion path.
Thickness= it is of the diffusion path.
FILTRATION
Small, polar drug molecules along with water molecules permeate through this process. Maximum weight of the molecules that can pass through the pores is 200. This process occurs in the interstitial space, endothelial lining of the blood vessels through pores.
ACTIVE TRANSPORT AND ENDOCYTOSIS
Large molecules such as peptides, amino acids and glucose are too large to diffuse through the lipid-soluble membranes into the cell or by filtration process. These molecules penetrate with the help of special protein carriers on the surface of the cell which carries them into the cell, leave s it and returns back to the surface to ferry more such molecules. This follows the process of endocytosis. Such large molecules are charged molecules and so this pathway is an energy-dependant process. It is specific for a particular type of drug structure. Drugs are transported against a gradient. I.e. from lower concentration to higher concentration. Drugs such as Vit.B12 attach with the carrier protein across the wall of the gut into the blood and penetrate the cell through the invagination formation. Iron attach with the protein transferrin, is transported into hemoglobin and synthesizes the Red Blood Cell precursor.
EXOCYTOSIS
There are several other cells which are not so selective to the membrane carriers. They are specially for expelling any foreign molecule by the process of Exocytosis through a vesicle formation on the cell membrane. The ABC family that is ATP- Binding Cassette Family contains P-glycoprotein or Multidrug-resistance Type=1 transporter. They are found in the brain, testes and other neoplastic cells. Another transport molecule of this family are known as Multi-Drug Resistance associated proteins that play an important role in the resistance of some drugs to chemotherapeutics drugs and metabolism of some drugs into urine and bile.
The four processes are:
1. ABSORPTION
When a drug is administered to a patient, the dosage form such as oral enters the mouth and further enters the bloodstream. This process is defined as “Absorption is the transfer of a drug from its site of administration to the bloodstream”
The rate of absorption can be determined by the site of administration and the formulation of drug. The mechanism of drug absorption can be Zero-order when the rate is independent of the amount of drug remaining in the gut and First Order when the full dose gets dissolved in the gastrointestinal fluids. The rate of absorption will be directly proportional to the gastrointestinal concentration.
If a person undergoes ‘Shock’, the blood flow during it reduces to the cutaneous tissue due to which the absorption by cutaneous administration reduces. Absorption of drug from intestine is more favored such that the area of the intestine is greater so that the drug absorption is more efficient. The rate of absorption of the drug decreases if any condition delays its transfer from the stomach to the intestine. Acidic drugs get absorbed in the acidic medium that is stomach and basic drugs in the basic medium that is the intestine.
The fraction of administered drug that reaches the circulation i.e. the plasma in a chemically unchanged form is known as Bioavailability.
Some drugs such as atenolol and acyclovir are too hydrophilic and lipophilic, possessing low bioavailability which is due to incomplete absorption i.e. they undergo First Pass Effect. While drugs administered parenterally undergo no first pass metabolism and their bioavailability is 100% which leads to complete absorption. Drugs such as Insulin are destroyed in the gastrointestinal due to the degradative enzymes present. So the chemical instability of a drug influences bioavailability.
The extent of bioavailability for a drug administered through a particular route can be measured by plotting the plasma concentration of drug against time on the graph known as Area Under Curve. The Area Under Curve of IVadministration is greater than the oral one. Bioavailability is calculated using the formula- AUC oral/ AUC injected X 100.
Drugs that show comparable bioavailability and similar times to reach the systemic circulation are Bioequivalent whereas two drugs that have differences in their bioavailability but have same effect are Bioinequivalent. On the other hand, two similar drugs which have similar rate of bioavailability and therapeutic equivalence are therapeutically equivalent.
The Extraction Ratio (ER) is expressed as the effect of first pass elimination on bioavailability-
ER= CL (liver)/Q .
Q= hepatic blood flow (90L/h in a 70kg person) and CL liver as liver is the main organ for metabolism.
2. DISTRIBUTION
‘It is the process by which the drug reversibly leaves the bloodstream and enters the interstitium and or the cells of the tissues. ’ Drugs such as iodine gets distributed in the thyroid gland. It is used to calculate the amount of drug needed to achieve the desired plasma concentration.
Selective distribution-
It occurs within the body because of special affinity between particular drugs and particular body constituents.
The drug after administration through any route distributes in the plasma compartment or extracellular fluid or intracellular fluid. The rate of blood flow to the tissue capillaries varies during cardiac output. The blood flow to the brain, kidney and liver is high as compared to the skeletal muscles. The brain capillary structure is continuous as it has no slit junctions between the endothelial cells. Lipophilic drugs such as alcohol, anesthetic gas, methyl DOPA can penetrate the CNS capillaries as they can dissolve in the lipid membrane whereas the charged hydrophilic drugs like atenolol cannot permeate as there are no slit junctions. “The tightly juxtaposed endothelial cells form tight junctions that constitute the blood brain barrier.”
In multiple drug therapy, the important concept that arises is the binding of drugs to the plasma protein which is Albumin. Albumin has strong affinity for weak acids (anionic) drugs and lipophilic drugs. The drugs existing in the bound state other then the free state are initially inactive and not metabolized because they are bound to the albumin. When the free drug gets eliminated, the bound drug detaches from the albumin and act as the free drug.
By taking warfarin and sulfonamide antibiotic as two different classes of drug, in case of multiple drug therapy, warfarin is slightly bound to albumin and Free State of it is less. When sulfonamide is administered, it substitutes warfarin and the bound warfarin gets detached and large amount of warfarin appears in free state. Volume of distribution relates the amount of drug in the body to the concentration of drug in the plasma. “It is a hypothetical volume of fluid into which a drug is dispersed.”
Vd= amount of drug in the body/ concentration
DISTRIBUTION KINETICS.
It has no physiologic process. Volume of distribution can be determined by sampling a reference compartment of the body.
Considering the distribution of drug in the absence of elimination, the drug by intravenous infusion moves from the plasma to the interstitium fluid into the cells causing decreased plasma concentration with respect to time. The concentration of drug on the body fluids remains constant as there is no elimination. It can be calculated using the formula,
C= D/Vd where C is the concentration of the drug in plasma and D is the total amount of drug in the body.
The next distribution is of the drug in the presence of elimination. Drug concentration initially decreases in the plasma due to rapid distribution and enters the interstitium and intracellular fluid. It slows the elimination phase.
Thirdly, if the distribution is instantaneous and the elimination process begins at the time of injection and continues throughout the distribution phase. The concentration of drug in plasma is extrapolated to time zero to determine the concentration of drug. The uneven distribution of drugs in several compartments of the body can be calculated by the formula-
Vd= (C2-C1) where C1 is the initial amount of drug in the body and C2 is the desired plasma concentration.
3. METABOLISM
The main organ involved in the drug metabolism is the LIVER. While sometimes metabolism through the gastro intestinal tract, lungs, skin, kidneys and brain also happens.
Drugs when administered orally, gets absorbed in the gastro intestinal tract and then enters the portal circulation. Then it passes to the liver and undergoes First Pass metabolism due to which the decreased amount of the unchanged form of drug enters the systemic circulation. Hence the First Pass Effect. Some drugs such as Pencillin are metabolized by the gastric acid and insulin by the digestive enzymes whereas drugs such as Midazolam undergo significant intestinal metabolism and Chlorpromazine undergo extensive metabolism in the intestine.
Metabolism leads to the Bioactivation (Inactive drug form to active drug form), Bioinactivation(active form to inactive drug form) or More-activation (conversion of diazepam to oxazepam). It also leads to prodrug formation e.g Phinacetine metabolizes and converts into Paracetamol.
The liver consists of microsomal enzymes found in the Smooth Endoplasmic Reticulum. They retain the morphological and functional characteristics of the intact membranes. The metabolism of drugs decreases its plasma concentration and therapeutic effect.
These enzymes are deficient in premature babies. It takes about three months for their development of full drug metabolism. They are more susceptible to side-effects. Drugs such as Chloramphenicol cause Grey Baby Syndrome.
The metabolism through the enzyme present in the liver can be categorized into two major reactions- Phase-I and Phase-II.
· PHASE-I REACTIONS
The Phase-I reactions form polar and readily excretable metabolites through non-synthetic reactions such as Oxidation, Reduction, and Hydrolysis whereby a non-polar group such as OH, NH2 attaches to it.
The Smooth Endoplasmic Reticulum contains Mixed Function Oxidases or Monooxygenases which is a class of enzyme present in the liver. The Smooth Endoplasmic Reticulum is rich in these enzymes that are responsible for oxidative-drug metabolism.
The cytochrome P450 acts as a terminal oxidase. It is abbreviated as P450 or CYP. It plays a key role in the metabolism of drugs as the sub enzymes known as isozymes belong to this family. Another enzyme that plays a key role is the Nicotinamide Dinucleotide Phosphate Cytochrome P450 Reductase which is a flavo protein. These enzymes play the key role in Oxidation-Reduction process where a single molecule is reduced per substrate molecule and a single Oxygen atom appears in the product and in the form of water while in the Reduction process, it binds to Carbon Monoxide and forms a complex.
The various isoforms that catalyzes the drug metabolism with respect to their percentage of the total P450 content in the liver are:
CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2D6, CYP2E1, and CYP3A4.
Out of these the CYP3A4 enzyme is the most important as it metabolizes almost 50% of the prescription drugs and xenobiotics.
Certain drugs act on the P450 enzymes and stimulate them which cause their increased rate of synthesis, increased activity in metabolizing the drug and decreased pharmacological activity. Such drugs are known as Inducers and the process is known as enzyme induction. Other than drugs, there are environmental chemicals and pollutants as well which act as inducers of P450 enzymes. Polycyclic aromatic hydrocarbons present in the tobacco smoke and other organic pycolysis products induce the CYP1A2 enzymes to alter the rates of drug metabolism.
To acquire the increased synthesis of the P450 enzymes, enhanced transcription, translation and heme synthesis is required. The nuclear receptor Perorusome Proliferator Receptor Alpha (PPRalpha) found in the liver and kidney uses drugs like fenofibrate and gomfibrozil as ligands. It functions as the regulatory receptor of fatty acids such as arachidonic acid metabolism which occurs by CYP4A enzymes. PPRalpha mediates the function of CYPA4 enzymes. Inducers also decrease the rate of degradation of the P450 enzymes for its increased metabolism.
Inversely, there are certain drugs that inhibit the activity of the cytochrome P450 enzymes known as Inhibitors and the process is known as enzyme induction. Inhibitors bind to the heme iron and inhibits the metabolism of the endogenous substances through competitive inhibition. CYP3A4 metabolize antibiotics such as erythromycin, troleandomycin; the inhibitors act to make them inactive. Proadifen is an inhibitor used in research.
There are certain inhibitors which actually act irreversibly on a mediator which further causes the inhibition of the P450 enzymes. These inhibitors are known as suicide-inhibitors such as the antibiotic- chloramphenicol is metabolized by CYP2B1. It includes certain steroids such as spironolactone, ethynyl estradiol, analgesic sedatives-allylisopropylacetylurea.
· PHASE-II REACTIONS
As stated in the book- Basic and clinical Pharmacology by Bertram G,Katzung- ‘conjugates are more polar molecules that are readily excreted and often inactive’
Phase-II reactions are conjugate reactions in which the parent drug or the metabolites formed by the Phase-I reactions undergo conjugation with an endogenous substances such as glucuronic acid to form conjugates. It includes specific transfer enzymes known as transferases located in the microsomes or in the cytosol. Phase-II reactions are faster reactions as compared to the Phase-I reactions. The Uridine 5-diphosphate glucuronosyl transferases enzyme catalyses the conjugating of an activated endogenous substance with a drug. Acyl glucuronidation of non steroidal anti-inflammatory drugs form reactive conjugates which are responsible for toxicity.
Metabolism Kinetics
The rate of drug metabolism is directly proportional to the concentration of free drug.
First order kinetics, it means a constant fraction of drug is metabolized per unit time. Rate of drug metabolism = Vmax [C]/Km where Km is the Michaelis constant and [C] is the concentration of drug. whereas high free drug concentration and so the constant metabolism rate over time is called zero order kinetics. Rate of drug metabolism, v= Vmax.
4. ELIMINATION
Metabolism and or conjugation leads to more polar form of drug in order to be readily excreted from the body. Drug elimination occurs mainly through the kidney. Bile, intestine, lungs and breast milk are also the organs from which the elimination occurs but to a little extent as compared to the Kidney.
When the lipophilic drugs undergo metabolism and get modified into polar drug- they block the polar groups in case of Phase-I reaction and do not get diffused back into the plasma from the kidney lumen in the case of Phase-II reaction.
Elimination through kidney takes place through three processes:
When the free drug flow through the capillary slits towards the Bowman space, it becomes part of the glomerular filterate. This Process is known as glomerular filtration.
When the free drug does not enter into the glomerular filtrate, it leaves the glomeruli thorugh the efferent arterioles and surrounds the nephric lumen in distal tubule by forming capillary plexus. The secretion in the distal tubule takes place by the two transport systems- anionic and cationic. The anionic system eliminates drugs like penicillin and the cationic system secrete drugs like Morphine. It is a carrier-mediated process. This process is known as the proximal tubular secretion.
When the drug starts moving from the proximal tubule, the drug concentration increases so that it exceeds the perivascular space. So the non polar drug reabsorption from the distal to convoluted tubule to plasma takes place by simply diffusion.
Plasma clearance- it is expressed as the volume of plasma from which all drug appears to be removed in a given time. It is equal to the amount of renal plasma flow multiply by the extraction ratio.
Total body clearance is the sum of the total clearances from various drug metabolizing and drug elimination organ.CL total= Cl(hepatic) + CL(pulmonary) + CL(renal) CL(others).
To decrease the amount of reabsorption of drug and increase in the clearance of undesirable drug; the pH of the urine can be modified to make the drug more polar. This is done by the ‘ion-trapping’ process where weak acids such as drug-Phenobarbital is eliminated by alkalizing the urine using the drug- Bicarbonate and weak basic drugs can be eliminated by acidification of urine such as incase of cocaine overdose and then ammonium chloride is used to acidify the urine.
Drugs such as tetracycline and neomycin taken orally are not completely absorbed from the gastrointestinal tract and they are excreted in the feces.
Some drugs such as erythromycin and phenolphthalein metabolized in the urine are excreted through bile.
Heavy metals are excreted through intestines and can produce intestinal ulceration.
Drugs through inhalation administration such as volatile general anesthetics get eliminated through lungs. Alcohol gets partially excreted through lungs.
Arsenic gets incorporated in hair follicles and it gets excreted through skin in small quantities.
Iodides and heavy metals are excreted in the saliva. It may cause stomatitis.
Drugs like penicillin and ampicillin appear in the breast milk in amounts sufficient that it could harm the infant adversely.
ELIMINATION KINETICS
Steady state is achieved when the plasma concentration of drug remains constant. It occurs when the rate of drug elimination is equal to the rate of administration (Intravenous infusion). Liver or kidney disease or any factor that increases the steady state concentration of an infused drug decreases clearance. It can be described by the equation-
Css= R0/KeVd = R0/ CLt
where Css= steady state concentration of drug, R0= infusion rate, Ke= first order elimination rate constant, Vd= volume of distribution.
Excretion rate= clearance x plasma concentration of drug.
Half-Life
By change in the rate of drug administration, half-life indicates the time required to attain the steady state.
The book basic and clinical Pharmacology by Bertram G.Katzung states that the “half life is the time required to change the amount of drug in the body by one-half during elimination or during constant infusion.”
The time course of drug in the body will depend on both volume of distribution and clearance. T1/2= 0.7xVd/CL
Thus, Pharmacokinetics help us understand what happens to the drug when it enters the body. The small, elegant, therapeutically effective structures! So important for a healthy living and pharmaceutical base yet so complex in its effect and mechanism.
BY
Maryam Akram
Doctor Of Pharmacy
The University Of Lahore