Introductory Pharmacology - Pharmacokinetics PDF

INTRODUCTORY PHARMACOLOGY
PHA 301
MRS JULIET OLAYINKA  PROCESSES IN PHARMACOKINETICS (start) Pharmacokinetics deals with what your body does to the drug. It includes: ABSORPTION→ DISTRIBUTION → METABOLISM → EXCRETION OF THE DRUGS.  DRUG ABSORPTION AND...

INTRODUCTORY PHARMACOLOGY
PHA 301
MRS JULIET OLAYINKA  PROCESSES IN PHARMACOKINETICS (start) Pharmacokinetics deals with what your body does to the drug. It includes: ABSORPTION→ DISTRIBUTION → METABOLISM → EXCRETION OF THE DRUGS.  DRUG ABSORPTION AND BIOAVAILABILITY ABSORPTION: Absorption is the movement of the drug from its site of adminstration into the systemic circulation. ABSORPTION→ SITE OF ADMINSTRATION → PLASMA → EFFECT ON ORGAN BIOAVAILABILITY: Is a term used to indicate the fraction of the unchanged drug reaching the systemic circulation following adminstration by any route. Bioavailability= Quantity of drug reaching the systemic circulation Quantity of drug adminstered The concept of bioavailabilty is that most drugs reach their site of action from the systemic circulation differently. For example intravenuously adminstered drugs are injected directly into the systemic circulation so the quantity of drug adminstered is equivalent to the amount reaching the systemic circulation, therefore , the bioavailability of such a drug is 1 or 100% In contrast, for orally adminstered drugs, factors such as incomplete absorption and hepatic metabolism could affect the bioavailability of such a drug, therefore bioavailability of such drug is less than 1 or less than 100%. Zero % bioavailability implies that no drug enters the systemic circulation where as 100% bioavailability means that all the dose is absorbed into the systemic circulation.  AREA UNDER CURVE (AUC): is a common measure of the extent of bioavailability of a drug given by a particular route. The total amount of drug in the systemic circulation is defined by the AUC. The AUC is expressed as 1 when the drug is adminstered intravenuosly ( ie there is 100% absorption) but the AUC is expressed as less than 100% when the drug is adminstered orally for two reasons which are : incomplete absorption and first pass elimination  Incomplete absorption: orally adminstered drugs may be incompletely absorbed due to lack of absorption through the gut wall, the drug being too hydrophilic or the drug being too lipophilic or due to a reverse transporter associated with p- glycoprotein (this process pumps drugs out of the gut wall into the gut lumen). First pass metabolism: following absorption through the gut wall, the portal blood delivers the drug to the liver before entering the systemic circulation. The liver now metabolizes the drug before it enters into the cirulation. Also, the liver can excrete the drug into the bile. These processes will reduce the bioavailability of the drug at the site of action. The drug can also be metabolized by the gut wall by cytochrome enzymes (Cyp3A4 enzymes) found in the gut wall  The AUC provides information about the amount or extent of drug absorbed. 1) AUC of drug A ˃ drug B ? What is the therapeutic significance? The AUC of drug A is greater than the AUC of drug B but drug B is ineffective because it did not reach the minimum effective concentration (MEC)  AUC A > B: Therapeutic Significance? MEC MEC: MINIMUN EFFECTIVE CONCENTRATION  2) AUC of drug A ˃ drug B but equally effective AUC of drug A is greater than AUC of drug B but drug B is still therapeutically effective because it exceeded the MEC. AUC A > B: Equally Effective MEC  BIOAVAILABILITY CURVE Effect of rate of absorption on peak concentration drug in plasma and duration of drug action.  CURVE EXPLAINED
The duration of action and peak plasma concentration of a drug can be affected markedly by the drug absorption rate. ▪ The three drugs (A,B and C) demonstrates different rates of absorption. ▪ Drug A is absorbed quickly and reaches the highest peak plasma concentration, as all of the drug is absorbed before significant elimination can take place ▪ Drug C is absorbed slowly and never achieves a high plasma concentration but it persists in the plasma for longer than drug A and B because the absorption still continues during the elimination phase. ▪ Drug B absorption rate is between those of drug A and C ▪ These drugs may have been adminstered through different routes but it is the same drug. Drug A could represent intravenous glucocorticoids, curve B could represent intramuscular injection while curve C could be an ultraslow-release subcutaneous formulation of the same drug.  FACTORS AFFECTING ABSORPTION AND BIOAVAILABILITY 1) Physicochemical properties of the drug 2) Nature of the dosage form 3) Pharmacogenetic factors 4) Disease states 1) PHYSICOCHEMICAL PROPERTIES OF THE DRUG. i) PHYSICAL STATE: Liquids are absorbed better than solids and crystalloids absorbed better than colloids. ii) LIPID OR WATER SOLUBILITY: Drugs in aqueous solution mix more readily than those in oily solution. However, at the cell surface, the lipid soluble are rapidly absorbed than the water soluble drugs. iii) IONIZATION: Unionized component is predominantly lipid soluble and is absorbed rapidly while an ionized component is often water soluble and is absorbed poorly. Most of the drugs are weak acids or bases.  Strong acids and strong bases are poorly absorbed by oral route , so has to be given using another route eg curare (a strong base) Drugs are usually designed as weak acids (aspirin) and weak bases (morphine). This is because they are more lipid soluble and so may diffuse across cell membrane readily. Weak acids are easily absorbed in the stomach because they are lipophilic at low PH. Weak bases are easily absorbed in the intestine because they are lipophilic at high PH.  The stomach is acidic with a PH of 1-3 and with small intestine which has a PH of 5-8. As acidity increases at night, this may increase the absorption of some drugs that are acid based. Taking drugs like antacids for ulcer patients will decrease the acid in the stomach, thereby decreasing the absorption of some drugs given together with them because these drugs will need an acid environment to be well absorbed.  2) NATURE OF THE DOSAGE FORMS i) PARTICLE SIZE: Drugs given in a dispersed or in an emulsified state are absorbed better ( small particle is important for drug absorption). Microfine tablets such as aspirin increases the rate of absorption. ii) DISINTEGRATION TIME AND DISSOLUTION TIME: Disintegration time is the rate of break up of the tablet or capsule into the drug granules. Dissolution time is the rate at which the drug goes into solution. Drugs may have a short duration of action if their dissolution time and absorption in the GIT is quick or metabolism is quick. Such drugs have to be given 2 or 3 times eg Nifedipine. But Nifedipine SR was produced after Nifedipine which has slower dissolution and prolonged delivery. Processes involved for drugs to be released from the gut to the circulation are these: The faster the dissolution time in the GIT, the faster the absorption and bioavailability. Processes involved for drugs to be released from the gut to the circulation Drugs released from dose form ↓ drug dissolution ↙ ↘ active membrane dissolution through Transport membrane ↓ Absorption  iii) FORMULATION: Substances like lactose, sucrose, starch and calcium phosphate are used as inert diluents in formulating powders or tablets. Tablets are compressed powder often formulated with starch to swell when in contact with water to aid dissolution. A faulty formulation can render a useful drug totally useless therapeutically  3) PHYSIOLOGICAL FACTORS i) GIT TRANSIT TIME: Rapid absorption occurs when the drug is given on an empty stomach. This is so because gastric content will determine the delay before absorption starts. ˃ Gastric content determines the rate of gastric emptying and also drug absorption in the a the GIT. Gastric emptying time can be slowed through the diet (fatty food and high bulk food ), disease state, pregnancy, certain drugs like alcohol, antimuscarines. With slow gastric emptying , there is more time for the drug to remain in the stomach longer and to be broken down by the stomach enzymes or acids in the stomach making the levels of the drug in the blood to be lower.  On the other hand, increased gastric emptying rate may promote the absorption of any drug and lead to increased blood levels of the drug. Gastric emptying rate can be promoted by Hunger, anxiety, certain drugs like metoclopromide. Although, in some cases the presence of food in the GIT tract aids absorption of certain drugs like propranolol, griseofulvin  However some irritant drugs like salicylates and iron preparations are deliberately adminstered after food to minimize the GIT irritation.. ii) PRESENCE OF OTHER AGENTS: Some certain drugs reduce or enhance absorption rate in the body. ˃ Eg vitamin C enhances the absorption of iron in the GIT. ˃ Calcium present in milk and in antacids form complexes with the tetracycline antiboitics and so reduces their absorption.  iii) AREA OF ABSORPTION SURFACE AND LOCAL IRRITATION: Drugs can be absorbed better from the small intestine than from the stomach because of the larger surface area of the former. Since the small intestine is the primary site for drug absorption and with presence of microvilli the longer the food stays in the small intestine, the greater the absorption of the drug. Increased vascular supply can increase surface area. iv) ENTEROHEPATIC CYCLING: some drugs move inbetween the intestine and the liver before getting to their site of action. This increases their bioavailability eg phenolphthalein V) METABOLISM OF DRUG/ FIRST PASS EFFECT: Rapid degradation of a drug by the liver during the first pass affects its bioavailability. Thus a drug though absorbed well may not be effective because of its first pass metabolism.  RELATIONSHIP BETWEEN FIRST PASS METABOLISM AND BIOAVAILABILITY 1) Drugs that are extensively metabolized have poor bioavailability after first pass metabolism. ˃ Eg Glyceryl trinitrate (nitroglycerin) which is used to treat angina. Because of its high first pass metabolism, it is not effective when taken orally but has to be given sublingually. Therefore larger doses of the drug has to be taken in other to increase the blood levels of such drug in the system. 2) On the other hand, drugs with high first pass metabolism have very high bioavailability in the blood in cases of liver diseases. ˃ eg in liver Cirrhosis: ↓ drug metabolism → ↑ bioavailability of drug in the system → ↑ Drug effect →toxicity.  4) PHARMACOGENETIC FACTORS: Individual variations occur due to genetically related reason in drug absorption and response. 5) DISEASE STATES: Disease state can slow absorption like in cases of CVS disease which leads to low perfusion or blood flow. This can slow drug absorption, other cases are malabsorption, liver cirrhosis etc. 6) Route of drug adminstration: The route in which the drug is administered can enhance or slow down absorption.  ORAL AVAILABILITY (F) This is the fraction of the dose of drug given orally that reaches the systemic circulation ie The (F) defines how much drug gets into the systemic circulation after oral ingestion. It is usually defined by comparing the AUC(t) curve in the systemic circulation after oral ingestion with the AUC after IV dosing ie the fraction (F) drug that gets into the body after oral(Po) versus (iv) adminstration. F= AUC(Po) AUC(IV ) The AUC is often less after oral adminstration compared with IV adminstration.  - Most drugs use this route - It could be passive non ionic diffusion or with specialized transporter - Absorption could take place in the stomach (to a lesser extent) and also in the small intestine (larger extent). Absorption in the small intestine is more passive than active Rate of absorption is 75% in 1-3hrs and depends on: › motility →diarrhoea decreases absorption. › blood flow › food → enhance or impair. › particle size and formulation › physicochemical factors – unionised or lipid soluble › rate of gastric emptying rate limiting step Determinants of (F) are: › Absorption › First pass metabolism  EFFECT OF FOOD ON ORAL DRUG ABSORPTION: Food affects absorption and first pass metabolism in opposite ways. While Food increases the (F) of drugs that are subject to high first pass eg grapefruit, food usually decreases the (F) of drugs that are poorly absorbed. Food can affect oral drug absorption in the following ways: › some drugs require acid environment to enhance absorption eg ketoconazole › Presence of fat or bile enhance absorption for some drugs eg carbamazepine › binding to fibre reduces absorption eg digoxin › binding to calcium (chelate) reduces absorption eg tetracyclines, quinolones. › poor acid stability : prolonged gastric exposure → degradation, therefore reducing absorption in some drugs eg erythromycin , azithromycin.  FIRST PASS METABOLISM OF ORAL DRUGS : This occurs in the liver and gut lumen. First pass metabolism in gut lumen: › Gastric acid inactivates benzylpenicillin › Proteolytic enzymes inactivate insulin. First pass metabolism in gut wall: Monamine oxidase → metabolises monoamine 1) CYP3A4: This enzyme in the gut wall can be blocked by grapefruit juice Also by some drug inducers, inhibitors, substrates. If 40mg simvastatin is adminstered with water, the extent of absorption will be low but if the same drug is adminstered with grapefriut, then the extent of absorption will be high. Administration of 40mg Simvastatin with ◦ Water Grapefruit juice  P- glycoprotein (enterocytes to gut lumen): Interactions between inhibitors (eg verapamil, macrolides) and substrates (eg digoxin). In the graph below, adminstering 0.75mg of digoxin with a placebo will decrease the rate of absorption and adminstering with clarithromycin will increase the rate of absorption HEPATIC FIRST PASS METABOLISM: Hepatic first pass metabolism reduces the amount of parent drug and forms metabolites Administration of 0.75mg digoxin with ◦ placebo clarithromycin  SUBLINGUAL ADMINSTRATION: There is rapid absorption (in minutes) because first pass metabolism is by passed. RECTAL FORMULATIONS: Erratic absorption because of rectal contents because it avoids first pass metabolism. PARENTERAL (INTRAVENOUS): No absorption is required because it goes directly to the plasma. It avoids first pass metabolism. PARENTERAL (SUBCUTANEOUS): It involves insoluble suspension, slow, even absorption.  DISTRIBUTION OF DRUGS Distribution is the penetration of a drug to the sites of action through the walls of blood vessels from the adminstered site after absorption. After a drug has been adminstered and absorbed into the systemic circulation, it has to be distributed into the blood from where it is carried to its site of action where the drug acts on. Blood is aqueous and only hydrophilic (water soluble) drugs will dissolve in it. For absorption, most drugs have to be lipid soluble. Lipophilic drugs are transported to the blood attached to the proteins.  Drugs are distributed through various body fluid compartment such as plasma, interstitial fluid compartment, transcellular compartment. Plasma water- 5%, Interstitial water- 16%, Intracellular water- 35%, Transcellular water -2%.  PROTEIN BINDING PROTEIN BINDING OF DRUGS: The active concentration of a drug is that part which is not bound to plasma proteins because it is only this fraction which is free to leave the plasma to the site of action. Plasma proteins such as albumin and globulin carry drugs in the systemic circulation. Acid drugs bind to albumin Basic drugs bind to globulin In the plasma, there is equilibrium between protein bound drug and free drug. › Free drugs are drugs that are not bound to the plasma proteins and can leave the blood stream to have effect on the organs.  Example, in a liver diseased condition, where bilirubin (yellow breakdown product of rbc) binds to plasma protein albumin, more bilirubin binds to albumin displacing the free drugs from their binding sites. When this happens, we have excess free drugs in circulation which may become toxic. Therefore, in cases of jaundice where high level of bilirubin binds to plasma proteins, the dose of the drug has to be reduced. › Also, drugs can compete for the same site on the plasma protein  Example, in cardiovascular disease where aspirin and warfarin are used as drugs for the treatment of CVS disease. Aspirin and warfarin have the same binding site in the plasma protein. Warfarin is used as an anticoagulant. The anticoagulant effect depends on the free warfarrin in circulation. Aspirin normally displaces warfarrin from their binding site causing high levels of warfarrin to be found in the circulation leading to haemorrhage. Therefore, using warfarin as a drug of combination in the treatment of CVS disease, the dosage has to be reduced.  Things to note about protein binding: i) only free drugs leave plasma to site of action. ii) binding of drugs to plasma proteins assists absorption. iii) protein binding acts as temporary store of a drug and tends to prevent large fluctuations in concentrations of unbound drug in the body fluids. iv) protein binding reduces diffusion of drug into the cell and thereby delays its metabolic degradation eg high protein bound drug like phenylbutazone is long acting while low protein bound drug like thiopental sodium is short acting.  PLASMA CONCENTRATION OF DRUG (PC): This represents both the drug bound to the plasma proteins ( albumins & globulins) and the drug in free form. The concentration of free drug in plasma does not always remain at the same level e.g i) After i.v adminstration of drug, plasma conc of drug falls sharply. ii) After oral adminstration, plasma conc rises and falls gradually Iii) After sublingual adminstration, plasma conc rise sharply and falls gradually.  FACTORS THAT AFFECT PROTEIN BINDING AND DRUG DISTRIBUTION 1) DECREASE IN BLOOD PROTEINS: Decrease in blood proteins normally lead to increase in free drug concentration which will lead to increase in the effect of the drug in the organ. This effect may go from therapeutic to toxic. Decrease in blood protein may be as a result of : › Insufficient dietary protein (malnutrition) › liver disease as a result of decreased protein synthesis. › burns When the blood protein is decreased, it is advisable for the dosage of the drug to be decreased in other to decrease the level of free drug in circulation from toxic to therapeutic.  2) INCREASE IN BLOOD PROTEINS: In increased blood proteins, the conc of free drug is decreased leading to no effect of the drug in the system. The effect of the drug goes below therapeutic levels. Example in multiple myeloma, where there is excessive production of immunoglobulin proteins. These proteins bind to the plasma proteins and inactivate the free drug. We now have less free drug in the circulation to produce a therapeutic response. In such a condition, it may be necessary to increase the dosage of the drug in other to bring about a therapeutic effect.  2) CLEARANCE : This is the volume of plasma cleared off the drug by metabolism and excretion per unit time. Protein binding reduces the amount of drug available for filtration at the glomeruli and hence delays excretion; therefore protein binding reduces clearance. 3) PHYSIOLOGICAL BARRIERS TO DISTRIBUTION: There are some specialized barriers in the body that could prevent the drug from distributing uniformly in the body such as : A) blood brain barrier: There are no gaps in between the endothelial cells lining blood vessels, so these form a barrier. Only lipid soluble drug can enter brain and CSF eg thiopental sodium could cross the BBB but not dopamine. B) placental barrier: which allows passage mostly lipid soluble drugs and some water soluble drugs eg antiepileptics, alcohol, morphine.  3) AFFINITY OF DRUGS TO CERTAIN ORGANS: The concentration of a drug in certain tissues after a single dose may persist even when its plasma concentration is reduced to low. In the body fat, the following drugs bind to it › lipid soluble drugs › stable reservoir eg anesthetics In the bone, › Adsorption into bone crystal surface › reservoir- slow release eg tetracyclines, heavy metals.  4) LIPID SOLUBILITY OF THE DRUG: In lipid solubility, drugs accumulate in fat tissues and they are also slowly released from the fat tissue. Example, Amiodarone used as an anti-arrhythmic drug. This drug is highly lipophilic and so accumulate in fat tissue. Even when the drug is stopped, amiodarone continues to be released from the fat tissues for weeks. Another example is Marijuana. It is very highly lipophilic and accumulate in fat tissues and is slowly released. Marijuana can still be detected in the blood 6 weeks after a single dose.  VOLUME OF DISTRIBUTION APPARENT VOLUME OF DISTRIBUTION ( Vd ) : This represents the volume that would be required to contain the total amount of absorbed drug in the body at the same concentration as that in the plasma. › Volume of distribution is the volume into which a drug appears to be distributed with a concentration equal to that of the plasma. › Volume of distribution gives the information on how the drug is distributed in the body › To calculate the Vd the loading dose and the target plasma conc (Cp) has to be known. › it is used to calculate or determine the loading dose Vd = Loading Dose [Drug]plasma  Example, two drugs A and B each adminstered at 100mg. These two drugs have different distribution and hence different Vd. If after adminstration, drug A has a plasma level of 20mg/ml and drug B has a plasma level of 2mg/ml. Drug A, Vd= 100mg = 5L 20mg/ml  Drug B, Vd = 100mg = 50L 2mg/ml With these results, we can say that drug B will be more widely distributed than drug A because it is less bound to the plasma protein (2mg/ml). The Vd of 50L tells you that drug B accumulates more in the tissues than drug A. › drugs with high Vd accumulate in fat tissues, are more distributed and cannot be removed by haemodialysis. › drugs with low Vd are more bound to plasma proteins, less distributed and so can be removed by haemodialysis. Eg Heparin is confined to the blood and so has a Vd of 0.06L/kg, therefore, can be removed by haemodialysis. Nortriptyline (an antidepressant) has a Vd of 14L/kg due to tissue binding and so cannot be removed by haemodialysis.  DRUG METABOLISM Metabolism is the process by which the body brings about changes in drug molecule. Drug metabolism is also known as drug biotransformaton. Need for metabolism: › To render non-polar (lipid soluble) substances polar (lipid insoluble). This is so because only water soluble drugs can be excreted from the kidney. The lipid soluble drugs are reabsorbed. › To produce inactive metabolites (metabolites are products of metabolism). This reduces the activity of the drug.  SITE OF METABOLISM: The primary site of drug metabolism is the liver. Others are the kidney, intestine, nasal mucosa, lungs. Biotransformation of drugs may lead to the following: Inactivation of an active drug: most drugs and their active metabolite are rendered inactive or less active eg ibuprofen, paracetamol. Conversion of an active drug to an active metabolite: many drugs are found to be partially converted to one or more active metabolite eg codeine to morphine. Activation of an inactive drug: some drugs are inactive as such need conversion in the body to one or more active metabolite. Such a drug is called a prodrug eg levodopa to dopamine.  INACTIVATION OF AN ACTIVE METABOLITE Example is paracetamol. This drug is considered to be a therapeutically safe drug. Normally most paracetamol is conjugated and excreted in urine. However, a very small amount of paracetamol is metabolized by the CYP enzymes to N-acetyl-p- benzoquinone imine (NAPQI), which is very toxic to the liver. But a small amount of NAPQI in the presence of glutathione can readily be excreted in the urine. When very high doses of paracetamol are adminstered, the glutathione gets used up, then NAPQI will now build up and become toxic to the liver. This main feature of paracetamol poisoning is toxic to the liver. DIAGRAM OF PARACETAMOL POISONING CONVERTING AN ACTIVE DRUG TO AN ACTIVE METABOLITE Some drugs produce pharmacologically active metabolite like morphine which is a pharmacologically active metabolite of heroin. Morphine when given orally, undergoes extensive first pass metabolism leaving lower conc of morphine in the blood; and making morphine not to relief pain and cause euphoria. Heroin is more lipid soluble than morphine and can enter the blood brain barrier readily than morphine. Once heroin is in the brain, it is metabolised to morphine which is made available in the brain to bring about pain relief and euphoria. This is why opioid addicts prefer heroin to morphine.  ACTIVATION OF AN INACTIVE DRUG (PRODRUG) Prodrugs are pharmacologically inactive drugs that have active metabolites. They are designed in such a way that maximum amount of drugs reach the site of action to produce an effect. An example is Enalaprilat. This drug is a potent inhibitor of Angiotensin Converting Enzyme (ACE) and used in treating CVS disease. Enalaprilat is not absorbed orally so is not given orally. To bypass this, the inactive prodrug, Enalapril is given orally becaused it is readily absorbed in the GIT. Enalapril is converted in the liver to Enalaprilat which is provided to give the beneficial effect we are looking for.  FIRST PASS METABOLISM This is the breakdown of a drug during its passage from the site of absorption into the systemic circulation. All orally adminstered drugs are exposed to drug metabolizing enzymes in the intestinal wall and liver.  Attributes of drugs with high first pass metabolism are: i) Oral dose is higher than sublingual or parenteral dose. ii) marked individual variation in the oral dose due to differences in first pass metabolism. iii) oral bioavailability is increased in patients with severe liver disease. iv) oral bioavailability of a drug is increased if another drug competing with it in first pass metabolism is given concurrently.  RELATIONSHIP BETWEEN FIRST PASS METABOLISM AND BIOAVAILABILITY A) Oral bioavailability is increased in patients with severe liver disease eg liver Cirrhosis. i) in this situation, there will be : Liver Cirrhosis → ↓ drug metabolism → ↑bioavailability → ↑ in drug effect → toxicity. Therefore dose of drug need to be ↓. B) Oral bioavailability is decreased in drugs that are extensively metabolized. Example is glyceryl trinitrate (nitroglycerin) used in treating angina has extensive first pass metabolism when taken orally, so is ineffective orally. Therefore this drug has to be taken sublingually. To bridge this problem, larger doses of the drug has to be taken, to increase the amount of drug that reaches the systemic circulation.  TYPES OF DRUG METABOLIZING ENZYMES There are two types of drug metabolizing enzymes involved in biotransformation. i) MICROSOMAL ENZYMES: These enzymes are located on the smooth endoplasmic reticulum, primarily, in the liver, kidney, intestinal mucosa and lungs eg the mono oxygenases, cytochrome p450, glucuronyl transferases etc. They catalyze most oxidations, reductions, hydrolysis and glucuronide conjugation reactions. They can be induced by drugs, diet and other agencies.  However, the metabolic pathway is mostly catalyzed by a group of enzymes called Cytochrome P450 system. The cytochrome P450 system is a superfamily of distinct enzymes known as CYP 1,2 and 3. They are found in the endoplasmic reticulum. The most common of the CYPs for drug metabolism is CYP3A4. In the nomenclature for CYP3A4, 3 is for the enzyme, A is for the subfamily, 4 is for the isoforms.  2) NON-MICROSOMAL ENZYMES: These enzymes are found in the cytoplasm and mitochondria of hepatic cells as well as in other tissues including plasma eg of such enzymes are the flavoproteins oxidases, esterases, amidases and the conjugases. They catalyze reactions such as some oxidation and reduction reactions and all conjugation reactions except glucuronidation They are not induced by drugs. Biotransformation reactions can be classified into two which are: 1) phase 1 reaction 2) phase 2 reaction  PHASE 1 REACTION Phase 1 reaction is also called non-synthetic reaction or functionalization reaction. This reaction leads : i) To the introduction of a functional group to the drug such that the drug now carries an OH, -COOH, - SH, -O-, NH2 group. ii) The drug is converted to a water soluble drug. iii) lead to biological inactivation of an active drug and also the activation of an inactive drug ( prodrug ). The phase 1 reactions are oxidation, reduction and hydrolysis  OXIDATION: This reaction involves the addition of oxygen or negatively charged radical to the drug or the removal of hydrogen or positively charged radical. Examples of oxidation reaction are hydroxylation, oxidative deamination, oxygenation at C, N or S atoms, dealkylation. Enzymes that carry out this reactions are flavin containing mono oxygenases in the liver, Cyp 450, epoxide hydrolase eg barbiturates. REDUCTION: This enzyme reaction is catalyzed by the enzyme reductase. It involves the CYP 450 working in the opposite direction. Alcohols, aldehyde are reduced  HYDROLYSIS: This is the splitting of drug molecule after adding water. Drug metabolism by hydrolysis is restricted to esters and amines ( by esterases and amidases). Example pethidine undergoes hydrolysis to form pethidinic acid. Ester + H20 Hydrolysis occurs in liver, intestines, plasma and other tissues.  PHASE II REACTIONS: the second phase of the metabolism is the conjugation. This is the phase where the product of phase 1 metabolism joins to another compound. This reaction is also called synthetic or non-functionalization reaction. This phase is very important because: 1) It produces a drug with increased water solubility 2) increased molecular weight and these will serve to facilitate the elimination of the drug from the tissues. 3) inactivates the drug. The most common conjugation products are GLUCURONIDES and SULPHATES Conjugation reactions have high energy requirements. Enzymes in phase ll reactions are glutathione-s-transferases ( GST ), UDP-glucuronosyltransferases ( UGT ), sulfotransferases ( SULT ), N- acetyltransferases ( NAT ) and methyltransferases ( MT ).  I) GLUCORONIDE CONJUGATION: Is the most important and commonest type of synthetic reaction. The reaction is carried out by a group of UDP- glucuronosyl transferases ( UGT ). Compounds with a hydroxyl or carboxylic acid group are easily conjugated with glucuronic acid which is derived from glucose eg paracetamol. Glucuronic acid is a product of glucose metabolism. When phase 1 metabolites conjugate with glucuronic acid they form glucuronide metabolite.  Some glucuronide metabolite are excreted by the kidney in the urine. However, many glucuronide metabolite are transported in the bile to the GIT and by so doing undergo enterohepatic recycling.  ENTEROHEPATIC RECYCLING IN THE LIVER When drug C arrives at the liver in the circulation, the drug C undergoes metabolism by conjugation with glucuronic acid to form drug C glucuronide which is now transported in the bile to the GIT tract. Once in the GIT, some of the glucuronide is excreted in the faeces while some may be deconjugated by the bacterial enzyme β-glucuronidase to yield back the active drug C, that can now be reabsorbed and taken back to liver, with some of the active drug re- entering the circulation  Many glucuronides undergo this recycling and this alters their kinetics. The plasma levels may be higher than expected because the drug is been recycled. Therefore,to counter this effect, the dosage of the drug may need to be lowered.  ll) ACETYLATION: Compounds having amino or hydrazine residues are conjugated with the help of acetyl co-enzyme A eg sulfonamides. Rate of acetylation shows genetic polymorphism ( slow or fast acetylators). Acetyltransferase is responsible for acetylation present in the kupffer cells of liver. Acetic acid is conjugated to drugs through its activation by CoA to form acetyl CoA. This acetyl CoA is then transferred to –NH2 group of drug eg dapsone. lll) METHYLATION: The amines and phenols can be methylated. Methionine and cysteine acting as methyl donors eg captopril.  iv) SULFATE CONJUGATION: The phenolic compounds and steroids are sulfated by sulfotransferases ( SULTs),eg chloramphenicol. The enzyme is present in liver, intestinal mucosa and kidney. It transfers sulfate group to drug molecule. V) GLYCINE CONJUGATION: Salicylates and other drugs having carboxylic acid group are conjugated with glycine.  Vi) GLUTATHIONE CONJUGATION: It forms a mercapturate. It serves to inactivate highly reactive quinone or epoxide intermediates formed during metabolism of certain drugs eg paracetamol. Phase 1 metabolite + conjugate glucuronic acid → glucuronide metabolite Vii) RIBONUCLEOSIDE/ NUCLEOTIDE SYNTHESIS: This pathway is very important for the activation of many purine and pyrimidine antimetabolites used in cancer chemotherapy.  ENZYME INDUCTION AND INHIBITION Enzyme induction and enzyme inhibition are two mechanisms of drug-drug interactions in drug biotransformations. ENZYME INDUCTION: Enzyme induction is a process by which a drug enhances the expression of an enzyme. When there is an enzyme induction, it means : i) there is an increased rate of metabolism of drugs by that enzyme ii) this will lead to decreased plasma conc of that drug iii) decreased intensity of drug action iv) the drug may become ineffective. If the drug A is metabolized by the microsomal enzymes, then it means that concurrent adminstration with a microsomal enzyme inducer ( drug B ) will result in enhanced metabolism of drug A. Examples of some enzyme inducers are rifampicin, phenobarbitone, phenytoin, cigarette smoking, diet and nutrition.  Eg warfarin ( anticoagulant ) + barbiturate ( enzyme inducer ) → decreased anticoagulation Another example is where cigarette smoke and charboiled meats induce CYP1A enzyme which is involved in the metabolism of paracetamol. If smokers take paracetamol, there will be lowered plasma level of paracetamol and pain relief will not be achieved because the cigarrette smoke will induce the enzyme CYP1A to metabolize paracetamol which will lower the plasma level of paracetamol. Paracetamol + cigarrette smoke (enzyme inducer) → ↓ in plasma levels of paracetamol → no relief in pain. St Johns’ Wort (enzyme inducer) + oral contraceptives → ↓ in plasma levels of oral contraceptives → no contraception → pregnancy. St John’s Wort (used in treating depression) induces the enzyme CYP3A4.  CONSEQUENCES OF MICROSOMAL ENZYME INDUCTION There will be decrease in the intensity or duration of action of drugs that are inactivated by metabolism. There will be increased intensity of action of drugs that are activated by metabolism. There will be tolerance if the drug induces its own metabolism (autoinhibition) eg carbamazepine, an antiepileptic drug. It induces the enzyme CYP3A4 and also is metabolized by the same enzyme. Some endogenuos substrate are also metabolized faster. Precipitation of acute intermittent porphyria. Enzyme induction increases porphyrin synthesis by depressing δ – aminolevulenic acid synthetase. Intermittent use of inducer may interfere with adjustment of dose of another drug prescribed on regular basis eg anticoagulants. Interference with chronic toxicity testing in animals.  POSSIBLE USES OF ENZYME INDUCTION 1) In cases of congenital non haemolytic jaundice : this is due to deficient glucuronidation of bilirubin; in these patients with hyperbilirubinemia, there is a reduction in the activity of glucuronyl transferase enzyme. Phenobarbitone hastens the treatment of jaundice by increasing the clearance of bilirubin which is due to the increased activity of glucuronyl transferase enzyme. 2) Cushings syndrome: phenytoin may reduce the manifestations by enhancing the degradation of adrenal steroids. 3) Chronic poisonings : by faster metabolism of the accumulated poisonous substance. Vitamin D is essential for calcium metabolism. With the interference of enzyme inducers, there is a diversion in the hepatic metabolism of vitamin D to biologically inactive metabolite. This has been used in vitamin D poisoning where agents like phenobarbitone has been used to decrease vitamin D concentration. 4) Liver disease : the ability to reduce drug metabolism may decrease with age.  Enzyme induction interferes with peripheral thyroid metabolism. Treatment with phenytoin will decrease the protein bound plasma iodine in man or animals. Phenobarbitone has been found to enhance insulin mediated glucose metabolism in healthy non-diabetic patients. Drugs like phenobarbitone, alcohol may influence serum lipid and apoliprotein concentration. This will result in increased concentration of high density lipoprotein ( HDL) which are cardioprotective and a reduction in low density lipoprotein ( LDL) which are atherogenic with a resultant increase in HDL : LDL ratio. Serum HDL concentration have been known to be directly proportional to cyp 450 content of human liver and also directly related to plasma antipyrine clearance. Therefore, enzyme inducing drugs by lowering the concentration of atherogenic lipid, may influence coronary heart disease  ENZYME INHIBITION: Enzyme inhibition is the inhibition of the expression of an enzyme by a drug or molecule. When there is enzyme inhibition, it means that the : i) activity of the enzyme is been inhibited ii) which will lead to decreased rate of metabolism iii) the plasma levels of the drug will be increased iv) leading to increased intensity of drug action v) which may sometimes become toxic. Some drugs that are enzyme inhibitors are disulfuram, isoniazid, cimetidine and other ulcer healing drugs etc.  Eg warfarin + metronidazole ( enzyme inhibito r ) → haemorrhage. Anti- HIV viral protease inhibitors + Ketoconazole (enzyme inhibitor)→ ↑in plasma conc of anti-viral protease inhibitors → toxic. Ketoconazole inhibits the enzyme CYP3A4 Another example is grapefruit which is a potent inhibitor of CYP3A4. This will lead to toxic levels of drugs metabolized by this enzyme.  FACTORS AFFECTING DRUG METABOLISM AGE : Neonatal jaundice results from deficiency of phase II enzyme UDP-GT (UDP-glucuronyl Transferase enzyme) Gray baby syndrome (Hemophilus influenza), infection in infants that is treated with chloramphenicol, an antiboitic that has a very toxic metabolite after phase I metabolism and requires phase II metabolism before it is excreted In the elderly, decline in the function of the liver is observed with age which is attributed to age related decrease in liver mass. GENDER: It has been reported that a decrease in oxidation of ethanol, estrogens, benzodiazepams in women in relation to men DIET: grapefruit taken with drugs that undergo high first pass metabolism  EXCRETION Excretion is the removal of drugs from the body either unchanged by the process of excretion or converted to metabolites. After oral adminstration, drugs are taken from the GIT in the circulation for metabolism in the liver. The metabolites are carried in the circulation to the kidney for excretion in the urine. The major routes of drug excretion includes : › the renal excretion, hepatobiliary excretion and pulmonary excretion. The minor routes of drugs excretion are : › saliva, sweat, tears, breast milk, vaginal fluid, nasal and hair.  DIFFERENT ROUTES OF DRUG EXCRETION 1) RENAL EXCRETION: The kidney is the most important organ for excreting drugs and their metabolites. It excretes only water soluble substances. Factors that may influence increase in drug excretion from the kidney are: › Increase in renal blood flow, › increase in glomerular filtration rate › decrease in plasma protein bound drug Excretion of drugs and their metabolites in the urine involves three distinct processes. i) Glomerular filtration ii) Active tubular secretion iii) Tubular reabsorption  Schematic representation of glomerular filtration, tubular secretion, reabsorption
Affernt Efferent arterioles arterioles Plasma protein Tubular Peritubular cell vessel PH FD-free drug, BD-bound drug, UD-unionized drug, ID- ionized drug, Dx-actively secreted orga Acid or base drug  i) GLOMERULAR FILTRATION: in the glomerulli, › only free drugs are filtered. › water soluble drugs are also filtered › lipid soluble drugs are not filtered › drugs bound to plasma proteins are not filtered Glomerula filtration rate (GFR): glomerular filtration rate of a drug depends on: › The drugs’plasma protein binding. › renal blood flow The normal glomerulli filtration rate is ~ 120ml/min. Glomerulli filtration rate is a good way to measure kidney function. Drug clearance is often proportional to GFR. GFR is reduced in newborn, elderly, kidney and heart disease.  ii) ACTIVE TUBULAR SECRETION: Secretion is the movement of substances into the tubule. Drug secretion takes place in the proximal convoluted tubule through active transport eg histamine. › positively charged cpds are secreted into the proximal tubule. › negatively charged cpds are also secreted  Non –specific transporters such as organic acid transporters (OAT) and organic base transporters (OBT) which are located on the proximal tubules transport organic acids (salicylate, penicillin) or bases (histamine, acetylcholine) Efflux transporters (p-glycoproteins)are also located in the luminal membrane of proximal tubular cells. Active transport of the drugs across tubules reduces the concentration of its free form in the tubular vessels and promotes the dissociation of protein bound drug which is secreted. Protein binding which is a hindrance for glomerular filtration of drug is not a hindrance to excretion by tubular secretion indicating that there is usually a rapid equilibrium between protein bound and unbound drug in the blood.  iii ) TUBULAR REABSORPTION: Reabsorption is the movement of substances out of the tubules into the interstitial fluid and hence to the blood. The tubular reabsorption takes place in the distal convoluted tubules. Lipid soluble drugs are reabsorbed. This can take place through: › Simple diffusion →drugs move from nephron lumen to blood. › Active transport →drugs move from urine to blood Tubular reabsorption depends on › lipid solubility › ionization of the drug at the existing urinary PH.  Changes in urinary PH will affect tubular reabsorption of drugs that are partially ionized in the sense that: When the urine is acidic → ↑ ionization of weak basic drug ↓ reabsorption → excretion. Conversely, when the urine alkaline → ↑ ionization of weak acidic drug ↓ reabsorption → excretion. Eg NaHCO3 increases PH and ionization of acids: in aspirin poisoning NH4Cl lowers PH and increases ionization of bases: in amphetamine poisoning. This principle will helps in eliminating the drug faster in drug poisoning.  If renal clearance of a drug is greater than the GFR (~120ml/min) then the drug is primarily secreted If renal drug clearance is less than the GFR, then the drug is reabsorbed by the kidney If renal drug clearance is equal to GFR, then it means that the drug is neither secreted or reabsorbed but the drug is filtered by the glomerulli eg insulin and creatinine  RENAL INSUFFICIENCY In renal insufficiency, the duration of action of drugs are prolonged due to low renal function. In cases of renal insufficiency, the levels of drugs such as digoxin and aminoglycoside which are excreted unchanged may increase in the body. Renal impairment may lead to ↓in drug excretion→↑in drug plasma levels→toxicity Renal function is low in the neonates and also low in older adults ≥ 65years, so require low doses of the drug.  Kidney function is assessed through the creatinine clearance. Creatinine clearance is the removal of creatinine from the body. The properties that make creatinine ideal for assessing kidney function are these: › Creatinine has a steady level in the blood and is freely filtered by the kidney. › In normal conditions, the plasma and urine levels of creatinine are the same but in renal insufficiency, the plasma levels of creatinine is increased while the urine levels is decreased. When creatinine clearance is inhibited, it shows renal insufficiency.  2) HEPATOBILIARY SECRETION: Conjugated drugs are excreted by hepatocytes into the bile. Molecular weight more than 300 daltons and polar drugs are excreted in the bile. Excretion of drugs through the bile provides a backup pathway when renal function is impaired. After the excretion of drug through the bile into the intestine, certain amount of drugs are reabsorbed into the portal vein leading to an enterohepatic cycling which can prolong the drug action eg chloramphenicol, oral estrogen are secreted into bile and largely reabsorbed and have long duration of action.  3) GASTROINTESTINAL EXCRETION: When a drug is adminstered orally, a part of the drug is not absorbed or excreted in the faeces. Drugs which do not undergo enterohepatic cycle after excretion into the bile are subsequently passed with the stool. Eg aluminium hydroxide changes the stool into white colour, ferous sulfate changes the stool into black and rifampicin into orange red.  4) PULMONARY EXCRETION: Drugs are readily vaporized such as many inhalation anesthetics and alcohols are excreted through the lungs. The rate of drug excretion through the lung depends on the volume of air exchanged , depth of respiration, rate of pulmonary blood flow and drug concentration gradient. 5) SWEAT AND SALIVA: A number of drugs are excreted into the sweat either by simple diffusion or active secretion eg rifampicin, heavy metals. 6) MAMMARY SECRETION: Many drugs, mostly weak bases are accumulated into the milk. The mammalian milk is more acidic than plasma. Therefore, lactating mothers should be cautious about the intake of these drugs because they may enter into the baby through the breast milk and produce harmful effects in the baby eg ampicillin, aspirin.  BLOOD LEVELS The ultimate aim of drug therapy is to achieve efficacy without toxicity. This involves achieving a plasma concentration (Cp) within the therapeutic window. The ideal plasma conc range is between the minimum effective concentration and the (MEC) and the minimum toxic concentration (MTC). MEC: Is the minimum plasma conc that is expected to produce a pharmacological response. MTC: Is the minimum plasma conc above which a drug can become toxic.  ELIMINATION HALF LIFE HALF LIFE (t1/2): It is the time taken for the drug concentration to fall to half its original value. The half life of a drug provides information of: › about the time course of a drug elimination › about the time course of a drug accumulation › Choice of dose interval  The elimination half life of a drug depends on : › rates of metabolism › elimination of that drug Therefore, slowly metabolized and slowly eliminated drugs will have long half lives whereas rapidly metabolized and rapidly eliminated drugs will have short half lives. The half life determines how frequently a drug needs to be adminstered. Drugs with shorter half lives need to be adminstered more often than those with longer half lives.  The relationship between half life (t1/2) and volume of distribution (Vd) and clearance (CL). The half life also depends on: › Vd → Which means that, the longer the half life, the larger the volume of distribution because it will take a longer time to remove drugs that have been distributed deep within the tissues › CL → But on the other hand, the shorter the half life, the greater the clearance, so it means that drugs which are fastly cleared from the plasma will have a shorter half life. t1/2 = Vd CL  This relationship can be turned into an equation by multiplying the right side by 0.693 t Therefore, 1/2 = 0.693 Vd CL The 0.693 is the natural logarithm of 2 (ie In 2) and gets into the equation because the t ½ involves a halving ie the inverse of 2 This equation on t1/2 indicates that: The t1/2 is dependent on Vd and CL Vd and CL are independent variables  LOADING DOSE This is a dose given initially to get above the MEC in other to achieve a therapeutic effect. It is called the statum dose that is given. It is usually a high dose so that the therapeutic concentration is reached early. When the beneficial effect of a drug is needed fastly, the loading dose is given to reach the MEC fastly while the maintenance dose is given to maintain the MEC. Loading doses are used for drugs with large Vd  Loading dose= Vd x Cp(target) Q1) What is the loading dose required for drug A if the target plasma concentration is 10mg/L. Vd is 0.75L/kg and patient weight is 75kg. Vd = 0.75L/kg x75kg= 56.25L Loading dose = 56.25L x 10mg/L=565mg This could be rounded off to 560 or 500mg  MAINTENANCE DOSE This is the small fixed dose given in other to sustain the therapeutic dose. With the use of a maintenance dose : › It may take longer time to reach the MEC as the plasma conc slowly builds up to reach the MEC and also to exceed the MEC. › Therefore, the maintenance dose is not the appropraite dose to use in cases of emergency, particularly, when immediate response is desired.  Maintenance dose= CL x CpSSav CpSSav is the target average steady state drug concentration. Maintenance dose will be in mg/hr. So for a total daily dose, you multiply by 24. Q2) What maintenance dose is required for drug A if target average steady state concentration is 10mg/L and clearance of drug A is 0.015L/kg/hr where patient weight is 75kg.  Maintenance dose= CL x CpSSav CpSSav is the target average steady state drug concentration. Maintenance dose will be in mg/hr. So for a total daily dose, you multiply by 24. Q2) What maintenance dose is required for drug A if target average steady state concentration is 10mg/L and clearance of drug A is 0.015L/kg/hr where patient weight is 75kg.  Maintenance Dose = CL x CpSS av
CL = 0.015 L/hr/kg x 75 = 1.125 L/hr
Dose = 1.125 L/hr x 10 mg/L
= 11.25 mg/hr
So will need 11.25 x 24 mg per day
= 270 mg  STEADY STATE STEADY STATE: Steady state occurs after a drug has been given for approximately four to five elimination half life. Steady state concentration is a reasonably even concentration achieved with repeat dosing or continual infusion, which gives a continual beneficial effect. At steady state, the rate of drug administration equals the rate of elimination and plasma conc-time curve found after each dose should be approximately superimpossable Rate in= rate out  CLEARANCE (stop) CLEARANCE: This is the ability of organs of elimination (eg kidney, liver) to clear drug from the bloodstream. › It is the volume of fluid which is completely cleared of drug per unit time. › It is used to determine the maintenance dose. › units are in L/hr or L/hr/kg. Clearance= volume of blood cleared of drug per unit time. If clearance=10L/hr Vd= 100L What is the elimination rate constant (K) CL= kV K= CL(Lhr-1) =10Lhr-1 V (L) 100L K= 0.1hr-1  Therefore it means that 10% of the volume of blood is cleared (of drug) per hour. Therefore (k)= is the fraction of drug in the body that is removed per hour. CL= ↓kV↑ It means that if V increases, then k must decrease as CL is a constant. CL and Vd are independent variables but (k) is a dependent variable. CL determines the maintenance dose.  FIRST ORDER AND ZERO ORDER KINETICS FIRST ORDER METABOLISM: Most drugs undergo first order metabolism. In such drugs, there is rapid fall in drug levels as most drugs are readily metabolised and there is excess of enzyme available for the metabolism. The enzymes are not saturated with drug. In first order kinetics:  Things to note about first order kinetics: › All the enzymes are working ›↑ in conc of drug in plasma→↑ rate of drug metabolism › rate in α rate out › rate of metabolism of drug is not constant › half life is constant › rate of metabolism is proportional to drug concentration because increasing the conc of the drug in the liver, will put more enzymes to work, therefore causing an increase in drug metabolism.  ZERO ORDER METABOLISM In zero order metabolism, there is limited amount of enzyme available to metabolize the drug. When that limit is reached, metabolism occurs at a constant rate. The enzyme is also saturated with drug Therefore, increasing the conc of the drug above a certain point does not increase the rate of metabolism eg alcohol, phenytoin, aspirin.  Things to note about zero order metabolism: › ↑ in plasma drug conc→ no ↑ in rate of drug metabolism → drug build up in the body→ toxicity. › rate of drug metabolism is constant › rate in is not α rate out › rate of metabolism is independent of drug concentration.  The pharmacokinetics parameters are Volume of distribution (Vd) Clearance (CL) Elimination Rate Constant (K) Half life (t1/2) Loading dose Maintenance dose Steady state  DRUG DOSAGE AND DOSE RESPONSE CURVES
Most drugs act by interacting with a cellular component called a receptor. Although not all drugs act through receptors, many therapeutic drugs exert their effect by combining with an enzyme or transport proteins and interfere with its function. Example inhibitors of angiotensin converting enzyme and serotonin uptake. These sites of drug action are not receptors. Receptors are macromolecules that interact with a drug and initiate a chain of events that bring about a resonse.  AFFINITY: A drug which is able to fit into a receptor is said to have affinity for that receptor. EFFICACY: Is the ability of a drug to produce an effect at a receptor site