Pharmacokinetics Lectures PDF

OUR LADY OF FATIMA UNIVERSITY College of Pharmacy BIOPHARMACEUTICS & PHARMACOKINETICS PHBP 311 Metabolism – Principles and Factors WEEK 4 UNIT OUTCOMES ► At the end of this module, the students are expected to: 1. Identify the dif...

OUR LADY OF FATIMA UNIVERSITY College of Pharmacy BIOPHARMACEUTICS & PHARMACOKINETICS PHBP 311 Metabolism – Principles and Factors WEEK 4 UNIT OUTCOMES ► At the end of this module, the students are expected to: 1. Identify the different pathways of metabolism 2. Identify and classify the major drug metabolizing enzymes and its implication on drug action 3. Identify the most important enzyme inducers and inhibitors 4. Describe other factors that influence the rate and extent of biotransformation UNIT OUTLINE 1. Identify the different pathways of metabolism 2. Identify and classify the major drug metabolizing enzymes and its implication on drug action 3. Identifythe most important enzyme inducers and inhibitors 4. Describeother factors that influence the rate and extent of biotransformation CHECKLIST ❑ Read course outcomes ❑ Read course guide prior to class attendance ❑ Proactively participate in discussions ❑ Watch videos related to the topic ❑ Participate in discussion board (Canvas) ❑ Answer and submit course unit tasks  LADM ER Metabolism Metabolism ► biotransformation ► The enzymatic or biochemical transformation of the drug substance to (usually less toxic) metabolic products, which may be eliminated more readily from the body ► Drug metabolism refers solely to the chemical biotransformation of a drug by the biological environment. ► In biochemistry, total amount of the biochemical reactions involved in maintaining the living condition of the cells in an organism. Metabolism serves three principal purposes: 1. to supply energy for body functions and maintenance 2. to break down ingested (foreign) compounds and biosynthesis of more complex molecules 3. To make compounds more polar water soluble Xenobiotics – foreign chemicals ANATOMY AND PHYSIOLOGY OF THE LIVER Liver ► Principal site of metabolism ► Both synthesizing and eliminating organ 1. Liver lobule ► Basic unit of the liver A. Large right lobe B. Left lobe 2. Hepatic artery ► Perfused blood in the liver ► Carries oxygen in the liver 3. Hepatic portal vein ► Collects blood from the various segments of the GIT that perfuse in the liver ► Carries nutrient to the liver 4. Sinusoids ► Large vascular capillaries ► Facilitates drug and nutrient removal before the blood enters the general circulation ► Metabolizing enzymes ► occur in the soluble, the mitochondrial, or the microsomal fractions ► metabolism can also take place in the bloodstream to some extent because some enzymes produced by the cell spill over into the extracellular fluid  Metabolism ► INACTIVE METABOLITES ► METABOLITES THAT RETAIN SIMILAR ACTIVITY ► METABOLITES WITH ALTERED ACTIVITY ► BIOACTIVATED METABOLITE ► REACTIVE METABOLITE 1. INACTIVE METABOLITES Procaine (local anesthetic) → p-aminobenzoic acid 6-mercaptopurine (immunosupressant) → 6-mercaptopuric acid amphetamine (CNS stimulant)→ phenylacetone phenobarbital (anticonvulsant)→ hydroxyphenobarbital 2. METABOLITES THAT RETAIN SIMILAR ACTIVITY Imipramine (Anti-depressant) → desipramine acetohexamide (antidiabetic) → L-hydroxyhexamide Codeine (analgesic,antitussive) → Morphine Procainamide (antiarryhtmic) → N-acetyl procainamide phenylbutazone (anti-inflammatory) → oxyphenbutazone 3. METABOLITES WITH ALTERED ACTIVITY Iproniazid (Anti-depressant) → isoniazid Retinoic acid (Vitamin A derivative) → Isotretinoin 4. BIOACTIVATED METABOLITE Enalapril → Enalaprilat (anti-hypertensive) Sulindac → active sulfide (anti-inflammatory) Levodopa → dopamine (anti-Parkinson drug) Prontosil → sulphanilamide (antibacterial drug) Hetacillin → ampicillin (antibacterial) Sulfasalazine → sulfapyridine + aminosalicylic acid (ulcerative colitis) 5. REACTIVE METABOLITE Acetaminophen → NAPQI Benzopyrene → Reactive epoxide metabolite Malathion → parathion  Metabolism ► Mostly, drug metabolites are more ionized than their parent structures and are, therefore in the form of water-soluble salts which have reduced lipid solubility. Three major components of a drug biotransformation 1. Reactant (drug or xenobiotics) 2. Product (metabolite) 3. Reaction catalyst (enzymes) REACTANT ENZYME PRODUCT PATHWAYS TO DRUG METABOLISM Pathways of Drug Metabolism Phase 1 Reaction Phase II Reaction ► Non-synthetic ► Synthetic or conjugation ► Which small chemical changes ► A molecule provided by the occur in one or more functional body is added to the drug groups of drug ► Glucoronidation, sulfation, ► Oxidation, reduction and amino acid conjugation, hydrolysis acetylation, glutathione conjugation, methylation Pathways of Drug Metabolism Phase 1 A. Oxidation Phase 2 B. Reduction A. Glucoronidation C. Hydrolysis B. Sulfate Conjugation C. Methylation D. Acetylation E. Glutathione Conjugation F. Amino Acid Conjugation Phase 1 Metabolism Cytochrome P450 ► Monooxygenase/mixed function oxidase enzyme ► Most common and most important class of enzymes for phase 1 metabolism ► responsible for REDOX rxn ► Located primarily in the endoplasmic reticulum  Phase 1 Reaction most common isoform INDIVIDUAL GENE Involve in drug SUBFAMILY ROOT metabolism CYP 1 FAMILY CYP 2 CYP 3 CYP 4  Acetaminophen, Diazepam, Fluvoxamine, Theophylline, CYP1A2 Methadone, Propranolol CYP2B1 Chlorpheniramine CYP2B6 Bupropion, Cyclophosphamide CYP2C8 Diclofenac, Omeprazole, Paclitaxel CYP2C9 S-warfarin, Tolbutamide, Ibuprofen CYP2C19 S-mephenytoin, Cimetidine, Fluoxetine CYP2D6 Antidepressants, Beta blockers, Captopril CYP2E1 Ethanol, Felbamate, GA, INH, ondansetron Acetaminophen, BZD, Busulfan, Amitriptylline, Amiodarone, CYP3A4 Azole antifungals  FUNCTIONALIZATION PHASE PHASE 1 ASYNTHETIC PHASE ADDING OR UNMASKING FUNCTIONAL GROUP AROMATIC HYDROXYLATION (PHENYTOIN) ALIPHATIC HYDROXYLATION(PHENOBARBITAL) OLEFENIC MOST DOMINANT PHASE 1 HYDROXYLATION(CARBAMAZEPINE) + O2 BENZYLIC HYDROXYLATION A. OXIDATION -H (TOLBUTAMIDE, IMIPRAMINE) LEORA ALLYLIC HYDROXYLATION (PENTAZOCINE, HEXOBARBITAL) HYDROXYLATION N-ALPHA TO A CARBONYL (DIAZEPAM, DOPAMINE)  OXIDATIVE DEAMINATION (AMPHETAMINE) N-DEALKYLATION (MORPHINE, EPHEDRINE) N-OXIDATION (ACETAMINOPHEN) O-DEALKYLATION (CODEINE, PAPAVERINE) S-DEALKYLATION (6- METHYLMERCAPTOPURINE) OXIDATION S-OXIDATION (CHLORPROMAZINE) DESULFURATION (THIOPENTAL) DEHALOGENATION (HALOTHANE, CHLORAMPHENICOL) OXIDATION OF ALCOHOLS (ETHANOL, ESTRADIOL) OXIDATION Of ALDEHYDE (ACETALDEHYDE, PGE2)  A. CARBONYL REDUCTION +H (ACETOHEXAMIDE) B. REDUCTION - O2 B. AZOREDUCTION (SULFASALAZINE) GEROA C. NITROREDUCTION (CHLORAMPHENICOL) A. ESTER HYDROLYSIS (PROCAINE) C. HYDROLYSIS ADDITION OF H2O B. AMIDE HYDROLYSIS (LIDOCAINE) RBC in metabolism ► to exchange oxygen for carbon dioxide at the tissue level ► equipped with numerous enzyme systems (energy supply for the cell protection of hemoglobin and cell membrane from oxidation) ► has no nucleus, no mitochondria, no ribosomes or m-RNA, it cannot synthesize protein ► no cytochrome P-450 PHASE 2 Phase 2 reactions ► AKA : Conjugation or Synthetic reactions ► conjugation or addition of a molecule from an endogenous substance to the parent compound. ► Enzymes: Transferase ► Cofactor – pertain to the substances that are needed by certain enzymes to carry out catalysis of a particular chemical reaction. GLUCURONIC ACID CONJUGATION ► AKA : Glucuronidation ► is a condensation of the drug or its primary metabolite with d-glucuronic acid. ► The reaction requires activation of glucoronic acid by synthesis of UDPGA (uridine diphosphate glucoronic acid). ► Enzyme: Glucoronyl transferase GLUCURONIC ACID CONJUGATION ► UDPGA is formed from UDP-glucose by a dehydrogenase found in the supernatant fraction of the liver. ► The reaction between UDPGA and the drug is catalyzed by glucuronyl transferase, a solubilized microsomal enzyme found in the liver, kidney, GI tract and skin. GLUCURONIC ACID CONJUGATION ► Alcoholsand phenols 🡪 ether type glucuronides ► aromatic and aliphatic carboxylic acids 🡪 ester type glucuronides GLUCURONIC ACID CONJUGATION ► Glucuronides ► are more water-soluble than the parent structures due to the large hydrophilic carbohydrate moiety ► more acidic than the parent molecule hence are more ionized ► less easily permeate through membranes and are poorly reabsorbed by the kidney tubule ► Excreted in tubular secretion, bile(minor) GLUCURONIC ACID CONJUGATION ► Microsomal drug inducers increase the activity of glucoronyl transferase, whereas microsomal enzyme inhibitors and some drugs reduce or inhibit glucoronyl transferase activity Condition wherein Glucuronidation is altered ► During pregnancy ► glucoronidation is reduced probably due to increase progesterone and pregnanediol levels ► Newborns ► very low glucuronyl transferase activity ► Gray baby syndrome ► kernicterus Gray Baby Syndrome and Kernicterus GLUCURONIC ACID CONJUGATION ► Drugs that compete for glucuronidation or reduce the already low glucuronyl transferase activity can precipitate Kernicterus. ► Treatment:Administration of Phenobarbital to the newborn or the mother before delivery Sulfate conjugation APS PAPS sulfate (adenosine-5- (3-phospho- adenosine-5- phosphosulfate) phosphosulfate) Sulfate conjugation ► Enzyme: sulfotransferases or sulfokinases ► found in the liver, kidney and GI tract ► limited and easily depleted ► with increasing drug doses sulfate conjugation becomes easily saturated Methylation ► minor pathway of metabolism ► of importance for many endogenous substances ► Enzyme: methyl transferase ► S-adenosylmethionine is formed which reacts with the drug in the presence of a methyl transferase ► occur in different tissues, such as liver, brain, kidney, skin, blood cells, glands, nerve fibers and lung Acetylation ► Conjugation of aromatic primary amines, aliphatic amines, amino acids, hydrazines, hydrazides and sulfonamides ► minor pathway of metabolism except for isoniazid and sulfonamides Acetylation ► Enzyme: acetyl transferases ► Expression of acetyl transferase is subject to genetic polymorphism (variation in the expression of genes between populations) ► slow acetylation and fast acetylators ► Newborns are not capable Glutathione conjugation A peptide of glutamyl- cysteine-glycine which is ► Detoxification of free radicals or in the important detoxification of reactive reactive oxygen oxygen and is the main intracellular molecule for ► E: Glutathione S transferaseprotection of cell against electrophilic compounds ►+ vit C Amino acid conjugation ►Glycineconjugation ►Glutamine Glycine conjugation ► most common endogenous amine for conjugation with organic acids (aromatic heterocyclic carboxylic acid drugs and aliphatic acids ► bile acids are conjugated with glycine ► limited Glycine conjugation ► Enzyme: Coenzyme A ► reduced in liver damage ► Reduced in newborn and aged ► Benzoic acid 🡪 hippuric acid Glutamine conjugation ► for conjugation with organic acids such as phenylacetic and related acids Maturation of Metabolic Processes in infant AGE METABOLISM CAPACITY DEVELOPED Birth Sulfation 1st week Reduction, Oxidation 1 month Acetylation 2 months Glucoronidation 3 months Glycine conjugation Glutathione conjugation Cysteine conjugation Enzyme induction Enzyme inhibition ENZYME INDUCTION ► Drug or chemical-stimulated increase in enzyme activity ► Alteration in the enzyme activity in liver microsomes resulting in a faster rate of metabolism ► not a disease; it is an adaptation of the body to exogenous material ►↑enzymes ↑ metabolism ↓ effect AUTO-INDUCTION ►A drug that stimulates its own metabolism EX. Phenobarbital is given repeatedly its metabolism is increased FOREIGN-INDUCTION ► oneenzyme inducer stimulate the rate of metabolism of another drug EX. If a dose of hexobarbital is then given its metabolism is also increased ENZYME INDUCERS P- Phenytoin P- Phenobarbital R- Rifampicin C- Carbamazepine (auto-induction) C- Cigarette smoking C – Chronic alcoholism ENZYME INHIBITION ► May be due to substrate competition o due to direct inhibition of drug metabolizing enzyme ► ↓enzyme↓ metabolism ↑ effect ENZYME INHIBITORS M- Metronidazole E- Erythromycin D- Disulfiram I- Isoniazid C- Cimetidine K- Ketoconazole V- Valproic acid G- Grapefruit juice (Naringin) A- Acute alcoholism FIRST PASS EFFECT ► Aka: PRESYSTEMIC METABOLISM ► initial BIOTRANSFORMATION of an active drug BEFORE reaching the systemic circulation ► reduces the systemic availability of the drug ► IV and PO ► eg. Propranolol, NTG, Morphine, Pentazocine, meperidine, Verapamil LEARNING CHECK Ms. Danvers takes Phenytoin as a maintenance drug for her epilepsy. She was recently diagnosed wit tuberculosis and she was prescribed with Isoniazid. After several days of taking Isoniazid, she had experienced ataxia and slurred speech which are neurotoxic effects of Phenytoin. How can you explain this phenomenon?  LADM E R Excretion Excretion ► thefinal LOSS of the drug substance or its metabolites from the body such as through the kidney (urine), intestines (feces), skin (sweat), saliva, and/or milk. Drug Elimination ► Irreversible removal of drug from the body by all routes of elimination. ► Collective term for metabolism and excretion ►Nonvolatile drugs ► are excreted mainly by renal excretion, into the urine ►Volatile drugs ► gaseous anesthetics, or drugs with high volatility, are excreted via the lungs into expired air Clearance ► theprocess of drug elimination from the body or from a single organ without identifying the individual processes involved. ► thevolume of fluid cleared of drug from the body per unit of time ► mL/min or L/hr. Kidney ► Located in the peritoneal cavity ► most important organ for excretion ► maintain normal fluid volume ► maintain salt and water balance ► with 2 endocrine fxn ► secretion of renin(regulates BP) ► secretion of erythropoetin(RBC production) Regulation of Renal Blood flow Blood flow to an organ is directly proportional to the arteriovenous pressure difference (perfusion pressure) across the vascular bed and indirectly proportional to the vascular resistance. Kidney Nephrons / malphigian bodies responsible for the removal of metabolic waste and the maintenance of water and electrolyte balance PARTS Tubular part Capillary part PCT Glomerulus Bowman's capsule LOH DCT CD ureter  Bowman’s capsule Proximal Convoluted Distal Convoluted Tubule Glomerulus Tubule Loop of Henle 3 kidney processes 1. Glomerular filtration 2. Active tubular secretion 3. Tubular reabsorption Glomerular Filtration ► passive process by which small molecules & drugs are filtered through glomerulus ► filtration of LMW molecules (MW