BIOL 2048/9 Pharmacokinetics Lecture 4 PDF

BIOL 2048/9 Pharmacology PHARMACOKINETICS Lecture 4 MyEngagement: Dr. Charles Birts [email protected] Learning Outcomes Explain the pharmacokinetic processes of absorption, distribution, metabolism and excretion (ADME). Apply mathematical princ...

BIOL 2048/9 Pharmacology PHARMACOKINETICS Lecture 4 MyEngagement: Dr. Charles Birts [email protected] Learning Outcomes Explain the pharmacokinetic processes of absorption, distribution, metabolism and excretion (ADME). Apply mathematical principles to the pharmacokinetic processes of absorption, distribution and elimination.  ELIMINATION 3. ELIMINATION Removal of the drug from the body and may involve: Metabolism (where the drug is transformed into a different molecule) Excretion (the drug molecule is expelled in the body’s liquid, solid or gaseous waste)  METABOLISM Lipid solubility is a useful property of drugs because it allows movement across lipid membranes (esp. if given via oral route) BUT lipid-soluble molecules are not easily eliminated and that is where METABOLISM comes into play METABOLISM: required for the elimination of lipid soluble drugs it allows a lipid-soluble drug to be converted to a water-soluble molecule water-soluble molecules are much more readily eliminated from the body in the urine Phases of drug metabolism OH OSO3- PHASE 1 PHASE 2 BENZENE PHENOL PHENYL SULPHATE Oxidation Sulphate conjugation 0% ionised 0.1% ionised 99.99% ionised lipid soluble lipid soluble water soluble PHASE 1 - oxidation, reduction, hydrolysis Introduces a functional side group (eg –OH) into the drug molecule that serve as sites for PHASE 2 metabolism PHASE 2 - conjugation reactions Formation of a covalent bond between the PHASE 1 metabolite and an endogenous substrate Phase 1 Metabolism – Cytochrome-P450 Membrane bound enzymes – in most tissues especially liver – part of the endoplasmic reticulum - They belong to a general class of enzymes called monooxygenases. Binds drug substrate + O2 or CO Phase 1 Metabolism – Cytochrome-P450 Membrane bound enzyme – in most tissues especially liver – part of the endoplasmic reticulum D = Drug M = Metabolite MGA = Metabolite glucuronide MS = Metabolite sulphate Phase 1 Metabolism – Cytochrome-P450 Membrane bound enzyme – in most tissues especially liver – part of the endoplasmic reticulum - They belong to a general class of enzymes called monooxygenases. Binds drug substrate + O or CO 2 Haemoprotein containing Fe Catalyses oxidation of drug Family of related enzymes with differing substrate specificities Phase 1 Metabolism – Cytochrome-P450 Membrane bound enzyme – in most tissues especially liver – part of the endoplasmic reticulum - They belong to a general class of enzymes called monooxygenases. Binds drug substrate + O or CO 2 Haemoprotein containing Fe Catalyses oxidation of drug Family of related enzymes with differing substrate specificities Genetically determined deficiencies Inducible by various compounds: drugs - phenobarbitone environmentally - smoking Inhibited by various compounds: cimetidine, quinine, grapefruit juice Drug extraction and metabolism Endoplasmic reticulum BLOOD FLOW drug Protein drug drug Protein drug BLOOD Space of Disse FLOW Brush border Increases SA Non P450 oxidations MAO R-CH2NH2 R-CHO ALCOHOL R-CH2OH R-CHO DEHYDROGENASE Reductions P450 MEDIATED IN LIVER GUT FLORA Hydrolysis Plasma pseudocholinesterase Tissue esterases Tissue amidases ESTERS R1-CH2.O.CO.-R2 R1-CH2OH + HOOC-R2 AMIDES R1-CH2.NH.CO-R2 R1-CH2NH2 + HOOC-R2 Procaine and Procainamide are similar drugs (Na+ Channels) – BUT…. Procaine is acted upon by ESTERASES, FAST acting = rapid hydrolysis and short duration of action PROCAINE LOCAL ANAESTHETIC ester bond C2H5 CO.O CH2-CH2 - N RAPID HYDROLYSIS H2N IN PLASMA C2H5 t1/2 = 25 mins Procainamide is acted upon by AMIDASES , SLOW acting = slow hydrolysis and longer duration of action PROCAINAMIDE ANTI-ARRHYTHMIC amide bond C2H5 SLOW HYDROLYSIS IN LIVER AND H2N CO.NH CH2-CH2 - N PLASMA C2H5 t1/2 = 3-4 hours Reduction Less common than oxidation reactions and occur at unsaturated Carbon atoms, Nitrogen and Sulphur CHO CH2OH -N=O -NH2 C=C CH-CH -NO2 -NH2 GROUPS =S=O =S -N=N- -NH2 H2N- REDUCED -S-S- -SH HS- Phase 2 metabolic reactions PHASE 2 - conjugation reactions Formation of a covalent bond between the PHASE 1 metabolite and an endogenous substrate Conjugation reactions Group on Reaction Product drug -OH, -COOH Drug -C6H9O7 -NH2 Glucuronidation Drug-O-SO3- -OH, -NH2 Drug-NH-SO3- Sulphation -NH2 , NHNH2 Drug-NH-COCH3 Acetylation Drug-O-CH3 -OH, -NH2 Drug-NH-CH3 Methylation -COOH Drug -CO-NH-AA Amino-acid conjugation Glucuronidation Discovered by Jaffe in 1874 COOH DRUG O O Glucuronic acid C6H9O6 replaces the H in OH -OH, -COOH, -NH2, -SO2NH-, -SH to give water soluble inactive products OH OH Requires activation of carbohydrate (UDPGA) UTP NAD G-6-P G-1-P UDP-Glucose UDP-Glucuronic acid UDP-Glucuronyl transferase (UDPGT)  Several forms  Microsomal in liver + gut + most other tissues Phase 1 Metabolism – Cytochrome-P450 Membrane bound enzyme – in most tissues especially liver – part of the endoplasmic reticulum UDP-Glucuronyl transferase D = Drug M = Metabolite MGA = Metabolite glucuronide MS = Metabolite sulphate Sulphation A sulphate group –SO3- replaces the H in R-OH, ArOH, ArNH2, ArNHOH to give very water soluble inactive - usually - excretory products Sulpho-transferase  Cytosolic enzyme in liver + gut + most other tissues  Found in nearly all species Acetylation Acetate CH3CO2- replaces the H in -NH2, -SO2NH2, -NHNH2  inactivates the functional group  but no real increase in water solubility Requires activation of acetate (AcetylCoA) N-acetyl transferase transfers the acetate to the drug N-acetyl transferase shows a genetic polymorphism i.e. some people are fast acetylators whilst other are slow acetylators. Fast acetylators will metabolise certain drugs faster. Caucasians 50% fast acetylators Japanese 95% fast acetylators Consequences of drug metabolism Once the drug has been metabolised it is a different compound and the activity due to the parent drug is removed The metabolite is a different compound and may have:  no activity (the usual case)  similar pharmacological activity  different pharmacological activity  a toxicological action PHASE 1 METABOLITES  are usually inactive, but may show the types of change in activity given above  frequently undergo phase 2 metabolism prior to excretion PHASE 2 METABOLITES  are nearly always inactive  are usually much more water soluble and readily excreted Variables affecting metabolism SPECIES rats and mice differ from humans  higher cardiac output  greater liver blood flow  higher rates of metabolism GENETICS ENZYME DEFECTS ENVIRONMENT OTHER DRUGS etc P450 SUBSTRATE INDUCER INHIBITOR CYP1A2 Theophylline Omeprazole Fluoroquinolones CYP2A6 Coumarin CYP2C9 Ethoxycoumarin Phenobarbitone Ketoconazole CYP2C19 Omeprazole Phenobarbitone Fluoxetine CYP2D6 Dextromethorphan Quinidine CYP2E1 Ethanol Ethanol Disulfiram CYP3A4 Terfenadine Carbamazepine Ketoconazole Variables affecting metabolism SPECIES rats and mice cf humans  higher cardiac output  greater liver blood flow  higher rates of metabolism GENETICS ENZYME DEFECTS ENVIRONMENT OTHER DRUGS etc especially very young (70 years) DISEASE especially liver disease Variables affecting metabolism Elderly: Size of liver and blood flow decreases with age Reduced phase 1 metabolic reactions especially relevant when prescribing lipid soluble drugs (undergo extensive phase 1 metabolism) Young: Drug metabolising enzymes are immature in the neonatal liver – glucuronidation First-pass metabolism is low Liver disease: Anatomical changes that impair rapid uptake of lipid-soluble drugs Intracellular enzyme activity is reduced  EXCRETION The drug molecule is expelled in the body’s liquid, solid or gaseous waste Routes of excretion LUNGS 1. Volatile compounds only e.g. general anaesthetics, alcohol KIDNEYS - urine 1. Low molecular weight (20kDa) + water soluble (glomerular F) 2. Active secretion + filtration of unbound (tubular secretion) 3. Reabsorption – affected by urine pH (reabsorption) BILE 1. High molecular weight (>50kDa) – esp. conjugates 2. Biliary metabolites may be metabolised in gut lumen and reabsorbed - entero-hepatic circulation Routes of excretion Bowmans Capsule Filtration of unbound drug Kidneys Proximal tubule Active secretion of acids + bases Loop of Henle pH dependent reabsorption Renal excretion 3 Processes: 1. Glomerular filtration 2. pH Dependent reabsorption 3. Renal tubular secretion 1. Glomerular filtration  Small molecules are filtered through pores 7-8 nm diameter  The glomerular filtrate contains 20% of the plasma volume delivered to the glomerulus and so 20% of all water-soluble, low-molecular-weight compounds.  Protein-bound drug is not filtered, but some dissociates when water is reabsorbed from the tubule, and the “new” free drug is filtered the next time it is carried back to the kidney 2. pH Dependent reabsorption  All lipid soluble compounds are reabsorbed from the tubule and returned to the circulation until they are metabolised to water-soluble products  Urine has a normal pH of about 6 compared with plasma at pH 7.4  Drug concentrates in the fluid in which it is most ionised (pH.partioning)  Weak acids can be reabsorbed into the plasma  Weak bases are excreted in the urine  NaHCO3 increases urine pH (alkalinases) and enhances elimination of acids  NH4Cl reduces urine pH (acidifies) and enhances elimination of bases WEAK ACID Urine pH 5-6 Plasma pH 7.4 ionised unionised unionised ionised Overall DIRECTION OF MOVEMENT WEAK BASE ionised unionised unionised ionised Overall DIRECTION OF MOVEMENT 3. Renal tubular secretion Blood Urine  There are different Free drug drug transporters for acids and bases Bound Free drug drug  An active process that Proximal tubule can strip a drug from Active secretion Bound of acids + bases protein binding sites Free drug drug  Many drug conjugates Bound are substrates for active Free drug drug secretion Bound drug Free drug Bound Active transport  Biliary excretion Drugs are eliminated in bile mixed with bile salts Large drugs or drug conjugates are eliminated in bile >32.5kDa mol.wt in rats >50kDa mol. wt in humans Elimination depends on the release of bile from the gall-bladder (which is not present in the rat) Entero-hepatic circulation The drug may be reabsorbed from the intestine and carried back to the liver via the hepatic portal vein A drug conjugate may be hydrolysed in the intestine back to the drug or an hydroxy- metabolite and then carried back to the liver via the hepatic portal vein Entero-hepatic circulation Entero-hepatic circulation of the drug helps to maintain concentrations in the systemic circulation To general circulation LIVER Drug Drug Conjugate STOMACH BILE Drug Conjugate Drug gut bacteria Lower Hydrolyse conjugate bowel Entero-hepatic circulation Initial peak due to absorption of oral dose Secondary peak(s) due to absorption of the bolus of drug (or conjugate) released into gut when the gall-bladder contracts Drug [ ] in plasma Drug with EHC Drug without EHC 0 6 12 18 24 30 36 42 48 Time in hours Drug with EHC provides a reservoir from which some continuously escapes to maintain plasma levels Learning Outcomes Lectures 1-4 Explain the Biological Basis of pharmacokinetic processes of Pharmacokinetics absorption, distribution, metabolism and Next time… excretion (ADME). Mathematical Basis of Pharmacokinetics Apply mathematical principles to the pharmacokinetic Recommended Text Chapter 9: Absorption and distribution of drugs Chapter 10: Drug metabolism and elimination Chapter 11: Pharmacokinetics Jove summary videos