Review of Pharmacokinetics Jean D. Deupree, Ph.D. Department of Pharmacology University Nebraska Medical Center 3014 DRC 559-4565 jddeupre@unmc.edu

Helpful Hints Be able to use the equations on p. 40-41. Equations will not be provided on the exam You will not be allowed to use a calculator on the exam Be able to use the pharmacokinetic terms which have been defined Be able to calculate the ionization of acid and bases in various biological media Understand the different types of drug-drug interactions that can occur Be able to apply the information. Exam question are not likely to be asked in the same manner as in this review When reviewing old exams make sure you understand why the wrong answers are wrong and the right answers are right

What are the differences between these transport processes? What types of molecules (charged, uncharged, chemical structural requirements) can be transported by these different mechanism? Bulk flow Passive diffusion Facilitated transport Active transport What are the differences between these transport processes?

Comparison of Transport Processes Transport Mechanism Energy Requirement Saturability and Selectivity Passive Diffusion None No Bulk Flow Facilitated Diffusion Yes Active Transport

Henderson-Hasselbalch Equation Know and be able to use these equations Weak acid Conjugate Base Acid Ionized Unionized pH - pKa = Log = Log Weak base pKa - pH = Log Conjugate Acid Base = Log Ionized Unionized

How much of a weak acid (pKa = 3) will diffuse from the stomach to the plasma? pH 2 Plasma pH 7 1 A CB- + H + A 1 CB- + H + 0.1 10,000 Log ------ = pH - pKa [CB-] [A] Log ------ = Log ------ = pH - pKa [CB-] [A] Log ------ = 2-3 = -1 7-3 = 4 [CB- ] [A] = 0.1 1 [CB- ] [A] 10,000 1 = [A] + [CB-] = 1.1 [A] + [CB-] = 10,001

How much of a weak base (pKa = 7) will be absorbed from the stomach? pH 2 Plasma pH 7 1 B + H+ CA+ B + H+ 1 CA+ 100,000 1 Log ------ = pKa - pH [CA+ ] [B] Log ------ = pKa - pH [CA+] [B] Log ------ = [CA+] [B] Log ------ = [CA+] [B] 7 - 2 = 5 7-7 = 0 100,000 1 [CA+] [B] = [CA+] [B] = 1 100,000 = 1 = [CA+] + [B] = 2 [CA+] + [B] = 100,001

When is ion trapping most likely to occur in these areas? Body Fluid Range of pH Breast Milk 6.4-7.6 Jejunum, ileum 7.5-8.0 Stomach 1.92-2.6 Prostatic secretions 6.5-7.4 Vaginal secretions 3.4-4.2 Urine 5.0-8.0 Note: You do not need to memorize the pH values for these tissues

Weak Acids and Bases % Protonation 100 50 pH 1 14 pKa 50 This is a plot of the % protonation of different drugs (weak acids and bases) at different pH. Every cruve on the graph represents a drug with a different pka. The pka of a drug is the pH where the drug is 50% protonated and 50% unprotonated. If you start at the pka and decrease the pH (go towards pH of 1) of the solution, all drugs will become more protonated because you are increasing the hydrogen ion concentration of the medium. If you raise the pH you will decrease the protonation. If the drug is a weak acid what will happen to the degree of ionization as you raise or lower the pH of the medium? Go to next slide for answer. pH 1 14 pKa

Weak Acid % Charged (Ionized) 100 50 pH 1 14 pKa 50 This is a plot of the degree of ionization of a weak acid as a function of the pH of the medium. Note for a weak acid as you lower the pH (increase protonation) the drug becomes more unionized. As the pH increases the acid molecules start to loses protons and become more ionized (more conjugate base is formed). If the drug is a weak base what will happen to the degree of ionization as you raise or lower the pH of the medium? Go to next slide for answer. pH 1 14 pKa

Weak Bases % Charged (Ionized) 100 50 pH 1 14 pKa 50 This is a plot of the degree of ionization of a weak base as a function of the pH of the medium. Note for a weak base as you lower the pH (increase protonation) the drug becomes more ionized (more conjugate acid is formed). As the pH increases the conjugate acid molecules start to loses protons and becomes more unionized (neutral base is formed). Click on the arrow to return to the slides from lecture pH 1 14 pKa

Will a weak base (pka 8) be more ionized at pH 6 or pH 8? Will a weak acid (pka 4) be absorbed better from a stomach where the pH is 2 or where the pH is 3? pH 2 What percent of a weak acid (pKa 4 ) will be in the ionized form at pH 3 [CB]/[A] = 0.1/1 % in ionized form = 0.1/1.1*100 = 9%

What are the advantages and disadvantages of the different routes of drug administration? See review on page 21

From which sites of drug administration do you get first-pass metabolism? First-pass hepatic metabolism?

What are the factors which affect the distribution of drugs to the different tissues of the body? Blood flow Size of the tissue compartment Ability of the drug to permeate the tissue Extent of ionization in the different tissue compartments Lipid solubility of the drug versus the lipid content of the tissue Extent of plasma protein binding

Which are the vessel rich organs and how does blood flow to an organ affect the distribution of drugs? Vessel-rich organs Brain Liver Heart Kidney Intermediate group Muscle Skin

What plasma proteins do drugs bind to? Basic Drugs Acidic Drugs Albumin Albumin Glycoproteins Lipoproteins

What are the characteristics of drugs which cross the placenta and enter the brain? Drugs which are lipid soluble can cross the blood brain barrier and the placenta by passive diffusion. The more lipid soluble the drug the faster it diffuses into the brain and the faster it crosses the placenta The more aqueous soluble drugs can only enter the brain or the fetus by active transport or facilitated diffusion. Note: Many drugs are not recognized by the transporters.

What are the phase I reactions? Oxidation Reduction Hydrolysis What are the phase II reactions? Conjugation

What are the consequences of drug metabolism? Pro-drug → Drug Drug → Active and inactive metabolite Drug → Reactive intermediate Hydrophobic → Hydrophilic Exposure of functional groups: -OH, -COOH, -NH2

Phase I and Phase II Reactions Elimination Drug Conjugation Active metabolite Conjugation Drug Inactive metabolite Drug Hydrophobic Hydrophilic What types of drug-drug interactions can occur?

What is the principle enzymes involved in Phase I reactions and what are some of the other names for this enzyme? Cytochrome P450 Microsomal mixed function oxidase Microsomal drug metabolizing enzyme

Is there more than one cytochrome P450 isozymes? How is the enzyme be regulated? Induction Inhibition Note: Not all drugs will induce or inhibit the CYP isozymes, only selective isozymes will be inhibited. NOTE: COMPETION OF ONE DRUG FOR ANOTHER ONLY BECOMES A PROBLEM IF THE DRUGS ARE IN HIGH CONCENTRATIONS. Remember for most drugs you are very low on the place of rate of elimination versus drug concentration Note: Induction or inhibition of a CYP 450 isozyme will affect the metabolism of all drugs metabolized by that isozyme

What CYP isozyme is induced by cigarette smoke? CYP1A family Which CYP isozyme is inhibited by grapefruit juice? CYP 3A4 in the wall of the intestine

What is the principle location of CYP isozymes? Liver What are the substrates, enzymes and cofactors of the CYP450 complex? Which isozyme is responsible for metabolism of over 50% of the drugs? CYP 3A4 What is the significance of the CYP 2D6 enzymes? Numerous polymorphism have been found

Cytochrome P450 Enzyme Complex

What are the other types of phase I reactions and what is the primary location of these enzymes? Oxidative reactions Flavin monooxygenases (liver) Amine oxidase (liver and nerve endings) Dehydrogenase (liver) Reductive reactions Liver Intestinal microflora Hydrolytic Esterases: Plasma Amidases: Liver

What are the different types of conjugation reactions that can occur? Glucuronidation Sulfate conjugation N-acetylation Methylation Glutathione conjugation Amino acid conjugation What are the enzymes involved in glucuronidation and hydrolysis of the glucuronide product?

Phase II reactions Glucuronidation R-OH + UDP-glucuronic acid UDP-glucuronyltransferase COOH O O-R (Liver) R-O-glucuronide Bile Kidney Intestine Excretion Beta-glucuronidase Intestinal mucosa Intestinal bacteria R-OH + glucuronic acid

How Does Age Affect Drug Metabolism? Less cyc P450 in Very young Elderly Phase II reactions are usually not affected in the elderly Decreased blood flow to liver in elderly

Factors Affecting Drug Metabolism Pharmacogenetic Genetic differences in the activities of many metabolic enzymes Altered activity of CPY enzymes and enzymes involved in conjugation 2D6 enzymes N-acetyltransferase: Slow vs fast acetylators

What factors determining the amount of drug excreted in the urine? Nephron Glomerular Filtration Tubular Reabsorption Secretion Excretion = glomerular filtration - tubular reabsorption + tubular secretion - passive reabsorption +passive secretion

How do you speed up the renal elimination of a weak acid? Make the urine alkaline

How are drugs or metabolites excreted from liver to bile? Passive diffusion Transporters for anions bile salts cations neutral organic compounds How do you enhance biliary excretion? polar groups high molecular weight Conjugation add polar groups and molecular weight

What drugs go through the enterohepatic recirculation pathway? Drugs which enter the bile and are: Lipid soluble enough to be reabsorbed from the wall of the gut Hydrolyzed in the gut and then reabsorbed Transported by transporters across the wall of the gut

What is the major route by which drugs are eliminated from the body? Kidney What are the minor routes? bile skin lungs sweat glands saliva breast milk

What are the basic types of pharmacokinetic drug- interactions? Involving metabolism Induction of cyc P450 enzymes Inhibition of cyc P450 enzymes Competition of two drugs for the same enzyme Depletion of endogenous compounds used in conjugation reactions Displacement from albumin Competition for transporters Blood flow to an organ Changes in pH of a body of fluid

0 2 4 6 8 10 12 5 4 3 2 1 Peak Drug Level Serum Concentration (mg/ml) Time to Peak Drug Level Time After Drug Administration (hr)

Time After Drug Administration (hr) Serum Concentration (mg/ml) 0 2 4 6 8 10 12 5 4 3 2 1 Duration of Therapeutic Effect Therapeutic Threshold

Time After Drug Administration (hr) Serum Concentration (mg/ml) 0 2 4 6 8 10 12 5 4 3 2 1 Toxic Effect Therapeutic Window Therapeutic Effect

What is the difference between therapeutic window and therapeutic index? Therapeutic window is the plasma concentration range where therapeutic effect occurs without toxic effects Therapeutic index is: TD50/ED50

How do you determine bioavailability? F = AUC for route being studied AUC after IV administration What factors will alter bioavailability?

What is the Volume of Distribution of a drug? Vd (L) = Xo mg Cp (mg/ml)

Plasma Drug Concentration How do you measure the concentration of drug in the plasma at time T = 0? 0 10 20 30 40 50 60 10 8 6 4 2 Time (Hours) Plasma Drug Concentration (μg/ml) Cp

What does it mean if a drug has the following Vd in a 70 kg person?

What are the units for kel? What is meant by t1/2? How many half-lives does it take for the drug concentration to reach steady-state? 4 to 5 half-lives How many half-lives does it take for the drug to be eliminated from the body? What is kel? Rate constant for elimination What are the units for kel? 1/time How does kel change with plasma concentration? It remains constant at low concentrations of drug

Is this drug being eliminated by first order or zero order kinetics? Time (Hrs) Concentration (μg/ml) Plasma Drug 2 4 6 8 100 80 60 40 20 Zero order What is the half-life of this drug? Half-life changes with plasma concentration If you double the dose how long does it take to eliminate the drug from the body? Twice as long What happens to steady state plasma levels if you double the dose? Greater than 2x plasma level

Concentration (μg/ml) Time (Hrs) Concentration (μg/ml) Plasma Drug 2 4 6 8 100 80 60 40 20

Effects of Doubling Dose Zero Order Kinetics 25 20 Plasma Concentration (mg/L) 15 10 8 16 24 32 40 48 56 64 72 80 88 96 Time (hrs) NOTE: Drug is entering the body faster than it leaves the body

Is this drug being eliminated by first order or zero order kinetics? 0 10 20 30 40 50 60 10 6 4 2 Time (Hours) Concentration (μg/ml) Plasma Drug 8 1st order What is the half-life of this drug? 10 hours If you double the dose how long does it take to eliminate the drug from the body? 4 to 5 half-lives or 40-50 hours --- time is independent of plasma levels What happens to steady state plasma levels if you double the dose? Plasma levels double

Effects of Doubling Dose 1st Order Kinetics 8 6 Plasma Concentration (mg/L) 4 2 8 16 24 32 40 48 56 64 72 80 88 96 Time (hrs) Note: Drug enters body at the same rate it is leaving the body

Is this drug being eliminated by first order or zero order kinetics? What is the half-life of this drug? 10 hours How long will it take to reach steady state plasma concentration if this drug is given by continual IV infusion? 40 to 50 hours

Concentration (μg/ml) 0 10 20 30 40 50 60 10 6 4 2 Time (Hours) Concentration (μg/ml) Plasma Drug 8

1st Order Zero Order Elimination Exponential Linear Half-life Constant with changing plasma concentrations Changes with plasma concentration Eliminated per unit time Constant Fraction Constant Amount Clearance Rate of elimination/Cp Maximum rate of elimination process Time required to eliminate drug 4-5 Half-lives Based on amount of drug given Double the dose Double plasma concentration Greater than twice plasma concentration

What is meant by the clearance of a drug from the body, how is it calculated and what are the units? Theoretical volume of fluid from which a drug is completely removed in a given period of time. For a drug eliminated by first order kinetics Clearance (CL)(ml/min) = Rate of elimination (mg/min) Concentration (mg/ml)

How is clearance related to Vd and t1/2? CLT = 0.7 X Vd t1/2 If CL changes but not Vd what happens to the t1/2 ? Half-life is inversely proportional to clearance. If volume of distribution does not change and clearance decreases then the half-life will increase.

How does rate of elimination change with drug concentration for a drug eliminated by zero order kinetics? V = Vmax Rate of elimination of the drug from the body is based on the maximum rate of the rate limiting step in the elimination process (metabolizing enzyme, transporter ect.)

What is meant by flow dependent elimination? Rate of clearance is dependent on the rate of blood flow through the organ

What is meant by saturation kinetics? Low doses first order kinetics occur As dose increases the half-life increases and elimination is by Michaelis-Menton kinetics At high doses rate of elimination is dependent on the maximum rate of the elimination pathway What is meant by flow dependent elimination? Rate of clearance is dependent on the rate of blood flow through the organ

Are drugs A or B being eliminated by first or zero order kinetics? 10.0 A 1st order Plasma Concentration µg/ml 1.0 B Zero order 0.1 25 50 75 100 125 150 Time (min) Why isn’t the curve for Drug B linear? Two compartment distribution

Two Compartment Model 1st Order Plasma Concentration One compartment a Time (min) a b 25 50 75 100 125 150 Plasma Concentration µg/ml 10.0 1.0 0.1 One compartment Two Compartment 1st Order What is determines the two phases of the curve for Drug B? Alpha-slope is due to redistribution of the drug from the vessel rich compartment to other compartments. Beta-phase is due to metabolism and/or excretion

Two Compartment Model for Drugs Acting on the CNS Note: The more lipid soluble the drug the faster the rate of on set of action More lipid soluble the faster the drug crosses the blood brain barrier Note: `The more lipid soluble the drug the shorter the duration of action The faster the redistribution phenomenon

How do you calculate the maintenance dose for IV administration? Dosing rate (mg/min/kg) = CL X Css How do you calculate loading dose? LD = Vd x Css How do you calculate the oral dose? Dose = (CLp (ml/min) x Cpavg (mg/ml)) x Dosing interval (Hr) F

How do decreases in body weight affect the following parameters? Vd CL Loading dose Oral dose IV maintenance dose No change Decrease Decrease Decrease Decrease Decrease

Given the following properties of a drug for a 70 kg man: Bioavailability = 0.8 CL = 3.5 L/hr Half-life = 8 hours Effective plasma concentration = 1 mg/ml What is the Vd for a 70 kg male? 0.7 x Vd t1/2 CL = Vd = CL x t1/2 0.7 Vd = 3.5 L/hr x 8 hr 0.7 Vd = 40 L

Given the following properties of a drug for a 70 kg man: Bioavailability = 0.8 CL = 3.5 L/hr Half-life = 8 hours Vd = 40 L Effective plasma concentration = 1 mg/ml What is the t1/2 for an individual whose CL is ½ the normal rate? 0.7 x Vd t1/2 CL = NOTE: If CL is decreased by 50%, t1/2 will increase by 50% = 0.7 x Vd CL t1/2 t1/2 = 0.7 x 40 L 1.75 L/hr t1/2 = 16 hours

Given the following properties of a drug for a 70 kg man: Bioavailability = 0.8 Vd = 40 L CL = 3.5 L/hr Half-life = 8 hours Effective plasma concentration= 1 mg/ml What is the loading dose for a 70 kg male? LD = Vd X Css LD = 40 L x 1 mg/ml x 1000 ml/L x 1 mg/1000 mg LD = 40 mg LD = 40 L x 1 mg/L = 40 mg LD = 40 mg/70 kg = 0.57 mg/kg

Given the following properties of a drug for a 70 Kg man: Bioavailability = 0.8 Vd = 40 L Cl = 3.5 L/hr Half-life = 8 hours Effective plasma concentration= 1 mg/ml What is the dose required for IV maintenance in a 100 kg male? Dose = CL/70 kg x Css x Body Weight Dose = 3.5 L/hr x 1 mg/L x (100 kg/70 kg) Dose = 5 mg/hr

Given the following properties of a drug for a 70 kg man: Bioavailability = 0.8 Vd = 40 L Cl = 3.5 L/hr Half-life = 8 hours Effective plasma concentration= 1 mg/ml What would the oral dose be for a 100 kg if you wanted to give the drug every 8 hours? CL x Cavg x Dosing interval x body weight F x 70 kg Dose = 3.5 L/hr x 1 mg/L x 8 hr x 100 kg 0.8 x 70 kg Dose = Dose = 50 mg every 8 hours

Fluctuations in Plasma Concentration with Dosing Intervals 25 20 Plasma Concentration (mg/L) 15 10 5 8 16 24 32 40 48 56 64 72 80 88 96 Time (hrs)