Session 7 - pharmacokinetics

Question 1

• Understand the importance of Pharmacokinetics applied to clinical practice
• Used the Interactive Clinical Pharmacology website icp.nz
• Recognise main routes of drug administration into the body
• Understand factors affecting drug Absorption and Distribution
• Recognise Equations used for Bioavailability and Volume of Distribution
• Understand factors affecting drug Metabolism and Excretion
• Recognise equations used for Clearance and drug Half Life
• Understand difference between Linear and Non-Linear Kinetics

Question 2

What is Pharmacokinetics

Answer 2

What the body does to drugs:

• Absorption (drug in)
• Distribution (drug in)
• Metabolism (drug out)
• Elimination (drug out)

Question 3

Recognise main routes of drug administration into the body LO

Answer 3

Enteral: Delivery into internal environment of body - GI Tract

Parenteral: Delivery via all other routes that are not the GI

Question 4

Understand factors affecting drug Absorption and Distribution LO

Drug Absorption

• Oral route - majority of formulations most convenient
• Normally little absorption in stomach - SA = 0.75 - 1m2
• Drug mixes with chyme enters small intestine
• Small intestine = 6-7 m in length x 2.5 cm diameter
• Total SA for absorption = 30-35 m2
• Constant GI movement - mixing - presenting drug molecules to GI epithelia

Question 5

Drug Absorption Small Intestine

Typical Transit Time = ?

Varying motility = ?

Weakly acidic pH = ?

Answer 5

3-5 hrs

1-10 hrs

6 -7 pH

Question 6

1. Molecular Level how do drugs pass the plasma membrane in the SI

Answer 6

1. • Passive Diffusion
     - Facilitated Diffusion
     - Primary / Secondary Active Transport
     - Pinocytosis

Question 7

Passive Diffusion 1

• Common mechanism for lipophilic drugs weak acids/ bases
• Lipophilic drugs e.g. steroids diffuse directly down concentration gradient into GI capillaries

Passive Diffusion 2

• Weak acids/bases protonated /deprotonated species can diffuse
• * E.g. Valproate : Anti -Epileptic Drug weak acid pKa = 5
• In gut at pH 6 - 10 % Valproate protonated - Lipophilic
• Lipophilic species crosses GI epithelia
• Over transit time 4-5 hrs and very large GI SA valproate diffuses into GI capillary bed

Question 8

Drug Absorption: Facilitated Diffusion: Solute Carrier (SLC) Transport

• Molecules (or Solutes) with nett ionic + or - charge within GI pH range can be carried across GI epithelia
• Passive process based on electrochemical gradient for that (solute) molecule SLCs are either OATs and OCTs
• Large family – expressed in all body tissue
• Pharmacokinetically important for drug absorption and elimination
• Highly expressed in GI, Hepatic and Renal Epithelia

Question 9

Secondary Active: Solute Carrier (SLC) Transport

• SLCs can also enable drug transport in GI by Secondary Active Transport
• Not utilise ATP - Transport driven by pre-existing electrochemical gradient across GI epithelial membrane e.g. Renal OATs and OCTs

Example

• Fluoxetine/Prozac - SSRI antidepressant co-transported with Na+ ion
• B-lactam antibiotics/Penicillin - co-transported with H+ ion

Question 10

Understand factors affecting drug Absorption and Distribution LO

1. Physicochemical Factors:
2. GI Physiology:
3. First Pass Metabolism by GI and Liver:

Answer 10

1. • GI length /SA

• Drug lipophilicity / pKa
• Density of SLC expression in GI

2. • Blood Flow: Increase post meal – drastically reduce shock/anxiety exercise

• GI Motility: Slow post meal - rapid with severe diarrhoea
• Food /pH: Food can reduce/increase uptake. Low pH destroy some drugs

3. • Gut Lumen: Gut/Bacterial Enzymes - can denature some drugs

• Gut Wall/Liver (larger expression) : Some drugs metabolised by two major enzyme groups Cytochrome P450s:

• Phase I Enzymes Conjugating
• Phase II Enzymes

Question 11

‘First Pass’ metabolism:

Answer 11

Reduces availability of drug reaching systemic circulation - therefore affects therapeutic potential

Question 12

Recognise Equations used for Bioavailability and Volume of Distribution LO

Drug Absorption: Bioavailability

1. Bioavailability Definition:
2. What is most common reference compartment
3. How do you work out bioavailability?

Answer 12

1. Fraction of a defined dose which reaches its way into a specific body compartment
2. Cvs/circulatory
3. IV bolus = 100%
   - No physical/metabolic barriers to overcome

• For other routes - compare amount reaching CVS by other route referenced to intravenous bioavailability
• Most common comparison oral or (O)/(IV)

Question 13

How do you work out Oral Bioavailability (F)?

Answer 13

Question 14

What is the first stage of drug distrubution?

Answer 14

First stage

• Bulk flow - Large distance via arteries to capillaries
• Diffusion - Capillaries to interstitial fluid to cell membranes to targets
• Barriers to Diffusion - Interactions /local permeability/non- target binding

Question 15

Understand factors affecting drug Absorption and Distribution LO

1. Major factors affecting drug distribution:

Answer 15

Drug Molecule Lipophilicity/Hydrophilicity

• If drug is largely lipophilic can freely move across membrane barriers
• If drug is largely hydrophilic journey across membrane barriers dependent on factors described for Absorption
• Capillary permeability
• Drug pKa & Local pH
• Presence of OATs/OCTs

Degree of drug binding to plasma and/or tissue proteins

• In circulation many drugs bind to proteins e.g.

• Albumin Globulins
• Lipoproteins Acid glycoproteins

Question 16

What are the differing levels of capillary permeability?

Answer 16

• Enables variation in entry by charged drugs into tissue interstitial fluid
• Then on to target site (s)

Question 17

Elaborate on why Degree of drug binding to plasma and/or tissue proteins affects distrubution of the drug

Answer 17

• Only free drug molecule can bind to target site(s)
• Binding in plasma/tissue decreases free drug available for binding
• Plasma/Tissue protein bound drug acts as ‘reservoir’
• Binding forces not strong – bound/unbound in equilibrium
• Binding can be up to = 100%

(Aspirin = 50%)

Question 18

Simplify body fluid compartments in to model with three main compartments

Answer 18

Question 19

Answer 19

Question 20

Increasing Penetration by Drug into Interstitial and Intracellular Fluid Compartments Leads to?

Answer 20

• Decreasing Plasma Drug Concentration
• Increasing Vd

Question 21

Recognise Equations used for Bioavailability and Volume of Distribution LO

1. What is Vd?
2. Smaller V_d values - ?
3. Larger Vd values - ?
4. What is the equation for volume of distrubution?
5. What is the units for V_d ?

Answer 21

1. Summarises movement out of Plasma -> Interstitial -> Intracellular Compartment
2. less penetration of Interstitial/Intracellular Fluid Compartment
3. less penetration of Interstitial/Intracellular Fluid Compartment
4. Look at the image

Question 22

When can V_d be affected?

Answer 22

Question 23

Drug Elimination
Elimination

• Term used to cover both Metabolic and Excretory Processes
• Both ‘flow’ processes closely integrated to optimise drug removal
• Elimination removes both exogenous and endogenous molecular species
• Evolutionary advantage in recognising xenobiotics – potential toxins
• Protective and Homeostatic function

Question 24

Understand factors affecting drug Metabolism and Excretion LO

1. Hepatic Drug Metabolism: Phase 1 and II

• Drug Metabolism largely takes place in Liver via ?
• Enzymes expressed ?
• Very large hepatic reserve – also ‘first port of call’ after GI absorption

2. Phase I and II Enzymes function and why?

Answer 24

1. Phase 1 and II enzymes

throughout body tissues

2. • Metabolise drugs - increase ionic charge enhance renal elimination

• Lipophilic drugs diffuse out renal tubules back into plasma
• Once metabolised - drugs usually inactivated *

Question 25

Drug Metabolism: Phase 1 1. Phase 1 Metabolism is carried out by Cytochrome ? 2. Phase 1 enzymes collectively refer to as ? 3. Large group of \> 50 isozymes located on ? 4. Catalyse: ? 5. CYP450s are versatile generalists, what does that mean? 6. What do the CYP450s do? 7. What happens to the drugs CYP450s have ? 8. \* Some ‘pro-drugs’ activated by Phase I metabolism to active species

Answer 25

1. P450 Enzymes 2. CYP450s 3. external face of ER 4. redox; dealkylation; hydroxylation reactions 5. metabolise very wide range of molecules 6. Metabolised drugs have increased ionic charge 7. Metabolised drug eliminated directly or go onto Phase II

Question 26

Drug Metabolism: Phase 1 Phase 1 Metabolism can activate prodrugs Some ‘pro-drugs’ activated by Phase I metabolism to active species 1. Example: ?

Answer 26

1. Codeine to Morphine In metabolisers = 0-15% Codeine metabolised by CYP2D6 to Morphine Morphine x 200 Codeine affinity for Opioid µ-Receptor CYP2D6 exhibits genetic polymorphism

Question 27

Drug Metabolism Phase II 1. Phase II Metabolism is carried out by ? Enzymes 2. Phase II enzymes - mainly (location) enzymes 3. Phase II still generalists but ? 4. Enhance hydrophilicity by ? 5. Catalyse: 6. Phase II metabolised drugs further increased ? 7. Phase II metabolism enhances?

Answer 27

1. Hepatic 2. cytosolic 3. exhibit more rapid kinetics than CYP450s 4. Further inc ionic charge - add to Phase I 5. Sulphation - Glucorinadation - Glutathione conjugation - Methylation N-acetylation 6. ionic charge 7. renal elimination

Question 28

Answer 28

Question 29

Cytochrome P450 Enzymes Cytochrome P450 enzymes include three superfamilies 1. Three superfamilies: 2. Isozyme members in each family coded by suffix: e.g. CYP3A4 3. Six isozymes metabolise = 90% prescription drugs 4. Other isozymes exhibit very variable hepatic expression 5. Each isozyme optimally metabolise specific drugs but do show overlap

Answer 29

1. CYP 1, 2 and 3 2.

Question 30

Understand factors affecting drug Metabolism and Excretion LO 1. Many Factors of Direct Clinical Relevance: 2. CYP450s: Induction and Inhibition and Genetic Factors:

Answer 30

1. • Age (Variable patterns in paediatric groups reduced in elderly) * Sex (gender differences drugs e.g. alcohol metabolism slower in women) * General Health/Dietary/Disease - especially Hepatic Renal CVS 2. • Other drugs (Rx/OTC) can induce or inhibit CYP450s • Genetic variability/polymorphism/ non-expression affects CYP450s

Question 31

Phase I Metabolism: CYP450 Induction 1. Concurrent administration of certain drugs (including just the one drug) can induce specific CYP450 isozymes. Induction mechanism via: ? 2. If another drug in body metabolised by induced CYP450 isozyme then its rate of elimination will be ? Plasma levels of drug will ? 3. For patient can have serious therapeutic consequences if levels drop significantly. Induction process typically occurs over ?

Answer 31

1. Inc transcription; inc translation; slower degradation 2. increased, fall 3. 1-2 weeks

Question 32

_Phase I Metabolism: CYP450 Induction_ Give an example of CYP450 Induction:

Answer 32

Carbamezepine (CBZ) * CBZ is an anti-epileptic metabolised by CYP3A4 * CBZ induces CYP3A4 – lowering its own levels affecting control of epilepsy * CBZ needs careful monitoring in first few month post prescription

Question 33

_Phase I Metabolism: CYP450 Inhibition_ CYP450 Inhibition 1. Concurrent administration of certain drugs (including just the one drug) can inhibit specific CYP450 isozymes. Inhibition mechanism via: ? 2. If another drug in body metabolised by inhibited CYP450 isozyme then its rate of elimination will be ? Plasma levels of drug will then ? 3. For patient can have serious side effects consequences if levels rise significantly. Inhibition process occurs within?

Answer 33

1. competitive/non-competitive inhibition 2. slowed down, increase 3. 1 to a few days

Question 34

_Phase I Metabolism: CYP450 Inhibition_ Examples of CYP450 Inhibition: ?

Answer 34

* Grapefruit Juice inhibits CYP3A4 * CYP3A4 metabolises Verapimil used to treat high blood pressure (BP) * Consequence can be much reduced BP and fainting

Question 35

_Phase I Metabolism: Genetic Factors_ Genetic Variation 1. CYP2C9: Not expressed in: 2. CYP2C19: Not expressed in: ? Prescriptive Practice Review • Need to consider safety/efficacy if not metabolised /rapidly metabolised

Answer 35

1. 1% Caucasians; 1% Africans • Metabolises NSAIDs, Tolbutamide, Phenytoin 2. 5% Caucasians; 30% Asians • Metabolises Omeprazole, Valium, Phenytoin

Question 36

_Drug Metabolism: Genetic Polymorphism_ - Genetic Polymorphism: Codeine and CYP2D6 ‘Pro-drugs’ activated by Phase I metabolism to active species Earlier example: Codeine to Morphine - CYP2D6 gene highly polymorphic - CYP2D6 variants categorized into: poor; normal/high; ultrarapid metabolisers Poor - codeine to morphine - may not experience pain relief Ultrarapid - codeine to morphine - lead to morphine intoxication/ADRs Genetic Variation/Polymorphism 1. - CYP2D6: Not expressed in: ? - Example of Polymorphism - Metabolises ?

Answer 36

1. 7% Caucasians; Hyperactive 30% East Africans Codeine, TCAs

Question 37

_Drug Elimination_ 1. Routes of Drug elimination ?

Answer 37

- Main route of drug elimination is **kidney** - Other routes: bile; lung; breast milk (deliver to baby); sweat, tears; genital secretions; saliva

Question 38

1. Renal Excretion. Three Processes:

Answer 38

1. - Glomerular Filtration - Active tubular secretion - Passive tubular reabsorption

Question 39

1. _Glomerular Filtration_ - Glomerulus = ?% renal blood flow - ? drug enter via Bowman’s capsule 2. _Proximal Tubular Secretion_ - Remaining ?% blood via peritubular capillaries - High Expression of ? Function? - Raison d’etre of Phase I and II metabolism - Facilitated Diffusion/Secondary Active Transport - Along tubule length water resorbed - In tubule [Solutes] inc - Therefore lipophilics pass back into blood - Henderson-Hasselbach + If tubular pH and molecule species pKa favourable + Get neutral AH or B species - reabsorbed by blood 3. Distal Tubular Reabsorption Examples: * OATs: ? * OCTs: ? * Transport subject to competition between drugs can effect pharmacokinetics/ therapeutics

Answer 39

1. - 20 - Unbound 2. - 80 - OATs and OCTs & Carry ionised molecules 3. - Urate (Gout); Penicillins; NSAIDs; Antivirals - Morphine; Histamine; Chlorpromazine

Question 40

Recognise equations used for Clearance and drug Half Life LO 1. What is Clearance? 2. Total Drug Clearance consists of that from all routes. For most drugs - Total Body Clearance = ?

Answer 40

1. The rate of Elimination of a drug from the body OR The Volume of Plasma that is completely cleared of the drug per unit time 2. Total Body Clearance = Hepatic Clearance + Renal Clearance

Question 41

Clearance is formally defined as: The Volume of Plasma that is completely cleared of the drug per unit time 1. CL measured in ? 2. But this ‘Volume’ is really referenced to ? 3. Real Plasma Volume is = ? Real Volume of Plasma cannot be ‘completely’ cleared of drug via glomerular filtration/ tubular secretion. In model, CL better thought of as ‘Apparent Rate of Elimination’.

Answer 41

1. ml/min or ml.min-1 2. Vd: the apparent volume of distrubution 3. 3 Litres

Question 42

CL and V_d 1. • Along with the concept of V_d clearance predicts how long drug will stay in body • Clinically Essential for informing: 2. In short answers ‘How long is drug in body and doing any therapeutic good? Together CL & V_d provide estimate of ?

Answer 42

1. - Designing dosing schedule - Therapeutic regimes levels - Minimising ADRs 2. ‘Drug Half-Life’ or t_1/2

Question 43

Drug Half Life 1. Defined as? 2. • T 1/2 is dependent on Vd & CL • If CL stays same and Vd increases then t 1/12 also ? • If CL increases & Vd stays the same then t1/2 ? 3. How do you work out T_1/2

Answer 43

1. The amount of time over which the concentration a drug in plasma decreases to one half of that concentration value it had when it was first measured 2. increases decreases 3. (Image)

Question 44

Question 45

Answer 45

Question 46

Linear Elimination Kinetics: Why are they Linear ?

Answer 46

* The rate of Metabolism or Excretion is proportional to Concentration of Drug * That is if there are Plenty of Phase I/II enzyme sites Plenty of OAT/OCT Transporters • Then the rate of metabolism /transport will be proportional to the number of molecules occupying a catalytic/ carrier site per unit time

Question 47

Why are they Linear ? Hypothetical Examples • Hepatic Phase I CYP450 Metabolism for Pretend Drug o If there are no molecules then the rate is none there is nothing to catalyse/transport o If the plasma concentration is 50 uM then lets say rate is 10 million molecules/second o If the plasma concentration is 100 uM then the rate is ? o If the plasma concentration is 200 uM then the rate is ? o Equally as molecules removed over time the plasma concentration decreases o Catalytic rate then also decreases in this exponential fashion (Linear Kinetics)

Answer 47

20 million molecules/second 40 million molecules/second

Question 48

_Linear Elimination Kinetics:_ What happens when Elimination Processes become Saturated?

Answer 48

* When processes are saturated they become _rate limited_ * They cannot go any faster said to be saturated * Parallels with o Computer processing speed o Car engine o Eating pies, doughnuts or chewing a toffee • When this happens the Elimination kinetics are referred to as Saturated or ***Zero Order***

Question 49

What is this graph showing?

Answer 49

Drug Elimination: Saturation or Zero Order Kinetics

Question 50

What is this graph showing?

Answer 50

Saturated Zero Order Drug Elimination Most drugs exhibit zero order kinetics at higher doses * NOTE : Y axis is Rate of Elimination * As [drug] increases Rate of elimination Levels off * Reaches limit of capacity –cannot chew toffees any faster ! * This has important clinical implications !

Question 51

Clinical Importance: Zero Order Kinetics * Drugs at or near therapeutic dose with saturation kinetics * More likely to result in ADRs/Toxicity * Fixed rate of elimination per unit time * Relatively small dose changes can - Produce large increments in plasma [drug] - Lead to serious toxicity * Half life is not calculable cannot easily predict dosage regimes * ‘Narrowing ‘ therapeutic window * Greater risk of drug-drug interactions due to taking up sites

Question 52

Clinical Importance: Zero Order Kinetics Drug Monitoring Patient Groups * Relatively few drugs in adults with Zero Order Kinetics at therapeutic dose * Problem in elderly /infants with decreased/immature capacity * Polypharmacy - competing at same processes * Similarly problem in seriously ill – cancer liver disease alcoholic * Reduced hepatic renal capacity easier to saturate * Example drugs: Phenytoin; Prozac; Alcohol; MDMA * Paracetamol at high dose (\>20 tablets) saturate Phase I & II – can be fatal

Question 53

Clinical Importance: Zero Order Kinetics Drug Monitoring Patient Groups * Relatively few drugs in adults with Zero Order Kinetics at therapeutic dose * Problem in elderly /infants with decreased/immature capacity * Polypharmacy - competing at same processes * Similarly problem in seriously ill – cancer liver disease alcoholic * Reduced hepatic renal capacity easier to saturate * Example drugs: Phenytoin; Prozac; Alcohol; MDMA * Paracetamol at high dose