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[AMS] Physiology and Pharmacology > S10-11) Pharmacokinetics > Flashcards

S10-11) Pharmacokinetics Flashcards

1
Q

Distinguish between pharmacodynamics and pharmacokinetics

A
  • Pharmacokinetics – what the body does to the drugs
  • Pharmacodynamics – what the drugs do to the body
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2
Q

What are the four main processes in pharmacokinetics?

A

- Drug in:

I. Absorption

II. Distribution

  • Drug out:

I. Metabolism

II. Elimination

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3
Q

What are the two forms of drug administration?

A
  • Enteral – delivery into internal environment of body (GI tract)
  • Paraenteral – delivery via all other routes that are not the GI
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4
Q

Identify the different types of drug administration

A
  • Paraenteral: intrathecal, intramuscular, transdermal, inhalation, intravenous, subcutaneous
  • Enteral: rectal, oral, sublingual

Mnemonic: OI, IT IS SIR

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5
Q

How are drugs absorbed?

A

Drug mixes with chyme, then enters small intestine where most absorption occurs

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6
Q

Which processes facilitate drug absorption?

A
  • Passive diffusion
  • Facilitated diffusion
  • Primary / secondary active transport
  • Pinocytosis
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7
Q

Describe drug absorption through passive diffusion

A

Lipophilic drugs (weak acids/bases) diffuse passively into GI capillaries

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8
Q

Describe drug absorption through facilitated diffusion

A
  • Molecules with net ionic charge can be carried across GI epithelia
  • Passive process based on electrochemical gradients
  • Solute carrier transporters are either OATs(-) and OCTs(+) which are highly expressed in GI, Hepatic and Renal Epithelia
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9
Q

Describe drug absorption through secondary active transport

A
  • SLCs facilitate this process (no ATP)
  • Transport driven by pre-existing electrochemical gradient across GI epithelial membrane
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10
Q

What are the physicochemical factors affecting drug absorption?

A
  • Surface area
  • Drug lipophilicity and pKa
  • Density of SLC expression in GI
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11
Q

What are the physiological factors affecting drug absorption?

A
  • Blood Flow: increases after a meal
  • GI Motility: decreases after a meal
  • Food/pH
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12
Q

What are the factors affecting drug absorption in terms of First Pass Metabolism by GI and Liver?

A
  • Gut Lumen: enzymes can denature drugs
  • Gut Wall/Liver: some drugs are metabolised by cytochrome P450s (phase I & phase II enzymes)
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13
Q

What is bioavailability?

A

Bioavailability is the fraction of a defined dose which reaches its way into a specific body compartment

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14
Q

What is the most common means of calculating bioavailability?

A
  • CVS (circulation) is most common reference compartment
  • Most common comparison is (O)/(IV)
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15
Q

How does one calculate oral bioavailability?

A

It is the amount of drug administered (AUC) via the oral route divided by the AUC via the IV route

I.e AUCO/AUCIV

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16
Q

Why do drugs distribute?

A

To reach and interact with therapeutic and non-therapeutic target

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17
Q

Outline the processes involved in the first stage of drug distribution

A
  • Bulk flow – large distance via arteries → capillaries
  • Diffusion – capillaries → interstitial fluid → cell membranes → targets
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18
Q

What are the major factors affecting drug distribution?

A

- Drug molecule hydrophilicity

I. Lipophilic drugs freely move across cell membrane

II. Hydrophilic drugs dependent on electrochemical gradients

  • Drug binding to plasma and/or tissue proteins e.g. albumin, lipoproteins, glycoproteins
19
Q

Describe drug binding to plasma and/or tissue proteins

A
  • Only free drug molecule can bind to target site(s)
  • Binding in plasma/tissue decreases free drug available for binding
  • Binding forces not strong (bound/unbound in equilibrium)
20
Q

What are the three main body fluid compartments?

A
21
Q

How can one calculate volume of distribution?

A

Vd = Drug dose/ [Plasma Drug]t=0

22
Q

What assumptions are made when calculating Vd?

A
  • Pretends that drug fully distributes throughout the body at time zero
  • Groups all fluid compartments into one compartment
  • Referenced to [plasma] (easiest to measure)
23
Q

What do smaller and larger Vd values indicate?

A
  • Smaller Vd values: lesser penetration of interstitial/intracellular fluid compartment
  • Larger Vd values: greater penetration of interstitial/intracellular fluid compartment
24
Q

Which units are used to measure apparent volume distribution?

A
  • Litres (assume ‘standard’ 70 kg body weight)
  • Litres/kg (more referenced to individual patient body weight)
25
Q

What are the factors affecting Vd?

A
  • Changes in regional blood flow
  • Pregnancy
  • Paediatrics/neonates/pre-term
  • Geriatrics
  • Cancer patients
26
Q

Describe the features of hepatic drug metablism

A
  • Drug metabolism occurs in liver via Phase 1 and II enzymes
  • Enzymes are expressed throughout body tissues
27
Q

Describe the action of Phase I and II enzymes

A
  • Metabolise (& inactivate) drugs
  • Increase ionic charge to enhance renal elimination
28
Q

How is Phase 1 Metabolism carried out by cytochrome P450 enzymes?

A
  • Phase 1 enzymes (CYP450s) catalyse redox; dealkylation; hydroxylation reactions
  • CYP450s are generalists – metabolise very wide range of molecules
29
Q

What are the properties and actions of metabolised drugs after Phase I metabolism?

A
  • Metabolised drugs have increased ionic charge
  • Metabolised drug eliminated directly or go onto Phase II
30
Q

How is Phase II Metabolism is carried out by hepatic enzymes?

A
  • Phase II enzymes (cytosolic enzymes) exhibit more rapid kinetics than CYP450s
  • Phase II metabolised drugs further increase ionic charge & enhance renal elimination
31
Q

What are the factors affecting drug metabolism?

A
  • Age
  • Sex e.g. alcohol metabolism slower in women
  • General health/dietary/disease (hepatic, renal, CVS)
32
Q

Identify the main routes of drug elimination

A
  • Main route of drug elimination is kidney
  • Other routes: bile; lung; breast milk (deliver to baby); sweat, tears; genital secretions; saliva
33
Q

What processes are involved in renal excretion?

A
  • Glomerular filtration
  • Active tubular secretion
  • Passive tubular reabsorption
34
Q

Briefly describe glomerular filtration

A

Unbound drug enters glomerulus via Bowman’s capsule for filtration

35
Q

Briefly describe proximal tubular secretion

A
  • Water reabsorbed along tube length
  • Lipophilic drugs diffuse out renal tubules back into plasma
36
Q

What is clearance?

A

Clearance is defined as the volume of plasma that is completely cleared of the drug per unit time (measured in ml/min or ml.min-1)

37
Q

Why is clearance better thought of as ‘Apparent Rate of Elimination’?

A

Renal volume of plasma cannot be ‘completely’ cleared of drug via glomerular filtration/ tubular secretion

38
Q

What is Drug Half Life (t1/2)?

A

Drug half life is 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

39
Q

Explain the relationship between drug half life, volume of distribution and clearance

A
  • t1/2 is dependent on Vd and CL
  • If CL stays same and Vd increases then t1/2 also increases
  • If CL increases and Vd stays the same then t1/2 decreases
40
Q

Explain the principles of linear elimination kinetics

A

The rate of metabolism / excretion is proportional to [drug] per unit time

41
Q

What are the features of linear elimination kinetics?

A
  • Plenty of Phase I/II enzyme sites
  • Plenty of OAT/OCT transporters
42
Q

What happens when elimination processes become saturated?

A
  • Saturated processes = rate limited
  • In elimination kinetics, this is referred to as saturated / zero order
43
Q

Outline the features of zero order kinetics

A
  • Drugs at/near therapeutic dose with saturation kinetics
  • Fixed rate of elimination per unit time
  • Relatively small dose changes can produce large increments in plasma [drug]

[AMS] Physiology and Pharmacology (13 decks)

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