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Pharmacology and Therapeutics > 4. Pharmacokinetics > Flashcards

4. Pharmacokinetics Flashcards

1
Q

What are the 4 (6) parts of the journey of a drug through the body?

A 
(• Administration)
• Absorption
• Distribution
• Metabolism
• Excretion
(• Voided)
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2
Q

What is the difference between systemic and local administration?

A
  • Systemic - entire organism exposed to drug

* Local - one area exposed to drug

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3
Q

What is the difference between enteral and parenteral administration?

A
  • Enteral - via GI tract e.g. sublingual, buccal, oral, rectal (easier)
  • Parenteral - everything apart from GI tract e.g. IV, inhalation etc. (more skill required)
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4
Q

How does the ionisation of drugs (most of which are weak acids or bases) effect their solubility?

A
  • Ionised - more water soluble

* Non-ionised - more lipid soluble

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5
Q

What mechanisms the can transfer of molecules across membranes occur by?

A
  • Passive diffusion (most common)
  • Facilitated diffusion
  • Active transport
  • Pinocytosis
  • Filtration (small water soluble molecules)
  • Paracellular transport
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6
Q

In what 2 ways do drug molecules move around the body?

A
  • Bulk Flow Transfer - move in bulk through the bloodstream to the tissues
  • Diffusion Transfer - molecule by molecule over short distances
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7
Q

How do lipid and aqueous (polar/ionised) molecules cross lipid barriers?

A
  • Lipid molecules diffuse through lipid barriers
  • Polar molecules diffuse through aqueous pores in the lipid barrier
  • Carrier molecules and pinocytosis can also be used
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8
Q

What does the ratio of ionised to non-ionised drug molecules depend on?

A
  • pH of environment

* pKa of molecules

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9
Q

Use the behaviour of aspirin to describe the pH Partition Hypothesis

A
  • Aspirin is acidic - has pKa of 3.4
  • Stomach has pH 1 - less than pKa of aspirin
  • Therefore aspiring is non-ionised in stomach - can diffuse easily across lipid bilayer
  • pH in small intestine is higher than pKa of aspirin - becomes ionised - slower absorption
  • Ratio of ionised:non-ionised dictated by pH of environment relative to pKa of drug
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10
Q

What is ion trapping?

A
  • The ionised form goes through the liver into systemic circulation
  • Aqueous environment and trapped in ionised form
  • Ion trapping creates another barrier to absorption
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11
Q

What 4 main factors influence drug distribution?

A
  • Regional blood flow
  • Extracellular binding (plasma protein binding)
  • Capillary permeability
  • Localisation in tissues
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12
Q

Outline the relationship between perfusion/activity of tissues with exposure/distribution?

A
  • Well perfused tissues - exposed to higher concentration of drug
  • Some tissues increase in perfusion when activity increases e.g. skeletal muscle
  • Highly metabolically active tend to have a greater blood flow & denser capillary network
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13
Q

What proportion of acidic drugs bind to plasma proteins?

A

50-80%

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14
Q

What form of the drug can albumin bind to?

A

Both ionised and non-ionised

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15
Q

How can ionised aspirin easily pass through endothelial cells?

A

Through small water-filled (aqueous) gaps between the cells

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16
Q

What are the 3 types of capillaries and why are they important in drug distribution?

A
  • Fenestrated - more permeable to drugs
  • Continuous - water-filled gap junctions
  • Discontinuous - large gaps between endothelial cells
17
Q

Where do lipophilic drugs tend to localise and why?

A
  • Fatty tissue e.g. brain, testes

* Not usually highly perfused - so very lipophilic environment

18
Q

Outline drug excretion in the kidneys

A
  • Drugs converted into water soluble molecules filtered in here
  • Most of the excretion of xenobiotics done here
  • However, most drugs enter urine via active secretion (not ultrafiltration) due to large, protein-bound drug complexes
  • Active secretion of acids and bases in proximal tubule
  • Lipid soluble drugs reabsorbed in proximal and distal tubules (passive diffusion)
19
Q

Outline drug excretion in the liver

A
  • Tends to be large molecular weight drugs
  • Drug concentrated in bile
  • Active transport - drugs use systems used for bile acids and glucuronides as they are non-polar and have a large molecule weight
  • Secreted into intestines
  • Enterohepatic cycling - drug/metabolite may be reabsorbed
  • Drug persists in body for longer than expected
20
Q

How can drugs be excreted, apart from the kidneys and liver?

A
  • Lungs - expired air
  • Gastrointestinal secretions
  • Saliva
  • Swear
  • Milk
21
Q

Why might treatment with IV sodium bicarbonate increase aspirin excretion?

A
  • Increased urine pH
  • Aspirin is ionised (equilibrium shifts right)
  • Aspirin becomes less lipid soluble
  • Less aspirin reabsorbed in proximal and distal tubules
  • Increased rate of excretion
22
Q

What is bioavailability?

A
  • Proportion of the administered drug that is available within the body to exert its pharmacological effect
  • Post-hepatic concentration
23
Q

What is the ‘Apparent Volume of Distribution’?

A
  • The volume in which a drug appears to be distributed

* Indicator of the pattern of distribution

24
Q

What is clearance?

A
  • Volume of plasma cleared of a drug per unit time
  • Related to volume of distribution and the rate at which the drug is eliminated
  • Incorporates excretion through all routes (kidneys, liver etc.)
25
Q 
Which of the following drugs would be least likely to penetrate lipid membranes:
• ionised drug
• non-ionised drug
• protein bound drug
• lipophilic drug
• hydrophilic drug
A

Protein bound drug

26
Q

What is ‘first-order kinetics’?

A
  • The rate of elimination of a drug where the amount of drug decreases at a rate that is proportional to the concentration of drug remaining in the body
  • Log of concentration (y) is proportional to time (x)
  • Most drugs follow first-order kinetics
27
Q

Why is almost impossible to determine the initial concentration of a drug in plasma?

A

Drug is instantaneously removed from the body

28
Q

What is ‘zero-order kinetics’?

A
  • A constant amount of drug is removed per unit time
  • drug concentration (y) vs. time (x)
  • First-order kinetics plotted on the same axis would be a curve, rather than a straight line
  • Zero-order kinetics shows that something is saturating the system - usually an enzyme
  • Alcohol follows zero-order kinetics (due to alcohol dehydrogenase etc.)
  • Rate of removal of the drug peaks and remains constant once enzymes are saturated

Pharmacology and Therapeutics (33 decks)

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