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2a- Pharmacology > Principles of pharmacokinetics > Flashcards

Principles of pharmacokinetics Flashcards

1
Q

What is pharmacology?

A

The study of how medicines work and how they affect our bodies

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2
Q

What are the 2 branches of pharmacology?

A

Pharmacokinetics
Pharmacodynamics

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3
Q

What is pharmacokinetics?

A
  • The fate of a chemical substance administered to a living organism
  • What the body does to the drug
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4
Q

What is pharmacodynamics?

A
  • The biochemical, physiological and molecular effects of a drug on the body
  • What the drug does to the body
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5
Q

What is the process of pharmacokinetics?

A
  1. Absorption
  2. Distribution
  3. Metabolism
  4. Excretion
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6
Q

What is absorption?

A

Transfer of a drug molecule from site of administration to systemic circulation

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7
Q

What are different routes of administration?

A
  • IV (intravenous)
  • IA (intra-arterial)
  • IM (intramuscular)
  • SC (subcutaneous)
  • PO (oral)
  • SL (sublingual)
  • INH (inhaled)
  • PR (rectal)
  • PV (vaginal)
  • TOP (topical)
  • TD (transdermal)
  • IT (intrathecal)
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8
Q

Which methods of administration does 100% of it reach the systemic circulation?

A

IV or IA

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9
Q

Why does every route except IV and IA not have 100% of the dose reach systemic circulation?

A

For any other route, drugs must cross at least one membrane to reach systemic circulation

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10
Q

What are 4 mechanisms for drugs to permeate membranes?

A
  1. Passive diffusion through hydrophobic membrane
  2. Passive diffusion aqueous pores
  3. Carrier mediated transport
  4. Pinocytosis
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11
Q

Which type of molecules enter by passive diffusion through hydrophobic membranes?

A

Lipid soluble molecules

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12
Q

Which type of molecules enter by passive diffusion through aqueous pores?

A
  • Very small water soluble drugs (eg. lithium in drugs to treat BPD)
  • Most drug molecules are too big
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13
Q

Which type of molecules enter through carrier mediated transport?

A

Proteins which transport sugars, amino acids, neurotransmitters and trace metals (and some drugs)

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14
Q

What 2 factors affect drug absorption?

A
  1. Lipid solubility
  2. Drug ionisation
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15
Q

Are ionised drugs more or less absorbed?

A

Less

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16
Q

Are lipid soluble drugs more or less absorbed?

A

More

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17
Q

Why are ionised drugs less absorbed?

A
  • Most drugs are weak acids or weak bases with ionisable groups
  • Ionised drug has poor lipid solubility and therefore is poorly absorbed
  • Proportion of ionisation depends on pH of the aqueous environment
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18
Q

Why are so many drugs taken orally?

A

Convenient, cost effective

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19
Q

What factors of the stomach affect oral drug absorption?

A
  • Gastric enzymes - drug molecule may be digested (peptides, proteins)
    • Eg. insulin and biologicals not given orally
  • Low pH - molecule may be degraded (benzylpenicillin, has to be given as injection)
  • Food (full stomach will generally slow absorption)
  • Gastric motility (altered by drugs and disease state)
  • Previous surgery reduce absorption and transit time (eg gastrectomy)
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20
Q

What factors of the intestine affect oral drug absorption?

A
  • Drug structure:
    • Lipid soluble/unionised
      molecules diffuse down
      concentration gradient
    • Large or hydrophilic molecules
      are poorly absorbed
  • Medicine formulation:
    • Capsule/tablet coating can
      control time between
      administration and drug
      release
    • Modified release controls
      (slows) the rate of absorption
      (less frequent dosing)
  • P-glycoprotein:
    • Protective protein
    • Removes substrates from
      intestinal endothelial cells back
      into lumen
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21
Q

What is first pass metabolism?

A

Metabolism of drugs preventing them reaching systemic circulation

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22
Q

What are the 4 main barriers in first pass metabolism?

A

▪ Intestinal lumen
▪ Intestinal wall
▪ Liver
▪ Lungs

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23
Q

What is bioavailability?

A

proportion of administered dose which reaches the systemic circulation

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24
Q

What is bioavailability affected by?

A
  • Dependent on extent of drug absorption and extent of first pass metabolism
  • NOT affected by rate of absorption
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25
What are pros of Rectal (PR) administration?
- Local administration - Avoids first pass metabolism - Good when nausea and vomiting and oral isn’t possible
26
What are cons of rectal (PR) administration?
- Absorption can be variable - Patient preference
27
What are features of inhaled (Inh) administration?
- Large SA and blood flow but limited by risks of toxicity to alveoli and delivery of non-volatile drugs - Largely restricted to volatiles such as general anaesthetics and locally acting drugs such as bronchodilators in asthma - Asthma drugs non-volatile so given as aerosol or dry powder
28
What are features of intramuscular (IM) administration?
- Depends on blood flow and water solubility - Increase in either enhances removal of drug from injection site - Can make a depot injection by incorporating drug into lipophilic formulation which releases drug over days or weeks
29
What are features of intradermal and subcutaneous (S/C) administration?
- Avoids barrier of stratum corneum - Mainly limited by blood flow - Small volume can be given - Use for local effect or to deliberately limit rate of absorption
30
What are pros of subcutaneous (S/C) administration?
- Faster onset than PO - Formulation can be changed to control rate of absorption
31
What is the con of subcutaneous (S/C) administration?
Not as rapid as IV
32
What are the pros of transdermal (TD) administration?
- Provides continuous drug release - Avoids first pass metabolism
33
What are the cons of transdermal (TD) administration?
- Only suitable for lipid soluble drugs - Slow onset of action
34
When a drug first enters systemic circulation where is it?
In plasma
35
What are the 4 compartments in the body?
- Fat - Plasma - Interstitial fluid - Intracellular fluid
36
What 3 properties of drugs influences their distribution?
- Molecule size - Lipid solubility - Protein binding
37
How does molecule size affect distribution?
smaller molecule more able to distribute
38
How does lipid solubility affect distribution?
lipid soluble drugs able to permeate across membranes and distribute into tissues
38
How does protein binding affect distribution?
- greater affinity to proteins (albumin) so cannot permeate membranes and distribute - Binding lowers the free concentration of drug and can act as a depot releasing the bound drug when the plasma concentration drops through redistribution or elimination - Some drugs bind irreversibly and cannot re-enter the circulation and is equivalent to elimination
39
Is binding to albumin for drugs reversible or irreversible?
Reversible
40
What is volume of distribution (Vd)?
- Theoretical volume a drug will be distributed in the body (apparent volume of distribution) - Volume of plasma required to contain the total administered dose
41
Do drugs that are well distributed have a high or low Vd?
High
42
What 3 ways can drugs reach the CNS through the BBB?
- High lipid solubility. e.g. Ѱ drugs usually very lipid soluble (therefore large Vd) - Intrathecal administration (e.g. baclofen in MS and spinal cord injury, chemotherapy) - Inflammation (causes barrier to become leaky)
43
What are adaptations of the BBB to protect the CNS?
- Continuous layer of endothelial cells with tight junctions - Efflux pumps remove water soluble molecules
44
What type of drugs can easily cross the placenta?
Lipid soluble drugs readily cross placenta
45
What type of molecules can't pass the placenta?
Large molecules do not cross placenta
46
What is the method of elimination in foetuses?
Foetal liver has low levels of drug metabolizing enzymes, so relies on maternal elimination
47
What is elimination?
the process by which the drug becomes no longer available to exert its effect on the body
48
What are the 2 forms of elimination?
- Excretion - Metabolism
49
Which type of drugs are excreted?
unchanged drugs (hydrophillic, polar molecules)
50
What is metabolism?
modification of chemical structure to form new chemical entity
51
What are the steps in metabolism?
Phase 1 Phase 2
52
What happens in phase 1?
Oxidation/reduction/hydrolysis to introduce reactive group to chemical structure
53
What happens in phase 2?
Conjugation of endogenous functional group to produce hydrophilic, inert polar molecule involving formation of covalent bond
54
What type of molecules is metabolism needed for?
Necessary for the elimination of lipid soluble drugs to then be excreted
55
What enzyme is used in phase 1 reactions?
CYP450 enzyme
56
How are drugs and metabolites excreted?
- Liquids (small, polar molecules): URINE, bile, sweat, tears, breast milk - Solids (large molecules): faeces (through biliary excretion) - Gases (volatiles): expired air
57
What are the 3 processes in renal excretion?
1. Glomerular filtration 2. Active tubular secretion 3. Passive reabsorption
58
What happens in glomerular filtration?
- 20% of plasma filtered - Free/unbound drug molecules - Very large molecules excluded
59
What happens in active tubular secretion?
- 80% of renal blood flow passes on to peritubular capillaries - Drug molecules transported by carrier systems: - Organic anion transporter (OAT) - Organic cation transporter (OCT) - Can clear protein bound drug - Most effective renal clearance mechanism
60
What happens in passive reabsorption?
- Diffusion down the concentration gradient from tubule into peritubular capillaries - Hydrophobic drugs will diffuse easily - Highly polar drugs will be excreted

2a- Pharmacology (10 decks)

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