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Nervous system > Pharmacokinetics > Flashcards

Pharmacokinetics Flashcards

1
Q

What is pharmacokinetics?

A

What the body does to the drug, depends on hepatic/kidney function, genetics, age, sex, body composition, weight, drug’s water/lipid solubility, molecular weight, protein binding ability

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2
Q

What is pharmacodynamics?

A

What the drug does to the body once it gets to the site of action

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3
Q

What are the four main steps of pharmacokinetics?

A

Absorption
Distribution
Metabolism
Excretion

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4
Q

Why is pharmacokinetics important?

A
  • Prescribing safely
  • Critical to new medicines research and development
  • Ability to enhance efficacy and reduce toxicity of drugs
  • Understanding what can go wrong with drug dosing and drug-interactions
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5
Q

Why is it beneficial to understand pharmacokinetics?

A
  • Choose appropriate drug/dose for patient with renal or hepatic impairment
  • Predict some drug-drug interactions and avoid giving certain combinations of drugs - potential for reduced efficacy or side effects
  • Explain why people react differently to some drugs
  • Understand the potential risks in dosing some drugs and monitor drug levels to ensure safety and efficacy - narrow therapeutic index drugs
  • Choose appropriate route of administration for a drug e.g. IV or oral
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6
Q

What is absorption?

A

How the drug enters the systemic circulation following administration

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7
Q

How do the majority of molecules move around the body?

A 
Bulk flow (blood, lymph, CSF)
Diffusion (molecule by molecule, short distances only)
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8
Q

What are the four main ways in which drugs are absorbed?

A

Passive diffusion through lipid
Facilitated passive diffusion
Active transport
Pinocytosis

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9
Q

With which molecules will passive diffusion work best to absorb a drug?

A

Small and limi-soluble molecules

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10
Q

What is the process of facilitated passive diffusion?

A

Polar molecules, drug substrate attaches to carrier and moves through cell membrane

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11
Q

What is pinocytosis?

A

Particle engulfed by cell, membrane encloses fluid and forms a vesicle which detaches and moves into cell - depends on energy, applicable to protein-based drugs

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12
Q

When will diffusion through a lipid occur most readily?

A
  • High concentration gradient
  • Low molecular weight
  • High lipid solubility
  • Low degree of ionization
  • Large surface area
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13
Q

What are the major routes of administration?

A
  • Oral
  • Sublingual (under tongue)
  • rectal
  • Topical
  • Transdermal
  • Inhalation
  • Injection (subcutaneous, intramuscular, intravenous, intrathecal - spinal column, intravitreal - eye)
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14
Q

Where does absorption of drugs mainly occur?

A

Small intestine

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15
Q

Factors affecting how much of a drug is absorbed from the GI tract

A
  • Particle site and formulation
  • Physiochemical factors
  • Gut content (fasted vs fed)
  • GI motility - gastric emptying determines how quickly drug delivered to small intestine
  • Splanchnic blood flow - blood flow to GI tract
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16
Q

Why is IV administration of a drug beneficial?

A
  • Direct administration into plasma
  • Rapid onset and full absorption of drug
  • When rapid effect needed or if oral absorption likely to be poor
  • When drug is rapidly metabolized/eliminated, a loading dose (bolus) may need to be given which is then followed with infusion to maintain blood concentrations
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17
Q

What is bioavailability?

A

The fraction of the administered dose that reaches the circulation as the parent drug

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18
Q

Why is the bioavailability of IV drugs 1?

A

100% of it reaches blood stream

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19
Q

Why might there incomplete bioavailability for oral administration of a drug?

A
  • Incomplete absorption and loss in feces - molecule is too polar to be absorbed or tablet did not release all contents
  • First pass metabolism (fraction of drug lost during absorption) in gut lumen, during passage across gut wall or by liver before drug reaches systemic circulation
  • Bioavailability has important therapeutic implications because it is the major factor determining the drug dosage fo different routes of administration
  • If drug bioavailability = 0.1, oral dose will need to be 10x higher than IV dose
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20
Q

What is the distribution of a drug?

A

How it is transferred between plasma and tissue

Mostly passive diffusion

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21
Q

What is the distribution of a drug dependent on?

A

Blood flow speed and lipid solubility

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22
Q

Which organs are normally well perfused?

A

Brain, liver, kidneys, GI tract

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23
Q

Which organs are normally poorly perfused?

A

Skin, skeletal muscle, bone, fat

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24
Q

How does a drug distribute when taken orally?

A

Even filling across all compartments

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25
Q

How does a drug distribute when injected?

A

Fills brain, heart etc initially, then redistributes

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26
Q

What is protein binding?

A

Many drugs bind to plasma to intracellular proteins
Usually reversible
Lowers free concentration of drug available (causes pharmacological effect)

27
Q

What is the volume of distribution?

A

Theoretical volume to contain total amount of administered drug equal to the concentration in the blood plasma

28
Q

Characteristics of low Vd drugs?

A

Large, water soluble, stay within plasma and well perfused organs

29
Q

Characteristics of high Vd drugs

A

Small, lipid soluble, distribute across all compartments

30
Q

What is elimination?

A

Metabolism + excretion

31
Q

How are larger molecules normally excreted?

A

Bile

32
Q

How are smaller molecules normally excreted?

A

Really

33
Q

What are the 3 phase 1 reactions?

A

oxidation, reduction, hydrolysis

34
Q

What are the 2 phase 2 reactions?

A

Sulphate, glucuronide

35
Q

Where is main site of drug metabolism?

A

Liver
Aims to make drug more soluble for kidneys
Lipid soluble molecule changes to water soluble

36
Q

What is phase 1 of elimination?

A

Non-synthetic reactions, aims to produce molecules suitable for phase 2

37
Q

What is phase 2 of elimination?

A

Addition of endogenous substances to increase water solubility - increases polarity and is more easily excreted

38
Q

Problems with first pass metabolism

A
  • Requiring significantly higher doses orally than IV due to reduced bioavailability
  • Marked individual variations occur in the extent of first-pass metabolism, both in the activities of drug-metabolizing enzymes and also as a result of variation in hepatic blood flow
  • Examples of drugs: lidocaine, salbutamol etc
39
Q

What is a pro-drug?

A

Metabolised for administration to a pharmacologically active metabolite, may be inactive until metabolised

40
Q

What is a first order reaction?

A

Elimination is proportional to drug concentration, loses constant fraction per time unit

41
Q

What is a zero order reaction?

A

Elimination of constant quantity per time unit

42
Q

What is half life?

A

Time taken for the plasma concentration fo a drug to decrease by 50%

43
Q

What is the elimination constant K?

A

Rate of drug removal from body

44
Q

What is the shape of the graph of ln against time for a first order reaction?

A

Straight line

45
Q

Characteristics of K for first order reaction?

A

Constant, independent of drug concentration

46
Q

What is clearance?

A

Volume of plasma in vascular compartment cleared of drug per unit time by metabolism and excretion

47
Q

How to calculate clearance

A

rate of elimination from body / drug concentration in plasma

48
Q

How to calculate clearance using K

A

Cl = K x Vd

49
Q

What is repeated administration?

A

Fixed dose at regular time interval

50
Q

What is a loafing dose?

A

Single dose needed to produce desired steady state concentration

51
Q

How to calculate loading dose

A

Css x Vd

52
Q

How would you change dosage if the patient had low cardiac output?

A

Reduce dose, give more slowly

53
Q

How would you change dosage if the patient had hepatic/renal impairment?

A

Increase dosage intervals, reduce dose

54
Q

Effects of drug interactions

A

Increase/decrease in drug effect or synergistic effects (combined effect is greater than sum of combined separate effects)

55
Q

How to calculate therapeutic index

A

(dose resulting in toxicity) / (dose giving therapeutic response)

56
Q

What is the therapeutic index?

A

Safety margin

57
Q

What does a high therapeutic index indicate?

A

Low risk of toxicity

58
Q

What does a low therapeutic index indicate?

A

High risk of toxicity

59
Q

Pharmacokinetic drug interactions in absorption

A

Physio-chemical reactions in GI tract

60
Q

Pharmacokinetic drug interactions in distribution

A

Protein binding displacement - increased free drug

61
Q

Pharmacokinetic drug interactions in elimination

A

Enzyme inhibition (enhanced activity) and enzyme induction (decreased activity)

62
Q

Examples of enzyme inhibitors

A

Grapefruit juice, excess alcohol, sodium valproate, erythromycin, cimetidine

63
Q

Examples of enzyme inducers

A

Alcohol consumption, cigarettes, sprouts, anticonvulsants, spironolactone

Nervous system (24 decks)

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