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Pharmacology > Pharmacokinetics > Flashcards

Pharmacokinetics Flashcards

1
Q

What is the process of Pharmacokinetics?

A

Dose administered - drug in bloodstream - drug in tissues- drug metabolised or excreted
OR
- Drug binding at site of action
- toxicity or efficacy

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2
Q

What is the therapeutic range?

A
  • aims to keep concentration of drug in the plasma within a certain range
  • minimum toxic concentration . above this = toxicity
  • Min therapeutic concentration = below = no beneficial effect
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3
Q

How do drugs move across membranes?

A
  • Through passive diffusion
  • small molecules move faster
    -solubility in the lipid bilayer
  • Inside - lipid bilayer = hydrophobic (not too much tho!)
  • Facilitated Transport (carrier mediated)
  • Passive
  • Bidirectional
  • Relies upon conc gradient
  • slc (solute carrier) = Organic Anion/Cation Transporters
  • Active transport
  • If drug is a substrate for transporter/carrier which pumps drugs back into lumen
  • ATP-driven pumps
    (ABC superfamily like P-glycoprotein
  • Can also occur in the kidney, brain`
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4
Q

What is the effect of pH in the stomach/intestine for weak acids?

A
  • Many drugs are weak acids
  • Charged drugs do not cross membranes
  • Low PH = h+ = high, EQUILIBRIUM GOES TOWARDS HA
  • VERY little ionised = lots absorbed
  • High pH, H+ = low. More ionised, less absorbed
  • Charged ions do not pass the membrane
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5
Q

What is the effect of pH in the stomach/intestine for weak bases?

A

Low pH= High H+ = equilibrium goes towards BH+
- Lots ionised
- Little absorbed
- High pH , less ionised , more absorbed

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6
Q

What is the effect of pH in the kidney?

A

Weak bases
- Charged drugs do not cross membranes
- At low pH , [H+] is high, equilibrium goes towards BH+
- Lots ionised = little absorbed
= At high pH [H+] is low
- Less ionised = more absorbed

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7
Q

What are the principles of Pharmacokinetics ?

A
  • Absorption from the site of administration
  • Distribution within the body
  • Metabolism
  • Excretion
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8
Q

How is the rate of absorption into systemic circulation modified?

A
  • site of administration
  • drug formation
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9
Q

What is the definition of absorption?

A
  • The transfer of drug from the site of administration to the systemic circulation
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10
Q

What is Bioavailability (F)?

A

F = Quantity of drug reaching systemic circulation as intact drug/ Quantity of drug administered (i.e. does)

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11
Q

What is bioequivalence?

A
  • If the drug preparation contains the same active ingredient
    AND
  • When the rates and extents of bioavailability of the active ingredient in the two products are not significantly different.
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12
Q

What factors affect absorption from the gut?

A
  • Gastric emptying ( taking meds on an empty stomach)
  • Gastrointestinal motility
  • Blood flow
  • Particle size and formulation
  • Physiochemical factors e.g. solubility, molecular weight, binding to calcium (esp. in milk) or fat in food
  • Metabolism ( enzyme-catalysed reactions)
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13
Q

What is the first pass effect?

A
  • When drugs are metabolised by enzymes in the gut wall or liver
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14
Q

What may metabolism in the gut or liver lead to?

A
  • Activation of pro drug- one that is converted to an active form by enzymes in the liver
    OR
    -Inactivation - when a drug converted to inactive metabolites.
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15
Q

What are the routes of absorption from the GI tract?

A
  • Mouth,Rectal - avoids first pass metabolism
    – Sub-lingual
    – Buccal
  • Oral
  • Duodenal
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16
Q

What is the sub-lingual (glyceryl trinitrate) route of administration?

A
  • undergoes first pass metabolism
  • Needs a quick response (does not go into stomach)
  • given as a spray or tablet under tongue
  • given for angina
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17
Q

What are sub-cutaneous routes of adminstrations (Examples)?

A
  • Local anaesthetics, that act locally
  • Can be given with vasoconstrictor to delay systemic absorption
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18
Q

What are intra-muscular depot preparations?

A
  • Contraceptives
  • Gradual release, sustained concentrations
  • Convenience
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19
Q

What are examples of rectal adminstered drugs?

A

Anti-epileptic drugs
- Diazepam for epilepsy when there is continuous fittings

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20
Q

What are examples of topic drugs?

A
  • Anti-inflammatories
  • Ibuprofen, avoids irritation to stomach
  • Corticosteroids, avoid systemic side effects
21
Q

What are examples of intra-articular anti-inflammatories?

A
  • Corticosteroids, direct to target, avoids systemic side effects.
22
Q

What is the effect of plasma protein binding on distribution?

A
  • Can slow down the elimination of the drug
23
Q

How are drugs distributed?

A
  • Different drugs distributed in different ways
  • Lipid soluble/water soluble drugs have a smaller distribution in the body = slower elimination in the body.
24
Q

What is the volume of distribution (Vd)?

A

The volume of plasma that would contain the total body content of the drug at a concentration equal to that of plasma.
- concentration = mass/ volume

25
Q

What is the common rule of Vd for lipid-soluble drugs?

A
  • The plasma concentration can be very small even though there is still a lot of drug in the body.
26
Q

What is the estimate Vd of large hydrophilic drugs (heparin, insulin)?

A

Plasma ~3.5L/70kg

27
Q

What is the estimate Vd for small hydrophilic drugs(e.g. gentamicin, theophylline) ?

A

Plasma + extracellular Vd ~ 14L/70 kg

28
Q

What is the estimate Vd for moderately lipid soluble drugs (E.g. ethanol, phenytoin)

A

Total body water ~ 14L/70 kg

29
Q

What is the estimate Vd for very lipid soluble drugs (e.g. morphine, cannabis)?

A

Concentrated in tissues/fat
Vd > 40 L/70 kg

30
Q

A novel drug has been discovered for the treatment of epilepsy in adults. The Vd is 12L. Will it be useful for epilepsy? Yes/No, why?

A
  • No , 12 L suggests that drug is hydrophilic. Drug needs to get inside of the brain and needs to be lipid soluble. Lipophilic Vd> 40L
31
Q

What is elimination?

A

The removal of the original drug from the bloodstream.
- Either by metabolism to a different chemical structure
- Or by excretion in kidney, breath, skin and hepatobiliary system

32
Q

Where are drugs metabolised?

A
  • Usually in the liver, but also the gut , plasma and lungs
33
Q

What is Phase 1 of drug metabolism?

A
  • often catalysed by cytochrome P450 enzymes (CYP). Important for non-polar drugs.
  • Oxidation, Hydroxylation, Dealkylation, Deamination, Hydrolysis
34
Q

What is Phase 2 of drug metabolism?

A
  • Increases drug solubility in water and therefore excretion in the kidney
35
Q

What happens when two drugs are given together and metabolised by the same enzyme?

A

One may inhibit the metabolism of the other.

36
Q

What happens if a drug is taken in the presence of an inhibitor of cytP450?

A

Its metabolism would be reduced

37
Q

What happens when CytP450 is induced?

A

The amount of enzyme increases- metabolism of drug is also increased.

38
Q

What does metabolism in the liver depend on?

A
  • Perfusion
  • Plasma protein binding
  • Secretion into bile
  • Presence and activity of transporters and enzymes
  • Low extraction ratio <0.3
  • High extraction ratio > 0.7
39
Q

What is renal elimination?

A
  • When blood is filtered at the glomerulus
  • Nutrients are then reabsorbed and lipid soluble drugs can diffuse back into the blood
  • Some drugs actively secreted from blood to urine
40
Q

What factors affect renal elimination?

A
  • Plasma protein binding can slow elimination
  • Drugs can compete for transporters
41
Q

What is renal clearance?

A

The efficiency of elimination , i.e. the ratio of the rate of elimination to the plasma concentration.

42
Q

What is the equation for renal clearance?

A

renal clearance = rate of elimination into urine mg/h / plasma concentration (mg/L)

43
Q

What is the equation for rate of elimination?

A

Rate of elimination = volume/time X conc (urine)

44
Q

What is the equation of hepatic clearance?

A

rate of elimination via liver (mg/h) / plasma concentration (mg/L)

45
Q

What is the definition of plasma clearance?

A
  • The volume of plasma from which all the drug molecules would need to be removed per unit time to achieve the overall rate of elimination of drug from the body.
  • Total CL = renal CL + hepatic CL
46
Q

What is the half life of a drug?

A

The time taken for the concentration to decrease by half.

47
Q

Under what circumstances is the half life constant ?

A

plasma = fluid in a bucket (single compartment)
inflow = absorption of fluid into the body
outflow = removal of fluid through filtration in the kidneys
volume of fluid in the bucket is constant

48
Q

Why is the half life for other drugs not constant?

A
  • The drug is redistributed into fat or tissues
  • The elimination mechanism becomes saturated

Pharmacology (14 decks)

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