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AD2 > Pharmacokinetics > Flashcards

Pharmacokinetics Flashcards

1
Q

What is oral bioavailability (F)?

A

The fraction of the orally administered drug that reaches the systemic circulation

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2
Q

Factors affecting oral bioavailability of a drug?

A

-Poor absorption from the gut
-Breakdown of drug in gut
-First pass effect

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3
Q

What is the first pass effect?

A
  • all drugs absorbed into intestine MUST first pass through liver via portal circulation
  • drugs may be broken down in liver
  • and so not all of drug may reach systemic circulation
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4
Q

What is the pKa of a drug?

A

the pH of a solution when 50% of the drug is ionized

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5
Q

What are the factors affecting drug absorption?

A
  • Lipid solubility, as need to cross membrane
  • pKa of drug: pH at which 50% is ionised
  • pH at absorbing surface
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6
Q

What is meant by the term drug distrubution?

A
  • penetration of drug into tissues and organs from the blood
  • can be considered in terms of body compartments
  • higher lipid solubility = higher ability to cross membranes therefore higher distribution
  • if drug binds to (eg) albumin easily it too will have a higher distribution
  • if high blood flow to that area, drug will have high distribution there
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7
Q

Apparent volume of distribution (Vd)

A

the volume of fluid which would be required to contain the amount of the drug in the body if it was equal to the concentration in the plasma

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8
Q

Vd =?

A

amount of drug in body/plasma drug concentration

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9
Q

What does a small Vd indicate?

A

that the drug is confined to body water, likely polar, and not well distributed
(conc is high in plasma)

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10
Q

What does a high Vd indicate?

A
  • a high concentration of drug in extravascular space (concentrated in tissues, not in the plasma)
  • most of the drug is contained in extraplasmic space & is unavailable for metabolism/elimination
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11
Q

What is drug elimination?

A

removal of the drug from the body

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12
Q

What 2 parameters control elimination?

A

Excretion and metabolism

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13
Q

How are drugs removed?

A

urine, faeces, milk, bile, vomit or if they are broken down

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14
Q

What does a drug with a short half life mean?

A

It is being eliminated by the body quickly
needs to be adminstered more often

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15
Q

What is clearance?

A

volume of plasma cleared of a substance per unit time

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16
Q

What are the features of first order kinetics?

A
  • Most common
  • t0.5 is constant
  • clearance not changing
  • higher clearance when higher Cp
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17
Q

What are the features of zero order kinetics?

A
  • Less common, rate of process is independent of drug concentration.
  • Happens when elimination process is saturated
  • Half life does change, eg if you give a smaller dose of the drug your half life will be smaller
18
Q

What is the First order elimination equation?

A

T0.5 = ( 0.693 * Vd) / clearance

19
Q

What is the main difference between the IV bolus curves and the oral admin curve?

A
  • Peak of oral curve shows when absorption and elimination are equal
  • No absorption phase in IV bolus as no Cp doesn’t go up
20
Q

What is intravenous infusion?

A
  • here the drug is eliminated as it is perfused
  • plasma conc goes up until Css is reached
21
Q

Rate of infusion =?

A

Css x Clearance

22
Q

Css

A

Steady state concentration
Rate of infusion = rate of elimination

23
Q

How many half lives does it take to reach steady state?

A

around 5

24
Q

What does a oral dosing graph look like?

A
  • Peaks and trough of oral dosing with every dose
  • But when these peaks and troughs are steady/ dont really change value, this is when the Css average is reached
25
Q

What will happen to the Css average of you double the oral dose?

A

remain the same

26
Q

What happens to drugs that are NOT lipid soluble?

A

excreted by the kidneys

27
Q

What happens to drugs if they are ionised in the nephron?

A
  • they are no longer lipid soluble
  • so will NOT be reabsorbed
28
Q

How can urinary excretion ofweak aciddrugs be increased

A
  • making urine more alkaline
  • if you increase the OH- in the urine, then this will combine with the H+ from the drug to form water
  • and the equation will be pulled to the right ie even more ionised drug is formed, so less absorbed
  • HA ⇌ A− + H+
29
Q

What can drugs be transformed into in the liver?

A

inactive metabolites

30
Q

What is phase 1 metabolism?

A
  • Phase I metabolism converts the drug into a derivative
  • By oxidation, reduction or hydrolysis
  • Often introducing a reactive site on the drug molecule
  • Making it MORE reactive
31
Q

What is phase 2 metabolism?

A
  • Phase II metabolism converts the derivative into a conjugate
  • Conjugation of the species formed in Phase I with polar molecules, making the metabolite less lipid soluble
  • Hence easier to excrete in urine
32
Q

What enzyme oxidates drugs in phase I?

A

cytochrome P450

33
Q

What process occurs in phase II?

A

Glucuronidation

34
Q

What happens if too much paracetamol is taken?

A

phase II reactions become saturated

35
Q

What pathway does paracetamol follow if phase II pathways are saturated?

A
  • phase I pathway
  • To a toxic intermediate that is extremely reactive
  • Called NAPQI
  • This can still be conjugated with glutathione but when this is depleted NAPQI reacts w cell proteins to cause hepatic cell damage
  • Fatal
  • In cats this molecule combines with Hb and stops it carrying oxygen
36
Q

What is Enzyme induction?

A
  • Some drugs induce increased cytochrome P450 enzymes (for Phase I)
  • Over days/ weeks
  • Can cause failure of some drugs
  • Because clearance has decreased due to higher oxidation levels, therefore reduced effect of drug
37
Q

What is enzyme inhibition?

A
  • Drugs directly inhibit cytochrome P450
  • Increase likelihood of toxic effects of drug
  • Increases the half life of the drug so it stays longer in the body
  • As isn’t oxidised as quickly
38
Q

What is Genetic polymorphism?

A
  • Poor ability to metabolise drugs by some groups of people
  • Polymorphism is a mutation of an amino acid, meaning defect in enzymes, meaning defect in drug metabolism enzymes
  • Means that high concentrations of drug gare still present after some time
39
Q

How does age effect drug metabolism?

A

the elderly have impaired liver functioning, while infants and children have immature livers
(metabolism lower in the very young and the elderly)

40
Q

What is a biologic?

A
  • Any virus, therapeutic serum, toxin, antitoxin, or analogous protein applicable to the prevention, treatment, or cure of disease or injuries of man MADE FROM LIVING ORGANISMS
  • large biological weight
41
Q

Biologics distribution

A
  • apparent vd is small
  • mechanisms of distributions is pavement in lymph
42
Q

Biologics elimination

A
  • long half life
  • mechanisms are proteolysis eg lysosomes INSIDE cells

AD2 (39 decks)

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