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Pharmacokinetics Flashcards

1
Q

Define absorption in pharmacokinetics

A

Process of taking drug from site of administration into the blood
Involves crossing of the barrier from site to blood and movement across the physical distance between the two.

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2
Q

What determins rate of diffusion I drug absorption

A

Rate directly proportional to conc gradient.membrane permeability.area / thickness of membrane

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3
Q

What processes are involved in drug absorption

A

Passive diffusion
Facilitated diffusion
Active uptake
Pinocytosis

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4
Q

What drug factors effect absorption (ability to pass through a membrane)

A

Lipid solubility
Ionisation
Size of molecule
Concentration gradient

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5
Q

What physiological factors influence drug absorption through a membrane

A

Ph
Carrier process
Pinocytosis

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6
Q

Where in the gi tract is effected by first pass metabolism

A

Oesophagus to upper rectum

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7
Q

Enthral routes of drug administration

A

Oral
Buccal
Rectal

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8
Q

Advantages of entral administration of drugs

A

Easily accessible
No equipment
Patient can self administer

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9
Q

Disadvantages of enteral administration of drugs

A

May not be appropriate route
Patient may not be able to swallow
Drug may be destroyed in gi tract
May subjected to first pass metabolism
Drug may be irritating to gi tract
Variable first pass effect and bioavailability
Effective dose unpredictable.

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10
Q

What is bioavailability

A

The amount of drug given by a route that reaches systemic circulation

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11
Q

Advantages of parenteral administration of drugs

A

Unaffected by first pass metabolism:
More predictable plasma levels
Doesn’t not need patient assistance or functioning gi tract

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12
Q

Disadvantages of parenteral drug administration;

A

Needs kit and training
Not usually self administered
Invasive

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13
Q

What effects absorption from an IM injection

A

Drug solubility in blood
Tissue binding
Protein binding
Blood flow to site

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14
Q

Advantages of IM injection

A

Easier than cannulation
Slows onset of effect
Prolongs effect

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15
Q

Advantages of transdermal administration

A

No kit or training
Not invasive
Avoids first pass metabolism
Slow prolonged effect avoiding peaks and troughs

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16
Q

What determins plasma platau level for a set dose of drug in a transdermal patch

A

Contact area

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17
Q

What factors determine distribution to individual tissues

A

Solubility of drug in those tissues
Plasma protein binding
Blood flow to tissue
Mass of tissue
Tissue/blood coefficient
Tissue protein binding
Facilitated transport
Drug ionisation
Drug molecular size

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18
Q

What does blood flow to the tissues influence

A

Speed of distribution
Amount of drug reaching tissue capilliaries and thus amount taken up into tissue

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19
Q

What are the three core compartments of drug distribution. What proportion of cardiac output does each compartment represent

A

Vessel rich group - 75%
Muscle group - 18%
Fat group - 5%

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20
Q

What are the two key factors in determining drug uptake by tissues

A

Blood concentration and tissue concentration
Tissue:blood partition coefficient

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21
Q

What effects speed of drug movement between compartments

A

Degree of concentration difference (distance away from distribution coefficient / equilibrium)
Ease of molecular movement (drug factors, membrane factors and size)
Size of tissue - larger means increased rate of transfer, increased total uptake but reduced time to reach equilibrium

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22
Q

Out of the three main compartments for drug distribution where would a typical iv agent first distribute, then where last

A

Rise and fall first in vessel rich group
Then muscle group
With steady climb in fat group

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23
Q

Why is a fall in drug concentration seen quite rapidly in the vessel rich distribution group follow8g an in bolus

A

As the drug distributes into the compartments plasma level falls so drug will start to move back out of the compartments towards equilibrium again

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24
Q

How is penicillin distributed? Implication

A

Active transport
Unidirectional process so doesn’t redistribute back to blood when concentration gradient reverses

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25
What sort of drugs tend to bind to albumin
Acidic drugs like aspirin
26
Which protein is most important for basic drug transport
Alpha 1 glycoprotein
27
What causes decreases in albumin? Consequences for albumin bound drugs
Chronic disease, liver failure, renal failure Increases free drug
28
What causes increases in alpha 1 glycoprotein
Obesity Trauma Burns Postop MI Carcinoma Inflammatory disease
29
What are the two key components of drug elimination
Bio transformation Excretion
30
How do Hepatocytes get the drugs they need where they need them
Active transport system that concentrates drugs and allows
31
Phases of biotrasformation with description
Phase 1 - simple reactions like hydrolysis, oxidation and reduction Phase 2 - more complex reactions that add molecular groups eg glucuronidation and acetylation
32
Where does phase 1 bio transformation occur What enzymes are involved
Redox in the liver - p450 Hydrolysis widespread eg in the blood - eg plasma cholinesterases
33
What do phase 2 reactions do to drugs
Make them more water soluble
34
What is the extraction ratio?
A measure of effectiveness with which a substance is removed or processed by an organ. ER = [arterial]-[venous] / [arterial]
35
What does the extraction of a drug depend on
Extraction ratio Enzyme activity Drug protein binding Red blood cell partitioning Perfusion
36
What is clearance
The volume of blood completely cleared by a drug per unit time Clearance = blood flow x extraction ratio
37
What influences clearance of a drug with a high extraction ratio vs low extraction ratio
High er = Blood flow Low ER = other factors such as enzymes, etc
38
What is drug excretion
Removal of active drug and metabolites from body
39
Routes of drug excretion
Urine Bile Lungs Faeces Sweat
40
What drugs are excreted via the lungs
Anaesthetic gases Ethanol
41
How does renal excretion occur What molecules are excreted this way
Molecules <69000 daltons can cross the basement membrane at the glomerulus depending on shape and charge Only really applies to water soluble molecules as lipid soluble substances will be at very low concentrations free in the plasma. Also very small amounts of highly protein bound substances excreted this way.
42
What happens to drug concentration in the urine as it moves through the nephron What is the importance of this
If less drug is reabsorbed than water then it will concentrate Little importance, it is the amount excreted that is important (though they are related as less reabsorption means more excretion)
43
What variables influence renal elimination of drugs? What doesn’t.
Yes - urine ph (influences reabsorption), renal blood flow and filtration rate, No - absolute volume of urine produced
44
What is the influence of probenecid on penicillin
Stops its active transport thus reduces excretion into urine and thus prolongs t1/2
45
Why are models necessary in pharmacokinetics
Easy to measure blood concentration in blood but hard elsewhere
46
What is the compartment model of pharmacokinetics
Based on body being a number of interconnected interlinked compartments, each with a specific volume and transfer rate for particular drug They assumed to have a consistent solubility within each compartment ie partition coefficients of 1
47
What is the one compartment model Basic calculations based off it
All tissues are a single compartment Drug administered, evenly distributes throughout Concentration = dose/volume Thus if you know dose and plasma concentration then volume of the compartment can be calculated.
48
What is the two compartment model
Body divided into central and peripheral compartments The central compartment is route in and out (dosing and elimination) and the peripheral compartment reached off the central compartment.
49
What is the three compartment model
Same as two compartment but with the addition of a deep peripheral compartment off the central compartment Communication with the deep peripheral compartment is much slower than the peripheral.
50
Which induction agent fits the two compartment model well? Which the three?
2 thiopental 3 propofol
51
What is the water analogue model of pharmacokinetics What do the height, cross section and volume of the containers represent What do the pipes represent
A model for inhaled anaesthetic agents as a series of interconnected cylinders with water height representing partial pressure of agent, cross sectional area of cylinder representing solubility off the agent in that tissue and amount of water representing amount of agent. Piping represent transfer characteristics between tissues (e.g. high resistance narrow pipe)
52
What does the water analogue model trend towards
Equilibrium where all tissues (containers) have the same volume of agent (water)
53
What is the assumption behind measuring an amount of drug in a sample from a compartment and inferring volume of that compartment from it?
That the drug is evenly distributed through the compartment
54
What is volume of distribution
A mathematical model/tool not an actual value, can easily exceed total volume of the body Represents how a drug is distributed
55
What are the two elimination kinetics? What type effecsts most drugs
Zero order - elimination system is saturated at clinical lieges so elimination rate is constant and unrelated to drug concentration. A fixed mass of drug is eliminated per unit time. First order - most common. Drug elimination is proportional to concentration.
56
What drugs are eliminated by zero order kinetics
Ethanol High levels of phenytoin, thiopental, salicylates Note when levels of above drop beneath saturation threshold then first order takes over.
57
What is the equation for first order kinetics
C = C0.e superscript -kel.t Concentration = concentration at time 0, Nat log, superscript elimination rate constant.time
58
What happens on a time vs concentration graph of a first order kinetics drug
Exponential decay, fast at first then slowing, never reaches zero
59
What constants can be determined from a first order time vs concentration graph
Half life - time taken for concentration to halve Time constant - time it would take for drug concentration to reach zero if elimination continued at rate of chosen starting point
60
What is the percentage of remaining drug at time constants 0 1 2 3 4
0 = 100 1 = 37 2 =13.5 3 = 5 4 = 1.8
61
What does a graph of time (x) vs ln concentration (y) appear like? What is the y intercept What can be calculated from this
A straight line decending with time Y intercept is concentration at time 0 Thus can be used to calculate volume of distribution assuming an instantaneous distribution.
62
If the two compartment model is used to produce a time (x) vs ln concentration (y) graph of a first order drug elimination how would it appear? What are the parts?
First initial steep decline due to rapid redistribution Then levels out into gentler straight decline due to elimination from central compartment
63
On a two compartment first order semilogarhythmic time x vs plasma conc y graph what are the derivable intercepts?
From extrapolation of first phase to y intercept gives Conc at time zero From extrapolation of second phase to y intercept gives conc at time zero if there was instantaneous redistribution - also considered volume of distribution at steady state (when infusion administration exactly equals elimination.
64
Is clearance first or zero order?
Clearance depends on concentration so is first order
65
What units are used for clearance
ml/kg/min ml blood per kg body weight per minute
66
What does TIVA use to calculate loading and maintenance doses
Loading dose = volume of distribution x desired concentration Maintainance dose = total clearance x desired concentration
67
What is a steady state infusion What happens on stopping
Infusion for long enough that all tissues are at equilibrium - elimination equals infusion rate and plasma concentration is constant On stopping slow terminal half life decline as drug is eliminated.
68
What commonly occurs with shorter infusions in terms of half life
Steady state not reached thus infusion has a context sensitive half life - the shorter the infusion the closer the half life is to the redistribution half life rather than terminal half life and thus more rapidly eliminated.
69
Why does a longer infusion have a slower context sensitive half life than a shorter one given the same plasma concentration
More drug distributed to tissues that re-enters plasma as plasma concentration falls maintaining plasma concentration for longer

Primary Pharmacology (18 decks)

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