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FNP > Absorption and distribution of drugs > Flashcards

Absorption and distribution of drugs Flashcards

1
Q

What are pharmacokinetics?

A

The study of the change in drug and metabolite concentrations in tissues and body fluids.

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2
Q

What is the therapeutic window?

A

If there is a high concentration of drug, it can be toxic.
If you only take a low concentration, it will have no effect.
The therapeutic window is below the maximum non-toxic concentration, and above the minimum therapeutic concentration, for the appropriately needed time.

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3
Q

How do therapeutic windows change?

A

Some drugs may need to act quickly and leave quickly, so the therapeutic window would be small.
For contraceptives, want long lasting, so therapeutic window would be longer.

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4
Q

What is therapeutic ratio?

A

Maximum non-toxic dose / Minimum therapeutic dose
The safest drugs have a high therapeutic ratio.
A high ratio will give a wide range of concentrations that will be helpful for the drug.

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5
Q

How can therapeutic ratio be calculated?

A

Therapeutic ratio = TD50 / ED50
TD50 = the toxic dose of a drug for 50% of the population
ED50 = the minimum effective dose for 50% of the population

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6
Q

What is ADME?

A

Absorption - movement of a drug from the point of administration to the plasma.
Distribution - from plasma to tissues and organs.
Metabolism - of drugs by enzymes in body.
Excretion - of drug/metabolite by the body

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7
Q

What is ADME important for?

A

Drug development
Understanding toxicity
Establishing route of administration
Establishing drug dose
Understanding possible drug interactions

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8
Q

What is bioavailability?

A

The fraction of unchanged drug that reaches systemic circulation.
It is a measure of how effective absorption of a drug is.

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9
Q

What is bioavailability decreased by?

A

Poorer absorption
First pass metabolism
IV drug delivery has 100% bioavailability.
But orally taken drugs might remain in the gut wall and are not absorbed - so have 0% bioavailability.

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10
Q

What does absorption depend on?

A

Route of administration
Chemical nature of the compound
Formulation
Blood flow rate to site of delivery.

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11
Q

How does the chemical nature of a compound affect absorption?

A

The higher lipophilicity the more absorptive.
The more uncharged the faster.
A smaller size is faster.

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12
Q

What are examples of formulation of drugs?

A

Liquids absorb faster than tablets because of the greater area for exchange.
Gelatin-coating tablets slows absorption and decreases local peak concentrations.
Capsules contain granules with differently soluble coatings allows controlled release.
Intramuscular injection of an oily depot of lipophilic drugs allows very slow release (as muscles is usually very fast).

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13
Q

What are examples of gelatin-coating tablets?

A

Non-steroidal anti-inflammatory drugs (NSAIDs)
e.g. aspirin, ibuprofen

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14
Q

What are examples of intramuscular injections?

A

Lipophilic drugs e.g. haloperidol
For example, used for anti-psychotic drugs.

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15
Q

What are the routes of absorption?

A

Intravenous - fast
Sublingual (sl) - under the tongue/buccal - medium
Enteral - gastrointestinal tract
Orally (po) /rectally (pr) - slow

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16
Q

What is the intravenous route?

A

Fast but invasive - need to access a vein.
Has an infection risk, as crossing an epithelial barrier.

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17
Q

What is the intramuscular(IM)/ subcutaneous (sc) route?

A

Easy, fairly fast, but can be invasive and painful.
It can cause local reactions.
It can be slowly absorbed if using a lipophilic drug dissolved in a lipid.
Subcutaneous is a space underneath the skin.

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18
Q

What is the sublingual route?

A

Fast, but only a very small amount can be absorbed, due to small surface area of tongue, so requires very potent drugs.
Can be spraying or holding a tablet under the tongue.
e.g. sleeping tablets, pain relief for angina.

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19
Q

What is the oral or rectal route?

A

Slow, with first pass metabolism, but non-invasive.
Mostly absorbed through the small intestine.

20
Q

What is the transcutaneous route?

A

Across the skin
A lipophilic carrier is required to cross the skin and carry the drug into the body.
Can take minutes to hours.

21
Q

What is enteral absorption?

A

Absorption from the gastrointestinal tract, administered orally or rectally.

22
Q

What needs considering related to enteral absorption?

A

Tablets can be taken before, during, or after food, because the rate of absorption will vary with the activity of the gut.
There are large pH changes on the route to the gastrointestinal tract, will can affect the H+ in the drugs.
Drugs can undergo first pass metabolism - metabolism of the drug in the liver before it reaches systemic circulation - decreases bioavailability.

23
Q

What is the effect of rapid absorption via the gut on plasma concentration?

A

Quick absorption, concentration of gut in gut falls until it reaches equilibrium with the circulating concentration.
In the body, drug concentration will have a high early peak then slowly removed.
Fast absorption causes high peak concentrations, can lead to toxicity, so absorption being slowed down can be helpful.

24
Q

What do the graphs of fast absorption look like?

A
25
Q

What is the effect of slow absorption via the gut on plasma concentration?

A

Slow down the rate of absorption, slow down rate at which the drug accumulates in the body.
Delays the peak concentration, and decreases the peak concentration.
As drugs are absorbed, they’re immediately entering other processes of distribution, metabolism and secretion, so are removed.

26
Q

What do the graphs of slow absorption look like?

A
27
Q

How can drugs enter cells?

A

Epithelial cell layer lines the gut wall.
A drug can get through the barrier paracellularly, if it is water soluble.
A drug can go across the barrier, if it is lipid soluble.
If not soluble, will require a transporter or channel to cross into cells, then protein pumps for absorption.

28
Q

How does pH affect the absorption of weak bases?

A

The protonated form is charged.
When pH becomes more alkaline, there will be more uncharged form, so the drug crosses the membrane.
e.g. NH3+ <–> NH2 + H+

29
Q

How does pH affect absorption of weak acids?

A

Protonated form is uncharged.
When pH becomes more acidic, there will be more uncharged form present, and the drug will cross membranes.
e.g. COOH <–> COO- + H+

30
Q

What is the general effect of pH on drug movement?

A

Differences in pH determine where drugs will accumulate.
Drugs tend to accumulate wherever they are ionised, as it is difficult for them to leave these compartments.

31
Q

What is enterohepatic circulation?

A

Drugs enter the gastrointestinal tract, travel through portal vein, to liver, then enter systemic circulation.
Some drugs can enter the bile, and go back into the gastrointestinal system - enterohepatic circulation, and is not yet absorbed.

32
Q

What is the effects of enterohepatic circulation?

A

Extended half life
Sensitive to bowel conjugation

33
Q

What are examples of drugs which undergo enterohepatic circulation?

A

Morphine, warfarin, indomethacin, cardiac glycosides.
Antimicrobial agents - clindamycin, rifampicin, erythromycin, metronidazole, ampicillin, ceftriaxone, and doxycycline.
Radiocontrast media.

34
Q

What is distribution?

A

How drugs equilibrate within the body after entering systemic circulation.

35
Q

When does no distribution occur?

A

Is not absorbed into the tissues.
For large drugs, cannot cross the membrane, or those binding to plasma proteins
e.g. heparin

36
Q

Where can drugs distribute?

A

Through the extracellular space, into muscle, skin, other tissues.
Throughout total body water - go everywhere in the body.
Accumulate in cells - passively.
Lipophilic drugs are dissolved in fat e.g. inhaled anaesthetics.

37
Q

How does distribution depend on blood flow?

A

Organs with high blood flow will receive the drugs faster - brain, heart, liver.
Low blood flow e.g. adipose tissue will receive drugs later.

38
Q

What is the volume of distribution?

A

The volume of fluid required to contain the total amount of drug in the body at the same concentration as that present in the plasma.
Can estimate the compartment in which a drug might be present.

39
Q

What is the equation for volume of distribution?

A

Vd = D/Cp
D = amount of drug in mg
Cp = concentration of drug in plasma, mg/L.

40
Q

When is volume of distribution measured?

A

Measured after absorption and distribution, but before significant metabolism and excretion.

41
Q

How is the volume of distribution related to heparin?

A

Heparin is large molecule confined to the plasma.
Vd = 0.05L/Kg, and when multiplied it is roughly the same as the volume of plasma.
Gentamicin is a polar molecule, so distributes in the extracellular space.
It’s Vd is the same as the extracellular space.

42
Q

What is the trend in Vd?

A

Drugs that accumulate in lipids have high Vd.
If it accumulates in plasma it has high Vd.

43
Q

How does distribution affect the concentration of drug?

A

Drug is injected, has high peak initial concentration, falls quickly as distributed to compartments of the body.
Will reach an equilibrium with the body concentration.
Elimination occurs - excretion and metabolism.

44
Q

What do the graphs of distribution look like?

A
45
Q

What is drug binding to circulating proteins?

A

Many drugs bind to circulating proteins, especially albumin.
It is the free concentration of a drug that determines its action.

46
Q

What does drug binding to circulating proteins do?

A

It decreases the initial free concentration.
Slows distribution
Slows clearance
Prolongs duration of action.

47
Q

What are examples of drugs bound by albumin?

A

Warfarin and salicylates (aspirin) bind to circulating plasma proteins.
When more than one of these drugs are present, there can be competition for biding, increasing the free drug concentration.

FNP (12 decks)

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