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Biomedical Pharmacology > Lecture 3: Drug Absorption And Distribution > Flashcards

Lecture 3: Drug Absorption And Distribution Flashcards

1
Q

What is drug disposition?

A

Drug disposition can be consider e to be the processes a drug is subjected to by the body upon administration

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2
Q

What are the processes of drug disposition?

A

Absorption Distribution Metabolism Excretion

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3
Q

What is excretion also known as?

A

Removal from body

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4
Q

What is the most common route of drug administration?

A

Oral or rectal

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5
Q

Why is the oral or rectal route of drug administration most common?

A

Fewest complication

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6
Q

Most drug movement is not

A

Unidirectional

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7
Q

What are the drug administration routes?

A

Oral, rectal, percutaneous, intravenous, intramuscular, intrathecal, inhalational

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8
Q

What are the drug elimination routes?

A

Urine, faeces, milk, sweat, expired air

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9
Q

What can the lung serve as?

A

Route of both administration and elimination

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10
Q

What 2 ways can the drug molecules move around the body?

A

Bulk flow (I.e. bloodstream, lymphatic, cerebrospinal fluid) Diffusion (I.e. molecule by molecule over short distances)

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11
Q

What is the ability to cross the hydrophobic diffusion barriers strongly influenced by?

A

Lipid solubility

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12
Q

What is the concept of Bioavailability?

A

The fraction (F) or the administered dose that reaches the systemic circulation

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13
Q

What is the bioavailability of an intravenous injection?

A

F=1, Bioavailability=100%

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14
Q

What is the bioavailability for other routes?

A

Less than 1

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15
Q

What does a bioavailability of less than 1 reflect?

A

Gastric instability of drug First pass metabolism Reduced absorption into the circulation

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16
Q

What are the factors affecting drug absorption?

A

Gastric motility Area of absorbing surface Blood flow at absorbing surface Ingestion of food Plasma protein Binding Nature of the drug Ionised/ non ionised form

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17
Q

Gastric motility

A

Affected by disease/food How quickly the drug/ substance passes through the GI tract

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18
Q

Area of absorbing surface

A

Large surface area means more absorption across any particular membrane

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19
Q

Blood flow at absorbing surface

A

Affected by food/exercise Direct blood flow away from GI slowing digestion towards lungs/heart and exercising muscles

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20
Q

Ingestion of food

A

Possible interactions

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21
Q

Plasma protein binding

A

Equilibrium between free drug and plasma bound membrane

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22
Q

Nature of the drug. Ionised/ non ionised form

A

Most are either weak acids or weak bases Exhibit PH-dependent ionisation - dependent on PH in different areas of the body Ionised species are not lipid soluble

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23
Q

What sort of drugs diffuse through aqueous channels?

A

Water soluble drugs

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24
Q

What is the most important method of absorption across a membrane?

A

Diffusion across a membrane

25
Q

Diffusion through aqueous pores/ Ion channels

A

Movement occurs down a concentration gradient The membrane contains small pores which allow water soluble molecules of less than 100Da through

26
Q

Examples of agents that use diffusion through aqeuous pores / Ion channels

A

Lithium - small monovalent ion

27
Q

Why is diffusion through aqeuous pores/ Ion channels not a major route for drug permeation?

A

Most drugs aren’t small enough

28
Q

Carrier mediated transport

A

Active transport/ facilitates diffusion

29
Q

What are the important sites for carrier mediated transport?

A

Important stores for such transfer, from a pharmacokinetic point of view include: renal tubule Blood brain barrier are important barriers drug must cross to be absorbed Gastrointestinal tract

30
Q

What can limit the rate of permutation for carrier mediated transport?

A

Saturation of transporters

31
Q

Pinocytosis

A

Concerned with the uptake of marcomolecules Engulfing membrane across very large molecules

32
Q

What size of drugs can enter via pinocytosis?

A

Drugs of large molecular weight (1000Da or more) can permeate via this route

33
Q

What is an example of of a drug that can permeate through pinocytosis?

A

Insulin crossing the blood brain barrier

34
Q

How have researchers tried to incorporate pinocytosis into drug uptake?

A

Incorporating the drug into lipid liposomes

35
Q

What ride drugs does diffusion through lipid Membranes allow for?

A

Only allow small lipid soluble non-polar drugs through

36
Q

What size drugs does diffusion through lipid Membranes allow for?

A

Surface area and blood flow. Degree of permeation is determined through Ficks Law Knowing the percentage of drug ionised

37
Q

What factors of Ficks Law are directly related to rate?

A

dQ/day - amount of drug diffusing with time Pk - permeability constant A - membrane area C1/C2 - concentration of drug either side of the membrane

38
Q

What factors of Ficks Law are indirectly related to rate?

A

Membrane thickness

39
Q

What is the effect of PH on drug ionisation?

A

Important as it allows prediction of absorption Most drugs are either weak acids or weak bases

40
Q

What is PKA ?

A

Dissociation constant - equal to the PH at which 50% of the drug molecules are ionised

41
Q

What does degree of ionisation give rise to?

A

Ion trapping

42
Q

What does a high degree of ionisation mean?

A

Little crosses the membrane

43
Q

Treating drug overdose

A

Alkaline urine means drugs won’t pass to the nephron this fam be used in the case of aspirin overdose. Urine can be alkalinized e.g. with NAOH changing ionised/unionised ratio

44
Q

What is 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

45
Q

What is Cp?

A

Plasma concentration

46
Q

How to calculate volume of distribution?

A

Vd = Dose/Cp

47
Q

What does it mean if you have high lipid solubility?

A

Higher amount of your compound partitions into the cell membrane Faster rate of movement across lipid soluble fell membrane and into the inside of the cell

48
Q

What makes up Fricks Law?

A

Steepness of the concentration gradient The surface area of the membrane across which diffusion is taking place The lipid solubility of the substance Membrane thickness

49
Q

Steepness of the concentration gradient

A

The bigger the concentration gradient, the steeper it is, the faster the rate of diffusion

50
Q

The surface area of membrane across which diffusion takes place

A

The larger the surface area, the greater the rate of diffusion

51
Q

The lipid solubility of the substance

A

The greater the lipid solubility of drug, the faster the substance can diffuse down its concentration gradient through lipid bilayer Smaller transmembrane concentration for insoluble lipid hence slower diffusion

52
Q

Membrane thickness

A

Thicker membrane means slower rate of diffusion Thinner membrane - fast rate of diffusion

53
Q

What is Ion trapping?

A

High PH of a drug that has relatively low PKA

54
Q

What are the 3 body compartments that a weak acid and a weak base would be distributed at equilibrium?

A

Plasma (PH 7.4) Alkaline urine (PH 8) Gastric juice ( PH 3)

55
Q

What does the ratio of ionised to unionised drug govern?

A

PKA is the drug and PH of that compartment

56
Q

What can cross the membrane?

A

Unionised species

57
Q

What cannot cross the membrane?

A

Ionised species

58
Q

How does drugs leave the circulation ?

A

Capillary filtration

59
Q

What are the different body compartments that the drug can distribute to?

A

Transceullar water Intracellular water Interstitial water Plasma water Fat

Biomedical Pharmacology (19 decks)

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