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Pharmacology > Drug metabolism and excretion > Flashcards

Drug metabolism and excretion Flashcards

1
Q

What is the main site of drug metabolism?

A

Liver

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2
Q

What is the main site of drug excretion?

A

Kidney

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3
Q

What are the two main processes of drug metabolism?

A
  1. Molecule is made more lipophilic to reduce the possibility of reabsorption in the renal tubules
  2. Conjugated to reduce its effects and aid excretion
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4
Q

What is a prodrug?

A

Inactive form of drug that becomes active through conversion in the body

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5
Q

Which reactions happen in phase I of metabolism?

A

Oxidation

Reduction

Hydrolysis

Glucyronidation

Glycosylation

Sulfanation

Methylation

Acetylation

Amino acid conjugation

Glutathione conjugation

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6
Q

Which phase I reactions are the most important?

A

The ones by the P450 system

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7
Q

What is required for reactions by the P450 system to take place?

A

Cytochrome P450 haem proteins

Molecular oxygen

cP450 reductase

NADPH

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8
Q

What reaction does P450 catalyse?

A

NADPH + oxidised cyto + H+ -> reduced cyto + NADP+

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9
Q

How can other drugs affect the P450 system?

A

Other drugs can increase/decrease the effect of P450

Some drugs can increase the effect of drugs through inhibiting the P450 system

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10
Q

What processes are involved in renal excretion?

A

Filtration at the glomerulus

Active tubular secretion

Reabsorption from the tubule

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11
Q

Describe how glomerular filtration helps to excrete drugs

A

Arterial blood pressure forces ultrafiltrate from the glomerular capillaries into Bowman’s capsule

Small drug molecules in both charged and uncharged states cross the glomerular membranes readily, achieving a concentration in the filtrate identical to their free concentration in plasma

Drug molecules bound to plasma proteins are not filtered

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12
Q

Describe how active tubular secretion helps to excrete drugs

A

Occurs at the PCR

Low-specificity carriers in the basolateral membrane transport endogenous substances and drugs from the plasma into the tubular cells from where they pass into the filtrate by facilitated diffusion

Tubular secretion of one drug may be competitively inhibited by another

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13
Q

Describe how reabsorption from the tubule affects drug excretion

A

Drugs within the tubules in the unionised and lipid form will be reabsorbed

A weak acid = more readily excreted in alkaline than acidic urine

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14
Q

Self-renewal of bile

A

Bile is produced at a daily rate of 0.5-1 litre

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15
Q

Describe the enterohepatic circulation and how that affects the bioavailability of drugs

A

Drugs may be reabsorbed from the bile back into the intestine to undergo further cycles of billiary excretion

This happens through active transport

This increases the drug’s half life

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16
Q

What determines the course of drug action?

A

The concentration of the drug at the site

An important exception - drugs that bind irreversibly, since the effect outlives the concentration

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17
Q

Describe the course of drug action in well and poorly perfused systems

A

Well perfused tissues can equilibrate with the plasma concentration quickly

Poorly perfused tissues will take up and release drugs slowly

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18
Q

What are ways drugs can enter cells?

A

Direct diffusion through the lipid bilayer

Carrier-mediated transport

Diffusion through aqueous pores

Pinocytosis

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19
Q

What determines the diffusion of a drug through membranes?

A

Concentration gradient

Diffusion coefficient

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20
Q

What is the Henderson Hasselback equation?

A

Determines the fractional ionisation of weak acids and bases

For a weak acid: log10 ci/cu = ph-pKa
For a weak base: log10 ci/cu = pKa – ph

21
Q

What is lipid trapping?

A

In different pH, the level of ionisation is different

Lipid bilayer prevents the movement of ionised molecules

Therefore, we can say that lipid bilayers act as traps preventing the movement of ions

22
Q

What is the importance of carriers for nutrition?

A

Carriers are important for membrane transport of essential nutrients that have low lipid solubility

23
Q

Which transporters are effective for drug transport?

A

Transporters found in the kidney

Important in the elimination of drugs from the body

24
Q

Example of a kidney transporter involved in the removal of drugs

A

P-glycoprotein

Involved in the removal of chemotherapeutic agents

25
Q

What are features of activa transporters?

A

Uphill transport

Finite number of transporters - leads to saturation

26
Q

What are the two main ways of drug administration?

A

Enteral administration

Factors controlling oral absorption

27
Q

What is enteral administration?

A

Oral absorption from the stomach or gut following swallowing

Most convenient/acceptable route

28
Q

What is an important consideration regarding enteral administration of drugs?

A

Oral absorption will deliver the drug directly to the liver, the major site of drug metabolism, via the portal circulation

This will lead to substantial first-pass elimination

29
Q

What type of administration prevent first-pass metabolism from the liver?

A

Sublingual or buccal administration

Lower rectal administration

30
Q

Example of drug given through the sublingual/buccal route

A

Glyceryl trinitrate absorbed quickly to relieve anginal pain

31
Q

Which factors control oral absorption of drugs?

A

Lipid solubility and ionisation

Drug formulation

Gastrointestinal motility

Bioavailability

32
Q

How does lipid solubility and ionisation affect the absorption of drug?

A

Weak acids will be mostly absorbed in the more alkaline intestine

Rapid passage of drug from the stomach to the intestine is likely to speed up drug absorption

33
Q

Fill in the blank

Ionised form of drugs are more -

A

Water soluble

34
Q

Fill in the blank

Unionised form of drugs are more -

A

Membrane permeable

35
Q

How does gastrointestinal motility affect drug absorption?

A

Stasis can slow oral absorption

Diarrhoea may allow insufficient time for complete absorption

36
Q

Causes of low bioavailability

A

Incomplete release of the dosage form

Destruction within the gut

Poor absorption

First-pass elimination

37
Q

What is bioavailability?

A

Proportion of the administered dose that reached the systemic circulation

38
Q

What are the uses of parenteral administration of drugs?

A

Rapid effects

Drugs that are poorly absorbed from the gut

Irritants

Localisation of action

39
Q

Different types of parenteral administration

A

Intravenous

Inhalation

Intramuscular and subcutaneous

Topical

40
Q

Advantage of intravenous administration

A

Most rapid route

41
Q

Advantage of intramuscular and subcutaneous drug administration

A

Utility in providing long-term therapy

42
Q

Describe the difference in drug distribution between small and large molecules

A

Large molecules cannot easily enter interstitial and intracellular spaces

Smaller, lipid-soluble molecules can spread rapidly throughout the whole body

43
Q

What does volume of distribution mean?

A

The volume of fluid required to hold the amount of drug in the body at the measured plasma concentration

44
Q

What is the equation for volume of distribution?

A

Vd = dose/cp

cp concentration of drug in the plasma after it has equilibrated in its distribution volume but before a significant fraction has been eliminated

45
Q

What are different ways drugs can interact with body tissues once administered?

A

Can bind in to proteins and other tissue components

Can accumulate in lipids

Can penetrate into the brain

46
Q

Which proteins bind to drugs?

A

Weak acids = albumin

Weak bases = a-acid glycoprotein

47
Q

What is the consequence of protein binding?

A

The bound drug is usually inactivated

The reduction in free drug concentraiton may reduce elimination or protein binding may serve to deliver the drug to the kidney and liver to enhance elimination

One drug may prevent the binding of another, enahncing the pharmacological activity

48
Q

What two aspects of the brain prevent drug entry?

A

Blood brain barrier - tight juncitons

Turnover of CSF - drugs that penetrate slowly will be removed by washout and achieve a steady-state concentration below the plasma concentration

Pharmacology (13 decks)

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