PD

Question 1

Define affinity

Answer 1

tendency of a drug to bind to a specific receptor (the concentration at which half the available receptors are bound) – which is Kd for agonists and Ki¬ for antagonists

Question 2

Define efficacy

Answer 2

the ability of a drug to produce a response as a result of the receptor being occupied

Agonist activity is referenced against themselves. Antagonists are said to have zero efficacy (they reduce the number of receptors available for agonists to bind). Partial agonists are referenced against the maximal efficacy of a named agonist.

Question 3

Define potency

Answer 3

dose required to produce the desired biological response (the smallest dose required the higher the potency)

• Agonist potency is the drug concentration at which 50% of the maximal response is obtained. This is the EC50. Because the response downstream of the receptor isn’t necessarily linear (there is redundancy in the receptors), it doesn’t always equal Kd.
• Antagonist potency is the concentration of drug that reduces maximal activation of a receptor by 50% (can only really be measured in vitro). You can get both competitive and non-competitive antagonism.

Question 4

Describe non-competitive and competitive antagonism and how potency changes

Answer 4

Competitive antagonism – antagonist competes with agonist for receptor sites – can reduce effect by increasing agonist concentration. EC50 changes but maximal effect stays the same

Non-competitive – maximal effect is depressed proportionally to the degree of antagonist present, EC50 stays the same
• Can bind at the site for agonist binding (irreversibly or unbinding very slowly)
• Or bind allosterically

Question 5

Describe how drug interactions affect the absorption of a drug

Answer 5

Agents affecting gut motility, or absorption from the gut.
Metoclopramide (acts as an antiemetic and gastro pro-kinetic) increases rate of gastric emptying and uptake from the small bowel

Question 6

Describe how drug interactions affect the distribution of a drug

Answer 6

Competition between drugs at protein binding sites (usually offset by increased clearance)

Phenytoin – non-linear pharmacokinetics and a narrow therapeutic window can lead to serious toxicity

drugs with greater Vd have lower plasma concs as they spread further throughout the body and are therefore less likely to interact with other drugs

Question 7

Describe how drug interactions affect the metabolism of a drug

Answer 7

Induction of CYP450 enzymes (increases transcription or translation, or slows degradation), leading to more rapid elimination and higher clearance. This usually takes 1-2 weeks.

• don’t withdraw the inducer without altering the dose of drugs that will now be metabolised slower – you risk toxicity
• may increase production of a toxic metabolite

Inhibition of CYP450 enzymes – this increases the half-life. Usually sets in over a few days.

Inducers – Pheytoin, Carbamazepine, Barbituates, Rifampicin, Alcohol (chronic use), Sulphonylureas, St John’s Wort

Inhibiters – Omeprazole, Disulfiram, Erythromycin, Valproate, Isoniazid, Cimetidine, Ciprofloxacin, Ethanol, Sulphonamides, Grapefruit Juice, Cranberry Juice

Question 8

Describe how drug interactions affect the excretion of a drug

Answer 8

Inhibition of tubular secretion

Changes in urine flow/pH

Question 9

Describe how drug disease interaction occur

Answer 9

• Renal disease –
Reduced GFR increased time to clear and so can stay in system longer and build up.
GFR can be further reduced by NSAIDs/ACEi so drugs are nephrotoxic.
Damaged kidneys can cause electrolyte disturbances interacting with drugs.

• Hepatic disease –
Reduced CYP450 activity so drugs have longer half-lives and accumulate leading to adverse effects.
Hypoalbuminaemia so higher free conc of drugs.

• Cardiac disease –
Heart failure leading to oedema allows drugs to accumulate in gut so it’s not distributed or accumulate elsewhere causing excessive effects.
Reduced hepatic and renal blood flow leads to problems with renal and hepatic disease discussed above.

Question 10

What are the types of ADR

Answer 10

On target ADRs are due to an exaggerated therapeutic effect of the drug, usually by acting on the same receptor (increased dosing or other factors). Examples include treatment of hypertension leading to hypotension (leads to dizziness, unsteadiness or temporary loss of consciousness), or antihistamines which work on the same receptor but in the CNS to cause drowsiness.

Off target ADRs – interact with other receptor types, or metabolites that act as toxins. For example paracetamol in overdose.

These are both type A – dose dependent

Type B are idiosyncratic ADRs – due to unique individual disposition or inappropriate immune responses.