Drug interactions

Question 1

Cimetidine: drug class

Answer 1

Anti acid

Question 2

Cimetidine: MOA

Answer 2

Histamine (H2) receptor antagonist

Question 3

Cimetidine: side effects pathophys

Answer 3

Potent inhibitor of cytochrome P450 enzymes (CYP1A2, CYP2D6, CYP3A4)
* Enzyme involved in CYP450 biotransformations
* Competitive and reversible inhibitor.
o Also irreversible reported

Question 4

Cimetidine: which cardiac drugs interact

Answer 4

Anti arrhythmics

Question 5

Cimetidine: effects on class IV

Answer 5

• Ca2+ channel blockers (Diltiazem/verapamil/felodipine/nifedipine) (class IV)
  o Hepatic metabolism: CYP450 (1st pass metabolism)
   Excreted by urine
  o ↑ blood levels → can affect HR and BP
   Liver disease can also affect metabolism
  o Amlodipine is not affected

Question 6

Cimetidine: effects on class IA

Answer 6

• Quinidine (class IA)
  o ↑ quinidine levels and risk of toxicity
   Inhibit hepatic metabolism
  o Most significant in
   Old patients
   Hepatic disease

Question 7

Cimetidine: effects on class II

Answer 7

o ↑ blood levels → potentiate B blocking action
 Inhibition of hepatic metabolism
 Reduction of hepatic blood flow

Question 8

Cimetidine: effects on class III

Answer 8

o ↑ blood levels

Question 9

Cimetidine: effects on class IB

Answer 9

• Lidocaine (class IB)
  o Lidocaine metabolism depend on
   Liver flow
• ↓ in CHF and B blockers
   Liver microsomal activity
  o Cimetidine, propranolol and halothane → ↓ hepatic clearance → ↑ blood levels
   Need to lower lidocaine dose

Question 10

Other drugs that will have similar effects to cimetidine on drug metabolism

Answer 10

 Ketoconazole
 Macrolides
 Protease inhibitors (ritonavir, verapamil)