16B08: Exam Report

Compare and contrast the pharmacology of ranitidine and omeprazole.

14% of candidates passed this question.

These agents are both commonly used in the ICU. The expectation was weighted towards the interesting and important aspects of pharmacology as outlined for category B drugs. It was expected candidates could detail that both drugs are used to suppress acid secretion in the stomach. Ranitidine is a competitive, reversible inhibitor of the action of histamine at the histamine H2 receptors found in gastric parietal cells. This results in decreased gastric acid secretion and gastric volume, and reduced hydrogen ion concentration.

In contrast to Omeprazole which is a proton pump inhibitor that irreversibly blocks the hydrogen potassium ATPase the gastric parietal cells.

Some general description of dosing, route of administration, pharmacokinetics and possible adverse effects was expected.

O2ii / 16B08: Compare and contrast the pharmacology of ranitidine and omeprazole

Ranitidine

Omeprazole

Use

Ranitidine

PUD

Stress ulcer prophylaxis

GORD

Dyspepsia

Omeprazole

H.Pylori infection

PUD, GORD, Dyspepsia

Zollinger-Ellison syndrome

Upper GI bleed

Presentation

Ranitidine

Tablets

Clear solution injection – 25mg/mL

Omeprazole

Capsules – 40mg powder vial

Dose

Ranitidine

50mg QID (IV)

150mg BD (PO)

Omeprazole

40mg OD or BD (PO)

40mg OD or BD (IV)

MoA

Ranitidine

  • Inhibits gastric acid secretion
  • Competitive Antagonist (actually an I.A) of H2 receptors of PARIETAL CELLS, which secrete HCl & IF
  • Histamine, ACh, gastrin cells stimulate Parietal cells but histamine most potent because:
    1. Blocks HCl secretion
    2. ↓volume of HCl secretion because ACh & gastrin action potentiated by histamine

Omeprazole

  • ↓Gastric acid acidity
  • Irreversibly binds H/K/ATPase pump of Parietal cells
  • NON-COMPETITIVE INHIBITION
  • Prevents H+ excretion

PD

Ranitidine

GI:

  • Gastric acid secretion
  • ↑LES tone (dose related)

Omeprazole

GI:

↓volume of gastric acid

PK

Ranitidine

A

OBA 60%

D

15% PPB, VD 1.2 – 1.8L/kg

M

small % metabolised by oxidation & methylation

E

mostly metabolised by kidneys

t ½ 1.6 – 2.5hrs 

Omeprazole

A

OBA 75%; rapid in 30 min peaks

D

95% PPB, VD 0.4L/kg

M

hepatic by CYP450, almost completely

E

urinary, metabolites excreted

t ½ 1hr

S/E

Ranitidine

Abnormal LFTs

Thrombocytopaenia

Confusion

Omeprazole

Confusion

Blurry vision

Drowsy

Arrhythmias