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Gastrointestinal Pharmacology - 2nd Year MBBS Notes

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PART 1: DRUGS USED IN PEPTIC ULCER DISEASE (PUD)

Classification

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1. Proton Pump Inhibitors (PPIs)
   • Omeprazole, Esomeprazole, Lansoprazole, Pantoprazole, Rabeprazole, Dexlansoprazole
2. H2 Receptor Antagonists
   • Cimetidine, Famotidine, Nizatidine (Note: Ranitidine withdrawn from market due to NDMA carcinogen contamination)
3. Anticholinergics (rarely used now)
   • Pirenzepine (selective M1 blocker)

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• Aluminium hydroxide, Magnesium hydroxide, Calcium carbonate, Sodium bicarbonate

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• Sucralfate
• Bismuth subsalicylate
• Misoprostol (Prostaglandin E1 analogue)
• Carbenoxolone

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• Amoxicillin, Clarithromycin, Metronidazole, Tetracycline, Bismuth compounds

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Pharmacological Basis of Drug Use in PUD

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PART 2: OMEPRAZOLE - DETAIL

Mechanism of Action

1. Administered as an inactive prodrug (enteric coated to protect from gastric acid)
2. Absorbed in the alkaline small intestine and transported to the parietal cell via blood
3. Being a lipophilic weak base (pKa ~4), it diffuses across lipid membranes and accumulates in the acidic secretory canaliculus of the parietal cell
4. In the acidic environment, it undergoes protonation and rapid molecular conversion to an active sulfenamide cation
5. This active form forms a covalent disulfide bond with cysteine residues on the H+/K+-ATPase (proton pump), causing irreversible inactivation

• Must be taken 30-60 minutes before meals (only active pumps are inhibited; fed state activates more pumps)
• Short plasma half-life (~0.5-1 hr) but acid inhibition lasts up to 24 hours (irreversible pump inactivation)
• New pumps take ~18 hours to synthesize
• Full effect takes 3-4 days of daily dosing (not all pumps blocked with first dose)
• Metabolized by CYP2C19 (genetic polymorphism affects response) (Katzung Basic & Clinical Pharmacology, 16th Ed.)

Therapeutic Uses of PPIs (Omeprazole)

1. Peptic ulcer disease (duodenal and gastric ulcers)
2. Gastroesophageal reflux disease (GERD) - first line
3. Zollinger-Ellison syndrome (massive acid hypersecretion)
4. H. pylori eradication - always used as the acid suppressor component of triple/quadruple therapy
5. NSAID-induced ulcer - treatment and prevention
6. Stress ulcer prophylaxis in ICU patients

Adverse Effects of Omeprazole

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PART 3: METOCLOPRAMIDE

Mechanism of Action

1. Dopamine D2 receptor antagonist (primary mechanism): Dopamine normally inhibits cholinergic smooth muscle stimulation in the GI tract. By blocking D2 receptors, metoclopramide releases this inhibition, restoring coordinated peristalsis
2. 5-HT4 receptor agonist: Enhances ACh release from myenteric plexus neurons, increasing peristaltic amplitude
3. 5-HT3 receptor antagonist (vagal and central): Contributes to antiemetic effect
4. Sensitizes muscarinic receptors on smooth muscle (possible additional prokinetic mechanism)
5. Blocks D2 receptors in the chemoreceptor trigger zone (CTZ)/area postrema - produces powerful antiemetic effect

Therapeutic Uses

1. Gastroparesis (diabetic, post-surgical) - first-line prokinetic
2. GERD - increases LES pressure; used with antisecretory agents in refractory cases
3. Prevention and treatment of nausea and vomiting (postoperative, drug-induced, migraine-associated)
4. Chemotherapy-induced emesis (high-dose IV)
5. Nonulcer dyspepsia - symptomatic relief
6. Facilitate nasoenteric tube advancement in hospitalized patients
7. Migraine - parenteral use for acute relief

Adverse Effects

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PART 4: PROKINETIC AGENTS

Definition

Classification and Examples

Therapeutic Uses of Prokinetics

1. Gastroparesis (diabetic, idiopathic, post-surgical) - primary indication
2. GERD - increase LES tone; used adjunctively with PPIs
3. Functional dyspepsia / nonulcer dyspepsia
4. Nausea and vomiting (metoclopramide as antiemetic)
5. Intestinal pseudo-obstruction (Ogilvie syndrome - neostigmine IV)
6. Facilitate enteral tube feeding
7. Pre-anesthesia (metoclopramide to reduce aspiration risk)

Adverse Effects (General Class)

• Metoclopramide/Domperidone: EPS reactions, tardive dyskinesia, hyperprolactinaemia (domperidone is safer - does not cross BBB as readily)
• Cisapride: Withdrawn due to fatal cardiac arrhythmias (QT prolongation - Torsades de pointes) from hERG channel block
• Erythromycin: Nausea, abdominal cramping, diarrhea, antibiotic resistance, drug interactions (CYP3A4)
• Bethanechol: Cholinergic effects (excessive salivation, bronchospasm, bradycardia, GI cramping)

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PART 5: LACTULOSE

Mechanism of Action

1. Not hydrolyzed by GI enzymes - reaches the colon intact
2. Colonic bacteria degrade lactulose into lactic acid, formic acid, and acetic acid (short-chain organic acids)
3. This lowers the intraluminal pH and increases osmotic pressure, drawing water into the colon
4. Results in: fluid accumulation, colon distension, soft stools, and defecation

• Acidification of colonic contents converts NH3 (ammonia, which is absorbed) into NH4+ (ammonium ion, which is non-absorbable and trapped in the colon)
• This reduces colonic ammonia production and absorption
• Colonic bacteria that produce ammonia are also suppressed by the acidic environment
• The cathartic effect shortens gut transit time, further limiting ammonia absorption (Lippincott Pharmacology, p. 1440)

Therapeutic Uses

1. Constipation - osmotic laxative
2. Hepatic encephalopathy - primary treatment to lower blood ammonia
3. Portal-systemic encephalopathy - chronic use to prevent recurrence
4. Sometimes used in hyperammonaemia of other causes

Adverse Effects

• Flatulence, bloating, and cramping (from bacterial fermentation and gas production) - most common; may reduce compliance
• Diarrhea (if dose is too high)
• Nausea and vomiting
• Electrolyte disturbances - hypokalemia, hyponatremia with excessive use
• Abdominal distension
• Long-term use: possible lactic acidosis (rare)
• Caution in diabetics - contains small amounts of lactose and galactose (generally safe but monitor)

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PART 6: LAXATIVES - CLASSIFICATION, MOA, AND INDICATIONS

Classification

• Psyllium (Isabgol), Methylcellulose, Bran

• Lactulose, Polyethylene glycol (PEG), Magnesium hydroxide (Milk of Magnesia), Magnesium citrate, Magnesium sulfate (Epsom salt), Sodium sulfate

• Senna (sennosides), Bisacodyl, Castor oil, Cascara sagrada, Dantron

• Docusate sodium, Docusate calcium

• Mineral oil, Glycerin (as suppository)

• Lubiprostone

• Linaclotide, Plecanatide

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MOA and Indications: Two Selected Classes

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CLASS 1: STIMULANT (IRRITANT) LAXATIVES

• Senna: Contains anthraquinone glycosides (sennosides). After oral ingestion, colonic bacteria hydrolyze sennosides to active anthrones. These act directly on nerve fibers in the colonic mucosa, stimulating the myenteric plexus and increasing peristalsis. Also increase electrolyte and water secretion into the bowel.
• Bisacodyl: Acts directly on nerve fibers in the mucosa of the colon, stimulating mass peristaltic movement. Available as suppositories (act within 15-60 min) and enteric-coated tablets (act in 6-10 hrs).
• Castor oil: Hydrolyzed in small intestine to ricinoleic acid, which is irritating to the GI mucosa, stimulating increased peristalsis. Onset 2-6 hours. Avoided now due to poor palatability and risk of intense cramping.

1. Acute constipation requiring rapid relief
2. Pre-procedural bowel preparation (colonoscopy, surgery)
3. Opioid-induced constipation - senna + docusate combination is standard
4. Bedridden patients needing regular bowel management
5. Post-hemorrhoidectomy/anorectal surgery (suppository form)

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CLASS 2: BULK-FORMING LAXATIVES

• These are hydrophilic colloids derived from indigestible plant material (cellulose, hemicellulose, gums)
• They absorb water in the large intestine, swell, and form a viscous gel
• This increases stool bulk and softness, distending the bowel
• Intestinal distension stimulates the defecation reflex and peristaltic activity
• Transit time is reduced
• They mimic the natural physiological effect of dietary fibre

1. Chronic constipation - safest for long-term use; drug of first choice
2. Irritable bowel syndrome (IBS-C)
3. Hemorrhoids and anorectal disorders (softens stool, reduces straining)
4. Diverticular disease - reduce intraluminal pressure
5. Post-bariatric surgery constipation
6. Hypercholesterolemia - psyllium reduces LDL cholesterol (additional benefit)
7. Diabetes mellitus - psyllium reduces postprandial blood glucose

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Contraindications of Laxatives (General)

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Therapeutic Uses of Laxatives (Summary)

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Quick Reference Summary Table

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• KD Tripathi, Essentials of Medical Pharmacology, 8th Ed. - Chapter on Drugs for Peptic Ulcer, GI Motility
• Lippincott Illustrated Reviews Pharmacology, 8th Ed. - Ch. 42 (GI Drugs)
• Katzung Basic & Clinical Pharmacology, 16th Ed. - Ch. 62 (GI Pharmacology)
• Goodman & Gilman's Pharmacological Basis of Therapeutics - Metoclopramide section

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DRUGS USED IN PEPTIC ULCER DISEASE

2nd Year MBBS Pharmacology - KD Tripathi Reference

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PATHOPHYSIOLOGY OVERVIEW (Basis for Drug Use)

• Acetylcholine (via vagus nerve, M1/M3 receptors) - raises intracellular Ca2+
• Histamine (from ECL cells, via H2 receptors) - raises cAMP via Gs-adenylyl cyclase
• Gastrin (from G cells) - raises intracellular Ca2+

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CLASSIFICATION OF DRUGS USED IN PEPTIC ULCER

GROUP I: DRUGS THAT REDUCE ACID SECRETION (Antisecretory Drugs)

• Omeprazole, Esomeprazole, Lansoprazole, Pantoprazole, Rabeprazole, Dexlansoprazole

• Cimetidine, Famotidine, Nizatidine
• (Ranitidine withdrawn due to NDMA carcinogen contamination)

• Pirenzepine (selective M1 blocker)
• Dicyclomine, Propantheline

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GROUP II: ANTACIDS (Acid Neutralizers)

• Aluminium hydroxide Al(OH)3
• Magnesium hydroxide Mg(OH)2
• Calcium carbonate CaCO3
• Sodium bicarbonate NaHCO3
• Combinations: Magaldrate (Mg-Al complex)

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GROUP III: MUCOSAL PROTECTIVE / CYTOPROTECTIVE AGENTS

• Misoprostol (PGE1 analogue)

• Bismuth subsalicylate, Colloidal bismuth subcitrate (CBS/De-Nol)

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GROUP IV: ANTI-H. PYLORI (ERADICATION THERAPY)

• Amoxicillin
• Clarithromycin
• Metronidazole / Tinidazole
• Tetracycline
• Bismuth compounds (dual role)

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PHARMACOLOGICAL BASIS OF EACH GROUP'S USE

1. Proton Pump Inhibitors

• Inhibit 90-98% of 24-hour acid secretion (vs ~70% by H2 blockers)
• Heal >90% of duodenal ulcers within 4 weeks
• Heal >90% of gastric ulcers within 6-8 weeks
• Essential component of H. pylori eradication regimens (also raise gastric pH, which increases antibiotic efficacy against H. pylori)

2. H2 Receptor Antagonists

• Suppress ~70% of 24-hour acid secretion
• Particularly effective against nocturnal acid (night-time secretion is mostly histamine-driven with little food stimulation)
• Healing duodenal ulcers in 4-8 weeks; best given as a single evening dose

3. Antacids

• Raise gastric pH above 4, at which point pepsin becomes inactive (additional benefit)
• Provide rapid symptomatic relief (within minutes)
• Do NOT reduce acid production; do NOT heal ulcers
• Used for symptomatic relief - taken after meals and at bedtime

4. Anticholinergics (Pirenzepine)

5. Misoprostol (Prostaglandin)

• Inhibit adenylyl cyclase via Gi → reduce cAMP → decrease acid secretion
• Stimulate mucus and bicarbonate secretion
• Maintain mucosal blood flow

6. Sucralfate

• Binds to proteins at the ulcer base forming a physical protective barrier against acid and pepsin
• Stimulates mucus and bicarbonate secretion
• Stimulates prostaglandin synthesis
• Adsorbs pepsin and bile acids

7. Bismuth

• Antimicrobial: Directly kills H. pylori (disrupts bacterial cell wall and membrane)
• Coats the ulcer crater with a protective layer
• Inhibits pepsin activity
• Stimulates mucus secretion

8. Antibiotics (H. pylori Eradication)

• Achieves >90% cure rate when combined with PPI
• Reduces ulcer recurrence from 60-100% per year to <15%
• Standard regimens:
  • Triple therapy (14 days): PPI + Clarithromycin + Amoxicillin (or Metronidazole)
  • Quadruple therapy (14 days - first line now): PPI + Bismuth + Metronidazole + Tetracycline
  • Concomitant quadruple (no bismuth): PPI + Amoxicillin + Clarithromycin + Metronidazole

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OMEPRAZOLE - DETAILED PHARMACOLOGY

Chemistry

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Mechanism of Action (Step-by-Step)

PRODRUG (inactive) → absorbed intestinally → enters parietal cell canaliculus
→ acid-activated to SULFENAMIDE → covalent bond to H+/K+-ATPase → IRREVERSIBLE inhibition
→ H+ secretion blocked → gastric pH rises

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Pharmacokinetics of Omeprazole

• Why short t½ but long action? The pump is inactivated irreversibly. Even though omeprazole is cleared in ~1 hour, acid suppression continues until new proton pumps are synthesized (~18 hours for new pump synthesis). Full acid secretion returns only 3-4 days after stopping the drug.
• Why take 30-60 minutes before meals? Only actively secreting pumps are in the canaliculus and susceptible to inhibition. Fasting state → only ~10% of pumps active. Meals stimulate pump activation. Taking omeprazole 30-60 minutes before breakfast ensures peak drug levels coincide with maximum pump activity.
• Why 3-4 days for full effect? Not all pumps are inactivated with the first dose (some pumps are in quiescent vesicles and not yet accessible). Progressive inactivation with each daily dose until steady-state suppression is achieved.
• CYP2C19 polymorphism: "Poor metabolizers" (higher drug levels) have better acid suppression. "Ultra-rapid metabolizers" may need higher doses.

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Therapeutic Uses of Omeprazole / PPIs

1. Peptic ulcer disease (gastric and duodenal ulcers) - first-line treatment
2. GERD / erosive esophagitis - first-line; heals erosions in 85-90% with once-daily dosing
3. Zollinger-Ellison syndrome (gastrinoma-driven massive acid hypersecretion) - high-dose PPIs
4. H. pylori eradication - always the PPI component in all regimens
5. NSAID-induced ulcers - treatment and prophylaxis (preferred over misoprostol)
6. Stress ulcer prophylaxis - in ICU patients (IV esomeprazole/pantoprazole)
7. GI bleed from peptic ulcer - high-dose IV PPI (omeprazole 80 mg bolus + 8 mg/hr infusion) reduces rebleeding
8. Barrett's esophagus - long-term acid suppression
9. Nonulcer dyspepsia - modest benefit

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Adverse Effects of Omeprazole

SHORT-TERM (Common, usually mild)

LONG-TERM (Important - high-yield for exams)

DRUG INTERACTIONS

CONTRAINDICATIONS

• Hypersensitivity to benzimidazoles
• Concurrent use with sucralfate (requires acidic pH for activation - PPIs reduce efficacy)
• Caution: severe hepatic impairment (dose reduction needed)

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SUMMARY TABLE: KEY DRUG COMPARISONS

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• Lippincott Illustrated Reviews Pharmacology, 8th Ed. - Ch. 42: Gastrointestinal Drugs (p. 1415-1428)
• Katzung Basic & Clinical Pharmacology, 16th Ed. - Ch. 62: Drugs Used in the Treatment of Gastrointestinal Diseases (p. 1699-1708)
• Goodman & Gilman's Pharmacological Basis of Therapeutics - Ch. 53: Pharmacotherapy of Gastric Acidity, Peptic Ulcers, and GERD (p. 1076-1088)
• KD Tripathi Essentials of Medical Pharmacology, 8th Ed. - Section IX: GI Pharmacology

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DRUGS USED IN PEPTIC ULCER DISEASE

2nd Year MBBS Pharmacology | KD Tripathi / Lippincott / Katzung Reference

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BACKGROUND: PATHOPHYSIOLOGY (Basis for Drug Therapy)

1. Helicobacter pylori infection (~90% of duodenal, ~70-80% of gastric ulcers)
2. NSAID/aspirin use (inhibit COX → reduce mucosal prostaglandins)

• Acetylcholine (from vagus nerve, via M1/M3 receptors) → raises intracellular Ca2+
• Histamine (from ECL cells, via H2 receptors) → activates Gs → adenylyl cyclase → raises cAMP
• Gastrin (from G cells, via CCK-B receptors) → raises intracellular Ca2+

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CLASSIFICATION OF ANTIULCER DRUGS

GROUP I: DRUGS THAT REDUCE ACID SECRETION (Antisecretory)

• Omeprazole, Esomeprazole, Lansoprazole, Pantoprazole, Rabeprazole, Dexlansoprazole

• Cimetidine, Famotidine, Nizatidine
• (Ranitidine withdrawn worldwide due to NDMA carcinogen contamination)

• Pirenzepine (selective M1 antagonist)
• Propantheline, Dicyclomine

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GROUP II: ANTACIDS (Acid Neutralizers)

• Aluminium hydroxide - Al(OH)3
• Magnesium hydroxide - Mg(OH)2
• Calcium carbonate - CaCO3
• Sodium bicarbonate - NaHCO3
• Combinations: Magaldrate (hydroxymagnesium aluminate)

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GROUP III: MUCOSAL PROTECTIVE / CYTOPROTECTIVE AGENTS

• Misoprostol (PGE1 analogue)

• Colloidal bismuth subcitrate (CBS / De-Nol)
• Bismuth subsalicylate (Pepto-Bismol)

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GROUP IV: ANTI-H. PYLORI DRUGS (Eradication Therapy)

• Amoxicillin
• Clarithromycin
• Metronidazole / Tinidazole
• Tetracycline
• Bismuth compounds (dual role - also cytoprotective)

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MOA + PHARMACOLOGICAL BASIS OF EACH GROUP

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1. PROTON PUMP INHIBITORS (PPIs)

1. Administered as enteric-coated inactive prodrug (protects from gastric acid)
2. Absorbed in the alkaline small intestine → enters blood
3. Being a lipophilic weak base (pKa ~4), diffuses across membranes and selectively accumulates in the acidic secretory canaliculus of parietal cells (concentrated >1000-fold by Henderson-Hasselbalch trapping)
4. In the acidic canaliculus, the prodrug is protonated and rapidly converted to an active sulfenamide cation
5. The active form forms a covalent disulfide bond with cysteine residues on the H+/K+-ATPase → irreversible inactivation

• H2 blockers only block histamine stimulation (one of three pathways)
• PPIs block the pump itself - blocking all stimuli simultaneously
• Heal >90% of duodenal ulcers in 4 weeks, >90% of gastric ulcers in 6-8 weeks
• PPIs raise gastric pH, which also increases the efficacy of co-administered antibiotics against H. pylori

Why take 30-60 min before breakfast? Only actively secreting pumps (in canaliculus) are susceptible. Fasting → only ~10% pumps active. A meal activates more pumps. Taking PPI before breakfast aligns peak drug levels with maximum pump activity.

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2. H2 RECEPTOR ANTAGONISTS

• Competitively and reversibly block H2 receptors on the basolateral membrane of parietal cells
• Histamine H2 receptor → Gs protein → adenylyl cyclase → cAMP → protein kinase A → activates proton pump
• H2 blockers block this entire signaling cascade by competitive antagonism at the H2 receptor
• They are selective for H2 receptors and have no effect on H1 receptors
• Reduce acid secretion by approximately 70% (basal, food-stimulated, and nocturnal)

• Nocturnal acid secretion is predominantly histamine-mediated (less food, vagal input minimal)
• H2 blockers are therefore particularly effective for nocturnal acid suppression - important in healing duodenal ulcers
• Evening single dose is sufficient for most patients
• Less potent than PPIs; cannot block gastrin- or ACh-stimulated acid completely
• Tolerance develops with continuous use (tachyphylaxis) - PPIs do not show this

• Oral: rapid absorption, peak levels in 1-3 hours
• Widely distributed (including breast milk, across placenta)
• Excreted mainly by kidneys - dose reduction in renal failure
• Famotidine available IV; cimetidine IV/IM

1. Peptic ulcer (duodenal > gastric) - healing in 4-8 weeks
2. GERD / heartburn (mild-moderate)
3. Stress ulcer prophylaxis (IV infusion in ICU patients)
4. Zollinger-Ellison syndrome (adjunct, high doses)

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3. ANTACIDS

• Weak bases that chemically neutralize HCl already present in the stomach lumen:
  • Al(OH)3 + 3HCl → AlCl3 + 3H2O
  • Mg(OH)2 + 2HCl → MgCl2 + 2H2O
  • CaCO3 + 2HCl → CaCl2 + H2O + CO2
• Raise gastric pH above 4 → pepsin becomes inactive (pepsin is inactive at pH >4) → additional benefit
• Do NOT reduce acid production; merely neutralize acid already secreted

• Provide rapid symptomatic relief (within minutes) - faster than any antisecretory drug
• Short duration of action (30-60 min if fasting; up to 3 hrs if taken after meals - food delays gastric emptying)
• Used after meals for maximum effectiveness
• Used as adjuncts - not primary healing agents

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4. ANTICHOLINERGICS (Pirenzepine)

• Pirenzepine is a selective M1 muscarinic receptor antagonist
• Blocks M1 receptors on parietal cells and on myenteric plexus neurons (which mediate vagal stimulation)
• Reduces vagal (ACh-mediated) stimulation of acid secretion by ~40-50%

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5. MISOPROSTOL (Prostaglandin Analogue)

• Synthetic analogue of Prostaglandin E1 (PGE1)
• PGE1 normally acts via Gi protein → inhibits adenylyl cyclase → reduces cAMP → decreases acid secretion from parietal cells
• Also directly stimulates mucus and bicarbonate secretion from gastric epithelial cells (cytoprotective effect)
• Maintains mucosal blood flow

• Specifically used for prevention of NSAID-induced gastric ulcers in high-risk patients (elderly, prior ulcer history, high-dose NSAID users)
• PPIs are now preferred for this indication due to better tolerability

• Diarrhea (dose-limiting, occurs in ~30%) - most common reason for discontinuation
• Abdominal cramping
• Uterine contractions - CONTRAINDICATED IN PREGNANCY (used medically for abortion/cervical ripening)
• Nausea, vomiting

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6. SUCRALFATE

• In an acidic environment (pH < 4), sucralfate undergoes polymerization into a viscous, sticky gel
• Binds to positively charged proteins at the base of the ulcer crater (especially in necrotic tissue), forming a physical protective barrier
• This barrier protects the ulcer from acid, pepsin, and bile salts
• Also stimulates mucus and bicarbonate secretion
• Adsorbs pepsin and bile salts directly
• Stimulates prostaglandin synthesis locally
• Promotes epithelial cell growth and healing

• Acts locally; minimal systemic absorption
• Effective for duodenal ulcers and stress ulcer prophylaxis

• Constipation (most common - due to Al content)
• Binds other drugs and impairs their absorption (tetracyclines, fluoroquinolones, digoxin, phenytoin) - take 2 hours apart
• Nausea, dry mouth

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7. BISMUTH COMPOUNDS

• Directly kills H. pylori - disrupts bacterial cell wall, prevents adhesion to gastric mucosa, inhibits urease, phospholipase, and proteases of the bacteria

• Coats the ulcer crater with a glycoprotein-bismuth complex
• Inhibits pepsin activity
• Stimulates mucus secretion
• Binds to glycoproteins in necrotic mucosal tissue

• Black stools and black tongue (bismuth sulfide - harmless, warn patients)
• Constipation
• Bismuth encephalopathy with excessive long-term use (now rare with recommended doses)
• Nausea

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8. ANTI-H. PYLORI DRUGS

• H. pylori causes 90% of duodenal ulcers. Eradication achieves true cure
• Recurrence rate with acid suppression alone = 60-100% per year
• Recurrence after successful eradication = <15%
• PPI is always co-administered to: (a) suppress acid directly, (b) raise gastric pH → increases antibiotic efficacy against H. pylori

• Triple therapy (14 days): PPI (BD) + Clarithromycin 500 mg (BD) + Amoxicillin 1g (BD) [or Metronidazole if penicillin allergy]
• Quadruple therapy (14 days, now first-line): PPI (BD) + Bismuth 524 mg (QID) + Metronidazole 500 mg (QID) + Tetracycline 500 mg (QID)
• Eradication success rate: >90% with quadruple therapy

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OMEPRAZOLE - COMPLETE DETAILED PHARMACOLOGY

Chemistry

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Mechanism of Action - Detailed (Most Asked in Exams)

PRODRUG (inactive, acid labile)
    ↓  [enteric-coated → intestinal absorption]
Bloodstream → parietal cell
    ↓  [accumulates in canaliculus, pH 1-2]
Active SULFENAMIDE CATION
    ↓  [covalent bond to Cys813/892]
H+/K+-ATPase - IRREVERSIBLY INACTIVATED
    ↓
No H+ secreted → Gastric pH rises

• Blocks all three stimulatory pathways (ACh, histamine, gastrin) simultaneously by acting at the final common pump
• H2 blockers only block histamine (one of three); gastrin and ACh pathways still active
• PPIs achieve 90-98% suppression of 24-hour acid; H2 blockers achieve only ~70%

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Pharmacokinetics

• Short t½ (~1 hr) but 24-hr action: Pump inactivation is irreversible. Even after omeprazole is cleared from plasma, the pump remains inactive until new H+/K+-ATPase is synthesized (~18 hours). Hence acid suppression outlasts drug levels.
• Why 3-4 days for full effect: Only actively secreting pumps (those in the canaliculus) are accessible to omeprazole. On day 1, many pumps are quiescent (in tubulovesicles) and not blocked. With each daily dose more pumps are inactivated as they activate. Full suppression (~70% of all pumps) is achieved by day 3-4.
• Take before breakfast: Meals stimulate pumps to move from vesicles to canaliculus (active state). Taking omeprazole 30-60 min before breakfast ensures peak drug concentration coincides with maximum pump activation, maximizing the number of pumps blocked.

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Therapeutic Uses of Omeprazole

1. Peptic ulcer disease (duodenal and gastric) - first-line; heals >90% in 4-8 weeks
2. GERD / erosive esophagitis - first-line; 85-90% symptom relief and healing with once-daily dose
3. Zollinger-Ellison syndrome - high doses (60-120 mg/day omeprazole) for massive acid hypersecretion from gastrinoma
4. H. pylori eradication - always the PPI component in all triple/quadruple regimens (acid suppression + enhances antibiotic efficacy)
5. NSAID-induced ulcers - treatment and prevention (preferred over misoprostol)
6. Stress ulcer prophylaxis in ICU patients (IV esomeprazole/pantoprazole)
7. GI bleed from peptic ulcer - high-dose IV PPI (80 mg bolus + 8 mg/hr infusion) reduces rebleeding by raising pH >6, promoting clot stability
8. Barrett's esophagus - long-term maintenance
9. Nonulcer dyspepsia - modest benefit

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Adverse Effects of Omeprazole

SHORT-TERM (Common)

LONG-TERM (High Yield - Exams)

DRUG INTERACTIONS (High Yield)

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COMPARATIVE SUMMARY TABLE

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QUICK MEMORY AID - Drug Classification (KD Tripathi Style)

ANTIULCER DRUGS
│
├─ REDUCE ACID SECRETION
│   ├─ PPIs: Omeprazole, Lansoprazole, Pantoprazole, Esomeprazole, Rabeprazole
│   ├─ H2 Blockers: Cimetidine, Famotidine, Nizatidine
│   └─ Anticholinergics: Pirenzepine (M1 selective)
│
├─ NEUTRALIZE ACID
│   └─ Antacids: Al(OH)3, Mg(OH)2, CaCO3, NaHCO3
│
├─ PROTECT MUCOSA (Cytoprotective)
│   ├─ Misoprostol (PGE1 analogue)
│   ├─ Sucralfate
│   ├─ Bismuth compounds
│   └─ Carbenoxolone
│
└─ ERADICATE H. PYLORI
    └─ Antibiotics: Amoxicillin, Clarithromycin, Metronidazole, Tetracycline + Bismuth

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• KD Tripathi, Essentials of Medical Pharmacology, 8th Ed. - Ch. Drugs for Peptic Ulcer
• Lippincott Illustrated Reviews Pharmacology, 8th Ed. - Ch. 42, pp. 1415-1428
• Katzung Basic & Clinical Pharmacology, 16th Ed. - Ch. 62, pp. 1699-1708
• Goodman & Gilman's Pharmacological Basis of Therapeutics - Ch. 53