Proton Pump Inhibitors, H2 blockers and Antacids pharmacology detailed notes

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Pharmacology Notes: PPIs, H2 Blockers, and Antacids

Sources: Goodman & Gilman's Pharmacological Basis of Therapeutics; Lippincott Illustrated Reviews: Pharmacology, 8e; Katzung's Basic and Clinical Pharmacology, 16e

Gastric Acid Physiology - The Foundation

Understanding these drugs requires knowing how acid is made. Three stimuli drive parietal cells to secrete H⁺:

Acetylcholine (via M3 muscarinic receptors) - Ca²⁺-dependent pathway
Histamine (via H2 receptors on parietal cells) - cAMP/PKA pathway
Gastrin (via CCK2 receptors) - Ca²⁺-dependent pathway

All three pathways converge on activation of the H⁺/K⁺-ATPase (proton pump), which exchanges H⁺ for K⁺ across the apical membrane of the parietal cell into the gastric lumen. The proton pump is the final common effector of acid secretion.

Parietal cell pharmacology showing all three stimulatory pathways and drug sites of action

Figure: Gastric acid secretion by the parietal cell, showing sites of drug action - Lippincott Illustrated Reviews: Pharmacology

Full pharmacological map of gastric regulation including ECL cells, G cells, ENS, and all drug targets

Figure: Pharmacological basis of therapy for acid-peptic disorders - Goodman & Gilman's

PART 1: PROTON PUMP INHIBITORS (PPIs)

Drugs in Class

Generic Name	Brand Name
Omeprazole	Prilosec
Esomeprazole (S-isomer of omeprazole)	Nexium
Lansoprazole	Prevacid
Dexlansoprazole (R-enantiomer of lansoprazole)	Dexilant
Pantoprazole	Protonix
Rabeprazole	Aciphex

All PPIs have equivalent efficacy at comparable doses.

Mechanism of Action

PPIs are prodrugs - they require acid activation to work.

Step-by-step activation:

Administered as acid-labile prodrugs (enteric-coated or delayed-release to survive stomach transit)
Absorbed in the small intestine → enter systemic circulation
Diffuse into parietal cell secretory canaliculi (the most acidic compartment in the body, pH ~1)
Protonation converts the prodrug into a tetracyclic sulfenamide (active form)
The sulfenamide forms covalent disulfide bonds with cysteine residues on the α-subunit of H⁺/K⁺-ATPase
This irreversibly inactivates the proton pump

Because the bond is irreversible, acid suppression persists until new pump protein is synthesized (~18-24 hours). This is why the duration of action far exceeds the plasma half-life (t½ ~1-2 hours, but effect lasts ~24 hours).

"The most potent suppressors of gastric acid secretion are inhibitors of the gastric H⁺,K⁺-ATPase or proton pump. These drugs diminish the daily production of acid (basal and stimulated) by 80% to 95%." - Goodman & Gilman's

Pharmacokinetics (ADME)

Property	Detail
Absorption	Small intestine (enteric coating protects from gastric acid)
Timing	Give 30 min before the first meal - food stimulates acid which activates PPIs; fasting increases H⁺/K⁺-ATPase expression
Protein binding	Highly protein-bound
Metabolism	Extensively by hepatic CYP2C19 and CYP3A4
Half-life	~1-2 hours (much shorter than duration of action)
CYP2C19 genetics	Asians/Oceanians: 25-60% are poor metabolizers → heightened efficacy/toxicity; Caucasians/Africans: ~15%
Second-generation PPIs	Dexlansoprazole, esomeprazole, and rabeprazole have less dependence on CYP2C19
IV forms	Esomeprazole sodium, omeprazole sodium, pantoprazole

Therapeutic Uses

Peptic ulcer disease (PUD) - duodenal and gastric ulcers; also used in H. pylori triple/quadruple therapy regimens
GERD and erosive esophagitis - first-line agent
Zollinger-Ellison syndrome - hypersecretory state; high-dose PPIs are required
NSAID-induced ulcer prevention - superior to H2 blockers
Stress ulcer prophylaxis in ICU
GI bleeding - IV PPIs used to stabilize clots by raising gastric pH
Barrett's esophagus - long-term maintenance

Adverse Effects

Effect	Mechanism / Notes
Headache, diarrhea, nausea	Most common GI side effects
Hypomagnesemia	Long-term use; monitor Mg²⁺, especially with diuretics
Vitamin B12 deficiency	Reduced gastric acid impairs B12 absorption (especially long-term, high-dose)
Iron, Ca²⁺ malabsorption	Reduced acid decreases solubility of non-heme iron and Ca²⁺ salts
Bone fractures (increased risk)	Multifactorial; Ca²⁺ malabsorption plays a role
Clostridium difficile infection	Reduced acid removes a barrier against ingested bacteria
Hypergastrinemia	Acid suppression removes negative feedback on gastrin release → rebound hypersecretion on stopping
ECL cell hyperplasia / fundic gland polyps	Consequence of hypergastrinemia
Community-acquired pneumonia	Reduced gastric acid may allow colonization
Rebound acid hypersecreretion	Occurs on discontinuation after prolonged use
Interstitial nephritis	Rare; all PPIs

Drug Interactions

Interaction	Mechanism
Clopidogrel	Omeprazole and esomeprazole inhibit CYP2C19, reducing clopidogrel activation → reduced antiplatelet effect. Boxed warning: avoid omeprazole/esomeprazole with clopidogrel. Use pantoprazole or an H2 blocker instead
Warfarin	Esomeprazole, lansoprazole, omeprazole, rabeprazole inhibit CYP-mediated warfarin clearance
Methotrexate	PPIs competitively inhibit methotrexate renal elimination → toxic levels
Atazanavir/nelfinavir	Reduced absorption due to raised pH
Ketoconazole, ampicillin esters, iron salts	Require acid for optimal absorption
Diazepam	Esomeprazole and omeprazole inhibit metabolism
Theophylline, imipramine	Omeprazole induces CYP1A2 → increased clearance
Phenytoin, disulfiram	Omeprazole inhibits CYP2C19 → decreased clearance

PART 2: H2 RECEPTOR ANTAGONISTS (H2 Blockers)

Drugs in Class

Generic Name	Brand Name	Status
Famotidine	Pepcid	Available
Cimetidine	Tagamet	Available (limited use)
Nizatidine	Axid	Limited availability
Ranitidine	Zantac	Withdrawn from market (NDMA contamination)

"Cimetidine was the first H2 receptor antagonist. However, its utility is limited by its adverse effect profile and drug-drug interactions." - Lippincott Illustrated Reviews

Mechanism of Action

H2 blockers are competitive, reversible antagonists at histamine H2 receptors located on the basolateral membrane of parietal cells.

Block the H2 receptor → reduce cAMP production → reduce protein kinase A activation → reduced H⁺/K⁺-ATPase stimulation
They are selective for H2 receptors; no effect on H1 receptors
Because histamine acts as a "permissive" stimulant that amplifies acetylcholine and gastrin signaling, H2 blockade dampens responses to all three stimuli (not just histamine alone)
Suppress basal and nocturnal acid secretion most effectively (~70% reduction over 24 hours)
Less potent than PPIs at suppressing meal-stimulated acid

Pharmacokinetics (ADME)

Property	Cimetidine	Ranitidine	Famotidine	Nizatidine
Onset (oral)	~1-3 h	~1-3 h	~1-3 h	~1-3 h
Duration	4-5 h (IV)	6-8 h (IV)	10-12 h (IV)	~10 h
t½	~1-2 h	~2-3 h	~2.5-3.5 h	~1-2 h
Elimination	Mainly renal (filtration + tubular secretion)	Renal	Renal	Renal
Protein binding	Low	Low	Low	Low
Hepatic metabolism	25-35%	<20%	<10%	<15%

Key point: Dose reduction required in renal impairment for all agents. Neither hemodialysis nor peritoneal dialysis significantly clears these drugs. Liver disease is generally NOT an indication for dose adjustment.

IV dosing (from Goodman & Gilman's Table 53-1):

Route	Cimetidine	Famotidine
Intermittent bolus	300 mg every 6-8 h	20 mg every 12 h
Continuous infusion	37.5-100 mg/h	1.7 mg/h

Therapeutic Uses

Peptic ulcer disease: All H2 blockers equally effective for duodenal and gastric ulcers. Evening dosing adequate for most cases (because nocturnal acid suppression is key for duodenal ulcer healing). Must combine with antibiotics if H. pylori present - H2 blockers alone do not prevent recurrence.
GERD / Heartburn: Effective for mild-moderate GERD; slower onset than antacids (up to 45 minutes). PPIs are now preferred.
Acute stress ulcer prophylaxis: IV infusion in ICU patients. Tolerance may develop with continuous use.
Zollinger-Ellison syndrome: Less effective than PPIs; rarely used now.
Over-the-counter heartburn: Still widely available OTC (particularly famotidine).

Adverse Effects

H2 blockers have an excellent safety record overall.

Cimetidine-specific adverse effects (most problematic of the class):

Antiandrogenic effects: Inhibits dihydrotestosterone binding to androgen receptors → gynecomastia, impotence, decreased libido in men; galactorrhea in women
CNS effects: Confusion, dizziness, headache (especially in elderly and critically ill)
Elevated serum creatinine: Blocks tubular secretion of creatinine (does not reflect true GFR change)
Most drug interactions of all H2 blockers (CYP inhibition)

Class effects:

Tolerance: Tachyphylaxis develops within days to weeks of continuous use (receptor upregulation/desensitization) - a limitation for long-term efficacy
Headache, diarrhea (uncommon)
Thrombocytopenia (rare)
Interstitial nephritis (rare)

Drug Interactions

Cimetidine is by far the most problematic:

Inhibits CYP1A2, CYP2C9, CYP2D6, CYP3A4 → reduces metabolism of:
- Warfarin, theophylline, phenytoin, carbamazepine, lidocaine, quinidine, beta-blockers, tricyclic antidepressants, calcium channel blockers, opioids
Inhibits renal tubular secretion of creatinine, procainamide, metformin

All H2 blockers:

May reduce absorption of drugs requiring acidic pH (ketoconazole, itraconazole)
Antacids can reduce absorption of H2 blockers if taken simultaneously (separate by at least 1 hour)

Tolerance (Tachyphylaxis)

A clinically important limitation: tolerance develops to H2 blockers within days of regular use. This does NOT occur with PPIs. The mechanism involves compensatory upregulation of H2 receptors and/or enhanced responsiveness to gastrin and acetylcholine pathways. This is why PPIs are now preferred for healing ulcers and erosive GERD.

PART 3: ANTACIDS

Drugs in Class

Drug	Formula	Key Properties
Aluminum hydroxide	Al(OH)₃	Causes constipation; slower onset
Magnesium hydroxide ("Milk of Magnesia")	Mg(OH)₂	Causes diarrhea
Calcium carbonate ("Tums")	CaCO₃	Fast onset; also used as Ca²⁺ supplement
Sodium bicarbonate	NaHCO₃	Fast but transient; NOT recommended (systemic alkalosis, Na load)
Combined Al(OH)₃ + Mg(OH)₂ (Maalox, Mylanta)	Mix	Balances bowel effects

Mechanism of Action

Antacids work by direct chemical neutralization of hydrochloric acid in the gastric lumen - they do not affect acid secretion.

Key reactions:

Al(OH)₃ + 3 HCl → AlCl₃ + 3 H₂O
Mg(OH)₂ + 2 HCl → MgCl₂ + 2 H₂O
CaCO₃ + 2 HCl → CaCl₂ + H₂O + CO₂
NaHCO₃ + HCl → NaCl + H₂O + CO₂ (produces gas/belching)

The efficacy depends on:

Acid-neutralizing capacity of the formulation
Fed vs. fasted state: Food delays gastric emptying → prolongs antacid contact time and duration of action. Antacids taken after meals last 2-3 hours vs. ~30 min if taken fasting.

Therapeutic Uses

Symptomatic relief of heartburn, GERD, peptic ulcer disease
Rapid onset - work within minutes; therefore useful for on-demand symptom relief
Give after meals for maximum duration of effect
Calcium carbonate preparations also used as calcium supplements for osteoporosis prevention
Anesthesia premedication (sodium citrate) to raise gastric pH before emergency surgery

Adverse Effects

Antacid	Main Adverse Effect	Notes
Aluminum hydroxide	Constipation	Also binds dietary phosphate → hypophosphatemia with long-term use (used therapeutically in dialysis patients)
Magnesium hydroxide	Diarrhea	Mg²⁺ accumulation in renal failure → hypermagnesemia
Calcium carbonate	Constipation, acid rebound, hypercalcemia	Milk-alkali syndrome with excessive use
Sodium bicarbonate	Metabolic alkalosis, sodium load, CO₂ bloating	Contraindicated in hypertension, heart failure, renal failure
Combined Al/Mg	Neutralizes bowel effects	Still risk of ion accumulation in renal impairment

Renal impairment: Accumulation of Mg²⁺, Al³⁺, and Ca²⁺ cations can cause toxicity. Use with caution or avoid in patients with renal failure.

Drug Interactions (Antacids)

Antacids affect absorption of many drugs by:

Raising gastric pH → reducing dissolution of weak bases (ketoconazole, itraconazole, atazanavir)
Chelation / adsorption: Al³⁺ and Mg²⁺ bind to and reduce absorption of:
- Tetracyclines, fluoroquinolones (ciprofloxacin), iron supplements, digoxin, levothyroxine, azithromycin
Separate antacids from these drugs by at least 2 hours

COMPARATIVE SUMMARY TABLE

Feature	PPIs	H2 Blockers	Antacids
Mechanism	Irreversible inhibition of H⁺/K⁺-ATPase	Competitive H2 receptor blockade	Direct acid neutralization
Acid suppression	80-95%	~70%	No effect on secretion
Onset of action	30-60 min (takes days for full effect with new pumps)	45-60 min	Within minutes
Duration	~24 h (outlasts plasma t½)	6-12 h	30 min (fasting) to 2-3 h (postprandial)
Tolerance	No	Yes (tachyphylaxis)	No
Best for	GERD, erosive esophagitis, ZE syndrome, H. pylori eradication	Mild GERD, nocturnal acid, ICU stress ulcer	Rapid symptom relief only
Requires acid for activation	Yes (prodrug)	No	No
Timing	30 min before first meal	Can be taken any time	After meals
Key adverse effects	Hypomagnesemia, B12 deficiency, C. diff, bone fractures	Antiandrogenic (cimetidine), CNS effects (elderly)	Constipation (Al), diarrhea (Mg), alkalosis (NaHCO₃)
CYP interactions	Clopidogrel (omeprazole)	Many (cimetidine)	Absorption chelation
Renal dosing	Not required	Required (all agents)	Caution (ion accumulation)

CLINICAL PEARLS

PPIs must be taken before eating - food stimulates acid production, activating newly synthesized pumps, making PPIs more effective. The first dose never achieves maximal effect; it takes 3-4 days of regular dosing to inhibit ~70% of pumps.
Stopping PPIs suddenly can cause rebound hypersecretion due to hypergastrinemia - taper when discontinuing after prolonged use.
Cimetidine is the worst for drug interactions - its broad CYP inhibition and antiandrogenic effects make it the least preferred H2 blocker.
Ranitidine was withdrawn worldwide due to NDMA (N-Nitrosodimethylamine) contamination.
Al hydroxide causes constipation; Mg hydroxide causes diarrhea - combining them balances bowel effects.
The clopidogrel-omeprazole interaction carries a boxed warning. Pantoprazole is the preferred PPI in patients on clopidogrel.
H2 blockers in renal failure require dose adjustment; PPIs do not (they are hepatically cleared).
Antacids are NOT long-term solutions - they provide no mucosal healing and relief is short-lived.
CYP2C19 poor metabolizers (common in East Asians) achieve higher plasma PPI levels → better acid suppression but also greater risk of adverse effects.
PPIs reduce Clostridium difficile defense - by raising gastric pH, they remove an important barrier to ingested spores. Limit use to lowest effective dose and duration.

Sources: Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14e (Chapter 53); Lippincott Illustrated Reviews: Pharmacology, 8e (Chapter 42); Katzung's Basic and Clinical Pharmacology, 16e (Chapter 62)

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