Pirenzepine: Selective M1 Muscarine Antagonist for Gastric Acid Reduction

Pirenzepine is a selective M1 muscarine receptor antagonist that has been used since the mid-1970s to treat peptic ulcers and gastric mucosal inflammation. Known brand names include Gastrozepin and Gastropax. It is considered a pioneer of targeted acid inhibition, but has been largely displaced clinically by the introduction of H2 antagonists (ranitidine, famotidine) and then proton pump inhibitors (omeprazole, pantoprazole).

Today, pirenzepine plays only a minor role in ulcer therapy, but remains available in Germany. In some indications outside the gastrointestinal tract, pirenzepine is discussed, such as for reduction of hyperhidrosis or in sialorrhea. An experimental application in ophthalmology for preventing myopia progression in children is the subject of ongoing studies, without current approval.

Mechanism of Action

In the human stomach, gastric acid secretion is triggered by three stimulation pathways: acetylcholine via muscarinic M1 and M3 receptors, histamine via H2 receptors, and gastrin directly on parietal cells. Pirenzepine selectively blocks M1 receptors on enterochromaffin-like (ECL) cells and ganglion cells, which indirectly reduces histamine and acid release.

In comparison to atropine or other non-specific muscarine antagonists, pirenzepine shows significantly fewer peripheral anticholinergic effects (dry mouth, tachycardia, accommodation disorders) because M2 and M3 receptors in the heart, salivary glands, and eye are less affected. This selectivity was historically a major advantage that led to clinical success in acid secretion inhibition.

Pharmacokinetically, oral bioavailability is low at approximately 25 percent because pirenzepine is polar and poorly membrane-permeable. Half-life approximately 11 hours, elimination renal in unchanged form, dose adjustment required for renal insufficiency.

Indications

• Peptic ulcer duodeni and ventriculi: historical indication, today displaced by PPIs
• Reflux esophagitis: in combination or as alternative with PPI intolerance
• Stress ulcer prophylaxis: in ICU patients as alternative to PPIs and H2 antagonists
• Hyperhidrosis and sialorrhea: off-label, especially in Parkinson disease and neurodegenerative disorders
• Myopia progression in children: experimental as eye drops, compared in studies with low-dose atropine

Dosage and Administration

Peptic ulcer in adults: 50 mg morning and evening 30 minutes before meals or 100 mg at night. Duration of therapy 4 to 6 weeks, extended if healing is insufficient.

Reflux esophagitis: 50 mg three times daily. With renal insufficiency: dose reduction with eGFR below 30 ml/min to half or less.

Administration: on an empty stomach 30 minutes before meals for optimal absorption. Food significantly reduces bioavailability. Take with sufficient water.

Side Effects

Common: mild dry mouth, accommodation disorders with blurred vision, drowsiness, dizziness, constipation, headaches.

Occasional: tachycardia, urinary retention especially in men with benign prostatic hyperplasia, allergic skin reactions.

Rare: acute angle-closure glaucoma, confusion especially in elderly, elevated liver transaminases, anaphylaxis.

Important: despite M1 selectivity, anticholinergic risks exist that can become clinically relevant in combination with other anticholinergic medications. In elderly patients and with polypharmacy, the indication should be critically reviewed.

Drug Interactions

• Other anticholinergic active substances (atropine, scopolamine, tricyclic antidepressants): additive anticholinergic effects
• Cholinesterase inhibitors (donepezil, galantamine): mutual counteraction of effects
• Antacids, sucralfate, H2 antagonists, PPIs: reduced absorption of pirenzepine through pH change, administer with 1 to 2 hours interval
• Levodopa: increased absorption possible
• Ketoconazole, itraconazole: reduced efficacy, as these antifungals require an acidic gastric environment

Special Notes

Pregnancy and breastfeeding: data limited, use only with strict indication. During breastfeeding, avoid use if possible, pirenzepine passes into breast milk in small amounts.

Angle-closure glaucoma and benign prostatic hyperplasia: despite M1 selectivity, these are relative contraindications because even minor anticholinergic effects can trigger acute glaucoma attack or urinary retention.

Helicobacter pylori: if an ulcer is caused by H. pylori, eradication with PPI-based triple or quadruple therapy is first-line treatment. Pirenzepine alone heals H. pylori-associated ulcers only insufficiently and is no longer recommended today.

Current status: pirenzepine is no longer first-line treatment in modern guidelines for ulcer therapy. It is mainly used in patients with intolerance to PPIs or H2 antagonists, or as a reserve option for specific off-label indications.

You may also be interested in

• Omeprazole, classic proton pump inhibitor
• Pantoprazole, another PPI with fewer interactions
• Famotidine, H2 receptor antagonist
• Sucralfate, mucosal protective therapy
• Scopolamine, another anticholinergic agent

Frequently Asked Questions

Sources

• Gelbe Liste, Pirenzepine Active Ingredient Profile
• DGVS Guideline Helicobacter pylori and Peptic Ulcer
• BfArM, Federal Institute for Drugs and Medical Devices
• EMA Product Information Pirenzepine Preparations