Omeprazole

Omeprazole (INN) (pronounced /oʊˈmɛprəzoʊl/) is a proton pump inhibitor used in the treatment of dyspepsia, peptic ulcer disease (PUD), gastroesophageal reflux disease (GORD/GERD) and Zollinger-Ellison syndrome. It was first marketed by AstraZeneca as the magnesium salt omeprazole magnesium under the trade names Losec and Prilosec, and is now also available from generic manufacturers under various trade names. Omeprazole is one of the most widely prescribed drugs internationally and is available over the counter in some countries.

Omeprazole is the racemate (containing equal amounts of both the S and R enantiomers). It has been proven that only the S-enantiomer is effective and the R-enantiomer is not. Omeprazole undergoes a chiral shift in vivo which converts the inactive R-enantiomer to the active S-enantiomer doubling the concentration of the active form. This chiral shift is accomplished by the CYP 2C19 isozyme of the cytochrome P450 enzyme which is not found equally in all human populations. Those who do not metabolize the drug effectively are called "poor metabolizers" and their distribution is as follows:

• Caucasians 10%
• Asian 20%
• South Pacific Islands 70%

Facing the loss of patent protection and competition from generic manufacturers, AstraZeneca developed, launched, and heavily marketed esomeprazole (Nexium), which is the S enantiomer in the pure form. The effect of esomeprazole on the proton pump is therefore equal in all patients, eliminating the "poor metabolizer effect".

In 1990, at the request of the United States Food and Drug Administration (FDA), the brand name Losec was changed to Prilosec to avoid confusion with the diuretic Lasix (furosemide).^[1] Unfortunately, the new name has led to confusion between omeprazole (Prilosec) and fluoxetine (Prozac), an antidepressant.^[1]

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Clinical use

Main article: Proton pump inhibitor

Use in Helicobacter pylori eradication

Omeprazole is combined with the antibiotics clarithromycin and amoxicillin (or metronidazole in penicillin-hypersensitive patients) in the one week eradication triple therapy for Helicobacter pylori. Infection by H. pylori is the causative factor in the majority of peptic and duodenal ulcers.^{[citation needed]}

Dosage forms

      Omeprazole is available as tablets and capsules (containing omeprazole or omeprazole magnesium) in strengths of 10 mg, 20 mg, and in some markets 40 mg; and as a powder (omeprazole sodium) for intravenous injection. Most oral omeprazole preparations are enteric-coated, due to the rapid degradation of the drug in the acidic conditions of the stomach. This is most commonly achieved by formulating enteric-coated granules within capsules, enteric-coated tablets, and the multiple-unit pellet system.

It is also available for use in injectable form (I.V.) in Europe, but not in the U.S. The injection pack is a combination pack consisting of a vial and a separate ampule of reconstituting solution. Each 10 ml clear glass vial contains a white to off-white lyophilised powder consisting of omeprazole sodium 42.6 mg equivalent to 40 mg of omeprazole.

Losec is manufactured by AstraZeneca, Södertälje, Sweden.

Multiple unit pellet system

Omeprazole tablets manufactured by AstraZeneca (notably Losec/Prilosec) are formulated as a "multiple unit pellet system" (MUPS). Essentially, the tablet consists of extremely small enteric-coated granules (pellets) of the omeprazole formulation inside an outer shell. When the tablet is immersed in an aqueous solution, as happens when the tablet reaches the stomach, water enters the tablet by osmosis. The contents swell from water absorption causing the shell to burst, releasing the enteric-coated granules. For most patients, the multiple-unit pellet system is of no advantage over conventional enteric-coated preparations. Patients for which the formulation is of benefit include those requiring nasogastric tube feeding and those with difficulty swallowing (dysphagia).

The granules are manufactured in a fluid air bed system. Sugar spheres in suspension are sequentially sprayed with aqueous suspensions of omeprazole, a protective layer, an enteric coating and an outer layer to reduce granule aggregation. The granules are mixed with other excipients and compressed into tablets. Finally, the tablets are film-coated to improve the stability and appearance of the preparation.

Immediate release formulation

In June 2004 the FDA approved an immediate release preparation of omeprazole/sodium bicarbonate that does not require an enteric coating. This preparation employs sodium bicarbonate as a buffer to protect omeprazole from gastric acid degradation. This allows for the production of chewable tablets. This combination preparation is marketed in the United States by Santarus under the trade name Zegerid. Zegerid is marketed as capsules, chewable tablets, and powder for oral suspension. Zegerid is most useful for those patients who suffer from nocturnal acid breakthrough (NAB) or those patients who desire immediate relief.

Side effects

As with all proton pump inhibitors, omeprazole is generally well tolerated, however it must be avoided by anyone with a suspicion of lesions or bleeding ulcers. Some of the most frequent side effects of omeprazole (experienced by over 1% of those taking the drug) are headache, diarrhea, abdominal pain, nausea and dizziness, although in clinical trials the incidence of these effects with omeprazole was mostly comparable to that found with placebo.^[2]

Proton pump inhibitors may be associated with a greater risk of hip fractures ^[3] and clostridium difficile diarrhoea.^[4]

Absorption and distribution

The absorption of omeprazole takes place in the small intestine and is usually completed within 3-6 hours. The systemic bioavailability of omeprazole after repeated dose is about 60%. Concomitant intake of food has no influence on the bioavailability. Plasma protein binding is about 95%.

Metabolism and excretion

Omeprazole is completely metabolised by the cytochrome P450 system, mainly in the liver. Identified metabolites are the sulphone, the sulphide and hydroxy-omeprazole, which exert no significant effect on the acid secretion. About 80% of an orally given dose is excreted as metabolites in the urine and the remainder is found in the feces, primarily originating from bile secretion.

Brand names

Zegerid
Prilosec, Losec, Prilosec OTC, Omez
Antra® in Italy
Nexium (esomeprazole, a S-enantiomer of omeprazole)
Ozid® in Indonesia )
OMEZ (Farhaad Panthaky; Dr Reddy's India)
Omepradex (Dexcel, Israel)

References

Ocid.Zydus Cadila.