Elephant specific information, if available, is in blue.

Chemistry – A substituted benzimidazole proton pump inhibitor, omeprazole has a molecular weight of 345.4 and pKa’s of 4 and 8.8.

Storage/Stability/Compatibility – Omeprazole tablets should be stored at room tempera­ture in light-resistant, tight containers. Omeprazole pellets found in the capsules are fragile and should not be crushed. If needed to administer as a slurry, it has been suggested to mix the pellets carefully with fruit juices and not water, milk or saline.

Pharmacology – A representative of a new class of agent, the substituted benzimidazoles, omeprazole is a gastric acid (proton) pump inhibitor. In an acidic environment, omepra­zole is activated to a sulphenamide derivative that binds irreversibly at the secretory sur­face of parietal cells to the enzyme, H+/K+ ATPase. There it inhibits the transport of hy­drogen ions into the stomach. Omeprazole inhibits acid secretion during both basal and stimulated conditions. Omeprazole also inhibits the hepatic cytochrome P-450 mixed function oxidase system (see Drug Interactions below).

Uses/Indications – Omeprazole is potentially useful in treating both gastroduodenal ulcer disease and to prevent or treat gastric erosions caused by ulcerogenic drugs (e.g., aspirin). Because of the drugs recent availability and high cost, experience is limited in domestic animals.

Pharmacokinetics – Omeprazole is rapidly absorbed from the gut; the commercial prod­uct is in an enteric coated granule form as the drug is rapidly degraded by acid. Peak serum levels occur within 0.5 to 3.5 hours and onset of action within 1 hour. Omeprazole is distributed widely, but primarily in gastric parietal cells. In humans, approximately 95% is bound to albumin and alpha1-acid glycoprotein. It is unknown whether omeprazole en­ters maternal milk.

Omeprazole is extensively metabolized in the liver to at least six different metabolites, these are excreted principally in the urine, but also via the bile into feces. Significant hepatic dysfunction will reduce the first pass effect of the drug. In humans with normal hepatic function, serum half life averages about 1 hour, but the duration of therapeutic ef­fect may persist for 72 hours or more.

Contraindications/Precautions/Reproductive Safety – Omeprazole is contraindicated in patients hypersensitive to it. Omeprazole should be used when the benefits outweigh the risks in patients with hepatic disease or a history of hepatic disease, as the drug’s half life may be prolonged and dosage adjustment may be necessary.

Omeprazole’s safety during pregnancy has not been established, but a study done in rats at doses of up to 345 times those recommended did not demonstrate any teratogenic ef­fects. Increased embryo-lethality has been noted in lab animals at very high dosages. It is unknown whether omeprazole is excreted in milk.

Adverse Effects/Warnings – While veterinary use is quite limited, the drug appears to be quite well tolerated in both dogs and cats at effective dosages. Potentially, GI distress (anorexia, colic, nausea, vomiting, flatulence, diarrhea) could occur as well as hematologic abnormalities (rare in humans), urinary tract infections, proteinuria, or CNS disturbances. Chronic very high doses in rats caused enterochromaffin-like cell hyperplasia and gastric carcinoid tumors; effects occurred in dose related manner. The clinical significance of these findings for long term low-dose clinical usage is not known. However, at the current time in humans, dosing for longer than 8 weeks is rarely recommended unless the benefits of therapy outweigh the potential risks.

Overdosage/Acute Toxicity – The LD50 in rats after oral administration is reportedly >4 g/kg. Humans have tolerated oral dosages of 360 mg/day without significant toxicity. Should a massive overdose occur, treat symptomatically and supportively.

Drug Interactions – Because omeprazole can inhibit the cytochrome P-450 enzyme sys­tem, omeprazole may decrease the hepatic clearance of diazepam, phenytoin or war­farin, thereby enhancing their effects and causing potential toxicity. Additional monitor­ing and dosage adjustments may be required. Because omeprazole can increase gastric pH, drugs that require low gastric pH for opti­mal absorption (e.g., ketoconazole, ampicillin esters or iron salts) may have their ab­sorption reduced. Although omeprazole causes bone marrow depression only rarely in humans, use with other drugs that cause bone marrow depression may lead to additive hematologic abnormalities.

Laboratory Considerations – Omeprazole may cause increased liver enzymes. Omeprazole will increase serum gastrin levels early in therapy.

Doses –

Horses:

For ulcer management:

a)   0.7 – 1.4 mg/kg PO once daily. While dosage has not been firmly established, these dosages have been shown to suppress gastric acid output for up to 24 hours. Use should probably be limited to those cases where the client prefers once a day only dosing. (Geor 1992)

Monitoring Parameters – 1) Efficacy; 2) Adverse Effects

Client Information – Brief client on drug’s cost before prescribing. Give before meals, preferably in the morning.

Dosage Forms/Preparations/FDA Approval Status/Withholding Times –

Veterinary-Approved Products: None

Human-Approved Products:

Omeprazole Oral Sustained Release 10 mg & 20 mg Capsules; Prilosec®  (Losec®  in Canada);  (Astra Merck); (Rx)