O
Olsalazine Sodium
Pharmacology and Mechanism of Action
Anti-inflammatory drug composed of two molecules of aminosalicylic acid joined by an azo bond. Each component is released in the colon by bacterial enzymes. The released drug is also known as mesalamine (see separate entry for Mesalamine). Mesalamine is also the active component of sulfasalazine, which is commonly administered for treatment of colitis. The action of mesalamine is not precisely known, but it appears to suppress the metabolism of arachidonic acid in the intestine. It inhibits both cyclo-oxygenase- and lipoxygenase-mediated mucosal inflammation. Systemic absorption is small; most of the action is believed to be local. Olsalazine has a similar effect as sulfasalazine but without the sulfonamide component. Other formulations of mesalamine include Asacol, Mesasal, and Pentasa. The others are coated tablets designed to release the active component in the intestine.
Indications and Clinical Uses
Olsalazine, like other forms of mesalamine, is used for treatment of inflammatory bowel disease, including colitis in animals. In small animals, most often sulfasalazine is used; however, in some animals olsalazine may be indicated. Use in animals has been primarily derived from empirical use and experience in humans. There are no well-controlled clinical studies or efficacy trials to document clinical effectiveness.
Stability and Storage
Store in a tightly sealed container, protected from light, and at room temperature.
Small Animal Dosage
Dose not established, but 5-10 mg/kg q8h have been used. (The usual human dose is 500 mg twice daily.)
Omeprazole
oh-mep′rah-zole
Trade and other names: Prilosec (formerly Losec; human preparation), GastroGard, and UlcerGard (equine preparations)
Pharmacology and Mechanism of Action
Proton pump inhibitor (PPI). Omeprazole inhibits gastric acid secretion by inhibiting the K+/H+ pump (potassium pump). Omeprazole is more potent and longer acting than most available antisecretory drugs. Other proton pump inhibitors include pantoprazole (Protonix), lansoprazole (Prevacid), and rabeprazole (Aciphex). They all act via similar mechanism and are equally effective. PPIs also have some effect for inhibiting Helicobacter organisms in the stomach when administered with antibiotics. Omeprazole is decomposed in the acid environment of the stomach. Oral absorption is decreased if administered with food. Formulations are designed to protect from acid to improve oral absorption (e.g., some capsules contain bicarbonate) and should not be modified (i.e., crushing of capsules) or the stability will be compromised.
Indications and Clinical Uses
Omeprazole, like other PPIs, is used for treatment and prevention of GI ulcers. It has been used in dogs, cats, and exotic species, but most efficacy data for animals have been produced in horses, in which it has been shown that omeprazole is effective for treating and preventing gastric ulcers. In foals, 4 mg/kg q24h suppressed acid secretion for 22 hours (by comparison, ranitidine suppressed acid up to 8 hours at a dose of 6.6 mg/kg). In dogs, 1 mg/kg q24h PO is as effective as pantoprazole (1 mg/kg) and famotidine (0.5 mg/kg q12h) for maintaining stomach pH >3-4. Repeated doses may be necessary (2-5 doses) for complete inhibition of acid secretion. Because of the long duration of effect, PPIs may be more effective than other drugs (e.g., histamine H2 blockers). In studies performed in horses, omeprazole was more effective than ranitidine for treating gastric ulcers. Effects were observed within 14 days, but recurrence was observed after treatment was discontinued. Omeprazole, like other PPIs, may be effective for preventing nonsteroidal anti-inflammatory drug (NSAID)–induced ulcers. Omeprazole has been used in combination with other drugs (antibiotics) for treatment of Helicobacter infections in animals.
Precautionary Information
Adverse Reactions and Side Effects
The only reported adverse effect in dogs has been diarrhea in some cases. Otherwise, adverse effects have not been reported in animals. However, in people there is concern about hypergastrinemia with chronic use. Horses have tolerated 20 mg/kg q24h for 91 days and 40 mg/kg q24h PO for 21 days. Dogs and cats also have tolerated the regimens listed in the dosing section. Overgrowth of Clostridium bacteria has been a concern from chronic use because of chronic gastric acid suppression, but the clinical importance of this concern in animals has not been established.
Contraindications and Precautions
One of the human formulations (Zegerid) is a packet to be mixed with water for oral administration. This formulation contains xylitol, which can be toxic to dogs if administered at high doses, or with other medications that contain xylitol. No contraindications have been reported for animals.
Drug Interactions
Although omeprazole has not been associated with drug interactions in animals, PPIs may inhibit some drug-metabolizing enzymes (CYP-450 enzymes). In people metabolism of clopidogrel to the active metabolite has been a concern. Omeprazole may also inhibit metabolism of other drugs. Because of stomach acid suppression, do not administer with drugs that depend on stomach acid for absorption (e.g., ketoconazole and itraconazole).
Instructions for Use
Omeprazole is the most common drug of this class used in animals. Other PPIs include pantoprazole (Protonix), lansoprazole (Prevacid), and abeprazole (Aciphex). No experience with these other products is reported for veterinary medicine. They are all considered equally efficacious. Pantoprazole and rabeprazole have the advantage in that they are available as tablets that can be crushed, and pantoprazole is less expensive than the other drugs in this class. Rectal administration has been studied in horses but has produced low and inconsistent response. Because there are no small animal formulations available, the human forms have been administered to dogs and cats and the equine formulation has been diluted in oil to 40 mg/mL for administration to small animals. Preliminary studies have shown that the equine formulation administered orally to dogs can be efficacious.
Patient Monitoring and Laboratory Tests
Omeprazole and PPIs are generally considered safe. No routine tests for monitoring adverse effects are recommended.
Formulations
Omeprazole is available in 20-mg capsules (human preparation) and in an equine paste, GastroGard. OTC equine paste is UlcerGard. Paste for horses is 370 mg/g of paste. A human oral formulation (Zegerid) is available either as 40 mg or 20 mg capsules with 1100 mg sodium bicarbonate. Zegerid is also available either as 40 mg or 20 mg single-dose packets of powder for oral suspension with 1680 mg sodium bicarbonate.
Stability and Storage
Omeprazole should be maintained in the manufacturer’s original formulation (capsules or paste) for optimum stability and effectiveness. It is stable at pH 11 but rapidly decomposes at pH <7.8. There are no small animal formulations available and either the human formulation or equine formulation has been administered to small animals. The equine formulation should be diluted (e.g., in oil) for small animal use because it is very concentrated. It has been diluted to 40 mg/mL by suspending the approved equine oral paste formulation in sesame oil at a ratio of 1:9 and stored at controlled cold temperature (7°C) and protected from light. However, the stability of extemporaneously prepared mixtures has not been evaluated. Intravenous forms of omeprazole have been formulated in sterile water and administered to experimental horses, but these formulations are not commercially available. Studies conducted on compounded oral equine formulations have shown that these formulations have low potency.
Large Animal Dosage
Horses
Regulatory Information
Not intended for administration to animals that produce food. Oral absorption in ruminants is not established. Withdrawal times are not established for animals that produce food. For extralabel use withdrawal interval estimates, contact FARAD at 1-888-USFARAD (1-888-873-2723).
Ondansetron Hydrochloride
Pharmacology and Mechanism of Action
Antiemetic drug from the class of drugs called serotonin antagonists. Like other drugs of this class, ondansetron acts by inhibiting serotonin type 3 (5-HT3) receptors. It is used primarily as an antiemetic during chemotherapy, for which it has been effective by blocking emetic stimuli that release serotonin. During chemotherapy, or following GI injury, there may be 5-HT released from the GI tract that stimulates vomiting centrally. This stimulus is blocked by this class of drugs. These drugs also have been used to treat vomiting from other forms of gastroenteritis, pancreatitis, and inflammatory bowel disease. Serotonin antagonists used for antiemetic therapy include granisetron, ondansetron, dolasetron, azasetron, and tropisetron.
Indications and Clinical Uses
Ondansetron, like other serotonin antagonists, is used to prevent vomiting. Although it may be effective for treating nausea and vomiting from other sources, the most common use is for preventing vomiting from chemotherapy. There is only limited efficacy information available for ondansetron effectiveness in animals. Oncologists have found it to be effective for managing vomiting from chemotherapy in animals.
Precautionary Information
Adverse Reactions and Side Effects
Ondansetron adverse effects have not been reported in animals. These drugs have little affinity for other 5-HT receptors. Because some severe adverse effects can occur from concurrent cancer drugs, it may be difficult to distinguish these from ondansetron effects.
Instructions for Use
Ondansetron has been used in dogs and cats. Granisetron is a similar drug that has been substituted for a similar purpose.
Formulations
Ondansetron is available in 4- and 8-mg tablets, 4-mg/5-mL flavored syrup, and 2-mg/mL injection.
Stability and Storage
Store in a tightly sealed container, protected from light, and at room temperature. Ondansetron is soluble in water. Solutions are stable, but pH should be <6 to prevent precipitation. Oral preparations have been mixed with syrups, juices, and other oral vehicles (e.g., Ora Sweet). It was stable for 42 days as long as pH remained low.
Orbifloxacin
Pharmacology and Mechanism of Action
Fluoroquinolone antimicrobial. Orbifloxacin acts via inhibition of DNA gyrase in bacteria to inhibit DNA and RNA synthesis. It is a bactericidal with a broad spectrum of activity. Spectrum includes staphylococci, gram-negative bacilli, and some Pseudomonas species. In dogs the half-life is 5.6 hours; in cats the half-life is 5.5 hours. In both species the oral absorption is nearly 100%. However, oral suspension absorption is not as high as tablets (see Instructions for Use section below). In horses, it has a half-life of 5 hours and oral absorption of 68%.
Indications and Clinical Uses
Orbifloxacin is registered for use in dogs and cats. Like other fluoroquinolones, it is used to treat susceptible bacteria in a variety of species. Treatment of infections has included skin, soft tissue, and UTIs in dogs and cats and soft tissue infections in horses.
Precautionary Information
Adverse Reactions and Side Effects
High concentrations may cause CNS toxicity, especially in animals with renal failure. It may cause some nausea, vomiting, and diarrhea at high doses. All of the fluoroquinolones may cause arthropathy in young animals. Dogs are most sensitive at 4 to 28 weeks of age. Large, rapidly growing dogs are the most susceptible. Blindness in cats has been reported from administration of some quinolones (nalidixic acid and enrofloxacin), but at doses up to 15 mg/kg (higher than approved label dose) orbifloxacin has not produced this effect.
Contraindications and Precautions
Avoid use in young animals because of risk of cartilage injury. Use cautiously in animals that may be prone to seizures.
Drug Interactions
Fluoroquinolones may increase concentrations of theophylline if used concurrently. Coadministration with divalent and trivalent cations, such as products containing aluminum (e.g., sucralfate), iron, and calcium, may decrease absorption. Do not mix in solutions or in vials with aluminum, calcium, iron, or zinc because chelation may occur.
Instructions for Use
At the registered label dose, orbifloxacin is active against most susceptible bacteria. Within the approved dose range, higher doses are needed for organisms with higher minimum inhibitory concentration (MIC) values. In the cat, orifloxacin oral suspension and orbifloxacin tablets are not bioequivalent. On a mg/kg basis, orbifloxacin oral suspension provides lower and more variable plasma levels of orbifloxacin than the tablets. The dose of orbifloxacin oral suspension in the cat is 7.5 mg/kg of body weight administered once daily, but tablet dose can be lower.
Patient Monitoring and Laboratory Tests
Susceptibility testing: CLSI break point for sensitive organisms is ≤1 mcg/mL. Other fluoroquinolones may be used in some cases to estimate susceptibility to this fluoroquinolone. However, other drugs may have lower MIC values for Pseudomonas aeruginosa. Against P. aeruginosa, ciprofloxacin has greater in vitro activity.
Formulations
Orbifloxacin is available in 5.7-, 22.7-, and 68-mg tablets. Oral suspension is 30 mg/mL.
Stability and Storage
Store in a tightly sealed container, protected from light, and at room temperature. Orbifloxacin is slightly water soluble. It has been mixed with various syrups, flavorings, and vehicles and was stable at room temperature for 7 days. Do not mix with vehicles that contain aluminum, calcium, or iron because this may decrease oral absorption via chelation.
Ormetoprim + Sulfadimethoxine
Pharmacology and Mechanism of Action
Antibacterial drug. Ormetoprim sulfadimethoxine is a synergistic combination similar to trimethoprim sulfonamide combinations. Ormetoprim inhibits bacterial dihydrofolate reductase, and sulfonamide competes with para-aminobenzoic acid (PABA) for synthesis of nucleic acids. Bactericidal/bacteriostatic. It has a broad antibacterial spectrum that includes common gram-positive and gram-negative bacteria. It also is active against some coccidia.
Indications and Clinical Uses
Ormetoprim + sulfadimethoxine is used in small animals to treat a variety of bacterial infections caused by susceptible organisms, including pneumonia, skin, soft tissue, and UTIs in dogs and cats. In horses, it may be administered orally for infections caused by susceptible gram-positive bacteria (Actinomyces, Streptococcus spp., and Staphylococcus spp.), but higher doses may be needed for gram-negative infections.
Precautionary Information
Adverse Reactions and Side Effects
Adverse effects associated with sulfonamides include allergic reactions, Type II and III hypersensitivity, arthropathy, anemia, thrombocytopenia, hepatopathy, hypothyroidism (with prolonged therapy), keratoconjunctivitis sicca, and skin reactions. Dogs may be more sensitive to sulfonamides than other animals because dogs lack the ability to acetylate sulfonamides to metabolites. Other, more toxic metabolites may persist. Ormetoprim has been associated with some CNS effects in dogs, which include behavioral changes, anxiety, muscle tremors, and seizures. In horses when IV doses were administered to experimental horses (IV formulation not commercially available), nervous system reactions such as tremors and muscle fasciculations were observed.
Instructions for Use
Doses listed are based on manufacturer’s recommendations. Controlled trials have demonstrated efficacy for treatment of pyoderma on a once-daily schedule.
Patient Monitoring and Laboratory Tests
Monitor tear production with long-term use. For susceptibility testing, break point ranges have not been determined for ormetoprim + sulfadimethoxine. Use a test for trimethoprim-sulfonamide as a guide for susceptibility to ormetoprim + sulfadimethoxine. CLSI break point for susceptible organisms is ≤2/38 mcg/mL (trimethoprim/sulfonamide).
Formulations
Ormetoprim + sulfadimethoxine is available in 120-, 240-, 600-, and 1200-mg tablets in 5 : 1 ratio of sulfadimethoxine : ormetoprim.
Stability and Storage
Store in a tightly sealed container, protected from light, and at room temperature.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
