Famciclovir

fam-sye′kloe-veer

Functional Classification: Antiviral

Pharmacology and Mechanism of Action

Antiviral drug. Famciclovir is a synthetic purine analogue (acyclic nucleoside analogue). It is converted to the antiviral drug penciclovir and has antiviral activity against herpes virus type 1 and 2. The action is related to the affinity for the enzyme thymidine kinase (TK), which converts penciclovir into penciclovir triphosphate, which inhibits viral DNA polymerase to prevent DNA chain elongation—thus inhibiting viral DNA chain elongation. It is used for treatment of various forms of herpes virus infection in humans and also has been used for treatment of viral infections in animals. However, feline herpes virus 1 (FHV1) is resistant to acyclovir and valciclovir, and studies are lacking on the susceptibility of other herpes viruses. Other antiviral drugs used in animals include acyclovir, penciclovir, and valacyclovir.

Indications and Clinical Uses

Although in vitro testing does not indicate high activity against the virus, famciclovir has been used to treat feline herpes virus (FHV1) associated with conjunctivitis, rhinosinusitis, keratitis, and FHV1-associated dermatitis. In treated cats there was improvement of conjunctivitis, decreased conjunctival inflammation, decreased ocular discomfort, and decreased tearing.

Precautionary Information

Adverse Reactions and Side Effects

No adverse effects were identified in limited studies performed in cats.

Contraindications and Precautions

Reduce dose in animals with compromised renal function.

Drug Interactions

No interaction identified.

Instructions for Use

The dose listed for cats is based on limited studies in which 62.5 mg per cat was studied initially, but later evidence suggested that a higher dose of 125 mg per cat was more effective.

Patient Monitoring and Laboratory Tests

Monitor BUN and creatinine during use.

Formulations

Famciclovir is available in 125-, 250-, and 500-mg tablets.

Stability and Storage

Store tablets and capsules in tightly sealed container, protected from light, and at room temperature.

Small Animal Dosage

Cats

• Treatment of feline herpes: 62.5 mg per cat, PO, q8h, for 3 weeks. However, a higher dose of 125 mg per cat q8h may be more effective.

Large Animal Dosage

Horses

• No doses have been established.

Regulatory Information

Because of mutagenicity, it should not be administered to animals intended for food.

Famotidine

fah-moe′tih-deen

Trade and Other Names: Pepcid

Functional Classification: Antiulcer agent

Pharmacology and Mechanism of Action

Histamine₂ antagonist (H₂ blocker). Stimulation of acid secretion in the stomach requires activation of histamine type 2 receptors (H₂ receptor), gastrin receptors, and muscarinic receptors. Famotidine and related H₂ blockers inhibit the action of histamine on the histamine H₂ receptor of parietal cells and inhibits gastric parietal cell gastric acid secretion. It increases stomach pH to help heal and prevent gastric and duodenal ulcers.

Indications and Clinical Uses

Famotidine, like other H₂-receptor blockers, is used to treat ulcers and gastritis in a variety of animals. Although it is often used for animals with vomiting, there are no efficacy data to indicate that it is effective. There are no efficacy data to support its use for preventing nonsteroidal anti-inflammatory drug (NSAID)–induced bleeding and ulcers. Famotidine has been used by veterinarians as a preferred H₂ blocker, but there is a lack of evidence to demonstrate superiority over other drugs in this class. Some studies have demonstrated efficacy at 1 mg/kg in dogs, while other studies have not demonstrated differences between a placebo in dogs for increasing stomach pH.

Precautionary Information

Adverse Reactions and Side Effects

Adverse effects usually are seen only with decreased renal clearance. In people, CNS signs may occur with high doses. The intravenous solution contains benzyl alcohol, aspartic acid, and mannitol. Give intravenous injections slowly to cats (over 5 minutes) because rapid intravenous injections may cause hemolysis.

Contraindications and Precautions

Intravenous solutions contain benzyl alcohol. IV injections to small animals, especially cats, should be done slowly.

Drug Interactions

Famotidine and other H2-receptor blockers block secretion of stomach acid. Therefore, they will interfere with oral absorption of drugs dependent on acidity, such as ketoconazole, itraconazole, and iron supplements. Unlike cimetidine, famotidine is not associated with inhibition of microsomal P450 enzymes.

Instructions for Use

Administer with food for best absorption. Clinical studies for famotidine have not been performed; therefore optimal doses for ulcer prevention and healing are not known. Dose recommendations are extrapolated from human use or from anecdotal experience. Experimental studies in dogs have shown that doses of 0.1-0.2 mg/kg inhibit stomach acid secretion, but other clinical studies have shown that doses of 1.0 mg/kg suppress stomach acid for 24 hours. For intravenous use, dilute with IV solutions (e.g., 0.9% saline) to a total volume of 5-10 mL.

Patient Monitoring and Laboratory Tests

No specific monitoring is necessary.

Formulations

Famotidine is available in 10-mg tablets, 8 mg/mL oral suspension, and 10-mg/mL injection.

Stability and Storage

Store in a tightly sealed container, protected from light, and at room temperature. Famotidine is soluble in water. Compounded formulations in cherry syrup have been stable for 14 days. Diluted intravenous solutions in saline are stable for 48 hours at room temperature.

Large Animal Dosage

Horses

• 1-2 mg/kg q6-8h PO.

Regulatory Information

No restrictions on use in animals not intended for food.

RCI Classification: 5

Febantel

feh-ban′tel

Trade and Other Names: Rintal and Vercom. Drontal Plus also contains two other drugs.

Functional Classification: Antiparasitic

Pharmacology and Mechanism of Action

Febantel is an antiparasitic that interferes with carbohydrate metabolism in parasitic worms. It suppresses mitochondrial reactions via inhibition of fumarate reductase and interferes with glucose transport. It is metabolized to a benzimidazole compound that binds to structural protein tubulin and prevents polymerization to microtubules, which results in incomplete digestion and absorption of nutrients by parasite.

A formulation of febantel, pyrantel, and praziquantel (Drontal Plus) has been used in cats for treatment of Giardia, roundworms, hookworms, and whipworms.

Indications and Clinical Uses

Febantel is indicated in the control and treatment of larvae and adult stages of intestinal nematodes. In horses, it is used for removal of large strongyles (Strongylus vulgaris, S. edentatus, S. equinus), ascarids (Parascaris equorum, sexually mature and immature), pinworms (Oxyuris equi, adult and fourth-stage larvae), and the various small strongyles.

In dogs and cats it is used for treatment of hookworms (Ancylostoma caninum and Uncinaria stenocephala), ascarids (Toxocara canis and Toxascaris leonina), and whipworms (Trichuris vulpis). In dogs, it is used in combination with praziquantel for treatment of hookworms (A. caninum and U. stenocephala), whipworms (T. vulpis), ascarids (T. canis and T. leonina), and tapeworms (Dipylidium caninum and Taenia pisiformis).

In cats it is used in combination with praziquantel for removal of hookworms (A. tubaeforme), ascarids (Toxocara cati), and tapeworms (D. caninum and Taenia taeniaeformis).

Febantel has been used with pyrantel (Drontal Plus) for treatment of Giardia.

Precautionary Information

Adverse Reactions and Side Effects

Vomiting and diarrhea may occur after dosing.

Contraindications and Precautions

Do not use in pregnant animals. Do not use in animals with liver or kidney dysfunction.

Drug Interactions

No drug interactions reported.

Instructions for Use

For horses, the paste may be administered on the base of the tongue or added to a portion of the normal grain ration. For most effective results, re-treat in 6-8 weeks. Febantel suspension may be used in combination with trichlorfon oral liquid when combining 1 part febantel suspension with 5 parts trichlorfon liquid.

Patient Monitoring and Laboratory Tests

No specific monitoring is necessary.

Formulations

Febantel is available in an equine paste: 45.5% febantel (455 mg/mL), suspension: 9.3% (2.75 g per ounce) febantel, and 27.2- and 163.3-mg tablets.

Febantel is also available in combinations; each gram of paste contains 34 mg of febantel and 3.4 mg of praziquantel.

Febantel, pyrantel, and praziquantel are available for small animals.

Stability and Storage

Store in a tightly sealed container, protected from light, and at room temperature.

Large Animal Dosage

Regulatory Information

Not for use in horses intended for food. No other regulatory restrictions are listed.

Felbamate

fel′bah-mate

Trade and Other Names: Felbatol

Functional Classification: Anticonvulsant

Pharmacology and Mechanism of Action

Anticonvulsant. The action to treat seizures in animals may be via antagonism at the N-methyl-D-aspartate (NMDA) receptor and block effects of excitatory amino acids. The half-life in dogs is 5-6 hours, which may require frequent administration.

Indications and Clinical Uses

Felbamate is used to treat epilepsy in dogs when they are refractory to other anticonvulsants. It has been used in conjunction with other anticonvulsants. However, the use in animals has declined because of the availability of other drugs to treat refractory seizures such as levetiracetam, zonisamide, gabapentin, and pregabalin.

Precautionary information

Adverse Reactions and Side Effects

Not documented with use in dogs. In people the most severe reactions have been hepatotoxicity and aplastic anemia.

Contraindications and Precautions

It may increase phenobarbital concentrations.

Drug Interactions

Possible interactions exist with drugs that either alter or are substrates for hepatic cytochrome P450 enzymes. See Appendix. It may increase phenobarbital concentrations if used concurrently.

Instructions for Use

Dosing has been empirically in dogs. There are no controlled studies to document efficacy, but it is usually administered when animals have been refractory to other drugs, such as phenobarbital or bromide.

Patient Monitoring and Laboratory Tests

Monitoring of plasma concentrations is helpful to assess therapy. Assays may be available in some commercial laboratories. Ideal plasma concentrations have not been established for animals. However, concentrations in humans of 24 to 137 mcg/mL in plasma have been effective (mean of 78 mcg/mL).

Formulations

Felbamate is available in 12- mg/mL oral liquid and 400- and 600-mg tablets.

Stability and Storage

Store in a tightly sealed container, protected from light, and at room temperature. Stability of compounded formulations has not been evaluated.

Large Animal Dosage

No large animal doses have been reported.

Regulatory Information

No withdrawal times have been established. For extralabel use withdrawal interval estimates, contact FARAD at 1-888-USFARAD (1-888-873-2723).

RCI Classification: 3

Fenbendazole

fen-ben′dah-zole

Trade and Other Names: Panacur and SafeGuard

Functional Classification: Antiparasitic

Pharmacology and Mechanism of Action

Benzimidazole antiparasitic drug. Like other benzimidazoles, fenbendazole produces a degeneration of the parasite microtubule and irreversibly blocks glucose uptake in parasites. Inhibition of glucose uptake causes depletion of energy stores in parasite, eventually resulting in death. However, there is no effect on glucose metabolism in mammals.

Indications and Clinical Uses

Fenbendazole is effective for treatment of numerous helminth intestinal parasites in animals, including Toxacara, Toxascaris, Ancylostoma, and Trichuris. In dogs it is effective for most intestinal helminth parasites and also against nematodes. Fenbendazole has been effective for treatment of Giardia, but higher doses are needed and there may be failure rates as high as 50%. It is effective in cats for treatment of lungworms, flukes, and a variety of helminth parasites.

Precautionary Information

Adverse Reactions and Side Effects

It has a good safety margin, but vomiting and diarrhea have been reported. When evaluated at doses of three and five times the recommended dose at three times the recommended duration, fenbendazole was well tolerated and no adverse effects were reported. It has been safe to use during pregnancy. There have been rare reports of pancytopenia associated with fenbendazole administration.

Contraindications and Precautions

No known contraindications. It may be used in all ages of animals.

Drug Interactions

There are no known drug interactions.

Instructions for Use

Dose recommendations based on clinical studies by manufacturer. Granules may be mixed with food. Paste may be given to horses and cattle. Presence of food does not affect oral absorption. In studies for treatment of Giardia, it was safer than other treatments.

Patient Monitoring and Laboratory Tests

Fecal monitoring may be performed to determine the efficacy of treatment for intestinal parasites.

Formulations Available

Fenbendazole is available in 22.2% (222 mg/g) Panacur granules, 10% (92 g/32 oz), and 100-mg/mL oral suspension.

Stability and Storage

Store in a tightly sealed container, protected from light, and at room temperature. Stability of compounded formulations has not been evaluated.

Regulatory Information

Cattle withdrawal time (meat): 8 days. There is no withdrawal period for milk.

Goat withdrawal time: 6 days meat; 0 days milk.

Fenoldopam mesylate

fe-nol’- doe- pam

Trade and Other Names: Corlopam

Functional Classification: Vasodilator

Pharmacology and Mechanism of Action

Fenoldopam is a dopamine agonist. It is specific for the dopamine D₁ receptors and therefore has been used to produce smooth muscle relaxation and vasodilation in vascular beds that have D₁ receptors. It has no activity on D₂ receptors and only a small effect on alpha-adrenergic receptors. Because of this activity, fenoldopam has more specificity than dopamine, which has been used for similar indications. The use of fenoldopam is usually confined to increasing renal perfusion to treat acute renal failure. The half-life in animals is very short (1-7 minutes in dogs), so it is usually administered via constant rate infusion.

Indications and Clinical Uses

The use of fenoldopam in veterinary medicine is limited to a few research studies (primarily in cats) and some anecdotal evidence of efficacy for treating acute renal failure. The use in people is to treat severe hypertension, to prevent renal ischemia, to increase gastrointestinal perfusion, and to treat acute renal failure. The use is limited to short-term in-hospital use when rapid treatment of hypertension is needed.

Precautionary Information

Adverse Reactions and Side Effects

The most common side effect is hypotension. The half-life of fenoldopam is short; therefore, if hypotension is observed, decrease the infusion rate. Other adverse effects have not been described for small animals. In people adverse effects can include increased intraocular pressure (risk in glaucoma), low potassium, and tachycardia.

Contraindications and Precautions

No known contraindications.

Drug Interactions

Use cautiously with other vasodilators; excessive hypotension can occus. Do not use with beta blockers.

Instructions for Use

Administer by constant rate infusion (CRI). Do not administer bolus doses. Onset of effects should occur within 15 minutes of starting the CRI. Dose recommendations are based on some limited research studies and anecdotal clinical experience from veterinarians. There have been no well-controlled studies of efficacy in animals, and the use is largely extrapolated from human recommendations. It is recommended that the solution is added to fluids to make a 40-mcg/mL solution for infusion. For example, add 1 mL (10 mg) to 250 mL.

Patient Monitoring and Laboratory Tests

Monitor patient’s heart rate and blood pressure during treatment.

Formulations Available

Fenoldopam is available as a 10-mg/mL injection. The pH range is 2.8 to 3.8.

Stability and Storage

Fenoldopam solution can be mixed with sodium chloride solution (0.9%) or 5% dextrose solution for infusion. Once mixed in fluids, it is stable under normal ambient light and temperature conditions for at least 24 hours. After 24 hours, discard the solution.

Small Animal Dosage

Dogs

0.8 mcg/kg/min CRI.

Cats

0.5 mcg/kg/min CRI.

Large Animal Dosage

Foals

0.04 mcg/kg/min.

Regulatory Information

There are no withdrawal times established for food animals. Because fenoldopam has a very short half-life, residues in food animals are not expected to be a problem.

Fentanyl Citrate

fen′tah-nil sih′trate

Trade and Other Names: Sublimaze and generic brands; Fentora buccal tablets

Functional Classification: Analgesic, Opioid

Pharmacology and Mechanism of Action

Synthetic opiate analgesic. Fentanyl is approximately 80 to 100 times more potent than morphine. Fentanyl is an agonist for the mu-opiate receptors on nerves and inhibits release of neurotransmitters involved with transmission of pain stimuli (such as Substance P). The central sedative and euphoric effects are related to mu-receptor effects in the brain. Fentanyl has a wide safety profile with doses as high as 300 times the recommended dose not being lethal in spontaneously breathing dogs. It is highly lipophilic—approximately 1000 times more lipophilic than morphine, which produces rapid diffusion into the CNS. In dogs the half-life is approximately 3-6 hours, and in cats it is approximately 2.5 hours. Clearance is high in dogs and oral absorption is very low. Fentanyl can be absorbed from the skin or oral mucous membrane, but it is not orally bioavailable if swallowed.

Indications and Clinical Uses

Fentanyl citrate is used as an intravenous bolus or as a constant rate infusion (CRI) in animals for relief of pain, an adjunct for anesthesia, or as a sedative in combination with other CNS sedatives. Fentanyl administered IV will produce antinociceptive effects for approximately 2 hours. Most of the doses are based on empiricism and experimental studies. Oral buccal tablets have been used in people for treatment of breakthrough pain, but there has been only anecdotal experience with this use in animals. See Fentanyl, Transdermal for information about transdermal form. Clinical use is primarily in dogs and cats. In horses at doses needed to produce analgesia it is associated with a high degree of restlessness, tachycardia, increased locomotor activity, and excitement (see Adverse Reactions and Side Effects section). In horses there has been poor efficacy at lower doses.

Precautionary Information

Adverse Reactions and Side Effects

Fentanyl has adverse effects similar to morphine. Like all opiates, side effects are predictable and unavoidable. Side effects include sedation, constipation, and bradycardia. Respiratory depression occurs with high doses. As with other opiates, a slight decrease in heart rate is expected. In most cases this decrease does not have to be treated with anticholinergic drugs (e.g., atropine), but it should be monitored. In horses, undesirable and even dangerous behavior can follow rapid intravenous opioid administration. Horses should receive a preanesthetic of acepromazine or an alpha₂ agonist.

Contraindications and Precautions

Fentanyl citrate is a Schedule II controlled substance. Tolerance and dependence occurs with repeated administration. Cats and horses are prone to excitement after administration, especially IV.

Drug Interactions

There are no specific drug interactions, but fentanyl will decrease other anesthetic requirements. Fentanyl will potentiate other opiates and CNS depressants.

Instructions for Use

No clinical studies have been reported. In addition to fentanyl injection, transdermal fentanyl is available. Oral transmucosal (buccal) forms and soluble muco-adhesive film strips are available for human administration but have not been adequately tested in animals. For SQ injection, fentanyl citrate may be mixed with bicarbonate solution (see stability section below) to decrease pain from injection.

Patient Monitoring and Laboratory Tests

Monitor analgesic response. Monitor patient’s heart rate and respiration. Although bradycardia rarely needs to be treated when it is caused by an opioid, atropine can be administered if necessary. If serious respiratory depression occurs, the opioid can be reversed with naloxone.

Formulations Available

Fentanyl citrate is available as a 250-mcg/5 mL injection (50 mcg/mL). Fentora buccal tablets are 100, 200, 400, 600, and 800 mcg. Fentanyl buccal soluble film (Onsolis) is 200, 400, 600, 800, and 1200 mcg per film strip.

Stability and Storage

Store in a tightly sealed container, protected from light, and at room temperature. It is soluble in water and slightly soluble in alcohol. It is a Schedule II drug, so store in a locked compartment. Compounded into a transmucosal gel has not been effective. The pH of fentanyl citrate is 5.2. If bicarbonate solution is added (1:10 or 1:20 dilution) the pH increases to 8.0, which may decrease pain from injection if injected immediately after mixing.

Large Animal Dosage

Small Ruminants

5-10 mcg/kg (0.005-0.010 mg/kg) IV.

Regulatory Information

Schedule II controlled drug by DEA. No withdrawal information is available for food-producing animals.

RCI Classification: 1

Fentanyl, Transdermal

fen′tah-nil

Trade and Other Names: Duragesic

Functional Classification: Analgesic, Opioid

Pharmacology and Mechanism of Action

Synthetic opiate analgesic. Fentanyl is approximately 80 to 100 times more potent than morphine. Fentanyl is an agonist for the mu-opiate receptors on nerves and inhibits release of neurotransmitters involved with transmission of pain stimuli (such as Substance P). The central sedative and euphoric effects are related to mu-receptor effects in the brain. The fentanyl transdermal system delivers fentanyl through the skin at a constant rate to produce systemic effects. Absorption is determined by the surface area for absorption.

Patches are available that deliver 25, 50, 75, and 100 mcg/hr. Absorption can be variable in animals (e.g., rate of release of fentanyl has varied from 27% to 98% [mean 71%] of the theoretical value). Cats absorbed the fentanyl at an average rate of approximately one third that of the theoretical delivery rate, but one patch will maintain consistent concentrations of fentanyl in the plasma for at least 118 hours. Fentanyl transdermal patches (two or three 100-mcg/hr patches) have been applied to the skin of horses to relieve pain. In horses, the duration is less than in dogs or cats and may have to be reapplied every 48 hours.

Indications and Clinical Uses

Transdermal absorption and effective plasma concentrations have been demonstrated for cats, dogs, horses, and goats. Fentanyl transdermal has the same properties as fentanyl citrate administered IV, except in this formulation it is administered transdermal to produce pain relief and as an adjunct to other drugs in perioperative patients and in patients with chronic pain. In dogs, fentanyl transdermal patches (50 mcg/hr) are appropriate for most average-size dogs. Transdermal fentanyl has been shown effective to relieve postoperative pain in dogs. Transdermal fentanyl has been well tolerated in cats. Fentanyl patches (25 mcg/hr) were effective and safe to relieve pain from onychectomy surgery in cats. Cats that have received fentanyl patches have had improvement in temperament, attitude, and appetite. Transdermal fentanyl has been used alone or combined with nonsteroidal anti-inflammatory drugs (NSAIDs) for treating severe pain in horses and may provide pain relief that is superior to NSAIDs alone.

Precautionary Information

Adverse Reactions and Side Effects

Severe adverse effects have not been reported. The patch may cause slight skin irritation at the site of application. If patch delivery of fentanyl is high, some signs of opiate overdose may occur (e.g., excitement in cats or sedation in dogs); however, these reactions are rare. Adverse effects have not been reported from the use in horses. If adverse effects are observed in animals (e.g., respiratory depression, excess sedation, or excitement in cats), remove patch and, if necessary, administer naloxone.

Contraindications and Precautions

Transdermal fentanyl is a Schedule II controlled substance. Use cautiously in animals of small body weight (e.g., small toy dogs and young or debilitated cats). Animal owners should be advised of the high risks to humans if transdermal patches are applied to humans. Fentanyl is absorbed through intact human skin.

Drug Interactions

There are no specific drug interactions, but transdermal fentanyl will decrease other anesthetic requirements. Transdermal fentanyl will potentiate other opiates and CNS depressants.

Instructions for Use

Transdermal fentanyl incorporates fentanyl into adhesive patches applied to skin of dogs and cats. Studies have determined that patches release sustained levels of fentanyl for 72-108 hours in dogs and cats. One 100-mcg/hr patch is equivalent to 10 mg/kg of morphine every 4 hours IM. Studies have determined that 25-mcg/hr patches are appropriate for cats. If rate of delivery is too high for cats and adverse reactions are suspected, covering half the adhesive surface area will reduce rate of delivery. A single 50-mcg/hr patch is appropriate for dogs weighing 10-20 kg. In horses, two or three 100-mcg/hr patches achieved rapid plasma concentrations within effective ranges in adults, but it was highly variable. Duration of effect in horses is less than in dogs or cats at only 48 hours. Follow manufacturer’s recommendations carefully when applying patches.

Patient Monitoring and Laboratory Tests

Monitor patient’s heart rate and respiration. Although bradycardia rarely needs to be treated when it is caused by an opioid, atropine can be administered if necessary. If serious respiratory depression occurs, the opioid can be reversed with naloxone. Monitor for signs of excitement in cats.

Formulations Available

Transdermal fentanyl is available in 25-, 50-, 75-, and 100-mcg/hr patches. In May 2009 the formulation was switched to a matrix vehicle instead of a reservoir system. These have been bioequivalent to previous formulations in people.

Stability and Storage

Store in a tightly sealed container, protected from light, and at room temperature. Do not open fentanyl patch membrane.

Small Animal Dosage

Dogs

• 10-20 kg: 50-mcg/hr patch q72h.

Cats

25-mcg patch q120 h.

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