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Medicating cats: Transdermal drugs and more (Proceedings)

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Aug 01, 2008

Long term administration of oral medications can be problematic in cats. Unfortunately, relatively few drugs have received FDA approval for cats and "cat friendly" drug formulations (e.g., small size and strength, attractive flavorings) are less common than desired by practitioners. In recent years, veterinarians and cat owners have sought alternative drug formulations, and compounding pharmacies have stepped in to fill the gap. But do they work?

Problems with Pills

Many cats resent the invasiveness of receiving oral medications, making it difficult or impossible for owners to be compliant with medication instructions. Even liquid medications may be difficult to administer to some cats as they may find the taste unpleasant, and salivate profusely. Adding medication to the cat's food may simply induce food aversion, or in a multi-cat household, the wrong cat may receive the drug.

Another significant problem is the risk of esophageal damage. Certain drugs given in tablet or capsule form, such as doxycycline and clindamycin, may induce esophagitis or esophageal stricture in cats (Melendez, Twedt et al. 2000; German, Cannon et al. 2005; Beatty, Swift et al. 2006). Studies have revealed a delayed transit time (longer than 30-60 seconds) in most cats that are dry-pilled (Graham, Lipman et al. 2000; Westfall, Twedt et al. 2001). More than 50% of cats in one study experienced entrapment of capsules in the mid-cervical region of the esophagus (Graham, Lipman et al. 2000). Cats given a water bolus or fed a small amount of food after a tablet or capsule do not experience prolonged retention of the drug in the esophagus and the risk of esophagitis is reduced or eliminated.

Flavored Suspensions and Chewables

Many pharmacies now offer compounding of a long list of drugs into flavored suspensions or chewables. In a commercial licensed formulation, the active ingredients and the excipients are tested and must meet FDA specifications to ensure the stability of the drug and uniform performance in the patient. In compounding, the addition of chemicals, flavorings, or vehicles may interfere with the stability of a drug, decrease potency, change distribution of the drug in the product, cause precipitation, compromise absorption, and thereby reduce efficacy or compromise safety. The same is true when the protective coating found on some tablets is disrupted to crush the tablets and make a suspension. Compounding pharmacists should be able to provide information to clinicians about the stability, safety and potency of any formulations offered. When veterinarians compound drugs in their own practices, the same standards apply.

Bitter tasting pills are a common problem in feline medicine. Flavored treats formulated to hide pills or capsules (e.g., Greenies® Pill Pockets®) are an alternative to crushing pills and adding a flavoring or vitamin/mineral supplement. Another option is to place the tablet inside an empty gelatin capsule to hide the taste. Empty gelatin capsules of various sizes are readily available from most pharmacies.

Transdermal Drug Administration

Transdermal administration of medications to cats appears to be an ideal solution to the difficulties with oral medications. This route of administration may also avoid gastric and intestinal irritation associated with some drugs. However, this is a relatively new area of veterinary therapeutics, and in many cases, scant information is available on safety and efficacy. In human medicine, a number of drugs are available as transdermal patches, such as fentanyl, scopolamine and nicotine. In veterinary medicine, there are several FDA-approved "pour-on" or "spot-on" antiparasitic drugs approved for cats.

Many factors influence transdermal drug absorption. Patient factors include skin thickness and lipid content, density of hair follicles, skin pH, blood flow, and integrity of stratum corneum. Drug factors include molecular weight, ionization, and the carrier vehicle. Drugs are developed for transdermal application in humans only if they are effective at doses under about 50 mg per day.

In order to evaluate a transdermal formulation, these questions must be answered:

1 Is the drug bioavailable – does it get absorbed?

2. Is the bioavailability consistent from patient to patient?

3. Does the drug require biotransformation for efficacy?

4.Is the formulation stable?