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Cats are not easy to medicate.
As a veterinarian specializing in cats, I constantly find myself
faced with the dilemma of whether to prescribe an
ideal medication that has to be given three times a day vs. a
less-spectacular medication that requires only once-daily medication. Invariably, I choose the second-rate medication.
Let’s face it: the vast majority of cat owners cannot reliably
pill their cat three times a day. The
best medication in the world won’t work if it doesn’t get into the
cat. Asking a cat owner to
medicate their cat once a day is reasonable.
Twice a day? Borderline. Three
times a day? Nearly
impossible.
Our ability to medicate animals
is rapidly changing, however, due to the increase in the number of
pharmacies now offering the compounding of veterinary medications.
Initially, these pharmacies offered to convert medications that
were traditionally available only in capsule or tablet form into a liquid
form, as many cat owners find it easier to administer liquid medications
to their cat. These
pharmacies soon developed chicken, liver, beef, and tuna-flavored carrier
liquids into which the medications were dissolved. Many cats liked the flavored liquids and would take them
voluntarily if offered, although some cats still resisted, not so much due
to the taste of the medication, but rather, due to the entire process of
medicating. Cats simply do
not like to be told to eat something when they don’t want to.
Pharmacologic therapy by way of
the skin offers unique opportunities to deliver medication to a cat.
Transdermal medication (delivered through the skin) is a relatively
new science, dating back approximately 20 years or so.
In human medicine, very few medications are prepared this way:
nicotine patches to help quit smoking, estrogen patches for hormone
replacement therapy, and nitroglycerines for heart disease are a few
common examples. The
tremendous growth in the use of compounding pharmacies has given
veterinarians the option of providing medications, doses, or formulations
that are not readily available from routine sources.
This has now gone beyond reformulating drugs into flavored liquids
or into doses that are more appropriate for veterinary patients.
Many of these pharmacies are advocating the use of transdermal gels
to deliver medication. As a
veterinarian, I need to know the basics: Do these medications work? What is the proper dosage to administer?
Are there side effects?
The physiologic function of the
skin is to provide a two-way barrier between what’s inside the body, and
what’s outside the body. Transdermal
drug absorption involves transgression of this barrier. Several variables affect the degree of transdermal drug
absorption, including the chemical characteristics of the drug, properties
of the skin, and the nature of the substance into which the drug has been
dissolved (the “vehicle”). For
example, fat-soluble drugs formulated at high concentration will be
absorbed to a greater extent than water-soluble drugs formulated at low
concentration. Healthy
intact skin generally functions as an effective barrier to drug
absorption, whereas diseased skin serves as an ineffective and
unpredictable barrier. The
vehicle within which the drug is contained greatly affects the rate and
extent of transdermal drug absorption.
Water, alcohol, and dimethyl sulfoxide (DMSO) have all been used,
as well as more complex formulations such as ointments, lotions, gels,
pastes, aerosols, and creams.
Hyperthyroidism is a common
glandular condition in cats. In
many cats, as they get older, the thyroid gland enlarges and produces
excessive amounts of thyroid hormones.
The elevated levels of thyroid hormone increase the body’s
metabolism, causing a number of clinical signs, the most common being
weight loss despite a ravenous appetite. Other clinical signs may include
excessive thirst and urination, hyperactivity, diarrhea, panting,
excessive shedding, fast heart rate, and a heart murmur.
Surgical removal of the thyroid gland cures the condition, as does
destruction of the thyroid gland using radioactive iodine, however, many
clients refuse to pursue these avenues due to excessive cost of the
procedure, undue risk to the cat, or both.
Most clients choose to administer medication to their cat instead.
The medication, methimazole (available in generic form, or as the brand
name Tapazole), is administered to cats twice daily.
Medical therapy does not cure the condition. Rather, medication
causes the thyroid gland to reduce production of thyroid hormones.
If the medication is not administered in a consistent manner,
thyroid hormone levels will begin to rise, and clinical signs will recur.
Because medical therapy of
hyperthyroidism is lifelong, and given the difficult nature of
administering tablets to cats, several veterinarians have suggested
once-daily dosing, citing similar remission rates between once daily
dosing and more frequent dosing regiments in human patients with
hyperthyroidism. A recent study presented at the 2001 American College of
Veterinary Internal Medicine (ACVIM) Forum, however, in which hyperthyroid
cats were given either 5 milligrams of methimazole once a day vs. 2.5
milligrams twice a day, suggested that once daily dosing is considerably
less effective than twice daily dosing.
Once again, we’re faced with our basic dilemma of feline
pharmacology: how to get
medication into a cat consistently and non-traumatically?
Research involving the use of
transdermal methimazole is currently underway, and preliminary results of
the studies are promising. A study presented at the 2001 ACVIM Forum, in
which 13 cats with hyperthyroidism were administered methimazole gel
(applied to the inside of the ear) resulted in clinical improvement in all
cats treated. Most cats
gained weight, and clinical signs in many cats improved or resolved.
None of the cats showed clinical side effects.
At the first recheck, approximately 4 weeks after initiating the
study, measurement of thyroid hormone levels showed a decrease in 9 of 10
cats, with 7 cats showing normal levels.
A study presented at the 2002
ACVIM Forum, however, in which methimazole was administered transdermally
to healthy cats, gave unexpected results.
In that study, healthy adult cats were given 5 milligrams of
methimazole either orally, intravenously, or by way of a transdermal gel.
Blood levels of methimazole were compared for each of the three
methods of drug delivery. The study showed, surprisingly, that methimazole absorption
following a single transdermal dose of methimazole gel was variable, but
poor in general. Only two of
six cats tested had any detectable methimazole in their bloodstream. These results seemingly contradict the results reported at
the previoius forum, and are in discordance with the increasing number of
anecdotal reports apparent efficacy of transdermal methimazole for the
treatment of hyperthyroidism. Reasons
for the poor absorption were not cited, however, the researchers
emphasized that this study, using healthy cats, involved giving a single
dose of transdermal methimazole. The
researchers hypothesized that repeated doses, as required for the
treatment of hyperthyroid cats, may enhance absorption of the drug due to
changes in the outer layer of the skin that occur when the gel is applied.
Additional studies are now underway to better characterize the dosing of
transdermal methimazole for the treatment of hyperthyroidism in cats.
Transdermal drug dosing will undoubtedly grow in importance in the
coming years, with other classes of drugs, for example, antibiotics, pain
medications, and cancer chemotherapy drugs, being formulated for such
usage. Even the most stubborn
and disagreeable cat is no match for a quick smear of a measured dose of
gel inside the ear.
Updated 2/9/06
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