DOSING
Table 1. Relative Glucocorticoid Potencies and Duration of Action of Selected Glucocorticoids*
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A large number of synthetic glucocorticoids have been developed with the goal of increasing efficacy while decreasing the
number of adverse effects (Table 1). In most cases, the mineralocorticoid-related effects of sodium and water retention and potassium excretion are unwanted
consequences of glucocorticoid use. Most synthetic glucocorticoids have been designed to reduce or eliminate these properties.
Prednisone vs. prednisolone
One of the most commonly used glucocorticoids in veterinary medicine is prednisone, or prednisolone, though this is based
more on familiarity and common practice than on a proven increased efficacy of this particular corticosteroid. No studies
in cats suggest that any particular glucocorticoid is more effective for the treatment of a specific disease, provided equipotent
doses are used. For prednisone to be effective, however, it first must be converted to the active form, prednisolone, in the
liver. In dogs, this process occurs efficiently, and prednisolone and prednisone can be considered to be bioequivalent.3 In cats, the absorption and conversion of prednisone to prednisolone is less efficient, and after the oral administration
of prednisone, only about 21% of the drug occurs in the bloodstream as the active form prednisolone.4 For this reason, prednisone and prednisolone should not be considered bioequivalent in cats, and the active form, prednisolone,
should be used preferentially.
Potency
The relative potency data presented in Table 1, while commonly cited in the veterinary literature, are based predominantly on studies performed in other species, as well
as clinical experience. Similar relative potencies are likely in cats, but no specific studies have been performed in this
area. In particular, a discrepancy exists regarding the estimated potency of triamcinolone.
In the veterinary literature, triamcinolone is commonly estimated to be five times more potent than hydrocortisone; however,
some authors think it may be up to 40 times more potent.5 This is supported by studies in people in which the ability of triamcinolone to suppress lymphocyte proliferation was used
as a measure of the immunosuppressive potential of a glucocorticoid. Triamcinolone was shown to be more potent than dexamethasone
in this respect.6
Moreover, the binding affinity of a glucocorticoid to the glucocorticoid receptor correlates with anti-inflammatory potential.
Triamcinolone also has a higher glucocorticoid-receptor binding affinity than dexamethasone, suggesting equivalent or increased
anti-inflammatory activity.6 I consider an estimate of 40 times that of hydrocortisone to be a more accurate approximation of triamcinolone's glucocorticoid
potency in dogs and cats, and I adjust doses accordingly.
Dosing and intended effect
Glucocorticoid doses are commonly separated into physiologic, anti-inflammatory, and immunosuppressive ranges. These terms
can be somewhat misleading because there is no exact dose at which a glucocorticoid changes from providing merely anti-inflammatory
to immunosuppressive effects. Some cited dose ranges in veterinary medicine may not specify to which species they refer, and
in most of these situations the doses are often most applicable to dogs. These ranges do provide a useful guide when selecting
an initial glucocorticoid dose, depending on the condition to be treated.
Physiologic doses of prednisolone, such as would be used as replacement therapy in dogs with hypoadrenocorticism, are estimated
to be 0.2 to 0.3 mg/kg given orally or parenterally once a day. Anti-inflammatory doses of prednisolone in dogs are cited
to be 0.55 to 1.1 mg/kg orally or parenterally once a day and would be appropriate for treating conditions such as atopic
dermatitis. Immunosuppressive doses in dogs range from 2.2 to 4.4 mg/kg once a day and are needed when treating immune-mediated
or autoimmune conditions such as pemphigus foliaceus or autoimmune hemolytic anemia.7
These dose ranges have not been verified scientifically in cats, but it is anecdotally noted that cats often require higher
glucocorticoid doses than dogs do in order to achieve equivalent effects. This is supported by a study that showed cats had
fewer numbers of glucocorticoid receptors than dogs in the organs that were evaluated (liver and skin).8 The binding affinity of the feline glucocorticoid receptor also was shown to be less than that of the dog.8 Consequently, many authors suggest that glucocorticoid doses in cats be doubled to achieve equivalent effects, resulting
in anti-inflammatory and immunosuppressive dose ranges of prednisolone extending to 2.2 mg/kg/day and 8.8 mg/kg/day, respectively.7,9,10 Doses for other glucocorticoids should be adjusted based on the relative potencies shown in Table 1 with the exception of triamcinolone, whose potency may be closer to that of dexamethasone, as noted above.
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