Once- vs. twice-daily dosing
Initial studies suggested that cortisol secretion in cats followed a circadian rhythm and that evening dosing would be preferred,
as it would most closely mimic this natural rhythm.11 However, subsequent larger studies failed to document a consistent pattern of cortisol secretion in cats, indicating that
the time of administration is probably not important.12,13 Likewise, no evidence of an advantage exists in dividing a once-daily glucocorticoid dose into two doses. Because it can
be difficult to administer pills to cats, giving glucocorticoids once daily is a reasonable treatment plan. A possible exception
occurs with higher, immunosuppressive doses as some authors think dividing the dose can reduce gastrointestinal irritation.14
Glucocorticoids are available in various formulations that can have marked effects upon their activity. Orally administered
glucocorticoid preparations usually have the same duration of effect as the base glucocorticoid (Table 1). However, parenterally administered glucocorticoids come bound to various compounds that affect solubility, absorption,
and duration of effect. Glucocorticoids often are bound to one of two highly water-soluble compounds, sodium succinate or
sodium phosphate. A common example is dexamethasone sodium phosphate. Glucocorticoids bound to highly water-soluble compounds
are absorbed rapidly. Their duration of action is similar to that of the base glucocorticoid.
In contrast, acetate and diacetate are poorly water-soluble, and glucocorticoids bound to these compounds accumulate in the
tissues and are slowly released during subsequent days and months. A common example in veterinary medicine is methylprednisolone
acetate. While the base glucocorticoid, methylprednisolone, has a duration of action of 12 to 36 hours, methylprednisolone
acetate's activity can last for three to six weeks.14
If long-term use of any glucocorticoid is deemed necessary, a common goal of veterinarians is to reach a treatment stage whereby
dosing every other day with short- and intermediate-acting glucocorticoids and dosing every 72 hours with long-acting glucocorticoids,
such as dexamethasone, are appropriate. While not proven to be important, such dosing may minimize the suppressive effects
of glucocorticoids on the hypothalamic-pituitary-adrenal (HPA) axis by allowing the HPA axis to recover on the "off" treatment
days. Such a strategy is impossible when using the long-acting repositol corticosteroids, such as methylprednisolone acetate.
Use of such glucocorticoids also decreases your ability to monitor and adjust the glucocorticoid dose based on the patient's
response. Oral or rapidly acting parenteral forms of glucocorticoids are preferred, with repositol formulations reserved for
cases in which oral dosing is not feasible because of patient or owner noncompliance.
Topical corticosteroids may also be beneficial in cats, although the propensity of this species to be fastidious groomers
can reduce the local efficacy of these products when applied to areas easily accessible to the tongue. Systemic absorption
may also be increased by this activity. Few studies specific to cats are available on the effects of topical corticosteroids,
but side effects such as cutaneous atrophy, alopecia, localized pyoderma, and percutaneous absorption are seen in other species.
Cutaneous atrophy in cats treated with systemic glucocorticoids can be severe, as discussed below, and is, therefore, also
of potential concern with the use of topical corticosteroids in this species. Studies are lacking to document how frequently
this occurs. The strength of topical corticosteroids increases with increasing glucocorticoid potency and vehicle occlusiveness.
Cutaneous atrophy would be expected to be more severe with potent topical formulations such as betamethasone dipropionate
or mometasone furoate ointments or creams. Although potent products may be necessary initially, attempt to switch to low-potency
products such as hydrocortisone sprays if long-term treatment is necessary.