Lauren Riester Pinchbeck,     Wilton, Connecticut

Effects of Glucocorticoids on the Immune System

Antiinflammatory actions are the most important therapeutic benefit of GCs; they decrease tissue responses to inflammation without affecting the cause of inflammation. GCs affect humoral and cell-mediated immune systems, but immunosuppressive actions are more pronounced on cell-mediated immunity. GCs inhibit phospholipase A₂ and the conversion of arachidonic acid to prostaglandin and leukotriene metabolites, suppress bacteriocidal and fungicidal actions of macrophages, decrease production of intracellular signaling cytokines, decrease expression of inflammatory cytokines, and increase expression of antiinflammatory cytokines.

Structure of Glucocorticoids

Orally administered GCs are formulated as a free base or an ester that is converted to a free base. The free base is absorbed, and its structure determines duration of action. Biologic half-life and duration of action are longer for high-potency GCs and for higher doses of administered GCs. These factors increase the risk of adverse effects. Injectable GCs are esters of acetate, diacetate, sodium phosphate, or sodium succinate. Parenteral sodium phosphate and sodium succinate esters are very soluble, attain serum levels quickly, and are metabolized rapidly. Acetate or diacetate esters are poorly water soluble and are absorbed and metabolized slowly; such repositol formulations provide prolonged therapeutic benefits but increase the risk of adverse effects.

Cortisone is converted in the liver to its active form, cortisol. Hydrocortisone is identical to cortisol and has a short duration of action (<12 hours) and low potency. The relative potency of synthetic GCs traditionally is compared with that of hydrocortisone. Prednisone, which is four times as potent as cortisol, is inactive and requires hydroxylation in the liver to the active form, prednisolone, which has an intermediate duration of action (12 to 36 hours). Methylprednisolone is formed by the addition of a methyl group to prednisolone at the C-6 position, which decreases salt-retaining effects and increases GC effects. Methylprednisolone is five times as potent as hydrocortisone and has an intermediate duration of action (12 to 36 hours). When given as the injectable acetate salt, the duration of effect is 3 to 6 weeks. Triamcinolone acetonide is a fluorinated synthetic GC with four to five times the potency of hydrocortisone but has no appreciable mineralocorticoid activity because of methylation at position C-16. Its duration of action is longer than 48 hours. Dexamethasone (30 times as potent as hydrocortisone) and betamethasone (25 to 30 times as potent as hydrocortisone) are other fluorinated synthetic GCs with potent GC effects and prolonged duration of action (48 hours). They demonstrate low mineralocorticoid activity.

Use of Glucocorticoids

Table 94-1 is a guideline to dosages of synthetic GCs commonly used in veterinary dermatology. Although sources often suggest twice-daily administration, the author prescribes oral GCs once daily in dogs and cats. It may be common to use injectable GCs in primary care practice, but the author rarely recommends this route because it does not allow for dosage adjustments or withdrawal in the event of severe adverse effects. Injectable GCs should be used only after careful consideration and when a patient shows no response to oral GCs.

Cats seem to tolerate GCs better than dogs. Dosages for cats also generally are higher because cats have fewer GC receptors and weaker receptor-binding affinity. In addition, cats may differ from dogs in the rate of GC metabolism or extent of oral absorption. A recent study demonstrated that cats do not absorb prednisone or do not convert prednisone to prednisolone effectively. Thus prednisolone, not prednisone, should be used in cats. In cats, the author finds dexamethasone to be effective for pruritic and inflammatory skin diseases.

In dogs, oral methylprednisolone often is substituted for prednisone if polyuria and polydipsia become problematic. These adverse effects seemingly are less dramatic with oral methylprednisolone. If a cat does not tolerate oral administration of prednisolone or dexamethasone, methylprednisolone acetate may be considered for pruritic or inflammatory skin diseases. It is recommended that this GC be administered no more often than every 2 months, with a maximum of three or four injections per year.

Common Indications for Glucocorticoid Use in Dermatology

Pruritus is a very common presenting problem, and there are numerous diseases for which it is the primary symptom. To manage pruritus in the dog or cat, one must identify, treat, and manage the primary disease(s). However, systemic GCs may be necessary to provide the patient relief from pruritus while one works toward a definitive diagnosis.