A comparison of the bioavailability and potency of dexamethasone phosphate and sulphate in man

The metabolic fate and ACTH-suppressant activity of two injectable dexamethasone esters, 21-phosphate and 21-sulphate, were studied in healthy men. After i.v. injection of 20mg free steroid alcohol, dexamethasone phosphate was efficiently hydrolyzed to free dexamethasone, reaching its peak plasma concentration within 5 min. About 9% of the administered dose appeared in the urine as free dexamethasone. By contrast, virtually no free dexamethasone was found in plasma and urine after injection of dexamethasone sulphate. Pharmacokinetic analysis showed that dexamethasone sulphate had a shorter plasma half-life and a higher metabolic clearance rate than free dexamethasone. A larger fraction (60%) of dexamethasone sulphate was rapidly excreted unmetabolized in urine. The plasma cortisol level was significantly suppressed for more than 24h after dexamethasone phosphate, while the plasma cortisol profile after dexamethasone sulphate merely showed physiological circadian variations. When the steroid esters were injected after pretreatment with metyrapone, a definite suppression of plasma ACTH was noted after dexamethasone phosphate, but again, dexamethasone sulphate was ineffective. These results cast serious doubt on the clinical value of dexamethasone sulphate as an injectable glucocorticoid, and critical reevaluation of this preparation is needed.