In normal subjects, intravenous administration of corticorelin results in a rapid and sustained increase of plasma ACTH levels and a near parallel increase of plasma cortisol. In addition, intravenous administration of corticorelin to normal subjects causes a concomitant and prolonged release of the related proopiomelanocortin peptides β- and γ-lipotropins (β -and γ-LPH) and β-endorphin (β -END). A number of dose-response studies have been performed on normal subjects using a range of corticorelin doses. In one study, doses of corticorelin ranging from 0.001 to 30 mcg/kg body weight were administered to 29 healthy volunteers. Blood samples were taken over a 2-hour period for determination of plasma ACTH and cortisol concentrations. There was a direct dose-dependent relationship that was more pronounced for ACTH than for cortisol. The threshold dose was 0.03 mcg/kg, the half-maximal dose was 0.3-1.0 mcg/kg and the maximally effective dose was 3-10 mcg/kg.

Plasma ACTH levels in normal subjects increased 2 minutes after injection of corticorelin doses of ≥0.3 mcg/kg and reached peak levels after 10-15 minutes. Plasma cortisol levels increased within 10 minutes and reached peak levels at 30 to 60 minutes. As the dose of corticorelin was increased, the rises in plasma ACTH and cortisol were more sustained, showing a biphasic response with a second lower peak at 2-3 hours after injection. Similar results were found in another study using 0.3, 3.0, and 30 mcg/kg doses. The duration of mean plasma ACTH increase after injection of 0.3, 3.0, and 30 mcg/kg was 4, 7, and 8 hours, respectively. The effect on plasma cortisol was similar, but more prolonged. Because there are differences in basal levels and peak response levels following a.m. or p.m. administration, it is recommended that subsequent evaluations in the same patient using the corticorelin stimulation test be carried out at the same time of day as the original evaluation.

Baseline ACTH and cortisol levels are usually higher in the morning. Pooled ACTH values from normal unstressed subjects (n=119) were 25 ± 7 pg/mL in the a.m. and 10 ± 3 in the p.m.; similar pooled cortisol values (n=170) were 11 ± 3 mcg/dL in the a.m. and 4 ± 2 mcg/dL in the p.m. The normal unstressed person has about seven to ten secretory episodes of ACTH each day. Most of them occur in the early morning hours and are responsible for the morning plasma cortisol surge. The following figure shows the daily circadian rhythm of ACTH and cortisol secretions in a normal unstressed person. Insulin, plasma renin activity, prolactin, and growth hormone release are not affected by corticorelin administration in humans.

Continuous 24-hour infusion of corticorelin (0.5, 1.0, and 3.0 mcg/kg/hr) increased plasma ACTH concentrations to a plateau of 15-20 pg/mL by the third hour and urinary-free cortisol reaches 173 ± 43 mcg/dL by 24 hours, comparable to those levels observed in patients with major depression, but less than levels noted in Cushing's disease. Continuous infusion did not abolish the circadian rhythm of plasma ACTH and cortisol, but did appear to desensitize the corticotroph. Intermittent doses of corticorelin (25 mcg every 4 hours for 72 hours), however, continued to elicit the expected ACTH and cortisol responses.

Intravenous administration of 1 mcg/kg corticorelin in combination with 10 pressor units intramuscular vasopressin had a synergistic effect on ACTH and a less marked synergistic effect on cortisol secretion.

The basal and peak response levels of ACTH and cortisol to a 1 mcg/kg or 100 mcg dose of corticorelin administered to normal volunteers in the morning and the evening are given below. These values were obtained by combining the results from 9 clinical trials conducted in the a.m. and 4 clinical trials conducted in the p.m.

The following table is to be used only as a general guide.