Cosyntropin combines with a specific receptor in the adrenal cell plasma membrane and, in patients with normal adrenocortical function, stimulates the initial reaction involved in the synthesis of adrenal steroids (including cortisol, cortisone, weak androgenic substances, and a limited quantity of aldosterone) from cholesterol by increasing the quantity of the substrate within the mitochondria. Cosyntropin does not significantly increase plasma cortisol concentration in patients with primary or secondary adrenocortical insufficiency.
Cosyntropin exhibits the full corticosteroidogenic activity of natural ACTH. Various studies have shown that the biologic activity of ACTH resides in the N- terminal portion of the molecule and that the 1-20 amino acid residue is the minimal sequence retaining full activity. Partial or complete loss of activity is noted with progressive shortening of the chain beyond 20 amino acid residue. For example, the decrement from 20 to 19 results in a 70% loss of potency. The pharmacologic profile of Cosyntropin is similar to that of purified natural ACTH. It has been established that 0.25 mg of Cosyntropin will stimulate the adrenal cortex maximally and to the same extent as 25 units of natural ACTH. Cosyntropin has less immunogenic activity than ACTH because the amino acid sequence having most of the antigenic activity of ACTH, i.e., amino acids 25-39, is not present in cosyntropin. The extra-adrenal effects which natural ACTH and Cosyntropin have in common include increased melanotropic activity, increased growth hormone secretion and an adipokinetic effect. These are considered to be without physiological or clinical significance.