1. Omeprazole, a substituted benzimidazole, is a potent inhibitor of gastric acid secretion which is currently being evaluated in patients with peptic ulcer and Zollinger-Ellison syndrome.

2. Drugs which possess an imidazole nucleus have previously been shown to inhibit cortisol release from the adrenal cortex, secondary to inhibition of mitochondrial cytochrome P-450 dependent hydroxylation reactions.

3. In a double-blind placebo-controlled crossover study in healthy male volunteers, omeprazole (60 mg daily for 7 days) did not alter basal cortisol levels. The peak cortisol response to ACTH stimulation was significantly reduced. Cortisol levels 60 min after ACTH were 824 ± 27 nmol/l on omeprazole (mean ± sem), and 929 ± 35 on placebo (P < 0.005).

4. In vitro, omeprazole caused a concentration-dependent inhibition of ACTH-stimulated cortisol release from isolated bovine adrenal cells (ED50 = 20 μg/ml). This was associated with a decrease in deoxycortisol synthesis. Therefore, unlike some other imidazole-containing drugs, the inhibitory effects of omeprazole are not entirely due to steroid 11 β-hydroxylase inhibition.

5. Substantial inhibition occurred at omeprazole concentrations which are higher than plasma levels normally achieved in clinical use. However, impairment of adrenocortical function may occur in patients on long-term high dose omeprazole treatment for Zollinger-Ellison syndrome.

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