Effects of ketoconazole on cholesterol synthesis

Studies have demonstrated that ketoconazole and related imidazoles block gonadal and adrenal steroidogenesis in humans by inhibiting several cytochrome P-450-dependent enzymes. Moreover, recent evidence suggests that cholesterol production in humans is also affected by ketoconazole. In the present experiments cultured normal human fibroblasts have been used to explore the effects of ketoconazole on cholesterol synthesis. Ketoconazole inhibited cholesterol synthesis (greater than 90% suppression in 1 hr) rapidly by blocking the conversion of methyl sterols to cholesterol. Dihydrolanosterol was the major methyl sterol which accumulated with ketoconazole. At high concentrations of ketoconazole, the conversion of squalene to methyl sterols was also inhibited. The inhibition of cholesterol synthesis was dose-dependent with an IC50 approximately 2.8 X 10(-8) M. In parallel to the inhibition of cholesterol synthesis, there was a reciprocal increase in methyl sterol production. The related imidazole antimycotic, clotrimazole, had similar effects, whereas the imidazole anesthetic, etomidate, had little effect on cholesterol synthesis. Confluent cells exposed to ketoconazole had a 90% fall in the activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase that declined with a T1/2 approximately 3.7 hr. In conclusion, ketoconazole has multiple effects on cholesterol synthesis, directly inhibiting late steps by blocking the conversion of methyl sterols to cholesterol and indirectly suppressing total sterol synthesis via feedback inhibition by sterol intermediates of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity.