Effect of ketoconazole, etomidate and other inhibitors of steroidogenesis on cytochrome P-450sccII-catalyzed reactions

Structural Analogues of Lanosterol from Marine Organisms of the Class Asteroidea as Potential Inhibitors of Human and Candida albicans Lanosterol 14α-demethylases

Lanosterol 14α-demethylases (hemoproteins of the cytochrome P450(51) family) are involved in biosynthesis of different membrane sterols, including animal cholesterol, fungal ergosterol and C24-modified plant and protozoa sterols. In this study we have investigated 10 structural analogs of lanosterol isolated from echinoderms belonging to the class Asteroidea as potential ligands (competitive inhibitors) of human and Candida albicans cytochromes P450(51). The study was performed using the surface plasmon resonance method, spectrophotometric titration and enzyme assay. Among the compounds tested we found several selective ligands for human and Candida albicans cytochromes. Between selective ligands of the human lanosterol 14α-demethylase we found two novel inhibitors of this enzyme: henricioside H1 and levisculoside G from Henricia derjugini. With due consideration of obtained data, we conclude that marine organisms from the class Asteroidea can be a valuable source of new lead compounds for creation of selective inhibitors of cytochromes P450(51) family with less side effects due to their selective action on these enzymes in different types of organisms.

Androgen synthesis in a songbird: a study of cyp17 (17alpha-hydroxylase/C17,20-lyase) activity in the zebra finch

Androgens and estrogens influence the maturation and function of numerous tissues in both male and female birds, especially the brains of the oscine songbirds. Although there exist a very large number of studies that have investigated circulating sex steroids in many species of wild and captive-held songbirds, there remain a significant number of questions about the sites of synthesis of the active steroids that act on the songbird brain. Estrogens are derived from androgen. Thus, the synthesis of androgen itself is critical for both androgen- and estrogen-dependent actions in both male and female songbirds. Therefore, we have undertaken studies of the enzyme 17alpha-hydroxylase/C17,20-lyase (Cyp17), the enzyme responsible for the synthesis of androgens from their progestin or pregnane precursors via their 17alpha-hydroxy intermediates. Here we have characterized optimal conditions for measuring Cyp17 in gonads of adult zebra finches via the conversion of tritiated [3H]progesterone into 17alpha-hydroxy P (17alpha-hydroxylase activity) and androstenedione and testosterone (C17,20-lyase) activity. Cyp17 activity is abundant in testis, with lesser amounts in ovary. Low levels of Cyp17 activity were also detected in male adrenals, but not in any other tissue, including brain. Testicular Cyp17 activity is readily inhibited in vitro by ketoconazole, a specific Cyp17 inhibitor. Ketoconazole works less well in vivo. In males castrated and/or treated with fadrozole, an inhibitor of aromatase, we detected no extragonadal sites of Cyp17 activity, although fadrozole appeared to increase circulating androgens in both castrated and intact males. Thus, we still do not know the site of androgen synthesis in these males. Further studies of Cyp17 will be useful in understanding more about the mechanisms of androgen delivery to neural circuits in adult and developing songbirds.

Different inhibitory effect of etomidate and ketoconazole on the human adrenal steroid biosynthesis

The narcotic agent etomidate and the antimycotic drug ketoconazole are known to block steroid biosynthesis in man. To study the different effects of these imidazole derivatives on human adrenal steroid biosynthesis we incubated slices of human adrenal glands with 3H-labeled precursors and increasing concentrations of etomidate or ketoconazole (0-2000 microM). After extraction the labeled metabolites were separated by thin-layer chromatography and quantified by scintillation counting. Etomidate inhibited most potently 11 beta-hydroxylase activity by suppressing the formation of corticosterone from 11-deoxycorticosterone to 1% of control [50% inhibitory concentration (IC50) 0.03 microM] while ketoconazole suppressed 11 beta-hydroxylase to only 39% of control activity (IC50 15 microM). Ketoconazole however, most potently blocked the conversion of 17 alpha-hydroxy-progesterone to androstenedione by C17,20-desmolase to about 15% of control activity (IC50 1 microM) while etomidate showed a much weaker effect on this enzyme with a suppression to 50% of C17,20-desmolase control activity at a concentration of 380 microM. Both imidazole drugs showed a similar strong inhibitory effect on the activity of 17 alpha-hydroxylase (IC50 6-18 microM) and 16 alpha-hydroxylase (IC50 4-8 microM) and did not affect 21-hydroxylase. These in vitro data indicate a predominant inhibitory effect of etomidate on corticosteroid biosynthesis by relative selective inhibition of 11 beta-hydroxylase and of ketoconazole on the adrenal androgen biosynthesis by a predominant inhibition of C17,20-desmolase. This differential inhibitory effect of etomidate and ketoconazole on human steroid biosynthesis may be of clinical importance for a possible therapeutic use of these imidazole derivatives in endocrine disorders.

Ketoconazole inhibition of the bifunctional cytochrome P450c17 does not affect androgen formation from the endogenous lyase substrate. The catalytic site remains refractory in the course of intermediary hydroxyprogesterone processing

The inhibition of the bifunctional steroidogenic cytochrome P450c17 (CYP17: steroid-17 alpha-hydroxylase/steroid-17,20-lyase) by the imidazole-type fungicide, [(+/-)-cis-1-acetyl-4-[4-[[2-(2,4-dichlorophenyl)-2-(1H-imidazol-1-yl- methyl)-1,3-dioxolan-4-yl]methoxy]phenyl]piperazine) (ketoconazole), was investigated with the aim of differentiating between effects on androgen formation from exogenously added and endogenously produced 17 alpha-hydroxyprogesterone. Using microsomal membranes from rat testis, turnover of progesterone by P450c17 was competitively inhibited by ketoconazole with KI = 0.40 microM. Ketoconazole did not affect the linear relationship between the ratio of productive events (corresponding to androgen formation rates) versus abortive events (corresponding to 17 alpha-hydroxyprogesterone formation rates) and the sum of catalytic events. This was an indication that this inhibitor did not interfere with intermediate processing by P450c17. Androgen formation from exogenous but not from endogenous 17 alpha-hydroxyprogesterone was competitively inhibited by ketoconazole. The simultaneous conversion of 1 microM each of [3H]progesterone and 17 alpha-hydroxy[14C]progesterone was also reduced by ketoconazole. Calculation of 3H/14C ratios in the 17 alpha-hydroxyprogesterone and androgen fractions revealed that the endogenous 17 alpha-hydroxyprogesterone pool was metabolized to androgens at rates 6.4, 11.6, 17.6 and 21.2-fold faster than the exogenous pool in the presence of 0.5, 1, 2 and 4 microM ketoconazole, respectively; this value was only 4.0 in controls. It is concluded that ketoconazole inhibits turnover of steroid ligands only when they approach the P450c17 active site in a substrate-state and that inhibition of androgen formation from progesterone is due to inhibition of the first catalytic step only. A model is described in which the P450c17 active site is refractory towards ketoconazole when the intermediary steroid is retained and being processed at that site.

Inhibition mechanism of reconstituted cytochrome P-450scc-linked monooxygenase system by antimycotic reagents and other inhibitors

The effects of various antimycotic reagents and some other reagents on a cytochrome P-450-linked monooxygenase system were investigated with respect to the activities of NADPH-ferricyanide reductase. NADPH-cytochrome c reductase of NADPH-adreno-ferredoxin reductase from NADPH to cytochrome c via adreno-ferredoxin, NADPH-cytochrome P-450-phenylisocyanide complex reductase, and the cholesterol side chain cleavage of the cytochrome P-450scc-linked monooxygenase system. No reagents inhibited the NADPH-ferricyanide reductase activity. Only cloconazole inhibited about 50% of NADPH-cytochrome c reductase activity. Cloconazole, econazole, clotrimazole, etomidate and ketoconazole inhibited both NADPH-cytochrome P-450-phenylisocyanide complex reductase and the side chain cleavage activity of cholesterol of the cytochrome P-450scc-linked monooxygenase system. Cloconazole, econazole, etomidate and ketoconazole behaved like non-competitive inhibitors for NADPH-cytochrome P-450-phenylisocyanide reductase activities and their Ki values were 10(-4)-10(-6) M. Cloconazole was a non-competitive inhibitor of NADPH-cytochrome c reductase and its Ki value was 8.3 x 10(-4) M. Cloconazole, clotrimazole, econazole, etomidate, ketoconazole and mitotane completely inhibited the side chain cleavage activity of cholesterol.

Mitochondrial P-450 activities in aldosteronoma tissues

Adrenal P-450 activities were measured by an in vitro reconstitution system from tissues obtained from human aldosteronomata, and the results compared with those of the normal adrenal tissues from patients with Grawitz's tumor. The P-45011 beta activity was significantly increased in adenoma tissue (55.6 +/- 5.3 vs 9.0 +/- 6.2 nmol corticosterone/mg of protein/min in the control tissues, P less than 0.01). P-450scc activity in adrenal adenomata was 13.4 +/- 2.0 nmol pregnenolone/mg of protein/min, significantly higher than control (P less than 0.05). The present results suggest that increased mitochondrial P-450(11 beta) activities may be characteristic of aldosterone-producing adenomata.

Inhibitory effect of some imidazole antifungal compounds on the synthesis of 16-ene-C19-steroid catalyzed by pig testicular microsomes

The activity of the enzyme (16-ene-C19-steroid synthesizing enzyme) responsible for the conversion of C21-steroids to 16-ene-C19-steroids, which was localized on pig testicular microsomes, was inhibited by some typical imidazole antifungal compounds such as clotrimazole, econazole, miconazole and ketoconazole which are known to be universal inhibitors of cytochrome P-450-dependent enzymes. The 50% inhibitory concentrations of clotrimazole, econazole and miconazole were 0.29, 0.36 and 1.25 microM, respectively for 16-ene-C19-steroid synthesizing enzyme activity. Clotrimazole was the most powerful inhibitor of all the compounds examined, which shows the competitive inhibition for 16-ene-C19-steroid synthesizing enzyme activity. The Ki-value was 0.26 microM for its activity. The degree of the inhibition by these imidazole compounds was very similar to the inhibition of 17 alpha-hydroxylase and C17,20-lyase activities on pig testicular microsomes.

High dose ketoconazole: endocrine and therapeutic effects in postmenopausal breast cancer

Ketoconazole, an antifungal agent, inhibits in vitro C17-C20 lyase, an enzyme involved in androgen biosynthesis. Since adrenal and ovarian androgens are the main precursors of oestrogens in postmenopausal women, the endocrine and therapeutic effects of high dose ketoconazole (400 mg three times a day) were evaluated in 14 postmenopausal women with advanced breast cancer. Testosterone levels were suppressed significantly (37%, P less than 0.025), as was dehydroepiandrosterone sulphate, and androstenedione levels showed a similar but non-significant fall. Seventeen hydroxyprogesterone levels rose significantly, as would be expected if C17-C20 lyase was inhibited. There was no suppression of cortisol or oestrone levels. There was a small suppression of oestradiol concentrations, reflecting a decrease in its precursor, testosterone. Sex hormone binding globulin levels rose, which may be due to a decrease in testosterone. All the changes are compatible with C17-C20 lyase as a major site of action in vivo. No responses occurred in 12 patients treated with ketoconazole alone, but in 2 patients who were progressing on aminoglutethimide, testosterone levels were suppressed and in one patient a partial response occurred. Ketoconazole was poorly tolerated due to gastrointestinal toxicity. This study shows that C17-C20 lyase is a potential target for hormone therapy, and that sequential blockade of enzymes involved in oestrogen biosynthesis should be further evaluated.