Comparative effects of the aromatase inhibitor R76713 and of its enantiomers R83839 and R83842 on steroid biosynthesis in vitro and in vivo.
R76713 (6-[(4-chlorophenyl)(1H-1,2,4-triazol-1-yl)methyl]-1-methyl-1H- benzotriazole) is a selective, non-steroidal aromatase inhibitor containing an asymmetric carbon atom. In this paper, we compare the effects of R76713 (racemate) with its enantiomers R83839 (the levo-isomer) and R83842 (the dextro-isomer) on steroid biosynthesis in rat cells in vitro and in the rat in vivo. In rat granulosa cells, aromatase activity was inhibited by 50% at concentrations of 0.93 nM of R76713, 240 nM of R83839 and 0.44 nM of R83842, revealing a 545-fold difference in activity between both enantiomers. Up to 1 microM, none of the compounds had any effect on steroid production in primary cultures of rat testicular cells. Above this concentration all three compounds showed a similar slight inhibition of androgen synthesis with a concomitant increase in the precursor progestins, indicative for some effect on the 17-hydroxylase/17,20-lyase enzyme. In rat adrenal cells none of the compounds showed any effect on corticosterone synthesis. At concentrations above 1 microM there was an increase in the levels of 11-deoxycorticosterone pointing towards an inhibition of the 11-hydroxylase enzyme. This increase was more pronounced for R83839 than for R76713 and R83842. In vivo, in PMSG-primed rats, R83842 reduced plasma estradiol by 50%. 2 h after oral administration of 0.0034 mg/kg, whereas 0.011 mg/kg of R76713 and 0.25 mg/kg of R83839 were needed to obtain the same result. Oral administration of up to 20 mg/kg of the compounds did not significantly affect plasma levels of adrenal steroids in LHRH/ACTH-injected rats. Plasma testosterone was lowered at 10 and 20 mg/kg of R83842 and at the highest dose (20 mg/kg) of R76713 and R83839. In conclusion, the present study shows that the aromatase inhibitory activity of R76713 resides almost exclusively in its dextro-isomer R83842. R83842 exhibits a specificity for aromatase as compared to other enzymes involved in steroid biosynthesis of at least a 1000-fold in vitro as well as in vivo. This confirms the extreme selectivity previously found for the racemate.
Wouters
W
W
Department of Endocrinology and Oncology, Janssen Research Foundation, Beerse, Belgium.
De Coster
R
R
van Dun
J
J
Krekels
M D
MD
Dillen
A
A
Raeymaekers
A
A
Freyne
E
E
Van Gelder
J
J
Sanz
G
G
Venet
M
M
D000305:Adrenal Cortex Hormones / Q000378:metabolism; D000324:Adrenocorticotropic Hormone / Q000494:pharmacology; D000818:Animals; D047072:Aromatase Inhibitors; D004958:Estradiol / Q000096:biosynthesis; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology; D006064:Gonadotropins, Equine / Q000494:pharmacology; D006107:Granulosa Cells / Q000187:drug effects / Q000378:metabolism; D066298:In Vitro Techniques; D008297:Male; D010053:Ovary / Q000187:drug effects / Q000378:metabolism; D051381:Rats; D013237:Stereoisomerism; D013250:Steroid Hydroxylases / Q000378:metabolism; D013256:Steroids / Q000096:biosynthesis*; D013737:Testis / Q000187:drug effects / Q000378:metabolism; D013739:Testosterone / Q000097:blood; D014230:Triazoles / Q000494:pharmacology*
D000305:Adrenal Cortex Hormones; D047072:Aromatase Inhibitors; D006064:Gonadotropins, Equine; D013256:Steroids; D014230:Triazoles; C060523:vorozole; D007987:Gonadotropin-Releasing Hormone; D013739:Testosterone; D004958:Estradiol; D000324:Adrenocorticotropic Hormone; D013250:Steroid Hydroxylases