Aromatase inhibitors: past, present and future

For the cellular physiology of sex steroid sensitive cells, the androgen/es trogen ratio may be more important than only one hormone action per se, in both sexes. This ratio is controlled in vertebrates by aromatase; its gene expression can be inhibited in different ways, and this is crucial for the treatment of estrogen-dependent diseases such as breast cancer, or gynecoma stia in males for instance. To reach this goal, new steroidal and non-stero idal inhibitors are continuously being developed, and some of them are used as first or second line agents. Aromatase inhibition is also an essential tool for studying the role of estrogens in the adult, or during development . Aromatase inhibitors have shown in particular that estrogens are essentia l also in males for skeletal maturation and bone mineralization, developmen t of masculine dendritic morphology in male brain linked to mating behaviou r, and testicular function. Testosterone is often the prohormone converted in situ in active estrogens, at these levels. Several strategies can be use d for aromatase inhibition. The first ones employed were blind screening or deductions from in vivo observations, which led fur instance to the discov ery of the role of aminoglutethimide in aromatase inhibition. Subsequently, in the years 1975-1990, the molecular modeling of compounds to mimic the s ubstrate shape of the enzyme constituted the major idea. Hundreds of chemic als were synthesized by numerous authors, ranging from the well-known and v ery efficient 4-OHA to complicated imidazole or indane derivatives tested b y sophisticated comparative molecular field analyses. Reticulum-bound activ e aromatase has not as yet been X-ray analyzed. Thus, aromatase inhibitors were also used more recently to probe and understand the active site confor mation of the enzyme and its modelization was obtained from comparisons wit h bacterial-related cytochromes. We developed a mammalian model considerabl y closer to human aromatase in order to study the active site shape with ne w potent aromatase non-steroidal inhibitors. This model is equine aromatase . This enzyme was biochemically characterized, purified, and cloned by our group. It allowed testing, by site-directed mutagenesis, predictive hypothe ses in human aromatase which contributed to designing of new inhibitors. Th e understanding of the functioning of an essential member of the cytochrome P450 family, which is necessary for cellular detoxification, was also faci litated. Inhibition of aromatase activity has also been carried out with an tibodies directed to the catalytic site and at the gene level by knock-out or by control of factor-specific promoters. This may result in different mR NA synthesized by alter native splicing. We have also obtained specific inh ibition of aromatase activity in human cells with antisense stable phosphor othioate oligodeoxynucleotides directed against aromatase mRNA tertiary str uctures. Besides known steroidal and non-steroidal inhibitors, the antiarom atase effects of compounds found in our daily environment such as dietary f lavonoids or xenobiotic pollutants have also been described. Finally, we un derline that all these aromatase inhibitors, or methods of aromatase inhibi tion, can modulate the estrogenic balance essential not only for female, bu t also for male physiology, including gonadal function. (C) 2001 Elsevier S cience Ireland Ltd. All rights reserved.