Oestrogen suppression--lessons from clinical studies

History of aromatase: saga of an important biological mediator and therapeutic target

Aromatase is the enzyme that catalyzes the conversion of androgens to estrogens. Initial studies of its enzymatic activity and function took place in an environment focused on estrogen as a component of the birth control pill. At an early stage, investigators recognized that inhibition of this enzyme could have major practical applications for treatment of hormone-dependent breast cancer, alterations of ovarian and endometrial function, and treatment of benign disorders such as gynecomastia. Two general approaches ultimately led to the development of potent and selective aromatase inhibitors. One targeted the enzyme using analogs of natural steroidal substrates to work out the relationships between structure and function. The other approach initially sought to block adrenal function as a treatment for breast cancer but led to the serendipitous finding that a nonsteroidal P450 steroidogenesis inhibitor, aminoglutethimide, served as a potent but nonselective aromatase inhibitor. Proof of the therapeutic concept of aromatase inhibition involved a variety of studies with aminoglutethimide and the selective steroidal inhibitor, formestane. The requirement for even more potent and selective inhibitors led to intensive molecular studies to identify the structure of aromatase, to development of high-sensitivity estrogen assays, and to "mega" clinical trials of the third-generation aromatase inhibitors, letrozole, anastrozole, and exemestane, which are now in clinical use in breast cancer. During these studies, unexpected findings led investigators to appreciate the important role of estrogens in males as well as in females and in multiple organs, particularly the bone and brain. These studies identified the important regulatory properties of aromatase acting in an autocrine, paracrine, intracrine, neurocrine, and juxtacrine fashion and the organ-specific enhancers and promoters controlling its transcription. The saga of these studies of aromatase and the ultimate utilization of inhibitors as highly effective treatments of breast cancer and for use in reproductive disorders serves as the basis for this first Endocrine Reviews history manuscript.

Focused Libraries of 16-Substituted Estrone Derivatives and Modified E-Ring Steroids: Inhibitors of 17ß-Hydroxysteroid Dehydrogenase Type 1

17beta-hydroxysteroid dehydrogenase type 1 (17beta-HSD1), an oxidoreductase which has a preferential reductive activity using NADPH as cofactor, converts estrone to estradiol and is expressed in many steroidogenic tissues including breast and in malignant breast cells. As estradiol stimulates the growth and development of hormone-dependent breast cancer, inhibition of the final step of its synthesis is an attractive target for the treatment of this disease. The parallel synthesis of novel focused libraries of 16-substituted estrone derivatives and modified E-ring pyrazole steroids as new potent 17beta-HSD1 inhibitors is described. Substituted 3-O-sulfamoylated estrone derivatives were used as templates and were immobilised on 2-chlorotrityl chloride resin to give resin-bound scaffolds with a multi-detachable linker. Novel focused libraries of 16-substituted estrone derivatives and new modified E-ring steroids were assembled from these immobilised templates using solid-phase organic synthesis and solution-phase methodologies. Among the derivatives synthesised, the most potent 17beta-HSD1 inhibitors were 25 and 26 with IC50 values in T-47D human breast cancer cells of 27 and 165 nm, respectively. Parallel synthesis resulting in a library of C5'-linked amides from the pyrazole E-ring led to the identification of 62 with an IC50 value of 700 nM. These potent inhibitors of 17beta-HSD1 have a 2-ethyl substituent which will decrease their estrogenic potential. Several novel 17beta-HSD1 inhibitors emerged from these libraries and these provide direction for further template exploration in this area. A new efficient diastereoselective synthesis of 25 has also been developed to facilitate supply for in vivo evaluation, and an X-ray crystal structure of this inhibitor is presented.