Tibolone and its delta-4, 7alpha-methyl norethisterone metabolite are reversible inhibitors of human aromatase.

Tibolone is used for the treatment of climacteric symptoms and osteoporosis in menopausal women. After ingestion, it is rapidly converted to a number of metabolites including 3alpha- and 3beta-hydroxy derivatives and the delta-4, 7alpha-methylnorethisterone (7alpha-MeNET) metabolite, which is rapidly cleared from circulation. Tibolone and some of its metabolites act in a tissue-selective manner to inhibit steroid sulphatase (STS) and 17beta-hydroxysteroid dehydrogenase Type 1 (17beta-HSD1) activities but also stimulate steroid sulphotransferase and 17beta-HSD2 activities. In the present study we have examined whether the ability of tibolone and its 7alpha-MeNET metabolites to regulate the activities of enzymes involved in oestrogen formation or inactivation extends to another key enzyme involved in oestrogen synthesis, the aromatase, which converts androstenedione to oestrone. Using JEG-3 choriocarcinoma cells, which have a high level of aromatase activity, tibolone and 7alpha-MeNET, but not the 3alpha- or 3beta-hydroxy metabolites, were found to inhibit aromatase activity in intact cells and also lysates prepared from these cells (up to 61% inhibition at 10muM). An investigation into the nature of aromatase inhibition by these compounds revealed that they inhibit aromatase activity by a reversible mechanism. Tibolone and 7alpha-MeNET also inhibited aromatase activity in MCF-7 breast cancer cells, which have a much lower level of aromatase activity than JEG-3 cells. It is concluded that, in addition to inhibiting STS and 17beta-HSD1, tibolone and 7alpha-MeNET may exert some of their tissue-selective effects in regulating oestrogen synthesis by also inhibiting aromatase activity.

The nature of inhibition of steroid sulphatase activity by tibolone and its metabolites.

Tibolone is used for hormone replacement therapy and acts in a tissue-specific manner being oestrogenic on CNS and bone but not on breast tissues or endometrium. The ability of tibolone and its metabolites to inhibit steroid sulphatase (STS) activity has a crucial role in regulating its tissue-specific effects. In this study, we have examined the ability of tibolone and its non-sulphated and sulphated metabolites to inhibit STS activity in different enzyme preparations and in intact cells. For this, we have used an 'extracellular' method, which measures the amount of product released into culture medium, and an 'intracellular' method, which assesses the extent of product formation within cells. In addition, the nature by which tibolone and some of its metabolites inhibit STS activity was investigated using intact cells and an enzyme kinetic method. In MCF-7 and T47D breast cancer cells and JEG-3 choriocarcinoma cells, which have high STS activity, tibolone and its metabolites were relatively potent inhibitors of STS activity (33-57% inhibition at 10 microM) using the extracellular assay method. In HOS-TE-85 osteoblast-like cells, tibolone and its Delta-4 metabolite were relatively inactive whereas the 3alpha/3beta-hydroxy metabolites and their sulphated conjugates inhibited activity by 39-55%. When STS activity was assessed in HOS-TE-85 cells using an 'intracellular' method tibolone and its 3beta-hydroxy metabolite were inactive. Pre-treatment of breast cancer cells and JEG-3 cells, and removal of drugs prior to assaying for STS activity, revealed that in these cells tibolone and its metabolites were acting mainly as reversible inhibitors. This finding was confirmed in an enzyme kinetic study to measure concentration-dependent STS inhibition. In HOS-TE-85 cells, pre-treatment of cells and removal of compounds before assaying for remaining STS activity indicated that some tibolone metabolites appeared to stimulate STS activity. Possible mechanisms by which this might occur are discussed but, if confirmed, this could contribute to the positive oestrogenic effects that tibolone has on bone.

The therapeutic potential of a series of orally bioavailable anti-angiogenic microtubule disruptors as therapy for hormone-independent prostate and breast cancers.

Therapies for hormone-independent prostate and breast cancer are limited, with the effectiveness of the taxanes compromised by toxicity, lack of oral bioavailability and drug resistance. This study aims to identify and characterise new microtubule disruptors, which may have improved efficacy relative to the taxanes in hormone-independent cancer. 2-Methoxy-3-O-sulphamoyl-17beta-cyanomethyl-oestra-1,3,5(10)-triene (STX641), 2-methoxy-3-hydroxy-17beta-cyanomethyl-oestra-1,3,5(10)-triene (STX640) and 2-methoxyoestradiol-3,17-O,O-bis-sulphamate (STX140) were all potent inhibitors of cell proliferation in a panel of prostate and breast cancer cell lines. STX641 and STX640 significantly inhibited tumour growth in the MDA-MB-231 xenograft model. STX641 inhibited both in vitro and in vivo angiogenesis. Despite good in vivo activity, STX641 was not as potent in vivo as STX140. Therefore, STX140 was evaluated in the prostate hormone-independent PC-3 xenograft model. STX140 had superior efficacy to docetaxel, 2-MeOE2 and bevacizumab. In contrast to vinorelbine, no significant toxicity was observed. Furthermore, STX140 could be dosed daily over a 60-day period leading to tumour regression and complete responses, which were maintained after the cessation of dosing. This study demonstrates that STX641 and STX140 have considerable potential for the treatment of hormone-independent breast and prostate cancer. In contrast to the taxanes, STX140 can be dosed orally, with no toxicity being observed even after prolonged daily dosing.