Steroidal Antioestrogens Research Articles

Assessment of the oestrogenic activity of the contraceptive progestin levonorgestrel and its non-phenolic metabolites

Levonorgestrel (13β-ethyl-17α-ethynyl-17β-hydroxy-4-gonen-3-one), a potent contraceptive progestin stimulates growth and proliferation of cultured breast cancer cells through a receptor-mediated mechanism, even though levonorgestrel does not bind to the oestrogen receptor (ER). To assess whether the oestrogen-like effects induced by this synthetic progestin are exerted via its metabolic conversion products, we studied the binding affinity of three A-ring levonorgestrel derivatives to the ER and their capability to transactivate an oestrogen-dependent yeast system co-transfected with the human ER gene and oestrogen responsive elements fused to a β-galactosidase reporter vector. The results demonstrated that the 3β,5α reduced levonorgestrel derivative and to a lesser extent its 3α isomer interact with the oestrogen receptor, with a significantly lower relative binding affinity (2.4% and 0.4%, respectively) than that of oestradiol (100%), while levonorgestrel does not. Both levonorgestrel metabolites were able to activate, in a dose-dependent manner, the β-galactosidase reporter gene in the yeast expression system, an effect that was precluded by a steroidal antioestrogen. The oestrogenic potency of levonorgestrel metabolites was significantly lower (750-fold) than that of oestradiol. Furthermore, high doses of 3β,5α levonorgestrel (2.5 mg/day/6 days) induced an increase of oestrogen-dependent progestin receptor in the anterior pituitary of castrated rats. The overall data offer a plausible explanation for the weak oestrogenic effects induced by high, non-pharmacological doses of levonorgestrel.

Functional inactivation of the oestrogen receptor by the antioestrogen, ZM 182780, sensitises tumour cells to reactive oxygen species.

Reactive oxygen species (ROS) play a fundamental role in both apoptotic and necrotic cell death. Their importance is highlighted by studies showing that they mediate cell death in response to radiotherapy and to some forms of chemotherapy. Here we provide the first evidence for a role of ROS in response to an antiendocrine agent currently undergoing clinical trials. Using the oestrogen receptor (ER) containing rat pituitary GH3 cell line, we show that cell death is induced by the pure steroidal antioestrogen, ZM 182780, and that this is blocked by the antioxidant, N-acetyl cysteine (NAC). By flow cytometry, we show that, prior to the onset of DNA breakdown measured by ELISA, ZM 182780 exposure has no significant effect on intracellular oxidant concentrations. In contrast, ZM 182780 exposure greatly increases sensitivity to oxidants generated by blocking cellular antioxidant pathways and from exogenous administration of hydrogen peroxide (H2O2). As both necrosis and apoptosis are controlled by mitochondrial function, further experiments conducted to determine mitochondrial membrane potential (Delta|gWm) have indicated that the ZM 182780-induced loss of ER function increases the ease with which oxidants collapse mitochondrial activity and, as a consequence, cell death.

Inhibition of Brain Oestrogen Receptors by RU 58668

The purpose of this study was to determine if injections of the 11 beta-substituted steroidal antioestrogen, RU 58668, would block two measures of oestrogen receptor action in ovariectomized adult female rats. Using an in vitro nuclear exchange assay, it was found that RU 58668 reduced cell nuclear [3H]-oestradiol binding in brain tissue 24 h after it was injected. However, pituitary cell nuclear [3H]-oestradiol binding was significantly reduced just 2 h after the antioestrogen was injected. Our results demonstrate that RU 58668 can reach the brain following a subcutaneous injection, but that it needs more time to reach the brain than it does to reach the pituitary. Since the levels of hypothalamic and pituitary progestin receptors are known to be regulated by oestradiol, cytosolic [3H]-R5020 binding was used as an in vitro assay of endogenous oestrogen receptor action. RU 58668 blocked induction by oestradiol of cytosolic [3H]-R5020 binding in both the brain and pituitary 48 h after it was injected. In the absence of oestradiol, RU 58668 did not stimulate cell nuclear [3H]-oestradiol binding or cytosolic [3H]-R5020 binding in either brain or pituitary. In conclusion, injections of RU 58668 blocked two measures of oestrogen receptor action in the brain and pituitary without showing oestrogenic activity itself.

Oestrogen/growth factor cross-talk in breast carcinoma: a specific target for novel antioestrogens.

Breast cancer cells express receptors for and are sensitive to a variety of steroids, polypeptide hormones and growth factors; the blocking of and/or the interference with their biochemical pathways could represent a new approach to breast tumour therapy. Antioestrogens achieve such a goal by competing with oestradiol for binding to the oestrogen receptors through which intracellular effects of the hormone are mediated. Tamoxifen has undergone the most extensive clinical evaluations and represents the treatment of choice for the endocrine management of breast cancer. Nevertheless, it is well known that tamoxifen retains agonist activity both in vitro and in vivo. To circumvent this disadvantage, new molecules with steroid-like structure, represented by ICI 164,384 and ICI 182,780, have been synthesized. In this review, Alessandra de Cupis and Roberto Favoni review data about the cross-talk between the two major families of breast cancer growth regulator: oestrogens and growth factors, focusing on the use of nonsteroidal antioestrogens and the new generation of steroidal antioestrogens as possible specifically targeted inhibitors of breast tumour proliferation.

Athymic nude mouse model for the study of new antioestrogens in breast cancer

Abstract Endocrine therapy utilizing agents that reduce oestrogen levels or block the oestrogen receptor remains an important modality in breast cancer management. While hormonal treatment are effective in many patients, de novo and acquired resistance are common problems. Model systems are needed that mimic the clinical situation to facilitate studies of the mechanisms of hormonal resistance. We have developed an in vivo model system for studies of endocrine therapy using the ER-positive MCF-7 human breast cancer cells growing subcutaneously in athymic mice. Similar to observations in patients, treatment of these mice by oestrogen withdrawal, tamoxifen, or steroidal antioestrogens (which have a pure antioestrogenic profile), inhibits growth of established tumours for several months. Eventually, tumours develop resistance and progression occurs. The steroidal antioestrogens offer more prolonged tumour control compared to tamoxifen and oestrogen withdrawal. Furthermore, these antioestrogens inhibit tumours that have developed resistance to oestrogen deprivation and/or tamoxifen. Studies from this model system support the hypothesis that pure steroidal antioestrogens may be effective antitumour agents in patients and that they may be useful in patients with tamoxifen resistance.

Pharmacology of new aromatase inhibitors

Recent investigations of novel aromatase inhibitors have addressed several important questions related to the biochemical effects of this class of drugs. While aromatase inhibitors such as aminoglutethimide, formestane (given by the oral and i.m. route), rogletimide and fadrozole administered at different doses have been found to inhibit in vivo aromatization by 60–93%, the novel aromatase inhibitors, letrozole and anastrozole, are shown to inhibit in vivo aromatization by about 98%. Contrary to previous studies on aromatase inhibitors reporting plasma oestrogens sustained at 30–50% of their control levels letrozole and anastrozole suppress plasma oestrone sulphate by approximately 95%. Future studies should explore cross-resistance of aromatase inhibitors and steroidal antioestrogens and between different types of aromatase inhibitors. A major goal is to evaluate alterations in tissue oestrogen concentration as well as growth factor expression in response to oestrogen deprivation with aromatase inhibitors.

New endocrine therapies for breast cancer

How do the new endocrine therapies stand up to the aims of modern endocrine therapy outlined in Table 1? We wish to see increased efficacy, decreased toxicity and improved general health in women taking a new agent. None of the new non-steroidal anti-oestrogens have shown unequivocal evidence of improved efficacy in the clinic to mirror their improved profiles over tamoxifen in preclinical studies. We know that toremifene is equivalent to tamoxifen, but we do not have any phase III data from the other four compounds in development. The specific steroidal antioestrogen, ICI 182,780, looks very promising, but is early in its developmental programme. The new aromatase inhibitors are likely to prove equal to tamoxifen or progestagens, but it is disappointing that improved oestrogen suppression has not led, to date, to improved efficacy. No comment can be made about adjuvant or preventative therapy for any of the new agents, although trials are planned for the new aromatase inhibitors in this clinical situation. Currently, the antiprogestins are disappointing and we will need to wait a considerable time for new agents in preclinical testing to reach the clinic. Many of the new agents are associated with decreased toxicity. It is likely that the NSAEs will be equitoxic with tamoxifen. The steroidal antioestrogen looks particularly non-toxic as do the new aromatase inhibitors, and thus we have an advance in terms of reduced toxicity. The effects of the new agents on the uterus, lipids and bone are in the early stages of testing. Raloxifene, ICI 182,780 and the new aromatase inhibitors are expected to have no proliferative effects on the endometrium, but only the new NSAEs are expected to have beneficial cardiovascular and skeletal effects. If the steroidal anti-oestrogens and new aromatase inhibitors become adjuvant therapies of choice, other agents to prevent osteoporosis and cardiovascular events may also have to be administered.

Lens opacification by antioestrogens: tamoxifen vs ICI 182, 780

The antioestrogen, tamoxifen, blocks volume-regulated chloride channels and reduces transparency in bovine lenses maintained in vitro. In contrast to tamoxifen, the steroidal antioestrogen, ICI 182780, did not block volume-regulated chloride currents in three cultured cell lines and required 10 fold higher concentration to induce significant opacification of bovine lenses maintained in vitro. These data suggest that ocular toxic side effects will be minimized by use of the steroidal (ICI 182780) rather than nonsteroidal antioestrogens (tamoxifen).

Interactions between oestradiol and epidermal growth factor in endometrial stromal proliferation and differentiation

The relationship between oestradiol and epidermal growth factor (EGF) in the control of endometrial proliferation and differentiation in cultures of human endometrial stromal cells was investigated. Oestradiol at a concentration of 10 nmol l-1 increased the incorporation of both [3H]thymidine and [3H]leucine but the differences were significantly different from control only for [3H]leucine incorporation. Concentrations of 0.16, 1.6 and 16 nmol EGF l-1 significantly increased both [3H]thymidine (P < 0.01) and [3H]leucine incorporation (P < 0.01). The pure steroidal antioestrogen, ICI 182,780, inhibited any increase in [3H]thymidine and [3H]leucine incorporation stimulated by oestradiol in endometrial stroma. The monoclonal antibody, ICR 16, directed against the EGF receptor did not inhibit the oestradiol action in stromal cells, indicating that, in this model system, oestradiol does not act by inducing synthesis or release of EGF. However, ICI 182,780 potently inhibited the incorporation of [3H]thymidine stimulated by EGF in endometrial stromal cells, suggesting interdependence between oestradiol and EGF in the control of endometrial stromal proliferation. Oestrogen-free conditioned medium from endometrial stromal cultures did not stimulate either [3H]thymidine or [3H]leucine incorporation, suggesting that oestradiol did not stimulate the secretion of a trophic factor from endometrial stromal cells.

Circumvention of doxorubicin resistance in multi-drug resistant human leukaemia and lung cancer cells by the pure antioestrogen ICI 164384

ICI 164384, a new steroidal antioestrogen, entirely devoid of oestrogenic activity, modulates doxorubicin resistance in vitro. At non-cytotoxic concentrations, ICI 164384 potentiated the cytotoxicity of doxorubicin in a dose-dependent manner in both the classical multi-drug resistant (MDR) human leukaemia cell lines CEM/VLB 100 and CEM/VLB 1000 and the human small cell lung cancer cell line H69 LX4. ICI 164384 had no effect on the two respective parental cell lines, CEM/CCRF and H69 P. None of these cell lines expressed the oestrogen receptor. In comparative studies at concentrations ranging from 1.25 to 10 μmol/l, ICI 164384 was significantly more effective (1.2–6-fold) than tamoxifen in reducing the IC 50 of doxorubicin in the CEM/VLB 100 line. In resistant cells, ICI 164384 increased 3H-daunomycin accumulation in a dose-dependent manner and was significantly more effective than tamoxifen at concentrations ranging from 2.5 to 10 μmol/l. ICI 164384 reduced the efflux of daunomycin from resistant cells more effectively than tamoxifen. These studies suggest that ICI 164384 is an effective modulator of MDR.