Cellular effects of tamoxifen in primary breast cancer

Modulation of epidermal growth factor receptor in endocrine-resistant, estrogen-receptor-positive breast cancer

An increasing body of evidence demonstrates that growth factor networks are highly interactive with estrogen receptor signaling in the control of breast cancer growth. As such, tumor responses to antihormones are likely to be a composite of the estrogen receptor and growth factor inhibitory activity of these agents. The modulation of growth factor networks during endocrine response is examined, and in vitro and clinical evidence is presented that epidermal growth factor receptor signaling, maintained in either an estrogen receptor-dependent or a receptor-independent manner, is critical to antihormone-resistant breast cancer cell growth. The considerable potential of the epidermal growth factor receptor-selective tyrosine kinase inhibitor Iressa (ZD 1839) to efficiently treat, and perhaps even prevent, endocrine-resistant breast cancer is highlighted.

Change in expression of ER, bcl-2 and MIB1 on primary tamoxifen and relation to response in ER positive breast cancer

Pre-treatment oestrogen receptor (ER) expression in breast cancer predicts for rate of response to endocrine therapy but not for the quality or duration of response (DofR). ER is known to be down-regulated by anti-oestrogens. This study has tested the hypothesis that the degree of down-regulation of ER and the ER-regulated marker bcl-2 are associated with the quality and duration of tamoxifen response. 80 patients with ER+ve breast cancer (H-score > 10) receiving primary tamoxifen (n = 51 Stage I-II elderly; n = 29 Stage III) underwent sequential tumour biopsies for immunocytochemical assessment of ER, bcl-2 and the proliferation marker MIB1. Median follow-up is 45 months. By 6-months on therapy three patients had attained complete response (CR), 27 partial response (PR); 44 static disease (SD) and six progression (PD) by UICC criteria. Greater decrease in ER and bcl-2 H-score from pre-treatment to 6 weeks (p = 0.035, p = 0.037) and ER and bcl-2 H-score from pre-treatment to 6 months (p = 0.058, p = 0.036) were significantly associated with better quality of response (CR/PR vs SD/PD). Greater 6-week and 6-month reduction in bcl-2 H-score (p = 0.041, p = 0.036) and 6-week reduction in MIB1 (p = 0.013) were significantly correlated with longer DofR. This study demonstrates that greater down-regulation of ER and the ER-regulated protein bcl-2 on primary tamoxifen are significantly associated with a better quality of response and bcl-2 and the proliferation marker MIB1 a longer duration of response in ER+ve breast cancer.

The role of estrogen receptor mutations in tamoxifen-stimulated breast cancer

During the past 20 years, the hormonal therapy of choice for the treatment of breast cancer has been the antiestrogen, tamoxifen. The use of tamoxifen has been proved to produce a favorable response and survival advantage in patients whose tumors are classified as estrogen receptor-positive (ER+)/progesterone receptor-positive (PR+). Additionally, tamoxifen is the only drug known to reduce the incidence of contralateral disease. This drug produces relatively few harmful side effects, while exhibiting several beneficial effects such as maintaining bone density and reducing the incidence of myocardial infarction in the postmenopausal woman. However, tumors eventually acquire a tamoxifen-resistant or tamoxifen-stimulated phenotype, resulting in disease recurrence. Several mechanisms have been proposed to account for tamoxifen-resistant breast cancer, in the hope of developing a more effective first-line or perhaps second-line treatment strategy. One popular theory is the occurrence of a mutation in the estrogen receptor, the drug target. A plethora of studies have reported the detection of estrogen receptor mRNA splice variants, and it has been suggested that the accumulation of these variant mRNAs are responsible for the development of tamoxifen-resistant breast cancer. In this review, several questions will be posed to address the suitability of both laboratory and clinical evidence to support this hypothesis. Although there is adequate data generated in the laboratory, there is, as yet, no compelling evidence to suggest that mutation of the estrogen receptor is the molecular mechanism producing tamoxifen-stimulated growth in human breast and endometrial cancer.

Pharmacokinetics, pharmacological and anti-tumour effects of the specific anti-oestrogen ICI 182780 in women with advanced breast cancer

We have assessed the pharmacokinetics, pharmacological and anti-tumour effects of the specific steroidal anti-oestrogen ICI 182780 in 19 patients with advanced breast cancer resistant to tamoxifen. The agent was administered as a monthly depot intramuscular injection. Peak levels of ICI 182780 occurred a median of 8-9 days after dosing and then declined but were above the projected therapeutic threshold at day 28. Cmax during the first month was 10.5 ng/ml-1 and during the sixth month was 12.6 ng ml-1. The AUCs were 140.5 and 206.8 ng day ml-1 on the first and sixth month of dosing respectively, suggesting some drug accumulation. Luteinising hormone (LH) and follicle-stimulating hormone (FSH) levels rose after withdrawal of tamoxifen and then plateaued, suggesting no effect of ICI 182780 on the pituitary-hypothalamic axis. There were no significant changes in serum levels of prolactin, sex hormone-binding globulin (SHBG) or lipids. Side-effects were infrequent. Hot-flushes and sweats were not induced and there was no apparent effect of treatment upon the endometrium or vagina. Thirteen (69%) patients responded (seven had partial responses and six showed "no change' responses) to ICI 182780, after progression on tamoxifen, for a median duration of 25 months. Thus ICI 182780, given by monthly depot injection, and at the drug levels described, is an active second-line anti-oestrogen without apparent negative effects on the liver, brain or genital tract and warrants further evaluation in patients with advanced breast cancer.

Oestrogen receptor: a stable phenotype in breast cancer

Oestrogen receptor (ER) expression in breast cancer is regarded as a phenotype that may change during the natural history of the disease or during endocrine therapy. It has been suggested that in up to 70% of tumours that show acquired resistance the mechanism may be changed in ER status from positive to negative. This paper proposes an alternative hypothesis that ER expression in a stable phenotype in breast cancer. The paper reviews the literature on ER expression during the natural history of breast cancer in patients and also presents data on the effect of endocrine therapy on ER expression. If the alternative hypothesis is true it has important implications for treatment from chemoprevention to acquired endocrine resistance in advanced disease. Equally, if the hypothesis is true, attempts to develop laboratory models of endocrine resistance where ER-positive tumours become ER negative need to be re-evaluated.

The antioxidant action of a pure antioestrogen: ability to inhibit lipid peroxidation compared to tamoxifen and 17 beta-oestradiol and relevance to its anticancer potential

The pure antioestrogen ICI 164,384, and nafoxidine (structurally related to tamoxifen) were good inhibitors of iron ion-dependent lipid peroxidation. In rat liver microsomes incubated with Fe(III)-ascorbate the overall order of effectiveness of the compounds tested as inhibitors of lipid peroxidation was 4-hydroxytamoxifen > 17 beta-oestradiol > nafoxidine > or = tamoxifen > ICI 164,384. When the microsomes were incubated with Fe(III)-ADP/NADPH, a similar order of effectiveness was observed. In ox-brain phospholipid liposomes incubated with Fe(III)-ascorbate the order was 4-hydroxytamoxifen > 17 beta-oestradiol > ICI 164,384 > tamoxifen > or = nafoxidine. The antioxidant ability of ICI 164,384, a steroidal oestrogen antagonist, is compared to that of tamoxifen (a non-steroidal antioestrogen and partial oestrogen agonist) and 17 beta-oestradiol and is discussed in relation to its anticancer action.

Carcinogenic antioxidants. Diethylstilboestrol, hexoestrol and 17 alpha-ethynyloestradiol

The synthetic oestrogens diethylstilboestrol, hexoestrol and 17 alpha-ethynyloestradiol are known to be carcinogenic, yet they all exert antioxidant properties in vitro in that they are good inhibitors of iron ion-dependent lipid peroxidation. In rat liver microsomes incubated with Fe(III)-ascorbate or Fe(III)-ADP/NADPH and also in ox-brain phospholipid liposomes incubated with Fe(III)-ascorbate; the overall order of effectiveness of the compounds tested as inhibitors of lipid peroxidation was diethylstilboestrol > hexoestrol > 17 alpha-ethynyloestradiol > 4-hydroxytamoxifen > 17 beta-oestradiol > tamoxifen. Compounds acting as antioxidants towards lipids may also exert pro-oxidant effects towards other molecules such as DNA and thus must never be assumed to be safe for human use.

Tamoxifen and related compounds decrease membrane fluidity in liposomes. Mechanism for the antioxidant action of tamoxifen and relevance to its anticancer and cardioprotective actions?

Tamoxifen and related compounds decrease membrane fluidity in ox-brain phospholipid liposomes: their order of effectiveness is, 4-hydroxytamoxifen > 17 beta-oestradiol > tamoxifen > cis-tamoxifen > N-desmethyltamoxifen > cholesterol. A good positive correlation was demonstrated between the decrease in membrane fluidity by these compounds and their antioxidant ability as inhibitors of liposomal and microsomal lipid peroxidation (correlation coefficient, r = 0.99, P < 0.001, in both cases). The ability of tamoxifen to decrease membrane fluidity is suggested to be the mechanism of its antioxidant action and is discussed in relation to its anticancer and cardioprotective actions.

Vitamin D is a membrane antioxidant. Ability to inhibit iron-dependent lipid peroxidation in liposomes compared to cholesterol, ergosterol and tamoxifen and relevance to anticancer action

Vitamin D is a membrane antioxidant: thus Vitamin D3 (cholecalciferol) and its active metabolite 1,25-dihydroxycholecalciferol and also Vitamin D2 (ergocalciferol) and 7-dehydrocholesterol (pro-Vitamin D3) all inhibited iron-dependent liposomal lipid peroxidation. Cholecalciferol, 1,25-dihydroxycholecalciferol and ergocalciferol were all of similar effectiveness as inhibitors of lipid peroxidation but were less effective than 7-dehydrocholesterol; this was a better inhibitor of lipid peroxidation than cholesterol, though not ergosterol. The structural basis for the antioxidant ability of these Vitamin D compounds is considered in terms of their molecular relationship to cholesterol and ergosterol. Furthermore, the antioxidant ability of Vitamin D is compared to that of the anticancer drug tamoxifen and its 4-hydroxy metabolite (structural mimics of cholesterol) and discussed in relation to the anticancer action of this vitamin.

Enhancement by tamoxifen of the membrane antioxidant action of the yeast membrane sterol ergosterol: relevance to the antiyeast and anticancer action of tamoxifen

The anticancer drug tamoxifen inhibits lipid peroxidation in ox-brain phospholipid liposomes, and is a good antiyeast agent, with clinical potential. We now report that the ergosterol-containing lipid fraction derived from yeast microsomal membranes (and the ergosterol separated from it) inhibited lipid peroxidation when introduced into ox-brain phospholipid liposomes. Inhibition of lipid peroxidation by the lipid fraction was greatly enhanced when yeast cell growth was inhibited with tamoxifen prior to lipid extraction. The ability of tamoxifen to enhance the membrane antioxidant ability of ergosterol is expressed in terms of a tamoxifen enhancement coefficient. Enhancement by tamoxifen of the membrane antioxidant action of ergosterol is discussed in relation to the antifungal and anticancer actions of tamoxifen.

Protective actions of tamoxifen and 4-hydroxytamoxifen against oxidative damage to human low-density lipoproteins: a mechanism accounting for the cardioprotective action of tamoxifen?

Tamoxifen and 4-hydroxytamoxifen protect isolated human low-density lipoproteins (LDLs) against copper-ion-dependent lipid peroxidation: 4-hydroxytamoxifen is more protective than tamoxifen or 17 beta-oestradiol. 4-Hydroxytamoxifen and 17 beta-oestradiol also prevent the increase in the electrophoretic mobility of LDL caused by exposure to copper ions, presumably by protection of the apoprotein B of LDL against oxidative modification. Our observations may help to account for the cardioprotective benefits reported to be associated with tamoxifen therapy and prophylaxis in breast cancer.

Tamoxifen inhibits lipid peroxidation in cardiac microsomes. Comparison with liver microsomes and potential relevance to the cardiovascular benefits associated with cancer prevention and treatment by tamoxifen

Tamoxifen and 4-hydroxytamoxifen were both good inhibitors of iron-dependent lipid peroxidation in rat cardiac microsomes. Tamoxifen was also a good inhibitor of lipid peroxidation in liposomes prepared from the phospholipid obtained from rat liver microsomes. In a modified rat liver microsomal system containing a sufficiently low amount of peroxidizable phospholipid to make it comparable with the rat cardiac microsomal system, tamoxifen and 4-hydroxytamoxifen were of similar effectiveness as in the cardiac system. Tamoxifen is known to lower serum cholesterol levels, and the findings reported here indicate that the drug might also protect heart cell membranes against peroxidative damage. Potential cardioprotective and antiatherosclerotic benefits of tamoxifen are discussed in relation to the drug's use in cancer prevention and treatment.