Non-steroidal Antioestrogen Research Articles

Which antidepressant agent can be used in patients receiving tamoxifen?

IntroductionDepression is frequently observed in carcinology. Many patients with breast cancer, receiving Tamoxifen, may need antidepressants to treat depression. Tamoxifen is a synthetic non-steroidal antioestrogen metabolized by the cytochrome P450 2D6(CYP2D6) to endoxifen which is the active metabolite of this drug. It was reported that the concomitant prescription of Tamoxifen and some antidepressant agents such as paroxetine and fluoxetine may decrease the anticancer effect of tamoxifen as they may inhibit the CYP2D6 pathway.ObjectivesThe objective of this case was to highlight the particularity of management of depression in patients under tamoxifen.MethodsCase report description of a patient treated with Tamoxifen for her breast cancer and who was admitted for major depression, followed by a literature review.ResultsA 36 –year- old woman, had breast cancer and she underwent a mastectomy followed by chemotherapy. Since September 2016, she received 20 mg per day of Tamoxifen as an antihormonal treatment. In November 2018, she was referred to our psychiatry department for depressive symptoms. The patient was sad, she reported social withdrawal, insomnia, anhedonia, and low self-esteem. She had no history of mania or hypomania. A major depressive episode was diagnosed. We prescribed Escitalopram 10 mg per day with clinical improvement. The psychiatric and oncologic status of the patient was stable after two years under tamoxifen and Escitalopram.ConclusionsThe choice of the adequate antidepressant agent in patients under Tamoxifen remains a challenge and requires a thorough knowledge of drug interactions.

Pretreatment with Tamoxifen does not Change Acute Epirubicin Cardiotoxicity in Rats

The use of anthracycline antibiotics as anticancer agents is limited by a cumulative dose-related cardiotoxicity (Wojtacki et al. 2000). Tamoxifen, a synthetic non-steroidal antioestrogen, in addition to its application in treatment and prevention of breast cancer, is also a modulator of multidrug resistance and an antioxidant (Perumal et al. 2005). Combination of epirubicin and tamoxifen has been used in breast cancer (Pico et al. 2004) and lung cancer therapy (Chen et al. 2000). Furthermore, high-dose tamoxifen have been used in phase I trials in patients with advanced malignancies (Perez et al. 2003). Clinical studies have not reported any increased cardiotoxicity with the combination that outweighs beneficial effects (Wils et al. 1999; Fargeot et al. 2004; Pico et al. 2004). Due to its multiple properties, tamoxifen can reverse resistance to anthracyclines (Lucci et al. 1999), but the possibility of increased cardiotoxicity of anthracyclines may represent a clinical problem. In this preliminary study the combination was examined in an isolated rat heart model.

Tamoxifen (ICI46,474) as a targeted therapy to treat and prevent breast cancer

Antihormonal therapy targeted to the oestrogen receptor (OER) is recognized as a significant advance in the treatment and prevention of breast cancer. However, the research method used to achieve the current successes seen in the clinic was not linear but was based on the changing fashions in research and the application of appropriate testing models. The discovery and investigation of nonsteroidal antioestrogens by the pharmaceutical industry during the 1960s was initially an exciting prospect for clinical development. The drugs were superb antifertility agents in laboratory animals, so the prospect of marketing a 'morning after' pill was a high priority. Unfortunately, the reproductive endocrinology of the rat was found to be completely different from that of the human. Antioestrogens, in fact, improved fertility by inducing ovulation in subfertile women so much of the drug development was discontinued. The successful reinvention of ICI46,474 from its origins as a failed contraceptive to a pioneering breast cancer treatment targeted to the OER presaged the development of the current menu of medicines targeted to a range of different survival mechanisms in cancer cells.

Therapeutic effect of tamoxifen and energy-modulating vitamins on carbohydrate-metabolizing enzymes in breast cancer

Cancer cells have an abnormal energetic metabolism. One of the earliest discovered hallmarks of cancer had its roots in bioenergetics, as many tumours were found in the 1920s to exhibit a high glycolytic phenotype. An animal with cancer shows significant and progressive energy loss from the host (i.e. noncancerous) tissues, which could occur by the establishment of a systemic energy-depriving cycle involving the interaction of tumour glycolysis and host gluconeogenesis. Tamoxifen (TAM) is a nonsteroidal antioestrogen that is widely used in adjuvant therapy for all stages of breast carcinoma. To improve the therapeutic efficacy of TAM and to expand its usage in the treatment of breast cancer, it is necessary to establish an energy-enhancing programme. In order to provide sufficient energy and to prevent cancer cachexia, TAM can be supplemented with energy-modulating vitamins (EMV). In this investigation the augmentation of the efficacy of TAM by the effects of EMV supplementation on carbohydrate-metabolizing enzymes, the mitochondrial Krebs cycle and respiratory enzymes was evaluated in the mammary gland of carcinoma-bearing rats. Female albino Sprague-Dawley rats were selected for the investigation. The experimental set-up included one control and four experimental groups. Mammary carcinoma was induced with 7,12- dimethyl benz(a)anthracene (25 mg), and TAM was administered orally (10 mg/kg body weight per day) along with EMV which comprised riboflavin (45 mg/kg per day), niacin (100 mg/kg per day) and coenzyme Q(10) (40 mg/kg per day). Measurements were made on tumour tissue and surrounding normal tissue in all experimental groups. Tumour tissue showed significant (P<0.05) increases in the glycolytic enzymes hexokinase, phosphoglucoisomerase and aldolase, and significant decreases in the gluconeogenic enzymes glucose-6-phosphatase and fructose-1,6-biphosphatase. In contrast, the surrounding tissue showed significant decreases in glycolytic enzymes and significant increases in gluconeogenic enzymes. The activities of the mitochondrial Krebs cycle enzymes isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase, succinate dehydrogenase and malate dehydrogenase, and respiratory chain enzymes NADH dehydrogenase and cytochrome c oxidase were significantly reduced in both tumour and surrounding tissue of the mammary carcinoma-bearing rats. These biochemical disturbances were effectively counteracted by supplementation with EMV, which restored the activities of all these enzyme to their respective control levels. Combination therapy of TAM with EMV not only alters carbohydrate metabolism but can also prevent body weight loss by enhancing the host energy metabolism.

Changing role of the oestrogen receptor in the life and death of breast cancer cells

The oestrogen receptor (ER) has proven to be an extraordinarily successful target for breast cancer treatment and prevention. The clinical use of tamoxifen, a nonsteroidal antioestrogen, demonstrated (1) that the strategic use of adjuvant tamoxifen in ER-positive patients could save lives and (2) that a selective ER modulator (SERM) could reduce the incidence of breast cancer in high-risk women. The ER is now the target for new and safer therapies such as the aromatase inhibitors and the pure antioestrogens that either block oestrogen synthesis or destroy the ER. However, the use of raloxifene, a SERM to prevent osteoporosis with the potential to prevent breast cancer has introduced a new dimension into preventive oncology. The widespread use of endocrine modulators (SERMs, aromatase inhibitors, and pure antioestrogens) raised the question of drug resistance. It is now clear that endocrine resistance can evolve through stages. Once a breast tumour becomes resistant to SERMs, the growth is stimulated by either the SERM or oestrogen. This is why an aromatase inhibitor is effective following SERM resistance and withdrawal. However, the extended use of repeated endocrine therapies now supersensitized the cells to oestrogen that causes apoptosis through the ER. We suggest that future clinical treatment strategies incorporate an ‘oestrogen purge’ to both enhance the actions of chemotherapy or completely reverse endocrine resistance and restore endocrine sensitivity. These new data build on the idea that breast cancer can be controlled as a chronic disease and will permit patients to live long and productive lives during targeted maintenance treatment.

Non‐steroidal anti‐oestrogens inhibit the differentiation of synovial macrophages into dendritic cells

Dendritic cells (DC) have been suggested to play an important role in the pathogenesis of rheumatoid arthritis (RA). Agents that inhibit DC differentiation and function may have a therapeutic value in the treatment of RA. To examine the effect of the non-steroidal anti-oestrogens toremifene and tamoxifen on the differentiation of synovial fluid (SF) macrophages into DC. SF macrophages from patients with RA were cultured with interleukin (IL)-4 and granulocyte/macrophage colony-stimulating factor (GM-CSF) in the presence or absence of anti-oestrogens. The expression of cell surface markers on SF antigen-presenting cells (APC) was studied by flow cytometry. The capacity of SF APC to stimulate allogeneic T cells was studied using the mixed lymphocyte reaction. The production of tumour necrosis factor-alpha, IL-10 and transforming growth factor-beta1 was studied using ELISA. Anti-oestrogens inhibited the differentiation of SF macrophages into DC and the capacity of SF macrophage-derived DC to stimulate allogeneic T cells. By inhibiting the differentiation of SF macrophages into DC, non-steroidal anti-oestrogens may have beneficial effects in RA.

Tamoxifen: a personal retrospective

Tamoxifen, originally described as an anti-oestrogen and antifertility agent in the rat, is now a pioneering medicine for the treatment and prevention of breast cancer. Its success is the result of an effective collaboration between laboratory research and clinical trial processes. However, this drug is more than just an anti-oestrogen to treat breast cancer. Laboratory and clinical research defined the concept of selective oestrogen receptor modulation in the 1980s. Non-steroidal anti-oestrogens show oestrogen-like activity in bones and lower cholesterol, but block oestrogen action in the breast and uterus. This realisation led to the development of chemical cousins, known as selective oestrogen receptor modulators. One of these compounds, raloxifene, is used for the prevention of osteoporosis, but is currently being tested as a preventive for breast cancer.

Radiation Recall: Another Call With Tamoxifen

Tamoxifen, a nonsteroidal antioestrogen, has been used as an adjuvant therapy in patients with oestrogen-receptor positive breast cancer for more than 10 years. Few cutaneous adverse side-effects of the skin are found with this therapy. In this study we present 20 cases of adverse skin effects in relation to tamoxifen during 1979-1997 reported to the Swedish Adverse Drug Reaction Register and 1160 skin side-effects reported to the World Health Organisation's International Collaborative Programme on Drug Monitoring. One new case report of radiation recall in conjunction to tamoxifen, with no sign of reactivation despite 18 months treatment with the tamoxifen analogue toremifene is also discussed in detail. This case illustrates that toremifene can be used as a second-line therapy in patients who have received radiation recall, on tamoxifen.

Inhibition of voltage-gated cationic channels in rat embryonic hypothalamic neurones and C1300 neuroblastoma cells by triphenylethylene antioestrogens

The effect of the non-steroidal antioestrogens tamoxifen and toremifene on voltage-gated cationic currents was examined in primary cultures of rat hypothalamic neurones and the C1300 mouse neuroblastoma cell line using the whole-cell patch clamp technique. When applied to the external bathing solution both tamoxifen and toremifene were able to inhibit TTX-sensitive sodium currents with IC 50 values of 1–2 μM and delayed rectifier type potassium currents (IC 50, 2–3 μM). However, only toremifene showed a significant inhibition of the I A current (IC 50 3 μM). Inhibition of voltage-gated cationic currents was significantly impaired when tamoxifen was applied in a serum-containing solution. The steroidal antioestrogen ICI 182,780 did not inhibit any of the currents at 10 μM.

Antioestrogens enhance tumour necrosis factor receptor 2 (TNF-R2) expression and TNF-R2-mediated proliferation in activated T cells.

In the present study we demonstrate that the non-steroidal antioestrogens toremifene and tamoxifen inhibit mitogen-induced proliferation and up-regulation of tumour necrosis factor (TNF) receptor family molecules on peripheral blood T cells. In activated T cells, however, toremifene and tamoxifen increase the surface expression of tumour necrosis factor receptor 2 (TNF-R2). This up-regulation is functionally important as TNF-R2-mediated proliferation is significantly enhanced in antioestrogen-treated activated T cells. The regulation of TNF-R2 expression in activated T cells seems to involve the c-Jun amino terminal kinase (JNK) pathway, as activation of JNK with anisomycin down-regulates TNF-R2. In activated T cells toremifene clearly inhibits phorbol 12-myristate 13-acetate (PMA)-induced JNK activity, suggesting that the JNK pathway may also be involved in the up-regulation of TNF-R2 expression by antioestrogens. Taken together, the enhancement of TNF-R2 expression and TNF-R2-mediated proliferation in activated T cells represents a novel feature for the effects of antioestrogens. The inhibitory effects of toremifene on the JNK pathway demonstrates that antioestrogens can influence not only cell growth, but also a variety of other cellular responses by inhibiting protein kinase C (PKC).

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.

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).

The antioxidant action of a pure antioestrogen: Ability to inhibit lipid peroxidation compared to tamoxifen and 17β-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 > 17β-oestradiol > nafoxidine ⩾ 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 > 17β-oestradiol > ICI 164,384 > tamoxifen ⩾ 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 β-oestradiol and is discussed in relation to its anticancer action.

Comparative affinity of steroidal and non-steroidal antioestrogens, cholesterol derivatives and compounds with a dialkylamino side chain for the rat liver antioestrogen binding site

Steroidal and non-steroidal antioestrogens, steroidal compounds with (disubstituted) dialkyl amino side chain, cholesterol derivatives, histaminic and (anti)-progestational compounds were tested for their ability to compete with [ 3H]tamoxifen for the specific antioestrogen binding site (AEBS) in the post-mitochondrial fraction of rat liver homogenates. Relative binding affinity was highest for compounds with diethylamino or pyrrolidino ethoxy side chains. Affinity decreased with shortening of this side chain. No connection could be established between the carbon backbone of the compound and affinity, except for the presence of (sometimes aromatic) ring structures. Steroidal ring structures do not seem to be necessary for binding. the cholesterol derivatives, tested in our laboratory, showed very little affinity for the rat liver AEBS. Histamine, melatonin and the (anti)-progestational compounds showed no affinity for the AEBS; and we therefore conclude that the AEBS is not identical to receptors for these compounds. Results of these experiments could be useful in other investigations on the development of resistance of breast cancer to antioestrogens.

Pharmacological characterization of a novel oestrogen antagonist, ZK 119010, in rats and mice

The oestrogenic and antioestrogenic effects of a new nonsteroidal compound ZK 119010 (2-(4-hydroxyphenyl)-3-methyl-1-[6-(1-pyrrolidinyl)-hexyl]-indol-5-ol) were evaluated and compared with those of tamoxifen and ICI 164384. In immature mice, ZK 119010 administered once daily for 3 days (s.c.) inhibited the uterotrophic and vaginotrophic effect of oestradiol in a dose-dependent manner and was distinctly more potent than tamoxifen or ICI 164384 in exerting antioestrogenic effects. When antioestrogens in combination with oestradiol were administered once daily for 5 days to ovariectomized adult rats, ZK 119010 at lower doses (less than or equal to 1 mg/kg) was slightly less effective than tamoxifen in inhibiting the uterotrophic effect of oestradiol. At the higher doses, however, ZK 119010 was strongly antioestrogenic, and ICI 164384 was less effective than ZK 119010 or tamoxifen. ZK 119010 at 10 mg/kg, like ICI 164384 at 30 mg/kg, caused an almost complete inhibition of the oestradiol-induced uterine growth in rats. The antioestrogenic effect of tamoxifen in rats was also limited by its inherent oestrogenic property. The oestrogenic activity of ZK 119010 was much below that of tamoxifen, whereas ICI 164384 did not show oestrogenicity. The present results indicate that ZK 119010 is a novel type of non-steroidal antioestrogen which has only a marginal oestrogenic effect in rats and mice. Such an antioestrogen may be useful for the treatment of oestrogen-sensitive diseases in man.

Glucocorticoids suppress and oestrogens enhance the lipopolysaccharide-induced increase in putrescine and N1-acetylspermidine in mouse liver

Previously we reported that administration of lipopolysaccharide (LPS) to mice increased the hepatic levels of putrescine (PUT) and N 1-acetylspermidine ( N 1-acetyl-SPD). In the current study, we examined the in vivo effects of some steroid hormones on the LPS-induced increase in PUT and N 1-acetyl-SPD. Corticosterone, hydrocortisone and dexamethasone suppressed the LPS-induced increase in PUT and N 1-acetyl-SPD in mouse liver in a dose-dependent manner, dexamethasone being the most effective among them. On the other hand, oesterone and oestradiol-17β enhanced the LPS-induced increase in PUT and N 1-acetyl-SPD in a dose-dependent manner. Oestradiol-17α and 16β-ethyl-oestradiol, as an inactive oestradiol isomer and an antioestrogen, respectively, likewise enhanced the increase in PUT and N 1-acetyl-SPD concentrations induced by LPS. 16α-hydroxy-oestradiol (oestriol), 16α-hydroxyestrone, 2-hydroxyoestradiol, 2-hydroxyoestrone, progesterone, testosterone, diethylstilboestrol and nonsteroidal antioestrogen such as tamoxifen and nafoxidine had no effect on the increase. Oestradiol-17β enhanced and corticosterone had little on the carbon tetrachloride-induced increase in PUT and N 1-acetyl-SPD. These results suggest that glucocorticoids suppress the increase by preventing the immunological injury by Kupffer cells on hepatocytes and that the stimulatory effect of oestrogens may not be associated with their oestrogenic activities mediated by the oestrogen receptor system.

Tamoxifen. A reappraisal of its pharmacodynamic and pharmacokinetic properties, and therapeutic use.

Tamoxifen, a non-steroidal antioestrogen, represents a significant advance in treatment of female breast cancer. In trials of tamoxifen as postsurgical adjuvant treatment of early breast cancer, disease-free survival is consistently prolonged, representing an enhanced quality of life in association with tamoxifen's favourable adverse effect profile. Moreover, overview analysis indicates a survival benefit of approximately 20% at 5 years for all women, most clearly evident in women over 50 years, while a survival benefit independent of menopausal, nodal or oestrogen receptor status has been demonstrated in some individual trials. Thus, for postmenopausal women, tamoxifen is clearly optimal adjuvant treatment, although the relative benefit of adjuvant chemotherapy in node-negative patients requires clarification. A survival benefit for women under 50 has not been clearly demonstrated in overview analysis, but is not precluded by these rather limited data, and adjuvant treatment of premenopausal women with tamoxifen may also warrant serious consideration. Response rates to tamoxifen in advanced breast cancer are around 30 to 35%, increasing with patient selection for oestrogen receptor positivity. Tamoxifen must be regarded as first-line endocrine treatment in postmenopausal women, and may represent an alternative to first-line ovarian ablation in premenopausal women. An emergent role in primary therapy of elderly and frail patients with operable disease is apparent. Tamoxifen is also of benefit following surgery in male breast cancer, and may have a role as first-line endocrine treatment. Tamoxifen also has a potential role in other hormone-sensitive malignancies such as pancreatic carcinoma, and in treatment of benign breast disease. Finally, tamoxifen has a place in treatment of male and female infertility. because of adverse effects is rarely necessary. The most frequent adverse effects are related to the drug's anti-oestrogenic activity, and include hot flushes, nausea and/or vomiting, vaginal bleeding or discharge, and menstrual disturbances in premenopausal patients. Thus, tamoxifen continues to play a major role in management of female breast cancer in both early and advanced stages of disease, with a place also in treatment of male breast cancer and of infertility.

Optimisation of antioestrogen therapy: laboratory and clinical concepts

SynopsisTamoxifen, a non-steroidal antioestrogen, is the first line endocrine therapy for breast cancer. Laboratory studies during the past decade have provided valuable clues to understand the mode of action of tamoxifen so that the drug can be used to its best advantage in the clinic. Animal models (carcinogen-induced rat mammary carcinoma models, spontaneous mouse mammary tumours, and athymic mice inoculated with hormone-dependent human breast cancer cell lines) all demonstrate that tamoxifen is a tumouristatic agent and long-term or indefinite therapy is required to prevent the appearance of tumours. However, when treatment is stopped, tumours appear spontaneously or can be encouraged to re-appear with oestrogen therapy. These data clearly support the view that adjuvant clinical trials with tamoxifen should employ an indefinite treatment policy or at least until the time of treatment failure. Several clinical trials organisations in Great Britain (CRC and Scottish trial) have recruited a significant number of premenopausal patients. However, unlike postmenopausal patients, long-term tamoxifen therapy of premenopausal women often causes an increase in circulating oestrogen. Since tamoxifen has been shown to be a competitive inhibitor of oestrogen action, strategies to reduce ovarian steroidogenesis (oophorectomy or Zodalex®) may provide an optimal environment to sustain the long-term effectiveness of adjuvant tamoxifen therapy.

Interaction of unsaturated fatty acids with anti-oestrogen-binding sites.

Specific high-affinity binding sites for non-steroidal anti-oestrogens such as tamoxifen have been identified in many animal and human tissues. The function of these binding sites and the nature of their endogenous ligands are currently unknown. Our laboratory has previously reported that unsaturated fatty acids at micromolar concentrations inhibited [3H]tamoxifen binding to the anti-oestrogen-binding sites in rat liver, raising the possibility that fatty acids might represent endogenous ligands for these sites. These studies have now been extended to examine the mechanism by which fatty acids inhibit [3H]tamoxifen binding to the anti-oestrogen-binding site. Saturation analysis revealed that increasing concentrations of oleic acid progressively decreased the apparent binding affinity of these sites for [3H]tamoxifen without decreasing the total number of binding sites; however, the apparent dissociation constant did not vary linearly with the prevailing oleic acid concentration, suggesting that the inhibition of [3H]tamoxifen binding by fatty acid was not competitive in nature. Kinetic studies of [3H]tamoxifen binding showed that oleic acid did not affect the rate of association, but increased the rate of dissociation of [3H]tamoxifen from the anti-oestrogen-binding site; the latter finding would not be expected if oleic acid acted as a competitive inhibitor. Furthermore, incubation of a rat microsomal fraction with [3H]oleic acid in the absence and presence of excess non-radioactively labelled tamoxifen also failed to demonstrate direct competition between oleic acid and tamoxifen for the same binding site. It is concluded that oleic acid, and presumably other unsaturated fatty acids, do not compete for the anti-oestrogen-binding site and probably reduce its tamoxifen-binding affinity by some other mechanism, such as perturbation of the lipid environment of the binding site. The biological significance of this interaction of unsaturated fatty acids with the anti-oestrogen-binding site remains to be elucidated.