A study on the effect of a single dose of tamoxifen on uterine hyperaemia and growth in the rat

Endometrial Surveillance in Tamoxifen and Letrozole Treated Breast Cancer Patients

Background: Our study aimed to assess the risk of endometrial pathologies after tamoxifen and aromatase inhibitors (AIs) adjuvant treatment for female breast cancer patients treated at Sultan Qaboos University Hospital in Oman.

Materials and Methods: A total of 457 patients diagnosed with estrogen positive breast cancer between January 2011 and December 2018 were screened. Two hundred and four patients met the inclusion criteria, and their detailed clinicopathological and endometrial surveillance data were collected from their electronic health records.

Results: All patients underwent endometrial assessment during tamoxifen or letrozole therapy. The mean diagnostic age of breast cancer patients is 43.6 years, ranging from 27-84 years. Eighty-three percent of those patients are premenopausal, and 17% are postmenopausal. The mean tamoxifen use duration was 33 months. The majority of patients, 123 (60.3%), have had tamoxifen for three years or less, 47 (23.1%) for 3-5 years, and only 22 (10.8%) were on tamoxifen for more than five years. Increased endometrial thickness was reported in 8% of the premenopausal and 14% of the postmenopausal group. Other endometrial pathologies that were detected are inactive endometrium three (1.47%), atrophic endometrium three (1.47%), serous carcinoma one (0.50%), endometrial cancer two (0.98%), and chronic endometritis one (0.50%), which were not significantly associated with tamoxifen or letrozole therapy duration. Two patients have developed endometrial cancer, and both are postmenopausal and > 60 years old.

Conclusions: Tamoxifen and letrozole did not increase the risk of endometrial cancer in premenopausal patients. Breast Cancer (BC) patients on tamoxifen or letrozole might need a pre-treatment endometrial evaluation and explanation of alarming symptoms to guide further endometrial surveillance.

Meta-analysis of the cumulative risk of endometrial malignancy and systematic review of endometrial surveillance in extended tamoxifen therapy

Background

Optimal management of the endometrium in patients with oestrogen receptor-positive breast cancer taking extended tamoxifen therapy (for 10 years) remains uncertain. A meta-analysis was performed to determine the cumulative risk ratio (RR) for endometrial malignancy following extended compared with standard tamoxifen treatment. A systematic review was undertaken to identify whether routine endometrial surveillance in patients receiving tamoxifen is associated with earlier detection and reduced incidence of endometrial malignancy.

Methods

Two independent searches were undertaken in the Cochrane Library, PubMed and MEDLINE. A meta-analysis was performed of RCTs reporting on endometrial malignancy risk in extended tamoxifen therapy. A systematic review included prospective studies investigating the benefit of endometrial surveillance during tamoxifen therapy.

Results

Four RCTs reported on endometrial risk in extended tamoxifen therapy. The cumulative risk of endometrial malignancy increased twofold from 1·5 to 3·2 per cent with extended therapy compared with the standard 5 years of tamoxifen (RR 2·29, 95 per cent c.i. 1·60 to 3·28; P < 0·001). Four studies analysed the value of endometrial screening in 5-year cohorts. Endometrial cancer rates of up to 2 per cent were reported, which is higher than rates in the large extended tamoxifen trials.

Conclusion

Extended adjuvant tamoxifen is associated with an increase in endometrial cancer. No clear benefit has been shown for routine endometrial surveillance in asymptomatic patients on tamoxifen therapy.

Ethylbromide tamoxifen, a membrane-impermeant antiestrogen, activates smooth muscle calcium-activated large-conductance potassium channels from the extracellular side

Smooth-muscle calcium-activated large-conductance potassium channels (BK channels) are activated by tamoxifen and 17-beta-estradiol. This increase in NP(o), the number of channels, N, multiplied by open probability, depends on the presence of the regulatory beta1-subunit. Furthermore, a previous study indicated that 17-beta-estradiol might bind an extracellular site on the beta1-subunit. Because tamoxifen and 17-beta-estradiol may share a common binding site, we hypothesized that tamoxifen activates BK channels through a site on the extracellular surface of the membrane. A membrane-impermeant analog of tamoxifen, ethylbromide tamoxifen, was synthesized and used to test this hypothesis in whole-cell, outside-out, cell-attached, and inside-out patches from canine colonic smooth muscle cells. Ethylbromide tamoxifen is positively charged and is therefore membrane-impermeant. In whole-cell experiments, ethylbromide tamoxifen increased K(+) current at potentials positive to +40 mV, which has previously been attributed to BK channels. Unlike tamoxifen, ethylbromide tamoxifen did not inhibit delayed rectifier current. In outside-out patches, ethylbromide tamoxifen increased BK channel NP(o) with an EC(50) value of 1 microM. Ethylbromide tamoxifen did not increase BK channel NP(o) in cell-attached or inside-out patches; however, subsequent addition of equimolar tamoxifen did. Both drugs diminished BK channel unitary conductance to a degree that paralleled the effect on NP(o), suggesting an additional interaction with the pore-forming alpha-subunit. An interaction of tamoxifen with the pore was supported by a right shift in the concentration-response curve for tetraethylammonium; similar results were evident with iberiotoxin and charybdotoxin block. Our data suggest that ethylbromide tamoxifen does not easily traverse the plasma membrane and that tamoxifen binding responsible for activation of BK channels is at an extracellular site. The tamoxifen binding site may be within the extracellular loop of the BK channel beta1-subunit or, alternatively, on an as-yet-unidentified mediator that has an extracellular binding site.

(Xeno)estrogen sensitivity of smooth muscle BK channels conferred by the regulatory beta1 subunit: a study of beta1 knockout mice

Estrogen and xenoestrogens (i.e. agents that are not steroids but possess estrogenic activity) increase the open probability (P(o)) of large conductance Ca(2+)-activated K(+) (BK) channels in smooth muscle. The mechanism of action may involve the regulatory beta1 subunit. We used beta1 subunit knockout (beta1-/-) mice to test the hypothesis that the regulatory beta1 subunit is essential for the activation of BK channels by tamoxifen, 4-OH tamoxifen (a major biologically active metabolite), and 17beta-estradiol in native myocytes. Patch clamp recordings demonstrate BK channels from beta1-/- mice were similar to wild type with the exception of markedly reduced Ca(2+)/voltage sensitivity and faster activation kinetics. In wild type myocytes, (xeno)estrogens increased NP(o) (P(o) x the number of channels, N), shifted the voltage of half-activation (V(12)) to more negative potentials, and decreased unitary conductance. These effects were non-genomic and direct, because they were rapid, reversible, and observed in cell-free patches. None of the (xeno)estrogens increased the NP(o) of BK channels from beta1-/- mice, but all three agents decreased single channel conductance. Thus, (xeno)estrogens increase BK NP(o) through a mechanism involving the beta1 subunit. The decrease in conductance did not require the beta1 subunit and probably reflects an interaction with the pore-forming alpha subunit. We demonstrate regulation of smooth muscle BK channels by physiological (steroid hormones) and pharmacological (chemotherapeutic) agents and reveal the critical role of the beta1 subunit in these responses in native myocytes.

Tamoxifen activates smooth muscle BK channels through the regulatory beta 1 subunit

Estrogen (17beta-estradiol; 17betaE) and xenoestrogens, estrogenic compounds that are not steroid hormones, have non-genomic actions at plasma membrane receptors unrelated to the nuclear estrogen receptor. The open probability (P(o)) of large conductance Ca(2+)/voltage-sensitive k(+)(BK) channels is increased by 17betaE through the regulatory beta1 subunit. The pharmacological nature of the putative membrane binding site is unclear. We probed the site by determining whether tamoxifen ((Z)-1-(p-dimethylaminoethoxy-phenyl)-1,2-diphenyl-1-butene; Tx), a chemotherapeutic xenoestrogen, increased P(o) in clinically relevant concentrations (0.1-10 microm). In whole cell patch clamp recordings on canine colonic myocytes, which express the beta1 subunit, Tx activated charybdotoxin-sensitive K(+) current. In single channel experiments, Tx increased the NP(o) (P(o) x number channels; N) and decreased the unitary conductance (gamma) of BK channels. Tx increased NP(o) (EC(50) = 0.65 microm) in excised membrane patches independent of Ca(2+) changes. The Tx mechanism of action requires the beta1 subunit, as Tx increased the NP(o) of Slo alpha expressed in human embryonic kidney cells only in the presence of the beta1 subunit. Tx decreased gamma of the alpha subunit expressed alone, without effect on NP(o). Our data indicate that Tx increases BK channel activity in therapeutic concentrations and reveal novel pharmacological properties attributable to the alpha and beta1 subunits. These data shed light on BK channel structure and function, non-genomic mechanisms of regulation, and physiologically and therapeutically relevant effects of xenoestrogens.

Tamoxifen increases plasma estrogen-binding equivalents and has an estradiol agonistic effect on histologically normal premenopausal and postmenopausal endometrium

OBJECTIVE

To determine the effects of single dose tamoxifen on plasma estrogen (E)-binding equivalents and endometrial estradiol (E2) and progesterone (P) receptors.

DESIGN

Controlled clinical study.

SETTING

Normal human volunteers were studied in an academic research environment.

PATIENTS

Premenopausal and postmenopausal women with histologically normal endometrium undergoing curettage or hysterectomy were selected.

INTERVENTIONS

Tamoxifen was administered orally; blood and endometrial samples were collected 4 to 96 hours after tamoxifen administration.

MAIN OUTCOME MEASURES

Plasma E-binding equivalents, endometrial cytosolic and nuclear E2 and P receptors.

RESULTS

(1) Plasma E-binding equivalents increased eightfold at 4 to 24 hours of tamoxifen administration and declined exponentially thereafter, reaching control levels at 73 to 96 hours. Plasma E-binding equivalents were not affected by endogenous E2 levels. (2) Endometrial total E2 and P receptor levels increased in all women 2.9 to 19.2-fold after tamoxifen. (3) Tamoxifen resulted in an increase in the fraction of the E2 receptor measured in the nuclear extract 2.1 to 7.5-fold in midcycle, secretory, and menopausal endometria but not in proliferative endometrium.

CONCLUSIONS

(1) Tamoxifen has an E2 agonistic effect on histologically normal human endometrium. (2) Irrespective of the total level of the endometrial E2 receptor, the nuclear capacity of that receptor in vivo is limited (approximately 75% to 80% of the total level).

The effects of thromboxane receptor antagonists on oestrogen-induced uterotrophic responses in the spontaneously hypertensive rat

1. The possible role of thromboxane in the uterotrophic response to oestrogen, in the spontaneously hypertensive rat was investigated by use of the thromboxane receptor antagonists EP092, AH23848 and BM 13.505. 2. The parameters studied were uterine blood flow (measured by the microsphere technique), uterine wet and dry weights and the concentrations of cytosolic and nuclear oestrogen receptors. 3. The antagonists attenuated oestradiol-induced uterine blood flow and significantly reduced both wet and dry uterine weight. These changes were accompanied by decreases in nuclear oestrogen receptor levels. 4. The results suggest a supportive role for thromboxane in oestradiol-induced uterine growth.

Modification of the rat uterine response to oestrogen and tamoxifen by thromboxane antagonists

1. The thromboxane receptor antagonists EP092, AH23848 and BM 13.505 were used to investigate the role of thromboxane in the uterotrophic response to oestradiol and tamoxifen. 2. The parameters examined were uterine blood flow (measured by the microsphere technique), uterine wet and dry weights and the concentrations of cytosolic and nuclear oestrogen receptors. 3. Only EP092 potentiated the hyperaemic response to oestrogen but all three thromboxane antagonists inhibited oestradiol-stimulated uterine growth. This inhibition was accompanied by a decrease in nuclear oestrogen receptor concentration. 4. The uterotrophic response to tamoxifen was unaffected by the thromboxane antagonists. 5. The mechanism by which the thromboxane antagonists may be exerting their growth inhibitory effect is discussed, although, whether this effect can be attributed to blockade of thromboxane receptors or to some other mechanism is not clear from this study.

The effect of a single dose of oestradiol on tamoxifen-induced uterine hyperaemia and growth in the rat

1. The effect of oestradiol on tamoxifen - (single doses of each agent) induced responses in the uterus of the mature ovariectomized rat was investigated. The parameters examined were uterine blood flow (measured by the microsphere technique), weight and oestrogen receptor concentrations. 2. Initially the addition of oestradiol to the tamoxifen regimen produced a reduction in uterine blood flow and dry weight which was reflected by a reduction in nuclear oestrogen receptor. However, as the tamoxifen response became more established from 48 h onwards the addition of oestradiol produced an additive effect. These changes were not mirrored by increases in nuclear oestrogen receptor. 3. The results suggest that tamoxifen and oestradiol may, in part, act via different mechanisms; the role of possible mediatory systems such as histamine release and eosinophil migration is discussed. Heterogeneous control systems may also be involved in the early and late uterine response.