Differential estrogen and antiestrogen responsiveness of the uterus during development in the fetal, neonatal and immature guinea pig

Tamoxifen and pregnancy: an absolute contraindication?

PURPOSE

Breast cancer is the most common malignancy among young women of reproductive age. Adjuvant treatment with tamoxifen reduces the risk of recurrence in hormone-sensitive breast cancer. However, the use of tamoxifen is considered contraindicated during pregnancy, because of a limited number of case reports demonstrating potential adverse effects on the fetus. The objective of this report is to give a more broad overview of the available data on the effect of tamoxifen exposure during pregnancy.

METHODS

A literature review was performed using PubMed and the databases of the Netherlands Pharmacovigilance Centre Lareb and of the International Network on Cancer, Infertility, and Pregnancy.

RESULTS

A total of 238 cases of tamoxifen use during pregnancy were found. Of the 167 pregnancies with known outcome, 21 were complicated by an abnormal fetal development. The malformations described were non-specific and the majority of cases concerned healthy infants despite exposure to tamoxifen.

CONCLUSION

There seems to be an increased risk of fetal abnormalities when taking tamoxifen during pregnancy (12.6% in contrast to 3.9% in the general population), but the evidence is limited and no causal relationship could be established. The possible disadvantage of postponing or discontinuing tamoxifen for the maternal prognosis is unclear. Patients should be counseled about the use of tamoxifen during pregnancy instead of presenting it as being absolutely contraindicated.

Transplacental estrogen responses in the fetal rat: increased uterine weight and ornithine decarboxylase activity

The synthetic estrogens, diethylstilbestrol (DES) and ethynylestradiol (EE2), are more potent than 17 beta-estradiol (E2) in inducing uterine weight gain in the neonatal rat, due to the binding of E2 to serum alpha-fetoprotein (AFP). However, all three hormones are equipotent in inducing neonatal uterine ornithine decarboxylase (ODC) activity. The present study assessed estrogen potency in fetal rats. Pregnant CD rats were injected sc daily on gestation days (GD) 16-20 with DES, EE2, or E2 in sesame oil. Both DES and EE2, but not E2, significantly increased uterine weight at birth, to more than twice that of controls. In addition, implants which continuously release E2 only slightly increased uterine weight at birth. Alternatively, dams were given a single estrogen injection on GD 20 and were sacrificed at various times after injection. Peak fetal uterine ODC activity occurred at 6-8 hours after maternal injection for all three estrogens. E2 had a relative potency about tenfold less than either DES or EE2 in stimulating fetal ODC activity, in contrast to equal potencies of the three estrogens in the postnatal rat uterus. Similar patterns were found following direct fetal injection with E2 or DES. In summary, these data demonstrate a transplacental induction of fetal uterine ODC activity and uterine weight gain by both DES and EE2. In addition, the lack of correlation between these endpoints in response to E2 suggests that they may be useful as selective indicators of potential toxicity of both natural and synthetic estrogens.

Pharmacodynamic and biological effects of anti-estrogens in different models

The biological response to anti-estrogens is very variable and depends on the animal species considered, the target organ, the parameter studied, and the experimental conditions. Anti-estrogens can bind specifically, (1) to the estrogen receptor, (2) to the typical anti-estrogen specific binding site, and (3) to low density lipoproteins in the plasma. Using a monoclonal antibody against the estrogen receptor, different immunological characteristics of the anti-estrogen-receptor complex can be observed. This difference could explain some of the different biological effects. Studies using different human mammary cancer cell lines (hormone-dependent) show that anti-estrogens are active in decreasing cell proliferation. Also, anti-estrogens can block proteins specifically produced by these cells. Some of these proteins could act as growth or inhibitory factors. Estrogen sulfates are the main precursors of estradiol in breast tissues and this conversion is significantly decreased by anti-estrogens. It is accepted that the main pathway of action of anti-estrogens is through the estrogen receptor, but recent information suggests the possibility that this is not the only step in the mechanism of action of anti-estrogens.

Estrogen- and antiestrogen-induced ornithine decarboxylase activity and uterine growth in the rat

The estrogen antagonists tamoxifen and monohydroxytamoxifen are also classified as partial estrogen agonists. In infantile rats, estradiol induced a single peak of uterine ODC activity at 6h following injection regardless of the extent of induction by various estradiol doses. By contrast, the timing of the ODC activity peak induced by tamoxifen and monohydroxytamoxifen was highly dependent upon the dosing conditions and was delayed to 18 h at lower tamoxifen doses. In immature rats, tamoxifen and monohydroxytamoxifen induced two peaks of uterine ODC activity resembling those induced by estradiol. Both ODC activity peaks were delayed by 9 h, without decreases in peak heights, by a 50-fold tamoxifen dose reduction. In all experiments the initial appearance of antiestrogen- and estradiol-induced ODC activity corresponded to initial uterine wet weight gain regardless of dosing condition. Thus, when dose-related temporal shifts are taken into account, tamoxifen and monohydroxytamoxifen are complete agonists with respect to induction of uterine weight gain and ODC activity.

Estrogen and antiestrogen modulation of the levels of mouse natural killer activity and large granular lymphocytes

We investigated the time course of the 17 beta-estradiol effect on mouse natural killer (NK) activity and the number of splenic large granular lymphocytes (LGL), a cell population recently associated with natural cytotoxicity and enriched in low density fractions of Percoll discontinuous density gradients. After 7 days of in vivo treatment with estrogen, an increased cytotoxicity against YAC-1 lymphoma cells was observed using only as effectors cells recovered from higher density fractions, usually devoid of NK activity. In contrast, after a 30-day treatment, augmented NK activity and an increase in LGL number were observed in the lower density Percoll fractions. Similar results were observed after a 30-day treatment with the antiestrogen tamoxifen. The cytotoxicity of both low density and high density splenocyte fractions was totally abrogated by treatment with antiserum to asialo GM1 plus complement, whereas anti-Thy 1.2 antibody treatment only partially decreased the reactivity. Further estrogen administration up to 60 days decreased both NK activity and LGL number. It is concluded that estradiol can induce opposite effects on NK activity depending on the time of treatment, with stimulation of NK activity during the first 30 days after treatment followed by depressed NK activity 1 month later.

Biological responses of tamoxifen in the fetal and newborn vagina and uterus of the guinea-pig and in the R-27 mammary cancer cell line

The biological and morphological responses of tamoxifen were studied in two models: the uterus and vagina of fetal and newborn guinea-pigs: R-27 cells--a mammary cancer cell line (tamoxifen resistant) derived from the MCF-7 cancer cell line. Tamoxifen (TAM) alone or in combination with estradiol (E2) was administered to pregnant (50-52 days of gestation) or to newborn (2-day-old) guinea-pigs for a long period (12 days). TAM alone produced a great trophic effect on the uterus and vagina which was markedly enhanced when TAM was administered together with E2. Histological studies showed that TAM provokes morphological changes in both the endometria and the myometria and this effect was also greater when TAM was administered together with E2. In the fetal uterus and vagina, the ultrastructural studies showed that TAM induces morphological alterations in different cytoplasmic organelles. This effect was much more intense in newborns where TAM provoked a significant vacuolization of the epithelial cells. Concerning progesterone receptor (PR) in the fetal or newborn tissues (uterus or vagina) TAM provoked a less intense effect than those provoked by E2, but TAM did not block the effect provoked by E2. It was observed that [3H]TAM binds specifically to the estrogen receptor (ER) of fetal guinea pig uterus and this complex is partially recognized by a monoclonal antibody which recognizes the activated form of this receptor, supporting the suggestion that the biological action of TAM is mediated by the ER. The biological and ultrastructural effects provoked by TAM (1 X 10(-6) M), estriol (E3)(5 X 10(-8) M) and the combination of TAM + E3 were studied in the R-27 mammary cancer cell line in culture. E3 stimulated the PR content by 7-10 times. However, TAM did not provoke a significant decrease in the concentration of PR, and in the mixture of TAM + E3 the concentration of PR was of the same order as that in E3 treatment. Ultrastructural observations indicate an intense concentration of ribosomes in the pericytoplasmic area after exposure to E3 and with exposure to TAM an increase in vacuoles and a significant enlargement of the size of the mitochondria were observed. It is concluded that TAM in the target tissues of fetal and newborn guinea pigs acts as a real estrogen and in the R-27 mammary cancer cell line TAM does not block the effect provoked by E3, however it does provoke intense ultrastructural modifications.