Risk Summary

SOLTAMOX can cause fetal harm when administered to a pregnant woman. There are no adequate and well- controlled studies of tamoxifen in pregnant women. There are limited postmarketing reports of vaginal bleeding, spontaneous abortions, birth defects, and fetal deaths in pregnant women taking tamoxifen. In a primate model administration of tamoxifen at doses 2 times higher than the maximum recommended human dose resulted in spontaneous abortion. In rat and rabbit studies, doses of tamoxifen less than or equal to human doses resulted in increased embryotoxicity, abortions, and altered learning behaviors. Additionally, rodent models showed reproductive tract changes often associated with DES in offspring of both sexes [see Data]. Advise pregnant women of the potential risk to a fetus, including the potential long-term risk of a DES-like syndrome.

The estimated background risk of major birth defects and miscarriage for the indicated population is unknown. In the U.S. general population, the estimated background risk of major birth defects and miscarriage in clinically recognized pregnancies is 2-4% and 15-20%, respectively.

Data

Animal Data

In embryofetal development studies in rats using doses significantly below those used in humans (0.01 to 0.03 times the recommended human dose on a mg/m2 basis), a lower incidence of embryo implantation and a higher incidence of fetal death or delayed in utero growth occurred, along with slower learning behavior in some rat pups when compared to historical controls. When pregnant rabbits received tamoxifen during organogenesis, abortion and premature delivery occurred at doses of greater than 0.125 mg/kg/day (about 0.05 times the daily maximum recommended human dose on a mg/m2 basis). Although reversible nonteratogenic skeletal variations occurred in rat reproductive studies at doses less than or equal to the human dose, there were no teratogenic changes in either rats or rabbits. In pregnant marmosets, administration of 10 mg/kg/day tamoxifen (about 2 times the daily maximum recommended human dose on a mg/m2 basis) during organogenesis or in the last half of pregnancy terminated pregnancy in some animals. In pregnancies that continued, there were no malformations or deformations in offspring.

In rodent models of fetal reproductive tract development, tamoxifen (at doses 0.002 to 2.4 times the daily maximum recommended human dose on a mg/m2 basis) caused changes in both sexes that are similar to those caused by estradiol, ethynylestradiol and diethylstilbestrol. Although the clinical relevance of these changes is unknown, some of these changes, especially vaginal adenosis, are similar to those seen in young women who were exposed to diethylstilbestrol in utero and who have a 1 in 1,000 risk of developing clear-cell adenocarcinoma of the vagina or cervix. To date, in utero exposure to tamoxifen has not been shown to cause vaginal adenosis, or clear-cell adenocarcinoma of the vagina or cervix, in young women; however, only a small number of young women have been exposed to tamoxifen in utero, and a smaller number have been followed long enough (to age 15 to 20) to determine whether vaginal or cervical neoplasia could occur as a result of this exposure.