Tamoxifen-stimulated tumor growth

A 5-year course of tamoxifen treatment provides protection superior to 1-2 years of treatment. Currently, 5 years of adjuvant tamoxifen is recommended to be optimal, since extending treatment beyond 5 years provides no further improvement [137,138], There are reports of tamoxifen-stimulated tumor growth occurring during the treatment of advanced (metastatic stage IV) breast cancer [139,140], but there is currently no evidence that extending tamoxifen beyond 5 years of adjuvant therapy increases the risk of tumor recurrence. Critically important, the protective effects of tamoxifen on breast cancer recurrence and mortality are persistent long after tamoxifen therapy is stopped. A meta-analysis of 15 years of follow-up of 10,386 women shows that 5 years of adjuvant tamoxifen in ER-positive disease versus not... 

Tamoxifen Estrogen receptor antagonist. Prevents endogenous estrogens from stimulating tumor growth. Used to treat estrogen receptor-positive breast cancer in postmenopausal women. 

When studied in a model of human endometrial carcinoma, such as EnCa 101 tumors in athymic mice, ICI 164384 not only showed no stimulatory activity on tumor progression but also blocked the tamoxifen-stimulated growth of the tumor (Gottardis et al. 1990). 

Idoxifene inhibits the growth of carcinogen-induced rat mammary tumors [293] and MCF-7 tumors grown in athymic mice [294, 295]. When compared to tamoxifen, idoxifene appears to have more antagonistic and less agonistic effects on ER in laboratory studies. Also, idoxifene has been reported to develop acquired antiestrogen resistance more slowly than tamoxifen [294]. However, there appears to be crossresistance in laboratory models of tamoxifen-stimulated growth [281]. 

Osborne, C.K., Jarman, M., McCague, R., Coronado, E.B., Hilsenbeck, S.G. and Wakeling, A.E. (1994) The importance of tamoxifen metabolism in tamoxifen-stimulated breast tumor growth. Cancer Chemotherapy and Pharmacology, 34, 89-95. 

In rats with estrogen-dependent mammary tumors, treatment with black cohosh did not stimulate cancerous growths (Freudenstein et al. 2002). Likewise, in rats with endometrial cancer treated with black cohosh alone or with tamoxifen, animals in both the black cohosh and black cohosh plus tamoxifen treatment groups had fewer metastases and smaller tumor mass than the untreated control animals (Nisslein and Freudenstein 2004). 

When used in tumor cells, fulvestrant was initially described as a potent, competitive growth inhibitor of ER-positive, human breast cancer MCF-7 cells, whose growth is stimulated by estradiol. The compound was ineffective in tumor cell lines without ER, such as MDA-MB-231. The inhibitory effects were more pronounced with fulvestrant than with tamoxifen in the same cell line (Wakeling et al. 1991). 

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