Breast cancer risk in postmenopausal women varies depending on the type of progestin used in hormone replacement therapy, according to a study published in Cancer Prevention Research (2010 Aug 10 [Epub ahead of print]). 

According to background information provided by the authors, synthetic progestins have different biological effects due to differences in their structure, stability, and how they interact in the body.

For the study, researchers at the University of Missouri used an animal model to investigate the effects of four clinically relevant progestins including medroxporgresterone acetate (MPA), norgesterel (N-EL), norethindrone (N-ONE), and megestrol acetate (MGA).

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Results of the study revealed that when administered using the same protocol as used for MPA, N-EL and N-ONE strongly inhibited tumor development. “Progestins, such as N-EL and N-ONE, may have potential as preventive agents in women with no history of breast disease or family history of breast cancer,” explained Salman Hyder, the Salk Endowed Professor in Tumor Angiogenesis and professor of biomedical sciences in the College of Veterinary Medicine and the Dalton Cardiovascular Research Center. “These progestins may also have a preventive role in post-menopausal women who are at an increased risk for developing breast cancer due to ingestion of combined estrogen/progestin (MPA) hormone replacement therapy and may have early stage disease.”