Postmenopausal Estrogen Alone Lowers Breast Cancer Cases, Deaths

Steve Cimino

July 30, 2020

A new follow-up study of menopausal hormone therapy found that prior use of conjugated equine estrogen (CEE) decreased both breast cancer incidence and mortality, while prior use of CEE plus medroxyprogesterone acetate (MPA) was associated with an increase in incidence.

"Prior use of CEE alone is, to our knowledge, the first pharmacologic intervention demonstrated to be associated with a statistically significant reduction in deaths from breast cancer," wrote Rowan T. Chlebowski, MD, PhD, of the Lundquist Institute for Biomedical Innovation in Torrance, Calif., and his coauthors. The study was published July 28 in JAMA. To further investigate the outcomes of the Women's Health Initiative in regard to hormone therapy and breast cancer risk, the researchers analyzed the long-term follow-up of two randomized trials that included 27,347 postmenopausal women with no prior breast cancer and negative mammograms at baseline. Their mean (SD) age was 63.4 (7.2) years. Enrollment took place from 1993 to 1998; participants were contacted for follow-up every 6 months through 2005 and annually from then on. Mortality data were gathered from follow-up and the National Death Index.

The first trial included 16,608 women with a uterus. Among these women, 8,506 received 0.625 mg/day of CEE plus 2.5 mg/day of MPA, and 8,102 received placebo. The second trial included 10,739 women who'd gotten a hysterectomy, 5,310 of whom received 0.625 mg/day of CEE alone and 5,429 of whom received placebo. The first trial ended in 2002 after a median intervention period of 5.6 years, and the second trial ended in 2004 after a period of 7.2 years.

An analysis in 2015 found that CEE alone was associated with lower risk of breast cancer and CEE plus MPA was associated with increased risk.

The current analysis confirmed that, after a median of 20.3 years of follow-up, and with mortality data now available for more than 98% of participants, CEE alone was associated with fewer cases of breast cancer (238 cases, annualized rate 0.30%), compared with placebo (296 cases, annualized rate 0.37%; hazard ratio 0.78; 95% confidence interval, 0.65-0.93; P = .005).

Furthermore, CEE alone was also associated with lower mortality (30 deaths, annualized mortality rate 0.031%), compared with placebo (46 deaths, annualized mortality rate 0.046%; HR 0.60; 95% CI, 0.37-0.97; P = .04).

By comparison, CEE plus MPA was linked with more cases of breast cancer (584 cases, annualized rate 0.45%) than placebo (447 cases, annualized rate 0.36%; HR 1.28; 95% CI, 1.13-1.45; P < .001). In regard to mortality, there was no statistically significant difference between CEE plus MPA (71 deaths, annualized mortality rate 0.045%) and placebo (53 deaths, annualized mortality rate 0.035%; HR 1.35; 95% CI, 0.94-1.95; P = .11).

"The big thing to think about is estrogen alone reducing breast cancer mortality by 40%," said Dr. Chlebowski in an interview. "None of the other interventions, including tamoxifen, had any change on mortality. This should change the way we look at breast cancer prevention, though we might have to be a little creative about it. I think you have to be a little away from menopause for it to reduce breast cancer. But we wanted to start that debate.

"On the other hand," he said, "a woman takes estrogen plus progestin and when you look at that curve, it's staying about 25% increased. You take it for 5.6 years and the increase continues through 20 years, so you're maybe buying a lifetime of increase in breast cancer by taking estrogen plus progestin for 5 years."

He also highlighted the comprehensiveness of the mortality data, noting that "when you hook up to the National Death Index, they find 98% of all deaths in the United States. That's really remarkable; you retain the whole power of the randomization. It means our data, between the death index and our follow-up of participants, is essentially complete."

Use of Hormone Therapy, and Decoding the Outcomes, Remains 'Complex'

Decades after the data were gathered from the Women's Health Initiative clinical trials, they continue to assist researchers and patients alike, wrote Christina A. Minami, MD, of Brigham and Women's Hospital in Boston and Rachel A. Freedman, MD, of the Dana-Farber Cancer Institute in Boston, in an accompanying editorial.

That said, in regard to the findings of this latest analysis, "many questions still remain on whether (and how) a hormone therapy intervention that occurred many years earlier may continue to affect breast cancer risk and mortality at 20 years," they wrote. They noted that it's "impossible" to isolate how exposure to certain therapies can impact long-term outcomes, and that a high percentage of patients who discontinued the drugs during each trial muddy the waters even further.

"Decisions to initiate these medications remain complex," they added, emphasizing that breast cancer risk is just one of many factors that physicians must consider when considering hormone therapy for their patients.

Dr. Chlebowski and his coauthors acknowledged their study's limitations, including the use of very specifically administered and formulated dosages making their findings "not necessarily generalizable to other preparations." In addition, they noted the significant percentage of patients – 54% with CEE alone and 42% with CEE plus MPA – who discontinued drug usage during their respective trials.

The Women's Health Initiative is supported by the National Institutes of Health and the Department of Health and Human Services. The authors reported numerous potential conflicts of interest, including receiving personal fees and grants from various government organizations, foundations, and pharmaceutical companies. The editorial's authors reported no conflicts of interest.

SOURCE: Chlebowski RT et al. JAMA. 2020 Jul 28. doi: 10.1001/jama.2020.9482.

This article originally appeared on MDedge.com.

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