Std Dose or Baby TAM for Breast Cancer Prevention?

Should 5 mg of tamoxifen — known as "baby TAM" — or the usual 20 mg dose be standard of care for breast cancer prevention in high-risk women?

Research to date clearly shows that tamoxifen can reduce the risk for breast cancer in high-risk individuals by 30%-50%. Recent evidence also indicates that this chemoprevention approach can reduce the risk of dying from breast cancer by as much as 57%.

In 2019, the US Preventive Services Task Force issued updated recommendations that clinicians offer risk-reducing medications, such as tamoxifen, raloxifene, or aromatase inhibitors, to women at an increased risk for breast cancer and a low risk for adverse medication effects.

However, this prophylactic strategy remains underused.

A major roadblock: The drug's side effects, which include venous thromboembolic events, endometrial cancer, as well as symptoms of menopause, such as hot flashes and sexual issues, have made uptake and adherence a challenge.

Offering women a lower dose of tamoxifen could allay fears about toxicities and improve uptake as well as reduce side effects and boost long-term adherence among those receiving baby TAM.

However, experts remain uncertain about whether patients need the standard dose to experience the full prevention benefit.

The Debate

Years ago, Andrea De Censi, MD, a breast cancer researcher at the Galliera Hospital in Genova, Italy, and his colleagues reasoned that, because tamoxifen is a competitive estrogen receptor inhibitor, it may indeed have a minimal effective dose below 20 mg/d.

The fruits of that line of thought were presented to the world in the TAM-01 trial, first published in 2019, which pitted tamoxifen 5 mg/d for 3 years against placebo in 500 women with high-risk lesions, including lobular and ductal carcinoma in situ.

De Censi and colleagues found that baby TAM reduced the risk for invasive breast cancer by 52% and the risk for contralateral breast cancer by 75%.

Treatment adherence was slightly higher in the baby TAM group at 65% vs 61% in the placebo group.

A recent 10-year follow-up showed ongoing benefits associated with baby TAM vs placebo — a 42% reduction in breast cancer and a 64% drop in contralateral lesions.

The baby TAM group vs placebo experienced a slight increase in hot flashes but no significant increase in other common side effects.

Regarding serious adverse events, the baby TAM arm had one case of stage I endometrial cancer (0.4% of patients) and 20 cases of endometrial polyps (5%) vs 13 cases of endometrial polyps in the placebo arm. But there were no significant differences in thrombosis, cataracts, bone fractures, and other serious events.

De Censi said he's surprised the baby TAM vs tamoxifen topic is still being debated. "Baby TAM, in my opinion, is a new standard of care for endocrine prevention of breast cancer in high-risk [women]," and baby TAM over 3 years is enough, said De Censi during a debate on the topic at the 2024 European Society for Medical Oncology Breast Cancer Congress in Berlin.

Gareth Evans, MD, a cancer genetics and prevention specialist at the University of Manchester, Manchester, England, however, isn't convinced.

During the debate, Evans explained that  his main concern was that the baby TAM trial was limited to women with high-risk lesions, not other common reasons for tamoxifen prophylaxis, such as a positive family history or BRCA mutations.

"In Manchester, we have put over a thousand women on tamoxifen who have a family history or other risk factors, not high-risk lesions," and there simply isn't definitive evidence for baby TAM in these women, Evans said.

The vast weight of evidence for tamoxifen prophylaxis, he added, is in trials involving tens of thousands of women, followed in some cases for 20 years, who received the 20 mg dose for 5 years.

As a result, women in Manchester are started on 20 mg and dropped down to 5 mg only for side effects. That way, Evans explained, we are not taking away the benefit among women who can tolerate 20 mg.

Meanwhile, there's no evidence that baby TAM improves medication adherence, he noted. Trials have reported similar adherence rates to baby TAM and standard dose tamoxifen as well as no definitive evidence that the risk for cancer and thrombosis is less with baby TAM, he said.

In fact, Evans noted, "many women take tamoxifen 20 mg for 5 years with no side effects."

Overall, "I don't think we've got the evidence yet to drop" dosages, particularly in women without high-risk lesions, Evans said. A real concern, he added, is poor metabolizers for whom 5 mg won't be enough to have a preventive effect.

De Censi noted, however, that there will likely never be a definitive answer to the question of baby TAM vs standard dosing because industry has no financial incentive to do a head-to-head trial because tamoxifen went off patent over 30 years ago.

Still, a poll of the audience favored Evans' approach — 80% said they would start high-risk women on 20 mg for breast cancer prophylaxis and reduce for side effects as needed.

De Censi didn't have any disclosures. Evans is a consultant/advisor for AstraZeneca, SpringWorks, Recursion, Everything Genetic, and Syantra.

M. Alexander Otto is a physician assistant with a master's degree in medical science and a journalism degree from Newhouse. He is an award-winning medical journalist who worked for several major news outlets before joining Medscape Medical News. Email: aotto@mdedge.com.