The Breast Cancer Index, a molecular test developed by Biotheranostics, successfully identified a group of women with hormone receptor (HR)-positive breast cancer who may benefit from long-term hormone therapy with tamoxifen, data from a Phase 3 trial shows.

Study findings were presented in a poster, titled “Breast Cancer Index predicts benefit from extended endocrine therapy in HR+ breast cancer,” at the 42nd Annual San Antonio Breast Cancer Symposium, in San Antonio, Texas.

Anti-estrogen therapies, such as tamoxifen, are a form of endocrine therapy that works by depriving cancer cells of the hormones, mainly estrogen, that promote their growth. For women with HR-positive breast cancer, these therapies can be lifesaving.

However, hormone therapy can also cause a series of unpleasant side effects, including osteoporosis, joint pain, blood clots, and endometrial cancer. For that reason, determining the best timing of treatment for each patient is important to ensure the maximum benefits and avoid undesirable side effects.

“Current clinical practice guidelines recommend 10 years of adjuvant endocrine therapy for most patients unless there are characteristics of low risk disease. However, approximately [two-thirds] of patients have favorable long-term outcomes after completing 5 years of adjuvant therapy,” the investigators said.

“Therefore, consideration of the risk-benefit profile for each patient is critical to identify who may be spared extended endocrine therapy and its associated toxicities, and which patients will benefit from an additional 5 years of endocrine therapy,” they added.

In an attempt to solve this problem, Biotheranostics created the Breast Cancer Index (BCI), a molecular test that is able to tell which genes are active inside tumors, to identify the patients who are most likely to respond well to extended endocrine therapy based on the genetic signature of their tumors.

The accuracy of the BCI test at selecting women with HR-positive breast cancer who may benefit from extended endocrine therapy has been put to the test in the large, randomized, prospective, Trans-aTTom Phase 3 trial (NCT00003678).

The study enrolled 6,953 women with HR-positive breast cancer who had completed at least four years of endocrine therapy with tamoxifen. Participants were randomly assigned to either stop treatment or continue therapy for five more years.

At the meeting, the company presented the latest findings from an expanded analysis of the Trans-aTTom trial, which continues to support the use of the BCI test to identify women who are likely to benefit from long-term endocrine therapy.

“We are excited to share these new data that provide further support and insight into the unique ability of the Breast Cancer Index to identify patients with the endocrine-responsive disease,” Catherine Schnabel, PhD, chief scientific officer of Biotheranostics, said in a press release.

The BCI test was performed in 2,445 women with HR-positive breast cancer, including 1,367 who were node-negative, 789 who were node-positive, and 289 who had an unknown node status. (Being node-positive means that some cancer cells have already spread from the breast to nearby lymph nodes).

In the overall population of 2,445 patients, final analyses to evaluate the predictive performance of the BCI test were underpowered due to the smaller-than-planned sample of patients which was not representative of the original population of the trial.

However, in the sub-group of 789 women who were node-positive, those who had been considered H/I-High by the BCI test were found to benefit from the additional five years of therapy with tamoxifen. These patients had an improvement of 9.7% in that time, without evidence of the cancer regrowing in the breast or elsewhere in the body — a measure called recurrence-free interval — with 10 years of therapy.

In contrast, those who were deemed as H/I-Low by the BCI test did not seem to benefit from extended endocrine therapy.

Of note, the term “H/I” is derived from the particular genes measured by the BCI test — HoxB13 and IL17BR.

“Biotheranostics remains committed to strengthening the clinical evidence supporting BCI towards individualizing care for each HR+ early-stage breast cancer patient and extension of endocrine therapy,” Schnabel said.

“We continue our commitment to furthering the validation of Breast Cancer Index for women with HR+ early-stage breast cancer.  The final results from the Trans-aTTom validation study further validate that Breast Cancer Index is the only predictive biomarker for extended endocrine therapy in early-stage HR+ breast cancer,” said Don Hardison, president and CEO of Biotheranostics.

“We are committed to ensuring that every breast cancer patient and her physician have information to help determine the best approach to individualize treatment,” he added.