Late Breaking Abstract – ASCO 2016 Extended Adjuvant AI Therapy Improves DFS in Postmenopausal Hormone Receptor Positive Breast Cancer

SUMMARY: Breast cancer is the most common cancer among women in the US and about 1 in 8 women (12%) will develop invasive breast cancer during their lifetime. Approximately, 246,660 new cases of invasive breast cancer will be diagnosed in 2016 and 40,450 women will die of the disease. Approximately 75% of patients with breast cancer are hormone receptor positive (Estrogen Receptor/Progesterone Receptor positive) and this is a predictor of response to endocrine therapy. These patients are often treated with anti-estrogen therapy as first line treatment. In premenopausal woman, the ovary is the main source of estrogen production, whereas in postmenopausal women, the primary source of estrogen is the Aromatase enzyme mediated conversion of androstenedione and testosterone to estrone and estradiol, in extragonadal/peripheral tissues. NOLVADEX® (Tamoxifen) is a nonsteroidal Selective Estrogen Receptor Modulator (SERM) and works mainly by binding to the Estrogen Receptor and thus blocks the proliferative actions of estrogen on the mammary tissue. ARIMIDEX® (Anastrozole), FEMARA® (Letrozole) and AROMASIN® (Exemestane) are Aromatase Inhibitors (AIs) that binds to the Aromatase enzyme and inhibit the conversion of androgens to estrogens in the extra-gonadal tissues. Postmenopausal women with hormone receptor positive early breast cancer are often treated with 5 years of Aromatase Inhibitor (AI) therapy either as up-front treatment or after 2-5 years of Tamoxifen. The benefit of extending treatment with an AI to 10 years may further reduce the risk of breast cancer recurrence, but this benefit was not previously known.

The Canadian Cancer Trials Group MA.17R is a double blind, placebo controlled, phase III trial, which tested the benefit of extending AI treatment, using FEMARA®, for an additional 5 years. This study involved 1,918 postmenopausal women with early stage breast cancer and included three patient groups – one group had no prior treatment with adjuvant Tamoxifen whereas the other two groups had adjuvant Tamoxifen for some duration of time. All patients however had recently received 4.5 to 6 years of adjuvant AI therapy. These patients were then randomly assigned to receive either extended adjuvant treatment with FEMARA® or placebo for an additional five years. The primary endpoint was Disease Free Survival (DFS).

After a median follow up of 6.3 years, the 5 year DFS rate for the extended FEMARA® group was 95% compared with 91% for the placebo group (HR=0.66; P=0.01). The improvement in DFS was significant among patients with node-positive disease, but not for those with node-negative disease. There was no difference in the 5 year Overall Survival between the two groups – 93% with FEMARA® versus 94% with placebo (HR 0.97; P=NS). The annual incidence rate of contralateral breast cancer was however significantly better in the FEMARA® group at 0.21%, compared with 0.49% with placebo (HR=0.42; P=0.007). Patients receiving extended treatment with FEMARA® had more frequent adverse events such as bone pain, elevation of alkaline phosphatase, and elevation of alanine transaminase. There was also a greater incidence of osteoporosis with FEMARA® than with placebo (11% vs 6%; P<0.0001) and fracture risk was higher in the FEMARA® group (14%) compared with 9% in the placebo group (P=0.001).

The authors concluded that this is the first study to show added benefit of improved Disease Free Survival, by extending an adjuvant AI beyond 5 years to 10 years, when compared with 5 years of AI treatment as initial therapy or preceded by 2-5 years of Tamoxifen. A randomized trial (MA.17R) of extending adjuvant letrozole for 5 years after completing an initial 5 years of aromatase inhibitor therapy alone or preceded by tamoxifen in postmenopausal women with early-stage breast cancer. Goss PE, Ingle JN, Pritchard KI, et al. J Clin Oncol 34, 2016 (suppl; abstr LBA1)