Using Androgen Receptor Blockers to Treat Triple-Negative Breast Cancer

Whereas the androgen receptor (AR) is a common target in the treatment of prostate cancer, Michael F. Press, M.D., Ph.D., shed light on the role of the receptor in breast cancer in a presentation during the 2017 Miami Breast Cancer Conference.

“The reason people are particularly interested in the possibility of using the AR in breast cancer has to do with the category of triple-negative breast cancers [TNBCs],” said Press, the Harold E. Lee Chair for Cancer Research in the Department of Pathology, Norris Comprehensive Cancer Center, University of Southern California.

At present, no targeted therapies are approved for the treatment of patients with TNBC and new treatment approaches are critically needed. “One of the potential targets is AR,” Press said. “A proportion of breast cancers that lack ER, PR and HER2 do show AR expression.”

Androgens are classified as male hormones, although they can also be expressed in the female body. Furthermore, they can be expressed in the breast epithelium. The hope is that AR blockers, most commonly used in the treatment of prostate cancer, could be used on breast cancers that express ARs.

“Since there are Food and Drug Administration-approved drugs that block the androgen receptor in men, the hope is that, if one could come up with a strategy that would demonstrate the effectiveness of these drugs in some women, that they could become clinically available for use in the clinic pretty quickly. This is under active investigation,” said Press. Current efforts to employ ARs in breast cancer are focused on three prostate cancer drugs.

Enzalutamide

A phase II study, presented at the 2015 ASCO Annual Meeting (MDV3100-11 trial), evaluated single-agent Xtandi (enzalutamide) — an AR inhibitor that impairs nuclear localization of AR — in patients with advanced AR-positive (IHC ≥10 percent and ≥1 post-baseline tumor assessment) TNBC. The clinical benefit rate (CBR) at 16 weeks served as the study’s primary endpoint.

Among the 75 evaluable patients, the CBR was 35 percent (n = 26) at 16 weeks (95 percent CI, 0.24-0.46) and 29 percent (n = 22) at 24 weeks (95 percent CI, 0.20-0.41). Six patients showed a complete or partial response (response rate of eight percent). The researchers found a median progression-free survival (PFS) of 14.7 weeks in the evaluable population (95 percent CI, 8.1-19.3).

The median age of patients in the intent-to-treat population (n = 118; those who received >1 dose of enzalutamide) was 57 years. The 16-month CBR for these patients was 25 percent, and the 24-month CBR was 20 percent. The response rate was six percent, and the median PFS was 12.6 weeks. Additionally, the median overall survival was 12 months.

Overall, 61 percent of patients (n = 72) experienced treatment-related adverse events (AEs), the most common of which were fatigue (30 percent), nausea (22 percent), and decreased appetite (11 percent).

Thus far, this study is the largest prospective trial to date of an AR inhibitor in TNBC. Xtandi represents one of the most promising agents evaluated in this space. The drug is currently being explored in the phase III ENDEAR trial, which will evaluate Xtandi in combination with paclitaxel or as a monotherapy versus placebo with paclitaxel in patients with locally advanced or metastatic TNBC.

In another study looking at Xtandi, which was published in Breast Cancer Research, the AR antagonist was used to elucidate the role of AR in preclinical models of ER-positive and ER-negative breast cancer.

The study authors examined a cohort of 192 women with ER-positive breast cancers, assessing their nuclear AR to ER protein ratios in primary breast cancers, in relation to response to treatment with endocrine therapy. The effects of AR inhibition with Xtandi were examined in vitro and in preclinical models of ER-positive and ER-negative breast cancers with AR expression.

The AR to ER ratio was an independent predictor of disease-free survival (HR, 4.04; 95 percent CI, 1.68-9.69; P = .002) and disease-specific survival (HR, 2.75; 95 percent CI, 1.11-6.86; P = .03). The findings demonstrated that the AR to ER ratio might influence a patient’s response to traditional endocrine therapy when being treated for breast cancer.

Bicalutamide

Casodex (bicalutamide), another hormone-based AR antagonist, was evaluated in a phase II trial involving patients with AR-positive, ER-negative metastatic breast cancer.

The study, published in Clinical Cancer Research, tested tumors from patients with ER/progesterone receptor (PgR)-negative advanced breast cancer for AR by IHC (>10 percent nuclear staining considered positive). If either the primary or a metastatic site was positive, patients were then eligible to receive Casodex at a dose of 150 mg daily. The primary endpoint was CBR, defined as the total number of patients who showed a complete response, partial response or stable disease more than six months. Secondary endpoints in the study included PFS and toxicity.

Of 424 patients with ER/PgR-negative breast cancer, 12 percent were found to have AR-positive disease. The six-month CBR was 19 percent (95 percent CI, 0.07-0.39) for Casodex and the median PFS was 12 weeks (95 percent CI, 11-22 weeks). The AR antagonist was well tolerated with no observable grade 4/5 treatment-related AEs.

These findings demonstrate proof of principle for the efficacy of minimally toxic androgen blockade in a select group of patients with ER/PgR-negative, AR-positive breast cancer. An ongoing study at Memorial Sloan Kettering Cancer Center is looking at Casodex in combination with the CDK4/6 inhibitor Ibrance (palbociclib) for women with AR-positive metastatic breast cancer (NCT02605486).

Zytiga

A phase II trial published in the Journal of Clinical Oncology evaluated the activity of Zytiga (abiraterone acetate), a second-generation AR inhibitor, in the treatment of postmenopausal women with ER-positive metastatic breast cancer. A total of 297 patients were randomized to receive abiraterone and prednisone (n = 89); abiraterone, prednisone, and exemestane (n = 106); or exemestane alone (n = 102). The primary endpoint of the trial was PFS.

Although the results showed that the overall response rate was higher among patients who received abiraterone, prednisone and exemestane (12.1 percent), compared with exemestane alone (6.3 percent), the benefit was not statistically significant (P = .366). Additionally, the clinical benefit rate was higher among patients who received abiraterone, prednisone and exemestane (22.7 percent) compared with exemestane alone (12.7 percent). Again, the data were not statistically significant (P = .137). The median PFS with exemestane alone was 3.7 months, compared with 4.5 months with abiraterone, prednisone and exemestane (HR, 0.96; 95 percent CI, 0.70-1.32; P = .795) and 3.7 months with abiraterone plus prednisone (HR, 1.1; 95 percent CI, 0.82- 1.60; P = .437).