Phase II trial of bicalutamide in combination with palbociclib for the treatment of androgen receptor (+) metastatic breast cancer

1017 Background: Genome-wide transcriptional analysis has identified a unique subset of androgen receptor (AR) +, estrogen receptor (ER)/progesterone receptor (PR)- breast cancer (BC). The functional role of AR was confirmed initially in preclinical models demonstrating that androgen-driven growth could be abrogated by antiandrogen therapy. TBCRC011 established the safety and efficacy of inhibiting AR with bicalutamide (B) in patients (pts) with AR+/ER/PR- metastatic BC (MBC) with a median progression free survival (PFS) of 12 weeks (wks) (95% CI, 11–22 wks). In preclinical data, palbociclib (P) has been shown to reduce growth of AR+/ER/PR- MDA-MB-453 BC cells. It has been shown that AR+ triple negative BC (TNBC) expresses a luminal profile and has intact Rb protein, the target of P activity. We conducted this Phase I/II trial of the AR inhibitor B in combination with the CDK4/6 inhibitor P in pts with AR+/ER/PR/HER2- BC (NCT02605486) to test the hypothesis that androgen blockade, paired with CDK4/6 inhibition would have increased efficacy in pts with androgen-dependent BC. Methods: Postmenopausal pts with AR+ TN MBC defined as IHC ≥ 1% nuclear staining (DAKO, Clone AR441 (5/2016-11/2016) then Ventana AR SP107 (11/2016-6/2018), ECOG ≤2, measurable/non-measurable disease were eligible for enrollment. Any number of prior regimens was permitted. Pts received B 150 mg daily and P 125 mg daily 3 wks on 1 wk off. Pts were evaluated for toxicity every 2-4 wks and for response every 8-12 wks. Primary endpoint: 6 month (mo) PFS. Secondary endpoints: clinical benefit rate, toxicity, correlative studies to better characterize AR+ TNBC. A Simon 2-stage minimax design that discriminates between 6 mo PFS rates of 20% and 40% was used. If ≥ 11/33 pts were PF at 6 mo then B+P would warrant further study. Results: As of 1.1.20 33 pts were enrolled on study with median (med) age 67 (42-79), performance status 0 (0-1). Number of pts with visceral metastases: 20, measurable disease: 22. AR% 1-9: 3, 10-50: 6; 51-100: 24. Med prior lines for MBC: 3 (0-9). Best response: (31 evaluable pts): 11 pts PF at 6mo: 10 SD > 6mo, 1 PR. Med wks on study: 14 (2-101). Toxicity > 10% grade >3 related: Number of pts with leukopenia: 21, neutropenia: 21, lymphocytopenia: 6, thrombocytopenia: 3. One pt with febrile neutropenia. One death due to disease progression within 30 days off study. Conclusions: In this selected subset of pts with AR+ TN MBC, this study met its prespecified endpoint with 11 pts PF at 6 mo on B 150 mg + P 125 mg. B+P has been well tolerated with no unexpected toxicity observed. Clinical trial information: NCT02605486 .