Phase 2 study of palbociclib (CDK 4/6 inhibitor) for ER positive, HER2-negative post-menopausal advanced breast cancer patients recurring after hormonal therapy (to reverse endocrine resistance - TREnd trial)

Background: Palbociclib (PD-0332991) is an orally active, potent and highly selective inhibitor of CDK4/6 that prohibits progression of the cell cycle from G1 into S phase and has shown activity in ER positive breast cancer in vivo and in vitro. Recently, in phase 2 settings, palbocilcib extended PFS in combination with letrozole as first-line hormonal treatment for advanced breast cancer (ABC) (Finn R.S. et al. Cancer Res., Dec 2012; 72: S1-6) and showed single agent activity in patients with ER positive ABC relapsed to several lines of therapy (DeMichele A. et al., ASCO 2013). Given pre-clinical evidence on the relevance of Cyclin D-CDK4/6-E2F signalling in the development of acquired resistance to hormonal therapy, CDK4-6 inhibition might be a strategy to overcome resistance in tumors who have progressed after hormonal therapy for metastatic disease.

Study design: This is an investigator initiated, open-label, multicenter, randomized, Phase 2 trial for patients with ER+ HER2 negative ABC who have experienced progression on first or second line hormonal therapy. Other eligibility criteria include: ≤1 previous line of chemotherapy for ABC; measurable disease; and available archival tumor tissue for translational studies. Consenting patients are randomized (1:1) to receive, until progression of disease, unacceptable toxicity or consent withdrawal, either ARM A) palbociclib 125 mg/d orally for 3 weeks followed by 1 week off treatment or ARM B) palbociclib in combination with the hormonal therapy to which the patient has experienced progression in the previous line of treatment (anastrozole 1 mg/d, letrozole 2.5 mg/d, exemestane 25 mg/d, fulvestrant 500mg/4 weeks). Patients are stratified according to: number or prior lines of hormonal therapy (1 vs. 2); disease site (visceral vs. non visceral); duration of prior line of hormonal therapy (>6 vs. ≤6 months); treating center.

The primary endpoint is clinical benefit (CB) (i.e. complete response, partial response, stable disease≥ 24 weeks), based on local assessment of response using RECIST 1.1 criteria. Key secondary endpoints are: objective response rate, duration of response, time to progression, progression-free survival, overall survival and assessment of exploratory tissue markers of response.

Statistical methods: A two-stage optimal phase 2 design is used to assess the activity of each of the two treatment regimens. Assuming P0≤20% (inactivity), P1≥ 40% (activity) and α = 10%, a sample size of 50 evaluable patients per treatment arm is required for a statistical power equal to 90%. In case of inactivity, enrolment could be stopped in each arm after 25 evaluable patients. This study does not provide adequate power for a two arm comparison.

Target accrual: the study is actively recruiting in 3 Italian centers and is expected to involve additional 7 centers in Italy by the end of 2013. The first patient was enrolled in October 2012 and, as per June 2013, a total of 9 patients have been enrolled, 8 remain on treatment.