Background: Mutations in PIK3CA, the gene encoding the p110alpha subunit of PI3K, have been associated with antiestrogen resistance in ER+ BC. In general, antiestrogen-resistant cancers retain ER and responsiveness to estradiol, suggesting that treatment of ER+/PI3K mutant BC should include PI3K inhibitors plus endocrine therapy.

Methods: We are conducting a phase Ib open-label trial of letrozole (2.5 mg/d) with the dual PI3K/ mTOR inhibitor BEZ235 (400 mg BID, sachet formulation) in post-menopausal patients with ER+/HER2 negative MBC. Upon toxicity, BEZ235 was reduced to 400 mg in AM/ 200 mg in PM (dose level 1), and 200 mg BID (dose level 2), in a standard 3+3 de-escalation design. Treatment continues until unacceptable toxicity or disease progression. Tumor assessments are performed every 2 months. All tumor biopsies are being analyzed for PI3K pathway alterations.

Results: To date, 9 patients have been accrued; 6 on dose level 1 (400 mg BID) and 3 on dose level 1 (400 mg in AM/ 200 mg in PM). Of note, the latter 3 patients have been on treatment for less than 2 weeks at the time of abstract submission. Median age was 55 years (range 40–65); all patients had disease progression on previous endocrine therapy, and 100% had bone metastases and 30% had visceral metastases. Preliminary toxicities for dose level 1 cohort (400 mg BID) are summarized in the table below. The DLT was grade 3 mucositis, which happened less than 3 weeks of study initiation, in a total of 3 out of 6 patients. All grade 3 mucositis cases improved after BEZ235 interruption for 7 days and subsequent dose reduction. These 3 events triggered activation of the dose level 1 cohort (400 mg AM/ 200 mg PM). The first 3 patients on dose level 1 cohort (2 of which have PIK3CA mutated tumors) had their tumor assessment scans at 8 weeks post treatment initiation; all patients were found to have stable disease.

Discussion: The combination of letrozole/BEZ235 is still being evaluated in patients with ER+/HER2MBC. Safety, tolerability and preliminary antitumor activity data will be updated at the meeting.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P6-10-05.