A pilot study of estradiol followed by exemestane for reversing endocrine resistance in postmenopausal women with hormone receptor-positive metastatic breast cancer.

BACKGROUND: Endocrine resistance is a frequent complication, and strategies to reverse it are a high research priority for metastatic breast cancer (MBC) that is hormone receptor positive. Preclinical data suggest re-exposure to estrogen induces tumor regression in tamoxifen-resistant tumors. We conducted a pilot study to determine whether short-term estradiol exposure would reverse endocrine resistance and resensitize tumors METHODS: Postmenopausal women with estrogen receptor-positive MBC whose disease had progressed after receiving at least one prior endocrine therapy were eligible for the study. Patients were initially treated with 6 mg/day estradiol, and those who had not progressed after 3 months were then switched to exemestane. RESULTS: Thirteen patients were evaluable for toxicity and response. No grade 3 or 4 toxicities were observed. Of the 13 patients who initiated estradiol therapy, 6 patients (46%) had not experienced disease progression at month 3 and were switched to exemestane. On exemestane, disease progression was documented in five patients, with one having stable disease as best response. Median progression-free survival for all patients was 4.8 months (range: 0.6-9.5 months). CONCLUSION: Treatment with an estrogen prior to resuming antiestrogen treatments was not effective at reversing hormone resistance; however, low-dose estradiol treatment had measurable clinical activity with minimal toxicity and should be considered as a therapeutic option for hormone-refractory MBC.