Randomized double-blind phase 2 trial of 3 doses of TAS-108 in patients with advanced or metastatic postmenopausal breast cancer.

Background: The objective of this study was to evaluate 3 different doses of (7α)-21-(4-[(diethylamino)methyl]-2 methoxyphenoxy)-7 methyl-19 norpregna-1,3,5(10)-trien-3-ol 2-hydroxy-1,2,3-propanetricarboxylate (TAS-108) in patients with recurrent, hormone-responsive breast cancer.

Methods: In this randomized, double-blind, multicenter study, TAS-108 was administered daily at a dose of 40 mg, 80 mg, or 120 mg to postmenopausal patients with locally advanced, or inoperable, or metastatic hormone-receptor positive breast cancer. The primary efficacy outcome was clinical benefit (CB), defined as the total number of patients who achieved a complete response, a partial response, or stable disease for ≥24 weeks. The study was a 2-stage design in which 19 patients per dose group were planned in the first stage. If at least 3 patients in any dose group achieved a CB, then that dose group was to be allowed to continue enrolling for the second stage, and the group could include up to a total of 60 patients.

Results: The 40-mg and 80-mg groups met the criterion and enrolled patients into the second stage. In the 40-mg group, there were 13 CB events in 60 patients (21.7%); and, in the 80-mg group, there were 12 CB events in 60 patients (20%). The 120-mg daily dose was stopped early, because it failed to achieve the criterion. For the 40-mg and 80-mg groups, the median time to progression was 15.0 weeks and 15.9 weeks, respectively. Only 1 drug-related serious adverse event (grade 3 hyperglycemia) was reported.

Conclusions: TAS-108 at 40 mg and 80 mg daily demonstrated clinical activity with an encouraging duration of benefit. Because of its superior safety profile, TAS-108 40 mg daily is recommended for further development.