TAS-108: a novel steroidal antiestrogen.

TAS-108 is a novel steroidal antiestrogen compound that has a strong binding affinity to the estrogen receptor and, in preclinical studies, has antitumor activity against tamoxifen-resistant breast cancer cell lines. Its molecular mechanisms of actions are different from those of tamoxifen and fulvestrant. TAS-108 showed tissue-selective agonist activity in the bone and cardiovascular systems and, in preclinical and phase I studies, did not show any effect on the endometrium. In a phase I study, TAS-108 was well tolerated at doses ranging from 40 to 160 mg/d with no maximum tolerated dose. Toxicities included hot flashes, headache, and nausea and vomiting. The drug has linear pharmacokinetics. In the phase I study, there was evidence of biological antitumor activity, with stable disease noted in several patients. A phase II study is ongoing, and phase III studies are being planned with the drug.