Pamela Munster, MD, Director, Early Phase Clinical Trials Unit and Leader, Developmental Therapeutics Program, UCSF Helen Diller Family Comprehensive Cancer Center, discusses results from a phase II study that examined the histone deacetylase (HDAC) inhibitor vorinostat in combination with tamoxifen for patients with hormone therapy-resistant breast cancer.

The trial enrolled 43 patients with estrogen receptor-positive metastatic breast cancer who had developed resistance to endocrine therapy, including tamoxifen and aromatase inhibitors. The trial also evaluated HDAC2 expression and histone acetylation, as possible markers.

The trial found that 19% of patients treated with vorinostat plus tamoxifen experienced tumor shrinkage and 21% had stable disease at 6 months. However, the examination of HDAC2 and histone acetylation showed that only approximately 55% of patients responded to treatment with the combination.

Munster adds that future trials examining the combination will implement testing for HDAC2 expression and histone hyperacetylation as part of the inclusion criteria.