Onapristone, a progesterone receptor antagonist, as first-line therapy in primary breast cancer.

The progesterone receptor antagonist, Onapristone, is an effective endocrine agent in experimental breast cancer models. This study aimed to investigate this agent as first-line endocrine therapy in patients with breast cancer. However, owing to the recognition in this and other clinical studies that some patients on Onapristone developed liver function test abnormalities, the development of this drug and recruitment to the study stopped in 1995. 19 patients either with locally advanced breast cancer (n = 12) or who were elderly, unfit patients with primary breast cancer (n = 7) received Onapristone 100 mg/day. Seventeen of the 19 tumours expressed oestrogen receptors (ER) whilst 12 of the 18 tumours tested expressed progesterone receptors (PgR). Tumour remission was categorised by International Union Against Cancer criteria. One patient was withdrawn after 4.5 months while her disease was static. Of the remaining 18 patients, 10 (56%) showed a partial response and 2 (11%) durable static disease (> or = 6 months), giving an overall tumour remission rate of 67%. The median duration of remission was 70 weeks. Transient liver function test abnormalities developed in a number of patients, mainly during the first 6 weeks of treatment. In conclusion Onapristone can induce tumour responses in human breast cancer.