US-based biopharmaceutical firm Arno Therapeutics has started dosing in its Phase II component of a modified Phase I/II clinical trial evaluating its lead compound onapristone, an oral, anti-progestin hormone blocker, in women with progesterone receptor expressing tumours.

About 29 patients with recurrent or metastatic endometrioid tumours that have been shown to express the activated progesterone receptor (APR), and who have received no more than one prior chemotherapy and no prior hormone therapy, will be enrolled in the Phase II trial.

Each patient in the Phase II component of the trial will receive 50mg of extended release onapristone twice daily.

The trial protocol provides that if at least two of the first ten patients have a confirmed response by RECIST 1.1 criteria, an additional 19 patients will be enrolled, for a total of 29 patients.

"Each patient in the Phase II component of the trial will receive 50mg of extended release onapristone twice daily."

The Phase II trial will also include a diagnostic test targeting women with tumours expressing the APR, which is intended to select those patients most likely to respond to onapristone treatment.

Arno Therapeutics chief executive officer Alex Zukiwski said: "We are pleased that we were able to quickly implement the Phase II study in patients with APR positive endometrioid tumours, which is the same patient population of our planned registration programme.

"The current study has been ongoing in France and we intend to open additional investigative sites both in France and the United States early next year."

Onapristone has been shown in previous Phase II clinical trials to exhibit anti-tumour activity in patients with breast cancer.

In pre-clinical trials, onapristone has been shown to block the activation of the progesterone receptor (PR), which is believed to be a mechanism that inhibits the growth of APR driven breast, endometrial and other, primarily gynaecological tumours.

The ongoing, open-label Phase I/II trial is currently enrolling patients and is designed to evaluate the safety of onapristone in the selected patient population, determine the recommended Phase II dose as well as evaluate any antitumor activity.

Currently, this multi-site trial is enrolling patients in France and is expected to start enrolling patients in the US in early 2015.