Arno Therapeutics, Inc. announced that data from a study supporting diagnostic development for its lead compound onapristone will be presented at a poster session at the 2014 San Antonio Breast Cancer Symposium (SABCS), being held December 9-13, 2014 in San Antonio, Texas. The study evaluated the impact of using a progesterone receptor (PR) isotype-specific immunohistochemical (IHC) diagnostic technique on the diagnosis of triple-negative breast cancer tumors, which are defined as estrogen receptor (ERα), PR and HER2 negative. PR cellular expression is thought to be linked to ERα expression in breast cancer.

Triple-negative breast cancer may express PR when tested with two forms of the PR: PRA and PRB antibodies. Routine IHC determination of PR utilizes "bispecific" antibodies which recognize epitopes common to both PRA and PRB. Ultimately, the accuracy of triple-negative phenotype determination depends in part on the ability of the antibody to detect both PRA and PRB expression in the tumor.

The study evaluated 775 archived breast cancer tumor samples from female patients diagnosed with invasive ductal breast cancer provided by Oscar Lambret Hospital in Lille, France. The samples were analyzed for ERα, PRA and PRB. Of the samples, 89 cases were determined to be ERα negative (by retesting) and HER2 negative (from patient records).

Evaluating the PR status with IHC staining demonstrated that 53 of the 89 cases (60%) were found to be 'true' triple negative using isotype specific antibodies for PRA and PRB. Furthermore, results showed that 21 of the 89 (24%) were PR positive using the PRA and PRB antibodies tested and 15 of the 89 (17%) were found to be PR positive with either the PRA or PRB antibody and not with the PR bispecific antibody used in standard PR testing. The use of PRA and PRB isotype-specific antibodies may reveal breast cancer cases that are not necessarily triple negative.