Prognostic Value of Estrogen Receptor Co-activator AIB1 in Metastatic Breast Cancer Patients Treated with a Combination of Ribociclib and Letrozole

Purpose The steroid receptor co-activator protein family members are transcriptional regulators that interact with steroid receptors, mainly estrogen receptors. We assessed the prognostic role of SRC-3, AIB1, in metastatic breast cancer patients treated with ribociclib and letrozole in combination therapy. Methods Forty-two patients were included in this retrospective study. AIB1 level was determined by immunohistochemical methods. The AIB1 expression score was calculated as the sum of a proportion score and an intensity score, representing the estimated proportion of stained cell nuclei. Results The median PFS (mPFS) of all patients was 22.4 months. Univariate cox regression analysis was performed to identify prognostic factors. Low AIB1 expression score (HR = 0.33, 95% CI 0.12-0.96, p = 0.042), low ER level (HR = 0.97, 95% CI 0.95-0.99, p = 0.001) and presence of liver metastasis (HR = 5.30, 95% CI 2.14-13.13, p < 0.001) were found to be poor prognostic factors. A statistically significant correlation was found between AIB1 expression score and prior chemotherapy, prior hormonotherapy, presence of bone metastasis, liver metastasis and PgR level (p = 0.08, p = 0.018, p = 0.041, p = 0.040, and p = 0.007, respectively). Conclusions We believe that the AIB1 score, which was investigated for the first time in this study and whose prognostic features have been presented, may be a guide in identifying high-risk patients requiring close follow-up in patients receiving the combination of ribociclib and letrozole and in investigating further targeted therapies.