Effect of AURKA targeting on nuclear PD-L1 and the efficacy of immune checkpoint inhibitors in triple-negative breast cancer.

e13153 Background: Aurora-A mitotic kinase (AURKA) plays a role in Triple Negative Breast Cancer (TNBC) progression through activation of epithelial to mesenchymal transition (EMT)-mediated cancer cell plasticity. Cancer cells that undergo EMT acquire a CD44 high /CD24 low and/or ALDH high cancer stem-like phenotype (Cancer Stem Cells) characterized by intrinsic drug resistance. Cancer Stem Cells (CSCs) shows high PD-L1 expression that induces tumor immune escape through CD8+ T-cells exhaustion. Significantly, “undraggable” nuclear PD-L1 may play a key role in induction of immune therapy resistance and cancer cell plasticity. Methods: (1) TNBC Models: MDA-MB 231, unique MDA-MB 231/LM Durvalumab resistant (DUV-R) and TNBC-M40 cells established from metastatic PDXs; (2) In Vitro Studies: TNBC cells were treated with the AURKA inhibitor alisertib (50nM). PD-L1 expression was evaluated by immunoblotting. Apoptosis was measured by immunofluorescence using Cleaved-PARP antibodies. To assess stemness capacity, TNBC cells were cultured under non-adherent conditions to form mammospheres. ALDH activity (functional stemness marker) was measured using the Aldefluor Kit. (3) In Vivo Studies: 1x10 6 MDA-MB 231/LM cells were injected into the mammary fat pad of humanized NSG-CD34+ female mice. Animals were treated with anti-PD-L1 ICI atezolizumab (20 mg/Kg) and alisertib (25 mg/Kg) as monotherapy or in combination. 1x10 6 MDA-MB 231/LM cells were also injected into the mammary fat pad of humanized NSG-CD34+ female mice and treated with durvalumab (20 mg/Kg) to develop durvalumab-resistant models. PD-L1, CD44, Vimentin expression and tumor infiltration of CD8+ T-cells were assessed by immunofluorescence. Results: Treatment of MDA-MB 231/LM xenografts with alisertib decreased the levels of CD44 and PD-L1 expression and increased CD8+ T-cells tumor infiltration. Because PD-L1 genetic targeting enhanced in vitro alisertib-induced apoptosis, MDA-MB 231/LM xenografts were treated with alisertib and the anti-PD-L1 ICI atezolizumab. Com- bination of atezolizumab and alisertib resulted in the reduced expression of the EMT marker vimentin that was linked to lack of organ metastasis. Combination of atezolizumab and alisertib also induced a significant reduction of ALDH High CSC in ex-vivo 3D-Organoids established from TNBC-M40 cells. MDA-MB 231/LM DUV-R cells showed increased nuclear PD-L1 compared to parental cells. The combination of alisertib with Immune Checkpoint Inhibitors (ICIs) induced apoptosis of MDA-MB 231/LM DUV-R cells through down-regulation of intra-tumoral nuclear PD-L1 expression. Conclusions: This study provides innovative pre-clinical rationale for combining AURKA inhibitors with ICIs to reduce “ undruggable” nuclear PD-L1 expression and impair cancer cell plasticity of metastatic TNBCs that currently have limited effective therapeutic options.