Abstract P5-04-06: Analysis of STEAP1 expression as a therapeutic target in breast cancer

Abstract Objective: The six transmembrane epithelial antigen of the prostate (STEAP1) is predominantly overexpressed in human prostate cancer. STEAP1 was first identified as a prostate-specific cell-surface antigen and found to be up-regulated in various cancers including lung, bladder, colon, and ovarian with little expression in normal tissue. An anti-STEAP1 monoclonal antibody linked to an antimitotic agent is currently in Phase I clinical trials for prostate cancer patients. Microarray data from our lab suggested that STEAP1 is also highly expressed in human breast cancers and bone marrow disseminated tumor cells. In this study we evaluate expression STEAP1 in primary tumors, and bone marrow (BM) from breast cancer patients. Experimental procedures: RNA was isolated from primary tumor, non-malignant breast tissue and bone marrow (BM) from stage II and III breast cancer patients, healthy volunteers, breast cancer cell lines and BM from patient derived xenographs (PDX). Disseminated tumor cells (DTCs) from patient BM were enriched by microfiltration and analyzed by RNA-in situ hybridization (ISH). STEAP1 RNA expression was analyzed by Nanostring nCounter and qRT-PCR using human specific probes. STEAP1 immunohistochemical (IHC) staining of human tissue was performed using standard protocols. Knockdown of steap1 expression was accomplished using a lentiviral system. Results: STEAP1 mRNA was up-regulated in 77% of tumors (28/36) compared to the corresponding normal tissue. STEAP1 protein was expressed in 100% of tumors (8/8) and was absent in non-malignant breast tissue (7/7) by IHC staining. STEAP1 mRNA was not expressed normal BM, but was detected in 8% (6/74) of BM from patients with early stage breast cancer. STEAP1 expression in the BM was associated with triple negative disease (3/6) and recurrent disease development (4/6, p=.028). STEAP1 expression was observed in individual DTCs isolated from patients BM, while no expression was observed in normal BM. In a PDX model of breast cancer, STEAP1 expression in BM was only observed in mouse who developed metastatic disease associated (7/10, p=.004). Knockdown of STEAP1 in the breast cancer cell line MDA-MB231 cells inhibited cell growth by 80-90%. Conclusion: STEAP1 is expressed in human breast tumors and disseminated tumor cells found in the bone marrow of breast cancer patients. Expression of STEAP1 in the BM is significantly associated with the development of metastatic disease in patients as well as in a mouse model of breast cancer. Our data indicate that STEAP1 could serve as a therapeutic target for the treatment of minimal residual disease in breast cancer. Citation Format: Aft R, Mudalagiriyappa C, Pillai S, Watson M. Analysis of STEAP1 expression as a therapeutic target in breast cancer. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P5-04-06.