Abstract P6-12-13: Single domain antibody (SBT-100) inhibits growth of human HER2+ and triple negative breast cancers (TNBC) in xenografts by binding STAT3 and P-STAT3

SBT-100 is a single domain antibody (sdAb), developed by Singh Biotechnology, that binds unphosphorylated signal transducer and activator of transcription 3 (STAT3) and phosphorylated STAT3 (P-STAT3). SBT-100 is approximately 13 kD or less than 1/10 th the size of a human IgG molecule, and is able to cross the cell membrane to bind intracellular STAT3 and P-STAT3. This in turn inhibits its effects on genes that promote malignant behavior of cancer cells. SBT-100 has a short serum half-life but a long biological half-life. Since certain types of human breast cancers express P-STAT3, we wanted to determine if SBT-100 could inhibit the growth of human breast cancers in vitro and in vivo by studying its effects on MCF-7 (ER+/PR+), BT474 (HER2+), and MDA-MB-231 (TNBC) cells. BACKGROUND: Many different types of human cancers (solid tumors, leukemias, and lymphomas) are dependent on constitutive expression of (P-STAT3) for their malignant phenotype. Growth factors, tyrosine kinase receptors, cytokines (IL-6, IL-11, IL-12, IL-23), BCR-ABL, and Src are some ways that STAT3 can be activated. In turn P-STAT3 turns on genes such as Cyclin D1 & D3, MMPs, Bcl-xL, Mcl-1, survivin, VEGF, and HIF-1 alpha. Constitutive expression of P-STAT3 has been shown to promote cancer cell proliferation, survival, angiogenesis, immune suppression, and metastasis. Additionally there is increasing evidence suggesting that unphosphorylated STAT3 contributes to malignant phenotype of cancers. STAT3 is also important for the survival of cancer stem cells as well as for some human breast cancers. METHODS: Immunoprecipitation and Western blot analyses were carried out to test whether SBT-100 binds cytoplasmic STAT3 and P-STAT3 in various malignant cell lines (e.g., MDA-MB-231, PANC-1, DU145, and HeLa). MTT assays were done to determine if SBT-100 could suppress the growth of different types of human breast cancers in vitro. Xenograft cancer models using ER+/PR+ (MCF-7), HER2+ (BT474), and TNBC (MDA-MB-231) cancer cells were used to evaluate treatment with SBT-100 1mg/kg/BID (IV and/or IP route). RESULTS: Immunoprecipitation and Western blot studies demonstrated that SBT-100 binds to both STAT3 and P-STAT3 in human cancers cells (MDA-MB-231, PANC-1, DU145, and HeLa). In a three day MTT assay, at least 90% growth suppression was achieved for all three subtypes of human breast cancer, which is highly significant. In the xenograft cancer models, SBT-100 (1mg/kg/BID) treatment for 28 days, yield growth suppression as follows: MDA-MB-231 44.8% (p CONCLUSION: Singh Biotechnology9s novel sdAb, SBT-100 suppresses growth of TNBC and HER2+ human breast cancers in vivo and suppresses growth of ER+/PR+, HER2+, and TNBC cells in vitro. The most significant anti-cancer effects of SBT-100 is observed against human TNBC. Citation Format: Singh S, Murillo G, Chen D, Singh A, Singh S, Singh A, Mehta R, Parihar A. Single domain antibody (SBT-100) inhibits growth of human HER2+ and triple negative breast cancers (TNBC) in xenografts by binding STAT3 and P-STAT3 [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P6-12-13.