Abstract 4699: Single domain antibody (sdAb) localizes in cancer cells to inhibit signal transducer and activator of transcription 3 (STAT3) resulting in therapeutic inhibition of multiple cancers

STAT3 is involved in the pathogenesis of many malignancies, so we developed an anti-STAT3 VHH (variable region of the heavy chain), SBT-100, that internalizes in cancer cells and binds unphosphorylated STAT3 (U-STAT3) and phosphorylated STAT3 (P-STAT3) and results in significant inhibition of multiple cancers. ATCC cell lines for triple negative breast cancers (MDA-MB-231, MDA-MB-468, MDA-MB-453), ER+/PR+ breast cancer (MCF-7), HER2+ breast cancer (BT474), pancreatic cancers (PANC-1, BX-PC3), murine mammary cancer (4T1), STAT3 null cells (PC-3), and castrate-resistant prostate cancer (DU145) were tested. Athymic nude mice were obtained from Envigo. The IL-6 Reporter Cell Assay was obtained from Promega. VEGF inhibition ELISA was done using retinal epithelial cells (ATCC). In vitro growth inhibition was done using a MTT assay. Immunoprecipitation (IP) and western blot studies in MDA-MB-231, PANC-1, HeLa, DU145, 4T1 and PC-3 showed that SBT-100 binds U-STAT3 and P-STAT3. No STAT3 binding was seen in PC-3. Binding to P-STAT3 was seen in lysates from the 4T1 cells, which have constitutively activated STAT3. Since rodent and human STAT3 have a 99% homology, rodents are excellent models for extrapolating to human disease for over production of P-STAT3. Within 24 hrs IL-6 assay showed that SBT-100 blocked the production of IL-6 (p