PO-038 PDGFRβ as a new biomarker for metastatic triple-negative breast cancer: development of a theranostic anti-PDGFRβ aptamer for imaging and suppression of metastases

Introduction Triple-negative breast cancers (TNBCs) are a heterogeneous group of aggressive tumours lacking oestrogen and progesterone receptors and HER2 receptor, thus excluding the possibility of using targeted therapy against these proteins. Mesenchymal-like (ML) subtype, characterised by a stem-like, undifferentiated phenotype, is more invasive and metastatic than other TNBC subtypes and has a strong tendency to form vasculogenic mimicry (VM). Recently, platelet derived growth factor receptor β (PDGFRβ) has been shown to play a role in VM of TNBC. Regrettably, therapies targeting PDGFRβ with tyrosine kinase inhibitors are not effective in treating TNBCs, thus developing new strategies to target PDGFRβ in TNBC patients is crucial to improve their chances of survival. Here, we describe the characterisation of the Gint4.T anti-PDGFRβ nuclease-resistant RNA aptamer as high efficacious theranostic tool for imaging and suppression of ML TNBC metastases. Material and methods Immunohistochemical analyses on a human TNBC tissue microarray was performed to correlate PDGFRβ expression with clinical and molecular features of different subtypes. Functional assays were conducted on PDGFRβ-positive ML BT-549 and MDA-MB-231 cells to investigate the effect of Gint4.T in interfering with cell growth in 3D conditions, migration, invasion and VM formation. Gint4.T was conjugated with near-infrared (NIR) fluorescent VivoTag-S680 and its binding specificity to receptor was confirmed both in vitro (confocal microscopy and flow cytometry analyses of TNBC cells) and in vivo (fluorescence molecular tomography in mice bearing TNBC xenografts). MDA-MB-231 cells were i.v. injected in nude mice and Gint4.T-NIR was used to detect lung metastases in mice untreated or i.v. injected with Gint4.T or a scrambled aptamer. Results and discussions The expression of PDGFRβ was observed in human TNBC samples characterised by higher metastatic behaviour. Treatment of TNBC cell lines with Gint4.T aptamer blocked their invasive growth and vasculogenic properties in 3D culture conditions, and strongly reduced cell migration/invasion in vitro and metastases formation in vivo . The Gint4.T-NIR was able to specifically bind to TNBC xenografts and detect lung metastases in vivo . Therefore, the aptamer revealed a high efficacious theranostic tool for imaging and suppression of TNBC metastases. Conclusion These studies indicate PDGFRβ as a new biomarker for ML and metastatic TNBC subtype and propose a novel targeting agent for the diagnosis and treatment of metastatic TNBCs.