(472a) Dendrimer-Mediated Multivalent Binding Enhances the Specificity and Sensitivity of Tumor Cell Detection

Accurate enumeration of circulating tumor cells (CTCs) is of clinical importance in diagnosis and prognosis of cancer metastasis; however the rareness of CTCs, comprising as few as one tumor cell in the background of 10⁶-10⁹ normal blood cells, has made the effective detection of the cells extremely difficult.  Multivalent interactions, simultaneous binding of multiple ligands toward their multiple receptors, commonly occur in physiological and pathological events, which have been utilized to enhance targeting efficacy.  The multivalent binding can be leveraged to develop an extremely sensitive device for detection of rare CTCs.    Here we report a novel surface chemistry that utilizes the multivalent effect for a highly stable cancer cell capture.  Poly(amidoamine) (PAMAM) dendrimers have been reported as an excellent multivalent binding mediator due to their deformability and multiple functional groups.  PAMAM dendrimers were conjugated with anti-epithelial cell adhesion molecules (aEpCAM), a commonly used antibody for CTC detection.  A direct, quantitative analysis using surface plasmon resonance (SPR) revealed that the aEpCAM-dendrimer conjugates exhibited a dramatically enhanced binding avidity by a million-fold, compared to the binding affinity of free aEpCAM.  The immobilization of dendrimers and aEpCAM on the surfaces were confirmed using x-ray photoelectron spectroscopy and fluorescence microscopy.  The surfaces were tested using in vitro breast cancer cell lines (MDA-MB-361, MCF-7, and MDA-MB-231 cells) as a CTC model.  Surface immobilization of the dendrimer-aEpCAM conjugates resulted in significantly enhanced tumor cell detection than the surfaces with the linear PEG-aEpCAM conjugates with higher binding stability over agitation (up to 69.1-fold).  The enhancement achieved through the multivalent binding effect was further increased up to 7-fold under flow with addition of E-selectin that effectively recruits the cancer cells onto the surface and induces their rolling.  This study demonstrates a novel surface engineering approach to exploiting the strong multivalent binding for sensitive detection of tumor cells, which has great potential for clinically significant detection of CTCs.