Abstract #LB-153: Evaluation of non-functional P2X7 receptor as a potential pan cancer therapeutic and diagnostic target (2009)

Objectives: Preclinical studies to determine the effectiveness of three anti-non-functional P2X7 (nf-P2X7) antibodies in binding to and inhibiting growth of a wide range of human tumors in vitro and in vivo. Methodology: The pan cancer nature of nf-P2X7 expression was determined using routine immunohistochemistry on a broad range of human tumors. The nf-P2X7 receptors expressed on the surface of the cells of many types of cancer are unable to form apoptotic pores upon exposure to the agonist ATP. Pore function may be determined by measuring the uptake of dyes such as ethidium bromide that are able to penetrate the open pore in functional receptors but not in non-functional receptors. Reactivity of three anti-nf-P2X7 antibodies with viable human tumor cells was confirmed by indirect immunofluorescence and flow cytometry on a panel of human cell lines representing a similar range of human tumor types. Growth inhibition studies were completed on these human tumor cell lines, in vitro, using the Cell Titer Blue (CTB)TMassay. Preliminary Data: Purinergic P2X7 receptors form ATP-gated cation-selective channels that can mediate programmed cell death, or apoptosis, in hematopoietic cells. Whereas cells from solid tissues such as prostate, breast, bowel, lung and skin are normally devoid of P2X7, malignant forms of these tissues are associated with nf-P2X7 expression. This expression of nf-P2X7 on the malignant tissues may result from the transformed cells circumventing normal apoptotic mechanisms and becoming immortal. Biosceptre\#8217;s antibodies, a mouse monoclonal and two fully human domain antibodies, are specific for epitopes unique to the non-functional form of P2X7. Using flow cytometry, these antibodies were shown to bind to 28 human tumor cell lines, including prostate, breast, colorectal, lung, melanoma, renal, ovarian, cervical, pancreatic, liver, brain and hematopoietic. In contrast, none of the nf-P2X7 specific antibodies bound to human peripheral blood lymphocytes, which express the functional form of P2X7. In vitro antibody mediated growth inhibition studies were completed on 14 of these cell lines (PC3, LNCaP, MCF-7, MDA-MB-231, Colo-205, NCI-H460, NCI-H520, NCI-H596, A375, MDA-MB-435, 786-O, SK-OV-3, PANC-1 and U937). Tumor cell growth in the presence of individual antibodies, was measured after 2-5 days, by the CTBTM assay. All of these cell lines were inhibited in both a time- and dose-dependent manner. In vitro growth inhibition occurred at antibody concentrations that are easily achievable in blood. In vivo growth inhibition studies with these 3 antibodies are currently in progress in xenograft models. Representative data, including the binding to a range of tissue and cell lines and the effect on cell growth in vitro and in vivo will be presented. Mechanisms of action of the different antibodies that include inhibition of cell division, apoptosis and ADCC will be discussed. Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr LB-153.