Abstract 3640: Potential cooperation of monoclonal antibody based targeting of gamma-glutamyl transferase (GGT1) with chemotherapy in kidney tumor xenografts

Gamma-glutamyl transferase (GGT1) is a cell membrane bound enzyme that functions in gamma-glutamyl cycling, to provide cells with intracellular glutathione (GSH). GGT1 has been recognized to participate in chemotherapeutic resistance of tumors to carboplatin and various alkylating agents by increasing GSH levels intracellularly, thus allowing the tumor to escape electrophilic damage. Cysteinylglycine generated by membrane bound GGT1 can bind to extracellular cisplatin and prevent it from entering the cell. Mechanisms that target GGT1 and alter its function have been postulated to increase tumor sensitivity to conventional chemotherapies. We have generated a novel monoclonal antibody (mAb), 8G9 [IgG2a, k1] that recognizes GGT1. An immunohistochemical (IHC) survey of benign adult human tissues revealed an antigen distribution in formalin-fixed, paraffin embedded tissues consistent with previous reports of GGT1 expression. A subsequent IHC screen of nearly 100 kidney tumors revealed that scoring based on 8G9 immunoreactivity showed a preference for clear cell (94.45%), multilocular cystic (100%) and papillary cell (95.83%) kidney tumors. A statistical association of 8G9 immunostaining with Fuhrman nuclear grade or pathology staging was not evident. FACS analysis confirmed that 8G9 recognizes a cell surface epitope. Cross-immunoprecipitations performed with 8G9 and a commercially available GGT1 antibody confirmed that 8G9 recognizes GGT1. 8G9 exhibited a unique preference for native GGT1 likely through recognition of a combined epitope formed by the association of the GGT1 heavy and light chains. Because 8G9 appears to recognize a native, surface accessible GGT1 epitope, we performed xenograft studies to test the efficacy of 8G9 in blocking kidney tumor growth either as a single agent or in combination with cisplatin or carboplatin. Athymic nude mice were engrafted with the human kidney tumor cell line Caki-1. Upon successful tumor engraftment, mice were treated with 8G9, cisplatin or carboplatin as single agents or with 8G9+ ciplatin or 8G9 + carboplatin. As a single agent, 8G9 treatment did not result in a reduction of relative tumor volume. Likewise, there was little difference in relative tumor volume in mice treated a combination of 8G9 and carboplatin. However, 8G9 + cisplatin treatment exhibited a modest reduction in tumor volume. These preliminary data suggest that targeting GGT1 may have a cooperative effect in reducing tumor growth in the presence of