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TMEFF2, a gene encoding a plasma membrane protein with 2 follistatin-like domains and one EGF-like domain, was identified to be highly over-expressed in prostate cancer by genome-wide expression profiling using the Eos Hu03, a customized Affymetrix GeneChip® (Henshall et al., 2003, Cancer Res 63:4196-4203). In normal tissues, TMEFF2 is exclusively expressed in prostate and central nervous system among over 70 different body tissues. In a survey of 57 different cancer cell lines and xenografts TMEFF2 was most highly expressed in the prostate cancer-derived lines LNCAP and CWR22. Antibodies raised against TMEFF2 were used in immunohistochemistry (IHC) of clinical specimens confirming the gene array data. IHC of prostate cancer patient samples demonstrated significant TMEFF2 protein expression in 73% of 216 elements of primary prostate tumors. Significant staining was also detected in 38% of 56 metastatic lesions, including lymph nodes and bone representing both hormone-naïve and castration resistant disease. The cell-surface localization and up-regulation in prostate cancer suggest. that TMEFF2 is a potential target for therapeutic antibodies. The exquisite tissue specificity of TMEFF2 expression would also allow for the use of a toxin-based antibody drug conjugate (ADC) strategy to target prostate cancer cells. We used an auristatin-E (Doronina et al., 2003, Nat. Biotechnol. 21:778-784; Bhaskar et al., 2003, Cancer Res. 63:6387-6394) conjugated anti-TMEFF2 antibody, Pr1-vcMMAE, to treat male SCID mice bearing either LNCAP tumors or CWR22 tumors. Our results show that Pr1-vcMMAE is a potent and selective agent against established xenografts. Treatment of LNCAP and CWR22 tumor-bearing mice with Pr1-vcMMAE at doses of 0.1 to 10 mg/kg resulted in significant and sustained tumor regression. This effect was specific since TMEFF2 expressing tumors were only responsive to ADCs directed to TMEFF2 and not to isotype control ADCs. Pr1-vcMMAE was well tolerated by the mice with no obvious toxicity observed. This finding is significant, considering that the anti-TMEFF2 specific antibodies recognize human and murine TMEFF2 equally well. Our results support TMEFF2 as a new target for antibody-drug conjugate therapy in advanced castration resistant prostate cancer. (In part supported by grants from the Prostate Cancer Foundation and PepsiCo to H.I. Scher and C. Cordon-Cardo)

[Proc Amer Assoc Cancer Res, Volume 45, 2004]