Abstract 2518: Folate hydrolase-1 is a novel target for J591-brachytherapy in Merkel cell carcinoma

Folate hydrolase-1 (FOLH1) is a type II transmembrane protein. Oncologically, FOLH1 is upregulated throughout prostate cancer cells; it is also luminally expressed by the neovasculature of most solid tumors but not by normal vessels. J591, a monoclonal antibody (AB), is specific to, and is effectively endocytosed after extracellular binding to, FOLH1. J591 is presently being developed in clinical trials as a vehicle for AB-based brachytherapy in FOLH1+ cancers. Merkel cell carcinoma (MCC) is a rare, neuroendocrine tumor; metastatic (m) MCC is associated with poor survival. We characterized FOLH1 expression in MCC to determine its target potential for J591-brachytherapy. Paraffin sections from primary (p) and mMCC were deparaffinized and rehydrated. Samples were stained with 3E6 (DAKO), a mouse IgG1 monoclonal anti-human FOLH1. Mouse IgG1 (10 ug/mL in 1% bovine serum albumin) was used as an isotype-matched negative control. Anti-CD31 (IgG1) was used as a positive control. Kaplan-Meier survival curves were calculated based on patient outcome data and FOLH1 expression. 81 MCC tumors were evaluated. 67% (54/81) of all cases with 77% (24/31) of pMCC and 60% (30/50) of mMCC tumors demonstrated FOLH1+ neovessels. No cellular staining of tumor cells was identified. 34 patients with FOLH1 +/- MCC were demographically homogeneous in terms of sex, age, immunosuppression status, prior therapies, stage at diagnosis, and local or distant recurrences. No significant differences were detected based on FOLH1 status, in regards to MCC specific survival (P=0.905), or overall survival (P=0.687), as measured from time of diagnosis. FOLH1 is expressed in the majority of pMCC and mMCC cases and is nonprognostic. Our findings support further investigation of targeted therapy with J591-brachytherapy for the management of MCC. Citation Format: Barbara Meier, Marigdalia K. Ramirez-Fort, Kristina Lachance, Candice D. Church, Sean S. Mahase, Joseph M. Jenrette, Christopher S. Lange, Lars E. French, Paul Nghiem, Neil H. Bander. Folate hydrolase-1 is a novel target for J591-brachytherapy in Merkel cell carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2518.