Folate hydrolase-1 (FOLH1) is a type II transmembrane protein which is luminally expressed by cancer neovessels, including malignant melanoma (MM). J591, a monoclonal antibody (AB) to FOLH1, is presently being developed in clinical trials as a vehicle for AB-based brachytherapy in cancers that express FOLH1. We aim to characterize FOLH1 expression in MM.
We studied FOLH1 expression levels in normal skin, primary (p), and metastatic (m) MM using RT-qPCR. The distribution in pMM and mMM tissue and co-localization were determined using immunohistochemistry (IHC) and immunofluorescence (IF). FOLH1 protein expression in primary MM cells was studied by IF. Human Umbilical Vein Endothelial Cells (HUVEC) were cultured with recombinant (r) TNF and with MM supernatant (SN) +/- an anti-TNF-AB. Proteomic analysis was performed using mass spectrometry. Clinical application of J591 was tested in 6 mMM patients by 111In-J591 planar imaging.
Our analysis showed significantly increased FOLH1 mRNA levels in pMM (10.6-fold; p=0.004) and mMM (18.2-fold; p=0.042) as compared to normal skin. FOLH1+ neovessels were identified in 4/11 (36%) of pMM and 9/14 (64%) of mMM; FOLH1 expression correlated with dysplasia and depth of invasion. IF identified a subset of non-endothelial FOLH1+/Melan-A+ cells. 2/6 MM cell lines showed FOLH1 protein expression in 10% to 40% of cells. Notably, 100% of these cells were positive for Oct4, a widely accepted stem cell marker. Culturing of HUVEC with MM cell SN resulted in a significant up-regulation (6.5-fold, p=0.005) of FOLH1 mRNA. Proteomic analysis showed protein expression of FOLH1 in HUVEC upon treatment with rTNF. Notably, anti-TNF-treatment before and during culturing in MM cell SN resulted in 50% inhibition of FOLH1 mRNA expression. In a first clinical trial, 6 patients demonstrated avid J591 uptake at sites of mMM.
Collectively, FOLH1 could be identified as a novel player and target for AB-based brachytherapy in malignant MM.

Folate hydrolase-1 is a novel player and target for antibody-based brachytherapy in malignant melanoma