MORAb-003 inhibits folate-driven growth mediated by the human folate receptor alpha

10108 Background: MORAb-003 is a therapeutic humanized antibody currently in Phase I clinical trials for advanced ovarian cancer, and is directed against the human folate receptor alpha (FRα), an antigen which is highly overexpressed on the surface of the majority of ovarian cancers. Overexpression of FRα has been shown to confer a growth advantage to tumorigenic cells in vitro, under conditions of reduced folate availability. We have previously determined that MORAb-003 elicits robust antibody-dependent cellular cytotoxicity (ADCC) and complement-directed cytotoxicity (CDC) in vitro, and is effective in mouse xenograft models of human ovarian cancer. We now show that MORAb-003 possesses novel, growth-inhibitory functions distinct from its immune effector properties. This activity can be recapitulated, using a cell-based assay to score inhibition of folate-dependent growth. Methods: Chinese Hamster Ovary (CHO) cells were engineered to express surface FRα. The cells were cultured under a low folate (< 20 nM) regimen, then re-fed medium supplemented with various concentrations (0–10 μM) of 5-methyltetrahydrofolate (5-MTF), a folate preferentially internalized by the FRα. Cellular proliferation was measured as a function of added 5-MTF, in the presence or absence of MORAb-003. Results: FRα-expressing CHO cells were found to require approximately 100-fold lower 5-MTF than a matched, non-expressing cell line. This increased proliferation could be reduced in a concentration-dependent manner by MORAb-003. Conclusions: MORAb-003 can antagonize the activity of human FRα, resulting in the loss of growth advantage conferred by overexpression of FRα under conditions which bracket physiological (10–100 nM) folate concentrations. [Table: see text]