Abstract 1772: Immunotherapy of colorectal cancer by the T-cell targeted DART® protein MGD007: Cellular mechanisms of action

Introduction MGD007 (glycoprotein A33 x CD3), a DART protein designed to redirect T cells to target gpA33-expressing colon cancer, is presently undergoing clinical evaluation (NCT02238805). The gpA33 target was selected based on its universal expression profile across primary and metastatic CRC, including expression on putative cancer stem cell (CSC) populations. MGD007 activity in CRC cell cytolysis and its prolonged PK in non-human primates have previously been reported (Cancer Res 2014;74(19 Suppl): Abstract nr 669.1). Here we further characterize MGD007 cellular mechanisms associated with redirected T-cell killing, cytokine responses and modulation with steroids. Method Redirected killing assays were performed using luciferase labelled gpA33+ve Colo205 or RECA0201-GF colorectal cancer stem-like cells (CSLC) with freshly isolated PBMC or fractionated T-cell populations; Treg cells (CD4+, CD127lo, CD25+) were expanded for 14 days in presence of IL-2 and rapamycin and confirmed to be suppressive; steroids (budesonide and dexamethasone) were evaluated at pharmacologically relevant concentrations; multi-parameter FACS and ELISA were performed to determine cell surface marker expression and cytokine levels respectively. Results MGD007 displays potent redirected T-cell killing of gpA33+ve CRC cells, including complete lysis of CRC stem cell-like models. Importantly, MGD007 mediated cytolysis can be supported by various T-cell populations, including Treg cells. Following prolonged in vitro exposure to MGD007 and gpA33+ve tumor cells, expanded T cells acquire a memory phenotype and retain potent CTL activity when challenged with fresh gpA33+ve target cells; however, much decreased cytokine release was observed compared to that observed following initial T-cell exposure. The addition of dexamethasone or budesonide to freshly isolated effector cells and gpA33+ve target cells also reduces cytokine release levels to baseline in the presence of MGD007, with minimal impact observed on MGD007-mediated killing. Discussion MGD007 supports targeted lysis of CRC, including CSC subpopulations, and can leverage suppressive Tregs in addition to conventional T cells for cytolytic activity. Biological activity modulation is also feasible through induction of cytolytic Tmem cells with diminished cytokine release potential via sequential exposure to MGD007 or the simultaneous exposure to low-dose steroids. These data support further clinical development of MGD007 for the treatme