Preclinical In Vitro and In Vivo Evidence Support a Therapeutic Role for the CD70 Directed Monoclonal Antibody (SGN-70) in Waldenström’s Macroglobulinemia (WM).

Introduction: WM is a B-cell disorder characterized by bone marrow (BM) infiltration of lymphoplasmacytic cells (LPC), along with excess mast cells (MC) which support the growth and survival of BM LPC through multiple TNF-family ligands including CD40L, APRIL and BLyS/BAFF. Importantly, BM LPC stimulate cell surface expression of TNF-family ligands through release of sCD27 which induces CD70 on MC. We therefore have sought the development of agents which could target CD27-CD70 interactions. As such, we examined the therapeutic potential of directly targeting CD70 using the fully humanized monoclonal antibody SGN-70 (Seattle Genetics, Inc., Bothell WA).

Methods-Results: As part of these studies, we used flow cytometric analysis to evaluate the expression of CD70 on primary WM patient BM LPC and MC, as well as 2 WM cell lines (BCWM.1 and WM-WSU). These studies demonstrated cell surface expression of CD70 on BM LPC and MC from 20/26 (77%) and 10/11 (90%) WM patients, respectively. We next assessed the ability of the SGN-70 antibody to eradicate primary WM LPC (n=5) and WM cell lines by assessing for direct induction of apoptosis, complement dependent cytotoxicity (CDC), antibody-dependent cell-mediated cytotoxicity (ADCC) as well as induction of TNF family ligands on primary WM MC and the LAD2 MC line. Following incubation of WM LPC with SGN-70 (0.01–20 μg/ml), no direct induction of apoptosis or CDC activity was observed. However, SGN-70 mediated significant dose-dependent ADCC against WM LPC and MC at concentrations of 0.1–20 ug/ml. Importantly, SGN-70 blocked sCD27-induced expression of CD40L and APRIL on primary WM MC and LAD2 MC. To further evaluate the therapeutic potential of SGN-70 in an in vivo model, SCID-hu mice bearing BCWM.1 WM cells were treated with SGN-70 (1 mg/kg, i.p., qOD) Serum human IgM and sCD27 levels were measured by ELISA to monitor for tumor engraftment and disease progression. SGN-70 initiated 6 weeks following tumor engraftment blocked tumor growth in 12/12 treated mice, whereas all 5 untreated mice demonstrated disease progression. The results of these studies provide the framework for clinical trials to examine the therapeutic potential of the SGN-70 monoclonal antibody in WM.