In Vivo Depletion of Cynomolgus Monkey B Cells Targeted by a Novel Monoclonal Antibody (Xi-20H5) Specific for a Shared Portion of the Human Immunoglobulin Variable Light Chain Lambda 1.

Genitope is developing a novel, personalized treatment for surface immunoglobulin (Ig) positive B cell malignancies. This treatment is based on a panel of monoclonal antibodies (Mabs) directed against shared epitopes expressed on different subsets of the variable regions of human Ig. The concept of targeting surface tumor Ig was explored in a series of clinical trials performed by Dr. Ronald Levy and his colleagues at Stanford. In these studies, Mabs directed against patient specific (idiotypic) determinants of the surface tumor Ig produced significant clinical benefit in relapsed follicular non-Hodgkin’s lymphoma patients. A number of these patients have had long term remissions. In Genitope’s planned clinical use, each patient will receive a single Mab from the panel selected based on its reactivity with the patient’s tumor. The selected Mab will react with the patient’s tumor and a minority of normal B cells, leaving the majority of the normal B cell repertoire intact. The ability of the panel members to provide therapeutic effect requires binding to tumor surface Ig in the presence of serum containing soluble Ig molecules. Mab Xi-20H5, a member of this panel of antibodies, is specific for a shared determinant on the human Ig variable light chain lambda 1. It binds to 25 to 35% of normal human B cells from peripheral blood and to 20 to 30% of normal cynomolgus monkey B cells from peripheral blood. In this study, we sought to demonstrate that, despite the presence of serum Ig, Mab Xi-20H5 would bind to surface Ig expressed on monkey B cells in vivo, resulting in specific depletion of target B cells. Six naïve cynomolgus monkeys received 8 intravenous infusions of the Mab Xi-20H5 at a dose of 40 mg/kg on days 1, 2, 3, 4, 7, 10, 14 & 17. Two naïve control animals received 8 infusions of vehicle only following the same schedule. The frequencies of lymphocyte sub-populations and of target B cells were monitored by flow cytometry on plasma-depleted whole blood samples. Samples were collected 23 hours after each infusion. In addition, two baseline samples were collected prior to treatment. The frequencies of lymphocyte sub-populations and of target B cells were compared to the average of the two baseline measurements. Frequencies of target B cells bound by Mab Xi-20H5 decreased in all treated animals while no significant change was detectable in the control animals. The bulk of the reduction in target B cell frequencies was observed 23 hours after the first infusion (range: 22% – 62%, average 41%). Frequencies of target B cells continued to decrease moderately with additional daily infusions (days 2 – 4), resulting in maximum reduction in target B cell frequency at 23 h post infusion 4 (range: 39% – 78%, average 54%). The frequencies of total B and T lymphocytes did not significantly change during the treatment. In vivo administration of Mab Xi-20H5 results in depletion of target B cells in a manner consistent with the expectation of an immunotherapeutic Mab aimed at treating surface Ig expressing B cell malignancies.