Rituximab therapy of lymphoma is enhanced by orally administered (1 → 3),(1 → 4)-d-β-glucan

Abstract

By activating complement, antitumor monoclonal antibodies coat tumor cells with iC3b. β-glucans, naturally occurring glucose polymers, bind to the lectin domain of the leukocyte receptor CR3, prime it for binding to iC3b, and trigger cytotoxicity of iC3b-coated tumor cells. We studied the combination of the complement-activating antibody rituximab with barley-derived (1 → 3),(1 → 4)-β-d-glucan (BG) against CD-20 positive lymphoma xenografts in SCID mice. Growth of established subcutaneous non-Hodgkin's lymphoma (NHL) (Daudi and EBV-derived B-NHL) or Hodgkin's disease (Hs445 and RPMI6666) was significantly suppressed in mice treated with a combination of intravenous rituximab and oral BG, when compared to mice treated with rituximab or BG alone. Survival of mice with disseminated lymphoma was significantly increased in the combination group as compared to other treatment groups. No clinical toxicity was observed. The therapeutic efficacy and lack of toxicity of this combination supports further investigation into its clinical utility.

Introduction

The chimeric anti-CD20 antibody rituximab is being evaluated in an increasing number of disorders. After clinical efficacy was initially demonstrated against relapsed and refractory follicular/low grade non-Hodgkin's lymphoma, responses to rituximab have been reported in other malignant and non-malignant B-cell disorders. Several mechanisms of action have been proposed including activation of apoptotic pathways, elaboration of cytokines and elicitation of host complement-dependent cytotoxicity (CDC) and antibody-dependent cell-mediated cytotoxicity (ADCC). Although many patients with B-cell disorders respond to rituximab, remissions are often transient. More than 50% of lymphomas recurrent after rituximab treatment failed to respond the second time. Mechanisms of resistance to rituximab are as yet unclear, and may include paucity or loss of target antigen, pharmacokinetic variations among individual patients, FcR polymorphism, resistance to complement activity or inherent gene expression of the lymphoma [1].

The iC3b receptor, CR3 is found on monocytes, macrophages, neutrophils, NK cells and cytotoxic T-lymphocytes. CR3 activation requires the engagement of two sites on its α-subunit (CD11b): the iC3b binding site within the I-domain at the N-terminus and a lectin site at the C-terminus. β-glucans, complex polymers of glucose extractable from barley, mushrooms, seaweed and yeasts bind specifically to the lectin site, as demonstrated initially by Ross et al. [2]. With bound β-glucan, CR3 is primed to engage iC3b fragments deposited on cells by complement-activating antibodies such as rituximab. CR3 mediates the diapedesis of leukocytes through the endothelium and stimulates phagocytosis, degranulation and tumor cytotoxicity. Many fungi present β-glucan or β-glucan-like CR3 binding ligands on their cell surface. Hence, when iC3b deposition occurs, both CD11b and lectin sites become engaged, and phagocytosis and respiratory burst is triggered. In contrast, tumor cells lack such molecules, and even when coated with iC3b do not generally activate CR3 and cannot activate leucocytes. Soluble forms of β-glucan bind to lectin sites and prime both phagocytic and NK cells to kill iC3b-coated tumor targets [3], [4].

(1 → 3),(1 → 4)-d-β-glucan (BG), a soluble, barley-derived β-glucan has advantages over previously studied (1 → 3),(1 → 6)-β-glucans, particularly efficacy when administered orally and a good safety profile [2]. We recently demonstrated in vivo synergism between BG and the complement-activating antibody 3F8 against human neuroblastoma xenografts [3], [4]. We now report on the synergism between BG and rituximab against lymphoma.