Past, Present, and Future of Rituximab-The World's First Oncology Monoclonal Antibody Therapy

Abstract

Rituximab is a chimeric mouse/human monoclonal antibody (mAb) therapy with binding specificity to CD20. It was the first therapeutic antibody approved for oncology patients and was the top-selling oncology drug for nearly a decade with sales reaching $8.58 billion in 2016. Since its initial approval in 1997, it has improved outcomes in all B-cell malignancies, including diffuse large B-cell lymphoma, follicular lymphoma, and chronic lymphocytic leukemia. Despite widespread use, most mechanistic data have been gathered from in vitro studies while the roles of the various response mechanisms in humans are still largely undetermined. Polymorphisms in Fc gamma receptor and complement protein genes have been implicated as potential predictors of differential response to rituximab, but have not yet shown sufficient influence to impact clinical decisions. Unlike most targeted therapies developed today, no known biomarkers to indicate target engagement/tumor response have been identified, aside from reduced tumor burden. The lack of companion biomarkers beyond CD20 itself has made it difficult to predict which patients will respond to any given anti-CD20 antibody. In the past decade, two new anti-CD20 antibodies have been approved: ofatumumab, which binds a distinct epitope of CD20, and obinutuzumab, a mAb derived from rituximab with modifications to the Fc portion and to its glycosylation. Both are fully humanized and have biological activity that is distinct from that of rituximab. In addition to these new anti-CD20 antibodies, another imminent change in targeted lymphoma treatment is the multitude of biosimilars that are becoming available as rituximab's patent expires. While the widespread use of rituximab itself will likely continue, its biosimilars will increase global access to the therapy. This review discusses current research into mechanisms and potential biomarkers of rituximab response, as well as its biosimilars and the newer CD20 binding mAb therapies. Increased ability to assess the effectiveness of rituximab in an individual patient, along with the availability of alternative anti-CD20 antibodies will likely lead to dramatic changes in how we use CD20 antibodies going forward.

Keywords: cancer; immunotherapy; lymphoma; monoclonal antibody; rituximab.

Figure 1

Rituximab development timeline. Key milestones leading to the development of rituximab and additional CD20 monoclonal antibodies (mAbs) for use to treat B-cell non-Hodgkin lymphoma.

Figure 2

CD20 is a transmembrane protein. The large and small extracellular loops and the general binding site of rituximab are depicted.

Figure 3

Engineered differences between Food and Drug Administration approved anti-CD20 monoclonal antibodies (mAbs). Rituximab is a chimeric mAb that is partially humanized, that has a human Fc portion but retains the murine variable region which recognizes CD20. Both ofatumumab and obinutuzumab are fully humanized mAbs, which reduces unintended immune responses against the therapies. Ofatumumab also has a glycoengineered Fc region which results in better binding with immune effector cells (106, 190).

Figure 4

Rituximab-mediated cell killing of CD20 expressing B-cells. (Top left) Binding of rituximab to CD20 can directly trigger apoptosis through both caspase-dependent and -independent mechanisms that are still not fully characterized. (Top right) Bound rituximab can recruit the C1 complex triggering the classical complement cascade which leads to insertion of the membrane attack complex (MAC) and ultimately leads to cell lysis, also known as complement-dependent cytotoxicity (CDC). (Bottom left) Bound rituximab can recruit natural killer cells via recognition by the FcγRIII leading to antibody-dependent cell-mediated cytotoxicity (ADCC). This facilitates release of perforin, which assembles into membrane compromising pores in the target cell, and granzyme B, which enters the target cell and triggers apoptosis by cleaving caspases and potentially by other methods. (Bottom right) Macrophages recognize CD20 bound rituximab through various Fcγ receptors which leads to antibody-dependent phagocytosis (ADP) of the target cell.

Figure 5

A simplified overview of rituximab manufacturing process. Once hybridoma cell lines are established from a single clone, the cultures are expanded to produce a single specific monoclonal antibody (mAb) on a massive scale. That mAb is then collected, purified, analyzed, and certified on a per lot basis.