Rituxan (rituximab) is a genetically engineered chimeric murine-human monoclonal antibody drug that recognizes and binds specifically to the CD20 molecule. CD20 is a protein biomarker displayed on B lymphocytes (B cells for short) — a type of immune cells.

Rituxan was approved by the U.S. FDA for the treatment of B cell non-Hodgkin’s lymphoma (of particular stages and types) and rheumatoid arthritis. About 63,000 new cases of non-Hodgkin’s lymphoma were diagnosed in the U.S. in 2007. There are presently about 2.4 million patients with rheumatoid arthritis in the U.S.

Rituxan targets CD20-expressing non-Hodgkin’s lymphoma B cells and normal mature B cells. It kills these cells by triggering apoptosis (programmed cell death or suicide) and marking them for destruction by powerfully armed immune reconnaissance and surveillance components of the immune system such as natural killer cells (NK cells). The mechanism of action of Rituxan in its therapeutic effects on rheumatoid arthritis is less clear. B cells play a secondary role in instigating inflammation of joints — what ails rheumatoid arthritis patients.

To patients’ relief, CD20 is not found on precursor/immature B cells. Therefore, precursor B cells can differentiate and mature and subsequently replenish the supply of normal mature B cells decimated by Rituxan.

Biogen Idec makes Rituxan. It is co-marketed by Genentech and Biogen Idec. It had worldwide sales of $3.1 billion ($2.2 billion to Genentech and $926 million to Biogen Idec) in 2007.

[Originally written for a company's weekly newsletter. With permission.]