Rituximab (Rituxan, Mab Thera) is the most commonly used preparation in this category, although others including a pill version are in clinical trials. Rituximab is a monoclonal antibody approved by the FDA in November 1997 for use in the treatment of mild cases of B-cell non-Hodgkin Lymphoma (NHL), a type of cancer, and in 2006 for Rheumatoid Arthritis. (1)
Rituximab reduces the number of B cells in your body. B cells are a type of white blood cell that, when activated, multiply and produce antibodies. Antibody production is triggered by antigens. An antigen and antibody fit together like a key in a lock. When an antigen lock (in this case CD20) and antibody key (anti-CD20 in rituximab) are joined in a B cell, the body eliminates the complex since the B cell's work is assumed to be done.
Since Rituxan reduces the number of B cells that contain CD20, it reduces the total number of cells that produce antibodies. This may include the antibodies that attach to platelets. There is also evidence that rituximab alters T cells, another type of white blood cell, and that this may be the reason that rituximab raises the platelet count of some patients with ITP. (5)
The short term response rate for Rituxan is about 60%. About 40% of patients achieve a longer term rise in platelet count.6
It can be given before or after a splenectomy. However, systemic infection is a concern 8, especially in combination with other multiple immune suppressive therapies.
Note: The B cells that are eliminated are not specific B cells that target cancer or ITP. Rituxan reduces the general population of all B cells that include CD20. It could take up to nine months for someone to replace those B cells and have their immune system and antibody production (including the helpful antibodies) back in working order.
Dosage
The usual dose for patients with ITP is 375 mg/m2 once weekly for four weeks, the same regimen used for NHL patients. In some small trials, lower doses have been shown to be as effective as the higher dose in raising platelet counts.3,4
Rituxan is to be given only as an IV infusion, not an IV push or bolus. Patients with pre-existing cardiac and pulmonary conditions or who experienced cardiopulmonary adverse events in the past need to be closely monitored 2
The manufacturer recommends premedication with acetaminophen (Tylenol) and an antihistamine (ex. Benadryl) before each infusion.2
Side Effects
About 77% of patients have infusion reactions such as fever, chills, weakness, nausea and headaches with the first dose. These reactions often decrease with subsequent doses.2 Rituximab can prompt serious (ex. serum sickness), sometimes fatal, infusion reactions. These usually occur from 30 to 120 minutes after administering the first dose.2 In a review of published studies about 3% of ITP patients treated with Rituxan had become extremely ill or died.6
"Physicians who are thinking about treating a patient with Rituxan for any condition should tell their patients about the chance of PML [ progressive multifocal leukoencephalitis] because there is no effective treatment for the disease."7
1. http://www.fda.gov/cder/drug/infopage/rituximab/rituximabQA.htm
2. http://www.gene.com/gene/products/information/pdf/rituxan-prescribing.pdf (package insert)
3. http://www.ncbi.nlm.nih.gov/pubmed/18403395 (Haematologica. 2008 Jun;93(6):930-3.
4. http://www.ncbi.nlm.nih.gov/pubmed/18055995 (Haematologica. 2007 Dec;92(12):1695-8)
5. http://www.ncbi.nlm.nih.gov/pubmed/17548576 (Blood. 2007 Oct 15;110(8):2924-30).
6. http://www.ncbi.nlm.nih.gov/pubmed/17200219 (Ann Intern Med. 2007 Jan 2;146(1):25-33.)
7. http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/psn/printer.cfm?id=501
8. http://www.ncbi.nlm.nih.gov/pubmed/19000786 (Int Immunopharmacol. 2009 Jan;9(1):10-25)
