How is microscopic polyangiitis (MPA) treated?

Medications that suppress the immune system form the foundation of treatment for MPA. There are a variety of immunosuppressive medications that are used in MPA, each of which has individual side effects.

People with MPA who have critical organ system involvement are generally treated with corticosteroids combined with another immunosuppressive medication such as cyclophosphamide (Cytoxan ®) or rituximab (Rituxan®). In patients who have less severe MPA, corticosteroids and methotrexate can be used initially. The goal of treatment is to stop all injuries that are occurring as a result of MPA. If disease activity can be completely "turned off," this is called "remission." Once it is apparent that the disease is improving, doctors slowly reduce the corticosteroid dose and eventually hope to discontinue it completely. When cyclophosphamide is used, it is only given until the time of remission (usually around 3 to 6 months), after which time it is switched to another immunosuppressive agent, such as methotrexate, azathioprine (Imuran®), or mycophenolate mofetil (Cellcept®) to maintain remission. The treatment duration of the maintenance immunosuppressive medication may vary between individuals. In most instances, it is given for a minimum of 1 to 2 years before consideration is given to possibly slowly reduce the dose toward discontinuation.

All of these medications are also used to treat other medical conditions. Azathioprine and mycophenolate mofetil is used to prevent organ transplant rejection. Methotrexate is used to treat rheumatoid arthritis and psoriasis. Both cyclophosphamide and methotrexate are given at high doses as a treatment for certain types of cancer and therefore are sometimes referred to as "chemotherapy." In cancer treatment, these medications work by killing or slowing the growth of rapidly multiplying cancer cells. In vasculitis, these medications are given at doses that are 10 to 100 times lower than those used to treat cancer, and their primary effect is to influence the behavior of the immune system in a manner that results in immunosuppression. Rituximab belongs to a class of medications called biologic agents that target a specific element of the immune system. Recent studies found that rituximab was as effective as cyclophosphamide for treating severe active MPA.

Because these medications suppress the immune system, there is an increased risk of developing serious infections. Each immunosuppressive drug also has a unique set of potential side effects. Monitoring for the side effects associated with each drug is critical to prevent or minimize their occurrence. Also, the fact that a patient may initially tolerate treatment does not guarantee that tolerance will remain the same over time. This makes ongoing monitoring essential, and in some instances, monitoring for long-term side effects may be important even after the drug is stopped.

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