Tecfidera (dimethyl fumarate) is licensed to treat relapsing remitting multiple sclerosis in adults. Dimethyl fumarate is known to cause severe lymphopenia in some patients.
The single case of PML reported with dimethyl fumarate to date occurred in a patient who had been receiving the drug for 4.5 years and had experienced severe lymphopenia for more than 3.5 years.
In view of the potential risk of PML associated with dimethyl fumarate, a full blood count (including lymphocytes) should be performed before starting treatment, and the risk of lymphopenia and PML should be explained to patients and carers. Prescribers should be aware that dimethyl fumarate has not been studied in patients with pre-existing lymphopenia or in combination with other immunosuppressants.
During treatment, full blood counts (including lymphocytes) should be performed at least every 6 to 12 months. Patients with lymphopenia should be monitored for symptoms of PML (eg, neurological dysfunction), which may be similar to features of multiple sclerosis as PML is also a demyelinating disease.
Treatment with dimethyl fumarate should be stopped immediately if PML is suspected.