Adverse reactions reported in clinical trials were generally mild and included nonserious infections (such as urinary tract, lower respiratory tract, gastrointestinal system, and vaginal infections), headache, depression, joint pains, and menstrual disorders. Serious adverse reactions were rare. Anaphylactic reaction with urticaria and bronchospasm can be rapidly reversed with antihistamines and cortisone. Delayed allergic reactions to natalizumab are associated with formation of neutralizing antibodies. Postmarketing data show that natalizumab can cause severe liver damage. A case of myocardial infarction following injection of natalizumab has been reported (08).
Primary central nervous system lymphoma, which is usually a disease of the elderly with no increased prevalence in multiple sclerosis, has been reported in a patient treated with natalizumab (35). The association, however, is not clear.
In 2005, Biogen Idec, the manufacturer of natalizumab, reported 1 confirmed fatal case and 1 possible case of progressive multifocal leukoencephalopathy in patients receiving natalizumab for multiple sclerosis. The diagnosis of multifocal leukoencephalopathy was confirmed on postmortem examination of the brain (20). JC viral DNA was detected in cerebrospinal fluid and was confirmed at autopsy. In another case of death following natalizumab therapy for Crohn disease, JC virus DNA had appeared in the serum 3 months after the initiation of therapy and 2 months before the appearance of symptomatic multifocal leukoencephalopathy (40). From these observations, it appears that testing for the appearance of JC virus in plasma, along with a high degree of clinical suspicion, may enable early diagnosis of multifocal leukoencephalopathy and discontinuation of natalizumab therapy with the possibility of recovery. No new cases of multifocal leukoencephalopathy have been observed since the initial 3 cases. Risk of progressive multifocal leukoencephalopathy in patients treated with natalizumab was estimated to be 1 in 1000 patients based on more than 3000 patients treated in clinical trials (46). A revised application for approval of the product submitted to the FDA contains integrated safety assessment of patients treated in clinical trials and a revised label and risk management plan. In 2006, the Peripheral and Central Nervous System Drugs Advisory Committee of the Food and Drug Administration voted unanimously to recommend reintroduction of natalizumab as a monotherapy for patients with relapsing forms of multiple sclerosis who have had an inadequate response to, or are unable to tolerate, alternative treatments (25). The risk of developing progressive multifocal leukoencephalopathy in a pooled clinical trial cohort has been estimated to be 1 person for every 1000 patients treated for an average of 17.9 months (13). A risk-benefit analysis concluded that more than 7-fold increase in actual risk of progressive multifocal leukoencephalopathy was required to decrease natalizumab's health gain below that of interferon beta-1a (38). In 2012, the FDA approved a product label change for Tysabri® (natalizumab) that will help enable individual benefit-risk assessment for patients with multiple sclerosis. The new label identifies positive anti-JCV antibody status as a risk factor for developing progressive multifocal leukoencephalopathy whereas negative anti-JCV antibody status indicates that exposure to the JC virus has not been detected. Irrespective of treatment, approximately 55% of multiple sclerosis patients are anti-JCV positive. Stratify JCV Antibody ELISA Testing Service (Quest Diagnostics) enables neurologists to determine their multiple sclerosis patients' anti-JCV antibody status and is the first blood test to be approved by the FDA for the qualitative detection of antibodies to the JC virus. Quantification of T and B lymphocytes can be used to monitor natalizumab therapy safety because new lymphocyte production indicates the integrity of immune surveillance (47). An unusually low percentage of l-selectin-expressing CD4+ T cells in peripheral blood is a possible biomarker for risk of development of progressive multifocal leukoencephalopathy in multiple sclerosis patients treated with natalizumab (34).
Plasma exchange and immunoadsorption have been used to accelerate clearance of natalizumab successfully, with improvement of symptoms in patients who developed progressive multifocal leukoencephalopathy after natalizumab monotherapy (21; 42). Although rapid withdrawal of the drug and administration of plasma exchange has enabled survival in some of the patients with progressive multifocal leukoencephalopathy, a new problem, immune reconstitution inflammatory syndrome, has been reported after drug withdrawal (19). This complication requires treatment with corticosteroids.
In 2012, there were 212 confirmed cases of progressive multifocal leukoencephalopathy among 99,571 patients treated with natalizumab; the risk was associated with anti-JC virus antibodies, previous use of immunosuppressants, and increased duration of natalizumab treatment, alone or in combination (03). Currently, more than 110,000 multiple sclerosis patients worldwide have been treated with natalizumab, and the risk of developing progressive multifocal leukoencephalopathy is approximately 2.7 per 1000 cases. Planned dosage interruptions of natalizumab have been recommended for decreasing the cumulative risk of multifocal leukoencephalopathy, but it is associated with clinical flares and recurrence of radiographic lesions. Some of these flares can be clinically severe, with a high number of contrast-enhanced lesions, suggesting a possible rebound of disease activity (43). Postmortem study in a patient with multiple sclerosis who died following natalizumab-associated progressive multifocal leukoencephalopathy showed coexistence and lack of interplay between lesions of chronic multiple sclerosis and progressive multifocal leukoencephalopathy (44).
Progressive multifocal leukoencephalopathy associated with natalizumab has better prognosis than that due to other causes. Early diagnosis, localized disease on MRI, and aggressive management may improve outcomes (41).
Natalizumab withdrawal because of progressive multifocal leukoencephalopathy triggers an immune reconstitution syndrome that can lead to neurologic complications or even death. According to a retrospective study, either disease-modifying treatment or “no treatment” can abolish reactivation of disease activity after discontinuation of natalizumab (06). RESTORE, a randomized, partially placebo-controlled exploratory study, showed that in most patients, gadolinium-enhancing MRI lesions during natalizumab interruption did not exceed pre-natalizumab levels or levels seen in historical control patients (18).
Because of the effect of natalizumab treatment on JCV seroconversion, monitoring of patients' JCV serology as well as incorporation of additional risk factors into the progressive multifocal leukoencephalopathy risk stratification is recommended (33). However, regardless of JCV antibody status, the possibility of progressive multifocal leukoencephalopathy should be considered in a natalizumab-treated patient with any new MRI lesion or neurologic symptoms (12).