Unexpected side effects when switching from natalizumab to a biosimilar

In May 2024, Imperial College Healthcare NHS Trust made extensive preparations to switch 80 patients with highly active, relapsing-remitting multiple sclerosis from an established monoclonal antibody drug to a biosimilar classified as interchangeable by the regulatory authorities. Within a few weeks, more than half of those affected reported unexpectedly severe and persistent side effects, so 42 patients were switched back to the original drug. After switching back, the symptoms improved in 90% of cases within two weeks. These observations suggest that despite formal interchangeability in clinical use, subtle differences in production, application and patient characteristics can lead to relevant variations in tolerability.