Alzheimer’s disease (AD) is the most common type of dementia. But despite a huge amount of effort being put into drug discovery programmes, no new treatments have emerged from the drug development pipeline for over 20 years. That is, until earlier this summer, when the US Federal Drug Administration (FDA) approved Aducanumab. Not surprisingly this news garnered a great deal of attention, not least because Aducanumab goes beyond what any other AD drug therapy offered; it promised to treat the cause of the disease, not just its symptoms.

Aducanumab is one of a class of drugs known as immunotherapies. As the name suggests, these elicit an immune response to a disease-causing agent. The approach has proved successful with some cancer treatments, where it is clear that the tumour cells are responsible for the disease. Things are not so clear with AD. We do know that during AD a particular protein in the brain misfolds and then aggregates, causing a build-up of hard insoluble plaques. One hypothesis is that these aggregates are toxic and so kill brain cells.

Aducanumab is designed to clear these amyloid plaques, flagging them to the immune system for “clear up”. Experiments from as far back as 2012 showed that this worked in mice, and from there clinical trials began. The FDA’s recent approval would appear to be testament to the validity of this immunotherapy approach.

Unfortunately, things are not so clear cut. The FDA’s decision has been marred by controversy, with one Harvard professor of medicine proclaiming it to be “probably the worst drug approval decision in recent US history”. Whilst plaques are certainly a feature of AD, it is not clear if they cause the disease or merely correlate with it. Moreover, the original clinical trial was deemed a failure in 2018. Then, only after data was reanalysed was a positive benefit for some AD patients found.

So, what now for Aducanumab? The FDA has conceded that it is only available as part of a nine-year post-approval confirmatory study, so we may have to wait a while to see if it actually works.