An effective drug against Alzheimer’s disease would be a blessing for millions. It would also be hugely profitable for the company that invented it. Hence the excitement over promising trial results from Eli Lilly. The US pharma group’s share price has jumped 12 per cent since the announcement on Monday, adding almost $18bn to its market value.

This was just a mid-stage trial involving a mere 272 patients. But the reaction reflects the potential importance of Alzheimer’s treatments. US sales for all of these could be as much as $50bn a year, increasing the overall pharmaceuticals market by a tenth, says Bernstein.

Eli Lilly’s drug donanemab is an antibody designed to remove clumps of the protein beta-amyloid from the brain. The trial results may increase the chances of success for other anti-amyloid drugs such as Roche’s gantenerumab and Biogen’s aducanumab. The latter’s shares jumped 6 per cent on Monday. The Massachusetts-based biotech expects a decision from US regulators on its drug within the next two months, though sceptics think a lot more evidence is needed to justify an approval.

The failure rate for drugs targeting the amyloid protein has been dauntingly high and not everyone is convinced by the argument that earlier treatments were given too late to prevent damage. Even if they work, improvements may be modest and side effects severe. Some experts draw a comparison with gruelling early chemotherapy treatments.

Other approaches are being explored. A Chinese drug oligomannate suppresses gut bacteria that may be linked to the disease. Eli Lilly, Biogen and AbbVie are developing drugs that aim to stop the build-up of a protein called tau in the brain.

But much rides on the theory that the amyloid protein plays a central role in the disease. This hypothesis has dominated Alzheimer’s research over the past 25 years. The prospects for drugs based on it, such as donanemab, are reaching a critical juncture. The next couple of years will be make-or-break.

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