The lack of definitive biological markers for the earliest stages of Alzheimer’s disease has made it impossible to enroll patients in drug-research trials, U.S. regulators said, slowing development of new treatments.

The methods for trials were established in 1984 and cover patients with full-blown dementia who suffer both cognitive and functional limitations, the Food and Drug Administration said in a statement today. The agency is proposing to relax guidelines for early-stage Alzheimer’s drug trials by letting researchers show that a medication slows the cognitive decline, rather than improvement in both cognitive and functional deterioration.

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