Top pharmaceutical companies like Alzheon and Biogen  (BIIB - Get Report) , shares of which plummeted in late afternoon trading on Thursday, are working to discover a cure for Alzheimer's.

The neurodegenerative disease currently affects over 45 million people worldwide, according to the World Alzheimer Report.

"It's an incredibly tough puzzle to crack," says P. Murali Doraiswamy, the head of biological psychiatry at Duke University. "I think a form of cure is more likely to come from delaying the onset rather than by growing new brain cells to repair lost tissue."

BAN2401, which is being developed by Biogen and Eisai Co. Ltd. (ESALY) , has been shown to reduce the rate of cognitive decline in patients with the neurodegenerative disease by 30 percent at the highest dose compared to placebo over an 18-month period, which exceeded analysts' expectations of a 15-20% reduction.

The drug showed statistically significant improvement across a few composite measures, including the Alzheimer's Disease Assessment Scale-cognitive sub scale, where it showed a 47% reduction in cognitive decline. BAN2401 was also shown to reduce the amount of amyloid plaque buildup by 93%.

Biogen is also working on another medication called Aducanumab, but results won't be out until 2020.

BAN2401 and Aducanumab are both antibodies that latch onto and reduce the amount of amyloid plaques that build up in the brain. Amyloid plaque buildup is currently the prevailing theory for the cause of Alzheimer's, and strong results could support the continuation of investigative studies based on the beta-amyloid hypothesis, analysts say.

Both Eli Lily (LLY - Get Report) and AstraZeneca (AZN - Get Report) have discontinued late-stage trials of their Alzheimer's drug, Lanabecestat.

Pfizer (PFE - Get Report) also dropped its search for an Alzheimer's cure earlier this year.