(Updated with MDVN stock quote.)

SAN FRANCISCO ( TheStreet) -- Medivation ( MDVN) shares were down more than 67% to $13.03 in late morning trading Wednesday, following news that Alzheimer's patients did not benefit from treatment with an experimental drug from the company, according to results from a phase III study.

The failure of the Dimebon study known as CONNECTION is a blow to Medivation's Alzheimer's disease program, which is partnered with Pfizer ( PFE - Get Report).

Medivation's stock closed Tuesday at $40.25, just short of its 52-week high.

"The results from the CONNECTION study are unexpected, and we are disappointed for the Alzheimer's community," said Medivation CEO David Hung, in a statement.

The phase III CONNECTION study compared six months of treatment with Dimebon on its own against a placebo in approximately 600 patients with mild to moderate Alzheimer's. Patients in the study were prohibited from taking any other drugs for Alzheimer's such as Pfizer's Aricept.

In the phase III study, Dimebon-treated patients were unable to demonstrate a statistically significant improvement compared to placebo-treated patients on two widely recognized measures of memory and cognitive function -- the so-called ADAS-cog and CIBIC-plus scales, Medivation said.

Medivation and Pfizer are conducting several other phase III studies of Dimebon in Alzheimer's patients, some which combine Dimebon with Aricept.

About 5 million Americans and 30 million people worldwide suffer today from Alzheimer's, the progressive neuro-degenerative disease. Those numbers are expected to triple by 2050. Current Alzheimer's drugs are only moderately effective but they still bring in about $5 billion in sales annually.

Elan ( ELN) and Johnson & Johnson ( JNJ - Get Report) are developing a competing drug, bapineuzumab, for Alzheimer's, with results from a phase III study expected next year. Bapineuzumab is also partnered with Pfizer.

-- Reported by Adam Feuerstein in Boston.

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