Elan Corp. Plc and Transition Therapeutics Inc. said Tuesday they've eliminated two of three doses used in a mid-stage study of a potential Alzheimer's disease treatment after nine patients died while receiving the higher doses.

Elan, based in Dublin, and Transition Therapeutics, of Toronto, said they would immediately withdraw patients who received 1,000-milligram and 2,000-milligram doses twice daily.

But those receiving a lower dose — 250 milligrams — would continue in the study along with placebo groups. Transition Therapeutics Chief Financial Officer Elie Farah said the trial ends in April, and the companies will decide whether to move into late-stage testing after looking at results.

The companies completed enrollment for this trial in October 2008.

A direct relationship between the drug compound and the deaths in the higher dose groups has not been established, the companies said in a statement.

The potential treatment, labeled ELND005, is an oral drug designed to treat mild to moderate Alzheimer's disease.

Transition Therapeutics is funding 30 percent of the development, while Elan pays for 70 percent, Farah said. The companies will split revenue the same way, but Transition Therapeutics has an option to increase its share to 40 percent.

Shares of Transition Therapeutics tumbled $3.81, or 48 percent, to $4.10 in late morning trading Tuesday. Elan's shares fell 14 cents, or 2 percent, to $6.56.

The Standard & Poor's 500 index was down less than 1 percent.