AstraZeneca Plc and Targacept Inc. announced on Monday that their treatment for Alzheimer’s disease did not have the expected results in a midstage trial, called Sirocco trial.

The companies said the trial on their drug called AZD3480 had inconclusive results. To be more specific, patients given the drug had no significant improvement compared to patients given Pfizer’s donepezil, sold under the brand name Aricept that is already marketed for Alzheimer’s treatment.

The trial “didn’t provide enough time to diminish the placebo effect, as a longer trial might have,” Dr. Donald deBethizy, the president and chief executive of Targacept, said during a conference call with analysts.

Moreover, patients given a placebo or a dummy pill showed an improvement in brain function, which also impacted the results of the study.

The trial involved 567 patients ages 60 to 85 with mild to moderate Alzheimer’s who were followed for 12 weeks.

The full results of the trial are to be discussed with leading medical experts over the coming months. A decision by AstraZeneca with respect to potential further development of AZD3480 is expected by December 2008.

AZD3480 is also studied as a treatment for cognitive dysfunction in schizophrenia patients and for attention deficit hyperactivity disorder. Results of that trial are expected by the end of 2008.

Shares of AstraZeneca fell 86 cents to $44.94 Monday. Shares of Targacept fell 63 cents, or 6.9 percent, to $8.46.

About 5.2 million Americans live with Alzheimer’s disease, a number that is expected to grow as the baby boom generation ages. About 16 million people are forecast to have Alzheimer’s by 2050, as existing drugs can ease symptoms but do not stop the disease from developing. The disease already afflicts one in eight people 65 and older and nearly one in two people over 85.