Pharmaceutical companies Wyeth and Elan Corp. on Tuesday announced results of a Phase II trial involving their experimental Alzheimer’s disease drug bapineuzumab.

According to the announcement, the drug didn’t improve Alzheimer’s symptoms more than a placebo. However, it was effective in a genetic sub-group of patients known as ApoE4 non-carriers who are known to be at increased risk of developing Alzheimer’s. These patients make up between 40 and 70 percent of the Alzheimer’s disease population.

Alzheimer’s disease causes a progressive loss of memory and mental faculties, which can be devastating for the patients concerned and those around them. About 5.2 million Americans currently live with this condition. Another 25 million people worldwide have Alzheimer’s. This number is expected to grow as the baby boom generation ages.

About 16 million people are forecast to have Alzheimer’s by 2050 in the U.S., 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.

The companies also said the phase II trail’s results supported their previous decision to continue late-stage studies, which were started last year. Their results will be presented at the International Conference on Alzheimer’s Disease in Chicago on July 29.

The drug is expected to become a blockbuster if it reaches the market, with some analysts forecasting annual sales of $13 billion. Also, it could be world’s first drug to modify the course of Alzheimer’s rather than just relieving its symptoms like many other drugs do.