SAN FRANCISCO (AP) _ It's tragedy enough that Pat Williams' mother has Alzheimer's disease. But Williams is also terrified because her chances of inheriting the disease are much better than average. So Williams eagerly enrolled her 90-year-old mother last year in a massive, 1,600-patient, 18-month clinical trial testing an experimental drug made by the biotechnology company Myriad Genetics Inc.

The drug, called Flurizan, slowed the mind-robbing disease in some of the 128 patients with mild Alzheimer's participating in a smaller test.

Based on those results, the company has gambled millions of research dollars on the largest-ever Alzheimer's drug trial. It aims to win an intense, international race among several biotech companies to find the first effective treatment to at least slow the disease's progression in the 4.5 million Americans who suffer from it.

Analysts predict the market for such a drug could reach $4 billion annually by 2013 and success for Myriad would lift the company's fortunes considerably. The Salt Lake City company is now best known for drilling deep into the Mormon community's detailed genetic history to develop a popular breast cancer test.

Myriad's Alzheimer's drug wasn't effective for patients with moderate forms of the disease, so the company is targeting patients who have just been diagnosed. Scientists are also using the latest in brain imaging and genetic technology to develop tests to find people like Williams who have above-average chances of coming down the disease.

``I'm hoping they will have a miracle pill so that I won't contract it,'' said Williams, who lives in Boston.

At least two other companies are hot on Myriad's tail.

Neurochem Inc., a Quebec company, has enrolled about 1,000 patients in its late-stage human test in Canada and the United States. It's also recruiting a similar number for a European trial. Neurochem hopes to complete the trial by January 2007, perhaps a few months ahead of Myriad.

``If everything goes well, we could have approval in 2008,'' said Neurochem spokeswoman Lise Hebert. Myriad hopes to have approval in 2008 as well.

Further behind is the Ireland-based Elan Pharmaceuticals, which is testing its drug in about 180 patients.

``I want somebody to win this,'' said Dr. Bill Thies, scientific director of the nonprofit Alzheimer's Association. ``I don't particularly care if it's Myriad or one of its competitors.''

All three are attempting to block production of the plaque-causing protein beta-amyloid. While the exact cause of Alzheimer's remains a mystery, most researchers think a buildup of the protein in the brain is what leads to the disease.

Thies said results of the companies' smaller human trials essentially showed they were safe and weren't enough to make firm predictions about how effective they will be. But he said Myriad's Flurizan has ``some elements that look really positive.''

What is known is that the immune system of a healthy person gets rid of most of the amyloid. But something happens in Alzheimer's patients that leads to a deadly amyloid buildup that coats and then kills brain cells.

Myriad is using an old class of anti-inflammatory drugs that includes the painkiller ibuprofen to prevent the buildup.

The company initially purchased commercial rights to Flurizan from its creators at Loma Linda University, near San Bernardino, because of its promise for fighting cancer. But it soon became clear in experiments with mice that the drug was also effective for Alzheimer's as Myriad scientists made key tweaks to the drugs. The company continues to research Flurizan's use in fighting prostate cancer.

``There is a feeling within the Alzheimer's field that if you limit amyloid accumulation you will change the course of the disease,'' Thies said.

The three drugs now approved in the United States for the disease temporarily alleviate Alzheimer's symptoms, rather than treating them.

Doctors are hopeful that experimental drugs like Flurizan can slow the march of Alzheimer's, which is expected to claim a staggering 14 million in the United States by 2050 if no advances are made, according to Thies.

Myriad hustled Flurizan, which is a modified arthritis drug that is more than 20 years old, into final testing last year after the positive results of the smaller test.

If it succeeds, it will most likely spark interest in other anti-inflammatory drugs, including controversial painkillers like Merck's Vioxx, which was pulled from the market in 2004 after it appeared likely to increase the risk of heart disease.

``These drugs are interesting and are real harbingers of a new class of Alzheimer's medication,'' said Dr. Robert C. Green, associate director of Boston University School of Medicine's Alzheimer's Disease Center.

Green is treating Williams' mother and leading Myriad's trial. Data from the trial will probably be released sometime next year and the drug could be on the market by 2009, Myriad said.

Alzheimer's is a considerable drain on the health care system because of the round-the-clock care the millions of patients require. Some estimates of the disease's cost rise to $100 billion annually when caretaker costs and time are considered.

Williams' mother, for instance, pays about $500 a month to attend an adult day center while Williams works as a legal secretary. If her disease progresses any more, she may have to go live in a nursing home.

Because of the typical design of such drug trials, Williams, and her mother Rose Turner, don't know whether they're receiving Myriad's Flurizan or a dummy pill. Williams said it would be nice if her mother was receiving the drug, but they're still proud to be in the study even if they're receiving the placebo. That's because Williams and millions of others are desperate for even a modicum of relief.

``If it's going to help people with Alzheimer's, then we've accomplished something special,'' Williams, 53, said. ``My mom right now is my new child.''