In a clinical trial that could lead to treatments that prevent Alzheimer’s disease, people who are genetically guaranteed to suffer from the disease years from now — but who do not yet have any symptoms — will for the first time be given a drug intended to stop them from developing it, federal officials announced Tuesday.

Experts say the study will be one of only a very few ever conducted to test prevention treatments for any genetically predestined disease. In Alzheimer’s research, the trial is unprecedented, “the first to focus on people who are cognitively normal but at very high risk for Alzheimer’s disease,” said Dr. Francis S. Collins, director of the National Institutes of Health. Most of the study’s participants will be drawn from an extended family of 5,000 people who live in Medellín, Colombia, and remote mountain villages outside that city. The family is believed to have more members who suffer from Alzheimer’s than any other in the world. Those who possess a specific genetic mutation begin showing cognitive impairment around age 45, and full-blown dementia around age 51. The 300 family members who participate in the initial phase of the trial will be years away from developing symptoms. Some will be as young as 30.

The $100 million study will run for five years, but results on sophisticated tests may indicate in as little as two years whether the drug is helping to delay memory decline or brain changes, said Dr. Eric M. Reiman, executive director of the Banner Alzheimer’s Institute in Phoenix, and a lead researcher on the study.

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