One of the most tragic aspects of Alzheimer's disease for the family is the way a patient's personality can change. Alzheimer's patients often become paranoid and belligerent, anxious and hostile.

Now, a team of researchers at the National Institute of Mental Health headed by Dr. Pierre N. Tariot has demonstrated that a drug normally used to treat depression seems to improve behavior in some patients with the brain-deteriorating illness.

NIMH psychiatrist Dr. Trey Sunderland said scientists had noted that a particular enzyme called monoamine oxidase-B (MAO-B) was elevated in the brains of Alzheimer's. The enzyme, along with the very similar MAO-A, has long been implicated in depression. It was hypothesized, Sunderland said, that an antidepressant drug known to inhibit the enzyme might have a use in Alzheimer's.

The inhibiting drug, used in an experiment described recently in the Archives of General Psychiatry, is called Deprenyl.

In the NIMH study, 17 Alzheimer's patients were given a low dose of the drug, a high dose of the drug and a placebo in alternating cycles. It was only the low dose of the drug, which inhibited MAO-B but not MAO-A, that had a significant effect on the Alzheimer's behavior. Patients were more communicative, easier to interact with, and had increased energy and less tension.

Although there were no significant increases in intellectual functioning, Sunderland said, the families of the approximately one third of the patients who improved behaviorally saw it "as a general clinical improvement." Long-term term trials are now under way.

Meanwhile, trials of a different Alzheimer's drug have been disappointing, scientists say. The drug bethanechol chloride, fed into the brain by surgically implanted pumps, has failed to lead to significant improvement, said Dr. Robert Harbaugh of the Dartmouth-Hitchcock Medical Center in Hanover, N.H. The drug was the subject of national attention in 1984 after four patients seemed to show improvement from it.

Alzheimer's disease afflicts an estimated 2.5 million American adults and kills more than 100,000 every year.