Seroquel is already approved to treat schizophrenia and bipolar disorder (formerly called manic-depressive illness).
The new studies were presented at the annual meeting of the American Psychiatric Association.
Major depression, which affects 15 to 19 million Americans, "is not just your typical blues. This is depression that lasts for a few weeks, even months," says researcher Richard Weisler, MD, of the University of North Carolina School of Medicine and Duke University Medical Center.
One study involved more than 700 people who had suffered from depression for at least one month but less than one year.
Patients were randomly assigned to take one of three doses of Seroquel or a placebo once a day for six weeks.
Those taking Seroquel showed greater improvement in depression symptoms than those on placebo.
"There was improvement in all the things that are impaired when you're depressed -- sleep, appetite, suicidal thoughts," Weisler tells WebMD.
After six weeks of treatment, 25% were in remission, he says.
Importantly, patients on Seroquel experienced a significant improvement in depressive symptoms as early as the fourth day of treatment, Weisler says.
"Typically, patients don't respond until they have had at least two weeks of antidepressant therapy. And often it can take up to a month," he says.
Seroquel Lowers Anxiety, Too
The observation led to a two-month study pitting Seroquel against placebo in nearly 1,000 patients with generalized anxiety disorder.
It's estimated that generalized anxiety disorder affects about 6.8 million Americans, with women twice as likely to be affected as men, according to the researchers.
Symptoms, which can linger for six months or longer, include constant worry and anxiety about things most people take in stride. Sufferers often can't get rid of their concerns even though they realize their level of anxiety is out of proportion with the situation at hand.
By one week after the study started, anxiety symptoms, such as trouble sleeping and relaxing, began to improve in patients given Seroquel compared with those given placebo. That improvement continued over the course of the trial, Weisler says.
Temple University's David Baron, DO, chairman of the committee that chose which studies to highlight at the meeting, says that new treatment choices for patients with major depression and anxiety are needed.
"A substantial proportion of patients don't respond to existing agents. Often patients who aren't helped by one drug will respond to another, even if it's in the same class," he tells WebMD.
AstraZeneca, the makers of Seroquel, funded the studies.