The Food and Drug Administration said yesterday it has approved new tests of the anti-AIDS drug DDC that will enable more patients -- particularly those who cannot take either of the two other AIDS drugs, AZT and DDI -- to gain access to the experimental therapy.

"The purpose of the new DDC study is to provide a drug that may be promising to patients who don't have any other good alternatives and who cannot get into the formal clinical trials," FDA spokesman Brad Stone said.

The agency said the new tests will allow AIDS patients to begin treatment with DDC if they are unable to qualify for the formal testing program and can tolerate neither AZT nor DDI, medical science's two principal weapons against the AIDS virus at this time.

DDC, which is short for dideoxycytidine, is being developed by Hoffmann-La Roche Inc. of Nutley, N.J., under a license from the federal government.

The license was granted in May 1987 and scientists at the time pronounced DDC a highly promising drug. Testing began in humans shortly after, but DDC was soon eclipsed by competing therapies including AZT and DDI.

The drug is regaining favor among AIDS researchers because its side effects are not as bad as those of the other drugs.

AZT, DDI and DDC are all close chemical relatives and all work in the same way, by inhibiting the ability of the AIDS virus to multiply inside infected cells.

AZT, made by London-based Wellcome PLC, is the sole drug to be FDA-licensed so far as safe and effective against AIDS.

Extensive human tests have found that it slows development of symptoms in infected individuals with no apparent signs of the disease, and prolongs survival and improves the quality of life in people with advanced AIDS.

DDI, being developed by New York-based Bristol-Myers Squibb Co. under a government license, also has been found in early clinical trials to suppress the AIDS virus and delay progress of the disease.

AZT can cause severe anemia and DDI is linked to gastrointestinal problems, painful nerve damage to the feet and occasional damage to the pancreas.

DDC also has side effects, including nerve damage in the feet. To minimize these, the FDA said it is now being tested at lower doses and in patients with less advanced AIDS.