YOKOHAMA, Japan -- With available AIDS drugs proving too toxic for patients or too weak to overcome the disease, scientists are now working on treatment ideas that sound like something out of "Star Trek."


Genetic manipulation of people's cells, alteration of the human immunodeficiency virus and direct injections of cells deliberately infected with other viruses are among the experiments being described this week at the 10th International Conference on AIDS, which ended Thursday.


With science pushing at the frontiers of understanding in its search for AIDS treatments, the dangers and pitfalls are unpredictable, Dr. Robert Gallo of the National Cancer Institute acknowledged.


But he told the conference, "I see harm in any therapy that has ever been tried on human beings. We have to be willing to take those risks."


In her laboratory at the University of California, San Diego, Dr. Flossie Wong-Staal has developed extraordinary mouse and bird viruses that she has genetically engineered to make special proteins, called ribozymes, that chop up AIDS viruses. In test tubes, she mixes these viruses with T-cells, white blood cells from the human immune system. When she subsequently adds HIV to those test tubes, the ribozyme-infected T-cells are able to destroy the AIDS viruses.


The effect is so dramatic in the test tube that Wong-Staal plans human tests before the end of this year to see if the ribozyme approach is safe and effective in people.


But the tremendous unknowns involved in such pioneering research make the U.S. Food and Drug Administration and the National Institute of Health nervous. There is concern that gene therapy, as such approaches are called, could result in human mutations, cancer, dangerous hyperallergenic reactions or other unique toxic responses. Though gene therapy has been tried in tiny trials on terminal cancer patients, it remains a medical technology so experimental that no one is certain of its safety.


Nevertheless, data presented at the meeting cast still greater doubt on existing HIV treatments by proving that the viruses can quickly mutate to evade all of them, even in combination.


Few researchers, however, believe gene-therapy treatments will be available soon -- if ever.


"All this work is being done with laboratory cell lines," which are mutant and unusual by virtue of having been manipulated in test tubes for many generations, said Dr. David Klatzman of the Universit? Pierre et Marie Curie in Paris.


"We have no idea what will happen in the real world, with real wild-type viruses and real human cells," he said.