A new type of genetically engineered AIDS drug is being tested starting Wednesday on human victims of the deadly disease at the National Cancer Institute. The first round of tests is designed to assess the safety of the drug, called CD4, rather than its effectiveness, but researchers hope the drug will open new avenues to attack the virus that causes acquired immune deficiency syndrome. The drug mimics a protein normally found on the surface of immune system cells that are susceptible to the AIDS virus. Researchers hope the experimental drug will, in effect, fool the deadly virus into harmlessly attaching itself to the injected substance rather than the similar receptors on immune system cells.