Doctors are genetically modifying the blood cells of patients with advanced cancer as part of the first U.S. study to use the gene-edited tool CRISPR to treat the disease.

Early results released on Wednesday show that immune system cells were successfully taken from the blood of three patients and genetically altered to help them recognize and fight cancer cells, and that the procedure appears to be safe.

The patients, two with the blood cancer multiple myeloma and one with a sarcoma, a cancer that forms in connective or soft tissue, were treated at the University of Pennsylvania Hospital in the study's first safety trial.

All three patients had previously gone through other treatments without apparent success -- one patient's cancer continued to worsen, another showed no change, and a third was treated too recently for any potential progress to be reported.

Dr. Edward Stadtmauer, an oncology professor at the University of Pennsylvania who is leading the study, told The Associated Press that the study was deliberately limited to three patients until it could be demonstrated that the treatment was safe.

"Our plan is to treat more patients and we've demonstrated that we've done it safely and that the cells have worked," says Stadtmauer. "In terms of responses, we still have a-ways to go to see whether these patients are benefiting from this treatment."

Rather than changing the DNA within a person's body, the treatment is a form of immunotherapy, removing the patient's cells before enhancing them in a lab in order to improve their cancer-fighting abilities.

While it is too early to determine whether the treatment will actually treat the cancer, the point of the study is to make sure the procedure is safe and feasible. Once given FDA approval, doctors say they will be able to target different genes to attack the cancer more effectively.

"Given that these cells have survived in the patients and have been successfully created, this can act as the basis of a whole new generation of clinical trials using different targets, different editing of genes, different techniques for different cancers," says Stadtmauer.

This is not the first time CRISPR has been used to treat a disease. The first published account of using the gene-editing tool to treat a disease in an adult appeared in the New England Journal of Medicine in September. In that report, Chinese researchers used CRISPR to try to cure a patient's HIV infection.

Doctors plan to treat 15 more patients to further test the safety and cancer-fighting effects of the treatment.

With files from The Associated Press