A type of herpes virus, designed by scientists, has shown promise in a preliminary study as a cancer cell destroyer, a researcher said last week at the European Society for Medical Oncology conference in Lugano, Switzerland.

The herpes simplex virus, called NV1020, has been modified by scientists to selectively replicate in cancer cells, killing them in the process. The virus does not harm normal cells, according to preliminary data.

“Our hope is that it will help fight cancers without causing side-effects in the rest of the body,” said Dr. Axel Mescheder, vice president of clinical research and development at the Munich-based biotech company MediGene, in a press release.

The virus was previously tested in animals and was shown to be effective at killing colorectal cancer cells and liver cancers. A current study, being led by Dr. Tony Reid from the University of California San Diego, is the testing the virus in humans at seven leading U.S. cancer centers.

At the ESMO conference, Mescheder presented preliminary safety results and a case report from an ongoing clinical trial in patients with colorectal cancer that had spread to the liver. These patients have not responded to chemotherapy alone.

Colorectal, also called colon cancer, is the third most common form of cancer and the second leading cause of death among cancers in the Western world. It causes 655,000 deaths worldwide per year.

Mescheder said a patient whose cancer had spread to 10 different places around the liver and four in the lungs was given the virus treatment in four weekly infusions directly into blood stream, followed by two cycles of approved chemotherapy.

Six months after treatment, scans showed the liver masses had nearly disappeared, he said. “The reduction in the tumor masses was really impressive in this patient,” Mescheder said. The patient survived for 12 months after treatment, he added.