Shares of Onyx Pharmaceuticals Inc jumped 25 percent on Wednesday after the cancer-fighting company announced it was proceeding with an advanced-stage trial on humans of its virus-based experimental treatment for recurrent head and neck cancer.

Shares of the Richmond, Calif.-based company were up 2-9/16 at 13 in late afternoon trading on Nasdaq.

In partnership with Warner-Lambert Co, Onyx said it was beginning to enroll patients in a clinical trial comparing a combination of chemotherapy and ONYX-015 to chemotherapy alone.

The two companies agreed last year to jointly develop and market the therapy.

Onyx says its corporate goal is discover and develop cancer therapies based on the genetic mutations that cause the disease.

ONYX-015 is an injected virus genetically engineered to replicate in the presence of tumour tissue.

``It is a very smartly designed virus. If it works, ONYX-015 would be able to kill cancer cells and leave other cells alone,'' said Sushant Kumar, an analyst at Mehta Partners.

The analyst made no recommendation on whether to buy Onyx shares, but called it an ``inexpensive'' stock.

ONYX-015 is a virus that has been designed to allow normal cells to continue replicating while seeking out and destroying cancer cells with a mutation in a gene, called p53, that is associated with tumour suppression, the analyst explained.

``About 50 percent of cancers have mutations in p53. So this could be useful in other cancers, but head and neck are the best indications since those tumours are easy to target,'' Kumar said.

Onyx said scientists conducting the trial will be looking for a variety of possible results including tumour responses, progression-free survival, overall survival and improvements in patients' quality of life.

Onyx and Warner-Lambert also are conducting earlier-stage clinical studies of ONYX-015 as a treatment for a a variety of other cancer indications.