THOUSAND OAKS, Calif. ( TheStreet) -- A herpes simplex virus re-engineered to kill cancer cells shrank tumors in patients with skin cancer, according to results from a phase III study announced by Amgen ( AMGN) on Tuesday night. Amgen is the first to demonstrate that a so-called "oncolytic virus" can shrink tumors in a late-stage clinical trial, but it's too early to tell how successful the therapy might be or if it will be approved at all. The treatment of melanoma, or skin cancer, has become much improved in recent years with the approval of targeted therapies and immunotherapies from Roche ( RHHBY) and Bristol-Myers Squibb ( BMY). Amgen shares rose $1.99, or 2%, to $94.40 in Wednesday morning trading. In the Amgen phase III trial, more than 400 patients with advanced skin cancer were randomized to treatment with the engineered virus known as talimogene laherparepvec (T-Vec) or the immune booster GM-CSF. Sixteen percent of the T-Vec patients reported meaningful tumor shrinkage for at least six months, compared to 2% for patients treated with GM-CSF. The difference in durable tumor response was statistically significant in favor T-Vec, achieving the primary endpoint of the study. Amgen said there was a trend towards T-Vec patients living longer in the study but the survival benefit over GM-CSF was not yet statistically significant. More mature survival data will be presented later this year, perhaps at the American Society of Clinical Oncology annual meeting in June. Two recently approved skin cancer drugs -- Roche's Zelboraf and Bristol Myers' Yervoy -- both prolong survival significantly. In order to compete and perhaps even to be approved at all, Amgen will have to demonstrate that T-Vec can help skin cancer patients live longer and not just shrink tumors. Amgen's development of T-Vec began in January 2011 when it acquired the therapy's original developer, BioVex, for $425 million plus another $575 million in potential future payments. In an interview at the time of the deal, Roger Perlmutter, Amgen's chief scientist, said, "What impresses me about about BioVex is the firm scientific approach the company has taken in engineering the virus but also the responses seen to date in clinical trials ."