Shares of Amgen Inc. gained more ground Thursday, two days after the company announced positive late-stage trials for a breakthrough melanoma treatment. The Thousand Oaks biotech announced that a vaccine derived from the herpes simplex virus, which causes cold sores, has been shown to reduce the size of melanoma cells in a wide-scale human trial. The company, which acquired the drug treatment when it bought Woburn, Mass. company Biovex Group Inc. in 2011, said the Phase 3 trial showed that the injected drug made tumors disappear or shrink by half for at least six months in 16 percent of the study’s participants. That’s compared to 2 percent of the patients in a control group receiving standard treatment. “These are the first Phase 3 results of this novel approach to cancer therapy,” said Dr. Sean Harper, executive vice president of research and development for Amgen, in a statement. “A high unmet need exists in melanoma and we believe the innovative mechanism of action talimogene laherparepvec may offer a promising approach for these patients.” The positive trial was good news for investors. While the company’s shares have been trading at record levels in recent months – credited primarily to an aggressive buyback program the company initiated in 2011 – recent setbacks in pipeline drug development have been discouraging. Last month, the Food and Drug Administration halted a study to determine the safety of pediatric use of its thyroid drug Senispar following the death of a 14-year-old patient in the study. In January it reported poor results in a trial for additional use of its blockbuster Aranesp. Amgen shares hit a 52-week high of $94.89 on Wednesday before settling back down. Share gained 26 cents on Thursday to close at $94.31 on the Nasdaq.