A pharmaceutical company has developed a novel immunotherapeutic agent for metastatic melanoma. Following successful preclinical toxicology studies, the company initiates a double-blind, randomized, multi-center trial enrolling 600 patients with advanced disease. Participants are randomly assigned to receive either the new agent or the current standard dacarbazine therapy. The primary endpoint is progression-free survival at 12 months. Which phase of clinical trials does this study represent?