Many tumors have oxygen-depleted areas; and the cells there are more likely to invade, spread, and resist current treatments. But a new experimental cancer drug, called TH-302, is activated when oxygen is low. In a phase I clinical trial of 34 people with melanoma, TH-302 shrank tumors in 7 people and kept tumors from getting worse in 12 more people. Now, researchers have begun a phase II trial that will include up to 40 people with melanoma. This drug could also treat a variety of cancers and phase III trials are currently underway for soft tissue sarcoma and pancreatic cancer.