Scientists are calling Plexxikon's experimental skin cancer drug a "breakthrough" after it successfully shrank tumors in clinical trial patients. Researchers at Sloan-Kettering Memorial Cancer Center in New York found that PLX4032 caused tumors to shrink in 70 percent of patients with a particular genetic mutation. Two of the 27 patients who participated in the study saw their tumors disappear completely. The results are a vast improvement over chemotherapy, which only shrinks tumors in 13 to 15 percent of melanoma patients. Melanoma is the deadliest form of skin cancer. PLX4032 works by blocking the activity of a cancer-causing gene mutation that is involved in 50 to 60 percent of melanomas. Approximately 40 percent of patients without the gene mutation did not respond to PLX4032. Researchers at Sloan-Kettering expect to start a larger Phase II trial at the end of the year. An international Phase III trial is planned for late 2009 or early 2010. The Phase III trial is expected to involve several hundred patients. Berkeley, CA-based Plexxikonis is developing PLX4032 in conjunction with Roche. PLX4032 fared better than Roche's cancer drug Avastin, which failed a Phase II clinical trial. Avastin had previously shown some promise against melanoma, but a last-minute review found that it ultimately did not confer a significant survival benefit. However, Avastin performed well in another clinical trial involving patients with colon cancer. The study found that Avastin shrank liver tumors in 78 percent of colon cancer patients. Melanoma is highly treatable if caught early. Companies such as Electro-Optical Sciences and Melanoscan are developing early diagnostic systems to catch melanomas while they are still in a treatable stage. Therapeutics for melanoma are being developed at BioVex, AVAX Technologies, Ganymed Pharmaceuticals, Peplin Limited, and AmpliMed Corporation.