Genentech, Inc. has exercised its option to further develop and commercialize Exelixis Inc.’s compound XL518, a selective and potent inhibitor of MEK that is currently in a Phase I trial. Under the terms of the agreement, Exelixis will continue that trial until the maximum tolerated dose (MTD) is determined. Genentech will then be responsible for completing the Phase I trial and subsequent clinical development.     “MEK inhibition is an exciting approach to cancer therapy. The MAP...