OncoMed Pharmaceuticals, the Redwood City, CA-based developer of drugs that aim for cancer stem cells, has struck a big partnership with Germany-based Bayer. The deal will provide OncoMed with $40 million in upfront cash, and also could offer milestone payments for as many as five different drug candidates, which have potential to generate as much as $387 million in milestone payments each for OncoMed. If any of the drug candidates become marketed products, OncoMed will receive a double-digit percentage royalty on sales. The companies will work together to develop small molecules, antibodies, or other protein drugs aimed at the Wnt signalling pathway, which is thought to play a role in the growth, recurrence, and spread of tumors.