OXiGENE's vascular disrupting agents, ZYBRESTAT® and OXi4503, are from the combretastatin family of compounds. Combretastatins were originally isolated from the bark of the South African Bush Willow tree by researchers from Arizona State University but they are now manufactured synthetically. OXiGENE has licensed broad, worldwide rights to combretastatins from the Arizona State University and has established extensive US and ex-US patent protection for these compounds.