Caffeic acid phenyl ester in propolis is a strong inhibitor of matrix metalloproteinase-9 and invasion inhibitor: isolation and identification - Jin, U H; Chung, T W; Kang, S K; Suh, S J; Kim, J K; Chung, K H; Gu, Y H; Suzuki, I; Kim, C H (Clin Chim Acta, 2005)