Synthesis and Evaluation of Anticancer Benzoxazoles and Benzimidazoles Related to UK-1

UK-1 is a structurally unique bis(benzoxazole) natural product isolated from a strain of Streptomyces. UK-1 has been reported to possess anticancer activity but no activity against bacteria, yeast, or fungi. Previous work has also demonstrated the ability of UK-1 to bind a variety of di- and tri-valent metal ions, particularly Mg2+ ions, and to form complexes with double-stranded DNA in the presence of Mg2+ ions. Here we report the activity of UK-1 against a wide range of human cancer cell lines. UK-1 displays a wide spectrum of potent anticancer activity against leukemia, lymphoma, and certain solid tumor-derived cell lines, with IC50 values as low as 20 nM, but is inactive against Staphylococcus aureus, a methicillin-resistant strain of S. aureus, or Pseudomonas aeruginosa. A series of analogues of the bis(benzoxazole) natural product UK-1 in which the carbomethoxy-substituted benzoxazole ring of the natural product was modified were prepared and evaluated for their anticancer and antibacterial properties. An analogue of UK-1 in which the carbomethoxy-substituted benzoxazole ring was replaced with a carbomethoxy-substituted benzimidazole ring was inactive against human cancer cell lines and the two strains of S. aureus. In contrast, a simplified analogue in which the carbomethoxy-substituted benzoxazole ring was replaced with a carbomethoxy group was almost as active as UK-1 against the four cancer cell lines examined but lacked activity against S. aureus. Metal ion binding studies of these analogues demonstrate that they both bind Zn2+ and Ca2+ ions about as well as UK-1. The non-cytotoxic benzimidazole UK-1 analogue binds Mg2+ ions 50-fold weaker than UK-1, whereas the simple benzoxazole analogue binds Mg2+ ions nearly as well as UK-1. These results support a role of Mg2+ ion binding in the selective cytotoxicity of UK-1 and provide a minimal pharmacophore for the selective cytotoxic activity of the natural product.