Synthesis and Evaluation of Anticancer Benzoxazoles and Benzimidazoles Related to UK-1 Devinder Kumar, a Melissa R. Jacob, b Michael B. Reynolds a and Sean M. Kerwin a, * a DivisionofMedicinalChemistryandInstituteofCellularandMolecularBiology,CollegeofPharmacy,TheUniversityofTexasatAustin, Austin, TX 78712, USA b National Center for Natural Products Research, School of Pharmacy, Thad Cochran Research Center, The University of Mississippi, University, MS 38677, USA Received 28 May 2002; accepted 5 July 2002 Abstract—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 Mg 2+ ions, and to form complexes with double- stranded DNA in the presence of Mg 2+ 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 IC 50 values as low as 20 nM, but is inactive against Staphylococcus aureus, a methicillin-resistant strain of S. aureus, or Pseu- domonas 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 Zn 2+ and Ca 2+ ions about as well as UK-1. The non-cytotoxic benzimidazole UK-1 analogue binds Mg 2+ ions 50-fold weaker than UK-1, whereas the simple benzoxazole analogue binds Mg 2+ ions nearly as well as UK-1. These results support a role of Mg 2+ ion binding in the selective cytotoxicity of UK-1 and provide a minimal pharmacophore for the selective cytotoxic activity of the natural product. # 2002 Elsevier Science Ltd. All rights reserved. Introduction The bis(benzoxazole) natural products are a structurally unique class of Streptomyces secondary metabolites that have recently been reported in the literature. 1 3 In the course of a screening program for new bioactive com- pounds, Taniguchi and co-workers isolated UK-1 from the acetone extracts of Streptomyces sp. 517–02. 1 Sub- sequently, Tsuji and co-workers isolated AJI9561 from Streptomyces sp. AJ956. 2 Both UK-1 and AJI9561 were reported to possess growth inhibitory activity against the murine cancer cell line P388, with IC 50 values in the 0.3–1.6 mM range. 1,2 Despite its cancer cell cytotoxic properties, UK-1 does not inhibit the growth of Gram- positive or Gram-negative bacteria, yeast, or fungi at concentrations as high as 250 mM. 1 However, the semi- synthetic derivatives methyl UK-1 (MUK-1) and deme- thyl UK-1 (DMUK-1) both have activity against Gram- positive and Gram-negative bacteria. 3,4 MUK-1 is also active against yeast and filamentous fungi. 3 The selective cytotoxicity of UK-1 towards cancer cells versus bacteria and fungi indicates that UK-1 may have 0968-0896/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0968-0896(02)00327-9 Bioorganic & Medicinal Chemistry 10 (2002) 3997–4004 *Corresponding author. Tel.: +1-512-471-5074; fax: +1-512-232- 2696; e-mail: skerwin@mail.utexas.edu