Bis(acridinylthiourea)platinum(II) Complexes: Synthesis, DNA Affinity, and Biological Activity in Glioblastoma Cells ToddM.Augustus, a Joel Anderson, b SuzanneM.Hess b and Ulrich Bierbach a, * a Department of Chemistry, Wake Forest University, Winston-Salem, NC 27109, USA b Department of Radiation Oncology, Wake Forest University School of Medicine, Winston-Salem, NC 27157, USA Received 9 November 2002; revised 16 December 2002; accepted 16 December 2002 Abstract—The preparation of two novel bis(acridine)platinum(II) complexes is reported. The 4+ charged conjugates associate strongly with double-stranded native DNA (K i > 10 6 ), possibly through bisintercalation. A cell viability assay was used to demon- stratethatbothcompoundsarecapableofmediatingcytotoxicityatmicromolarconcentrationsinSNB19braintumorcells. # 2003ElsevierScienceLtd.Allrightsreserved. Recently, we have reported a new class of DNA-tar- geted hybrid platinum–acridine agents that show cyto- toxic activity at nano-to-micromolar concentrations in solid tumor and leukemia cancer cell lines. 1 The proto- type, conjugate 1, was synthesized by replacing one chloro leaving group in [PtCl 2 (en)] (en=ethane-1,2-dia- mine), a cisplatin analogue, with the novel 9-amino- acridine derivative, 1-[2-(acridin-9-ylamino)ethyl]-1,3- dimethylthiourea (2a). 1 Unlike intercalator-tethered cis- diaminedichloro complexes reported previously, 26 1 does not induce bifunctional covalent adducts (cross- links) in DNA but acts through a mechanism that involves monofunctional platination and intercalation of the planar chromophore into the DNA base stack. This type of adduct, which causes local unwinding of double-stranded DNA by 21  , is considered a potential cytotoxic lesion of the drug. 7 The sequence and groove specificity of platinum binding are currently under investigation. In unpurified preparations of conjugate 1, obtained from reactions of equimolar amounts of platinum pre- cursorandacridine,aminorimpuritywasobserved.We werenowabletoidentifythesideproductasthecorres- pondingbis(acridinylthiourea)platinum(II)complex, 3a. Monoactivation of [PtCl 2 (en)], achieved by abstraction of one chloro ligand with silver ion, obviously does not completely prevent the unwanted substitution of the second chloride by 2a (Scheme 1). The formation of 3a in the above reaction mixtures is most likely due to the high nucleophilicity of thiourea sulfur. 8 Sulfur donors exhibit a high affinity to divalent platinum and are knowntoreplacechloroligandswithoutpriorsolvolysis ofthePt-Clbond. 9 To investigate the DNA interactions and possible bio- logical effects of bis(acridinylthiourea)platinum(II) complexeswesynthesized 3a andthenewderivative, 3b, containing an acridinylthiourea with an extended pro- pylene flexible linker chain (2b). The tethering of two acridine moieties to platinum was achieved after removal of chloride in the precursor with two equiva- lents of silver nitrate, affording both complexes as their nitrate salts (Scheme 2). The new acridine derivative, 1-[3-(acridin-9-ylamino)propyl]-1,3-dimethylthiourea(2b, HNO 3 salt), was generated using the synthetic scheme developed for the prototype, 2a. 1 Briefly, the synthesis involved selective protection of the primary and 0960-894X/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0960-894X(02)01078-8 Bioorganic & Medicinal Chemistry Letters 13 (2003) 855–858 Scheme 1. *Corresponding author. Tel.: + 1-336-758-3507; fax: + 1-336-758- 4656; e-mail: bierbau@wfu.edu