Guanine and Plasmid DNA binding of Mono- and Trinuclear fac-[Re(CO) 3 ] + Complexes with Amino Acid Ligands Fabio Zobi, Bernhard Spingler, and Roger Alberto* [a] Introduction In the last few decades metal-based antitumor drugs have been playing an important role as therapeutic agents in anti- blastic chemotherapy. Cisplatin and other Pt II -based complexes remain the most effective inorganic drugs used in clinics but other transition-metal ions have received much attention as well. Among the more recent metal complexes that have been studied for their therapeutic potential, the octahedral geome- try is often encountered. Ruthenium(ii ) and (iii ) compounds, for example, have raised great interest, and complexes of the type [trans-RuCl 4 (X)(X’)] + (X = imidazole, indazole; X’ = Me 2 SO, indazole) have already entered phase I clinical trials. [1] Although the mode of action of these complexes is not yet well under- stood, there is evidence for DNA as a likely target, in a manner similar to the well-established platinum drugs. [2–19] Dinuclear rhodium acetate [Rh 2 (m-(O 2 CCH 3 ) 4 )(H 2 O) 2 ] and related com- plexes also showed good antitumor activity, [20] and structural studies suggest an analogous activity to that of cisplatin by binding two adjacent purines on DNA. [21–24] In the context of developing metal-based chemo- and radio- pharmaceuticals, our attention was attracted to recent studies that reported the cytotoxicity of a number of different com- pounds, all based on the fac-[Re(CO) 3 ] + core. Studies on L1210 lymphoid leukemia and other cell lines indicated that rheni- um(i ) alkoxo/hydroxo carbonyl complexes were effective in suppressing DNA synthesis through the inhibition of dihydro- folate reductase and other enzymes in the purine and pyrimi- dine pathways. Interaction with DNA, however, was not ruled out and it was suggested that the compounds may bind to the purine bases after displacement of the alkoxide or hydrox- ide ligands. [25] Similarly, the cytotoxicity of rhenium(i ) carbonyl 2-(dimethylamino)ethoxide complexes may involve binding to DNA bases or side chains of amino acid residues in peptides, [26] while phosphine-derivatized amine complexes seem unlikely to act as alkylating agents. [27] We have recently shown that the [M(CO) 3 ] + moiety (M = Re, 99 Tc) can bind two guanine bases in a cis fashion, [28] and X-ray crystallography confirmed that the two bases assume both a head-to-head (HH) and a head-to-tail (HT) conformation around the Re core. [29] The two bases can freely rotate about the Re N(7) bond, and neither intramolecular hydrogen bond- ing nor steric hindrance imposed by the carbonyl oxygen atom of the coordinated guanines are driving forces for the preference of one or the other conformation in the octahedral complex. [29] In order to understand whether the cytotoxicity exhibited by some fac-[Re(CO) 3 ] + based complexes is due to an alkylat- ing event resulting in the formation of inter- or intrastrand links between DNA bases, we have studied the interaction of a series of rhenium tricarbonyl complexes (Scheme 1) with FX174 plasmid DNA. For these studies we have prepared and fully characterized new complexes, three of which are based on l-proline (Pro) and N,N-dimethylglycine (dmGly). The com- plexes [Re(Pro)(CO) 3 ] 3 (1), [Re(dmGly)(CO) 3 ] 3 (2), and [Re(Pro)(9- MeG)(CO) 3 ](3 ; 9-MeG = 9-methylguanine) are rare examples of structurally characterized complexes with an amino acid bound to an organometallic Re moiety. As [Re(H 2 O) 3 (CO) 3 ] + (5) interacts unspecifically with potential coordination sites of proteins in human serum, this drug is not available in relevant concentrations for therapeutic use unless the coordination sites are protected. Once the prodrug is inside a cell, the protecting ligands should be released (for ex- ample, due to a decreased pH value), thereby setting free the active form of the metal drug. Amino acids seem to be good candidates for this purpose, mainly for two reasons: they afford robust complexes and are not foreign to biological sys- tems. Once displaced from the metal, they will not exhibit [a] F. Zobi, Dr. B. Spingler, Prof.Dr. R. Alberto Institute of Inorganic Chemistry, University of Zürich Winterthurerstrasse 190, 8057 Zürich (Switzerland) Fax:(+ 41)1-635-6803 E-mail:ariel@aci.unizh.ch We have synthesized and fully characterized four new complexes comprising the fac-[Re(CO) 3 ] + moiety and the ligands NH 3 , l-pro- line (Pro), or N,N-dimethylglycine (dmGly). The reaction of [Re- (H 2 O) 3 (CO) 3 ] + with the two amino acids gives trinuclear com- plexes of general formula [Re(L)(CO) 3 ] 3 (where L = amino acid). We have studied the in vitro behavior of these compounds with guanine and DNA in order to understand whether the cytotoxici- ty exhibited by certain rhenium complexes based on the fac- [Re(CO) 3 ] + core is due to the formation of nucleobase complexes and inter- or intrastrand links between DNA bases. We have per- formed model studies with guanine and studied the structural ef- fects induced by different rhenium(i ) tricarbonyl complexes on FX174 plasmid DNA by electrophoretic methods. Our results show that rhenium complexes with two available coordination sites interact with plasmid DNA to form a stable adduct that is likely to involve two bases. ChemBioChem 2005,6,1397–1405 DOI: 10.1002/cbic.200400453 # 2005 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim 1397