ELSEVIER Biological Properties of Two Gold(III) Complexes:AuC13(Upm)and AuC12(pm) Paola Calamai, Stofania Carotti, Annalisa Guerri, Luigi Messori, Enrico Mini, Pierluigi Orioli, and Gian Paolo Speroni PC, AG, LM, PO, GPS. Department of Chemistry, University of Florence, Florence, Italy.--SC, EM. Department of Pharmacology, University of Florence, Florence, Italy Abstract The reactivity in solution of two recently characterized gold(Ill) complexes, AuCl3(Hpm) and AuCl2(pm) , has been investigated in view of their potential use as anti- cancer agents. In water, both compounds undergo rela- tively fast hydrolysis of the bound chlorides without loss of the beterocycle ligand,, the process is much faster within a physiological buffer. When the two gold(Ill) complexes react with proteins like albumin or transfer- tin, reduction of gold(lll) to gold(I) and~ or hydrolysis is observed. On the other band, both complexes bind rapidly and tightly to either polynucleotides or calf thymus DNd, with gold remaining in the +3 oxidation state. Circular dicbroism investigations reveal a large perturbation of DNA confbrmation upon gold(liD binding; preferential binding to GC sequences is sbo~vn. Cytotoxicity studies on a number of tumor cell lines demonstrate a good activity of these gold(HI) complexes compared to cisplatin. How- ever, quick hydrolysis and~ or reduction of these com- pounds under plrysiological ¢onditious may represent a severe limitation to their use. Journal of Inorganic Bio- chemistry 66, 103-109 (1997) © 1997 Elsevier Science Inc. Introduction Owing to their strict similarity to platinum(II) complexes, d 8 square planar gold(III) complexes are of potential interest as anticancer agents. Unfortunately, in spite of the abundant literature existing on gold drugs (predomi- nantly gold(I) drugs), only a few reports concern the biological actions of gold(III) complexes [1]. The paucity of data on gold(III) compounds mainly derives from the high redox potential and the relatively poor stability of these compounds that make their use rather cumbersome under physiological conditions. However, from the few data presently available, and by comparison with plat- inum(II) complexes, it may be suggested that the biologi- cal action of the gold(III) complexes, and specifically their antitumor activity, is probably mediated by a direct interaction with DNA. For instance, Mirabelli et al. [2] reported that trichloropyridine gold(III) reacts with a number of different conformations of pBR322 DNA to Address correspondence to: Prof. Pierluigi Orioli, Department of Chemistry, University of Florence, via Gino Capponi 7, 50121 Flo- rence, Italy, E-mail: orioli@riscl.lrm.cnr.fi.it. produce interstrand crosslinks and single-strand breaks. Dabrowiak [3] showed that Et3PAuBr 3 binds to Hind III/Ncil, a 139 base pair restriction fragment from pBR322. Moreover, the binding of Au(III) complexes to DNA has been modeled through crystallographic and spectroscopic investigations of gold(III) complexes with nucleosides and nucleotides [4, 5]. A number of studies, based on different physicochemical techniques, suggest that probable binding sites for Au(III) are N1/N7 of adenine, N(7) or C(6)O of guanine, N(3) of cytosine, and N(3) of thymidine which are analogous to the binding sites for the isolelectronic platinum(II) ion [5-7]. Yet, it remains to be established to which extent gold(III) com- pounds are able to reach their presumed ultimate target, i.e., DNA, as such. Apparently, most of them are subject to hydrolysis and reduction to gold(I), this representing a major drawback for clinical applications. In order to further elucidate these aspects of gold(III) chemistry under physiological conditions and, specifi- cally, the reactivity of gold(III) compounds with biomolecules, we have prepared the complexes AuCl3(Hpm) and AuC12(pm) (Fig. 1), and investigated their reactions with proteins, DNA, and polynucleotides, and their cytotoxicity in vitro on a number of representa- tive tumor cell lines. The choice of these two compounds is dictated by their strict structural similarity to cisplatin as it emerges from the recently solved X-ray structure of AuC13(Hpm) [8], by the fact that both of them are neutral, and by the relatively high stability of the oxida- tion number +3 here achieved. For comparison purposes, we have also investigated the reactivity of the parent compound AuC14 which, different from the above com- pounds, exists in solu6on as a monoanionic species. Experimental Section Pbysicocbemical Studies The gold(Ill) complexes AuCl3(Hpm) and AuC12(pm) were synthesized according to the reported procedures [9]: a solution of 2-pyridylmethanol in water was added dropwise to a stirred solution of NaAuCI4-2H20 and sodium chloride in water at 0°C. A yellow solid separated immediately; the stirring went on for 45 min. The solid was filtered off, washed with water, and extracted with diethyl ether. The yellow ether solution was allowed to evaporate at room temperature. The orange diethyl ether-insoluble residue was AuCI 2 (pm). The purity of the resulting products was tested through elemental analysis. © 1997 Elsevier Science Inc. 0162-0134197 I$17.00 655 Avenue of the Americas, New York, NY 10010 Pl! S0162-0134(96)00190-0