Aquation of the Ruthenium-Based Anticancer Drug NAMI-A: A Density Functional Study Neva Bes ˇker, Cecilia Coletti, Alessandro Marrone, and Nazzareno Re* Dipartimento di Scienze del Farmaco, UniVersita ` degli Studi “G. D’Annunzio”, I-66100 Chieti, Italy ReceiVed: January 16, 2008; In Final Form: February 20, 2008 We carried out density functional theory (DFT) calculations to investigate the thermodynamics and the kinetics of the double aquation reaction of the anticancer drug NAMI-A. Three explicit water molecules were included in the calculations to improve the PB solvation energies. Our calculations show that the chloride substitution reactions on the considered Ru(III) octahedral complex follow a dissociative interchange mechanism, I d , passing through a loose heptacoordinate transition state. We calculated an activation enthalpy and free energy for the first aquation step of 101.5 and 103.7 kJ mol -1 , respectively, values that are in good agreement with the available experimental results. The activation enthalpy and free energy for the second aquation step were found significantly higher, 118.7 and 125.0 kJ mol -1 , again in agreement with the experimental evidence indicating a slower rate for the second aquation. Introduction In attempts to find a new, metal-based anticancer drug with activity complementary to cisplatin, several ruthenium com- plexes have recently been investigated for their antitumor activity. 1-4 Among them, some ruthenium(III) complexes, designed on the model of cisplatin, have demonstrated favorable antitumor properties toward a number of in vitro and in vivo tumor models while showing lower systemic toxicity than platinum(II) compounds. 1-4 In particular, Keppler-type complexes (HL)[trans-RuCl 4 L 2 ] with L as a heterocyclic nitrogen ligand 5-7 and the related tetrachlororuthenium(III) dmso complexes of the type (X)[trans- RuCl 4 (dmso-S)L] (L ) imidazole; X ) Na or HL, NAMI or NAMI-A, respectively) 8-9 have shown interesting antimetastatic properties. NAMI-A is the first ruthenium antitumor complex that has entered clinical testing and has recently successfully completed a phase I trial. 9c The spectrum of the antitumor action of this complex differs significantly from that of cisplatin and presents an important antimetastatic rather than citotoxic activ- ity. 8,9 Although the mechanism of action of the NAMI-A-type complexes (HL)[trans-RuCl 4 (dmso-S)L] is not yet understood, they appear to be prodrugs that hydrolyze rapidly in vivo, forming a number of potentially active species. 10-12 The investigation of the hydrolytic properties of this type of complex is therefore important for determining the nature of the active species and could be useful to optimize the protocols for the administration of the drugs. NAMI-A is a hexacoordinate complex with pseudooctahedral geometry showing a square-planar arrangement of the four chlorides and the remaining dmso and imidazole ligands in the axial positions (Figure 1). In physiological conditions, it has been shown to undergo a hydrolysis reaction in which up to two chloride ligands are substituted by water leading to more reactive aquated species. 10-12 A deeper insight into the aquation of NAMI-A is important to understand its mechanism of action in vivo and may be useful to design new ruthenium-based anticancer drugs. To this end, we carried out density functional theory (DFT) calculations to investigate the thermodynamics and the kinetics of NAMI-A aquation (see Scheme 1). Although ruthenium antitumor com- pounds have been the subject of intense experimental research, very few quantum mechanical studies have been performed, either at semiempirical level 13a or on a specific organometallic ruthenium complex. 13b While the manuscript was in preparation, a theoretical study, the first one to the best of our knowledge, on the hydrolysis of NAMI-A was published by Chen et al. 14 However, that study presented a disagreement between the calculated activation enthalpy for the second aquation step, lower than the value for the first aquation step, and the experimental evidence showing a slower rate for the second step 12 and attributed to the inaccuracy of the employed solvent model. Moreover, our study has been carried out at significantly higher level of theory: we performed geometry optimization and transition state search in solution, using also diffuse functions on both ruthenium and main group atoms, and considered three explicit water molecules to account more accurately for specific hydrogen bond interactions between the ruthenium ligands including the leaving chloride anion and the solvent. Computational Details All calculations were performed with the Jaguar 6.0 quantum chemistry package, 15 using density functional theory (DFT) with the B3LYP hybrid functional, 16 which is known to give good descriptions of reaction profiles for transition metal-containing compounds. 17 The 1s-4d core electrons of the ruthenium atom were described with the Hay and Wadt core-valence relativistic effective core potential (ECP), 18 leaving the outer electrons to be treated explicitly by a basis set of double- quality plus one * To whom correspondence should be addressed. E-mail: nre@unich.it. Tel.: +3908713554603. Fax: +3908713554614. 3871 2008, 112, 3871-3875 Published on Web 03/11/2008 10.1021/jp800411g CCC: $40.75 © 2008 American Chemical Society