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NAMI-A is a ruthenium-based drug endowed with the unique property of selectively targeting solid tumour metastases. Although two clinical studies had already been completed, limited information exists on the behavior of NAMI-A after injection into the bloodstream. PK data in humans informs us of a rather low free drug concentration, of a relatively high half-life time of elimination and of a linear relationship between the administered dose and the corresponding AUC for up to toxic doses. In the present study, we examined the chemical kinetics of albumin binding with or without the presence of reducing agents, and we evaluated how these chemical aspects might influence the in vivo PK and the in vitro ability of NAMI-A to inhibit cell migration, which is a bona fide, rapid and easy way to suggest anti-metastatic properties. The experimental data support the binding of NAMI-A to serum albumin. The reaction is facilitated when the drug is in its reduced form and, in agreement with already reported data, the adduct formed with albumin maintains the biological activity of the ruthenium drug. The formation of the adduct is favored by low ratios of NAMI-A : HSA and by the reduction of the drug with ascorbic acid. The difference in in vivo PK and the faster binding to albumin of the reduced NAMI-A seem to suggest that the drug is not rapidly reduced immediately upon injection, even at low doses. Most probably, cell and protein binding prevail over the reduction of the drug. This observation supports the thesis that the reduction of the drug before injection must be considered relevant for the pharmacological activity of NAMI-A against tumour metastases.

The ruthenium anticancer compound NAMI-A, imidazolium [trans-RuCl4(1H-imidazole)(DMSO-S)], is currently undergoing advanced clinical evaluation. As with other Ru(iii) chemotherapeutic candidates, interactions with human serum albumin (HSA) have been identified as a key component of the speciation of NAMI-A following intravenous administration. To characterize coordination to HSA, we have performed electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopic analysis of deuterium-labelled isotopologues of both NAMI-A and its bis-DMSO analogue, [(DMSO)2H][trans-RuCl4(DMSO-S)2] (Ru-bis-DMSO). Samples were prepared using phosphate buffered saline, in the presence of HSA, and with the individual amino acids histidine, cysteine, and alanine. Analysis of (1)H ENDOR spectra shows characteristic hyperfine interactions from DMSO, water, and imidazole ligands. Furthermore, coordination of imidazole ligands was confirmed from diagnostic (14)N ENDOR signals. Combined with the EPR data from the complexes following incubation in the presence of histidine, the ENDOR data demonstrate that both complexes bind to HSA via histidine imidazoles. Furthermore, the protein-bound species are shown to have water ligands and, in the case of Ru-bis-DMSO, one species has a remaining coordinated DMSO.

Triggered by the resounding success of cisplatin, the past decades have seen tremendous efforts to produce clinically beneficial analogues. The recent achievement of oxaliplatin for the treatment of colon cancer should, however, not belie the imbalance between a plethora of investigated complexes and a very small number of clinically approved platinum drugs. Strategies opening up new avenues are increasingly being sought using complexes of metals other than platinum such as ruthenium or gallium. Based on the chemical differences between these metals, the spectrum of molecular mechanisms of action and potential indications can be broadened substantially. Other approaches focus on complexes with tumour-targeting properties, thereby maximizing the impact on cancer cells and minimizing the problem of adverse side effects, and complexes with biologically active ligands.

The behaviour of the antimetastatic Ru(III) complex imidazolium [trans-RuCl₄(1H-imidazole)(DMSO-S)] (NAMI-A) under physiological conditions and its interactions with human serum albumin (hsA) have been studied using electron paramagnetic resonance spectroscopy (EPR). In physiological buffer at pH 7.4, these experiments demonstrate that the DMSO ligand is replaced rapidly by water, and spectra from the subsequent formation of five other Ru(III) complexes show further aquation processes. Although EPR spectra from mono-nuclear Ru(III) complexes are visible after 24 h in buffer, a significant decrease in the overall signal intensity following the first aquation step is consistent with the formation of oxo-bridged Ru(III) oligomers. Incubation with hsA reveals very rapid binding to the protein via hydrophobic interactions. This is followed by coordination through ligand exchange with protein side chains, likely with histidine imidazoles and at least one other specific site. Similar behaviour is observed when the complex is incubated in human serum, indicating that hsA binding dominates speciation in vivo. The addition of ascorbic acid to NAMI-A in buffer leads to quantitative reduction, producing EPR-silent Ru(II) complexes. However, this process is prevented when the complex binds coordinatively to hsA. Together, these results demonstrate the key role that hsA plays in defining the species found in vivo following intravenous treatment with NAMI-A, through prevention of oligomerization and maintenance of the oxidation state, to give protein-bound mono-nuclear Ru(III) species.

Ruthenium anticancer drugs have attracted an increasing interest in the last 20 years and two of them have entered clinical trials. Compared to platinum drugs, the complexes based on ruthenium are often identified as less toxic and capable of overcoming the resistance induced by platinum drugs in cancer cells. These activities were attributed to the transportation to tumour cells by transferrin and to the selective activation to more reactive species by the reducing environment of solid tumours as compared to healthy tissues. Ruthenium anticancer drugs have been almost always designed to mimic platinum drugs, particularly for targeting DNA. Indeed, none of the above properties has never been clearly demonstrated even for the ruthenium drugs that entered clinical trials. The suggestion for the future is to change the perspective when designing new chemical entities, abandoning the philosophy that guided the actual panel of ruthenium drugs and to look further into the fine mechanism by which the most relevant ruthenium complexes available kill the target tumour cells, then focusing on targets selective of tumour cells and responsible for cell growth and malignancy.

The imidazolium trans-tetrachloro(dimethylsulfoxide)imidazoleruthenate(III) complex [ImH][Ru(III)Cl(4)(DMSO)(Im)], NAMI-A, has shown an interesting antimetastatic activity. Since Ru(III) complexes are coordinatively more inert than the corresponding Ru(II) derivatives, an "activation by reduction" mechanism has been proposed to explain the biological activity of NAMI-A, thus acting as a pro-drug. We report here an electrochemical study on NAMI-A in aqueous solutions which emphasizes the structural and chemical consequences accompanying the easy Ru(III)/Ru(II) electron transfer (e.g., axial imidazole/water exchange in acidic solution in the short timescale of cyclic voltammetry followed by equatorial chloride/water exchange in the longer timescale of macroelectrolysis).