GREMMLENZ UNICAMP

The application of gold in medicine traces back ancient times, and modernly Au^I coordination compounds have been applied clinically to the treatment of rheumatoid arthritis [1,2]. Recently, research into gold compounds for biomedical application has seen a renaissance due to their high antibacterial [3,4], antiprotozoal [5,6], antitumor [5,7] and anti-HIV [[8], [9], [10]] in vitro activities. Also, a different mode of actions in comparison to classical Pt^II antineoplastic compounds is reported for gold compounds.

In spite of their impressive antiproliferative effects, high toxicity is also observed, mainly due to the remarkable reducing potential of Au^III/Au^I and Au^I/Au⁰ leading to extensive cell damage and drug inactivation [11,12]. In addition, Au^I and Au^III complexes undergo fast substitution reactions, which turns difficult to trace the active species and also limits their activity in animal models [9,13].

Several biomolecular targets are described for gold compounds, depending on the ligands, oxidation state, total charge, and geometry [1]. Thus, gold is an interesting example of activity modulation through coordination chemistry. For example, the gold^I lipophilic cations accumulate in the mitochondria and inhibit the selenoenzyme thioredoxin reductase, [14,15] while Au^I aliphatic compounds inhibit reverse transcriptase [16]. Moreover, aurothioglucose inhibits transcription factor NF-κB [17,18] whilst the phosphinegold(I) with S-donor ligand, auranofin, is involved in different phases of the immune reactions [19,20] and phosphinegold(I) with N-donor ligands replaces zinc in the zinc finger proteins [10]. Further, as Au^III complexes reduction in the biological milieu is always a possibility, the use of multidentate ligands, and nowadays organomettalic chemistry [6], have been described as a good strategy to decrease the reduction of Au^III to Au^I, besides DNA, RNA, zinc fingers, and thioredoxin have been suggested as potential targets [7,8,21].

Considering this scenario, synthetic inorganic chemists are engaged in finding methods and ligands to stabilize oxidation state and slow down ligand exchange reactions of gold complexes. Classically, phosphines have been used for the gold^I complexes stabilization, being the development of auranofin the most important example. Also, different families of ligands and organometallic chemistry have been developed to stabilize gold^I and gold^III complexes, allowing to the design of more secure and active gold drugs [5,6].

Tris(2-carboxyethyl)phosphine P(CH₂CH₂COOH)₃ (TCEP) is a reducing agent frequently used in biochemistry to reduce disulfide bonds in proteins, and peptides [22]. Its aqueous solubility, odorless, stoichiometric application and activity in a wide range of pH turns TCEP application very convenient. TCEP forms complexes with transition metals and the investigation of Ni^II, Cu^II, Zn^II, Cd^II, Pb^II, Pt^II, Co^III and Fe^I complexes were reported, with crystallographic structure determination for Zn^II, Cd^II, Pt^II, Co^III and Fe^I complexes. [[23], [24], [25], [26]] In almost of all these compounds, metals coordinate to the TCEP by the phosphorus (P) and carboxylate groups (COO⁻) except for the Fe^I, cis-[PtCl₂(TCEP)₂] and trans-[PtCl₂(TCEP)₂] complexes which coordinate only through the P atom [25].

TCEP has been previously reported promoting the reaction between cisplatin with Sp1 zinc finger [27] and between cisplatin and Atox 1 (Cu^II transporter ATP7B). The formation of a platinate adduct [Pt(TCEP)₂(NH₃)] was observed, which one TCEP was coordinated through P atom and COO⁻ group in a bidentate bond, and another one coordinated through phosphorus [28].

No Au^I/III complexes of TCEP have been reported yet. Isolation of a compound of gold in its oxidized states (+1 or +3) with TCEP is difficult due to the ability of TCEP to reduce gold ions to metal gold, forming gold nanoparticles, stabilized by TCEP ligands coordinated in the surface [29].

In this work we report the synthesis, characterization, crystal structure, solution behavior of the Au^I linear complex with TCEP (AuTCEP). The biophysical evaluation was performed by the interaction with amino acids [N-acetyl-l-cysteine (nac), l-histidine (his)], peptides [HIV-1-NCp7 C-terminal zinc finger, the Sp1 third zinc finger] and proteins [bovine serum albumin (BSA) and Bacillus cereus β-lactamases]. Besides, the in vitro antiproliferative activity of AuTCEP was evaluated against a panel of nine tumorigenic and one non-tumorigenic cell lines while the antibacterial activity was performed against four bacterial strains, and the combinatory therapy of ampicillin and AuTCEP was evaluated by the determination of minimum growth inhibitory concentration (MIC) in B. cereus strains.

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