Self-assembly of organometallic Pd(II) complexes via CH3⋯π interactions: The first example of a cyclopalladated compound with herringbone stacking pattern

New insights into the mechanism of action of the cyclopalladated complex - CP2 in Leishmania: Calcium Dysregulation, Mitochondrial Dysfunction and Cell Death

The current treatment of leishmaniasis is based on few drugs that present several drawbacks such as high toxicity, difficult administration route, and low efficacy. These disadvantages raise the necessity to develop novel antileishmanial compounds allied to a comprehensive understanding of their mechanisms of action. Here, we elucidate the probably mechanism of action of the antileishmanial binuclear cyclopalladated complex [Pd(dmba)(μ-N₃)]₂ (CP2) in Leishmania amazonensis. CP2 causes oxidative stress in the parasite resulting in disruption of mitochondrial Ca²⁺ homeostasis, cell cycle arrest at S-phase, increasing the ROS production and overexpression of stress-related and cell detoxification proteins, collapsing the Leishmania mitochondrial membrane potential and promotes apoptotic-like features in promastigotes leading to necrosis or directs programmed cell death (PCD)-committed cells toward necrotic-like destruction. Moreover, CP2 is able to reduce the parasite load in both liver and spleen in Leishmania infantum-infected hamsters when treated for 15 days with 1.5 mg/Kg/day CP2, expanding its potential application in addition to the already known effectiveness on cutaneous leishmaniasis for the treatment of visceral leishmaniasis, showing the broad spectrum of action of this cyclopalladated complex. The data herein presented bring new insights into the CP2 molecular mechanisms of action, assisting to promote its rational modification to improve both safety and efficacy.

Synthesis and Structure of Multinuclear Pd(II) Complexes Bridged by Phosphide or Azide Ligands

The synthesis and structure of phosphide- and azide-bridged multinuclear Pd(II) complexes bearing phosphine ligands [PdX(μ-X')(PR₃)] _n (X = Cl and N₃; X' = PR₂ ^' and N₃; n = 2 and 4) are reported. The oxidative addition of R₂ ^'PCl to Pd(PMe₃)₂ furnished the phosphide-bridged dinuclear Pd(II) complexes [PdCl(μ-PR₂ ^')(PMe₃)]₂ [R' = ⁱ Pr (1a) and Cy (1b)]. However, the oxidative addition of (o-tolyl)₂PCl to Pd(PMe₃)₂ produced a nonbridged mononuclear Pd(II) complex with the bis(o-tolyl)phosphinic ligand, trans-[Pd(PMe₃)₂{P(O)(o-tolyl)₂}] (2), via oxidation of the phosphinyl ligand. The reaction of the chloride-bridged dinuclear Pd(II) complexes [PdCl(μ-Cl)(PR₃)]₂ [PR₃ = PEt₃ (3a) and PPhMe₂ (3b)] with NaN₃ afforded the azide-bridged dinuclear and tetranuclear Pd(II) complexes [Pd(N₃)(μ-N₃)(PEt₃)]₂ (4) and [Pd(N₃)(μ-N₃)(PPhMe₂)]₄ (5). Comparisons of the X-ray structures of 4 and 5 show that the square-planar molecular geometry of the Pd(II) centers of 4 are more distorted than those of 5. Density functional theory calculations suggest that the tetranuclear eight-membered ring structure like 5 is more stable than the dinuclear four-membered ring structure like 4 in the gas phase in both PEt₃ and PPhMe₂ systems. However, because the relative energy difference between the four-membered and eight-membered ring structures is small in the PEt₃ system with smaller steric hindrance compared with PPhMe₂, it is assumed that this difference is compensated by the crystal packing energy, and the dinuclear four-membered ring complex 4 is actually obtained.

Cyclopalladated compounds containing 2,6-lutidine: Synthesis, spectral and biological studies

Bridge splitting reactions between [Pd(C²,N-dmba)(μ-X)]₂ (dmba = N,N-dimethylbenzylamine; X = Cl, I, N₃, NCO) and 2,6-lutidine (lut) in the 1:2 molar ratio at room temperature afforded cyclopalladated compounds of general formulae [Pd(C²,N-dmba)(X)(lut)] {X = Cl^- (1), I^-(2), NNN^-(3), NCO^-(4)}, which were characterized by elemental analyses and infrared (IR), ¹H NMR spectroscopy. The molecular structures of all synthesized palladacycles have been solved by single-crystal X-ray crystallography. The cytotoxicity of the cyclopalladated compounds has been evaluated against a panel of murine {mammary carcinoma (4T1) and melanoma (B16F10-Nex2)} and human {melanoma (A2058, SK-MEL-110 and SK-MEL-5) tumor cell lines. All complexes were about 10 to 100-fold more active than cisplatin, depending on the tested tumor cell line. For comparison purposes, the cytotoxic effects of 1-4 towards human lung fibroblasts (MRC-5) have also been tested. The late apoptosis-inducing properties of 1-4 compounds in SK-MEL-5 cells were verified 24 h incubation using annexin V-Fluorescein isothiocyanate (FITC)/propidium iodide (PI). The binding properties of the model compound 1 on human serum albumin (HSA) and calf thymus DNA (ct-DNA) have been studied using circular dichroism and fluorescence spectroscopy. Docking simulations have been carried out to gain more information about the interaction of the palladacycle and HSA. The ability of compounds 1-4 to inhibit the activity of cathepsin B and L has also been investigated in this work.

Efficacy of a Binuclear Cyclopalladated Compound Therapy for Cutaneous Leishmaniasis in the Murine Model of Infection with Leishmania amazonensis and Its Inhibitory Effect on Topoisomerase 1B

Leishmaniasis is a disease found throughout the (sub)tropical parts of the world caused by protozoan parasites of the Leishmania genus. Despite the numerous problems associated with existing treatments, pharmaceutical companies continue to neglect the development of better ones. The high toxicity of current drugs combined with emerging resistance makes the discovery of new therapeutic alternatives urgent. We report here the evaluation of a binuclear cyclopalladated complex containing Pd(II) and N,N'-dimethylbenzylamine (Hdmba) against Leishmania amazonensis The compound [Pd(dmba)(μ-N3)]2 (CP2) inhibits promastigote growth (50% inhibitory concentration [IC50] = 13.2 ± 0.7 μM) and decreases the proliferation of intracellular amastigotes in in vitro incubated macrophages (IC50 = 10.2 ± 2.2 μM) without a cytotoxic effect when tested against peritoneal macrophages (50% cytotoxic concentration = 506.0 ± 10.7 μM). In addition, CP2 was also active against T. cruzi intracellular amastigotes (IC50 = 2.3 ± 0.5 μM, selective index = 225), an indication of its potential for use in Chagas disease therapy. In vivo assays using L. amazonensis-infected BALB/c showed an 80% reduction in parasite load compared to infected and nontreated animals. Also, compared to amphotericin B treatment, CP2 did not show any side effects, which was corroborated by the analysis of plasma levels of different hepatic and renal biomarkers. Furthermore, CP2 was able to inhibit Leishmania donovani topoisomerase 1B (Ldtopo1B), a potentially important target in this parasite. (This study has been registered at ClinicalTrials.gov under identifier NCT02169141.).

Two-dimensional assembly of magnetic binuclear complexes: a scanning tunneling microscopy study

Mono- and binuclear metal-organic compounds bearing long alkyl chains were synthesized and studied at the liquid/graphite interface using scanning tunneling microscopy. Two different lamellar surface patterns as well as a star like structure were obtained driven by van der Waals interactions of the alkyl chains and weak hydrogen bonds of the phenoxy moieties. In the case of the star like structure solvent molecules (1,2,4-trichlorobenzene) are supposed to play an important role for the stabilization of the created pattern. Magnetic investigation of the bulk material by a superconducting quantum interference device magnetometer revealed magnetic moments up to 1.7 mu(B) (NiCo) and most likely antiferromagnetic coupling between the two metals within a single complex. The presented two-dimensional crystallization of the binuclear complexes may provide an easy access to new designable materials in molecular electronics.

Antitumor and antimycobacterial activities of cyclopalladated complexes: X-ray structure of [Pd(C2,N-dmba)(Br)(tu)] (dmba=N,N-dimethylbenzylamine, tu=thiourea)

The reactions between [Pd(C(2),N-dmba)(micro-X)](2) (dmba=N,N-dimethylbenzylamine; X=Cl, Br) and thiourea (tu) in the 1:2 molar ratio at room temperature resulted in the mononuclear compounds [Pd(C(2),N-dmba)(Cl)(tu)] (1) and [Pd(C(2),N-dmba)(Br)(tu)] (2), which were characterized by elemental analyses and infrared (IR), (1)H- and (13)C{(1)H} NMR spectroscopies. The crystal and molecular structures of 2 were determined by single-crystal X-ray diffraction. In vitro cytotoxicity assays of the compounds 1, 2, tu, dmba and cisplatin were carried out using two murine tumor cell lines, namely mammary adenocarcinoma (LM3) and lung adenocarcinoma (LP07). The compounds 1, 2, tu and dmba were also tested against Mycobacterium tuberculosis and their MIC values were determined.