Synthesis and Structure-Activity Relationship Study of Organometallic Bioconjugates of the Cyclic Octapeptide Octreotate

Study on Chemical Modifications of Glutathione by Cold Atmospheric Pressure Plasma (Cap) Operated in Air in the Presence of Fe(II) and Fe(III) Complexes

Cold atmospheric pressure plasma is an attractive new research area in clinical trials to treat skin diseases. However, the principles of plasma modification of biomolecules in aqueous solutions remain elusive. It is intriguing how reactive oxygen and nitrogen species (RONS) produced by plasma interact on a molecular level in a biological environment. Previously, we identified the chemical effects of dielectric barrier discharges (DBD) on the glutathione (GSH) and glutathione disulphide (GSSG) molecules as the most important redox pair in organisms responsible for detoxification of intracellular reactive species. However, in the human body there are also present redox-active metals such as iron, which is the most abundant transition metal in healthy humans. In the present study, the time-dependent chemical modifications on GSH and GSSG in the presence of iron(II) and iron(III) complexes caused by a dielectric barrier discharge (DBD) under ambient conditions were investigated by IR spectroscopy, mass spectrometry and High Performance Liquid Chromatography (HPLC). HPLC chromatograms revealed one clean peak after treatment of both GSH and GSSH with the dielectric barrier discharge (DBD) plasma, which corresponded to glutathione sulfonic acid GSO₃H. The ESI-MS measurements confirmed the presence of glutathione sulfonic acid. In our experiments, involving either iron(II) or iron(III) complexes, glutathione sulfonic acid GSO₃H appeared as the main oxidation product. This is in sharp contrast to GSH/GSSG treatment with DBD plasma in the absence of metal ions, which gave a wild mixture of products. Also interesting, no nitrosylation of GSH/GSSG was oberved in the presence of iron complexes, which seems to indicate a preferential oxygen activation chemistry by this transition metal ion.

Ru(ii)-Peptide bioconjugates with the cppH linker (cppH = 2-(2'-pyridyl)pyrimidine-4-carboxylic acid): synthesis, structural characterization, and different stereochemical features between organic and aqueous solvents

Three new Ru(ii) bioconjugates with the C-terminal hexapeptide sequence of neurotensin, RRPYIL, namely trans,cis-RuCl2(CO)2(cppH-RRPYIL-κNp) (7), [Ru([9]aneS3)(cppH-RRPYIL-κNp)(PTA)](Cl)2 (8), and [Ru([9]aneS3)Cl(cppH-RRPYIL-κNp)]Cl (11), where cppH is the asymmetric linker 2-(2'-pyridyl)pyrimidine-4-carboxylic acid, were prepared in pure form and structurally characterized in solution. The cppH linker is capable of forming stereoisomers (i.e. linkage isomers), depending on whether the nitrogen atom ortho (No) or para (Np) to the carboxylate on C4 in the pyrimidine ring binds the metal ion. Thus, one of the aims of this work was to obtain pairs of stereoisomeric conjugates and investigate their biological (anticancer, antibacterial) activity. A thorough NMR characterization clearly indicated that in all cases exclusively Np conjugates were obtained in pure form. In addition, the NMR studies showed that, whereas in DMSO-d6 each conjugate exists as a single species, in D2O two (7) or even three if not four (8 and 11) very similar stable species form (each one corresponding to an individual compound). Similar results were observed for the cppH-RRPYIL ligand alone. Overall, the NMR findings are consistent with the occurrence of a strong intramolecular stacking interaction between the phenol ring of tyrosine and the pyridyl ring of cppH. Such stacking interactions between aromatic rings are expected to be stronger in water. This interaction leads to two stereoisomeric species in the free cppH-RRPYIL ligand and in the bioconjugate 7, and is somehow modulated by the less symmetrical Ru coordination environments in 8 and 11, affording three to four very similar species.

Designing metallodrugs with nuclease and protease activity

The accidental discovery of cisplatin some 50 years ago generated renewed interest in metallopharmaceuticals. Beyond cisplatin, many useful metallodrugs have been synthesized for the diagnosis and treatment of various diseases, but toxicity concerns, and the propensity to induce chemoresistance and secondary cancers make it imperative to search for novel metallodrugs that address these limitations. The Amino Terminal Cu(ii) and Ni(ii) (ATCUN) binding motif has emerged as a suitable template to design catalytic metallodrugs with nuclease and protease activities. Unlike their classical counterparts, ATCUN-based metallodrugs exhibit low toxicity, employ novel mechanisms to irreversibly inactivate disease-associated genes or proteins providing in principle, a channel to circumvent the rapid emergence of chemoresistance. The ATCUN motif thus presents novel strategies for the treatment of many diseases including cancers, HIV and infections caused by drug-resistant bacteria at the genetic level. This review discusses their design, mechanisms of action and potential for further development to expand their scope of application.

Binding of Cobaltocenium-containing Polyelectrolytes with Anionic Probes

Cationic cobaltocenium-containing polyelectrolytes have a unique ability to form ionic complex with various anionic species. We carried out two sets of model study to compare the relative binding strength of a cobaltocenium-containing polyelectrolyte. First, the nature and relative strength of intermolecular interaction between cobaltocenium-containing polyelectrolytes and different anionic probes were investigated by spectroscopic methods. A dye-displacement method was used to monitor absorbance and fluorescence emissions. Second, the binding strength of this cobaltocenium-containing polyelectrolyte was compared with a classical quaternary ammonium polymer. Formation of polyelectrolyte complex between the cobaltocenium-containing polyelectrolyte and a common anionic polyelectrolyte at various concentrations was examined by optical absorption and light scattering.

A Green Light-Triggerable RGD Peptide for Photocontrolled Targeted Drug Delivery: Synthesis and Photolysis Studies

We describe for the first time the synthesis and photochemical properties of a coumarin-caged cyclic RGD peptide and demonstrate that uncaging can be efficiently performed with biologically compatible green light. This was accomplished by using a new dicyanocoumarin derivative (DEAdcCE) for the protection of the carboxyl function at the side chain of the aspartic acid residue, which was selected on the basis of Fmoc-tBu SPPS compatibility and photolysis efficiency. The shielding effect of a methyl group incorporated in the coumarin derivative near the ester bond linking both moieties in combination with the use of acidic additives such as HOBt or Oxyma during the basic Fmoc-removal treatment were found to be very effective for minimizing aspartimide-related side reactions. In addition, a conjugate between the dicyanocoumarin-caged cyclic RGD peptide and ruthenocene, which was selected as a metallodrug model cargo, has been synthesized and characterized. The fact that green-light triggered photoactivation can be efficiently performed both with the caged peptide and with its ruthenocenoyl bioconjugate reveals great potential for DEAdcCE-caged peptide sequences as selective drug carriers in the context of photocontrolled targeted anticancer strategies.

Metal-containing and related polymers for biomedical applications

A survey of the most recent progress in the biomedical applications of metal-containing polymers is given. Due to the unique optical, electrochemical, and magnetic properties, at least 30 different metal elements, most of them transition metals, are introduced into polymeric frameworks for interactions with biology-relevant substrates via various means. Inspired by the advance of metal-containing small molecular drugs and promoted by the great progress in polymer chemistry, metal-containing polymers have gained momentum during recent decades. According to their different applications, this review summarizes the following biomedical applications: (1) metal-containing polymers as drug delivery vehicles; (2) metal-containing polymeric drugs and biocides, including antimicrobial and antiviral agents, anticancer drugs, photodynamic therapy agents, radiotherapy agents and biocides; (3) metal-containing polymers as biosensors, and (4) metal-containing polymers in bioimaging.

Using Hydrazine to Link Ferrocene with Re(CO)3: A Modular Approach

Acetyl ferrocene and diacetyl ferrocene both readily react with an excess of hydrazine to afford the corresponding hydrazone compounds. These compounds can then be linked to Re(CO)₃ via a metal-mediated Schiff base reaction, resulting in a series of ferrocene-Re(CO)₃ conjugates with different stoichiometries. Conjugates with 1:1, 1:2, and 2:1 ferrocene: Re(CO)₃ ratios can be produced via this "modular" type synthesis approach. Several examples of these conjugates were structurally characterized, and their spectroscopic, electrochemical, and spectroelectrochemical behaviors were investigated. The electronic structures of these compounds were also probed using DFT and TDDFT calculations.

Vesicular disruption of lysosomal targeting organometallic polyarginine bioconjugates

Compounds which are able to destabilize the lysosomal membrane have been proposed as interesting candidates for targeted anticancer drugs due to the pronounced lysosomal changes in cancer cells. For this purpose, metallocene derivatives of a cell penetrating polyarginine peptide M–(Arg)9(Phe)2Lys–NH2 (where M = ferrocene carboxylate or ruthenocene carboxylate) were designed and their biological activities were investigated in detail. The ferrocenoyl- and ruthenocenoyl polyarginine bioconjugates were synthesized via Fmoc solid-phase peptide synthesis (SPPS) protocols on a microwave-assisted synthesizer. After HPLC purification >98% purity was observed for all conjugates. Their interaction with supported biomimetic membranes was investigated on a quartz crystal microbalance (QCM) and revealed a very strong binding of the metallocene peptides and their metal-free congeners to an artificial eukaryotic membrane model (DMPC–cholesterol). To demonstrate their antiproliferative utility as cytotoxic compounds for a targeted anticancer drug, cell viability (by the crystal violet assay), apoptosis (flow cytometry, Ann V/PI staining), induction of reactive oxygen species (ROS, by flow cytometry with dihydroethidium staining), and changes in cancer cell metabolism, e.g. respiration and glycolysis, were studied. Our results reveal only a weak toxicity for the metal-free polyarginine peptide, which could be significantly enhanced (to ca. 50 μM against HeLa cells in the best case) by coupling ferrocene or ruthenocene carboxylates to the N-terminus of the peptide. The investigation of the cellular uptake and intracellular localization by fluorescence microscopy revealed an enhanced vesicular disruption by the metallocene bioconjugate compared to the metal-free derivative which could be triggered by light and chemicals. Further studies of apoptosis, respiration, glycolysis and ROS formation reveal the superior characteristics of the metallocene compounds. While most cells remain viable even at 300 μM of the metal free bioconjugate 1, most cells are dead or in late stages of apoptosis at 200 μM of the ruthenocene derivative 3, and at 100 μM of the most active ferrocene derivative 2, however, all show very little sign of necrosis. Also, the metal free compound 1 does not induce ROS formation but both metallocene–polyarginine bioconjugates are clearly associated with enhanced intracellular ROS levels, with levels for the redox-active ferrocene derivative being two times higher than for the structurally very similar but redox-silent ruthenocene derivative. We propose that such metallocene–polyarginine peptides induce lysosomal membrane permeabilization and thereby could be developed towards targeted anticancer drugs.

Integrin-targeted delivery into cancer cells of a Pt(IV) pro-drug through conjugation to RGD-containing peptides

Conjugates of a Pt(IV) derivative of picoplatin with monomeric (Pt-c(RGDfK), 5) and tetrameric (Pt-RAFT-{c(RGDfK)}4, 6) RGD-containing peptides were synthesized with the aim of exploiting their selectivity and high affinity for αVβ3 and αVβ5 integrins for targeted delivery of this anticancer metallodrug to tumor cells overexpressing these receptors. Solid- and solution-phase approaches in combination with click chemistry were used for the preparation of the conjugates, which were characterized by high resolution ESI MS and NMR. αVβ3 and αVβ5 integrin expression was evaluated in a broad panel of human cancer and non-malignant cells. SK-MEL-28 melanoma cells were selected based on the high expression levels of both integrins, while CAPAN-1 pancreatic cancer cells and 1BR3G fibroblasts were selected as the negative control. Internalization experiments revealed a good correlation between integrin expression and the cellular uptake of the corresponding fluorescein-labeled peptides and that the internalization capacity of the tetrameric RGD-containing peptide was considerably higher than that of the monomeric one. Cytotoxic experiments indicated that the antitumor activity of picoplatin in melanoma cells was increased by 2.6-fold when its Pt(IV) derivative was conjugated to c(RGDfK) (IC50 = 12.8 ± 2.1 μM) and by 20-fold when conjugated to RAFT-{c(RGDfK)}4 (IC50 = 1.7 ± 0.6 μM). In contrast, the cytotoxicity of the conjugates was inhibited in control cells lacking αVβ3 and αVβ5 integrin expression. Finally, cellular uptake studies by ICP-MS confirmed a good correlation between the levels of expression of integrins, intracellular platinum accumulation and antitumor activity. Indeed, accumulation and cytotoxicity were much higher in SK-MEL-28 cells than in CAPAN-1, being particularly higher in the case of the tetrameric conjugate. The overall results highlight that the great ability of RAFT-{c(RGDfK)}4 to bind to and to be internalized by integrins overexpressed in SK-MEL-28 cells results in higher accumulation of the Pt(IV) complex, leading to a high antitumor activity. These studies provide new insights into the potential of targeting αVβ3 and αVβ5 integrins with Pt(IV) anticancer pro-drugs conjugated to tumor-targeting devices based on RGD-containing peptides, particularly on how multivalency can improve both the selectivity and potency of such metallodrugs by increasing cellular accumulation in tumor tissues.

Expanding the scope of strained-alkyne chemistry: a protection–deprotection strategy via the formation of a dicobalt–hexacarbonyl complex

A protection–deprotection strategy for strained alkynes is reported. A strained alkyne can be protected with dicobalt–octacarbonyl and we demonstrate for the first time that the a strained alkyne can be re-formed and isolated under mild conditions for further bioorthogonal reactivity.