Microwave-assisted solid-phase synthesis, cellular uptake, and cytotoxicity studies of cymantrene–peptide bioconjugates

A solid support-based synthetic strategy for the site-selective functionalization of peptides with organometallic half-sandwich moieties

The number of donor atoms available on peptides that can competitively coordinate to metal centers renders the site-selective generation of advanced metal-peptide conjugates in high purity a challenging venture. Herein, we present a transmetalation-based synthetic approach on solid support in which an imidazolium proligand can be used to selectively anchor a range of transition metal half-sandwich complexes onto peptides in the presence of multiple coordinative motifs. Amenable to solid support, a range of N-terminus and/or lysine conjugated metal-peptide conjugates were obtained in high purity after cleavage from the resin. The metalated peptides were evaluated for their anticancer properties against human cancer cell lines. While no cytotoxic activity was observed, this platform has the potential to i) provide a pathway to site-selective peptide labelling, ii) be explored as a biorthogonal handle and/or iii) generate a new strategy for ligand design in transition metal catalysts.

A novel type of organometallic 2-R-2,4-dihydro-1H-3,1-benzoxazine with R = [M(η5-C5H4)(CO)3] (M = Re or Mn) units. Experimental and computational studies of the effect of substituent R on ring-chain tautomerism

Novel 2-cyrhetrenyl and cymantrenyl-2,4-dihydro-1H-3,1-benzoxazines.

Synthesis of monofunctional platinum(iv) carboxylate precursors for use in Pt(iv)-peptide bioconjugates

Herein we present platinum(iv) bioconjugates with polyarginine peptides as prospective prodrug delivery systems. Asymmetrical platinum(iv) complexes 3 were obtained via oxidation of parent platinum(ii) complexes 2 with N-bromosuccinimide (NBS) in the presence of succinic anhydride. The combination of these two oxidation reagents furnishes the platinum(iv) environment with two different axial ligands, one of which bears a free carboxylic acid. All platinum(ii) and (iv) compounds were characterized by FT-IR, ESI-MS, HPLC, ¹H-, ¹³C- and ¹⁹⁵Pt-NMR. Standard solid-phase peptide chemistry was used for the synthesis of polyarginine (R₉) peptides. Coupling of the platinum complexes with peptides N-terminally afforded peptide monoconjugates, which were purified by semi-preparative HPLC and characterized by analytical HPLC and ESI-MS. Platinum(iv)-peptide bioconjugates as well as platinum(ii) and platinum(iv) complexes were tested as cytotoxic agents against two different human cancer cell lines (MCF-7, HepG2) and normal human fibroblasts cell lines (GM5657T). Preliminary in vitro data showed that all platinum(iv) complexes exhibit lower activity than their platinum(ii) precursors towards most cell lines. Interestingly, in the case of HepG2 cells, the Pt(iv)-(R)₉-G-A-L bioconjugate (4a) showed even higher activity compared to the non-targeting platinum(iv) parent compound.

Cyclopentadienyl iron dicarbonyl (CpFe(CO)2) derivatives as apoptosis-inducing agents

CpFe(CO)₂ complexes kill cancer cells while leaving normal cells unharmed.

Facile solid phase peptide synthesis with a Re-lysine conjugate generated via a one-pot procedure

We have synthesized a Re(CO)3-modified lysine via a one-pot Schiff base formation reaction that can be used in the solid phase peptide synthesis. To demonstrate its potential use, we have attached it to a neurotensin fragment and observed uptake into human umbilical vascular endothelial cells.

Identification of the structural determinants for anticancer activity of a ruthenium arene peptide conjugate

Organometallic Ru(arene)-peptide bioconjugates with potent in vitro anticancer activity are rare. We have prepared a conjugate of a Ru(arene) complex with the neuropeptide [Leu(5)]-enkephalin. [Chlorido(η(6)-p-cymene)(5-oxo-κO-2-{(4-[(N-tyrosinyl-glycinyl-glycinyl-phenylalanyl-leucinyl-NH2)propanamido]-1H-1,2,3-triazol-1-yl)methyl}-4H-pyronato-κO)ruthenium(II)] (8) shows antiproliferative activity in human ovarian carcinoma cells with an IC50 value as low as 13 μM, whereas the peptide or the Ru moiety alone are hardly cytotoxic. The conjugation strategy for linking the Ru(cym) (cym=η(6)-p-cymene) moiety to the peptide involved N-terminal modification of an alkyne-[Leu(5)]-enkephalin with a 2-(azidomethyl)-5-hydroxy-4H-pyran-4-one linker, using Cu(I)-catalyzed alkyne-azide cycloaddition (CuAAC), and subsequent metallation with the Ru(cym) moiety. The ruthenium-bioconjugate was characterized by high resolution top-down electrospray ionization mass spectrometry (ESI-MS) with regard to peptide sequence, linker modification and metallation site. Notably, complete sequence coverage was obtained and the Ru(cym) moiety was confirmed to be coordinated to the pyronato linker. The ruthenium-bioconjugate was analyzed with respect to cytotoxicity-determining constituents, and through the bioconjugate models [{2-(azidomethyl)-5-oxo-κO-4H-pyronato-κO}chloride (η(6)-p-cymene)ruthenium(II)] (5) and [chlorido(η(6)-p-cymene){5-oxo-κO-2-([(4-(phenoxymethyl)-1H-1,2,3-triazol-1-yl]methyl)-4H-pyronato-κO}ruthenium(II)] (6) the Ru(cym) fragment with a triazole-carrying pyronato ligand was identified as the minimal unit required to achieve in vitro anticancer activity.

Somatostatin subtype-2 receptor-targeted metal-based anticancer complexes

Conjugates of a dicarba analogue of octreotide, a potent somatostatin agonist whose receptors are overexpressed on tumor cells, with [PtCl(2)(dap)] (dap = 1-(carboxylic acid)-1,2-diaminoethane) (3), [(η(6)-bip)Os(4-CO(2)-pico)Cl] (bip = biphenyl, pico = picolinate) (4), [(η(6)-p-cym)RuCl(dap)](+) (p-cym = p-cymene) (5), and [(η(6)-p-cym)RuCl(imidazole-CO(2)H)(PPh(3))](+) (6), were synthesized by using a solid-phase approach. Conjugates 3-5 readily underwent hydrolysis and DNA binding, whereas conjugate 6 was inert to ligand substitution. NMR spectroscopy and molecular dynamics calculations showed that conjugate formation does not perturb the overall peptide structure. Only 6 exhibited antiproliferative activity in human tumor cells (IC(50) = 63 ± 2 μM in MCF-7 cells and IC(50) = 26 ± 3 μM in DU-145 cells) with active participation of somatostatin receptors in cellular uptake. Similar cytotoxic activity was found in a normal cell line (IC(50) = 45 ± 2.6 μM in CHO cells), which can be attributed to a similar level of expression of somatostatin subtype-2 receptor. These studies provide new insights into the effect of receptor-binding peptide conjugation on the activity of metal-based anticancer drugs, and demonstrate the potential of such hybrid compounds to target tumor cells specifically.

Influence of the metal center and linker on the intracellular distribution and biological activity of organometal-peptide conjugates

Organometallic complexes conjugated to cell-penetrating peptides (CPPs) are promising systems for diagnostic imaging and therapeutic applications in human medicine. Recently, we reported on the synthesis of cymantrene(CpMn(CO)(3))-CPP conjugates with biological activity on different cancer cell lines. However, the precise mechanism of cytotoxicity remained elusive in these studies. To investigate the role of the metal center and the linker between the CpM(CO)(3) moiety and the peptide, a number of derivatives with manganese replaced by rhenium and the keto linker originally used substituted by a methylene group were prepared and fully characterized by (1)H NMR spectroscopy, infrared spectroscopy, electrospray ionization mass spectrometry, and elemental analysis as well as X-ray structure determination. The organometal-peptide conjugates as well as carboxyfluorescein-labeled derivatives thereof were prepared by solid-phase peptide synthesis, purified by high-performance liquid chromatography, and analyzed by mass spectrometry. Fluorescence microscopy studies of MCF-7 human breast cancer cells revealed an efficient cellular uptake and pronounced nuclear localization of the bioconjugates with the methylene linker compared with systems with the keto group. In addition, the latter also showed a higher cytotoxicity. In contrast, the variation of the metal center from manganese to rhenium had a negligible effect. The structure-activity relationships determined in the present work will aid in the further tuning of the biological activity of organometal-peptide conjugates.

Photocontrolled DNA binding of a receptor-targeted organometallic ruthenium(II) complex

A photoactivated ruthenium(II) arene complex has been conjugated to two receptor-binding peptides, a dicarba analogue of octreotide and the Arg-Gly-Asp (RGD) tripeptide. These peptides can act as "tumor-targeting devices" since their receptors are overexpressed on the membranes of tumor cells. Both ruthenium-peptide conjugates are stable in aqueous solution in the dark, but upon irradiation with visible light, the pyridyl-derivatized peptides were selectively photodissociated from the ruthenium complex, as inferred by UV-vis and NMR spectroscopy. Importantly, the reactive aqua species generated from the conjugates, [(η(6)-p-cym)Ru(bpm)(H(2)O)](2+), reacted with the model DNA nucleobase 9-ethylguanine as well as with guanines of two DNA sequences, (5')dCATGGCT and (5')dAGCCATG. Interestingly, when irradiation was performed in the presence of the oligonucleotides, a new ruthenium adduct involving both guanines was formed as a consequence of the photodriven loss of p-cymene from the two monofunctional adducts. The release of the arene ligand and the formation of a ruthenated product with a multidentate binding mode might have important implications for the biological activity of such photoactivated ruthenium(II) arene complexes. Finally, photoreactions with the peptide-oligonucleotide hybrid, Phac-His-Gly-Met-linker-p(5')dCATGGCT, also led to arene release and to guanine adducts, including a GG chelate. The lack of interaction with the peptide fragment confirms the preference of such organometallic ruthenium(II) complexes for guanine over other potential biological ligands, such as histidine or methionine amino acids.

Influence of the metal complex-to-peptide linker on the synthesis and properties of bioactive CpMn(CO)3 peptide conjugates

By combining organometallic groups and peptides, a large number of conjugates with interesting new biological properties can be prepared. Especially, attachment to cell-penetrating peptides (CPP) that act as efficient cell delivery vehicles has come to the fore. However, the presence of the metal moiety in such systems can interfere with standard conjugate synthesis procedures which therefore need to be optimized for every new compound. In this work, we report on the preparation of six new cymantrene-sC18 peptide bioconjugates that were prepared by solid phase peptide synthesis (SPPS) techniques. The cymantrene complexes were chosen for their different linker to the peptide, to study the influence of the linker group on cellular uptake and cell viability of the conjugates. Interestingly, the attachment of the metal complex leads to a non-standard cleavage of the Rink amide linker used in the SPPS protocol under trifluoroacetic acid (TFA) treatment, resulting in peptide amides that are N-alkylated at the C-terminus. Furthermore, we found that depending on the type of cymantrene moiety attached, the formation of reactive carbocations which result from decomposition of the resin linker is facilitated and can alkylate the metal complex moiety. Both effects were analyzed by MS/MS studies and cleavage mixtures for efficient elimination of this byproduct formation were identified. Moreover, initial biological testing of the cytotoxicity of one of the bioconjugates gave promising results. Concentration-dependent cell viability studies of Cym1-sC18 on human MCF-7 breast adenocarcinoma cells gave an IC(50) value of 59.8 (+/- 6.7) microM and demonstrate their potential in anticancer chemotherapy.