Modulation of the stacking interaction of MN4 (M=Pt, Pd, Au) complexes with tryptophan through N-heterocyclic ligands

What is holding back the development of antiviral metallodrugs? A literature overview and implications for SARS-CoV-2 therapeutics and future viral outbreaks

In light of the Covid-19 outbreak, this review brings together historical and current literature efforts towards the development of antiviral metallodrugs. Classical compounds such as CTC-96 and auranofin are discussed in depth, as pillars for future metallodrug development. From the recent literature, both cell-based results and biophysical assays against potential viral biomolecule targets are summarized here. The comprehension of the biomolecular targets and their interactions with coordination compounds are emphasized as fundamental strategies that will foment further development of metal-based antivirals. We also discuss other possible and unexplored methods for unveiling metallodrug interactions with biomolecules related to viral replication and highlight the specific challenges involved in the development of antiviral metallodrugs.

Zinc finger domains as therapeutic targets for metal-based compounds - an update

Zinc finger proteins are one of the most abundant families of proteins and present a wide range of structures and functions. The structural zinc ion provides the correct conformation to specifically recognize DNA, RNA and protein sequences. Zinc fingers have essential functions in transcription, protein degradation, DNA repair, cell migration, and others. Recently, reports on the extensive participation of zinc fingers in disease have been published. On the other hand, much information remains to be unravelled as many genomes and proteomes are being reported. A variety of zinc fingers have been identified; however, their functions are still under investigation. Because zinc fingers have identified functions in several diseases, they are being increasingly recognized as drug targets. The replacement of Zn(ii) by another metal ion in zinc fingers is one of the most prominent methods of inhibition. From one side, zinc fingers play roles in the toxicity mechanisms of Ni(ii), Hg(ii), Cd(ii) and others. From the other side, gold, platinum, cobalt, and selenium complexes are amongst the compounds being developed as zinc finger inhibitors for therapy. The main challenge in the design of therapeutic zinc finger inhibitors is to achieve selectivity. Recently, the design of novel compounds and elucidation of the mechanisms of zinc substitution have renewed the possibilities of selective zinc finger inhibition by metal complexes. This review aims to update the status of novel strategies to selectively target zinc finger domains by metal complexes.

Synthesis and biological evaluation of substituted 3-(2'-benzimidazolyl)coumarin platinum(II) complexes as new telomerase inhibitors

Eight new platinum(II) complexes Pt1-Pt8 with substituted 3‑(2'‑benzimidazolyl) coumarins were successfully synthesized and characterized by single crystal X-ray diffraction analysis, nuclear magnetic resonance spectroscopy (NMR), electrospray ionization-mass spectrometry (ESI-MS), infrared spectrophotometry (IR) and elemental analysis. Crystallographic data of these Pt1-Pt8 complexes showed that the Pt(II) has distorted four-coordinated square planar geometry. Pt1-Pt8 were found to display high cytotoxic activity in vitro against the cisplatin-resistant SK-OV-3/DDP cancer cells with a low IC₅₀ from 1.01-10.32 μM, but low cytotoxicity on the normal HL-7702 cells. Further studies revealed that Pt1-Pt3 induced apoptosis in SK-OV-3/DDP cancer cells via mitochondria dysfunction signaling pathways. Our findings also indicated that Pt1 was a telomerase inhibitor targeting c-myc promoter elements.

Interaction of the HIV NCp7 Protein with Platinum(II) and Gold(III) Complexes Containing Tridentate Ligands

The human immunodeficiency virus (HIV) nucleocapsid protein (NCp7) plays significant roles in the virus life cycle and has been targeted by compounds that could lead to its denaturation or block its interaction with viral RNA. Herein, we describe the interactions of platinum(II) and gold(III) complexes with NCp7 and how the reactivity/affinity of potential inhibitors can be modulated by judicious choice of ligands. The interactions of [MCl(N₃)]ⁿ⁺ (M = Pt²⁺ (n = 1) and Au³⁺ (n = 2); N₃ = tridentate chelate ligands: bis(2-pyridylmethyl)methylamine (Mebpma, L¹) and bis(2-pyridylmethyl)amine (bpma, L²) with the C-terminal zinc finger of NCp7 (ZF2) were investigated by electrospray ionization-mass spectroscopy (ESI-MS). Mass spectra from the incubation of [MCl(Mebpma)]ⁿ⁺ complexes (PtL¹ and AuL¹) with ZF2 indicated that they were more reactive than the previously studied diethylenetriamine-containing analogues [MCl(dien)]ⁿ⁺. The initial product of reaction of PtL¹ with ZF2 results in loss of all ligands and release of zinc to give the platinated apopeptide {PtF} (F = apopeptide). This is in contrast to the incubation with [PtCl(dien)]⁺, in which {Pt(dien)}-peptide adducts are observed. Incubation of the Au³⁺ complex AuL¹ with ZF2 gave Au_xFⁿ⁺ species (x = 1, 2, 4, F = apopeptide) again with loss of all ligands. Furthermore, the formally substitution-inert analogues [Pt(N₃)L]²⁺ (L = 4-methylpyridine (4-pic), 4-dimethylaminopyridine (dmap), and 9-ethylguanine (9-EtGua)) were prepared to examine stacking interactions with N-acetyltryptophan (N-AcTrp), the Trp-containing ZF2, and the "full" two-finger NCp7 itself using fluorescence quenching titration. Use of bpma and Mebpma gave slightly higher affinity than analogous [Pt(dien)L)]²⁺ complexes. The dmap-containing complexes (PtL¹a and PtL²a) had the greatest association constants (K_a) for N-AcTrp and ZF2 peptide. The complex PtL¹a had the highest K_a when compared with other known Pt²⁺ analogues: [Pt(dien)(9-EtGua)]²⁺ < [Pt(bpma)(9-EtGua)]²⁺ < [Pt(dien)(dmap)]²⁺< PtL²a < PtL¹a. A K_a value of ca. 40.6 ± 1.0 × 10³ M^-1 was obtained for the full NCp7 peptide with PtL¹a. In addition, the mass spectrum of the interaction between ZF2 and PtL¹a confirms formation of a 1:1 PtL¹a/ZF2 adduct. The reactivity of selected complexes with sulfur-containing amino acid N-acetylcysteine (N-AcCys) was also investigated by ¹⁹⁵Pt and ¹H NMR spectroscopy and ESI-MS. The precursor compounds [PtCl(N₃)]⁺ PtL¹ and PtL² reacted readily, whereas their [Pt(N₃)L]²⁺ analogues PtL¹a and PtL²a were inert to substitution.

Enhancement of the physicochemical properties of [Pt(dien)(nucleobase)]2+ for HIVNCp7 targeting

Physicochemical properties of coordination compounds can be exploited for molecular recognition of biomolecules. The inherent π-π stacking properties of [Pt(chelate)(N-donor)]²⁺ ([PtN₄]) complexes were modulated by systematic variation of the chelate (diethylenetriamine and substituted derivatives) and N-donor (nucleobase or nucleoside) in the formally substitution-inert PtN₄ coordination sphere. Approaches to target the HIV nucleocapsid protein HIVNCp7 are summarized building on (i) assessment of stacking interactions with simple tryptophan or tryptophan derivatives to (ii) the tryptophan-containing C-terminal zinc finger and (iii) to the full two-zinc finger peptide and its interactions with RNA and DNA. The xanthosine nucleoside was identified as having significantly enhanced stacking capability over guanosine. Correlation of the LUMO energies of the modified nucleobases with the DFT π-stacking energies shows that frontier orbital energies of the individual monomers can be used as a first estimate of the π-stacking strength to Trp. Cellular accumulation studies showed no significant correlation with lipophilicity of the compounds, but all compounds had very low cytotoxicity suggesting the potential for antiviral selectivity. The conceptual similarities between nucleobase alkylation and platination validates the design of formally substitution-inert coordination complexes as weak Lewis acid electrophiles for selective peptide targeting.

The dynamics of zinc sites in proteins: electronic basis for coordination sphere expansion at structural sites

The functional role assumed by zinc in proteins is closely tied to the variable dynamics around its coordination sphere arising by virtue of its flexibility in bonding. Modern experimental and computational methods allow the detection and study of previously unknown features of bonding between zinc and its ligands in protein environment. These discoveries are occurring just in time as novel biological functions of zinc, which involve rather unconventional coordination trends, are emerging. In this sense coordination sphere expansion of structural zinc sites, as observed in our previous experiments, is a novel phenomenon. Here we explore the electronic and structural requirements by simulating this phenomenon in structural zinc sites using DFT computations. For this purpose, we have chosen MPW1PW91 and a mixed basis set combination as the DFT method through benchmarking, because it accurately reproduces structural parameters of experimentally characterized zinc compounds. Using appropriate models, we show that the greater ionic character of zinc coordination would allow for coordination sphere expansion if the steric and electrostatic repulsions of the ligands are attenuated properly. Importantly, through the study of electronic and structural aspects of the models used, we arrive at a comprehensive bonding model, explaining the factors that influence coordination of zinc in proteins. The proposed model along with the existing knowledge would enhance our ability to predict zinc binding sites in proteins, which is today of growing importance given the predicted enormity of the zinc proteome.

Dinuclear platinum complexes containing planar aromatic ligands to enhance stacking interactions with proteins

In an approach to design drugs with higher affinity for π-π stacking and electrostatic interactions with targeted biomolecules, complexes of the type [{cis-Pt(A)2 (L)}2 -μ-{trans-1,4-dach}](NO3 )4 ((A)2 =(NH3 )2 or ethylenediamine (en), L=quinoline (quin) or benzothiazole (bztz), dach=trans-1,4-diaminocyclohexane) were synthesized. The quinoline complex, [{cis-Pt(en)(quin)}2 -μ-(dach)](NO3 )4 (9) was synthesized from the precursor K[PtCl3 (quin)] (1), while the benzothiazole complexes, [{cis-Pt(A)2 (bztz)}2 -μ-(dach)](NO3 )4 ((A)2 =(NH3 )2 (10) and (A)2 =en (11)) were synthesized from the precursors cis-[Pt(A)2 Cl(bztz)] ((A)2 =(NH3 )2 (7) and (A)2 =en (8)). Their interactions with N-acetyltryptophan and a model pentapeptide (N-Ac-WLDSW-OH), modeled on the pentapeptide recognition sequence (FSDLW) of p53-mdm2 interaction, were examined by fluorescence spectroscopy. The dinuclear complexes were found to be significantly stronger at quenching the fluorescence of tryptophan than their mononuclear Pt-based analogues indicating stronger binding. Molecular modeling suggests a "sandwich" mode of binding, and the flexibility of the dinuclear motif can allow the design of more selective and stronger-binding complexes. Based on these results a further prototype, [{Pt(en)(9-EtGua)}2 μ-H2 N(CH2 )6 NH2 ](4+) , incorporating the purine 9-ethylguanine (9-EtG) as a stacking moiety, was prepared which showed good cytotoxicity in A2780 and OsACL tumor cell lines.