A multi-methodological inquiry of the behavior of cisplatin-based Pt(IV) derivatives in the presence of bioreductants with a focus on the isolated encounter complexes

Machine Learning-Based Prediction of Reduction Potentials for PtIV Complexes

Some of the well-known drawbacks of clinically approved PtII complexes can be overcome using six-coordinate PtIV complexes as inert prodrugs, which release the corresponding four-coordinate active PtII species upon reduction by cellular reducing agents. Therefore, the key factor of PtIV prodrug mechanism of action is their tendency to be reduced which, when the involved mechanism is of outer-sphere type, is measured by the value of the reduction potential. Machine learning (ML) models can be used to effectively capture intricate relationships within PtIV complex data, leading to highly accurate predictions of reduction potentials and other properties, and offering significant insights into their electrochemical behavior and potential applications. In this study, a machine learning-based approach for predicting the reduction potentials of PtIV complexes based on relevant molecular descriptors is presented. Leveraging a data set of experimentally determined reduction potentials and a diverse range of molecular descriptors, the proposed model demonstrates remarkable predictive accuracy (MSE = 0.016 V2, RMSE = 0.13 V, R2 = 0.92). Ab initio calculations and a set of different machine learning algorithms and feature engineering techniques have been employed to systematically explore the relationship between molecular structure and similarity and reduction potential. Specifically, it has been investigated whether the reduction potential of these compounds can be described by combining ML models across different combinations of constitutional, topological, and electronic molecular descriptors. Our results not only provide insights into the crucial factors influencing reduction potentials but also offer a rapid and effective tool for the rational design of PtIV complexes with tailored electrochemical properties for pharmaceutical applications. This approach has the potential to significantly expedite the development and screening of novel PtIV prodrug candidates. The analysis of principal components and key features extracted from the model highlights the significance of structural descriptors of the 2D Atom Pairs type and the lowest unoccupied molecular orbital energy. Specifically, with just 20 appropriately selected descriptors, a notable separation of complexes based on their reduction potential value is achieved.

Investigation of Siderophore-Platinum(IV) Conjugates Reveals Differing Antibacterial Activity and DNA Damage Depending on the Platinum Cargo

The growing threat of bacterial infections coupled with the dwindling arsenal of effective antibiotics has heightened the urgency for innovative strategies to combat bacterial pathogens, particularly Gram-negative strains, which pose a significant challenge due to their outer membrane permeability barrier. In this study, we repurpose clinically approved anticancer agents as targeted antibacterials. We report two new siderophore-platinum(IV) conjugates, both of which consist of an oxaliplatin-based Pt(IV) prodrug (oxPt(IV)) conjugated to enterobactin (Ent), a triscatecholate siderophore employed by Enterobacteriaceae for iron acquisition. We demonstrate that l/d-Ent-oxPt(IV) (l/d-EOP) are selectively delivered into the Escherichia coli cytoplasm, achieving targeted antibacterial activity, causing filamentous morphology, and leading to enhanced Pt uptake by bacterial cells but reduced Pt uptake by human cells. d-EOP exhibits enhanced potency compared to oxaliplatin and l-EOP, primarily attributed to the intrinsic antibacterial activity of its non-native siderophore moiety. To further elucidate the antibacterial activity of Ent-Pt(IV) conjugates, we probed DNA damage caused by l/d-EOP and the previously reported cisplatin-based conjugates l/d-Ent-Pt(IV) (l/d-EP). A comparative analysis of these four conjugates reveals a correlation between antibacterial activity and the ability to induce DNA damage. This work expands the scope of Pt cargos targeted to the cytoplasm of Gram-negative bacteria via Ent conjugation, provides insight into the cellular consequences of Ent-Pt(IV) conjugates in E. coli, and furthers our understanding of the potential of Pt-based therapeutics for antibacterial applications.

Protonated Forms of Naringenin and Naringenin Chalcone: Proteiform Bioactive Species Elucidated by IRMPD Spectroscopy, IMS, CID-MS, and Computational Approaches

Naringenin (Nar) and its structural isomer, naringenin chalcone (ChNar), are two natural phytophenols with beneficial health effects belonging to the flavonoids family. A direct discrimination and structural characterization of the protonated forms of Nar and ChNar, delivered into the gas phase by electrospray ionization (ESI), was performed by mass spectrometry-based methods. In this study, we exploit a combination of electrospray ionization coupled to (high-resolution) mass spectrometry (HR-MS), collision-induced dissociation (CID) measurements, IR multiple-photon dissociation (IRMPD) action spectroscopy, density functional theory (DFT) calculations, and ion mobility-mass spectrometry (IMS). While IMS and variable collision-energy CID experiments hardly differentiate the two isomers, IRMPD spectroscopy appears to be an efficient method to distinguish naringenin from its related chalcone. In particular, the spectral range between 1400 and 1700 cm^-1 is highly specific in discriminating between the two protonated isomers. Selected vibrational signatures in the IRMPD spectra have allowed us to identify the nature of the metabolite present in methanolic extracts of commercial tomatoes and grapefruits. Furthermore, comparisons between experimental IRMPD and calculated IR spectra have clarified the geometries adopted by the two protonated isomers, allowing a conformational analysis of the probed species.

Binding Modes of a Cytotoxic Dinuclear Copper(II) Complex with Phosphate Ligands Probed by Vibrational Photodissociation Ion Spectroscopy

The dinuclear copper complex bearing a 2,7-disubstituted-1,8-naphthalenediol ligand, [(HtomMe){Cu(OAc)}₂](OAc), a potential anticancer drug able to bind to two neighboring phosphates in the DNA backbone, is endowed with stronger cytotoxic effects and inhibition ability of DNA synthesis in human cancer cells as compared to cisplatin. In this study, the intrinsic binding ability of the charged complex [(HtomMe){Cu(OAc)}₂]⁺ is investigated with representative phosphate diester ligands with growing chemical complexity, ranging from simple inorganic phosphate up to mononucleotides. An integrated method based on high-resolution mass spectrometry (MS), tandem MS, and infrared multiple photon dissociation (IRMPD) spectroscopy in the 600-1800 cm^-1 spectral range, backed by quantum chemical calculations, has been used to characterize complexes formed in solution and delivered as bare species by electrospray ionization. The structural features revealed by IRMPD spectroscopy have been interpreted by comparison with linear IR spectra of the lowest-energy structures, revealing diagnostic signatures of binding modes of the dinuclear copper(II) complex with phosphate groups, whereas the possible competitive interaction with the nucleobase is silenced in the gas phase. This result points to the prevailing interaction of [(HtomMe){Cu(OAc)}₂]⁺ with phosphate diesters and mononucleotides as a conceivable contribution to the observed anticancer activity.

The Strange Case: The Unsymmetric Cisplatin-Based Pt(IV) Prodrug [Pt(CH(3)COO)Cl(2)(NH(3))(2)(OH)] Exhibits Higher Cytotoxic Activity with respect to Its Symmetric Congeners due to Carrier-Mediated Cellular Uptake

The biological behavior of the axially unsymmetric antitumor prodrug (OC-6-44)-acetatodiamminedichloridohydroxidoplatinum(IV), 2, was deeply investigated and compared with that of analogous symmetric Pt(IV) complexes, namely, dihydroxido 1 and diacetato 3, which have a similar structure. The complexes were tested on a panel of human tumor cell lines. Complex 2 showed an anomalous higher cytotoxicity (similar to that of cisplatin) with respect to their analogues 1 and 3. Their reduction potentials, reduction kinetics, lipophilicity, and membrane affinity are compared. Cellular uptake and DNA platination of Pt(IV) complexes were deeply investigated in the sensitive A2780 human ovarian cancer cell line and in the corresponding resistant A2780cisR subline. The unexpected activity of 2 appears to be related to its peculiar cellular accumulation and not to a different rate of reduction or a different efficacy in DNA platination and/or efficiency in apoptosis induction. Although the exact mechanism of cell uptake is not fully deciphered, a series of naïve experiments indicates an energy-dependent, carrier-mediated transport: the organic cation transporters (OCTs) are the likely proteins involved.

Elusive intermediates in cisplatin reaction with target amino acids: Platinum(II)-cysteine complexes assayed by IR ion spectroscopy and DFT calculations

The reactivity of a widely used metal based antineoplastic drug, cisplatin, cis-PtCl₂(NH₃)₂, with L-cysteine (Cys) has been investigated using a combination of electrospray ionization mass spectrometry (ESI-MS), IRMPD gas phase ion spectroscopy and DFT calculations. The cysteine lateral chain represents one of the main platination sites in proteins, which is believed to be related to the resistance mechanisms to cisplatin. The vibrational features of the mass-selected substitution product cis-[PtCl(NH₃)₂(Cys)]⁺ and the intercepted cis-[PtCl(NH₃)₂(H₂O)(Cys)]⁺ intermediate complex were compared to calculated IR spectra, enabling the assessment of the sampled ions structures. In cis-[PtCl(NH₃)₂(Cys)]⁺, cysteine was found to bind platinum through the sulfur atom as a thiolate zwitterion, highlighting the enhanced acidity of the cysteine thiol group upon metal coordination. The cis-[PtCl(NH₃)₂(H₂O)(Cys)]⁺ structure complies with the non-covalent encounter complex, formed by cis-[PtCl(NH₃)₂(H₂O)]⁺ and neutral cysteine. This species is able to undergo the substitution process to produce cis-[PtCl(NH₃)₂(Cys)]⁺ when activated as a mass-isolated ion suggesting its participation in the reaction mechanism of cisplatin with cysteine in solution. Finally, the DFT-calculated energy profile for the substitution reaction was correlated with the peculiar gas-phase reactivity of this non-covalent complex, resulting to be 10-fold less reactive toward substitution than the corresponding methionine complex.

Unusually Chemoselective Photocyclization of 2-(Hydroxyimino)aldehydes to Cyclobutanol Oximes: Synthetic, Stereochemical, and Mechanistic Aspects

Photocyclization of carbonyl compounds (known as the Norrish-Yang reaction) to yield cyclobutanols is, in general, accompanied by fragmentation reactions. The latter are predominant in the case of aldehydes so that secondary cyclobutanols are not considered accessible via the straightforward Norrish-Yang reaction. A noteworthy exception has been reported in our laboratory, where cyclobutanols bearing a secondary alcohol function were observed upon UV light irradiation of 2-(hydroxyimino)aldehydes (HIAs). This reaction is here investigated in detail by combining synthesis, spectroscopic data, molecular dynamics, and DFT calculations. The synthetic methodology is generally applicable to a series of HIAs, affording the corresponding cyclobutanol oximes (CBOs) chemoselectively (i.e., without sizable fragmentation side-reactions), diastereoselectively (up to >99:1), and in good to excellent yields (up to 95%). CBO oxime ether derivatives can be purified and diastereomers isolated by standard column chromatography. The mechanistic and stereochemical picture of this photocyclization reaction, as well as of the postcyclization E/Z isomerization of the oxime double bond is completed.

Microwave-Assisted Synthesis: Can Transition Metal Complexes Take Advantage of This “Green” Method?

Microwave-assisted synthesis is considered environmental-friendly and, therefore, in agreement with the principles of green chemistry. This form of energy has been employed extensively and successfully in organic synthesis also in the case of metal-catalyzed synthetic procedures. However, it has been less widely exploited in the synthesis of metal complexes. As microwave irradiation has been proving its utility as both a time-saving procedure and an alternative way to carry on tricky transformations, its use can help inorganic chemists, too. This review focuses on the use of microwave irradiation in the preparation of transition metal complexes and organometallic compounds and also includes new, unpublished results. The syntheses of the compounds are described following the group of the periodic table to which the contained metal belongs. A general overview of the results from over 150 papers points out that microwaves can be a useful synthetic tool for inorganic chemists, reducing dramatically the reaction times with respect to traditional heating. This is often accompanied by a more limited risk of decomposition of reagents or products by an increase in yield, purity, and (sometimes) selectivity. In any case, thermal control is operative, whereas nonthermal or specific microwave effects seem to be absent.

Cation-π Interactions between a Noble Metal and a Polyfunctional Aromatic Ligand: Ag+ (benzylamine)

The structure of an isolated Ag⁺ (benzylamine) complex is investigated by infrared multiple photon dissociation (IRMPD) spectroscopy complemented with quantum chemical calculations of candidate geometries and their vibrational spectra, aiming to ascertain the role of competing cation-N and cation-π interactions potentially offered by the polyfunctional ligand. The IRMPD spectrum has been recorded in the 800-1800 cm^-1 fingerprint range using the IR free electron laser beamline coupled with an FT-ICR mass spectrometer at the Centre Laser Infrarouge d'Orsay (CLIO). The resulting IRMPD pattern points toward a chelate coordination (N-Ag⁺ -π) involving both the amino nitrogen atom and the aromatic π-system of the phenyl ring. The gas-phase reactivity of Ag⁺ (benzylamine) with a neutral molecular ligand (L) possessing either an amino/aza functionality or an aryl group confirms N- and π-binding affinity and suggests an augmented silver coordination in the product adduct ion Ag + ( benzylamine ) ( L ) .

Photo-Enhanced Chemotherapy Performance in Bladder Cancer Treatment via Albumin Coated AIE Aggregates

The implementation of cisplatin-based neoadjuvant chemotherapy (NAC) plays a key role in conjunction with surgical resection in preventing bladder cancer progression and recurrence. However, the significant dose-dependent toxic side effects of NAC are still a major challenge. To solve this problem, we developed a photoenhanced cancer chemotherapy (PECC) strategy based on AIEgen ((E)-3-(2-(2-(5-(4-(diphenylamino)phenyl)thiophen-2-yl)vinyl)-1,1-dimethyl-1H-3λ⁴-benzo[e]indol-3-yl)propane-1-sulfonate), which is abbreviated as BITT. Multifunctional BITT@BSA-DSP nanoparticles (NPs) were employed with an albumin-based nanocarrier decorated with the cisplatin(IV) prodrug and loaded to produce strong near-infrared fluorescence imaging (NIR FLI), and they exhibited good photoenhancement performance via photodynamic therapy (PDT) and photothermal therapy (PTT). In vitro results demonstrated that BITT@BSA-DSP NPs could be efficiently taken up by bladder cancer cells and reduced to release Pt (II) under reductase, ensuring the chemotherapy effect. Furthermore, both in vitro and in vivo evaluation verified that the integration of NIR FL imaging-guided PECC efficiently promoted the sensitivity of bladder cancer to cisplatin chemotherapy with negligible side effects. This work provides a promising strategy to enhance the sensitivity of multiple cancers to chemotherapy drugs and even achieve effective treatments for drug-resistant cancers.

Ligation Motifs in Zinc-Bound Sulfonamide Drugs Assayed by IR Ion Spectroscopy

The sulfonamide–zinc ion interaction, performing a key role in various biological contexts, is the focus of the present study, with the aim of elucidating ligation motifs in zinc complexes of sulfa drugs, namely sulfadiazine (SDZ) and sulfathiazole (STZ), in a perturbation-free environment. To this end, an approach is exploited based on mass spectrometry coupled with infrared multiple photon dissociation (IRMPD) spectroscopy backed by quantum chemical calculations. IR spectra of Zn(H2O+SDZ−H)+ and Zn(H2O+STZ−H)+ ions are consistent with a three-coordinate zinc complex, where ZnOH+ binds to the uncharged sulfonamide via N(heterocycle) and O(sulfonyl) donor atoms. Alternative prototropic isomers Zn(OH2)(SDZ−H)+ and Zn(OH2)(STZ−H)+ lie 63 and 26 kJ mol−1 higher in free energy, respectively, relative to the ground state Zn(OH)(SDZ)+ and Zn(OH)(STZ)+ species and do not contribute to any significant extent in the sampled population.

Pt(IV) antitumor prodrugs: dogmas, paradigms, and realities

Platinum(II)-based drugs are widely used for the treatment of solid tumors, especially in combination protocols. Severe side effects and occurrence of resistance are the major limitations to their clinical use. To overcome these drawbacks, a plethora of Pt(IV) derivatives, acting as anticancer prodrugs, have been designed, synthesized and preclinically (often only in vitro) tested. Here, we summarize the recent progress in the development and understanding of the chemical properties and biochemical features of these Pt(IV) prodrugs, especially those containing bioactive molecules as axial ligands, acting as multi-functional agents. Even though no such prodrugs have been yet approved for clinical use, many show encouraging pharmacological profiles. Thus, a better understanding of their features is a promising approach towards improving the available Pt-based anticancer agents.

19F NMR Allows to Investigate the Fate of Platinum(IV) Prodrugs in Physiological Conditions

Pt(IV) prodrugs can overcome resistance and side effects of conventional Pt(II) anticancer therapies. By 19 F-labeling of a Pt(IV) prodrug (Pt-FBA, FBA = p -fluorobenzoate), the activation under physiological conditions could be investigated. It is found that, unlike single-electron reductants, multi-electron agents can efficiently promote the two electrons reduction of Pt(IV) to Pt(II). Moreover, the activation of Pt-FBA in cell lysate is highly dependent upon the type of cancer cells. When administered to E. coli , Pt-FBA is reduced intracellularly and free FBA can shuttle out of the cell. Interestingly, the reduction rate greatly increases by inducing metallothionein overexpression and is lowered by addition of Zn(II) ions. Finally, when injected into mice, Pt-FBA undergoes fast reduction in the bloodstream accompanied by metabolic degradation of FBA; nevertheless, unreduced Pt-FBA can accumulate to detectable levels in liver and kidneys. The proposed 19 F-NMR approach has the advantage of avoiding the interference of all background signals.

From Preassociation to Chelation: A Survey of Cisplatin Interaction with Methionine at Molecular Level by IR Ion Spectroscopy and Computations

Methionine (Met) plays an important role in the metabolism of cisplatin anticancer drug. Yet, methionine platination in aqueous solution presents a highly complex pattern of interconnected paths and intermediates. This study reports on the reaction of methionine with the active aqua form of cisplatin, cis-[PtCl(NH₃)₂(H₂O)]⁺, isolating the encounter complex of the reactant pair, {cis-[PtCl(NH₃)₂(H₂O)]⁺·Met}, by electrospray ionization. In the unsolvated state, charged intermediates are characterized for their structure and photofragmentation behavior by IR ion spectroscopy combined with quantum-chemical calculations, obtaining an outline of the cisplatin-methionine reaction at a molecular level. To summarize the major findings: (i) the {cis-[PtCl(NH₃)₂(H₂O)]⁺·Met} encounter complex, lying on the reaction coordinate of the Eigen-Wilkins preassociation mechanism for ligand substitution, is delivered in the gas phase and characterized by IR ion spectroscopy; (ii) upon vibrational excitation, ligand exchange occurs within {cis-[PtCl(NH₃)₂(H₂O)]⁺·Met}, releasing water and cis-[PtCl(NH₃)₂(Met)]⁺, along the calculated energy profile; (iii) activated cis-[PtCl(NH₃)₂(Met)]⁺ ions undergo NH₃ departure, forming a chelate complex, [PtCl(NH₃)(Met)]⁺, whose structure is congruent with overwhelming S-Met ligation as the primary coordination step. The latter process involving ammonia loss marks a difference with the prevailing chloride replacement in protic solvent, pointing to the effect of a low-polarity environment.

Can an Elusive Platinum(III) Oxidation State be Exposed in an Isolated Complex?

Platinum(IV) complexes are extensively studied for their activity against cancer cells as potential substitutes for the widely used platinum(II) drugs. Pt^IV complexes are kinetically inert and need to be reduced to Pt^II species to play their pharmacological action, thus acting as prodrugs. The mechanism of the reduction step inside the cell is however still largely unknown. Gas-phase activation of deprotonated platinum(IV) prodrugs was found to generate products in which platinum has a formal +3 oxidation state. IR multiple photon dissociation spectroscopy is thus used to obtain structural information helping to define the nature of both the platinum atom and the ligands. In particular, comparison of calculations at DFT, MP2 and CCSD levels with experimental results demonstrates that the localization of the radical is about equally shared between the d_xz orbital of platinum and the p_z of nitrogen on the amino group, the latter acting as a non-innocent ligand.

The effect of charge on the uptake and resistance to reduction of platinum(IV) complexes in human serum and whole blood models

cis- and trans-Platinum(iv) complexes with diaminetetracarboxylate coordination spheres possess the highly desirable property of exhibiting unusual resistance to reduction by blood serum components and endogenous reductants such as ascorbate. At the same time they are rapidly reduced in the intracellular environment of cancer cells. Consequently, they can potentially be tuned to remain intact in vivo until arrival at the tumour target where they are rapidly reduced to yield the active platinum(ii) species. However, in order to achieve this, uptake must be largely restricted to tumour cells and therefore uptake by healthy cells including red blood cells must be prevented. In this proof of concept study, we report on the effect of net charge as a means of controlling the uptake by red blood cells. Using 1H NMR spectroscopy we found that modifying the net charge of the complex does not influence the rate of reduction of the complexes by an excess of ascorbate. Using XANES spectroscopy we found that modifying the net charge of the platinum(iv) complexes decreased the extent of reduction in whole blood, although probably not to the degree needed for the optimal delivery to tumours. Therefore, it is likely to be necessary to adopt higher charges and/or additional strategies to keep platinum(iv) prodrugs out of blood cells.