Biological activity of PtIV prodrugs triggered by riboflavin-mediated bioorthogonal photocatalysis

Oxaliplatin(IV) Prodrugs Functionalized with Gemcitabine and Capecitabine Induce Blockage of Colorectal Cancer Cell Growth-An Investigation of the Activation Mechanism and Their Nanoformulation

The use of platinum-based anticancer drugs, such as cisplatin, oxaliplatin, and carboplatin, is a common frontline option in cancer management, but they have debilitating side effects and can lead to drug resistance. Combination therapy with other chemotherapeutic agents, such as capecitabine and gemcitabine, has been explored. One approach to overcome these limitations is the modification of traditional Pt(II) drugs to obtain new molecules with an improved pharmacological profile, such as Pt(IV) prodrugs. The design, synthesis, and characterization of two novel Pt(IV) prodrugs based on oxaliplatin bearing the anticancer drugs gemcitabine or capecitabine in the axial positions have been reported. These complexes were able to dissociate into their constituents to promote cell death and induce apoptosis and cell cycle blockade in a representative colorectal cancer cell model. Specifically, the complex bearing gemcitabine resulted in being the most active on the HCT116 colorectal cancer cell line with an IC50 value of 0.49 ± 0.04. A pilot study on the encapsulation of these complexes in biocompatible PLGA-PEG nanoparticles is also included to confirm the retention of the pharmacological properties and cellular drug uptake, opening up to the possible delivery of the studied complexes through their nanoformulation.

Flavin-Conjugated Pt(IV) Anticancer Agents

In situ activation of Pt(IV) to Pt(II) species is a promising strategy to control the anticancer activity and overcome the off-target toxicity linked to classic platinum chemotherapeutic agents. Herein, we present the design and synthesis of two new asymmetric Pt(IV) derivatives of cisplatin and oxaliplatin (1·TARF and 2·TARF, respectively) bearing a covalently bonded 2',3',4',5'-tetraacetylriboflavin moiety (TARF). ¹H and ¹⁹⁵Pt NMR spectroscopy shows that 1·TARF and 2·TARF can be effectively activated into toxic Pt(II) species, when incubated with nicotinamide adenine dinucleotide, sodium ascorbate, and glutathione in the dark and under light irradiation. Density functional theory studies of the dark Pt(IV)-to-Pt(II) conversion of 2·TARF indicate that the process involves first hydride transfer from the donor to the flavin moiety of the complex, followed by electron transfer to the Pt(IV) center. When administered to MDA-MB-231 breast cancer cells preincubated with nontoxic amounts of ascorbate, 2·TARF displays enhanced toxicity (between 1 and 2 orders of magnitude), suggesting that the generation of oxaliplatin can selectively be triggered by redox activation. Such an effect is not observed when 2 and TARF are coadministered under the same conditions, demonstrating that covalent binding of the flavin to the Pt complex is pivotal.

Photoinduced Reduction of Novel Dual-Action Riboplatin Pt(IV) Prodrug

Controlled photoreduction of Pt(IV) prodrugs is a challenging task due to the possibility of targeted light-controlled activation of anticancer agents without affecting healthy tissues. Also, a conjugation of photosensitizers and clinically used platinum drugs into one Pt(IV) prodrug allows combining photodynamic therapy and chemotherapy approaches into one molecule. Herein, we designed the cisplatin-based Pt(IV) prodrug Riboplatin with tetraacetylriboflavin in the axial position. A novel Pt(IV) prodrug is able to act both as a photodynamic therapy (PDT) agent through the conversion of ground-state ³O₂ to excited-state ¹O₂ and as an agent of photoactivated chemotherapy (PACT) through releasing of cisplatin under gentle blue light irradiation, without the requirement of a reducing agent. The light-induced behavior of Riboplatin was investigated using an electrochemical sensor in MCF-7 tumor spheroids. Photocontrolled cisplatin release and ROS generation were detected electrochemically in real time. This appears to be the first confirmation of simultaneous photoactivated release of anticancer drug cisplatin and ROS from a dual-action Pt(IV) prodrug observed from the inside of living tumor spheroids.

Recent Advances in Light-Controlled Activation of Pt(IV) Prodrugs

Pt(IV) prodrugs remain one of the most promising alternatives to conventional Pt(II) therapy due to their versatility in axial ligand choice and delayed mode of action. Selective activation from an external source is especially attractive due to the opportunity to control the activity of an antitumor drug in space and time and avoid damage to normal tissues. In this review, we discuss recent advances in photoabsorber-mediated photocontrollable activation of Pt(IV) prodrugs. Two main approaches developed are the focus of the review. The first one is the photocatalytic strategy based on the flavin derivatives that are not covalently bound to the Pt(IV) substrate. The second one is the conjugation of photoactive molecules with the Pt(II) drug via axial position, yielding dual-action Pt(IV) molecules capable of the controllable release of Pt(II) cytotoxic agents. Thus, Pt(IV) prodrugs with a light-controlled mode of activation are non-toxic in the absence of light, but show high antiproliferative activity when irradiated. The susceptibility of Pt(IV) prodrugs to photoreduction, photoactivation mechanisms, and biological activity is considered in this review.

A Self‐Assembling Flavin for Visible Photooxidation

A new flavin‐based gelator is reported which forms micellar structures at high pH and gels at low pH. This flavin can be used for the photooxidation of thiols under visible light, with the catalytic efficiency being linked to the self‐assembled structures present.

Flavin-mediated photoactivation of Pt(IV) anticancer complexes: computational insights on the catalytic mechanism

The mechanism for the photocatalytic activation of Pt(IV) anticancer prodrugs by riboflavin in the presence of NADH has been investigated by DFT. In the first step of the reaction, the oxidation kinetics of NADH to afford the catalytically active riboflavin hydroquinone is dramatically favoured by generation of the flavin triplet excited state. In the triplet, formation of a π-π stacked adduct promotes the hydride transfer from NADH to riboflavin with an almost barrierless pathway (2.7 kcal mol^-1). In the singlet channel, conversely, the process is endergonic and requires overcoming a higher activation energy (19.2 kcal mol^-1). In the second half of the reaction, the reduction of the studied Pt(IV) complexes by riboflavin hydroquinone occurs via an inner sphere mechanism, displaying free energy barriers smaller than 10 kcal mol^-1. Pt reduction by bioreductants such as NADH and ascorbate involve instead less stabilized transition states (22.2-38.3 kcal mol^-1), suggesting that riboflavin hydroquinone is an efficient reducing agent for Pt(IV) derivatives in biological settings.

Metal substrate catalysis in the confined space for platinum drug delivery

Catalysis-based approaches for the activation of anticancer agents hold considerable promise. These principally rely on the use of metal catalysts capable of deprotecting inactive precursors of organic drugs or transforming key biomolecules available in the cellular environment. Nevertheless, the efficiency of most of the schemes described so far is rather low, limiting the benefits of catalytic amplification as strategy for controlling the therapeutic effects of anticancer compounds. In the work presented here, we show that flavin reactivity within a hydrogel matrix provides a viable solution for the efficient catalytic activation and delivery of cisplatin, a worldwide clinically-approved inorganic chemotherapy agent. This is achieved by ionically adsorbing a flavin catalyst and a Pt(iv) prodrug as substrate into porous amino-functionalized agarose beads. The hydrogel chassis supplies high local concentrations of electron donating groups/molecules in the surrounding of the catalyst, ultimately boosting substrate conversion rates (TOF >200 min-1) and enabling controlled liberation of the drug by light or chemical stimuli. Overall, this approach can afford platforms for the efficient delivery of platinum drugs as demonstrated herein by using a transdermal diffusion model simulating the human skin.

Platinum(IV)-azido monocarboxylato complexes are photocytotoxic under irradiation with visible light

Complexes trans,trans,trans-[Pt(N3)2(OH)(OCOR)(py)2] where py = pyridine and where OCOR = succinate (1); 4-oxo-4-propoxybutanoate (2) and N-methylisatoate (3) have been synthesized by derivation of trans,trans,trans-[Pt(OH)2(N3)2(py)2] (4) and characterised by NMR and EPR spectroscopy, ESI-MS and X-ray crystallography. Irradiation of 1-3 with green (517 nm) light initiated photoreduction to Pt(ii) and release of the axial ligands at a 3-fold faster rate than for 4. TD-DFT calculations showed dissociative transitions at longer wavelengths for 1 compared to 4. Complexes 1 and 2 showed greater photocytotoxicity than 4 when irradiated with 420 nm light (A2780 cell line IC50 values: 2.7 and 3.7 μM) and complex 2 was particularly active towards the cisplatin-resistant cell line A2780cis (IC50 3.7 μM). Unlike 4, complexes 1-3 were phototoxic under green light irradiation (517 nm), with minimal toxicity in the dark. A pKa(H2O) of 5.13 for the free carboxylate group was determined for 1, corresponding to an overall negative charge during biological experiments, which crucially, did not appear to impede cellular accumulation and photocytotoxicity.

Enhancing the Photocatalytic Conversion of Pt(IV) Substrates by Flavoprotein Engineering

Our recent work demonstrates that certain flavoproteins can catalyze the redox activation of Pt(IV) prodrug complexes under light irradiation. Herein, we used site-directed mutagenesis on the mini singlet oxygen generator (mSOG) to modulate the photocatalytic activity of this flavoprotein toward two model Pt(IV) substrates. Among the prepared mutants, Q103V mSOG displayed enhanced catalytic efficiency as a result of its longer triplet excited-state lifetime. This study shows, for the first time, that protein engineering can improve the catalytic capacity of a protein toward metal-containing substrates.

Toward supramolecular nanozymes for the photocatalytic activation of Pt(IV) anticancer prodrugs

A supramolecular nanozyme for the photocatalytic conversion of a Pt(iv) anticancer complex to cisplatin is described herein. We employed 1.9 nm Au nanoparticles decorated with thiol ligands bearing a TACN (1,4,7-triazacyclononane) headgroup to encapsulate FMN (riboflavin-5'-phosphate). In the presence of an electron donor, flavin-loaded nanoparticles photocatalyzed the reductive activation of the prodrug cis,cis,trans-[Pt(NH3)2(Cl2)(O2CCH2CH2COOH)2] to cisplatin, achieving turnover frequency values of 7.4 min-1.

Transfer hydrogenation catalysis in cells

Hydrogenation reactions in biology are usually carried out by enzymes with nicotinamide adenine dinucleotide (NAD(P)H) or flavin mononucleotide (FAMH2)/flavinadenine dinucleotide (FADH2) as cofactors and hydride sources. Industrial scale chemical transfer hydrogenation uses small molecules such as formic acid or alcohols (e.g. propanol) as hydride sources and transition metal complexes as catalysts. We focus here on organometallic half-sandwich RuII and OsII η6-arene complexes and RhIII and IrIII η5-Cp x complexes which catalyse hydrogenation of biomolecules such as pyruvate and quinones in aqueous media, and generate biologically important species such as H2 and H2O2. Organometallic catalysts can achieve enantioselectivity, and moreover can be active in living cells, which is surprising on account of the variety of poisons present. Such catalysts can induce reductive stress using formate as hydride source or oxidative stress by accepting hydride from NAD(P)H. In some cases, photocatalytic redox reactions can be induced by light absorption at metal or flavin centres. These artificial transformations can interfere in biochemical pathways in unusual ways, and are the basis for the design of metallodrugs with novel mechanisms of action.

Functionalizing NaGdF4:Yb,Er Upconverting Nanoparticles with Bone-Targeting Phosphonate Ligands: Imaging and In Vivo Biodistribution

Lanthanide-doped upconverting nanoparticles (UCNPs) transform near infrared light (NIR) into higher-energy UV and visible light by multiphotonic processes. Owing to such unique feature, UCNPs have found application in optical imaging and have been investigated for the NIR light activation of prodrugs, including transition metal complexes of interest in photochemotherapy. Besides, UCNPs also function as magnetic resonance imaging (MRI) contrast agents and positron emission tomography (PET) probes when labelled with radionuclides such as 18F. In this contribution, we report on a new series of phosphonate-functionalized NaGdF4:Yb,Er UCNPs that show affinity for hydroxyapatite (inorganic constituent of bones), and we discuss their potential as bone targeting multimodal (MRI/PET) imaging agents. In vivo biodistribution studies of 18F-labelled NaGdF4:Yb,Er UCNPs in rats indicate that surface functionalization with phosphonates favours the accumulation of nanoparticles in bones over time. PET results reveal leakage of 18F− for phosphonate-functionalized NaGdF4:Yb,Er and control nanomaterials. However, Gd was detected in the femur for phosphonate-capped UCNPs by ex vivo analysis using ICP-MS, corresponding to 6–7% of the injected dose.

Cisplatin: The first metal based anticancer drug

Cisplatin or (SP-4-2)-diamminedichloridoplatinum(II) is one of the most potential and widely used drugs for the treatment of various solid cancers such as testicular, ovarian, head and neck, bladder, lung, cervical cancer, melanoma, lymphomas and several others. Cisplatin exerts anticancer activity via multiple mechanisms but its most acceptable mechanism involves generation of DNA lesions by interacting with purine bases on DNA followed by activation of several signal transduction pathways which finally lead to apoptosis. However, side effects and drug resistance are the two inherent challenges of cisplatin which limit its application and effectiveness. Reduction of drug accumulation inside cancer cells, inactivation of drug by reacting with glutathione and metallothioneins and faster repairing of DNA lesions are responsible for cisplatin resistance. To minimize cisplatin side effects and resistance, combination therapies are used and have proven more effective to defect cancers. This article highlights a systematic description on cisplatin which includes a brief history, synthesis, action mechanism, resistance, uses, side effects and modulation of side effects. It also briefly describes development of platinum drugs from very small cisplatin complex to very large next generation nanocarriers conjugated platinum complexes.

Catalysis Concepts in Medicinal Inorganic Chemistry

Catalysis has strongly emerged in the field of medicinal inorganic chemistry as a suitable tool to deliver new drug candidates and to overcome drawbacks associated to metallodrugs. In this Concept article, we discuss representative examples of how catalysis has been applied in combination with metal complexes to deliver new therapy approaches. In particular, we explain key achievements in the design of catalytic metallodrugs that damage biomolecular targets and in the development of metal catalysis schemes for the activation of exogenous organic prodrugs. Moreover, we discuss our recent discoveries on the flavin-mediated bioorthogonal catalytic activation of metal-based prodrugs; a new catalysis strategy in which metal complexes are unconventionally employed as substrates rather than catalysts.