Antiproliferative activity of a series of cisplatin-based Pt(IV)-acetylamido/carboxylato prodrugs

Platinum(IV) combo prodrugs containing cyclohexane-1R,2R-diamine, valproic acid, and perillic acid as a multiaction chemotherapeutic platform for colon cancer

The complex [PtCl2(cyclohexane-1R,2R-diamine)] has been combined in a Pt(IV) molecule with two different bioactive molecules (i.e., the histone deacetylase inhibitor 2-propylpentanoic acid or valproic acid, VPA, and the potential antimetastatic molecule 4-isopropenylcyclohexene-1-carboxylic acid or perillic acid, PA) in order to obtain a set of multiaction or multitarget antiproliferative agents. In addition to traditional thermal synthetic procedures, microwave-assisted heating was used to speed up their preparation. All Pt(IV) complexes showed antiproliferative activity on four human colon cancer cell lines (namely HCT116, HCT8, RKO and HT29) in the nanomolar range, considerably better than those of [PtCl2(cyclohexane-1R,2R-diamine)], VPA, PA, and the reference drug oxaliplatin. The synthesized complexes showed pro-apoptotic and pro-necrotic effects and the ability to induce cell cycle alterations. Moreover, the downregulation of histone deacetylase activity, leading to an increase in histone H3 and H4 levels, and the antimigratory activity, indicated by the reduction of the levels of matrix metalloproteinases MMP2 and MMP9, demonstrated the multiaction nature of the complexes, which showed biological properties similar to or better than those of VPA and PA, but at lower concentrations, probably due to the lipophilicity of the combo molecule that increases the intracellular concentration of the single components (i.e., [PtCl2(cyclohexane-1R,2R-diamine)], VPA and PA).

The Strange Case: The Unsymmetric Cisplatin-Based Pt(IV) Prodrug [Pt(CH3COO)Cl2(NH3)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.

Application of the anthraquinone drug rhein as an axial ligand in bifunctional Pt(IV) complexes to obtain antiproliferative agents against human glioblastoma cells

Octahedral Pt(IV) prodrugs are an effective way to combine cisplatin-like moieties and a second drug to obtain selective and stimuli responsive bifunctional antiproliferative compounds. Recently, two bifunctional Pt(IV) complexes have shown interesting in vitro and in vivo effects in glioblastoma, the most aggressive primary brain tumor. An interesting observation indicates that 4,5-dihydroxy-9,10-dioxo-9,10-dihydroanthracene-2-carboxylic acid (rhein) can inhibit in vivo glioma tumor progression. Furthermore, a prodrug in which cisplatin was combined with two molecules of rhein showed a potency higher than that of cisplatin toward cisplatin-resistant lung carcinoma cells. However, the high lipophilicity of this type of complex affects their solubility and bioavailability. To overcome these limits, in the present work, three Pt(IV) derivatives were obtained by differently linking one molecule of rhein and one acetato ligand at the axial position to a cisplatin core. The complexes proved to be similar to or more potent than the parent cisplatin and rhein, and the reference drug temozolomide on two human glioblastoma cell lines (U87-MG and T98G). They retained their activity under hypoxia and caused a significant reduction in the motility of both cell lines, which can be related to their ability to inhibit MMP2 and MMP9 matrix metalloproteinases. Finally, physicochemical and computational studies indicated that these Pt(IV) derivatives are more prone than rhein to cross the blood-brain barrier.

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.

Optimization of axial ligands to promote the photoactivation of BODIPY-conjugated platinum(IV) anticancer prodrugs

Carboplatin-based platinum(IV) prodrugs containing axial carboxylates are relatively resistant to reduction to release active platinum(II) species and kill cancer cells. To facilitate the activation process, a boron dipyrromethene (BODIPY) ligand has been utilized as a photoabsorber at the axial position to photoactivate carboplatin-based platinum(IV) complexes. However, the influence of the axial ligands on the photoactivation rate of the platinum center and the subsequent biological activity are still unknown. In this study, we report the design and synthesis of a series of carboplatin-based photoactivable platinum(IV) prodrugs containing BODIPY axial ligands with different lengths. The resulting BODIPY-conjugated platinum(IV) prodrugs OH2C-OH8C bearing hydroxido ligands at the opposite axial position are slightly less stable in the dark than the corresponding prodrugs AC2C-AC8C containing acetato ligands. The prodrugs OH3C-OH8C can be photoactivated under irradiation in eight minutes, and the photoactivation rate is further improved in prodrugs AC3C-AC8C where only twenty seconds are needed. Moreover, the prodrug AC3C, in which the linker between the BODIPY photoabsorber and the platinum center has an appropriate length, is photoactivated the quickest among the acetylated prodrugs AC2C-AC8C. The high cellular accumulation may contribute more to the moderate photocytotoxicity of these prodrugs. Our research highlights the way to promote the photoactivation of BODIPY-conjugated platinum(IV) anticancer prodrugs by optimization of axial ligands and may contribute to the future rational design of photoactivable platinum-based complexes.

Unsymmetric Cisplatin-Based Pt(IV) Conjugates Containing a PARP-1 Inhibitor Pharmacophore Tested on Malignant Pleural Mesothelioma Cell Lines

Cisplatin is widely employed as a first-line chemotherapeutic agent for many solid tumors, including malignant pleural mesothelioma (MPM). However, its clinical use is limited by heavy side effects and acquired resistance, the latter being mainly related to enhanced DNA repair. Many clinical trials using combinations of platinum drugs and PARP-1 inhibitors (PARPis) have been carried out, with the hope that such combinations might lead to improved therapeutic efficacy against tumors. Here, the synthesis and efficacy in reducing MPM cell viability of four cisplatin-based Pt(IV) prodrugs containing the PARPi 3-aminobenzamide (3-ABA) fragment are described. The most promising conjugate is more effective than cisplatin or cisplatin/3-ABA combination, administered in equimolar doses, in inhibiting PARP-1 activity and inducing apoptosis in BRCA1/2 wild type MPM cells, grown as monolayer or as multicellular spheroids.

Pt(iv) complexes based on cyclohexanediamines and the histone deacetylase inhibitor 2-(2-propynyl)octanoic acid: synthesis, characterization, cell penetration properties and antitumor activity

The Pt(iv) complexes based on (SP-4-2)-dichlorido(cyclohexane-1,4-diamine)platinum(ii) (kiteplatin) and the histone deacetylase inhibitor 2-(2-propynyl)octanoic acid (POA) were investigated. Since POA contains a chiral carbon, all the possible Pt(iv) isomers were prepared and characterized, and their antiproliferative activity on six cancer cell lines was compared with that of the corresponding Pt(iv) complexes containing the cyclohexane-1R,2R-diamine equatorial ligand. To justify the very good antiproliferative activity (nanomolar IC₅₀), the polarity, lipophilicity, permeability, and cell accumulation of the complexes were studied. Overall, the two series of Pt(iv) complexes showed similar cell penetration properties, being significantly better than that of the Pt(ii) reference compounds. Finally, a representative compound of the whole set of complexes (i.e., that based on cyclohexane-1R,2R-diamine and racemic POA) was tested in vivo on mice bearing Lewis lung carcinoma, showing good tumor growth inhibition with negligible body weight loss.

Cis,cis,trans-[PtIVCl2(NH3)2(perillato)2], a dual-action prodrug with excellent cytotoxic and antimetastatic activity

Two Pt(iv) conjugates containing one or two molecules of perillic acid (4-isopropenylcyclohexene-1-carboxylic acid), an active metabolite of limonene, were synthesized both with traditional and microwave-assisted methods and characterized. Their antiproliferative activity was tested on a panel of human tumor cell lines. In particular, cis,cis,trans-[Pt^IVCl₂(NH₃)₂(perillato)₂] exhibited excellent antiproliferative and antimetastatic activity on A-549 lung tumor cells at nanomolar concentrations. A number of in vitro biological tests were performed to decipher some aspects of its mechanism of action, including transwell migration and invasion as well as wound healing assay.

Diverse and unexpected outcomes from oxidation of the platinum(II) anticancer agent [Pt{(p-BrC6F4)NCH2CH2NEt2}Cl(py)] by hydrogen peroxide

Oxidation of the anti-tumour agent [Pt{(p-BrC₆F₄)NCH₂CH₂NEt₂}Cl(py)], 1 (py = pyridine) with hydrogen peroxide under a variety of conditions yields a range of organoenamineamidoplatinum(II) compounds [Pt{(p-BrC₆F₄)NCH=C(X)NEt₂}Cl(py)] (X = H, Cl, Br) as well as species with shared occupancy involving H, Cl and Br. Thus, oxidation of the -CH₂-CH₂- backbone (dehydrogenation) occurs, often accompanied by substitution. Oxidation of 1 with H₂O₂ in acetone yielded 1:1 co-crystallized [Pt{(p-BrC₆F₄)NCH=CHNEt₂}Cl(py)], 1H and [Pt{(p-BrC₆F₄)NCH=C(Cl)NEt₂}Cl(py)], 1Cl. The former was obtained pure in low yield from the oxidation of 1 with (NH₄)₂[Ce(NO₃)₆] in acetone, and the latter was obtained from 1 and H₂O₂ in CH₂Cl₂ at near reflux. From the latter reaction under vigorous refluxing [Pt{(p-BrC₆F₄)NCH=C(Br)NEt₂}Cl(py)], 1Br was isolated. In refluxing acetonitrile, oxidation of 1 with H₂O₂ yielded [Pt{(p-BrC₆F₄)NCH=C(H₀.₂₅Br_0.75)NEt₂}Cl(py)], 1H_0.25Br_0.75, in which the alkene is mainly substituted by Br in a dual occupancy. Treatment of 1 with H₂O₂ and tetrabutylammonium hydroxide in acetone at room temperature formed [Pt{(p-HC₆F₄)NCH₂CH₂NEt₂}Cl(py)], 2. Oxidation of [Pt{(p-HC₆F₄)NCH₂CH₂NEt₂}Br(py)], 3 with H₂O₂ in boiling acetonitrile gave the ligand oxidation product [Pt{(p-HC₆F₄)NCH=C(Br)NEt₂}Br(py)], 3Br. All major products were identified by X-ray crystallography as well as by ¹H and ¹⁹F NMR spectra. In cases of mixed crystals or dual occupancy compounds, the ¹⁹F and ¹H NMR spectra showed dissociation into the components in the solution in the same proportions as in isolated crystalline material.

S. Paqui M, Amorim AL, P. de Paula C, Rocha MC, Malavazi I, Cunha A, S. Santana F, Ribeiro RR, Gariani RA, Mendes SR, Xavier FR. Polypyridyl iron(iii) complexes containing long alkyl chains: synthesis, characterization, DFT calculations and biological activity

A bis(picolyl)amine ligand was functionalized with a long alkyl chain, and two iron(iii) complexes were prepared and their biological activity was presented.

BODI-Pt, a Green-Light-Activatable and Carboplatin-Based Platinum(IV) Anticancer Prodrug with Enhanced Activation and Cytotoxicity

Platinum drugs are widely used in clinics to treat various types of cancer. However, a number of severe side effects induced by the nonspecific binding of platinum drugs to normal tissues limit their clinical use. The conversion of platinum(II) drugs into more inert platinum(IV) derivatives is a promising strategy to solve this problem. Some platinum(IV) prodrugs, such as carboplatin-based tetracarboxylatoplatinum(IV) prodrugs, are not easily reduced to active platinum(II) species, leading to low cytotoxicity in vitro. In this study, we report the design and synthesis of a carboplatin-based platinum(IV) prodrug functionalized with a boron dipyrromethene (bodipy) ligand at the axial position, and the ligand acts as a photoabsorber to photoactivate the platinum(IV) prodrug. This compound, designated as BODI-Pt, is highly stable in the dark but quickly activated under irradiation to release carboplatin and the axial ligands. A cytotoxic study reveals that BODI-Pt is effective under irradiation, with cytotoxicity 11 times higher than that in the dark and 39 times higher than that of carboplatin in MCF-7 cells. Moreover, BODI-Pt has been proven to kill cancer cells by binding to the genomic DNA, arresting the cell cycle at the G₂/M phase, inducing oncosis, and generating ROS upon irradiation. In summary, we report a green-light-activatable and carboplatin-based Pt(IV) prodrug with improved cytotoxicity against cancer cells, and our strategy can be used as a promising way to effectively activate carboplatin-based platinum(IV) prodrugs.

Stability, Reduction, and Cytotoxicity of Platinum(IV) Anticancer Prodrugs Bearing Carbamate Axial Ligands: Comparison with Their Carboxylate Analogues

Platinum(IV) complexes containing carboxylate and carbamate ligands at the axial position have been reported previously. A better understanding of the similarity and difference between the two types of ligands will provide us with new insights and more choices to design novel Pt(IV) complexes. In this study, we systematically investigated and compared the properties of Pt(IV) complexes bearing the two types of ligands. Ten pairs of unsymmetric Pt(IV) complexes bearing axial carbamate or carboxylate ligands were synthesized and characterized. The stability of these Pt(IV) complexes in a PBS buffer with or without a reducing agent was investigated, and most of these complexes exhibited good stability. Besides, most Pt(IV) prodrugs with carbamate axial ligands were reduced faster than the corresponding ones with carboxylate ligands. Furthermore, the aqueous solubilities and lipophilicities of these Pt(IV) complexes were tested. All the carbamate complexes showed better aqueous solubility and decreased lipophilicity as compared to those of the corresponding carboxylate complexes, due to the increased polarity of carbamate ligands. Biological properties of these complexes were also evaluated. Many carbamate complexes showed cytotoxicity similar to that of the carboxylate complexes, which may derive from the lower cellular accumulation but faster reduction of the former. Our research highlights the differences between the Pt(IV) prodrugs containing carbamate and carboxylate axial ligands and may contribute to the future rational design of Pt-based anticancer prodrugs.

Quantum theory of atoms in molecules, electron localization function, and localized-orbital locator investigations on trans-(NHC)PtI2(para-NC5H4X) complexes

In this study, the MPW1PW91 method is applied to analyze the quantum theory of atoms in molecules, the electron localization function, and the localized-orbital locator in trans-(NHC)PtI2( para-NC5H4X) (X = H, F, COOH, CN, NO2, Me, OH, NH2) complexes. The substituent effect is assessed in the presence of electron-withdrawing groups and electron-donating groups and their influence on the Pt–C and Pt–N bonds of the molecules is analyzed using quantum theory of atoms in molecules, electron localization function, and localized-orbital locator methods. In addition, the eta index (η) is used to evaluate the Pt–C and Pt–N bonds in the studied complexes. The correlations between electron localization function, localized-orbital locator, and the η index values of Pt–C and Pt–N bonds with Hammett constants (σp) and dual parameters (σI and σR) are given.

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

The study of Pt(IV) antitumor prodrugs able to circumvent some drawbacks of the conventional Pt(II) chemotherapeutics is the focus of a lot of attention. This paper reports a thorough study based on experimental methods (reduction kinetics, electrochemistry, tandem mass spectrometry and IR ion spectroscopy) and quantum-mechanical DFT calculations on the reduction mechanism of cisplatin-based Pt(IV) derivatives having two hydroxido (1), one hydroxido and one acetato (2), or two acetato ligands (3) in axial position. The biological reductants glutathione and ascorbic acid were taken into consideration. The presence of a hydroxido ligand resulted to play an important role in the chemical reduction with ascorbic acid, as verified by ¹⁵N-NMR kinetic analysis using ¹⁵N-enriched complexes. The reactivity trend (1 > 2 > 3) does not reflect the respective reduction peak potentials (1 < 2 < 3), an inverse relationship already documented in similar systems. Turning to a simplified environment, the Pt(IV) complexes associated with a single reductant molecule (corresponding to the encounter complex occurring along the reaction coordinate in bimolecular reactions in solution) were characterized by IR ion spectroscopy and sampled for their reactivity under collision-induced dissociation (CID) conditions. The complexes display a comparable reduction reactivity ordering as that observed in solution. DFT calculations of the free energy pathways for the observed fragmentation reactions provide theoretical support for the CID patterns and the mechanistic hypotheses on the reduction process are corroborated by the observed reaction paths. The bulk of these data offers a clue of the intricate pathways occurring in solution.Graphic abstract.

Synthesis of 2-deoxy-d-glucose coated Fe3O4 nanoparticles for application in targeted delivery of the Pt(iv) prodrug of cisplatin – a novel approach in chemotherapy

Pt(IV) prodrug of cisplatin was loaded on 2DG functionalized silica coated Fe₃O₄ nanoparticles. The formulation alone exhibited biocompatibility whereas Pt(IV) loaded formulation exhibited cytotoxicity comparable with cisplatin.

Elusive Intermediates in the Breakdown Reactivity Patterns of Prodrug Platinum(IV) Complexes

Kinetically inert platinum(IV) complexes are receiving growing attention as promising candidates in the effort to develop safe and valid alternatives to classical square-planar Pt(II) complexes currently used in antineoplastic therapy. Their antiproliferative activity requires intracellular Pt(IV)-Pt(II) reduction (activation by reduction). In the present work, a set of five Pt(IV) complexes has been assayed using mass spectrometry-based techniques, i.e., collision-induced dissociation (CID), and IR multiple photon dissociation (IRMPD) spectroscopy, together with ab initio theoretical investigations. Breakdown and reduction mechanisms are observed that lead to Pt(II) species. Evidence is found for typically transient Pt(III) intermediates along the dissociation paths of isolated, negatively charged (electron-rich) Pt(IV) prodrug complexes.

Synthesis and Cytotoxic Study of a Platinum(IV) Anticancer Prodrug with Selectivity toward Luteinizing Hormone-Releasing Hormone (LHRH) Receptor-Positive Cancer Cells

Platinum drugs including cisplatin are widely used in clinics to treat various types of cancer. However, the lack of cancer-cell selectivity is one of the major problems that lead to side effects in normal tissues. Luteinizing hormone-releasing hormone (LHRH) receptors are overexpressed in many types of cancer cells but rarely presented in normal cells, making LHRH receptor a good candidate for cancer targeting. In this study, we report the synthesis and cytotoxic study of a novel platinum(IV) anticancer prodrug functionalized with LHRH peptide. This LHRH-platinum(IV) conjugate is highly soluble in water and quite stable in a PBS buffer. Cytotoxic study reveals that the prodrug selectively targets LHRH receptor-positive cancer cell lines with the cytotoxicities 5-8 times higher than those in LHRH receptor-negative cell lines. In addition, the introduction of LHRH peptide enhances the cellular accumulation in a manner of receptor-mediated endocytosis. Moreover, the LHRH-platinum(IV) prodrug is proved to kill cancer cells by binding to the genomic DNA, inducing apoptosis, and arresting the cell cycle at the G₂/M phase. In summary, we report a novel LHRH-platinum(IV) anticancer prodrug having largely improved selectivity toward LHRH receptor-positive cancer cells, relative to cisplatin.

Antiproliferative Activity of Pt(IV) Conjugates Containing the Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) Ketoprofen and Naproxen †

Cisplatin and several non-steroidal anti-inflammatory drugs (NSAIDs) have been proven to act synergistically or at least additively on several tumor cell lines. Dual-action cisplatin-based Pt(IV) combos containing ketoprofen and naproxen offer good antiproliferative performance on a panel of human tumor cell lines, including a malignant pleural mesothelioma (MPM) one, a very chemoresistant tumor. The main reason of the increased activity relies on the enhanced lipophilicity of these Pt(IV) conjugates that in turn promotes increased cellular accumulation. A quick Pt(IV)→Pt(II) reduction generates the active cisplatin metabolite. The NSAID adjuvant action seems to be almost independent from cyclooxygenase-2 (COX-2) expression in the tumor cells under investigation (lung A-549, colon HT-29, HCT 116, SW480, ovarian A2780, and biphasic MPM MSTO-211H), but it seems to rely (at least in part) on the activation of the NSAID activated gene, NAG-1 (a member of the transforming growth factor beta, TGF-β, superfamily), which has been suggested to be involved in NSAID antiproliferative activity.

Pt(IV) Bifunctional Prodrug Containing 2-(2-Propynyl)octanoato Axial Ligand: Induction of Immunogenic Cell Death on Colon Cancer

The synthesis, characterization, and in vitro activity of a cyclohexane-1 R,2 R-diamine-based Pt(IV) derivative containing the histone deacetylase inhibitor rac-2-(2-propynyl)octanoato, namely, ( OC-6-44)-acetatodichlorido(cyclohexane-1 R,2 R-diamine)( rac-2-(2-propynyl)octanoato)platinum(IV), are reported together with those of its isomers containing enantiomerically enriched axial ligands. These Pt(IV) complexes showed comparable activity, of 2 orders of magnitude higher than reference drug oxaliplatin on three human (HCT 116, SW480, and HT-29) and one mouse (CT26) colon cancer cell lines. In vivo experiments were carried out on immunocompetent BALB/c mice bearing the same syngeneic tumor. The complex ( OC-6-44)-acetatodichlorido(cyclohexane-1 R,2 R-diamine)( rac-2-(2-propynyl)octanoato)platinum(IV) showed higher tumor mass Pt accumulation than oxaliplatin, due to its higher lipophilicity, with negligible nephro- and hepatotoxicities when administered intravenously. A remarkable tumor mass invasion by cytotoxic CD8+ T lymphocytes, following the Pt(IV) treatment, indicated a strong induction of immunogenic cell death.

Synthesis and characterization of cyclohexane-1R,2R-diamine-based Pt(iv) dicarboxylato anticancer prodrugs: their selective activity against human colon cancer cell lines

Three pairs of asymmetric dicarboxylato derivatives based on the cisplatin and oxaliplatin-like skeletons have been synthesized de novo or re-synthesized. The axial ligands consist of one medium-chain fatty acid (MCFA), namely clofibrate (i.e. 2-(p-chlorophenoxy)-2-methylpropionic acid, CA), heptanoate (HA) or octanoate (OA), respectively, and an inactive acetato ligand that imparts acceptable water solubility to such conjugates. Stability tests provided evidence for the partial formation of two hydrolyzed products, corresponding to two monoaqua diastereomers derived from the substitution of an equatorial chlorido ligand with a water molecule. The complexes have been tested on three different colon cancer cell lines having different histological history, and also on the cisplatin-sensitive A2780 ovarian cancer cell line for comparison. This allowed the evaluation not only of the increase in activity on passing from Pt(ii) to Pt(iv) derivatives, but also the selectivity towards colon cancer cells brought about by the cyclohexane-1R,2R-diamine carrier ligand.

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.

Hybrid inorganic (nonporous silica)/organic (alginate) core-shell platform for targeting a cisplatin-based Pt(IV) anticancer prodrug

Nonporous silica nanoparticles with an external shell containing the 3‑aminopropyl arm (SiNP) were further decorated with alginic acid (SiNP-ALG) as a potential biocompatible delivery system for Pt antitumor agents. Such particles were coupled with the prodrug (OC‑6‑44)‑acetato(β‑alaninato)diamminedichloridoplatinum(IV), 1, through the formation of amide bonds between the pendant carboxylate groups on SiNP-ALG and the free amino group of the complex. Cytosol extracted from tumor cells was able to quickly and efficiently reduce the Pt(IV) prodrug, and produces the active metabolite cisplatin. SiNP-ALG-Pt conjugate was more active than both cisplatin and 1, due to its more efficient cell uptake, whereas the SiNP-ALG unplatinated nanoparticles were deprived of any nonspecific toxicity.

An unsymmetric cisplatin-based Pt(iv) derivative containing 2-(2-propynyl)octanoate: a very efficient multi-action antitumor prodrug candidate

The design, synthesis, characterization and biological properties of a Pt(iv) complex containing the very active inhibitor of histone deacetylase (2-propynyl)octanoic acid, POA, as an axial ligand are reported here. The title complex, namely (OC-6-44)-acetatodiamminedichlorido(2-(2-propynyl)octanoato)platinum(iv), 1, containing POA in racemic or in enantiomeric forms, was one/two orders of magnitude more active than cisplatin, depending on the chemo-sensitivity of the cancer cell lines. Moreover, 1 exhibited similar or even better antiproliferative activity than (OC-6-33)-diamminedichloridobis(2-propylpentanoato)platinum(iv), 2, containing two molecules of the well-known histone deacetylase inhibitor 2-propylpentanoic (valproic) acid. The high potency of 1 is likely due to its high cellular accumulation and to the synergism between the DNA-damaging cisplatin and the histone deacetylase inhibitor POA, both released upon the intracellular reduction of 1. Prodrug 1, after oral administration, caused an impressive reduction of the tumor mass (94%) in a model of solid tumor (murine Lewis lung carcinoma), compared to that of the control, whereas (intraperitoneal) cisplatin induced a tumor regression of 75% only. A good accumulation of 1 was observed in the tumor mass. The time course of the body weight attested that cisplatin induced elevated anorexia, whereas treatment with 1 did not induce significant body weight loss throughout the therapeutic experiment.

Synthesis, Structure, and Cytotoxicity of Oxaliplatin-Based Platinum(IV) Anticancer Prodrugs Bearing One Axial Fluoride

Fluorine plays more and more important roles in drug design and development. In recent years, fluorine-containing organic drugs have already been applied in a broad range of therapeutic areas. Herein, we report our attempt to introduce an axial fluorine ligand to Pt(IV) complexes by oxidizing oxaliplatin with electrophilic fluorinating reagents in different protic solvents. The crystal structure of one representative complex is presented. The fluorinated Pt(IV) complexes are further expanded by functionalization with different anhydrides, and their analogues bearing one different axial ligand (OAc or OH group) are also synthesized. Further investigations show that the axial fluorine atom has dramatic effects on the chemical properties of these prodrugs. These new fluorinated Pt(IV) complexes are proved to be stable in physiological conditions. For most of the fluorinated Pt(IV) complexes, a higher reduction potential indicates its greater tendency to be reduced by ascorbate. Introducing an axial fluorine ligand in Pt(IV) complexes does not lead to the increase of their lipophilicity. Moreover, these new fluorinated Pt(IV) complexes show better cytotoxicity than nonfluorinated analogues which may derive from their higher cellular accumulation in cancer cells. Therefore, the good stability and high cytotoxicity of these fluorinated Pt(IV) prodrugs indicate their great potential as a building block for further functionalization.

How to obtain Pt(iv) complexes suitable for conjugation to nanovectors from the oxidation of [PtCl(terpyridine)]. Dalton Trans 2018;46:10246-10254. [PMID: 28737785 DOI: 10.1039/c7dt01706e]

Oxidation of [Pt^(II)Cl(terpy)]⁺ (terpy = 2,2':6',2''-terpyridine) has been attempted with several oxidizing agents and under different experimental conditions in order to obtain a Pt(iv) complex suitable for the conjugation to nanovectors to be used in drug delivery targeting for anticancer therapy. The best compromise in terms of yield and purity of the final complex was obtained by microwave-assisted reaction at 70 °C in 50% aqueous H₂O₂ for 2 h. Under these conditions the quantitative formation of [Pt^(IV)Cl(OH)₂(terpy)]⁺ was observed. The subsequent synthetic steps were, (i) functionalization of [Pt^(IV)Cl(OH)₂(terpy)]⁺ in the axial position with succinic anhydride to obtain [Pt^(IV)Cl(OH)(succinato)(terpy)]⁺ and (ii) reaction of the latter with nonporous silica nanoparticles (SNPs) with an external shell containing primary amino groups to obtain a nanovector able to transport the Pt(iv) antitumor prodrug in the form of a conjugate Pt-SNP. Finally, the antiproliferative activity and cell accumulation of [Pt^(II)Cl(terpy)]⁺, [Pt^(IV)Cl(OH)₂(terpy)]⁺, and the Pt-SNP conjugate were measured on three cancer cell lines. Despite highly effective accumulation of Pt-SNP in cells, a modest increase in activity was observed with respect to the molecular species. Further experiments showed that the Pt-SNP conjugate can release [Pt^(II)Cl(terpy)]⁺ upon reduction, but this metabolite may undergo hydrolysis, and the resulting aquo complex could coordinate once again the free amino groups of the SNPs. In the resulting tetraamine form, the Pt(ii) complex conjugated to the SNPs cannot completely perform its antiproliferative activity.

The cisplatin-based Pt(iv)-diclorofibrato multi-action anticancer prodrug exhibits excellent performances also under hypoxic conditions

The cisplatin/clofibrato combos are multi-action Pt(iv) complexes active on a panel of human tumor cell lines, also under hypoxic conditions.

A computational mechanistic investigation into the reduction of Pt(iv) prodrugs with two axial chlorides by biological reductants

The reduction of the Pt(iv) prodrugs with two axial chlorido ligands by biological reductants does not always proceed via the chloride-bridge inner-sphere mechanism.

May glutamine addiction drive the delivery of antitumor cisplatin-based Pt(IV) prodrugs?

A small series of Pt(IV) prodrugs containing Gln-like (Gln=glutamine) axial ligands has been designed with the aim to take advantage of the increased demand of Gln showed by some cancer cells (glutamine addiction). In complex 4 the Gln, linked through the α-carboxylic group is recognized by the Gln transporters, in particular by the solute carrier transporter SLC1A5. All compounds showed cellular accumulation, as well as antiproliferative activity, related to their lipophilicity, as already demonstrated for the majority of Pt(IV) prodrugs, that enter cells mainly by passive diffusion. On the contrary, when the Gln concentration in cell medium is near or lower to the physiological value, complex 4 acts as a Trojan horse: it enters SLC1A5-overexpressing cells, where, upon reduction, it releases the active metabolite cisplatin and the Gln-containing ligand, thus preventing any possible extrusion by the L-type amino acid transporter LAT1. This selective mechanism could decrease off-target accumulation of 4 and, consequently, Pt-associated side-effects.

On the stability and biological behavior of cyclometallated Pt(IV) complexes with halido and aryl ligands in the axial positions

A series of cyclometallated platinum(IV) compounds (3a, 3a' and 3b') with a meridional [C,N,N'] terdentate ligand, featuring an halido and an aryl group in the axial positions has been evaluated for electrochemical reduction and preliminary biological behavior against a panel of human adenocarcinoma (A-549 lung, HCT-116 colon, and MCF-7 breast) cell lines and the normal bronquial epithelial BEAS-2B cells. Cathodic reduction potentials (shifting from -1.463 to -1.570V) reveal that the platinum(IV) compounds under study would be highly reluctant to be reduced in a biological environment. Actually ascorbic acid was not able to reduce complex 3a', the most prone to be reduced according its reduction potential, over a period of one week. These results suggest an intrinsic activity for the investigated platinum(IV) complexes (3a, 3a' and 3b'), which exhibit a remarkable cytotoxicity effectiveness (with IC₅₀ values in the low micromolar range), even greater than that of cisplatin. The IC₅₀ for A-549 lung cells and clog P values were found to follow the same trend: 3b'>3a'>3a. However, no correlation was observed between reduction potential and in vitro activity. As a representative example, cyclometallated platinum(IV) compound 3a', exercise its antiproliferative activity directly over non-microcytic A-549 lung cancer cells through a mixture of cell cycle arrest (13% arrest at G1 phase and 46% arrest at G2 phase) and apoptosis induction (increase of early apoptosis by 30 times with regard to control). To gain further insights into the mode of action of the investigated platinum(IV) complexes, drug uptake, cathepsin B inhibition and ROS generation were also evaluated. Interestingly an increased ROS generation could be related with the antiproliferative activity of the cyclometallated platinum(IV) series under study in the cisplatin-resistant A-549 lung and HCT-116 cancer cell lines.

Functionalized nonporous silica nanoparticles as carriers for Pt(iv) anticancer prodrugs

Conjugate Pt(iv)–silica nanoparticles exhibited better antiproliferative activity than cisplatin and Pt(iv) precursors, due to their more efficient cellular uptake.