Dinuclear platinum complexes with N,N′-bis(aminoalkyl)-1,4-diaminoanthraquinones as linking ligands. Part II. Cellular processing in A2780 cisplatin-resistant human ovarian carcinoma cells: new insights into the mechanism of resistance

Cancer 3D Models for Metallodrug Preclinical Testing

Despite being standard tools in research, the application of cellular and animal models in drug development is hindered by several limitations, such as limited translational significance, animal ethics, and inter-species physiological differences. In this regard, 3D cellular models can be presented as a step forward in biomedical research, allowing for mimicking tissue complexity more accurately than traditional 2D models, while also contributing to reducing the use of animal models. In cancer research, 3D models have the potential to replicate the tumor microenvironment, which is a key modulator of cancer cell behavior and drug response. These features make cancer 3D models prime tools for the preclinical study of anti-tumoral drugs, especially considering that there is still a need to develop effective anti-cancer drugs with high selectivity, minimal toxicity, and reduced side effects. Metallodrugs, especially transition-metal-based complexes, have been extensively studied for their therapeutic potential in cancer therapy due to their distinctive properties; however, despite the benefits of 3D models, their application in metallodrug testing is currently limited. Thus, this article reviews some of the most common types of 3D models in cancer research, as well as the application of 3D models in metallodrug preclinical studies.

Drug delivery strategies in maximizing anti-angiogenesis and anti-tumor immunity

Metronomic chemotherapy has been shown to elicit anti-tumor immune response and block tumor angiogenesis distinct from that observed with maximal tolerated dose (MTD) therapy. This review delves into the mechanisms behind anti-tumor immunity and seeks to identify the differential effect of dosing regimens, including daily low-dose and medium-dose intermittent chemotherapy (MEDIC), on both innate and adaptive immune populations involved in observed anti-tumor immune response. Given reports of VEGF/VEGFR blockade antagonizing anti-tumor immunity, drug choice, dose, and selective delivery determined by advanced formulations/vehicles are highlighted as potential sources of innovation for identifying anti-angiogenic modalities that may be combined with metronomic regimens without interrupting key immune players in the anti-tumor response. Engineered drug delivery mechanisms that exhibit extended and local release of anti-angiogenic agents both alone and in combination with chemotherapeutic treatments have also been demonstrated to elicit a potent and potentially systemic anti-tumor immune response, favoring tumor regression and stasis over progression. This review examines this interplay between various cancer models, the host immune response, and select anti-cancer agents depending on drug dosing, scheduling/regimen, and delivery modality.

Monofunctional Platinum(II) Anticancer Agents

Platinum-based anticancer drugs represented by cisplatin play important roles in the treatment of various solid tumors. However, their applications are largely compromised by drug resistance and side effects. Much effort has been made to circumvent the drug resistance and general toxicity of these drugs. Among multifarious designs, monofunctional platinum(II) complexes with a general formula of [Pt(3A)Cl]⁺ (A: Ammonia or amine) stand out as a class of "non-traditional" anticancer agents hopeful to overcome the defects of current platinum drugs. This review aims to summarize the development of monofunctional platinum(II) complexes in recent years. They are classified into four categories: fluorescent complexes, photoactive complexes, targeted complexes, and miscellaneous complexes. The intention behind the designs is either to visualize the cellular distribution, or to reduce the side effects, or to improve the tumor selectivity, or inhibit the cancer cells through non-DNA targets. The information provided by this review may inspire researchers to conceive more innovative complexes with potent efficacy to shake off the drawbacks of platinum anticancer drugs.

Chemical Imaging of Platinum-Based Drugs and their Metabolites

Platinum-based drugs (cisplatin, carboplatin, and oxaliplatin) are widely used therapeutic agents for cancer treatment. Even though the platinum (Pt)-drugs are routinely used clinically, a clear picture of their distribution within tumor tissues is lacking. The current methods to image the distribution of Pt drugs are limited and do not enable the discrimination of the drug from its metabolites. In this manuscript, we demonstrate a methodology that enables chemical imaging of a Pt drug and its metabolites simultaneously and specifically. Matrix-Assisted Laser Desorption/Ionization (MALDI) Mass Spectrometry Imaging (MSI) is combined with an on-tissue chemical derivatization using diethyldithiocarbamate (DDTC). DDTC abstracts the Pt atom to generate ionizable complexes that can be imaged by MALDI MSI. We demonstrate that Pt drugs and their metabolites can be specifically imaged. This approach was successfully applied to map the penetration and metabolism of oxaliplatin in hyperthermic intraperitoneal chemotherapy (HIPEC)-like treated 3D colorectal tumor mimics. The distribution of cisplatin and carboplatin was mapped in additional 3D tumor mimics. We demonstrate that the approach can also be used to image the distribution of copper ions in cells. This method has the potential to be used to evaluate the penetration and distribution of a wide range of compounds.

Azide vs Alkyne Functionalization in Pt(II) Complexes for Post-treatment Click Modification: Solid-State Structure, Fluorescent Labeling, and Cellular Fate

Tracking of Pt(II) complexes is of crucial importance toward understanding Pt interactions with cellular biomolecules. Post-treatment fluorescent labeling of functionalized Pt(II)-based agents using the bioorthogonal Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction has recently been reported as a promising approach. Here we describe an azide-functionalized Pt(II) complex, cis-[Pt(2-azidobutyl)amido-1,3-propanediamine)Cl2] (1), containing the cis geometry and difunctional reactivity of cisplatin, and present a comparative study with its previously described alkyne-functionalized congener. Single-crystal X-ray diffraction reveals a dramatic change in the solid-state arrangement with exchange of the alkyne for an azide moiety wherein 1 is dominated by a pseudo-chain of Pt-Pt dimers and antiparallel alignment of the azide substituents, in comparison with a circular arrangement supported by CH/π(C≡C) interactions in the alkyne version. In vitro studies indicate similar DNA binding and click reactivity of both congeners observed by fluorescent labeling. Interestingly, complex 1 shows in vitro enhanced click reactivity in comparison to a previously reported azide-appended Pt(II) complex. Despite their similar behavior in vitro, preliminary in cellulo HeLa studies indicate a superior imaging potential of azide-functionalized 1. Post-treatment fluorescent labeling of 1 observed by confocal fluorescence microscopy shows nuclear and intense nucleolar localization. These results demonstrate the potential of 1 in different cell line localization studies and for future isolation and purification of Pt-bound targets.

Synthesis, characterization, and anticancer properties of organometallic Schiff base platinum complexes

A small family of organometallic platinum complexes containing a chloride, cis-cyclooctene, and a Schiff base ligand have been prepared and characterized fully. Three aliphatic amines and four aromatic amines were chosen as representative examples. All complexes were stable in air except for 7, derived from the pinacol-protected 4-aminophenylboronate ester 4-H2NC6H4Bpin (pin = 1,2-O2C2Me4), which decomposed via B–C bond cleavage. Both complexes 4 (derived from aniline) and 7 were further characterized by single-crystal X-ray diffraction studies and confirmed the square planar nature of the platinum centre whereby the chloride ligand lies trans to the deprotonated hydroxyl group of the Schiff base ligand. The imine functionality is trans to the organic cyclooctene group. Complex 3, which contained the longest aliphatic chain studied (an octyl group), was the most promising for inducing apoptosis in the malignant MB231 breast cancer cell line. Conversely, complexes 4–6, which contained aromatic groups, were the most active against renal cell carcinoma (RCC) cell lines.

Fluorometric imaging methods for palladium and platinum and the use of palladium for imaging biomolecules

Palladium and platinum metals have been used to facilitate novel bioimaging methods.

Fluorescent silver(I) and gold(I)-N-heterocyclic carbene complexes with cytotoxic properties: mechanistic insights

Silver(I) and gold(I)-N-heterocyclic carbene (NHC) complexes bearing a fluorescent anthracenyl ligand were examined for cytotoxicity in normal and tumor cells. The silver(I) complex exhibits greater cytotoxicity in tumor cells compared with normal cells. Notably, in cell extracts, this complex determines a more pronounced inhibition of thioredoxin reductase (TrxR), but it is ineffective towards glutathione reductase (GR). Both gold and silver complexes lead to oxidation of the thioredoxin system, the silver(I) derivative being particularly effective. In addition, the dimerization of peroxiredoxin 3 (Prx3) was also observed, demonstrating the ability of these compounds to reach the mitochondrial target. The fluorescence microscopy visualization of the subcellular distribution of the complexes shows a larger diffusion of these molecules in tumor cells with respect to normal cells.

NanoSIMS multi-element imaging reveals internalisation and nucleolar targeting for a highly-charged polynuclear platinum compound

Simultaneous multi-element imaging using NanoSIMS (nano-scale secondary ion mass spectrometry), exploiting the novel combination of (195)Pt and (15)N in platinum-am(m)ine antitumour drugs, provides information on the internalisation and subcellular localisation of both metal and ligands, and allows identification of ligand exchange.

Facile preparation of mono-, di- and mixed-carboxylato platinum(IV) complexes for versatile anticancer prodrug design

Facile strategies were developed for the versatile functionalization of platinum(IV) axial sites, allowing for easy accessibility to unsymmetric mono- and mixed-carboxylato, as well as symmetric di-substituted platinum(IV) complexes. The first method involves the direct oxidation and carboxylation of the platinum(II) center using an appropriate peroxide and the carboxylate of choice to firstly yield a monocarboxylato monohydroxido platinum(IV) complex. This platinum(IV) intermediate can undergo further carboxylation to give rise to a mixed-carboxylato platinum(IV) complex. The second method involves the activation of the carboxylate of choice by a common carbodiimide coupling reagent, and its reaction with a dihydroxido platinum(IV) precursor to give the monocarboxylato platinum(IV) complex. Uronium salts can be employed to promote efficient dicarboxylation of the dihydroxido platinum(IV) precursor. Lastly, an axial azide pendant group was demonstrated to be suitable for orthogonal "click" conjugation reactions.

Getting to the core of platinum drug bio-distributions: the penetration of anti-cancer platinum complexes into spheroid tumour models

Elemental mapping and fluorescence imaging techniques are frequently employed to probe the distribution of platinum-based chemotherapeutics within biological systems. Although useful, these techniques have unique limitations: elemental mapping methods, such as those that use particle beams, typically require rigorous sample preparation that can alter chemical distributions, whilst in situ visible fluorescence studies require fluorescent-tagging of the platinum component and may be confounded by factors such as ligand loss. The present study aimed to establish reliable methods for accurately probing the bio-distribution of platinum compounds within the model tumour micro-environment of the well characterised DLD-1 colorectal cancer cell spheroids. 3D X-ray fluorescence computed micro-tomography (XRF-CT) was performed on intact untreated spheroids to determine the effect of physical sectioning and chemical fixation on elemental distributions. It was revealed for the first time that cisplatin can readily penetrate through DLD-1 spheroids and accumulate in the central hypoxic and necrotic regions of the spheroids. Furthermore, formalin fixing was shown to cause significant changes to the distributions and concentrations of the elements, particularly in the cases of platinum and zinc. This effect was not observed in the cryo-fixed and cryo-sectioned samples. X-ray fluorescence microscopy (XFM) was used to re-examine the fate of platinum in the previously reported fluorescence distribution studies of platinum(ii) complexes tagged with fluorescent anthraquinone moieties. In contrast to the fluorescence distributions, in which fluorescence was observed predominantly around the periphery of the spheroids, the XFM revealed a high level of platinum in the spheroid centre, indicating that ligand exchange occurred within the peripheral cell layers. Both the platinum maps and the fluorescence images exhibit similar diffusion trends, supporting the conclusion that charge on the compound can slow cellular uptake can enhance tumour penetration.

Mono- and dinuclear platinum(II) compounds with 5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine. Structure, cytotoxic activity and reaction with 5'-GMP

Mono- and dinuclear platinum(II) coordination compounds of formula cis-[PtCl(2)(NH(3))(dmtp)], 1, cis-[PtCl(2)(dmtp)(2)], 2 and {H(+)[C(28)H(32)Cl(2)N(16)Pt(2)](2+)(NO(3))(3)(H(2)O)(6)}, 3, in which dmtp is 5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine have been synthesized and characterized by infrared and by (1)H, (13)C, (195)Pt NMR spectroscopy. The coordination units of the cationic species of formula [Pt(2)(mu-dmtp)(2)Cl(2)(dmtp)(2)](2+) are built up by two platinum atoms in a square-planar environment. Two sites are occupied by two 5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine (dmtp) bridging ligands which are linked to both metal atoms through their nitrogen atoms in positions 3 and 4. The other two positions are occupied by one monodentate 5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine (dmtp) molecule and a coordinated chloride. This compound is the first in which the same triazolopyrimidine ligand (dmtp) coordinates to a metal ion in two different ways, i.e. bridging bidentate and non-bridging monodentate. In addition, the interaction of compounds 1 and 2 with 5'-GMP was investigated in solution by (1)H NMR spectroscopy. The cytotoxicity of all the new platinum(II) compounds were studied by using two different cell lines: T47D (breast cancer) and HCV29T (bladder cancer).

BCRP expression does not result in resistance to STX140 in vivo, despite the increased expression of BCRP in A2780 cells in vitro after long-term STX140 exposure

The anti-proliferative and anti-angiogenic properties of the endogenous oestrogen metabolite, 2-methoxyoestradiol (2-MeOE2), are enhanced in a series of sulphamoylated derivatives of 2-MeOE2. To investigate possible mechanisms of resistance to these compounds, a cell line, A2780.140, eightfold less sensitive to the 3,17-O,O-bis-sulphamoylated derivative, STX140, was derived from the A2780 ovarian cancer cell line by dose escalation. Other cell lines tested did not develop STX140 resistance. RT-PCR and immunoblot analysis demonstrated that breast cancer resistance protein (BCRP) expression is dramatically increased in A2780.140 cells. The cells are cross-resistant to the most structurally similar bis-sulphamates, and to BCRP substrates, mitoxantrone and doxorubicin; but they remain sensitive to taxol, an MDR1 substrate, and to all other sulphamates tested. Sensitivity can be restored using a BCRP inhibitor, and this pattern of resistance is also seen in a BCRP-expressing MCF-7-derived cell line, MCF-7.MR. In mice bearing wild-type (wt) and BCRP-expressing tumours on either flank, both STX140 and mitoxantrone inhibited the growth of the MCF-7wt xenografts, but only STX140 inhibited growth of the MCF-7.MR tumours. In conclusion, STX140, a promising orally bioavailable anti-cancer agent in pre-clinical development, is highly efficacious in BCRP-expressing xenografts. This is despite an increase in BCRP expression in A2780 cells in vitro after chronic dosing with STX140.

Binding of [Pt(1C3)(dien)](2+) to the duplex DNA oligonucleotide 5'-d(TGGCCA)-3': the effect of an appended positive charge on the orientation and location of anthraquinone intercalation

The binding of a platinum intercalator complex [Pt(1C3)(dien)](2+) (1C3 = 1-[(3-aminopropyl)amino]-anthracene-9,10-dione, dien = 3-azapentane-1,5-diamine) to DNA and to the self-complementary oligonucleotide 5'-d(TGGCCA)-3' has been investigated by UV-visible spectrophotometry and 2D NMR spectroscopy, respectively. The uncomplexed anthraquinone, 1C3, has an apparent DNA binding constant of 1.4 x 10(4), similar to that of ethidium bromide. Addition of the coordinatively saturated {Pt(dien)} moiety increases the binding constant to 3.7 x 10(5) M(-1), showing the effect of the increased positive charge introduced by this moiety. Multiple binding modes are evident from the lack of isosbestic points in the titration spectra and the non-linear nature of the half-reciprocal plot used to calculate the binding constant. [Pt(1C3)(dien)](2+) forms a 2 : 1 adduct with 5'-d(TGGCCA)-3' and is shown by 2D NMR to intercalate primarily in the TG:CA base pairs at the ends of the oligonucleotide with the side chain and {Pt(dien)} situated in the minor groove.

The role of cellular accumulation in determining sensitivity to platinum-based chemotherapy

The platinum (Pt) drugs cisplatin and carboplatin are heavily employed in chemotherapy regimens; however, similar to other classes of drugs, a number of intrinsic and acquired resistance mechanisms hamper their effectiveness. The method by which Pt drugs enter cells has traditionally been attributed to simple passive diffusion. However, recent evidence suggests a number of active uptake and efflux mechanisms are at play, and altered regulation of these transporters is responsible for the reduced accumulation of drug in resistant cells. This review suggests a model that helps reconcile the disparate literature by describing multiple pathways for Pt-containing drugs into and out of the cell.

Developing new metal-based therapeutics: challenges and opportunities

Unexpected side effects and problems experienced in clinical trials have created a difficult environment for those developing new pharmaceuticals and, as a consequence, the number of new chemical entities being registered has fallen to a historic low. Those developing metal-based therapeutics will face the same difficulties, but this environment also provides many opportunities for creative solutions to be applied to overcoming the problems and concerns. In this perspective, these challenges and opportunities are considered in the context of examples of a number of classes of metal-based therapeutics.

Cytotoxic efficacy of an anthraquinone linked platinum anticancer drug

Platinum complexes are widely used in cancer chemotherapy; however, they are associated with toxicity, high "non-specific" reactivity and relatively poor pharmacokinetic profiles. In particular, their low cellular uptake and rapid metabolic inactivation means that the amount of "active" drug reaching the nuclear compartment is low. Our strategy to facilitate nuclear accumulation was to introduce a hydrophobic anthraquinone (1C3) moiety to the Pt-complex. Anthraquinones are known to readily intercalate into DNA strands and hence, the Pt-1C3 complex may represent an effective system for the delivery of the platinum moiety to nuclear DNA. Efficacy of the complex was determined by measuring the extent and potency of cytotoxicity in comparison to cisplatin and an anthraquinone based anticancer drug, doxorubicin. The Pt-1C3 complex generated higher levels of cytotoxicity than cisplatin, with a potency of 19 +/- 4 microM in the DLD-1 cancer cell line. However, this potency was not significantly different to that of the 1C3 moiety alone. To examine the reason for the apparent lack of platinum related cytotoxicity, the cellular distribution was characterised. Confocal fluorescence microscopy indicated that the Pt-1C3 complex was rapidly sequestered into lysosomes, in contrast to the nuclear localisation of doxorubicin. In addition, there was negligible DNA associated Pt following administration of the novel complex. Thus, the addition of a 1C3 moiety generated sequestration of the complex to lysosomes, thereby preventing localisation to the nucleus.

Application of fluorescence microscopy for investigation of cellular distribution of dinuclear platinum anticancer drugs

The dinuclear platinum complexes with aliphatic diamines [{cis-Pt(NH(3))(2)Cl}(2)(mu-H(2)N(CH(2))(6)NH(2))](NO(3))(2) (1,1/c,c) and [{trans-Pt(NH(3))(2)Cl}(2)(mu-H(2)N(CH(2))(4)NH(2))](NO(3))(2) (1,1/t,t), which are known to be highly active in vitro against several cancer cell lines, have been modified with a fluorogenic reporter (carboxyfluorescein diacetate, CFDA) and a hapten (dinitrophenyl, DNP). These labeled complexes have been designed for fluorescence microscopy investigation of cellular pathways of promising dinuclear platinum anticancer drugs and present the first example of labeling biologically active dinuclear platinum complexes with a fluorescent reporter. The modified compounds interact with a guanine model base similarly to the label-free parent complexes. The uptake of the complexes with a fluorescent label and the respective unlabeled complexes in the U2-OS human osteosarcoma cell line and its cisplatin-resistant derivative, U2-OS/Pt cell line has been investigated. Cellular processing of the CFDA- and DNP-modified dinuclear platinum complexes in U2-OS and U2-OS/Pt cells has been studied.

Metal complex-DNA interactions: from transcription inhibition to photoactivated cleavage

Metal ions and complexes, because of their cationic character, three-dimensional structural profiles, and propensity for performing hydrolysis, redox, or photoreactions, have a natural aptitude for interacting with DNA. Indeed, the need for cellular regulation of DNA led to the evolution of metallonucleases to catalyze and repair DNA strand breaks. Moreover, inorganic constructs such as cisplatin and bimetallic rhodium acetate exert antitumor activity by inner-sphere coordination to DNA. Because binding and cleavage of DNA is at the heart of cellular transcription and translation, it is an obvious target for therapeutic intervention and the development of diagnostic structural probes. To this end, new metal complexes have been designed that utilize or create open coordination positions for DNA binding and hydrolysis, generate reactive oxygen-containing species or other radicals for DNA oxidation, or perform direct redox reactions with DNA. The recent emerging themes are the development of bifunctional architectures containing multiple metal-binding or reactive sites, specialized ligand implementation, or incorporation of site-specific targeting substructures. This review describes their employment in novel reaction strategies that do not require bimolecular cofactors and as site-specific probes or cleavage agents.

Dinuclear platinum anticancer complexes with fluorescent N,N′-bis(aminoalkyl)-1,4-diaminoanthraquinones: cellular processing in two cisplatin-resistant cell lines reflects the differences in their resistance profiles

The biological activity of N,N'-bis(aminoalkyl)-1,4-diaminoanthraquinones (aminoalkyl is 2-aminoethyl, 3-aminoprop-1-yl and 4-aminobut-1-yl) and their dinuclear platinum complexes has been evaluated in the U2-OS human osteosarcoma cell line and its cisplatin-resistant U2-OS/Pt subline. All the compounds have been found to exhibit high cytotoxicity in the sensitive cell line, and to overcome cisplatin resistance in U2-OS/Pt cells. Cellular processing of N,N'-bis(2-aminoethyl)-1,4-diaminoanthraquinone and the respective dinuclear platinum complex in the sensitive and resistant U2-OS cells has been studied over time using digital fluorescence microscopy. Cellular processing of the compounds has been found to be similar in sensitive and resistant U2-OS cells, which is in agreement with the lack of cross-resistance in the U2-OS/Pt cell line. Both the platinum complex and the free ligand quickly enter the cell and accumulate in the nucleus. The platinum complex is excreted from the cell via the Golgi apparatus, while the weakly basic anthraquinone ligand accumulates in the Golgi complex, where it is taken up by lysosomes and then transported to the cell surface. The cellular distribution of the fluorescent anthraquinones and their dinuclear platinum complexes in the sensitive/resistant pair of U2-OS osteosarcoma cell lines is compared with the earlier studied cellular processing in the sensitive/resistant pair of A2780 ovarian carcinoma cell lines. In the A2780cisR cell line, the platinum complexes (and not the free ligands) are sequestered in lysosomes, which is not the case in A2780 sensitive cells. The differences in cellular distribution of the compounds in these two sensitive/resistant pairs of cell lines most likely result from different resistance profiles in A2780cisR and U2-OS/Pt cells. Lysosomes of A2780cisR cells are less acidic than lysosomes of A2780 sensitive cells, which is likely to be the cause of a defect in endocytosis. The disruption of normal endocytosis might facilitate sequestration of the platinum complexes in lysosomes, which partly confers the cross-resistance of these complexes with cisplatin in the A2780cisR cell line. In contrast, sequestration in acidic vesicles does not occur in U2-OS/Pt cells that do not exhibit enhanced lysosomal pH and which are likely to have normal endocytosis.

Dinuclear platinum complexes with N,N′-bis(aminoalkyl)-1,4-diaminoanthraquinones as linking ligands. Part I. Synthesis, cytotoxicity, and cellular studies in A2780 human ovarian carcinoma cells

A series of N, N'-bis(aminoalkyl)-1,4-diaminoanthraquinones (aminoalkyl=2-aminoethyl, 3-aminoprop-1-yl and 4-aminobut-1-yl) was functionalized with trans-platinum DNA-binding moieties. Cytotoxicity testing in A2780 human ovarian carcinoma cells revealed high anticancer activity of the formed cationic dinuclear platinum complexes. The cationic dinuclear platinum complexes with the shortest aminoalkyl chain were shown to be the most active, which agrees with the structure-activity relationship found for the corresponding free ligands without platinum. The N, N'-bis(aminoalkyl)-1,4-diaminoanthraquinones partly circumvent cisplatin resistance, whereas their dinuclear platinum complexes were found susceptible to the resistance mechanisms in A2780cisR. The platinum complexes have resistance factors comparable to the control dinuclear complex BBR3005 [(trans-PtCl(NH3)2)2)(micro-(NH2(CH2)6NH2))](NO3)2. The 1,4-diaminoanthraquinone moiety is fluorescent, and thus the cellular processing of the compounds could be monitored by time-lapse digital fluorescence microscopy. The intercalators without platinum were shown to enter the cells within minutes. The platinum complexes enter the cells more slowly. Most likely, the positive charges of the platinum complexes hamper the diffusion through the membrane. Interestingly, the platinum complexes are processed differently than the platinum-free compounds by the cells. After 24 hours the fluorescent platinum complexes are encapsulated in large vesicles in the cytosol. Co-localization of the anthraquinone fluorescence with Lysotracker Green DND-26 shows that these vesicles are acidic compartments, probably lysosomes.