Drug accumulation and DNA platination in cells exposed to aquated cisplatin species

Common Chemical Inductors of Replication Stress: Focus on Cell-Based Studies

DNA replication is a highly demanding process regarding the energy and material supply and must be precisely regulated, involving multiple cellular feedbacks. The slowing down or stalling of DNA synthesis and/or replication forks is referred to as replication stress (RS). Owing to the complexity and requirements of replication, a plethora of factors may interfere and challenge the genome stability, cell survival or affect the whole organism. This review outlines chemical compounds that are known inducers of RS and commonly used in laboratory research. These compounds act on replication by direct interaction with DNA causing DNA crosslinks and bulky lesions (cisplatin), chemical interference with the metabolism of deoxyribonucleotide triphosphates (hydroxyurea), direct inhibition of the activity of replicative DNA polymerases (aphidicolin) and interference with enzymes dealing with topological DNA stress (camptothecin, etoposide). As a variety of mechanisms can induce RS, the responses of mammalian cells also vary. Here, we review the activity and mechanism of action of these compounds based on recent knowledge, accompanied by examples of induced phenotypes, cellular readouts and commonly used doses.

Fortification of blood plasma from cancer patients with human serum albumin decreases the concentration of cisplatin-derived toxic hydrolysis products in vitro

While cisplatin (CP) is still one of the world's bestselling anticancer drugs, its intravenous administration is inherently associated with severe, dose limiting toxic side-effects. Although the molecular basis of the latter are not well understood, biochemical transformations of CP in blood and the interaction of the generated platinum species with plasma proteins likely play a critical role since these processes will ultimately determine which platinum-species reach the intended tumor cells as well as non-target cells. Compared to healthy subjects, cancer patients often have decreased plasma human serum albumin (HSA) concentrations. Little, however, is known about how the plasma HSA concentration will affect the metabolism of CP. To gain insight, we obtained blood plasma from healthy adults (n = 20, 42 ± 4 g HSA per L) and pediatric cancer patients (n = 11, 26 ± 7 g HSA per L). After the incubation of plasma at 37 °C, a pharmacologically relevant dose of CP was added and the Pt-distribution therein was determined by size-exclusion chromatography coupled on-line to an inductively coupled plasma atomic emission spectrometer. At the 2 h time point, a 5.9% increase of toxic CP-derived hydrolysis products was detected in pediatric cancer patient plasma, while 9.8% less platinum was protein bound compared to plasma from healthy controls. These in vitro results suggest that the elevated concentration of highly reactive free CP-derived hydrolysis products in plasma may cause the toxic side-effects in cancer patients. More importantly, the deliberate increase of the plasma HSA concentration in cancer patients prior to CP treatment would represent a simple strategy to possibly alleviate the fraction of patients that suffer from drug induced toxic side-effects.

Say no to DMSO: dimethylsulfoxide inactivates cisplatin, carboplatin, and other platinum complexes

The platinum drugs cisplatin, carboplatin, and oxaliplatin are highly utilized in the clinic and as a consequence are extensively studied in the laboratory setting. In this study, we examined the literature and found a significant number of studies (11%-34%) in prominent cancer journals utilizing cisplatin dissolved in DMSO. However, dissolving cisplatin in DMSO for laboratory-based studies results in ligand displacement and changes to the structure of the complex. We examined the effect of DMSO on platinum complexes, including cisplatin, carboplatin, and oxaliplatin, finding that DMSO reacted with the complexes, inhibited their cytotoxicity and their ability to initiate cell death. These results render a substantial portion of the literature on cisplatin uninterpretable. Raising awareness of this significant issue in the cancer biology community is critical, and we make recommendations on appropriate solvation of platinum drugs for research.

Polymeric drug delivery of platinum-based anticancer agents

Platinum-based anticancer agents such as cisplatin and carboplatin are in widespread clinical use but associated with many side effects. Improving the delivery of cytotoxic platinum compounds may lead to reduced side effects and achieve greater efficacy at lower doses. Polymer-based therapeutics have been investigated as potential drug delivery vehicles for platinum-based drugs. Against a background of the chemistry and pharmacology of cytotoxic platinum compounds, this review discusses the formation and properties of platinum-polymer complexes, dendrimers, micelles, and microparticulates.

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.

Modification and uptake of a cisplatin carbonato complex by Jurkat cells

The interactions of Jurkat cells with cisplatin, cis-[Pt(15NH3)2Cl2]1, are studied using 1H-15N heteronuclear single quantum coherence (HSQC) NMR and inductively coupled plasma mass spectrometry. We show that Jurkat cells in culture rapidly modify the monocarbonato complex cis-[Pt(15NH3)2(CO3)Cl]- (4), a cisplatin species that forms in culture media and probably also in blood. Analysis of the HSQC NMR peak intensity for 4 in the presence of different numbers of Jurkat cells reveals that each cell is capable of modifying 0.0028 pmol of 4 within approximately 0.6 h. The amounts of platinum taken up by the cell, weakly bound to the cell surface, remaining in the culture medium, and bound to genomic DNA were measured as functions of time of exposure to different concentrations of drug. The results show that most of the 4 that has been modified by the cells remains in the culture medium as a substance of molecular mass <3 kDa, which is HSQC NMR silent, and is not taken up by the cell. These results are consistent with a hitherto undocumented extracellular detoxification mechanism in which the cells rapidly modify 4, which is present in the culture medium, so it cannot bind to the cell. Because there is only a slow decrease in the amount of unmodified 4 remaining in the culture medium after 1 h, -1.1 +/- 0.4 microM h(-1), the cells subsequently lose their ability to modify 4. These observations have important implications for the mechanism of action of cisplatin.

Experimental and Theoretical Studies on the Pharmacodynamics of Cisplatin in Jurkat Cells

For Jurkat cells in culture exposed to cisplatin (1), we measured the number of platinum adducts on DNA and showed that it is proportional to the AUC, the area under the concentration vs time curve, for cisplatin. The number of platinum-DNA adducts is measured immediately following exposure to drug. The AUC is calculated either as the product of the initial cisplatin concentration and the exposure time or as the integral under the concentration vs time curve for the unreacted dichloro species, which decreases exponentially. We also show that the number of adducts correlates with decreases in respiration, with the amount of DNA fragmentation, and with cell viability, all measured 24 h after exposure to the drug. To study the reactions of cisplatin at concentrations approaching clinical relevance (65 microM), we use two-dimensional [1H15N]HSQC NMR and the 15N-labeled form of the drug, cis-Pt(15NH3)2Cl2, 1. In the absence of cells, 1 reacts with components of the growth medium and also transforms slowly (k(h) = 0.205 h-1 at 37 degrees C) into the chloro-aquo species, cis-[Pt(15NH3)2Cl(H2O)]+ (2), which at the pH of the medium (pH 7.15), is mainly in the deprotonated chloro-hydroxy form, cis-Pt(15NH3)2Cl(OH) (4). The concentration of 2 (4), as measured by HSQC NMR, decreases due to reaction with components of the medium. In the presence of 5 million or more cells, the concentration of 1 decreases with time, but the NMR signal for 2 (4) is not seen because it is rapidly removed from solution by the cells, keeping its concentration very low. These experiments confirm that the species preferentially removed from the medium by cells is 2 (4) and not 1. Our findings are discussed in the context of a kinetic model for platination of nuclear DNA by cisplatin, which includes aquation of cisplatin outside the cell, passage of 2 (4) through the cell membrane, reaction of reactive platinum species (RPS) in the cytosol with thiols, formation of adducts between RPS and accessible sites on genomic DNA, and removal of platinum from DNA by repair. Some of the rate constants involved are measured, but others can only be estimated. Calculations with this model show that little of the platinum reacts with intracellular thiols before reaching the nuclear DNA, indicating that binding to thiols is not important in cisplatin resistance. The model also predicts the circumstances under which the amount of platination of nuclear DNA is proportional to AUC.

Impaired hydrolysis of cisplatin derivatives to aquated species prevents energy-dependent uptake in GLC4 cells resistant to cisplatin

It has been widely stated that cisplatin enters cells by passive diffusion, despite some reports supporting a carrier-mediated mechanism. We have determined the rate of uptake of carboplatin (CBDCA), of cisplatin (CDDP) and of aquated forms, at different values of the extracellular pH, in the small lung-cancer cells GLC4 and GLC4/CDDP, cells resistant to CDDP. The rate of CDDP uptake is about 2-fold lower in resistant cells than in sensitive ones; in ATP-depleted cells this rate is about the same for both cell lines. The rate of CBDCA uptake is about 10-fold lower than that of CDDP and is the same in both cell lines independently of the ATP status of the cells. On the other hand, the rate of uptake of the aquated form of CDDP is approximately 40-fold higher than that of CDDP and is the same in both cell lines, but decreases dramatically in ATP-depleted cells. The plot of the initial rate of uptake of the aquated species as a function of its extracellular concentration shows a tendency to be saturable with k(m)=1.9 mM. In conclusion, our data show that, in sensitive GLC4 cells, passive diffusion of CDDP, probably in its neutral dichloro form, and active uptake of the aquated form contribute to the platinum uptake. The active transport of CDDP involves at least two steps: (1). the hydrolysis of the dichloro species in a deficient Cl(-) space at the level of the plasma membrane, which is the limiting step, and (2). the active transport of the aquated species. In resistant cells, step (1). should be deficient whereas step (2). is the same as in sensitive cells. For CBDCA this mechanism holds; however, step (1). is so low that the active transport does not contribute to the uptake of CBDCA by cells.

Tumour-inhibiting platinum complexes--state of the art and future perspectives

Thirty years after the onset of the first clinical studies with cisplatin, the development of antineoplastic platinum drugs continues to be a productive field of research. This article reviews the current preclinical and clinical status, including a discussion of the molecular basis for the activity of the parent drug cisplatin and platinum drugs of the second and third generation, in particular their interaction with DNA. Further emphasis is laid on the development of third generation platinum drugs with activity in cisplatin-resistant tumours, particularly on chelates containing 1,2-diaminocyclohexane (DACH) and on the promising and more recently evolving field of non-classic ( trans- and multinuclear) platinum complexes. The development of oral platinum drugs and drug targeting strategies using liposomes, polymers or low-molecular-weight carriers in order to improve the therapeutic index of platinum chemotherapy are also covered.

Interaction of the anti-cancer drug cisplatin with phosphatidylserine in intact and semi-intact cells

The anti-cancer drug cisplatin (cis-diamminedichloroplatinum(II)) forms a stable coordination complex with phosphatidylserine (PS) in model membrane systems (Speelmans et al., Biochemistry 36 (1997) 10545-10550). Because a similar interaction in vivo would be expected to have important physiological implications we studied cisplatin-PS interaction in human erythrocytes and tumor cell lines. Although cisplatin was efficiently taken up by intact erythrocytes, a cisplatin-PS complex was only detected in cells which had lysed as a result of prolonged storage or hypotonic shock. Despite the use of highly sensitive detection methods, and despite efficient cellular uptake of cisplatin, a complex could also not be detected in four human tumor cell lines, unless cells were permeabilized. In experiments in which cisplatin was incubated with PS-containing liposomes in the presence of an alternative cellular substrate, such as reduced glutathione, the relative affinity of cisplatin for PS was found to be low. Moreover, loading erythrocyte ghosts with physiological concentrations of glutathione strongly reduced cisplatin-PS complexation. Thus, in intact (tumor) cells a complex is not detected, most likely, because of the presence of higher affinity substrates. Though a transient complexation of cisplatin to PS cannot be excluded, our data suggest that cisplatin-PS does not play a direct role in the cellular (cyto)toxicity of cisplatin.

The kinetics and cytotoxicity of cisplatin and its monohydrated complex

This paper examines the monohydrated complex of cisplatin (MHC) with respect to kinetics and cytotoxicity. Equilibrium mixtures of cisplatin and hydrated species have been used in previous studies of a similar nature. To our knowledge, this is the first paper examining MHC after isolation and quantification. This was accomplished using liquid chromatography with porous graphitic carbon. MHC and cisplatin were quantified over time in a suspension of the small-cell lung cancer cell line U-1285. Cytotoxicity was evaluated using the fluorescent microculture cytotoxicity assay. MHC was significantly more cytotoxic than cisplatin at the high end of the drug concentrations tested. In culture media with low chloride ion concentrations, the stability of MHC was related to changes in pH. At a pH of between 6.0 and 7.2, MHC was rapidly converted to cisplatin. In culture media with a pH above 7.2, MHC was considerably more stable. These findings might have clinical significance given that MHC circulates in the blood stream of patients receiving cisplatin infusions and that solid tumours often have environments that are extremely acidotic.