Effects of the anti-neoplastic agent ET-18-OCH3 and some analogs on the biophysical properties of model membranes

Lipids and Membrane Microdomains: The Glycerolipid and Alkylphosphocholine Class of Cancer Chemotherapeutic Drugs

Synthetic antitumor lipids are metabolically stable lysophosphatidylcholine derivatives, encompassing a class of non-mutagenic drugs that selectively target cancerous cells. In this chapter we review the literature as relates to the clinical efficacy of these antitumor lipid drugs and how our understanding of their mode of action has evolved alongside key advances in our knowledge of membrane structure, organization, and function. First, the history of the development of this class of drugs is described, providing a summary of clinical outcomes of key members including edelfosine, miltefosine, perifosine, erufosine, and erucylphosphocholine. A detailed description of the biophysical properties of these drugs and specific drug-lipid interactions which may contribute to the selectivity of the antitumor lipids for cancer cells follows. An updated model on the mode of action of these lipid drugs as membrane disorganizing agents is presented. Membrane domain organization as opposed to targeting specific proteins on membranes is discussed. By altering membranes, these antitumor lipids inhibit many survival pathways while activating pro-apoptotic signals leading to cell demise.

Anticancer Agent Edelfosine Exhibits a High Affinity for Cholesterol and Disorganizes Liquid-Ordered Membrane Structures

Edelfosine is an anticancer drug with an asymmetric structure because, being a derivative of glycerol, it possesses two hydrophobic substituents of very different lengths. We showed that edelfosine destabilizes liquid-ordered membranes formed by either 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine, sphingomyelin (SM), and cholesterol (1:1:1 molar ratio) or SM and cholesterol (2:1 molar ratio). This was observed by differential scanning calorimetry in which phase transition arises from either of these membrane systems after the addition of edelfosine. The alteration in the liquid-ordered domains was characterized by using a small-angle X-ray diffraction that revealed the formation of gel phases as a consequence of the addition of edelfosine at low temperatures and by a wide-angle X-ray diffraction that confirmed changes in the membranes, indicating the formation of these gel phases. The increase in phase transition derived by the edelfosine addition was further confirmed by Fourier-transform infrared spectroscopy. The effect of edelfosine was compared with that of structurally analogue lipids: platelet-activating factor and 1-palmitoyl-2-acetyl- sn-glycero-3-phosphocholine, which also have the capacity of destabilizing liquid-ordered domains, although they are less potent than edelfosine for this activity, and lysophosphatidylcholine, which lacks this capacity. It was concluded that edelfosine may be associated with cholesterol favorably competing with sphingomyelin, and that this sets sphingomyelin free to undergo a phase transition. Finally, the experimental observations can be described by molecular dynamics calculations in terms of intermolecular interaction energies in phospholipid-cholesterol membranes. Higher interaction energies between asymmetric phospholipids and cholesterol than between sphingomyelin and cholesterol were obtained. These results are interesting because they biophysically characterize one of the main molecular mechanisms to trigger apoptosis of the cancer cells.

Comparative Experimental and Computational Study of Monoalkyl Chain Phosphatidylcholine-Containing Thermoresponsive Liposomes

Liposomes containing lysophospholipids are intensively studied as drug delivery systems that are stable at normal body temperature but exhibit fast release of their drug load at slightly elevated temperatures. In this study, the stability and release properties of dipalmitoylglycerophosphocholine (DPPC)-based liposomes incorporating the commonly used lysophosphatidylocholine (lyso-PC), and a series of monoalkyl chain ether-linked phosphatidylcholine, i.e., the biologically relevant monoalkyl chain platelet activating factor (PAF) and its derivatives lyso-PAF and methyl-PAF, were investigated. To this end a series of PEGylated small unilamellar liposomes with DPPC:monoalkyl lipid compositions of 5% and 10% molar ratio were prepared and compared with regard to stability (37 °C) and release properties at elevated temperatures (38-43 °C). All systems were characterized with respect to size distribution, ζ-potential, and phase transition characteristics. The presence of ether-lipids endows liposomes with superior (∼10% increase) release properties at 5% incorporation compared to lyso-PC, while at 10% molar ratio the formulations do not differ significantly, the release being close to 90%. The findings are supported by atomistic molecular dynamics simulations that suggest a correlation between the enhanced permeability and increased penetration of water molecules within the bilayers with density fluctuations resulting from the increased area-per-lipid and the disorder of the lysolipids alkyl chains.

Disruption of lipid domain organization in monolayers of complex yeast lipid extracts induced by the lysophosphatidylcholine analogue edelfosine in vivo

The lysophosphatidylcholine analogue edelfosine is a potent antitumor and antiparasitic drug that targets cell membranes. Previous studies have shown that edelfosine alters membrane domain organization inducing internalization of sterols and endocytosis of plasma membrane transporters. These early events affect signaling pathways that result in cell death. It has been shown that edelfosine preferentially partitions into more rigid lipid domains in mammalian as well as in yeast cells. In this work we aimed at investigating the effect of edelfosine on membrane domain organization using monolayers prepared from whole cell lipid extracts of cells treated with edelfosine compared to control conditions. In Langmuir monolayers we were able to detect important differences to the lipid packing of the membrane monofilm. Domain formation visualized by means of Brewster angle microscopy also showed major morphological changes between edelfosine treated versus control samples. Importantly, edelfosine resistant cells defective in drug uptake did not display the same differences. In addition, co-spread samples of control lipid extracts with edelfosine added post extraction did not fully mimic the results obtained with lipid extracts from treated cells. Altogether these results indicate that edelfosine induces changes in membrane domain organization and that these changes depend on drug uptake. Our work also validates the use of monolayers derived from complex cell lipid extracts combined with Brewster angle microscopy, as a sensitive approach to distinguish between conditions associated with susceptibility or resistance to lysophosphatidylcholine analogues.

Liposomal codelivery of a synergistic combination of bioactive lipids in the treatment of acute myeloid leukemia

Aim: The aim of this work was to develop a liposomal formulation to facilitate delivery of a synergistic safingol/C2-ceramide combination in the treatment of acute myeloid leukemia (AML). Materials & methods: Liposomes were prepared using the extrusion method and the bioactive lipids were encapsulated passively. Drug concentrations were determined by liquid chromatography tandem mass spectrometry. Antileukemic activity was evaluated using human leukemic cell lines, patient samples and U937 leukemic xenograft models. Results: A stable liposome formulation was developed to coencapsulate safingol and C2-ceramide at 1:1 molar ratio with >90% encapsulation efficiency. The liposomal safingol/C2-ceramide was effective in AML cell lines, patient samples and murine xenograft models of AML, compared with liposomal safingol or liposomal C2-ceramide alone despite a dose reduction of 33%. Conclusion: Our study provided proof-of-concept evidence to deliver synergistic combination of bioactive lipid to achieve complete remission in AML.

Original submitted: 27 February 2013; Revised submitted: 25 June 2013

Exogenous ether lipids predominantly target mitochondria

Ether lipids are ubiquitous constituents of cellular membranes with no discrete cell biological function assigned yet. Using fluorescent polyene-ether lipids we analyzed their intracellular distribution in living cells by microscopy. Mitochondria and the endoplasmic reticulum accumulated high amounts of ether-phosphatidylcholine and ether-phosphatidylethanolamine. Both lipids were specifically labeled using the corresponding lyso-ether lipids, which we established as supreme precursors for lipid tagging. Polyfosine, a fluorescent analogue of the anti-neoplastic ether lipid edelfosine, accumulated to mitochondria and induced morphological changes and cellular apoptosis. These data indicate that edelfosine could exert its pro-apoptotic power by targeting and damaging mitochondria and thereby inducing cellular apoptosis. In general, this study implies an important role of mitochondria in ether lipid metabolism and intracellular ether lipid trafficking.

Testing the hypothesis that amphiphilic antineoplastic lipid analogues act through reduction of membrane curvature elastic stress

The alkyllysophospholipid (ALP) analogues Mitelfosine and Edelfosine are anticancer drugs whose mode of action is still the subject of debate. It is agreed that the primary interaction of these compounds is with cellular membranes. Furthermore, the membrane-associated protein CTP: phosphocholine cytidylyltransferase (CCT) has been proposed as the critical target. We present the evaluation of our hypothesis that ALP analogues disrupt membrane curvature elastic stress and inhibit membrane-associated protein activity (e.g. CCT), ultimately resulting in apoptosis. This hypothesis was tested by evaluating structure-activity relationships of ALPs from the literature. In addition we characterized the lipid typology, cytotoxicity and critical micelle concentration of novel ALP analogues that we synthesized. Overall we find the literature data and our experimental data provide excellent support for the hypothesis, which predicts that the most potent ALP analogues will be type I lipids.

Edelfosine is incorporated into rafts and alters their organization

The effect of edelfosine (1- O-octadecyl-2- O-methyl-rac-glycero-3-phosphocholine or ET-18-OCH3) on model membranes containing 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine/sphingomyelin/cholesterol (POPC/SM/cholesterol) was studied by several physical techniques. The sample POPC/SM (1:1 molar ratio) showed a broad phase transition as seen by DSC, X-ray diffraction, and 2H NMR. The addition of edelfosine to this sample produced isotropic structures at temperatures above the phase transition, as seen by 2H NMR and by 31P NMR. When cholesterol was added to give a POPC/SM/cholesterol (at a molar ratio 1:1:1), no transition was observed by DSC nor X-ray diffraction, and 2H NMR indicated the presence of a liquid ordered phase. The addition of 10 mol % edelfosine increased the thickness of the membrane as seen by X-ray diffraction and led to bigger differences in the values of the molecular order of the membrane detected at high and low temperatures, as detected through the M 1 first spectral moment from 2H NMR. These differences were even greater when 20 mol % edelfosine was added, and a transition was now clearly visible by DSC. In addition, a gel phase was clearly indicated by X-ray diffraction at low temperatures. The same technique pointed to greater membrane thickness in this mixture and to the appearance of a second membrane structure, indicating the formation of two separated phases in the presence of edelfosine. All of these data strongly suggest that edelfosine associating with cholesterol alter the phase status present in a POPC/SM/cholesterol (1:1:1 molar ratio) mixture, which is reputed to be a model of a raft structure. However, cell experiments showed that edelfosine colocalizes in vivo with rafts and that it may reach concentrations higher than 20 mol % of total lipid, indicating that the concentrations used in the biophysical experiments were within what can be expected in a cell membrane. The conclusion is that molecular ways of action of edelfosine in cells may involve the modification of the structure of rafts.

Combination of the anti-tumour cell ether lipid edelfosine with sterols abolishes haemolytic side effects of the drug

Edelfosine (1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine) is an anti-tumour cell ether lipid with surface-active properties. Pure edelfosine can be dispersed in aqueous media in the form of micelles. One important, negative side effect of edelfosine is that it is highly haemolytic. In this paper, we show that edelfosine can be co-dispersed in water with certain lipids (particularly cholesterol, campesterol or beta-sitosterol) so that it gives rise to liposomes. Surface pressure measurements demonstrate that edelfosine is slowly released from these liposomes. In liposomal form, edelfosine remains apoptogenic for a variety of leukemia cell lines, while its haemolytic effect is abolished. The phenomenon is explained on the basis of the complementarity of the molecular geometries of sterols and edelfosine.

Surface-active properties of the antitumour ether lipid 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (edelfosine)

The surface activity and interaction with lipid monolayers and bilayers of the antitumour ether lipid 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (edelfosine) have been studied. Edelfosine is a surface-active soluble amphiphile, with critical micellar concentrations at 3.5 microM and 19 microM in water. When the air-water interface is occupied by a phospholipid, edelfosine becomes inserted in the phospholipid monolayer, increasing surface pressure. This increase is dose-dependent, and reaches a plateau at ca. 2 microM edelfosine bulk concentration. The ether lipid can become inserted in phospholipid monolayers with initial surface pressures of up to 33 mN/m, which ensures its capacity to become inserted into cell membranes. Upon interaction with phospholipid vesicles, edelfosine exhibits a weak detergent activity, causing release of vesicle contents to a low extent (<5%), and a small proportion of lipid solubilization. The weak detergent properties of edelfosine can be related to its very low critical micellar concentrations. Its high affinity for lipid monolayers combined with low lytic properties support the use of edelfosine as a clinical drug. The surface-active properties of edelfosine are similar to those of other "single-chain" lipids, e.g. lysophosphatidylcholine, palmitoylcarnitine, or N-acetylsphingosine.