Increased membrane permeability for an antitumoral alkyl lysophospholipid in sensitive tumor cells

Synthetic Glycosylated Ether Glycerolipids as Anticancer Agents

Glycosylated antitumor ether lipids (GAELs) are a class of synthetic antitumor ether lipids (AELs) with a sugar moiety in place of the phosphocholine found in the prototypical AEL, edelfosine. This chapter reviews the development of GAELs as antitumor agents. Studies on structure–activity relationships, mechanism of induction of cell death, metabolism, selectivity against cancer cells, toxicity, hemolysis and thrombogenic effects are discussed. The requirements for significant cytotoxic activity include a glycerol moiety, a cationic sugar other than mannose and an O- or C-glycosidic bond with either α- or β-configuration. Compounds with S- and N-glycosidic linkages are not very active. The most active GAEL to date, 1-O-hexadecyl-2-O-methyl-3-O-(2′-amino-2′-deoxy-α-d-galactopyranosyl)-sn-glycerol, displays greater in vitro activity than edelfosine, the AEL “gold standard”. The unique properties of GAELs as antitumor agents include their apoptotic-independent mechanism of inducing cell death and the ability to kill cancer stem cells. These characteristics of GAELs offer the potential for their development into chemotherapeutic agents to prevent the recurrence of tumors as well as for treatment against drug-resistant cancers.

Inward Translocation of the Phospholipid Analogue Miltefosine across Caco-2 Cell Membranes Exhibits Characteristics of a Carrier-mediated Process

Miltefosine (hexadecylphosphocholine, HePC) is the first effective oral agent for the treatment of visceral leishmaniasis. The characteristics of HePC incorporation into the human intestinal epithelial cell line Caco-2 were investigated in order to understand its oral absorption mechanism. The results provide evidence for the involvement of a carrier-mediated mechanism, since the association of HePC at the apical pole of Caco-2 cells was (1) saturable as a function of time with a rapid initial incorporation over 5 min followed by a more gradual increase; (2) saturable as a function of concentration over the range studied (2-200 microM) with a saturable component which followed Michaelis-Menten kinetics (apparent K (m) 15.7 micromol/L, V (max) 39.2 nmol/mg protein/h) and a nonspecific diffusion component; (3) partially inhibited by low temperature and ATP depletion, indicating the temperature and energy-dependence of the uptake process. Moreover, we demonstrated, by an albumin back-extraction method, that HePC is internalized via translocation from the outer to the inner leaflet of the plasma membrane and that HePC may preferentially diffuse through intact raft microdomains. In conclusion, our results suggest that incorporation of HePC at the apical membrane of Caco-2 cells may occur through a passive diffusion followed by a translocation in the inner membrane leaflet through an active carrier-mediated mechanism.

Reduction of tamoxifen resistance in human breast carcinomas by tamoxifen-containing liposomes in vivo

We investigated whether it is possible to reduce anti-estrogen resistance using liposomally encapsulated tamoxifen in vivo. Small liposomal vesicles containing up to 5.1 mg tamoxifen/ml liposomal suspension, together with an alkylphospholipid to enhance the cellular uptake, were prepared and characterized. Mice transplanted with different tumor models were treated with tamoxifen liposomes administered i.p. or orally as a bolus dose of 50 mg/kg once a week or as a daily dose of 10 mg/kg/day, both during a 4-week period. After orally administered tamoxifen liposomes, tumor growth was significantly reduced for the 3366/tamoxifen (acquired resistance) and for the MCF-7 (inherent resistance) models to 47 and 16%, respectively (treated to control value of relative tumor volume). Intraperitoneal treatment with tamoxifen liposomes revealed similar results. Investigation of biodistribution revealed especially an accumulation of liposomal tamoxifen in MCF-7 tumors and livers of the treated mice. These liposomes had uterotrophic properties comparable to the dissolved compound. This study demonstrates for the first time that a liposomal formulation of tamoxifen was able to induce pharmacological effects and to improve the therapeutic efficacy in several anti-estrogen-resistant xenografts.

Undifferentiated HL-60 cells internalize an antitumor alkyl ether phospholipid more rapidly than resistant K562 cells

In this study, we confirmed a previous finding that 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (methyl-PAF) expresses higher antineoplastic activity against the promyelocytic leukemia cell line HL-60, than against the erythroleukemic cell line K562, and intended to clarify the reason for this. Using an albumin back-exchange method, we measured the rates of binding and internalization of [3H]methyl-PAF by HL-60 and K562 cells. We found that methyl-PAF associated very rapidly and to similar extents with the two types of cells at low concentrations of extracellular bovine serum albumin, but that when bound to the cell surface, it was internalized into HL-60 cells faster than into K562 cells. The internalization of methyl-PAF by HL-60 cells was concentration-independent, intracellular ATP-independent and susceptible to thiol group-modifying reagents and cytochalasin B. Thus the inward transbilayer movement of methyl-PAF seems to occur by cytochalasin B-sensitive protein-mediated mechanism based on passive diffusion not requiring energy, in which SH-groups of protein play a critical role. We also found that the internalization of 1-hexadecanoyl-2-(4,4-difluoro-5,7- dimethyl-4-bora-3a, 4a-diaza-s-indacene-3-pentanoyl)-sn-glycero-3-phosphocholine (Bodipy-C5-PC), whose structure resembles that of methyl-PAF, into HL-60 cells was faster than that into K562 cells. Using a combination of an albumin back-exchange method and observation by confocal laser scanning microscopy, we next examined the intracellular distribution of this fluorescent phospholipid probe after its internalization. Intracellular membranes, especially those peripheral to nuclei, were fluorescence-labeled in both HL-60 and K562 cells, but fluorescence of the nuclear membranes was weak, suggesting that this probe seems mainly to accumulate in intracellular granules, and may interact directly with several key enzymes for phospholipid metabolism, leading to cell injury. Because the difference between the internalization rates of methyl-PAF in HL-60 and K562 cells was correlated with their different susceptibilities to the cytotoxic effect of methyl-PAF, we suggest that the capacities for uptake of methyl-PAF and its accumulation in intracellular membranes are critical factor for its induction of apoptosis. (c) 1998 Elsevier Science B.V.

Thymotrophic effect of ether lipid 1-O-octadecyl-2-O-methoxy-rac-glicero-3-phosphocholine in the mouse

1-O-octadecyl-2-O-methoxy-rac-glicero-3-phosphocholine (ET-18-OCH3) is a synthetic derivate of 2-lysophosphatidyl-choline, endowed with some immunomodulatory and anticancer effects. In the present work we report that a chronic (1 microgram/g b.w. for 2 weeks) or acute single dose injection of ET-18-OCH3 produced a recovery of thymus weight and thymocytes cellularity in two different strains of mice, C57BL6 and Swiss mice, undergoing thymus age-dependent involution. This effect was significant when the thymus weight was reduced at 50% and it was without effect on thymus lacking age dependent involution, such as young mice. The ability of ET-18-OCH3 to produce thymus weight and thymocyte cellularity recovery was also demonstrated in adult mice showing hypotrophy of thymus induced by chronic corticosterone treatment, suggesting that this compound could be effective against thymus hypotrophy induced by external stimuli. This thymotrophic effect of ET-18-OCH3 was not dependent on direct action on thymocyte proliferation, but probably it was dependent on its action on thymic epithelial cells to produce hormone thymulin, which level was found significantly increased in the plasma. These results provide further immunomodulatory propriety of ET-18-OCH3 and open the possibility to use this compound to counteract thymus hypotrophy.

Characterization of an HL-60 cell variant resistant to the antineoplastic ether lipid 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine

A resistant cell line (HL-60R) was selected by incubating HL-60 cells with increasing concentrations of 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3) and used to examine the mechanism of resistance to the antineoplastic ether-linked lipid. The HL-60R cells exhibited a > 10-fold increase in resistance when measured by [3H]-thymidine incorporation in comparison to the HL-60 cell line. ET-18-OCH3 binding occurred at 4 degrees C and was not saturable at the concentrations tested (1-100 microM), indicating that the binding was receptor-independent. At 4 degrees C, association of ET-18-OCH3 was low for each cell line. AT 37 degrees C, uptake in the HL-60 cells was approximately 5-fold greater in comparison to HL-60R cells at each concentration tested. However, when the cellular content of ET-18-OCH3 was equal, both cell lines experienced similar declines in cell growth. Cellular incorporation of ether lipid was determined using serum-free media and in the presence of serum albumin or lipoproteins. Reduced uptake by the resistant cell line was observed only in the presence of albumin. A greater proportion of ether lipid could be removed from prelabeled HL-60R cells than from HL-60 cells, by an albumin wash procedure, indicating an increased rate of internalization and retention by the sensitive cell line. ET-18-OCH3 uptake in the HL-60 cell line was also more sensitive to treatment with endocytic (chloroquine, monensin) or metabolic (NaF, KCN) inhibitors. These results suggest that uptake is the principal determinant influencing sensitivity of the resistant cell line and consists of receptor-independent binding followed by internalization. Differential uptake requires the presence of serum albumin and is dependent on the energy-dependent endocytosis of the ether lipid.

Severe modifications of ether- ester- or diester-linked glycerolipid and non glycerolipid synthesis in human neuroblastoma LAN-1 cells cultured with octadecylmethylglycerophosphocholine

The growth rate of the human neuroblastoma LAN-1 cells was decreased half after 48 h of incubation with dialkylglycerophosphocholine 1-O-octadecyl 2-O-methyl-sn-glycero-3-phosphocholine (ET-18-O-CH3), at 4 microM. Four radiolabelled precursors, [3H]hexadecanol, [3H]hexadecanoic, [3H]arachidonic acids, or N-acetyl[14C] neuraminic acid were added in the culture medium to follow their cell incorporation among various glycerolipids, gangliosides and eicosanoids. Several modifications of the glycerophospholipid synthesis induced by ET-18-O-CH3 were observed. The inhibition of 1-O-[3H]hexadecanoyl 2-O-acyl glycerophosphocholine synthesis provoked the accumulation of diacylglycerophosphoethanolamine. The inhibition of 1-O-[3H]hexadecyl 2-O-acyl glycerophosphocholine and ethanolamine synthesis enhanced the synthesis of non phosphorous glycerolipids with 1-O-[3H]hexadecyl-sn-glycerol backbone. The eicosanoid synthesis was not disturbed. Alterations of ether- ester- and diester-linked glycerophospholipid and non phosphorous glycerolipid synthesis could modify the lipid membrane distribution and affect the enzymatic pathway of the ceramide synthesis. An excess of [3H]hexadecanoyl-amide molecular species of ceramides in mono- and disialo-gangliosides was observed. By contrast the N-acetyl [14C]neuraminosyl-linkage in these two groups of gangliosides never reached the hypersialylation process described during sialoglyco-peptide synthesis in murine carcinoma kidney cell lines. Both metabolic disturbances of the glycerolipid and ganglioside synthesis reported for the neuroblastoma LAN-1 cell line sensitive to ET-18-O-CH3 extend and confirm the previous studies with other more resistant cell lines from the murine Meth A sarcoma and the rat colon carcinoma.

Analogs of alkyllysophospholipids: chemistry, effects on the molecular level and their consequences for normal and malignant cells

In the search for new approaches to cancer therapy, the first alkyllysophospholipid (ALP) analogs were designed and studied about two decades ago, either as potential immunomodulators or as antimetabolites of phospholipid metabolism. In the meantime, it has been demonstrated that they really act in this way. However, their special importance is based on the fact that, in addition, they interfere with key events of signal transduction, such as hormone (or cytokine)-receptor binding or processing, protein kinase C or phospholipase C function and phosphatidylinositol and calcium metabolism. There are no strict structural requirements for their activity. Differences in the cellular uptake or the state of cellular differentiation seem to be mainly responsible for higher or lower sensitivities of cells towards ALP analogs. Consequences of the molecular effects mentioned on the cellular level are cytostasis, induction of differentiation (while in contrast the effects of known inducers of differentiation such as 12-O-tetradecanoylphorbol-13-acetate are inhibited, probably as a consequence of protein kinase C inhibition) and loss of invasive properties. Already in sublytic concentrations, alterations in the membrane structure were observed, and lysis may begin at concentrations not much higher than those causing the other effects described. Few ALP analogs have already entered clinical studies or are in clinical use. ALP analogs are the only antineoplastic agents that do not act directly on the formation and function of the cellular replication machinery. Therefore, their effects are independent of the proliferative state of the target cells. Because of their interference with cellular regulatory events, including those failing in cancer cells, ALP analogs, beyond their clinical importance, are interesting model compounds for the development of new, more selective drugs for cancer therapy.

Synthesis of heterocyclic platelet activating factor analogues

The synthesis of heterocyclic analogues of the platelet activating factor is described. The preparation starts with acylating rac-tetrahydro-1,3-thiazine-4-carboxylic acid ethyl ester, with palmitoyl chloride to form the amide linkage. Following ester reduction, the phosphocholine part is introduced via 2-chloro-2-oxo-1,3,2-dioxaphospholane and subsequent ring opening with trimethylamine under pressure. Furthermore, the related L-thiazolidine analogue is prepared using the same procedure. In addition the sulfinyl and sulfonyl derivatives of this compound are obtained by oxidation with 3-chloro-perbencoic acid. From one sulfinyl intermediate the diastereomeres are separated and their conformations are determinated by 13C-NMR spectroscopy.

Ether lipids in biomembranes

Plasmalogens (1-O-1'-alkenyl-2-acylglycerophospholipids) and to a lesser extent the 1-O-alkyl analogs are ubiquitous and in some cases major constituents of mammalian cellular membranes and of anaerobic bacteria. In archaebacteria polar lipids of the cell envelope are either diphytanylglycerolipids or bipolar macrocyclic tetraether lipids capable of forming covalently linked 'bilayers'. Information on the possible role of ether lipids as membrane constituents has been obtained from studies on the biophysical properties of model membranes consisting of these lipids. In addition, effects of modified ether lipid content on properties of biological membranes have been investigated using microorganisms or mammalian cells which carry genetic defects in ether lipid biosynthesis. Differential utilization of ether glycerophospholipids by specific phospholipases might play a role in the generation of lipid mediators that are involved in signal transduction. A possible function of plasmalogens as antioxidants has been demonstrated with cultured cells and might play a role in serum lipoproteins. Synthetic ether lipid analogs exert cytostatic effects, most likely by interfering with membrane structure and by specific interaction with components of signal transmission pathways, such as phospholipase C and protein kinase C.

Synthesis of heterocyclic platelet activating factor analogues

The synthesis of heterocyclic analogues of the platelet activating factor is described. The preparation starts with acylating rac-tetrahydro-1,3-thiazine-4-carboxylic acid ethyl ester, with palmitoyl chloride to form the amide linkage. Following ester reduction, the phosphocholine part is introduced via 2-chloro-2-oxo-1,3,2-dioxaphospholane and subsequent ring opening with trimethylamine under pressure. Furthermore, the related L-thiazolidine analogue is prepared using the same procedure. In addition the sulfinyl and sulfonyl derivatives of this compound are obtained by oxidation with 3-chloro-perbencoic acid. From one sulfinyl intermediate the diastereomeres are separated and their conformations are determinated by 13C-NMR spectroscopy.

Unidirectional membrane uptake of the ether lipid antineoplastic agent edelfosine by L1210 cells

We have studied the cellular uptake of edelfosine (1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine; ET-18-OCH3), a membrane active anticancer drug of the ether lipid family, by L1210 murine leukemia cells. Initial unidirectional linear uptake velocity was 1.1 nmol/min per 2 x 10(6) cells; at about 30 min it reached a steady-state phase of accumulation of approximately 5 nmol/2 x 10(6) cells. Concentration studies indicated no saturation kinetics from 0 to 40 microM. Studies with metabolic inhibitors displayed no energy dependence. There was no effect of chloroquine, monensin or cytochalasin B, which are known inhibitors of endocytosis. The inhibitory effect of lower temperature on uptake was moderate in extent and compatible with passive diffusion. There was no efflux of drug from preloaded cells which indicates intense binding of incorporated drug to cells. In human serum, edelfosine bound to several protein components, primarily high density lipoprotein and albumin, and this may explain why cellular uptake was slowed considerably by the presence of serum or albumin in the incubation medium. We conclude that the lipophilic ether lipid derivative edelfosine is taken up by passive diffusion by the L1210 cell. It is tightly bound to cellular structures, probably by insertion into the membrane lipid bilayer.

The role of intracellular free calcium mobilization in the mechanism of action of antitumour ether lipids SRI 62-834 and ET18-OMe

Membrane-active antitumour ether lipids such as ET18-OMe (1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine) and SRI 62-834 ((+-)-2-(Hydroxy[tetrahydro-2-(octadecyloxy) methylfuran-2-yl] methoxyl phosphinyloxy)-N,N,N-trimethylethaniminium hydroxide) are selectively cytotoxic to tumour cells in vitro. Their precise mechanisms of action are unclear, but they are known to have effects on cell membranes and cell signalling. A previous report suggested that ether lipids cause a biphasic sustained rise in intracellular free calcium [Lazenby et al., Cancer Res 50: 3327-3330, 1990]. We show here that the second phase is an experimental artefact due to cell membrane permeabilization by ether lipids in serum-free buffers. In serum-free medium, the membrane toxicity of antitumour ether lipids was increased 50-60 fold, when compared to medium containing 10% serum. Membrane disruption was neither dependent on extracellular calcium, nor modulated by preloading cells with the calcium chelators bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid or 2-[[2-[bis(carboxymethyl)amino]-5-methylphenoxy]methyl]-6- methoxy-8-[bis(carboxymethyl)amino]quinoline. This indicates that the mechanism of membrane damage by ether lipids does not involve changes in calcium homeostasis. Using indo-1 and fura-2 as calcium probes, we established that lower concentrations of antitumour ether lipids do elicit a genuine monophasic and transient rise in intracellular free calcium, predominantly mobilized from internal stores. This acute calcium agonist activity of ether lipids is distinct from the inhibitory effects on cell signalling reported previously after more prolonged exposure. It appears that the calcium elevation induced by antitumour ether lipids is unlikely to be instrumental in their selective and potent antitumour activity.

The 1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphocholine causes a differential incorporation of hexadecanol into neutral ether ester glycerolipids of 2 variant cell lines of rat colon carcinoma

The 1-O-octadecyl 2-O-methyl-sn-glycerophosphocholine (ET-18-O-CH3), when incubated for 24-48h with cells in culture, exerts a highly selective cytotonic activity against a variety of tumor cells that is not seen in normal ones. In this study, we present data which indicate that this exogenous molecule altered the endogenous synthesis of the neutral ether, ester-sn-glycerols, in 2 variant cell lines of a rat colon carcinoma. ET-18-O-CH3, at 20 microM in the medium and for an incubation of 48h, inhibited the growth rates of the PRO cells which have the ability to metastasize and of the REG cells (the regressive cell line), by, respectively, 54 and 67%, as measured after [3H] thymidine uptakes. The synthesis of the ether, ester-glycerolipids was followed after an incorporation of [3H] hexadecanol into the cell lipids. The radiospecific activity of the alcohol in the ether, ester-glycerolipids was higher for the PRO cells than for the REG cells. ET-18-O-CH3 activated the incorporation of [3H] hexadecanol in the neutral ether, ester-sn-glycerols: 1.55 fold in the PRO cells, but 2.15 fold in the REG cells. No change was observed in the alkyl (alkenyl) acyl-sn-glycerophospholipids. Most of the transformed cells have a low etherase activity and are known to accumulate the ether, ester-glycerolipids, (neutral and ionic structures).(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of protein kinase C by ether-linked lipids is not correlated with their antineoplastic activity on WEHI-3B and R6X-B15 cells

To test the hypothesis that the action of antineoplastic ether-linked lipids in leukemic cells is associated with their ability to inhibit protein kinase C (PKC), we have compared the effects of two ether-linked lipids, 1-O-hexadecyl-2-O-methyl-sn-glycero-3-phosphocholine (ET16-OCH3-GPC) and 1-O-hexadecyl-2-O-methyl-sn-glycero-3-(S-beta-D-1'- thioglucopyranosyl)-sn-glycerol (ET16-OCH3-beta-thio-Glc), on two different leukemic cell lines (WEHI-3B and R6X-B15). ET16-OCH3-GPC killed WEHI-3B cells with an EC50 value of 2.5 microM, whereas it was far less effective against R6X-B15 cells (EC50 = 40 microM). In contrast, the beta anomer of ET16-OCH3-beta-thio-Glc did not kill either cell line at concentrations up to 40 microM. Both ET16-OCH3-GPC and ET16-OCH3-thio-Glc inhibited 12-O-tetradecanoylphorbol 12,13-dibutyrate (TPA)-induced PKC translocation in both WEHI-3B and R6X-B15 cells. When WEHI-3B cells were first exposed to TPA, and then to ET16-OCH3-GPC, no significant decrease in PKC activity in the particulate fraction was noticed. When, however, the cells were first exposed to ET16-OCH3-GPC and then to TPA, the enzyme activity in the particulate fraction was decreased by 20-30%. A phorbol dibutyrate binding assay showed that the decrease in membrane-associated PKC activity and the increase in cytosolic PKC activity did not result from impeded enzyme translocation. These results suggest that the similar PKC inhibitory potency of ET16-OCH3-GPC and ET16-OCH3-beta-thio-Glc: (a) is not correlated with the widely different cytotoxicities of these agents and (b) is probably due to interference with the binding of diacylglycerol/phosphatidylserine or TPA to PKC. Taken together, these results suggest that the ether-linked lipids compete with diacylglycerol/phosphatidylserine or TPA for binding sites on PKC required for enzyme activation.

Synthesis of the spin-labeled derivative of an ether-linked phospholipid possessing high antineoplastic activity

We report here the complete synthesis of the spin-labeled derivative of an antitumor ether phospholipid, 1-O-octadecyl-2-O-(4'-doxylpentyl)-rac-glycerol-3-phosphocholine. This also represents the first time that the synthesis of a nitroxide spin-labeled diether phospholipid is described. In vitro experiments showed that at micromolar concentrations, this new analog is readily incorporated into the plasma membranes of human HL60 and mouse E8/AK.D1 leukemic cells, and subsequently kills the cells. The availability of this new probe should permit the electron spin resonance spectroscopic approach to investigate ways by which anti-tumor ether phospholipids selectively destroy the tumor cells.

Alterations in red blood cell sugar transport by nanomolar concentrations of alkyl lysophospholipid

Acyl lysolipids presented in vitro to red blood cells in amounts comparable to blood serum levels inhibit protein-mediated glucose transport (Naderi, A., Carruthers, A. and Melchior, D.L. (1989) Biochim. Biophys. Acta 985, 173-181). In this study, an alkyl lysolipid (2-O-methyl-1-O-octadecyl-sn-glycero-3- phosphocholine; ALP), was found to be an order of magnitude more effective in inhibiting sugar transport than the most potent acyl lysolipid. Bilayer concentrations of ALP as low as 5 ALP molecules per transporter (0.1 mol% of total membrane lipid) result in a 50% inhibition of transport activity. ALP acts as a competitive inhibitor of exchange L-glucose transport, of CCB binding to the glucose transporter and of D-glucose inhibition of CCB binding to the transporter. Inhibition of zero-trans sugar uptake by ALP is noncompetitive. The two enantiomers of ALP show a different ability to inhibit sugar transport. The action of ALP is consistent with a mechanism in which ALP interacts with a transmembrane portion of the sugar transport molecule resulting in a competitive displacement of D-glucose or cytochalasin B from the cytosolic facing side of the transport molecule. The simplest explanation of our findings is a direct interaction of the ALP molecule with the transport protein.

Chirospecific syntheses of 2H- and 13C-labeled 1-O-alkyl-2-O-alkyl'-sn-glycero-3-phosphoethanolamines and 1-O-alkyl-2-O-alkyl'-sn-glycero-3-phosphocholines

A convenient sequence for the synthesis of 1-O-alkyl-2-O-alkyl'-sn-glycero-3-phospholipids was demonstrated starting from 2,3-O-isopropylidene-sn-glycerol, which was first alkylated with 1-bromohexadecane, then converted to the corresponding benzylidene analog. Other less convenient methods to prepare 2,3-O-benzylidene-1-O-hexadecyl-sn-glycerol were also investigated. The key step in the synthesis was the reduction of 2,3-O-benzylidene-1-O-hexadecyl-sn-glycerol with lithium aluminum hydride-aluminum chloride to give 3-O-benzyl-1-O-hexadecyl-sn-glycerol as the major product in 79% yield. The syntheses of 1-O-hexadecyl-2-O-hexadecyl-(1',1'-d2,-sn-glycero-3-phosphoethanolamine and 1-O-hexadecyl-2-O-hexadecyl-(1'-13C)-sn-glycero-3-phosphoethanolamine as well as the correspondingly labeled sn-glycero-3-phosphocholine analogs were then performed. The optical purities of the synthetic intermediates and the ether lipids were established by a novel 1H-NMR method.

Molecular dynamics of antitumor ether-linked phospholipids in model membranes: a spin-label study

We have synthesized a spin-labeled derivative of ET-18-OCH3, a known antitumor ether-linked phospholipid. The spin-labeled analog was shown to be as potent as ET-18-OCH3 in inhibiting 3H-thymidine uptake of HL60 leukemic cells. Electron spin resonance (ESR) studies showed that the mobility of this ether-linked phospholipid in the membrane is more restricted when compared to its ester-linked counterparts. It is probable that the absence of the bulky carbonyl oxygens allows closer packing of the two alkyl chains in the ether-linked phospholipid, thereby reducing the angular amplitude of the motion of the alkyl chains. These findings may be of importance in elucidating mechanisms by which the antitumor ether-linked phospholipids perturb the structure of cellular membranes.