rac-1,2-DIACYLOXYPROPYL-3-ARSONIC ACIDS: ARSONOLIPID ANALOGUES OF PHOSPHONOLIPIDS

Synthesis of Novel Arsonolipids and Development of Novel Arsonoliposome Types

Arsonolipids represent a class of arsenic-containing compounds with interesting biological properties either as monomers or as nanostructure forming components, such as arsonoliposomes that possess selective anticancer activity as proven by in vitro and in vivo studies. In this work, we describe, for the first time, the synthesis of novel arsono-containing lipids where the alkyl groups are connected through stable ether bonds. It is expected that this class of arsonolipids, compared with the corresponding ester linked, will have higher chemical stability. To accomplish this task, a new methodology of general application was developed, where a small arsono compound, 2-hydroxyethylarsonic acid, when protected with thiophenol, can be used in an efficient and simple way as a building block for the synthesis of arsono-containing lipids as well as other arsono-containing biomolecules. Thus, besides the above-mentioned arsonolipid, an arsono cholesterol derivative was also obtained. Both ether arsonolipid and arsono cholesterol were able to form liposomes having similar physicochemical properties and integrity to conventional arsonoliposomes. Furthermore, a preliminary in vitro anticancer potential assessment of the novel ether arsonolipid containing liposomes against human prostate cancer (PC-3) and Lewis lung carcinoma (LLC) cells showed significant activity (dose- and time-dependent), which was similar to that of the conventional arsonoliposomes (studied before). Given the fact that novel arsonolipids may be more stable compared to the ones used in conventional arsonoliposomes, the current results justify further exploitation of the novel compounds by in vitro and in vivo studies.

Organic Arsenicals as Functional Motifs in Polymer and Biomaterials Science

Arsenic (As) exhibits diverse (bio)chemical reactivity and biological activity depending upon its oxidation state. However, this distinctive reactivity has been largely overlooked across many fields owing to concerns regarding the toxicity of arsenic. Recently, a clinical renaissance in the use of arsenicals, including organic arsenicals that are known to be less toxic than inorganic arsenicals, alludes to the possibility of broader acceptance and application in the field of polymer and biomaterials science. Here, current examples of polymeric/macromolecular arsenicals are reported to stimulate interest and highlight their potential as a novel platform for functional, responsive, and bioactive materials.

A high yield procedure for the preparation of arsonolipids (2,3-diacyloxypropylarsonic acids)

The crucial step in the preparation of the title arsonolipids starting from the dichloromethane-soluble dithioarsonite CH(2)(OH)CH(OH)CH(2)-As(SPh)(2) is to avoid an internal cyclization during the acylation which protects the primary -OH group from being acylated. This was to a large extent accomplished by using fatty acyl chloride in the presence of the weak base pyridine and controlling the temperature and rate of the acyl chloride addition, giving approximately 70% yields of arsonolipids. The presence of catalytic amounts of 4-dimethylaminopyridine boosted the yields to 82-85%. This yield is a great improvement over the yields (20-55%) previously achieved. The acylating systems (RCO)(2)O or RCOCl and BF(3).Et(2)O gave only moderate yields (25-60%) of arsonolipids.

Arsonoliposomes: Effect of Lipid Composition on Their Stability and Morphology

The influence of the lipid composition of arsonoliposomes on their membrane integrity was investigated to evaluate whether it is possible to combine their action with drugs that can be encapsulated in their aqueous interior. This was investigated by measuring the retention of vesicle-encapsulated calcein (100 mM) during incubation, in the absence and presence of serum proteins. Liposomes containing various concentrations of arsonolipid (with the palmitoyl side chain) as well as egg-lecithin (phosphatidylcholine, PC) and cholesterol (lipid/chol 2:1 mol:mol) were prepared. In some experiments, PC was replaced by the synthetic phospholipid DSPC. All PC/arsonoliposomes tested are stable after 24 h of incubation in buffer at 37 degrees C. After incubation in the presence of serum proteins, arsonoliposomes that contain low amounts of arsonolipid (up to 5 mol% of the lipid content without cholesterol) are stable, whereas increased release of calcein is observed when vesicle arsonolipid concentration is raised (from 5 to 15 mol%). Further increase of arsonolipid content results in immediate decrease of calcein latency while the remaining calcein is rapidly released during incubation. DSPC/arsonoliposomes are comparably more stable, and membrane integrity is independent of the vesicle arsonolipid content, in the range investigated (15-40 mol% of the lipid content without cholesterol). Thereby, we conclude that more stable arsonoliposomes that incorporate high arsonolipid concentrations may be produced when PC is replaced by DSPC. The latter arsonoliposomes provide a system that may be used for combining arsonolipid activity with the activity of other drugs.

In vivo distribution of arsonoliposomes: Effect of vesicle lipid composition

Sonicated arsonoliposomes were prepared using arsonolipid with palmitic acid acyl chain (C16), mixed with 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC-based), and cholesterol (Chol) with a molar ratio C16/DSPC/Chol 8:12:10. PEG-lipid (1,2-distearoyl-sn-glycero-3-phosphoethanolamine conjugated to polyethylenoglycol 2000) containing vesicles (Pegylated-arsonoliposomes) were also prepared. DSPC-based and Pegylated-arsonoliposomes, were administered by intraperitoneal injection in balb/c mice (15 mg arsenic/kg) and the distribution of As in the organs was measured by atomic absorption spectroscopy. Results demonstrate that a high portion of the dose administered is rapidly excreted since 1 h post-injection only about 30-40% of the dose was detected cumulatively in animal tissues. After this, the whole body elimination of arsenic was a slow process with a half-life of 27.6 h for Pegylated-arsonoliposomes, and 83 h, for the DSPC-based ones. For both arsonoliposomes, arsenic distribution was greater in intestines, followed by liver, carcass+skin stomach, spleen, kidney, lung and heart. Different arsenic kinetics in blood between the two liposome types were observed. Compared to the results obtained previously with PC-based arsonoliposomes, both the DSPC-based and Pegylated-arsonoliposomes have better bioavailability. This proves that arsonoliposome lipid composition (and consequently their integrity) influences their pharmacokinetic profile. Thus, the proper arsonoliposome composition should be used according to the intended application.

Trypanocidal activity of arsonoliposomes: Effect of vesicle lipid composition

Sonicated arsonoliposomes were prepared using an arsonolipid with palmitic acid acyl chain (C16), mixed with phosphatidylcholine (PC-based) or 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC-based), and cholesterol (Chol) with a molar ratio C16 /PC or DSPC/ Chol 8:12:10. PEG-lipid (1,2-distearoyl-sn-glycero-3-phosphoethanolamine conjugated to polyethylenoglycol 2000) containing vesicles (pegylated-arsonoliposomes) were also prepared. The in vitro and in vivo trypanocidal activity of the various types of arsonoliposomes was evaluated. Although PC-based arsonoliposomes exhibited in vivo activity on an acute trypanosomiasis animal model, no evidence of activity was demonstrated for DSPC-based or pegylated-arsonoliposomes on a chronic model. Despite the fact that DSPC-based and pegylated-arsonoliposomes have better bioavailability compared to PC-based ones, their in vitro activity is lower than that of PC-based arsonoliposomes, indicating the importance of arsonoliposome lipid composition on their trypanocidal activity and suggesting that further arsonoliposome structure design is required to overcome these disadvantages.

Preparation of dl-2,3,4-trihydroxybutylarsonic acid and dl-2,3-dihydroxybutane-1,4-bis(arsonic acid): starting compounds for novel arsonolipids

The reaction of DL-1,3-butadiene diepoxide and of DL-1,4-dibromo-2,3-butanediol with aqueous alkaline sodium arsenite, "Na(3)AsO(3)", gave mixtures of the title arsonic acids which can be separated by anion exchange resin. Characterization of by-products leads to a better understanding of these reactions. These compounds are valuable intermediates for the preparation of novel arsonic acids and bis(arsonic acids).

Physical Stability of Sonicated Arsonoliposomes: Effect of Calcium Ions

The physical stability of sonicated arsonoliposomes in the absence and presence of Ca(2+) ions is evaluated. Cholesterol-containing arsonoliposomes composed of arsonolipids [having different acyl chains (C(12)-C(18))], or mixtures of arsonolipids with phospholipids (phosphatidylcholine or distearoyl-phosphatidylcholine) were prepared, and physical stability was evaluated in the absence and presence of CaCl(2), by vesicle dispersions turbidity measurements and cryo-electron microscopy morphological assessment. In some cases, vesicle zeta-potential was measured, under identical conditions. Results demonstrate that self-aggregation of the vesicles studied is low and influenced by the acyl chain length of the arsonolipid used, whereas calcium-induced aggregation is higher, correlating well with the decreased values of vesicle zeta-potential in the presence of Ca(2+) ions (weaker electrostatic repulsion). Acyl chain length of arsonolipids used has a significant quantitative effect on Ca(2+)-induced vesicle aggregation mainly for arsonoliposomes that contain phospholipids (mixed), compared with the vesicles that consist of plain arsonolipids (significant effect only for initial aggregation at time 0). Another difference between plain and mixed arsonoliposomes is that for mixed arsonoliposomes Ca(2+)-induced increases in turbidity are irreversible by ethylenediaminotetraacetic acid, suggesting that vesicle fusion is taking place. This was confirmed by cryo-electron microscopy observations. Finally, when phosphatidylcholine is replaced by distearoyl-phosphatidylcholine, arsonoliposomes are more stable in terms of self-aggregation, but in the presence of calcium, the turbidity and morphology results are similar.

The effect of pH on the electrophoretic behaviour of a new class of liposomes: arsonoliposomes

Herein we report the effect of pH on the surface charge of a new class of liposomes: arsonoliposomes. Plain or mixed arsonoliposomes with cholesterol (Chol) and distearoyl-phosphatidylcholine (DSPC) in 1:1 molar ratio were prepared with lauryl-(C12), myristoyl-(C14) and palmitoyl-(C16) acyl side chain arsonolipids. The one step hydration method was used for vesicle preparation and zeta potential measurements were performed in the pH range from 3 to 9. The results revealed that these lipids hold a negative surface charge at all pH values investigated. The presence of cholesterol in 1:1 molar ratio results in higher zeta potential compared with plain arsonoliposomes with the exception of palmitoyl-(C16) acyl chain arsonolipids in neutral and slightly basic pH values. Oppositely, the DSPC (1:1 molar ratio) containing arsonoliposomes had lower values of zeta potential compared with plain arsonoliposomes. Concluding, the experimental results reveal that zeta potential of arsonoliposomes is indeed modified when the vesicles are incubated in environments with different acidity. In most cases these changes are in accordance with the ionization pattern of the arsonolipid headgroup, while some peculiar deviations may be connected with the known difference in the structure between some of the vesicle types studied.

In vivo distribution of arsenic after i.p. injection of arsonoliposomes in balb-c mice

We recently showed that arsonoliposomes (novel arsenic containg liposomes) demonstrate differential toxicity towards various types of cancer and normal cells, in cell culture studies, as well as anti-parasitic activity. In this study, the in-vivo distribution of the active moiety of these vesicles, As, is evaluated. Sonicated arsonoliposomes were prepared using the arsonolipid with palmitic acid acyl chain (C16) mixed with egg-phosphatidyl choline (PC) and cholesterol (Chol) [C16/PC/Chol at 8:12:10 mol/mol/mol]. A dose of arsonoliposomes, corresponding to 5 mg arsenate/kg was administered by intraperitoneal injection in balb-c mice. At various time points post-injection the mice were sacrificed and the distribution of As in the organs was measured, by atomic absorption spectroscopy. Results demonstrate that a high portion of the dose administered is rapidly excreted; since 1-h post-injection only about 30% of the dose administered was detected cumulatively in the animal tissues. After this the elimination of arsenic was a slow process with a total body elimination rate constant of 0.023 h(-1), corresponding to a half-life of 30 h. Tissues with the highest arsenic concentration during the study period are: spleen-kidneys-stomach, followed by lung, liver, intestines-heart, carcass+skin and finally blood. No acute toxicity, or effect on the body or organ weight of the mice was observed.

Arsenolipids

Natural arsenolipids are analogues of neutral lipids, like monoglycerides, glycolipids, phospho- and also phosphonolipids. They have been found in microorganisms, fungi, plants, lichens, in marine mollusks, sponges, other invertebrates, and in fish tissues. This review presented structures of natural arsenolipids (and derivatives), their distribution, biogenesis in algae and invertebrates, synthesis, and also biological activity. Arsenolipids are thought to be end products of arsenate detoxification processes, involving reduction and oxidative methylation and adenosylation. The proposed biogenesis of arsenolipids is based on the natural occurrence of arsenic metabolites, and all the intermediates in the proposed pathway have been identified as natural products of algal origin. Different arseno species are shown to be inhibitors of glycerol kinase, bovine carbonic anhydrase, and also is an effective therapy for acute promyelocytic leukemia, and there has been promising activity noted in other hematologic and solid tumors. Arsonoliposomes demonstrated high anti-trypanosomal activity against Trypanosoma brucei and inhibit growth of some types of cancer cells (HL-60,C6 and GH3).

In-vitro antileishmanial and trypanocidal activities of arsonoliposomes and preliminary in-vivo distribution in BALB/c mice

We have studied the antiprotozoal activity of some recently prepared and characterized arsonoliposome formulations. Plain arsonoliposomes and phosphatidylcholine arsonoliposomes prepared with palmitoyl- (C16) or lauroyl-(C12) acyl side chain arsonolipids showed in-vitro antileishmanial activity after a 72-h incubation period against wild-type promastigote forms of Leishmania donovani. The IC50 values ranged from 0.40 to 11.6 microM arsonolipid. Interestingly, all preparations tested were found to be significantly more potent against amphotericin B- or miltefosine-resistant promastigote forms of L. donovani, with IC50 values ranging between 0.21- and 2.33 microM arsonolipid. When tested in-vitro against Trypanosoma brucei brucei, all arsonoliposome formulations were found to have anti-trypanosomal activity after a 24-h incubation period. The fact that the corresponding arsonolipids (dissolved in dimethyl sulfoxide) were found not to be potent against the Leishmania promastigotes or the trypanosomes tested suggested that the formation of liposomes possibly influenced the mode of interaction between the active lipid and the parasites modulating their potency. In addition, a preliminary in-vivo study in BALB/c mice was performed for the initial evaluation of the biodistribution of arsonoliposomes. The accumulation of arsenic in the BALB/c mouse liver in relatively high amounts was an additional advantage of this approach for anti-protozoal therapy, especially for visceral leishmaniasis where parasites are located mainly in the liver.

Arsonoliposomes: effect of arsonolipid acyl chain length and vesicle composition on their toxicity towards cancer and normal cells in culture

Arsonolipid-containing liposomes were investigated in order to characterize the influence of the lipid acyl-chain length and liposome composition on cytotoxicity. Three types of cancer cells (HL-60, C6 and GH3), and two types of normal cells (HUVEC and RAME) were used. Liposomes containing the lauroyl, myristoyl and stearoyl side chain arsonolipids (with different lipid compositions) were incubated with a given number of cells and cell viability was estimated (MTT assay and trypan blue exclusion). Morphological studies were also performed in some cases. In addition, the interaction between some of the prepared arsonoliposomes and HUVEC cells was assessed. Results reveal that all the studied arsonoliposomes cause a dose dependent inhibition of survival in all three malignant cell lines studied (initiated at 10(-6) M). The corresponding toxicity against normal cells (HUVEC and RAME) is much lower for all arsonoliposomes, except for the lauroyl side chain arsonoliposomes which were demonstrated to be relatively toxic towards normal cells, especially RAME. The microscopic observations that these vesicles possibly cause apoptosis of most cell types studied, as well as the different speed of their cytotoxic activity, imply a different mechanism of action for this arsonoliposome type. Taking the results of this study in conjunction with our previous results on arsonoliposome physical stability and cytotoxicity, it is recommended that palmitoyl-arsonolipid arsonoliposomes be used for further investigations in vivo towards the development of an anticancer product.

Preparation of pseudo-arsonolipids: 2-acyloxypropane-1,3-bis(arsonic acids). The crystal structure of 2-hydroxypropane-1,3-bis(arsonic acid)

Epihalohydrins react with alkaline arsenite to give in very good yields 2-hydroxypropane-1,3-bis(arsonic acid) (7), a key compound for the synthesis of pseudo-arsonolipids and more complex arsinolipids. Through a series of reduction to -As(SPh)(2), acylation, and oxidation to -AsO(3)H(2), pseudo-arsonolipids, i.e. 2-acyloxypropane-1,3-bis(arsonic acids), were obtained. These pseudo-lipids are very sensitive to bases, being de-acylated. The bis(arsonic acid) (7) crystallizes in the orthorombic space group P2(1)2(1)2(1) with unit cell constants a=6.911(3), b=17.496(8), c=7.002(3) A. Both arsenic atoms are essentially tetrahedral being bound to three oxygens and one carbon. All hydrogen atoms have been located. There is no intramolecular but only intermolecular hydrogen bonding involving all the As=O, As-OH, and C-OH groups. The C-OH group acts as a hydrogen donor to an acidic As-OH, and this As-OH, in turn, acts as a hydrogen donor to an As=O group.

Synthesis of arsinolipids. II. A non-isosteric analogue of fully acylated cardiolipin

2-Hydroxypropane-1,3-bis(arsonic acid) after six successive reactions gives the arsinolipid 2-acyloxy-As, As'-bis[2,3-di(acyloxy)propyl]propane-1,3-diylbis(arsinic acid) in 20-40% overall yields. This arsinolipid is a non-isosteric analogue of the fully acylated cardiolipin. The R- and S-glycidol, used to create the backbone of the lipid, give the optically active RR and SS, respectively, arsinolipids, while the rac-glycidol produces a mixture of diastereomers (a racemic pair, RR and SS, and two meso forms with an RS configuration). Some properties of these arsinolipids are described, from which the most interesting are the facile hydrolysis of the middle acyl group and their tendency to absorb environmental water.

Preparation and properties of arsonolipid containing liposomes

Arsonolipids are analogs of phosphonolipids which have a chemically versatile head group. In preliminary cell culture studies, liposomes composed solely of arsonolipids or of phosholipid-arsonolipid mixtures, demonstrate a specific toxicity against cancer cells (Gortzi et al., unpublished results). The possibility of using such formulations as an alternative of arsenic trioxide with or without combination of other cytostatic agents (encapsulated in their aqueous interior) prompted the investigation of their physicochemical characteristics. Herein we compared the characteristics of arsonolipid containing vesicles with different lipid compositions. Experimental results and morphological observations reveal that non-sonicated formulations have different structures and stability (when both membrane integrity and aggregation are taken into account) depending on the acyl chain length of the arsonolipid. When phospholipids and especially cholesterol are included in their membranes almost all arsonolipids studied produce more stable vesicles. An interesting aspect of these arsonolipid containing vesicles is also their negative surface charge, which may be modulated by mixing phospholipids with arsonolipids. Sonicated vesicles have smaller sizes and profoundly higher stability, especially when containing cholesterol and phosphatidylcholine mixed with arsonolipids. The only exception is that of the arsonolipid with the C(12) acyl chain which was observed to produce long tubes which break down to cubes by sonication. In conclusion, these initial studies demonstrate that sonicated vesicles composed of arsonolipid and phospholipid mixtures mixed with cholesterol posses the stability required to be used as an arsonolipid delivery system. In addition, although cryo-electron microscopy demonstrated that the sonicated vesicles are elliptical in shape, their encapsulation efficiency is not significantly lower than sonicated phospholipid liposomes. Thereby, these vesicles may be also used for the delivery of other drug molecules which can be sufficiently retained in their aqueous interior.