Synthesis and apoptogenic activity of fluorinated ceramide and dihydroceramide analogues

A review of ceramide analogs as potential anticancer agents

Ceramide serves as a central mediator in sphingolipid metabolism and signaling pathways, regulating many fundamental cellular responses. It is referred to as a 'tumor suppressor lipid', since it powerfully potentiates signaling events that drive apoptosis, cell cycle arrest, and autophagic responses. In the typical cancer cell, ceramide levels and signaling are usually suppressed by overexpression of ceramide-metabolizing enzymes or downregulation of ceramide-generating enzymes. However, chemotherapeutic drugs as well as radiotherapy increase intracellular ceramide levels, while exogenously treating cancer cells with short-chain ceramides leads to anticancer effects. All evidence currently points to the fact that the upregulation of ceramide levels is a promising anticancer strategy. In this review, we exhibit many anticancer ceramide analogs as downstream receptor agonists and ceramide-metabolizing enzyme inhibitors.

Chemical tools to investigate sphingolipid metabolism and functions

Sphingolipids comprise an important group of biomolecules, some of which have been shown to play important roles in the regulation of many cell functions. From a structural standpoint, they all share a long 2-amino-1,3-diol chain, which can be either saturated (sphinganine), hydroxylated at C4 (phytosphingosine), or unsaturated at C4 (sphingosine) as in most mammalian cells. N-acylation of sphingosine leads to ceramide, a key intermediate in sphingolipid metabolism that can be enzymatically modified at the C1-OH position to other biologically important sphingolipids, such as sphingomyelin or glycosphingolipids. In addition, both ceramide and sphingosine can be phosphorylated at C1-OH to give ceramide-1-phosphate and sphingosine-1-phosphate, respectively. To better understand the biological and biophysical roles of sphingolipids, many efforts have been made to design synthetic analogues as chemical tools able to unravel their structure-activity relationships, and to alter their cellular levels. This last approach has been thoroughly studied by the development of specific inhibitors of some key enzymes that play an important role in biosynthesis or metabolism of these intriguing lipids. With the above premises in mind, the aim of this review is to collect, in a systematic way, the recent efforts described in the literature leading to the development of new chemical entities specifically designed to achieve the above goals.

Stereoselective synthesis of (2S,3S,4Z)-4-fluoro-1,3-dihydroxy-2-(octadecanoylamino)octadec-4-ene, [(Z)-4-fluoroceramide], and its phase behavior at the air/water interface

BACKGROUND

Sphingolipids belong to the most important constituents of the membranes of eukaryotic cells. As intermediates in sphingolipid metabolism, sphingosine and its N-octadecanoyl-derivative, ceramide, exhibit a variety of biological functions. These compounds play a crucial role in many essential biological processes such as cell growth, cell differentiation, cell recognition and apoptosis. More specifically, sphingolipids are crucial e.g. for the function of the skin because they contribute to the formation of the water permeability barrier consisting of a highly organized multilaminar lipid matrix of free fatty acids, cholesterol and ceramides containing additional hydroxyl groups in the sphingosin part and longer fatty acid amide functions.

RESULTS

In a short synthetic route (2S,3S)-4-fluorosphingosine and 4-fluoroceramide, the fluorinated analogues of the natural products, D-erythro-sphingosine and ceramide, have been prepared. The key step of the synthetic sequence is an asymmetric aldol reaction of (Z)-2-fluorohexadec-2-enal, prepared in three steps from tetradecanal, with an enantiopure N-protected iminoglycinate. Deprotection of the imino function and reduction of the ester group led to the 4-fluorosphingosine, which on acetylation with stearoyl chloride gave 4-fluoroceramide. After careful HPLC purification of the latter compound its phase behavior was investigated by Langmuir film balance technique and compared to that of natural ceramide. While the isotherms are quite similar in shape, they differ significantly in the starting point of increasing film pressure (56 or 67 A(2)/molecule) and in the film collapse pressure (38 or 56 mN/m) for ceramide and 4-fluoroceramide, respectively. Moreover, the hysteresis curves are very different. While consecutive isothermic compression - expansion cycles are reversible for the 4-fluoro derivative, substantial substance loss into the subphase or irreversible formation of multi-layers was observed for natural ceramide.

CONCLUSIONS

Asymmetric aldol reaction proved to be successful for the preparation of enantiopure 4-fluoroceramide. Surface/pressure isotherms and hysteresis curves of ceramide and its 4-fluoro derivative showed that the presence of fluorine leads to stronger intermolecular interactions between the hydrophobic chains of neighboring molecules, and therefore to increasing stability of the monolayer of 4-fluoroceramide at the air water interface.

Sphingolipid players in the leukemia arena

Sphingolipids function as bioactive mediators of different cellular processes, mostly proliferation, survival, differentiation and apoptosis, besides being structural components of cellular membranes. Involvement of sphingolipid metabolism in cancerogenesis was demonstrated in solid tumors as well as in hematological malignancies. Herein, we describe the main biological and clinical aspects of leukemias and summarize data regarding sphingolipids as mediators of apoptosis triggered in response to anti-leukemic agents and synthetic analogs as inducers of cell death as well. We also report the contribution of molecules that modulate sphingolipid metabolism to development of encouraging strategies for leukemia treatment. Finally we address how deregulation of sphingolipid metabolism is associated to occurrence of therapy resistance both in vitro and in vivo. Sphingolipids can be considered promising therapeutic tools alone or in combination with other compounds, as well as valid targets in the attempt to eradicate leukemia and overcome drug resistance.

Synthesis of constrained ceramide analogs and their potent antileukemic activities

Constrained ceramide analogs were designed and synthesized by binding terminal alcohol and amine of ceramide with additional carbonyl functional group as 3-acetyl (3), 3-propionyl (4), 3-benzoyl (5), and 3-hexadecanoyl-4-(1-hydroxyhexadec-2-enyl)-oxazolidin-2-ones (6). Compounds 4 and 5 showed potent antileukemic activities against human leukemia HL-60 cells with good correlation between cell death and DNA fragmentation.

Apoptotic activities of C2-ceramide and C2-dihydroceramide homologues against HL-60 cells

The apoptotic activities of non-natural ceramide homologues, C2-homo-ceramide, C2-homo-dihydroceramide, C2-bishomo-ceramide and C2-bishomo-dihydroceramide, were examined using human leukemia HL-60 cells. The apoptotic activity was in order of C2-ceramide>C2-homo-ceramide approximately C2-bishomo-ceramide and the activities of the L-erythro- and D-erythro-ceramide homologues were similar. The morphological features of the cells, DNA fragmentations, proteolytic processing of pro-caspase-3 and the cleavage of PARP as the result of treatments with these homologues indicated that cell death was induced by apoptosis.

Designing anticancer drugs via the achilles heel: ceramide, allylic ketones, and mitochondria

Published reports are reviewed as the basis of a proposal that an effective antineoplastic drug should contain several features: (a) resemblance to the natural lipid, ceramide; (b) an allylic alcohol and/or allylic ketone moiety; (c) a hydroxyl and/or a nitrogen atom near the allylic group; (d) conjugated double bonds as part of the allylic region. The drug should produce reactive oxygen species in tumor mitochondria, stimulate the generation of ceramide in the tumor, and condense with mitochondrial glutathione. It is pointed out that some antibiotics with these features are also active against cancer cells; perhaps anticancer drugs with these features will prove useful as antibiotics. Common problems in working with lipoidal substances are discussed.

Ceramide analogues in apoptosis: a new strategy for anticancer drug development

A survey on the role played by ceramide within the sphingolmyelin pathway is here reported, taking into account its importance as an intracellular effector molecule in apoptosis. Recently, several analogs of ceramide, able to pass the cell membrane and then to induce apoptosis, have been developed as a new potential approach in anticancer therapy.

Synthesis of non-natural C2-homo-ceramide and its apoptotic activity against HL-60 cells

Non-natural ceramide analogues, C2-homo-ceramide and C2-homo-dihydroceramide, were prepared from L-aspartic acid via L-homo-serine. The apoptotic activities of the synthesized ceramide analogues were examined in HL-60 human leukemia cells. C2-homo- and C2-bishomo-ceramide indicate low but considerable apoptotic activities in comparison with C2-ceramide.

Conformationally restrained ceramide analogues: effects of lipophilic modifications on the antiproliferative activity

Conformationally restrained analogues of ceramide containing thiouracil or uracil moieties in their polar head, substituted with an ethyl group in their 6-positions, proved to inhibit cell proliferation and induce apoptosis. A series of new thiouracil and uracil analogues of ceramide possessing several 6-alkyl- or 6-arylalkyl-substituents, were synthesized and tested as inhibitors of cell proliferation. The lipophilic substituents introduced in the 6-position were pure alkyls (n-propyl, n-butyl, i-butyl, neo-pentyl), or aryl-alkyls (2-phenylethyl). Although a significant antiproliferative activity was maintained in most compounds synthesized, none of them showed any improvement with respect to their 6-ethyl-substituted counterparts.

Structure-activity relationship of short-chain sphingoid bases as inhibitors of sphingosine kinase

Short-chain sphinganine analogues 8, 9, 18, and 19, as well as 3-fluoro-sphingosine analogues 25 and 26 were synthesized. Their potential as sphingosine kinase inhibitors was investigated, in combination with previously synthesized sphingosine and fluorinated sphinganine analogues.