Structure-activity relationships of four anti-cancer alkylphosphocholine derivatives in vitro and in vivo

Alkylphospholipids are Signal Transduction Modulators with Potential for Anticancer Therapy

BACKGROUND

Alkylphospholipids (APLs) are synthetically derived from cell membrane components, which they target and thus modify cellular signalling and cause diverse effects. This study reviews the mechanism of action of anticancer, antiprotozoal, antibacterial and antiviral activities of ALPs, as well as their clinical use.

METHODS

A literature search was used as the basis of this review.

RESULTS

ALPs target lipid rafts and alter phospholipase D and C signalling cascades, which in turn will modulate the PI3K/Akt/mTOR and RAS/RAF/MEK/ERK pathways. By feedback coupling, the SAPK/JNK signalling chain is also affected. These changes lead to a G2/M phase cell cycle arrest and subsequently induce programmed cell death. The available knowledge on inhibition of AKT phosphorylation, mTOR phosphorylation and Raf down-regulation renders ALPs as attractive candidates for modern medical treatment, which is based on individualized diagnosis and therapy. Corresponding to their unusual profile of activities, their side effects result from cholinomimetic activity mainly and focus on the gastrointestinal tract. These aspects together with their bone marrow sparing features render APCs well suited for modern combination therapy. Although the clinical success has been limited in cancer diseases so far, the use of miltefosine against leishmaniosis is leading the way to better understanding their optimized use.

CONCLUSION

Recent synthetic programs generate congeners with the increased therapeutic ratio, liposomal formulations, as well as diapeutic (or theranostic) derivatives with optimized properties. It is anticipated that these innovative modifications will pave the way for the further successful development of ALPs.

Antitumor Lipids--Structure, Functions, and Medical Applications

Cell proliferation and metastasis are considered hallmarks of tumor progression. Therefore, efforts have been made to develop novel anticancer drugs that inhibit both the proliferation and the motility of tumor cells. Synthetic antitumor lipids (ATLs), which are chemically divided into two main classes, comprise (i) alkylphospholipids (APLs) and (ii) alkylphosphocholines (APCs). They represent a new entity of drugs with distinct antiproliferative properties in tumor cells. These compounds do not interfere with the DNA or mitotic spindle apparatus of the cell, instead, they incorporate into cell membranes, where they accumulate and interfere with lipid metabolism and lipid-dependent signaling pathways. Recently, it has been shown that the most commonly studied APLs inhibit proliferation by inducing apoptosis in malignant cells while leaving normal cells unaffected and are potent sensitizers of conventional chemo- and radiotherapy, as well as of electrical field therapy. APLs resist catabolic degradation to a large extent, therefore accumulate in the cell and interfere with lipid-dependent survival signaling pathways, notably PI3K-Akt and Raf-Erk1/2, and de novo phospholipid biosynthesis. They are internalized in the cell membrane via raft domains and cause downstream reactions as inhibition of cell growth and migration, cell cycle arrest, actin stress fibers collapse, and apoptosis. This review summarizes the in vitro, in vivo, and clinical trials of most common ATLs and their mode of action at molecular and biochemical levels.

Differential effects of erufosine on proliferation, wound healing and apoptosis in colorectal cancer cell lines

The alkylphosphocholine, erucylphospho-N,N, N-trimethylpropanolamine (erufosine), has demonstrated anticancer effects in various cell lines, including leukemia, multiple myeloma, bladder, breast and oral squamous cell carcinoma cells. The purpose of the present study was to investigate its antiproliferative, antimigratory and pro-apoptotic effects in colorectal cancer cell lines, SW480 and CC531. The antiproliferative effect was determined by (3-[4,5-dimethylthiazol-2-yl]-2, 5-diphenyltetrazolium bromide) (MTT) dye reduction assay following exposure to erufosine (3.1-100 µM) for 24-72 h. The antimigratory effect of erufosine (1.6-6 µM) was investigated by a wound healing assay for 12-48 h. Caspase-3/-7 activity was measured to detect apoptotic cell death. Erufosine inhibited cell proliferation in a dose- and time-dependent manner. The IC50 values following 72 h of incubation were 3.4 and 25.4 µM for SW480 and CC531 cells, respectively. erufosine at concentrations of 50 and 100 µM induced caspase-3/-7 activity concentration-dependently in SW480 cells, but only at 100 µM in CC531 cells. Incubation of SW480 cells with erufosine (1.56 µM) for 48 h inhibited migration into the scratched area by 54% as compared to the untreated cells; whereas in CC531 cells, the wound width in the erufosine-treated (1.56-6.25 µM) cells following 48 h was closed 2-fold slower than the rate in the untreated group. Erufosine (25 µM) attenuated osteonectin expression and abolished COL1A1 expression in CC531 cells. Erufosine appears to be a promising treatment agent for colorectal cancer. Rat CC531 cells are less sensitive to erufosine than human SW480 cells.

Erufosine, a novel alkylphosphocholine, in acute myeloid leukemia: single activity and combination with other antileukemic drugs

PURPOSE

Alkylphosphocholines represent a new class of cytostatic drugs with a novel mode of action. Erufosine (ErPC3), the first compound of this class that can be administered intravenously, has recently been shown to be active against human tumor and leukemic cell lines.

METHODS

In order to evaluate the antileukemic potential of ErPC3 in acute myeloid leukemia (AML) the lethal concentration 50% (LC 50) was determined using WST-1 assay. For analysis of cell death, staining for Annexin V and activated caspase 3 was performed. An interaction analysis was performed by calculation of combination index and construction of isobolograms.

RESULTS

The LC 50 was 7.4 microg/ml after 24 h and 3.2 microg/ml after 72 h in HL 60 cells and 30.1 and 8.6 microg/ml, respectively, in 19 fresh samples from patients with AML. ErPC3 was found to be cytotoxic in HL60 cells with distinct activation of caspase 3. ErPC3 was not cross-resistant with cytarabine, idarubicine and etoposide as shown by the linear relation of respective LC 50s. The latter agents, however, exerted an additive cytotoxicity in combination with ErPC3 as revealed by isobologram analysis and combination index, although results are uneven for idarubicine.

CONCLUSION

Based on these data ErPC3 appears as a novel antileukemic candidate drug, which needs to be explored further in the treatment of AML.

Toxicity study of erucylphosphocholine in a rat model

PURPOSE

To investigate the effect of intraocular erucylphosphocholine (ErPC) on the retina, the retinal pigment epithelium (RPE), and the choroid in an in vivo rat model.

METHODS

Adult male Brown Norway rats were injected intravitreally with ErPC dissolved in balanced salt solution (BSS) at a final concentration of 10 or 100 microM with BSS serving as control. Adverse effects on the anterior and posterior segment were assessed by slit-lamp biomicroscopy and ophthalmoscopy. Retinal toxicity was assessed by electroretinography (ERG), retinal ganglion cell (RGC) quantification, and histology 7 days after intravitreal administration of ErPC.

RESULTS

There was neither a statistically significant difference in the clinical examination nor in the ERG waves of treated versus control rats 7 days after intravitreal administration of ErPC. Correspondingly, the number of RGC after BSS injection did not differ significantly from ErPC-injected animals. Histologic sections of the posterior segment of 10 and 100 microM ErPC-injected rats did not show any signs of retinal toxicity. Electron microscopy did not display a difference between the 10 microM and the control group. Only the 100 microM-injected animals showed a discrete irregularity of the Müller cell and the retinal ganglion cell cytoplasm at the ultrastructural level.

CONCLUSIONS

ErPC can safely be injected into the vitreous of adult rats at a concentration of 10 microM without any retinal toxicity. Even a 10-fold increase in ErPC concentration leads only to a discrete cytoplasmic irregularity of the innermost retinal layers.

Structure-activity relationships of antineoplastic ring-substituted ether phospholipid derivatives

PURPOSE

Previous studies have shown that alkylphosphocholines (APCs) exhibit strong antineoplastic activity against various tumour cell lines in vitro and in several animal models. The current study was designed to investigate the influence of cycloalkane rings on the antiproliferative activity of APCs against a panel of eight human and animal cell lines (PC3, MCF7, A431, Hela, PC12, U937, K562, CHO). Specifically, we explored the effect of the presence of 4-alkylidenecyclohexyl and cycloalkylidene groups in alkoxyethyl and alkoxyphosphodiester ether lipids, respectively. In addition, the haemolytic activity of the new ring-substituted ether phospholipids (EP) was evaluated.

METHODS

Cells were exposed to various concentrations of the compounds for 72 h. The cytotoxicity was determined with the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] dye reduction assay. Similarly, red blood cells were distributed in 96-well microplates and treated with the test compounds at concentrations ranging from 100 to 6.25 microM for 1 h. After centrifugation, the absorbance of the supernatants was measured at 550 nm.

RESULTS

The majority of the compounds tested exhibited significant cytotoxic activity which depended on both the ring size and position with respect to the phosphate moiety, as well as the head group. Among the cycloalkylidene series the 11-adamantylideneundecyl-substituted N-methylmorpholino EP 13 was the most potent and exhibited broad-spectrum anticancer activity comparable to or superior to that of hexadecylphosphocholine (HePC). All the adamantylidene-substituted EPs were nonhaemolytic (concentration that exhibits 50% haemolytic activity, HC(50), >100 microM). Furthermore, the cyclohexylidene-substituted analogues were more potent against the cell lines tested, with the exception of U937 and K562, than the cyclodecapentylidene-substituted compounds. Hydrogenation of the double bond in the cycloalkylidene-substituted EPs (compounds 14 and 15) resulted in improvement of anticancer activity. Among the 2-(4-alkylidenecyclohexyloxy)ethyl EPs, 2-(4-hexadylidenecyclohexyloxy)ethyl phosphocholine (22) possessed the highest broad-spectrum cytotoxic activity than all the other analogues of this series and was nonhaemolytic (HC(50) >100 microM). In general, the 2-(4-alkylidenecyclohexyloxy)ethyl-substituted EPs were more active against the more resistant cell lines U937, K562 and CHO than HePC.

CONCLUSIONS

The presence of cycloalkane rings in the lipid portion of APCs reduces haemolytic effects compared to HePC and in several analogues results in improved antineoplastic activity.

Effects of miltefosine and other alkylphosphocholines on human intestinal parasite Entamoeba histolytica

The protozoan parasite Entamoeba histolytica is the cause of amoebic dysentery and liver abscess. It is therefore responsible for significant morbidity and mortality in a number of countries. Infections with E. histolytica are treated with nitroimidazoles, primarily with metronidazole. At this time, there is a lack of useful alternative classes of substances for the treatment of invasive amoebiasis. Alkylphosphocholines (alkyl-PCs) such as hexadecyl-PC (miltefosine) were originally developed as antitumor agents, but recently they have been successfully used for the treatment of visceral leishmaniasis in humans. We examined hexadecyl-PC and several other alkyl-PCs with longer alkyl chains, with and without double bond(s), for their activity against two strains of E. histolytica. The compounds with the highest activity were oleyl-PC, octadecyl-PC, and nonadecenyl-PC, with 50% effective concentrations for 48 h of treatment between 15 and 21 microM for strain SFL-3 and between 73 and 98 microM for strain HM-1:IMSS. We also tested liposomal formulations of these alkyl-PCs and miltefosine. The alkyl-PC liposomes showed slightly lower activity, but are expected to be well tolerated. Liposomal formulations of oleyl-PC or closely related alkyl-PCs could be promising candidates for testing as broad-spectrum antiprotozoal and antitumor agents in humans.

Separation and quantification of alkylphosphocholines by reversed phase high performance liquid chromatography

Alkylphosphocholines represent a new class of drugs with remarkable antineoplastic and antiprotozoal activity. For instance, hexadecylphosphocholine has been approved for the topical treatment of skin metastasis. In addition, it was successfully studied in India for the treatment of leishmaniasis. Different phase-I and phase-II-trials resulted in cure rates of more than 97%. To optimize antitumor or antiprotozoal activity, we have prepared alkylphosphocholines differing in chain length and unsaturation. For the qualitative and quantitative analysis of these longer chain analogues, we have used isocratic high performance liquid chromatography. The separation of the alkylphosphocholines with different chain lengths in this reversed phase HPLC system was achieved on a YMC-TMS column with a mobile phase consisting of methanol-water (85:15; v/v) at a flow rate of 1.0 ml/min. Furthermore the cis-/trans-isomers such as oleylphosphocholine and elaidylphosphocholine were clearly separated on a YMC-C8 column with a methanol-water mixture (80:20; v/v) as mobile phase. In the described reversed phase HPLC systems simple refractive index detection and UV detection allow the sensitive and quantitative determination of alkylphosphocholines. These methods are very important for reproducible identification and quantitative determination of saturated and mono-unsaturated alkylphosphocholines with alkyl residues containing up to 25 carbon atoms.

Alkylphosphocholines: Effects on human leukemic cell lines and normal bone marrow cells

The anti-leukemic activity of a series of alkylphosphocholines (APCs) was studied against a panel of human leukemic cell lines (HL-60, K-562, Reh, MOLT-4, Jurkat, Ramos and Raji). Cytotoxic efficacy was measured by the MTT cell survival assay. All cell lines were found to be sensitive, except the multipotential CML-derived K-562 cell line. Flow cytometry of HL-60 cells showed a significant decrease of cells in S phase and the formation of a sub-G fraction. DNA fragmentation typical for programmed cell death was detected by DNA gel electrophoresis in these cells but not in any of the other leukemic lines. At concentrations below the cytotoxic range, mitogenic effects were seen in HL-60 cells after 14-hr exposure. Colony formation by K-562 cells revealed an augmented clonogenicity after exposure to APC with a short alkyl chain. In contrast, cells of lymphoid origin did not undergo DNA fragmentation or show mitogenic stimulation after exposure to APC. Normal bone marrow cells were also investigated for mitogenic and genotoxic effects. No decrease was found in the number of hematopoietic progenitors in long-term bone marrow cell cultures after exposure to APC. On the contrary, a significant increase was found after short exposure. Dodecylphosphocholine, hexadecylphosphocholine (HPC) and (octadecyl-[2-(N-methylpiperidino)-ethyl]phosphate exhibited a mild clastogenicity at equimolar high doses on murine bone marrow cells in vivo, which is unusual for the majority of classical DNA-interacting anti-cancer drugs. In conclusion, APCs are agents with a broad spectrum of in vitro anti-leukemic effects, which lack hematological toxicity.

Comparison of hexadecylphosphocholine with fish oil as an antitumor agent

Hexadecylphosphocholine (HePC) reduced the growth of the human mammary tumor, MX-1, in the athymic nude mouse similar to the fish oil, MaxEPA. When used together, HePC and MaxEPA were additive towards reducing tumor growth. An unsaturated alkylphosphocholine mixture, ShisoPC, was not as effective as HePC in reducing tumor growth. MaxEPA reduced tumor PGE2 levels greater than 90%, while HePC and the ShisoPC only reduced tumor PGE2 40-60% with HePC being slightly better than ShisoPC. MaxEPA markedly increased the cellular omega 3 fatty acids and decreased 20:4 omega 6, the substrate for PGE2. HePC did not alter the tumor fatty acid composition, but it significantly lowered the total fatty acid concentration of the tumor by about 47%. In addition, phosphatidylcholine and sphingomyelin decreased in tumors from animals treated with HePC, and alterations in other phospholipids also were noted. These data suggest that different mechanisms exist for HePC and fish oil in reducing tumor growth.

Growth arrest vs direct cytotoxicity and the importance of molecular structure for the in vitro anti-tumour activity of ether lipids

A panel of 25 different lipid agents was evaluated for in vitro activity against HT29 human colon carcinoma and HL60 promyelocytic leukaemia cells by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The structure-activity relationships seen with this series, including those for four sets of positional or stereoisomers, indicate that specific receptor proteins are unlikely as targets for anti-tumour lipid (ATL) action. Additional data confirm the lack of involvement of the platelet-activating factor receptor in particular and suggest that metabolic stability is a most important determinant of ATL activity. More detailed studies, with 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET18-OCH3) and (+/-)-2-(Hydroxy[tetrahydro-2-(octadecyloxy)methylfuran-2- yl]methoxyphosphinyloxy)-N,N,N,-trimethylethaniminium hydroxide (SRI 62-834), suggest three different modes of activity, depending on drug concentration and exposure time. Low doses of up to 5 microM in standard serum-containing medium cause population growth arrest after prolonged exposure. Growth arrest was associated with a leaky G2/M block as determined by flow cytometry. These effects are reversible. Intermediate concentrations (5-40 microM) were cytotoxic, causing a net reduction in cell numbers after 2-3 days. At even higher concentrations, all lipids caused rapid, direct membrane lysis. When the clonogenic assay was used to assess the effects of ATLs, most agents reduced colony formation at concentrations above 5 microM. However, some compounds proved stimulatory at nanomolar concentrations, suggesting that they might possess mitogenic properties. These results, particularly those concerning the concentration and time dependence, may be relevant to current clinical trials with ether lipids.

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.

Antineoplastic activity of alkylphosphocholines (APC) in human breast carcinomas in vivo and in vitro; use of liposomes

This study examines the in vitro and in vivo activity of alkylphosphocholines (APC) in experimental human breast carcinomas. Three analogs, hexadecylphosphocholine (HPC), octadecylphosphocholine (OPC) and eicosanylphosphocholine (EPC) were investigated. Three hormone receptor negative cell lines were sensitive to all three APCs in vitro whereas the receptor positive MCF-7 line was more resistant. Sensitivity was seen in 4/6 hormone receptor negative tumors in vivo, with HPC being the most active analog. There were no antitumor effects in the four receptor positive models. The reasons for these differences in response between hormone receptor negative and -positive lines are not yet understood and require further study. Gastrointestinal toxicity and hemolysis, the major side effects of the APCs, were reduced by the use of liposomal preparations.

Hexadecylphosphocholine differs from conventional cytostatic agents

Alkylphosphocholines, and especially their main representative hexadecylphosphocholine (HPC), show high anticancer activity in methylnitrosourea (MNU)-induced autochthonous rat mammary carcinoma. The regression of MNU-induced rat mammary carcinoma during HPC treatment can be evaluated by computed tomography and sonography. This allows a noninvasive monitoring of therapy in vivo (tumor size, morphology, and blood supply). Both diagnostic modalities can show a rapid concentric decrease in tumor volume as well as the appearance of cystic, scarry, and necrotic areas in the tumor tissue as a result of HPC treatment. In addition, prior to, during and after therapy tumor perfusion can be assessed by color Doppler sonography in vivo. A more than 4-fold difference in HPC efficacy was observed when the colony growth of explanted MNU-induced mammary carcinoma cells was measured in the methylcellulose colony assay (IC50 = 180 mumol HPC/l) and the Hamburger Salmon colony assay (IC50 = 740 mumol HPC/l). In the latter assay, growth of concomitantly seeded untransformed cells, especially of fibroblasts, is much lower than in the methyl-cellulose colony assay. We therefore assume that the antitumor efficacy of HPC against MNU-induced mammary carcinoma is enhanced by neighboring cells such as fibroblasts. Cell culture experiments with the three MNU-induced rat mammary carcinoma cell clones 1-C-2, 1-C-30, and 1-C-32 revealed IC50 values in the range of 50-70 mumol HPC/l. The volume of 1-C-2 cells increased up to 4-fold after 72 h of permanent exposure to 100 mumol HPC/l, a concentration that completely inhibited proliferation of tumor cell numbers without being cytotoxic. Nucleotide triphosphate levels dropped significantly after 24 h and were slowly restored in spite of continued exposure. After 72 h, they nearly reached those levels observed in plateau-phase cells. This suggests that HPC-induced growth inhibition has similarities with physiologically occurring growth arrest. Finally, replication of RNA viruses and DNA viruses was suppressed 30-fold and 7-fold, respectively, at low concentrations of HPC (12 mumol/l), which caused no or negligible growth inhibition in the virus-harboring cells, thus demonstrating specific antiviral activity of HPC. From these observations we conclude that HPC differs in many important aspects from conventional cytostatic agents and is certainly worth following-up in further investigations.