Synthesis and structure-activity relationship effects on the tumor avidity of radioiodinated phospholipid ether analogues

Radioiodinated phospholipid ether analogues have shown a remarkable ability to selectively accumulate in a variety of human and animal tumors in xenograft and spontaneous tumor rodent models. It is believed that this tumor avidity arises as a consequence of metabolic differences between tumor and corresponding normal tissues. The results of this study indicate that one factor in the tumor retention of these compounds in tumors is the length of the alkyl chain that determines their hydrophobic properties. Decreasing the chain length from C12 to C7 resulted in little or no tumor accumulation and rapid clearance of the compound in tumor-bearing rats within 24 h of administration. Increasing the chain length had the opposite effect, with the C15 and C18 analogues displaying delayed plasma clearance and enhanced tumor uptake and retention in tumor-bearing rats. Tumor uptake displayed by propanediol analogues NM-412 and NM-413 was accompanied by high levels of liver and abdominal radioactivity 24 h postinjection to tumor-bearing rats. Addition of a 2-O-methyl moiety to the propanediol backbone also retarded tumor uptake significantly. A direct comparison between NM-404 and its predecessor, NM-324, in human PC-3 tumor bearing immune-compromised mice revealed a dramatic enhancement in both tumor uptake and total body elimination of NM-404 relative to NM-324. On the basis of imaging and tissue distribution studies in several rodent tumor models, the C18 analogue, NM-404, was chosen for follow-up evaluation in human lung cancer patients. Preliminary results have been extremely promising in that selective uptake and retention of the agent in tumors is accompanied by rapid clearance of background radioactivity from normal tissues, especially those in the abdomen. These results strongly suggest that extension of the human trials to include other cancers is warranted, especially when NM-404 is radiolabeled with iodine-124, a new commercially available positron-emitting isotope. The relatively long physical half-life of 4 days afforded by this isotope appears well-suited to the pharmacodynamic profile of NM-404.

Phase I and pharmacokinetic study of the cytotoxic ether lipid ilmofosine administered by weekly two-hour infusion in patients with advanced solid tumors

PURPOSE

A Phase I trial was performed to determine the dose-limiting toxicity and maximum tolerated dose, and to describe the pharmacokinetics of the alkyl-lysophospholipid, ilmofosine, when administered as a weekly 2-h infusion in patients with solid tumors.

EXPERIMENTAL DESIGN

Thirty-nine patients were entered into a trial of ilmofosine administered weekly for 4 weeks followed by a 2-week rest period. Dose escalation occurred in 10 levels from 12 to 650 mg/m(2).

RESULTS

Thirty-six patients were evaluable for toxicity. The median number of cycles per patient was 1 (range, 1-4). Dose-limiting gastrointestinal toxicity occurred at 650 mg/m(2) with grade 3 nausea in two patients and grade 3 vomiting and diarrhea in one patient. Grade 2 diarrhea was observed in four of six patients treated at 550 mg/m(2). In addition, two patients treated at 550 mg/m(2) and two patients treated at 650 mg/m(2) experienced a decline in performance status of two or more levels that was determined to be due to treatment. There were no tumor responses. Stabilization of disease for at least 8 weeks occurred in six patients. Plasma concentrations of ilmofosine and its sulfoxide metabolite were evaluated by high-pressure liquid chromatography. The elimination of both compounds was biexponential with terminal half-lives of approximately 40 h for ilmofosine and 48 h for the sulfoxide. The area under the concentration-time curve was dose-proportional for each compound, and there was no evidence of saturable kinetics.

CONCLUSIONS

The dose-limiting toxicity of ilmofosine is gastrointestinal and the recommended dose for Phase II trials is 450 mg/m(2) as a 2-h weekly infusion. The relatively long half-life of ilmofosine and its active metabolite support the use of this intermittent schedule.

Quantitative determination of the antitumor alkyl ether phospholipid edelfosine by reversed-phase liquid chromatography–electrospray mass spectrometry: application to cell uptake studies and characterization of drug delivery systems

Edelfosine is a synthetic alkyl ether phospholipid that represents a promising class of antitumor agents. However, analytical methods to measure these type compounds are scarce. The lack of a reliable methodology to quantify edelfosine is a major problem in ongoing and scheduled preclinical and clinical trials with this drug. We evaluated the applicability of high-performance liquid chromatography-mass spectrometry to determine edelfosine in biological samples and polymeric delivery systems. Sample pre-treatment involved polymer precipitation or cell lysis with methanol. HPLC separation was performed on an Alltima RPC(18) narrow-bore column and edelfosine quantification was done by electrospray ionization mass spectrometry (ESI-MS) using positive ion mode and selected ion monitoring. Assays were linear in the tested range of 0.3-10 microg/ml. The limit of quantification was 0.3 ng/sample in both matrices, namely biological samples and polymeric delivery systems. The interassay precision ranging from 0.79 to 1.49%, with relative errors of -6.7 and 12.8%. Mean extraction recovery was 95.6%. HPLC-ESI-MS is a reliable system for edelfosine analysis and quantification in samples from different sources, combining advantages of full automation (rapidity, ease of use, no need of extensive extraction procedures) with high analytical performance and throughput.

Lem3p is essential for the uptake and potency of alkylphosphocholine drugs, edelfosine and miltefosine

The alkylphosphocholine class of drugs, including edelfosine and miltefosine, has recently shown promise in the treatment of protozoal and fungal diseases, most notably, leishmaniasis. One of the major barriers to successful treatment of these infections is the development of drug resistance. To understand better the mechanisms underlying the development of drug resistance, we performed a combined mutant selection and screen in Saccharomyces cerevisiae, designed to identify genes that confer resistance to the alkylphosphocholine drugs by inhibiting their transport across the plasma membrane. Mutagenized cells were first selected for resistance to edelfosine, and the initial collection of mutants was screened a second time for defects in internalization of a short chain, fluorescent (7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD))-labeled phosphatidylcholine reporter. This approach identified mutations in a single gene, YNL323W/LEM3, that conferred resistance to alkylphosphocholine drugs and inhibited internalization of NBD-labeled phosphatidylcholine. Loss of YNL323W/LEM3 does not confer resistance to N-nitroquinilone N-oxide or ketoconazole and actually increases sensitivity to cycloheximide. The defect in internalization is specific to NBD-labeled phosphatidylcholine and phosphatidylethanolamine. Labeled phosphatidylserine is internalized at normal levels in lem3 strains. LEM3 is a member of an evolutionarily conserved family and has two homologues in S. cerevisiae. Single point mutations that produce resistance to alkylphosphocholine drugs and inhibition of NBD-labeled phosphatidylcholine internalization were identified in several highly conserved domains. These data demonstrate a requirement for Lem3p expression for normal phosphatidylcholine and alkylphosphocholine drug transport across the plasma membrane of yeast.

Activity, pharmacokinetics and tissue distribution of TLC ELL-12 (liposomal antitumor ether lipid) in rats with transplantable, s.c. methylnitrosourea-induced tumors

TLC ELL-12 is a liposomal formulation of the novel antineoplastic compound 1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphocholine (L-ET-18-OCH(3)). The purpose of these studies was to evaluate the activity and tissue distribution of L-ET-18-OCH(3) when administered i.v. as TLC ELL-12 to rats bearing solid tumors. Growth-inhibitory activity of L-ET-18-OCH(3) and TLC ELL-12 against methylnitrosourea (MNU)-induced tumors grown in vitro was evaluated. Female Buffalo rats were injected s.c. with transplantable MNU-induced tumor cells. Four days later, animals were treated i.v. with L-ET-18-OCH(3) administered as TLC ELL-12 once daily for 5 consecutive days. Another group of MNU-tumor bearing rats was given a single 12.5 mg/kg dose of TLC ELL-12 containing [14C]L-ET-18-OCH(3) by i.v. injection into a tail vein. The 50% growth inhibitory concentration for TLC ELL-12 against MNU tumor cells in vitro was 63 microM (about 30 microg/ml). Tumor growth was significantly inhibited in ELL-12-treated rats versus controls. After a single dose, whole blood L-ET-18-OCH(3) concentrations declined in a multiphasic fashion with C(max) and terminal half-life values of approximately 91.1 microg L-ET-18-OCH(3)/ml and 13.1 h, respectively. Tumor L-ET-18-OCH(3) levels increased through the first 16-24 h post-dosing to about 23 microg/g and remained elevated at the terminal time point with little evidence of metabolism. Concentration-time profiles for selected tissues indicate rapid distribution of L-ET-18-OCH(3) from the circulation into tissues with highest concentrations in spleen, liver, lungs, kidneys and gastrointestinal tract. L-ET-18-OCH(3) as TLC ELL-12 shows both in vitro and in vivo activity against the MNU tumor line. When i.v. administered, L-ET-18-OCH(3) from ELL-12 is well distributed and slowly eliminated by metabolism in tissues.

Alkyl-lysophospholipid resistance in multidrug-resistant Leishmania tropica and chemosensitization by a novel P-glycoprotein-like transporter modulator

Drug resistance has emerged as a major impediment in the treatment of leishmaniasis. Alkyl-lysophospholipids (ALP), originally developed as anticancer drugs, are considered to be the most promising antileishmanial agents. In order to anticipate probable clinical failure in the near future, we have investigated possible mechanisms of resistance to these drugs in Leishmania spp. The results presented here support the involvement of a member of the ATP-binding cassette (ABC) superfamily, the Leishmania P-glycoprotein-like transporter, in the resistance to ALP. (i) First, a multidrug resistance (MDR) Leishmania tropica line overexpressing a P-glycoprotein-like transporter displays significant cross-resistance to the ALP miltefosine and edelfosine, with resistant indices of 9.2- and 7.1-fold, respectively. (ii) Reduced expression of P-glycoprotein in the MDR line correlates with a significant decrease in ALP resistance. (iii) The ALP were able to modulate the P-glycoprotein-mediated resistance to daunomycin in the MDR line. (iv) We have found a new inhibitor of this transporter, the sesquiterpene C-3, that completely sensitizes MDR parasites to ALP. (v) Finally, the MDR line exhibits a lower accumulation than the wild-type line of bodipy-C(5)-PC, a fluorescent analogue of phosphatidylcholine that has a structure resembling that of edelfosine. Also, C-3 significantly increases the accumulation of the fluorescent analogue to levels similar to those of wild-type parasites. The involvement of the Leishmania P-glycoprotein-like transporter in resistance to drugs used in the treatment of leishmaniasis also supports the importance of developing new specific inhibitors of this ABC transporter.

Intracellular triggering of Fas, independently of FasL, as a new mechanism of antitumor ether lipid-induced apoptosis

Antitumor ether lipid 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH(3); edelfosine) induces apoptosis in cancer cells, sparing normal cells. We have found that the apoptotic action of ET-18-OCH(3) required drug uptake and Fas in the target cell. Failure to accomplish one of these requirements prevents cell killing by the ether lipid. In human lymphoid leukemic cells, ET-18-OCH(3) does not promote Fas or FasL expression and ET-18-OCH(3)-induced apoptosis is not inhibited by pre-incubation with an anti-Fas blocking antibody that abrogates cell killing mediated by Fas/FasL interactions. ET-18-OCH(3)-resistant normal human Fas-positive fibroblasts do not incorporate ET-18-OCH(3), but undergo apoptosis upon ET-18-OCH(3) microinjection. Murine fibroblasts L929 and L929-Fas, stably transfected with human Fas cDNA, do not incorporate ET-18-OCH(3) and are resistant to its action when added exogenously. Microinjection of ET-18-OCH(3) induces apoptosis in L929-Fas cells, but not in wild-type L929 cells. Confocal laser scanning microscopy shows that ET-18-OCH(3) induces Fas clustering and capping during triggering of ET-18-OCH(3)-induced apoptosis. Microinjection-induced apoptosis and Fas clustering are specific for the molecular structure of ET-18-OCH(3). Our data indicate that ET-18-OCH(3) induces apoptosis via Fas after the ether lipid is inside the cell, and this Fas activation is independent of the interaction of Fas with its natural ligand FasL. This explains the selective action of ET-18-OCH(3) on tumors since only cancer cells incorporate sufficient amounts of the drug.

Decreased sensitivity to 1-O-octadecyl-2-O-methyl-glycerophosphocholine in MCF-7 cells adapted for serum-free growth correlates with constitutive association of Raf-1 with cellular membranes

We have previously shown that inhibition of MCF-7 cell proliferation by 1-O-octadecyl-2-O-methyl-glycerophosphocholine (ET-18-OCH3) is linked to a drug-induced decrease in membrane Raf-1 levels and the subsequent inhibition of mitogen-activated protein (MAP) kinase activation in response to growth factor stimulation. We now report that adaptation of MCF-7 cells for growth in a serum-free formulation results in decreased sensitivity to growth inhibition by ET-18-OCH3. The decrease in ET-18-OCH3 sensitivity occurred progressively during the adaptation process and correlated with the presence of increasing amounts of inactive Raf-1 that stably associated with MCF-7 cell membranes. ET-18-OCH3 sensitivity could be restored by growing the adapted cells in serum-containing medium, which resulted in the loss of membrane-associated Raf-1. In human normal mammary epithelial cells, which are insensitive to ET-18-OCH3, Raf-1 was also associated with membranes in quiescent cells. In both cell types, incubation with ET-18-OCH3 had no effect on the membrane-Raf-1 levels, suggesting that ET-18-OCH3-induced reduction of Raf-1 levels in growth factor-stimulated MCF-7 cells is due to inhibition of Raf translocation. The activation and termination of the MAP kinase pathway in response to growth factors in the adapted MCF-7 cells and HNME cells occurred without changes to membrane Raf-1 levels. Because membrane translocation is not required to activate Raf in these cells, inhibition of Raf translocation by ET-18-OCH3 subsequent to cell stimulation has no effect on the activation of the membrane-bound Raf and, consequently, the activation of the MAP kinase pathway. The ability of the cells to activate the MAP kinase pathway in the presence of the drugs enables them to resist the growth-inhibitory effects of the drug, leading to the observed ET-18-OCH3 insensitivity of the cells.

Intravitreal pharmacokinetics in rabbits of the foscarnet lipid prodrug: 1-O-octadecyl-sn-glycerol-3-phosphonoformate (ODG-PFA)

PURPOSE

To evaluate the intraocular distribution and metabolism of the lipid prodrug of foscarnet, 1-O-octadecyl-sn-glycerol-3- phosphonoformate (ODG-PFA), following intravitreal administration.

METHODS

Twenty rabbits received ODG-[14C]PFA intravitreal injection, yielding 0.632 mM resultant intravitreal concentration. Two animals per group were sacrificed at different intervals post-injection. The drug levels in ocular tissues were determined with counting the radioactivity by Tracor Mark III Liquid Scintillation Counter. Four rabbits were used for analysis of the drug metabolism in vitreous by lipid extraction technique.

RESULTS

The drug level in vitreous was 526 microM at day one and 227 microM at the fifth week. The vitreous half life was approximately four to five weeks. The retinal level of the drug was 292 microM at day one, 75 microM at the fifth week and 32 microM at the tenth week, which was still more than ten times higher than the IC90 against HCMV. Lipid extraction analysis showed that, in vivo, both ODG-PFA and PFA were present in vitreous, but in in vitro incubations with vitreous, ODG-PFA conversion to PFA was negligible.

CONCLUSION

ODG-PFA possesses a long vitreous half life and sustained high drug level in retina. The vitreous did not metabolize drug but acted as a drug reservoir. Intravitreal liposomal ODG-PFA may be expected to be a long acting potent local therapy for CMV retinitis.

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.

Functional interactions between synthetic alkyl phospholipids and the ABC transporters P-glycoprotein, Ste-6, MRP, and Pgh 1

The ABC superfamily of transporters includes the mammalian P-glycoprotein family (Class I and Class II P-gps), the multidrug resistance-associated protein (MRP), the Pgh-1 product of Plasmodium falciparum gene pfmdr1, all of which are associated with cellular pleiotropic drug resistance phenomena. STE6, the yeast transporter for the farnesylated peptide pheromone a, is also a member of this family. Structural similarities in this family translate into functional homology as expression of mouse Mdr3S (P-gp), P. falciparum Pgh-1, and human MRP partially restore mating in a sterile yeast mutant lacking a functional STE6 gene. The demonstration that Class II P-gps function as phosphatidylcholine (PC) translocators raise the possibility that other ABC transporters may also interact with physiological lipids. We report the identification of the synthetic lipid and PC analog ET-18-OCH3 (edelfosine) as a substrate for not only Class II P-gp but also for Class I P-gps and surprisingly for the other ABC transporters MRP, Pgh-1, and STE6. Expression of these proteins in the yeast Saccharomyces cerevisiae JPY201 was found to confer cellular resistance to cytotoxic concentrations of this lipid by a factor of 4-20-fold in a growth inhibition assay. The noted activity of ABC transporters toward this synthetic lipid was specific as a mutant variant of Mdr3 (Mdr3F) with reduced activity could not convey cellular resistance to ET-18-OCH3. ET-18-OCH3 was also found capable of blocking a-peptide pheromone transport and STE6 complementation by these ABC proteins. The inhibitory effect of ET-18-OCH3 on cell growth and a-factor transport could be abrogated by incubation with the lipid acceptor protein BSA or by enzymatic cleavage by microsomal alkylglycerol mono-oxygenase (MAMO). MAMO and BSA reversal of the ether lipid effect was only seen in the presence of a functional transporter. These results suggest that the group of cytotoxic synthetic PC analogs studied reveal possible structural and functional aspects common to the ABC transporters tested. Furthermore, the studies with BSA and MAMO suggest that the mechanism of transport of ET-18-OCH3 by these ABC transporters may be related to the flippase mechanism of PC transport by Mdr2.

Selective induction of apoptosis in cancer cells by the ether lipid ET-18-OCH3 (Edelfosine): molecular structure requirements, cellular uptake, and protection by Bcl-2 and Bcl-X(L)

The ether lipid 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3; Edelfosine) has been shown to be a rapid inducer of apoptosis in human leukemic cells and has been considered as a promising drug in cancer treatment. Here we have found that ET-18-OCH3 induced apoptosis not only in human tumor cell lines but also in primary tumor cell cultures from cancer patients. Human leukemic cells were highly sensitive to ET-18-OCH3, whereas normal cells remained unaffected. Among the distinct modifications of the ET-18-OCH3 molecule assayed, we found that substitutions in positions sn-2 and sn-3 of the glycerol backbone resulted in a complete loss of its capacity to induce apoptosis, highlighting the importance of the molecular structure of ET-18-OCH3 in its apoptotic effect. Induction of apoptosis by ET-18-OCH3 was very well correlated with the uptake of this ether lipid. ET-18-OCH3-resistant 3T3 fibroblasts became sensitive and incorporated significant amounts of the ether lipid following transformation with the SV40 virus. ET-18-OCH3-induced apoptosis as well as ET-18-OCH3 uptake were not mediated through binding of the ether lipid to the platelet-activating factor receptor. Overexpression of bcl-2 or bcl-xL by gene transfer in the human erythroleukemic HEL cells abrogated apoptosis induced by ET-18-OCH3. ET-18-OCH3 did not affect the expression of bcl-2, bcl-xL, or bax in HEL and HL-60 human leukemic cells but induced expression of c-myc, an important effector of apoptosis in several systems. Thus, ET-18-OCH3 behaves as a potent and highly selective antitumor drug able to induce an apoptotic pathway of cell death in tumor cells but not in nonmalignant cells.

Exogenously added alkylmethylglycerophosphocholine and alkylmethylcarbamylglycerophosphocholine accumulate in plasma membranes more than in intracellular membranes of rabbit platelets

We found that extracellular addition of 2% bovine serum albumin (BSA) to a suspension of rabbit platelets after stimulation with platelet-activating factor resulted in a biphasic extraction of [3H]1-O-alkyl-2-O-methyl (or 2-O-methylcarbamyl)-sn-glycero-3-phosphocholine. A fast phase of extraction of the phospholipid probe by BSA was found to be mainly due to removal of the probe remaining in an outer layer of platelet plasma membrane, whereas a second phase of extraction of the probe by BSA was mostly attributed to redistribution of the probe which had been flipped across the plasma membrane. On the basis of analysis of the biphasic extraction by BSA of 1-O-alkyl-2-O-methyl (or methylcarbamyl)-sn-glycero-3-phosphocholine at various times after its addition, we suggested that the radioactive phospholipid accumulated in plasma membrane more than in intracellular membranes of rabbit platelets. In similar experiments with guinea-pig polymorphonuclear leukocytes, we observed a monophasic extraction of 1-O-alkyl-2-O-methyl (or methylcarbamyl)-sn-glycero-3-phosphocholine by BSA, indicating its unidirectional movement across the plasma membrane.

Biological disposition and imaging of a radioiodinated alkylphosphocholine in two rodent models of breast cancer

Iodine-125-12-[m-iodophenyl]-dodecylphosphocholine (NM-324) has been shown to accumulate in a variety of animal tumor models. Moreover, preliminary pharmacokinetic studies with NM-324 are being conducted in cancer patients. The present study was undertaken to examine the potential application of NM-324 as a breast tumor-imaging agent.

METHODS

Two animal models of breast cancer were utilized: namely, syngenic inbred Lewis female rats bearing the rat mammary tumor (RMT) and athymic mice with HT-39 human tumor xenografts. After i.v. administration of NM-324, the tissue distribution of radioactivity was determined at various time points. Gamma camera scintigrams were also acquired to confirm the biodistribution results. Macro- and microautoradiography were used to analyze cellular distribution of radioactivity in tumors.

RESULTS

In the rat mammary tumor model, levels of radioactivity in the tumor reached a maximum at 24 hr after i.v. administration (1.65% ID/g, tumor-to-blood 6.4). These tumors could be visualized by gamma camera scintigraphy as early as 1 hour after administration. In the nude mouse model, levels of radioactivity in tumor reached a maximum at 48 hr after i.v. administration (4.96 %ID/g, tumor-to-blood 5.5). Tissues expected to interfere with the resolution of breast lesions such as fat, heart, lung and muscle displayed much lower concentrations of the radioactivity. Gamma camera scintigraphy confirmed the results observed from biodistribution experiments. Lipid extraction of the tumors and major organs in both animal models showed the sole presence of unchanged NM-324. Microautoradiographic analysis of slices of rat mammary and HT-39 tumors provided additional information regarding the intratumoral distribution of radioactivity.

CONCLUSION

The ability of radioiodinated phospholipid analogs to accumulate in breast tumors reinforces the need for further investigation of this type of radiopharmaceutical as tumor imaging agents.

Synthesis and biological evaluation of radioiodinated phospholipid ether stereoisomers

Radioiodinated phospholipid ethers have shown the remarkable ability to selectively accumulate in a variety of animal tumors as well as in human tumor xenografts. It has been suggested that this tumor avidity may arise as a consequence of metabolic differences between tumor and corresponding normal tissue. One such compound, 1-O-[12-(m-iodophenyl)dodecyl]-2-O-methyl-rac-glycero-3-phosphocholine (NM-294), contains a chiral center at the sn-2 position. The unnatural S- and natural R-enantiomers (4 and 5, respectively) of NM-294 were synthesized in order to provide further information on the mechanism(s) responsible for the tumor avidity of phospholipid ethers. In vitro cytotoxicity studies demonstrated a lack of stereospecificity. Biodistribution studies in rats bearing the Walker 256 tumor demonstrated the S- and R-isomers to have similar tissue uptake at 24 and 48 h after administration. Tumor-to-blood ratios at 24 h were 11.1 and 11.0 for the S- and R-isomers, respectively. In addition, gamma-camera scintigrams of tumor-bearing rats at various time points after iv administration of the S- and R-isomers did not show any qualitative differences in the distribution of radioactivity. Prior studies have shown that rac-NM-294 was not a substrate for phosphatidylcholine specific phospholipase C, but was a substrate for two forms of phospholipase D (PLD). Therefore, metabolism studies with 4 and 5 with various forms of PLD were performed. PLD from cabbage demonstrated a degree of stereoselectivity. In the presence of 1% ethanol, the R-isomer was metabolized to the greatest extent, followed by rac-NM-294 and the S-isomer. PLD isolated from Streptomyces chromofuscus failed to demonstrate any stereoselectivity. The results suggest that the mechanism(s) of retention of these compounds in tumors may not involve a highly stereoselective component.

Influence of serum levels on leukemic cell destruction by the ether lipid ET-18-OCH3

The effect of serum on the antineoplastic action of the alkyl-lysophospholipid 1-octadecyl-2-O-methyl-sn-glycerol-3-phosphocholine (ET-18-OCH3) was studied in two human leukemia cell lines, HL60 and K562, and in leukemic cells of patients. Decreasing amounts of serum in the culture medium enhanced the cytotoxic action of ET-18-OCH3 dramatically in both cell lines and in the leukemic cells, as measured by cell survival and proliferation. Uptake of ET-18-OCH3 was likewise increased at reduced serum levels. Similar effects were obtained when fetal calf serum (FCS) in the culture medium was substituted by bovine serum albumin (BSA, fatty acid free). Selectivity of the alkyl-lysophospholipid at reduced serum or BSA level was demonstrated by clonogenic assays of normal marrow progenitor cells. Our study provides an optimalization of the purging conditions in autologous bone marrow transplantation, by using a low concentration of BSA during ET-18-OCH3 treatment (20 micrograms/ml for 4 h) in serum-free culture medium.

Synthesis and biological evaluation of radioiodinated phospholipid ether analogs

Previous work has shown that radioiodinated phospholipid ether analogs with the iodine-125 substituted on the meta position of the aromatic ring readily localized in a variety of animal tumors. In an effort to ascertain the importance of such meta substitution, three phospholipid ether analogs with the iodine-125 in the para position were synthesized for evaluation as potential tumor-localizing imaging agents. 12-(p-Iodophenyl)dodecyl phosphocholine, 1-O-[12-(p-iodophenyl)dodecyl]-1,3-propanediol-3-phosphocholine, and 1-O-[12-(p-iodophenyl)dodecyl]-2-O-methyl-3-rac-glycerophosphocholine were synthesized and labeled with iodine-125 via an isotope exchange procedure. Similar to previous results with the meta substituted analogs, tissue distribution studies with the three para analogs demonstrated tumor localization and retention of radioactivity at 24 h after i.v. injection. In all three cases, the para isomers showed greater tumor avidity than the meta isomers and clearance of the radiotracer from the tumor was much slower than the clearance from nontarget tissue. 12-(p-Iodophenyl)dodecyl phosphocholine afforded the greatest tumor-to-nontarget tissue ratio. For example, the tumor-to-blood and tumor-to-liver ratios at 24 h were 10.96 and 1.85, respectively. As a result of such selective tumor retention, it was possible to clearly delineate the tumor using gamma-camera scintigraphy.

Biodistribution, metabolism, and excretion of radioiodinated phospholipid ether analogs in tumor-bearing rats

The phospholipid ether analog, [125I]-1-O-[12-(m-iodophenyl)dodecyl]propanediol-3-phosphocholine (NM-295) was synthesized and evaluated for its ability to visualize tumors. Preliminary studies were performed in rats bearing the Walker 256 carcinosarcoma. Most of the radioactivity was cleared from the animals during the first 24 hours. However, the tumor showed a decreased rate of clearance of radioactivity when compared with non-target tissue. This difference in the clearance rate allowed for excellent images of the tumor at 24 hours. Scintigraphic images compared favorably with other radioiodinated phospholipid ether analogs such as [125I-rac-1-O-[12-(m-iodophenyl)dodecyl]-2-O-methylglycero-3- phosphocholine (NM-294) and [125I]-12-(m-iodophenyl)dodecyl phosphocholine (NM-324). In contrast with the latter two compounds, however, tissue distribution studies revealed that NM-295 cleared at a much faster rate from all tissues, including tumor. In addition, within 24 hours following administration of NM-295, over 70% of the radioactivity was excreted as compared to 50% and 20% for NM-294 and NM-324, respectively. The majority of excreted radioactivity appeared in the urine for all three compounds. Thin-layer chromatography of urine and fecal extracts showed the presence of metabolites only. In contrast, lipid extracts of either liver or tumor demonstrated only the presence of the parent compound. Therefore, these data suggest that in each case it was the parent phospholipid analog that was taken up and retained by the tissues, while the metabolic product(s) was cleared and excreted from the animal.

Membrane lipid modification and sensitivity of leukemic cells to the thioether lipid analogue BM 41.440

Since the ether lipid anticancer drugs are membrane targeted, we examined the effect of membrane lipid structural alteration on their cytotoxicity. Enrichment with docosahexaenoic acid increased the sensitivity to the thioether lipid BM 41.440, compared to control cells enriched with oleic acid. The effect was dependent upon drug concentration, time, and the extent of cellular fatty acid enrichment. Other polyunsaturated fatty acids had a similar effect, which was proportional to the degree of unsaturation of the molecule inserted. Depletion of cellular glutathione with buthionine sulfoximine increased the sensitivity to ether lipid, but prooxidants such as Fe2+ and antioxidants such as vitamin E had little effect. The addition of serum to the incubation medium markedly diminished the cytotoxicity of ether lipids for cells modified with both docosahexaenoic acid and oleic acid, probably due to binding of the drug to serum components. The toxicity of another ether lipid, 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine, was not affected appreciably by membrane alteration. Drug uptake studies with a radiolabeled BM 41.440 analogue, 1-[3H]hexadecylthio-2-ethyl-rac- glycero-3-phosphocholine, demonstrated no difference in transport at early time points and no difference in accumulation up to 60 min. We conclude that increases in cellular and/or membrane fatty acid polyunsaturation heighten the cytotoxic effect of a membrane-active ether lipid. The effect is not due to a change in drug transport or accumulation. It may be related to a change in oxidative events. These observations provide further confirmation of the membrane being the target of ether lipid action, using biochemical rather than morphological techniques. Most importantly, this observation offers a potential innovative approach to therapy.

Selective localization of radioiodinated alkylphosphocholine derivatives in tumors

We have designed and synthesized two radioiodinated analogs of hexadecylphosphocholine in order to evaluate their tumor imaging potential. 12-(m[125I]iodophenyl)dodecyl phosphocholine (NM-324) and hexadecyl-2-[N,N-dimethyl-N-(m[125I]iodobenzyl)-ammonium] ethyl phosphate (NM-326) demonstrated the ability of such compounds to localize in and thereby visualize the Walker 256 tumor in rats. However, the tumor avidity of NM-324 was far superior to NM-326. In addition, NM-324 showed excellent tumor localization in athymic mice bearing subcutaneous human tumors.

Distribution of hexadecylphosphocholine and octadecyl-methyl-glycero-3-phosphocholine in rat tissues during steady-state treatment

The distribution of the alkylphosphocholine hexadecylphosphocholine (He-PC) and the (alkyl)lysophospholipid 1-0-octadecyl-2-0-methyl-rac-glycero-3-phosphocholine (ET18-OCH3) was analyzed in rats. The compounds were given orally at a daily dose of 75 mumol/kg body weight. After 6, 11, and 18 days, three rats in each treatment group were killed and the drug concentration in various tissues and fluids was determined. With the exception of the kidney (He-PC) and brain (He-PC and ET18-OCH3), steady-state levels of the drugs could be achieved in all organs investigated and in serum. Maximal concentrations of He-PC were found in the kidney, adrenal glands, and spleen, whereas the highest concentrations of ET18-OCH3 were detected in the adrenal glands, spleen, and small intestine. The concentrations of He-PC exceeded those of ET18-OCH3 in most tissues by a factor of about 2-25. Since samples of urine and feces did not contain detectable amounts of the compounds, the absorption of both lipid analogues was assumed to be complete. The total amount of He-PC recovered after 6, 11, and 18 days was 15%, 12%, and 6%, respectively, and that of ET18-OCH3 was 1.3%, 0.8%, and 0.3%, respectively. This indicates that the bioavailability of He-PC and ET18-OCH3 is not controlled by differences in the uptake of the two drugs, but by differences in their metabolism. The results could explain the differing efficacy of these two compounds in their antitumor action in animal models.

[The pharmacologic and biochemical effects of unsaturated phospholipids]

The review deals with the analysis of the recent publications indicating the important biological function of essential phospholopids (EP) in the cell and in the organism as a whole. The protective mechanism of action of EP in diseases of the liver, atherosclerosis, ischemic heart disease, myocardial infarction determined by their high biological activity in some pathological conditions associated with lipid metabolism disturbance is discussed. The efficiency of the biological action of EP is determined by the depends on the type of phospholipids, the form of phospholipid preparations, their charge, and also on the routes of their administration.

Potential tumor or organ-imaging agents. 30. Radioiodinated phospholipid ethers

A radioiodinated analogue of a naturally occurring alkyl lysophospholipid (ALP) was synthesized for evaluation as a potential tumor-localizing imaging agent. rac-1-[12-(m-Iodophenyl)dodecyl]-2-methylglycero-3-phosphocholine (ET-12IP-OMe, 14) was radiolabeled with iodine-125 via an isotope-exchange procedure. Tissue distribution studies with [125I]ET-12IP-OMe in tumor-bearing rats revealed an immediate tumor uptake of radioactivity. Although radioactivity was also present in nontarget tissues at this time, clearance of tracer from the tumor was much slower and thus provided a suitable tumor to nontarget tissue ratio at 24 h. As a result of this selective accumulation, it was possible to clearly delineate the tumor with gamma-camera scintigraphy.

Determination of the alkyl lysophospholipid derivative ET-18-OCH3, a new antineoplastic drug, in plasma

We describe a sensitive method for measuring the concentration of the new antineoplastic drug ET-18-OCH3 in plasma. After plasma lipids are extracted, ET-18-OCH3 is separated from the excess of endogenous lipids by thin-layer chromatography and specific enzymatic hydrolysis of sphingomyelin by the action of sphingomyelinase. Analytical recovery after the complete isolation was 73.5% (CV = 9.8%, n = 15). [3H]-ET-18-OCH3 is used as internal standard. A densitometric method in which 8-anilino-1-naphthalenesulfonate, Mg salt, is used as fluorescent agent (excitation at 367 nm and emission greater than 390 nm) allows the sensitive determination of ET-18-OCH3 down to 0.1 mg/L (CV greater than 30%). The day-to-day CV is 25% for concentrations of 0.15 to 0.625 mg/L, 12% for 1.5 to 5.0 mg/L. Preliminary pharmacokinetic data reveal gastrointestinal absorption of ET-18-OCH3 after multiple oral administration.

Metabolism and disposition of (RS)-2-methoxy-3-(octadecylcarbamoyloxy)propyl 2-(3-thiazolio)ethyl phosphate (MOTP) in rats and dogs

1. Metabolites (RS)-4-[(3-hydroxy-2-methoxy)propoxycarbonylamino]butanoic acid (I) and (RS)-2-[(3-hydroxy-2-methoxy)propoxycarbonylamino]acetic acid(II) were isolated from urine after i.v. administration of (RS)-2-methoxy-3-(octadecyl-[14C]carbamoyloxy)propyl 2-(3-thiazolio)ethyl phosphate (14C-MOTP) to rats and characterized by t.l.c., g.l.c.-mass spectrometry and p.m.r. spectrometry. 2. After i.v. administration of 14C-MOTP, the plasma concentration of the drug declined biphasically with half-lives of 0.22 and 3.94 h in rats, and 0.81 and 8.00 h in dogs. In rats and dogs, unchanged MOTP was the main 14C component in the plasma, together with a small amount of I and II. 14C-MOTP was highly bound to plasma protein of both animals. 3. Five min after i.v. administration of 14C-MOTP to rats, 14C was widely distributed in tissues, with the highest conc. in the lung and the lowest in the eye. The distribution of 14C was relatively slow in some tissues. In most tissues, 14C decreased to low levels at 96 h, except in the Harder's gland. 4. Elimination of 14C-MOTP was almost complete within 120 h in rats and 144 h in dogs. In both species, the administered 14C was excreted largely in the urine as I and II, with the remainder appearing in the faeces and the expired air. Biliary excretion and reabsorption of 14C were detected in rats. 5. During repeated i.v. administration of 14C-MOTP to rats for 7 days, the conc. of 14C in plasma and most tissues attained steady state within 5 days, except in Harder's gland, where the level rose gradually until the seventh day of dosing. Within 6 days after the last dosing, 96% of the injected dose was eliminated from the body.

The degradation of platelet-activating factor and related lipids: susceptibility to phospholipases C and D

1-O-Octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3) is an ether-linked lipid that exhibits selective cytotoxicity toward several types of tumor cells and is relatively inactive toward normal cells under the same conditions of treatment. The mechanis of this selective cytotoxicity is unknown. We conducted studies to determine whether this compound is metabolized by phospholipases C and D and, if so, whether sensitive and resistant cells differ in their ability to degrade ET-18-OCH3 by these enzymes. We have examined the metabolism of the L-isomer of ET-18-OCH3, 1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphocholine (L-ET-18-OCH3), by lysophospholipase D of rat liver microsomes and by a phospholipase D from the marine bacterium Vibrio damsela. The metabolism of L-ET-18-OCH3 was also examined in cell culture using Madin-Darby canine kidney cells, human promyelocytic leukemia cells and human myelocytic leukemia cells. In these studies, L-ET-18-OCH3 and related 1-O-alkyl-linked phosphocholine analogs radiolabeled with 3H in the 1-O-alkyl chain were used. L-ET-18-OCH3 was not hydrolyzed by lysophospholipase D from rat liver microsomes under conditions where cleavage of 1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine was observed. However, phospholipase D from the marine bacterium V. damsela readily hydrolyzed L-ET-18-OCH3 to 1-O-[3H]octadecyl-2-O-methyl-sn-glycero-3-phosphate, demonstrating that L-ET-18-OCH3 can be degraded by a phospholipase D. Platelet-activating factor (PAF) and lyso-PAF were also substrates for the bacterial phospholipase D.(ABSTRACT TRUNCATED AT 250 WORDS)

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

We have investigated cellular sensitivity to the antitumoral alkyl lysophospholipid (ALP) 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3) in vitro. The permeation of this lipid into the cell was not influenced by metabolic inhibitors of ATP biosynthesis. ET-18-OCH3 uptake was not saturable within sublytic concentrations, but could be inhibited in part by cytochalasin B (CB) and dipyridamole. The activation energy of the CB-sensitive uptake process was increased up to threefold compared to CB-insensitive uptake. ET-18-OCH3 influx and equilibrium binding of ET-18-OCH3 were decreased in a fibrosarcoma cell variant (MethA) selected for ET-18-OCH3 resistance. The resistant MethA cells were also less sensitive to cytolysis by lysophosphatidylcholine and other ALP. After 72 hr, the resistant MethA cells had metabolized only 11.8% more of the absorbed ET-18-OCH3 than sensitive MethA cells. However, they tolerated at least a 30-fold concentration of this ALP. The uptake mechanism, which could be inhibited by CB, was less active in resistant MethA cells and several other ALP-resistant cell lines. The concentration of CB, required for maximal uptake inhibition, was increased more than four times in the ALP-sensitive tumor cell lines. CB-specific ET-18-OCH3 uptake was also enhanced after virus transformation of 3T3 fibroblasts by SV 40. Dipyridamole retarded the ET-18-OCH3-mediated cell destruction.

Accumulation of an alkyl lysophospholipid in tumor cell membranes affects membrane fluidity and tumor cell invasion

Tumor cells grown in the presence of 1-O-alkyl-2-O-methylglycero-3-phosphocholine (AMG-PC) accumulated this ether lipid in their membranes. Depending on the cell type and the dose of the compound, up to 17% of the total phospholipids of the purified plasma membranes consisted of authentic AMG-PC. Extensive incorporation of the agent resulted in a decrease in plasma membrane fluidity and inhibition of tumor cell invasiveness in embryonic chick heart fragments. The extent of AMG-PC incorporation and fluidity change was not strictly correlated with the degree to which tumor cell invasion was inhibited.

Metabolism of ether phospholipids and analogs in neoplastic cells

The ether phospholipid 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (OM-GPC) is known to be a potent inhibitor of cell growth. Metabolic studies in both Raji and L1210 leukemic cells on OM-GPC, 3H-labeled in the methyl groups of the choline moiety, showed a (diacyl)-phosphatidylcholine as the only labeled metabolite. Since the formation of radiolabeled (diacyl)-phosphatidylcholine showed a direct correlation with cell death, we tested other lipid analogs. One of these compounds, hexadecylphosphocholine (He-PC), which was 3H-labeled in the methyl-choline groups, showed a formation of labeled (diacyl)-phosphatidylcholine similar to that found with OM-GPC. Again, there was a direct linear correlation between the formation of the labeled product and cell death. He-PC was found to be a potent cell toxin in in vitro experiments on cell cultures. However, analogs with an elongated phosphor to trimethylammonium distance showed no toxicity towards the cells in in vitro experiments. From the data, we conclude that the ether phospholipids are substrates for a phospholipase C or related enzyme. This substrate property may be responsible for the toxicity of the compounds in neoplastic cells.

Pharmacokinetics of the thioether phospholipid analogue BM 41.440 in rats

BM 41.440 (1-hexadecylmercapto-2-methoxymethyl-rac-glycero-3-phosphocholine) is a cytotoxic thioether phospholipid analogue that recently has entered phase I trials in cancer patients. The objective of this study was to evaluate the pharmacokinetics of this compound in female rats after administration of a single oral dose (15 mg/kg body weight [bw] ). Furthermore, BM 41.440 serum concentrations were determined under a daily oral treatment of up to 13 weeks. Blood samples were obtained via permanent catheters from the femoral arteries before and after drug administration for a total of 120 hr. Urine was collected in 24 hr-intervals for 120 hr; the volume was measured, and aliquots were stored at -20 C until analytical determination of the thioether derivative. BM 41.440 was assayed in serum and urine by means of a specific, newly developed reverse-phase high pressure liquid chromatography technique. Mean maximum serum concentrations (1.7 micrograms/ml, n = 4 animals) were attained after seven hr. A terminal half-life of ca. 27 hr was calculated from the rate constant for the terminal elimination phase (lambda z approximately 0.026/hr). The mean serum BM 41.440 concentration-time-area-under-the-curve was 52.9 mg X hr/l. The ratio of total body clearance to absorption fraction was 4.7 ml/min X kg bw. Only a small amount of the drug was found in the urine. The quantity excreted in the urine during a 24 hr-interval never exceeded 1.5% of the administered dose.(ABSTRACT TRUNCATED AT 250 WORDS)