Multigram synthesis of 1-alkylamido phospholipids.

Phospholipids containing a 1-alkylamido linkage have shown promising in vitro neoplastic cell growth inhibitory properties and anti-human immunodeficiency viral activity. We have synthesized a series of alkylamido ether lipid analogues on a milligram scale for initial evaluation, but for further in vivo testing of these bioactive phospholipids, synthesis on a larger scale is required. The multigram synthesis of 1-alkylamido ether phospholipids was accomplished by modifying reaction conditions in the amidation step and changing reagents and solvent systems in both the detritylation and phosphorylation steps. This was most crucial in the phosphorylation step, where in the multigram synthesis 2-bromoethyl dichlorophosphate in diethyl ether/tetrahydrofuran (7:3, vol/vol) gave much improved yields as compared to the 2-chloro-2-oxo-1,3,2-dioxaphospholane reagent. The modifications also resulted in a product that could be more easily purified in sufficient quantities for use in in vivo inhibition studies.

Synthesis and evaluation of novel ether lipid nucleoside conjugates for anti-HIV-1 activity.

Combinations of an amidoalkylphosphocholine, 8, and AZT have been found to cause an apparent synergistic action in suppressing infectious HIV-1 replication. In addition, amidoalkyl, oxyalkyl, and thioalkyl ether lipids have been chemically linked to anti-HIV-1 nucleosides (AZT and DDI) through phosphate and phosphonate linkages. These conjugates have shown promising in vitro anti-HIV-1 activity. Also, the conjugates have a 5-10-fold reduction in cell cytotoxicity compared to AZT alone. The most active compound, an amidoalkyl ether lipid-AZT conjugates, 4A, was found to have a differential selectivity of 1793 in a syncytial plaque assay. In comparison, AZT alone has a value of 1281.

Ether lipids inhibit the effects of phorbol diester tumor promoters.

Recent studies have shown that the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA) stimulates protein kinase C (PKC), whereas the ether-linked phospholipid 1-O-octadecyl-2-O-methyl-rac-glycerol-3-phosphocholine (ET-18-OCH3) inhibits PKC activity in vitro. Therefore, the antitumor effects of ET-18-OCH3 could be due to its inhibition of PKC activity and the effects of tumor promotion. TPA stimulates arachidonic acid release, prostaglandin synthesis, phosphatidylcholine synthesis and the degradation of phosphatidylcholine by phospholipase C in Madin Darby canine kidney (MDCK) cells. Therefore, we have determined the effects of ET-18-OCH3 on these consequences of TPA stimulation. Preliminary experiments determined that ET-18-OCH3 inhibited PKC partially purified from MDCK cells by ion-exchange chromatography on DEAE-cellulose. In addition, ET-18-OCH3 inhibited the TPA-stimulated phosphorylation of a 40,000-dalton protein in intact MDCK cells. These data indicate that ET-18-OCH3 is an effective inhibitor of PKC activity in MDCK cells. In addition, ET-18-OCH3 was found to inhibit arachidonic acid release and prostaglandin synthesis. The inhibition of prostaglandin synthesis appears to be secondary to inhibition of arachidonic acid release, since ET-18-OCH3 does not inhibit TPA-stimulated synthesis of prostaglandin H synthase or the activity of the enzyme directly (Parker, J., Daniel, L. W., and Waite, M. [1987] J. Biol. Chem. 262, 5385-5393). ET-18-OCH3 also inhibits TPA-stimulated phosphatidylcholine synthesis and phosphatidylcholine degradation by phospholipase C.(ABSTRACT TRUNCATED AT 250 WORDS)

1-O-alkyl-2-N-methylcarbamyl-glycerophosphocholine: a biologically potent, non-metabolizable analog of platelet-activating factor.

We prepared unlabeled and 3H-labeled analogs of platelet-activating factor (PAF) containing a N-methylcarbamyl residue at the sn-2 position. PAF and its methylcarbamyl analog competed for binding to high affinity receptors on human polymorphonuclear neutrophils; their respective dissociation constants for these receptors were 0.2 and 1.1 nM. The binding affinities of the two analogs correlated precisely with their capacities to stimulate neutrophil degranulation responses. Unlike PAF, however, the methylcarbamyl analog completely resisted metabolic inactivation by neutrophils and by human sera. Thus, these compounds' biological potencies are determined predominantly by receptor binding: cellular metabolism of the ligands neither contributes to nor appreciably limits their stimulating actions.

Human neutrophils incorporate arachidonic acid and saturated fatty acids into separate molecular species of phospholipids.

The incorporation of radiolabeled arachidonic acid and saturated fatty acids into choline-linked phosphoglycerides (PC) of rabbit and human neutrophils was investigated by resolving the individual molecular species by reversed-phase high performance liquid chromatography. PC from neutrophils incubated with a mixture of [3H]arachidonic acid and [14C]stearic or [14C]palmitic acid contains both radiolabels; however, double labeling of individual molecular species is minimal. After labeling for 2 h, the [3H]arachidonate is distributed almost equally between diacyl and 1-O-alkyl-2-acyl species, but it is incorporated into diacyl species containing unlabeled stearate or palmitate at the sn-1 position. In contrast, labeled saturated fatty acids are incorporated only into diacyl species and contain predominantly oleate and linoleate at the sn-2 position. Labeled linoleate is not incorporated into ether-linked species, but is found in the same species as labeled stearate. The findings suggest that mechanisms exist in neutrophils for specific shunting of exogenous arachidonic acid into certain phospholipid molecular species and support the concept that the 1-O-alkyl-2-arachidonoyl species may be a functionally segregated pool of arachidonic acid within the PC of neutrophils.

Synthesis of sulfur analogues of alkyl lysophospholipid and neoplastic cell growth inhibitory properties.

Five sulfur-containing phospholipid analogues (compounds 1-5) of alkyl lysophospholipid (1-O-alkyl-2-O-methyl-rac-glycero-3-phosphocholine, ALP) were synthesized and tested for inhibition of neoplastic cell proliferation with two human ovarian carcinoma cell lines in a clonogenic assay and with the HL-60 promyelocytic leukemia cell line. Compared with 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OMe), the most active reference analogue, these thio analogues are at least as active against HL-60 cells, and the 1-S-hexadecyl-2-O-ethyl analogue (2) is twice as active in the clonogenic assays.

Binding and metabolism of platelet-activating factor by human neutrophils.

Human polymorphonuclear neutrophils rapidly incorporated radiolabeled platelet-activating factor, 1-O-[hexadecyl-9, 10-3H2]-2-acetyl-sn-glycero-3-phosphocholine ([3H]PAF), and then metabolized it into its sn-2-fatty acyl derivative. Fractionation of radiolabel-pretreated cells over Percoll gradients revealed that virtually all of the intact [3H]PAF was located in nongranule membranes that were enriched with alkaline phosphatase and cell surface glycoproteins. While still membrane associated, the ligand was rapidly converted to its acyl derivative and then more slowly transferred to specific granules and, to a lesser extent, azurophilic granules. In contrast, neutrophils did not metabolize [3H]PAF at 4 degrees C but rather gradually accumulated it in their alkaline phosphatase-enriched membrane subfractions. These same subfractions contained receptors for the ligand, as determined by their capacity to bind [3H]PAF specifically. Binding was readily saturated, partially reversible, and fit a two receptor model; dissociation constant (Kd) values for high and low affinity sites were 0.2 and 500 nM, respectively. Receptors with similar affinities were detected in whole cells. Furthermore, the potencies of several structural analogues in inhibiting binding of [3H]PAF to membranes correlated closely with their respective potencies in stimulating degranulation responses. Finally, quantitative studies suggested all or most of the cell's receptors were membrane associated. We conclude that PAF rapidly enters cellular membranes to bind with specific receptors that trigger function. The intramembranous ligand is also deacetylated, acylated, and then transferred to granules. This metabolism may be sufficiently rapid to limit ligand-receptor binding and distort quantitative analyses of receptors.

Desorption chemical ionization mass spectrometry of 1-O-alkyl-2-O-acetyl-sn-glycero-3-phosphocholines and analogues.

Ammonia desorption chemical ionization of ether-linked phospholipids of the type 1-O-alkyl-2-O-acetyl-sn-glycero-3-phosphocholine (platelet-activating factors) and a series of analogues revealed a systematic fragmentation pattern that is characteristic for these compounds. The predominant ions included the protonated molecular ion and a series of fragments derived from the molecular ion having the following nominal mass losses: MH-14, MH-42, MH-59, and MH-183. Deuterated ammonia was used to elucidate the nature of several fragments. In addition, desorption chemical ionization was used to quantitate 1-O-hexadecyl-2-O-acetyl-sn-glycero-3-phosphocholine at the nanogram/sample level.

Facile synthesis of platelet-activating factor and racemic analogues containing unsaturation in the sn-1-alkyl chain.

Platelet-activating factor, 1 (PAF, 1-O-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine), and octadecyl-PAF were synthesized chemically as the racemates. The sn-1-O-alkyl isomers were isolated after treatment of the racemates with phospholipase A2 and subsequent reacetylation of the 1-O-alkyl-2-lyso-sn-glycero-3-phosphocholines released. Analogues of PAF containing unsaturated alkyl moieties at the sn-1 position (2, 4, 5) were synthesized by utilizing the methoxyethoxymethyl protecting group as a novel method for preparing unsaturated alkyl lipids. This procedure provides a facile means for preparing unsaturated either phospholipids of defined structure that may be tritiated to high radiospecific activity for metabolic studies. Unsaturation in the alkyl chain had minimal effect on the bioactivities examined in this study.

Halogenated biphenyl transport by blood components.

Halogenated biphenyl transport by components of rat blood was studied under both in vivo and in vitro conditions. Fractionation of plasma components by gel filtration, ultracentrifugation, and chromatography on a column of fine glass beads indicate that halogenated biphenyls are associated with each major class of plasma proteins but are most concentrated in the lipoproteins. A significant portion of the total halogenated biphenyl in whole blood is also associated with the cellular component. Halogenated biphenyls are readily exchanged between plasma and the cellular component and between lipoproteins and other classes of plasma proteins. Partition of a series of halogenated biphenyls between lipoproteins and other plasma proteins indicated that the relative affinity of a biphenyl for each fraction was proportional to the lipid solubility of the biphenyl involved. Halogenated biphenyls in blood are not thought to be bound to specific sites on blood proteins but rather they are believed to be associated with hydrophobic sites on plasma proteins or the cellular component of blood. The rapid transfer of these compounds to tissues is thought to be by partition to similar sites on cellular proteins.

Platelet-activating factor and analogues: comparative studies with human neutrophils and rabbit platelets.

Platelet-activating factor and 12 structural analogues stimulated rabbit platelets to aggregate and release [14C]-serotonin. They likewise caused human neutrophils to aggregate, degranulate, and take up [3H]-deoxyglucose. Their respective potencies, which varied by 4-5 orders of magnitude, correlated highly (r greater than or equal to 0.93) in all assays. These compounds also selectively desensitized neutrophils to the degranulating actions of platelet-activating factor but not to C5a or a formylated oligopeptide. Three other analogues with structures quite similar to platelet-activating factor were unable to activate or desensitize the cells. Hence, the structure-activity relations of the analogues in several assays of platelet and neutrophil function were similar and they stimulated neutrophils by a common activation mechanism that differed from those used by C5a or formylated oligopeptides. These data are consistent with the notion that platelet-activating factor activates and desensitizes various target cells through stereospecific receptors. Apparently, these putative receptors on neutrophils and platelets have similar structural specificities for platelet-activating factor and its analogues.

Structural features of platelet activating factor (1-alkyl-2-acetyl-sn-glycero-3-phosphocholine) required for hypotensive and platelet serotonin responses.

A number of analogs similar in structure to biologically active 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine (platelet activating factor) were tested for their relative effectiveness in lowering the blood pressure of spontaneous hypertensive rats and in releasing [3H]serotonin from rabbit platelets. Except for some minor variations, the two test systems gave the same pattern of relative responses for a given analog. Both biological activities (the antihypertensive response and release of platelet serotonin) appear to be stereospecific since the unnatural isomer exhibited no detectable effects. Other structural modifications that greatly reduced (greater than 500-fold) or eliminated the two biological activities were: 1) removal of the acetate group, 2) substitution of the acetate group with long-chain acyl, methoxy, benzoxy, N-formyl, N-trifluoroacetyl, N-hexadecanoyl, or deoxy groups such as dimethyl, propyl, isopropyl, or isobutyl, and 3) replacement of the sn-1 0-alkyl group with an acyl moiety. Biological activities were reduced to a lesser extent (100- to 500-fold) when the sn-2 carbon contained butyrate or hexanoate groups or if ethanolamine was substituted for the choline base. Replacing the sn-2 acetate group with an ethoxy group lowered the activity 48- and 120-fold for the blood pressure and serotonin release, respectively. Substitution of propionate for acetate at the sn-2 carbon gave a compound that was at least, if not more, biologically active than the parent structure; although an N-acetyl analog possessed biologic activities, the responses were only 0.27 to 1.0% of those obtained with the O-acetyl lipid. Maximum biological activity for both hypotensive effects and platelet serotonin release required a glycerolipid having an alkyl ether at the sn-1, acetate or propionate groups at the sn-2, and phosphocholine at the sn-3 positions.