This invention relates to a method for preparing phosphodiester conjugates
useful for preparing immunoactive liposomes; more particularly, it relates to a
method for preparing certain phosphodiester conjugates from certain phospholipids,
ligands and analytes, as well as to the preparation of immunoactive liposomes from
Liposome immunoassay utilizes immunoactive liposomes (i.e. liposomes
with covalently attached antigen, antibody or other analyte of interest). Preparation
of the immunoactive liposomes involves preparing analyte derivatives of phosphatidylethanolamine
(PE) or other phospholipids and subsequently incorporating specific quantities
of this phospholipid-analyte conjugate into the liposome surface membrane during
the formation of the liposome vesicle.
U.S. Patent No. 4,480,041 describes a method for preparing immunoactive
liposomes utilizing phosphotriester intermediates of phospholipids derivatized
with an analyte. Synthesis of the phosphotriester intermediates and conversion
to the targeted phospholipid-analyte derivative is a five or six step process.
U.S. Patent No. 4,193,983 describes the preparation of a conjugate
of a carboxylate-containing phenobarbitol and dipalmitoyl phosphatidyl ethanolamine
(DPPE) by (1) reacting the phenobarbitol with oxalic acid and then (2) reacting
the resultant reaction product with DPPE to produce the conjugate. This patent
additionally describes the derivatization of cardiolipin to a cardiolipin hemisuccinate
through the reaction of the hydroxyl group of the glycerol moiety in the cardiolipin
with succinic anhydride. This hemisuccinate is then activated by oxalyl chloride
and subsequently reacted with fluoresceinamine to produce a cardiolipin-succinate-fluorescein
In O'Connell et al, "A Highly Sensitive Immunoassay System Involving
Antibody-Coated Tubes and Liposome-Entrapped Dye, Clin. Chem. 31/9, 1424-1426 (1985),
the authors describe the direct coupling of DPPE and 3-Ketodigoxigenin to form
a Schiff base which was then reduced with sodium cyanogenborohydride to produce
a DPPE-digoxigenin conjugate.
In Haga et al, "Drug Sensor: Liposome Immunosensor for Theophylline",
Anal. Biochem. 118, 286-293 (1981), the authors describe a process for the
preparation of a theophyllinephosphatidylethanolamine (PE) conjugate by (1) reacting
theophylline and thionyl chloride to produce an acid chloride of theophylline
(a theophylline analog with a carboxylate group); and then (2) reacting the acid
chloride of theophylline with PE to produce the theophylline-PE conjugate.
It is the object of the present invention to provide a novel method
of preparing phosphodiester-ligand-analyte conjugates useful in preparing immunoactive
This invention relates to a method for producing a phosphodiester-ligand-analyte
conjugate, which comprises:
a) reacting a polar phospholipid of the formula:
wherein R&sub1; and R&sub2; are hydrogen, hydroxyl, R&sec;, OR&sec; or O R&sec;
wherein R&sec; is a saturated or unsaturated, branched or straight-chain alkyl
or alkene group of from 1 to 24 carbon atoms and wherein at least one of R&sub1;
and R&sub2; is O R&sec;
or OR&sec; ; and
wherein R&sub3; is a branched or straight-chain alkyl amine of from 1 to 24 carbons,
with a ligand to produce a polar phospholipid-ligand composite; and
b) reacting the polar phospholid-ligand composite with an analyte capable of
coupling with the ligand of the polar composite, to produce the phosphodiester-ligand-analyte
conjugate, wherein the ligand is reactive with the R&sub3; moiety of the phospholipid
and reactive with the analyte.
Preferably, R&sub1; and R&sub2; are O R&sec;
OR&sec;; R&sub3; is a straight-chain alkyl amine of from 2 to 4 carbon atoms,
and R&sec; is a saturated straight-chain alkyl of from 10 to 20 carbon atoms.
More preferably, R&sub1; and R&sub2; are O R&sec;
; R&sub3; is aminoethyl;
and R&sec; is a saturated straight-chain alkyl of 15 carbon atoms.
The ligand useful in the process of this invention acts as a connector,
or spacer, between the phospholipid and the analyte, i.e., the ligand is a bifunctional
compound which enables the attachement at one end to the desired analyte and at
the other end to the phospholipid. The ligands are preferably alkyl groups of
from 2 to 20 carbon atoms, branched or unbranched, saturated or unsaturated, which
contain 0-20 heteroatoms, normally nitrogen, oxygen, sulfur, or phosphorous, and
two distinct functional groups comprising carboxyl, amine, hydroxyl, sulfhydryl,
imidate, maleimide, 2-pyridylthio, halide, isothiocyanate, or anhydride residues,
preferably carboxyl, amine, hydroxyl or sulfhydryl residues, preferably at the
opposite ends of the ligand. One end of the ligand is designed for attachment
to the phospholipid, preferably via a carboxyl group. The actual mechanism for
attachment of the ligands to the phospholipids and analytes will depend on the
nature of the reactive components. Certain ligands need to be activated (derivatized
to form a reactive group capable of coupling to the phospholipid) prior to reacting
the ligand with the phospholipid. Certain phospholipid-ligand composites also need
to be be activated (derivatizing the ligand of the phospholipid-ligand composite
to form a reactive group capable of coupling to the desired analyte) prior to
reacting the phospholipid-ligand composite with the analyte. Anhydrides, thionylchloride,
sulfonic acid derivatives, N-hydroxysuccinamides, carbodiimides, and other means
of activation (or reactive group formation) known in the art can be used to prepare
the ligand for reaction with the phospholipid or prepare the phospholipid-ligand
composite for reaction with the analyte. The type of activation (or derivatization)
necessary will depend on the particular phospholipid, ligand and analyte combination
to be used.
Useful ligands include hetero- and homo- bifunctional cross-linking
reagents and cyclic acid anhydrides. Examples of preferred ligands include succinimyl
4(p-maleimidophenyl)-butyrate (SMPB); N-succinimydyl 3(2-pyridyldithio)proprionate;
2-iminothiolane; succinic anhydride, and the like.
The analytes useful in the method of this invention are those materials
which are capable of coupling to the ligand of the polar phospholipid-ligand composite.
Examples of suitable analytes include antigens, circulating hormones, antibiotics
and other therapeutic drugs, and derivatives thereof. Useful analytes include
folate, digoxin, B-12 and theophylline. Preferred analytes are amino-containing
analytes such as thyroxine and triiodothyronine. Certain analytes, such as e.g.,
folate, digoxin, or B-12, can require activation (derivatization) in order for
these analytes to be coupled to the ligand-containing phospholipid. The type of
derivatization necessary will depend on the specific analyte-ligand combination
to be used.
In a preferred embodiment of the method of this invention, the phospholipid
is a phosphatidylethanolamine (PE) compound having the formula:
Reaction of the PE with a cyclic anhydride such as succinic anhydride yields a
phosphatidyldiester derivative (PE-Suc) having the formula:
The carboxyl group of the PE-Suc derivative can be activated by reaction with an
activating reagent such as ethyl chloroformate (to form a mixed anhydride), dicyclohexylcarbodiimide
and N-hydroxysuccinimide (to form an active ester) or oxalyl chloride or thionyl
chloride (to form an acyl chloride).
Activation of the PE-Suc derivative by the mixed anhydride method
can be conducted in an aprotic solvent, preferably chloroform. The activating reagent
can be a branched or straight-chain alkyl chloroformate, preferably the alkyl group
is ethyl. Addition of an organic base is required to neutralize any hydrogen chloride
generated. This activation is preferably conducted at a temperature of from about
-20°C to about 25°C, more preferably from about 0°C to about 10°C, for a period
of time from about 5 minutes to about 90 minutes, most preferably from about 20
minutes to about 30 minutes.
The analyte, e.g., thyroxin, is then added in equimolar amounts or
in molar excess of the amount of PE-Suc derivative because separation of the desired
conjugate (e.g., PE-Suc-T&sub4;) from excessive, unreacted analyte (e.g., T&sub4;)
is easier than from unreacted PE-Suc.
Immediately after adding the analyte, a polar, aprotic solvent, e.g.,
dimethylformamide (DMF), in volume ranging from one-half to three times that of
the original reaction mixture should be added. The yield of the desired conjugate
is improved by addition of the solvent. After the addition of the solvent, the
reaction is kept at room temperature for about 0.5 hours to about 16 hours.
PE-Suc can also be activated by an active ester method using a carbodiimide,
such as dicyclohexylcarbodiimide (DCC), and N-hydroxysuccinimide (NHS) in equimolar
or excess amounts of the PE-Suc derivative. PE-Suc is initially reacted with DCC
in an aprotic, polar or non-polar, preferably non-polar, solvent such as chloroform,
at room temperature or preferably, a lower temperature, for about 5 to about 30
minutes, preferably about 10 to about 15 minutes, to form a reaction adduct between
PE-Suc and DCC. NHS is then added to this reaction adduct and reacted at room
temperature overnight to produce the desired ester, PE-Suc-NHS, which has the following
and dicyclohexylurea as a by-product. NHS can be added to the reaction mixture
in solid form followed by addition of a polar, aprotic solvent such as DMF. Alternatively,
NHS can first be dissolved in the polar solvent and then added to the reaction
Prior to the addition of the analyte (e.g., thyroxin) the aprotic
solvents are removed from the reaction mixture, e.g., by evaporation. The analyte
is then added in equimolar or molar excess amounts of the PE-Suc derivative, to
the reaction mixture. Immediately, after addition of the analyte, a polar, protic
solvent, preferably methanol in the presence of an organic base such as triethylamine,
is added in excess. The reaction is then conducted at room temperature overnight
to produce the desired conjugate, e.g. PE-Suc-T&sub4;.
To separate the resultant desired conjugate from the reaction mixture,
a purification procedure, such as preparative thin layer chromatography (PLC),
In a preferred embodiment employing a PLC procedure, the crude reaction
mixture containing the desired conjugate is first dissolved in a solvent system
of sufficient polarity to facilitate even adsorption of the material throughout
the thickness of the silica gel of a preparative thin layer chromatography plate.
Preferably, a pre-coated silica gel (60 angstrom pore-sized particles, 2mm thickness)
preparative thin layer chromatography plates (20 x 20 cm) with fluorescence indicator
is utilized for the PLC purification. The solvent system utilized should be equal
to or more polar than a 20% methanol/chloroform solvent system, preferably capable
of being enhanced by the addition of water. A preferred solvent system is chloroform/methanol/water
in a ratio of 73:24:3, respectively. The crude reaction mixture is loaded onto
the plates in amounts of between 125-250 mg/plate, preferably 150-200 mg/plate,
in a volume of between 1-4 ml/plate, preferably, between 2-3 ml/plate.
Purification of a preferred conjugate, PE-Suc-T&sub4;, from the crude
reaction mixture utilizes an organic acid containing solvent system, such as, for
example, chloroform/methanol/acetic acid (60:20:3) which separates PE-Suc-T&sub4;
and unreacted T&sub4; into two distinct bands on the plate. The PE-Suc-T&sub4;
band is then stripped off the plate and effectively recovered by eluting in a
polar solvent system such as, for example, chloroform/methanol/water (55:40:5).
Frequently, the PE-Suc-T&sub4; so produced will be contaminated with
unreacted PE-Suc. To separate the PE-Suc-T&sub4; from PE-Suc a non-acid containing
polar solvent system is utilized, such as, for example, chloroform/methanol/water
(55:40:5), on a second PLC plate on which the eluted band from the first PLC plate
is loaded. The PE-Suc-T&sub4; is stripped and eluted. This PLC step to separate
PE-Suc-T&sub4; from unreacted PE-Suc is repeated until a PE-Suc-T&sub4;/chloroform
solution (0.5 mg/ml) exhibits an absorbance at A300 of 1.05 or higher. The PE-Suc-T&sub4;
so produced is then useful for thyroxin-liposome preparation.
Other preparative chromatographic procedures which can be used to
separate the desired conjugate prepared by the method of this invention from the
crude reaction mixture include low to medium-pressure short column chromatography,
flash chromatography and high performance liquid chromatography (HPLC). HPLC,
although requiring expensive equipment, can be expected to be a very efficient
and simple procedure once optimal elution conditions are determined.
Liposomes incorporating the conjugates prepared according to the
method of this invention can be prepared using procedures known in the art. For
example, several useful procedures are described in Szoka et al., Ann. Rev. Biophys.
Bioeng. 9, pp. 467-508 (1980). A preferred method for preparing unilavellar and
oligolamellar liposomes is described in copending U.S. Application Serial No.
940, 519, filed on December 10, 1986.
The following Examples are presented to illustrate the method of
EXAMPLE 1Preparation of N-(3-Carboxypropionyl)(1,2-dipalmitoyl-3-rac-phosphatidyl)ethanolamine
A mixture of (1,2-dipalmitoyl-3-rac-phosphatidyl)ethanolamine (PE,
440 mg, 0.637 mmole), succinic anhydride (76 mg, 0.764 mmole), triethylamine (0.09
ml, 0.637 mmole) in 40 ml of DMF:CHCl&sub3; (1:1) was stirred, heated at 65°C for
1.5 hours, and evaporated in vacuo. The oily residue was taken up with 4 ml of
20% methanol in chloroform and streaked on silica gel preparative TLC plates (Merck
& Co., Inc., Rahway, N.J., Catalog No. 5717-7) at the loading ratio of 0.25
g/plate. The plates were developed with chloroform/methanol/water (55:40:5). A
faint UV band (Rf = 0.30) was stripped, eluted with chloroform/methanol/water (54:40:5).
The eluent was evaporated and the residue triturated with 30 ml of chloroform
and filtered. The filtrate was evaporated to give the desired product PE-Suc (470
EXAMPLE 2Preparation of N-[3-(α-carboxy-3,3&min;, 5,5&min;-tetraiodo-L-thyronyl)
propionyl](1,2-dipalmitoyl-3-rac-phosphatidyl)-ethanolamine (PE-Suc-T&sub4;)(A) Mixed Anhydride Method:
A solution of PE-Suc (402 mg, 0.508 mmole, from Example 1) in 10
ml of chloroform was treated with triethylamine (100 ul, 0.70 mmole), cooled in
an ice-bath and then treated with ethyl chloroformate (65 ul, 0.70 mmole). The
reaction mixture was stirred for 20 minutes, treated with L-thyroxin (sodium salt,
406 mg, 0.508 mmole) and 10 ml of dimethylformamide. The ice-bath was removed.
A complete solution was obtained in 10-15 minutes. The reaction mixture was stirred
at room temperature overnight, coevaporated with 10 ml of xylene 2 times to complete
dryness. The desired PE-Suc-T&sub4; was isolated by repeated PLC as follows: The
dried reaction mixture was dissolved in 10 ml of chloroform/methanol/water (73:24:3),
streaked on silica gel preparative TLC plates (Merck & Co., Inc., Rahway, N.J.,
Catalog No. 5717-7) at the loading ratio of 200 mg/plate, and developed with chloroform/methanol/acetic
acid (60:20:3). The desired product was identified as a major UV absorbing band
(Rf = 0.42) which also gave a positive test with a molybdenum spray (test for
organophosphate). The band was stripped, and eluted with 150 ml of chloroform/methanol/water
(54:40:5). The residue obtained from the evaporation of the eluent was subjected
to a second PLC purification using chloroform/methanol/water (65:25:4) to develop
the plates. The desired band was identified, stripped, eluted, and evaporated
in the same manner as the first PLC. The residue so obtained was triturated with
30 ml of chloroform and filtered through Whatman #1 filter paper by gravity. The
filtrate was evaporated to give the pure desired product PE-Suc-T&sub4; (202 mg,
Anal. Calc for C&sub5;&sub6;H&sub8;&sub7;N&sub2;I&sub4;O&sub1;&sub4;P:
C, 43.36; H, 5.65; N, 1.81.
C, 43,51: H, 5.90; N, 1.67.
(B) Active Ester Method:
A solution of PE-Suc (100 mg, 0.126 mmole, from Example 1) in 5 ml
of chloroform was cooled in ice bath, treated with DCC (39 mg, 0.19 mmole) in 1
ml of chloroform. After 10 min, NHS (22 mg, 0.19 mmole) in 1 ml of DMF was added.
The reaction was stirred at room temperature overnight, coevaporated with 2 ml
of xylene to complete dryness. To the residue was added thyroxin (sodium salt,
150 mg, 0.19 mmole) and triethylamine (90 ul, 0.63 mmole) in 10 ml of methanol.
The reaction mixture was stirred at room temperature overnight and evaporated to
The desired PE-Suc-T&sub4; (50 mg, 25.6%) was isolated by repeated
PLC in the same manner as described in (A) above.
(C) Active Ester Method:
The same procedure as described in (B) was used except that the desired
PE-Suc-T&sub4; was isolated and purified from the reaction mixture by flash chromatography
on a 100-ml glass column (Aldrich Chemical Co., Inc., Milwaukee, WI, Catalog No.
Z14, 735-4), packed with a slurry of silica gel (40 um, J.T. Baker Chemical Co.,
Phillipsburg, N.J., Catalog No. 7024-1), in chloroform/methanol/acetic acid (73:24:3)
under 10-15 psi nitrogen. The fractions containing pure PE-Suc-T&sub4; were pooled,
evaporated to dryness. The residue was repurified by PLC using chloroform/methanol/water
(55:40:5) to develop the plate. The yield of pure PE-Suc-T&sub4; was 16.2%.
EXAMPLE 3Preparation of N-[3-(α-carboxy-3,3&min;,5-triiodo-L-thyronyl)-propionyl](1,2-dipalmitoyl-3-rac-phosphatidyl)ethanolamine
A solution of PE-Suc (360 mg, 0.45 mmole from Example 1) in 7 ml
of chloroform was treated with triethylamine (78 ul, 0.54 mmole), cooled in an
ice-bath and treated with ethyl chloroformate (54 ul, 0.54 mmole). The reaction
was stirred for 25 minutes, treated with T&sub3; (293 mg, 0.45 mmole) and 7 ml
of dimethylformamide. The ice-bath was removed. A complete solution was obtained
in 10-15 minutes. The reaction mixture was stirred at room temperature overnight,
coevaporated with 10 ml of xylene to complete dryness.
The desired PE-Suc-T&sub3; (65 mg, 9.5%) was isolated by PLC in the same manner
as described in Example 2A above.
EXAMPLE 4Preparation of Immunoactive T&sub4; Liposomes Using PE-Suc-T&sub4;A. Preparation of Lipid Film
A solution of PE-Suc-T&sub4; (0.95 mg., OD 300 = 2.0/ul of chloroform,
from Example 2A), L-dipalmitoyl phosphatidylcholine (DPPC) (125 mg) (Avanti Polar
Lipids, Inc., Birmingham, AL), cholesterol (67.5 mg) (Sigma Chemical Co., St. Louis,
MO), and L-dipalmitoyl phosphatidylglycerol (11.5 mg) (Avanti Polar Lipids, Inc.,
Birmingham, AL), in 5 ml of chloroform was evaporated on an 8 cm diameter crystallizing
dish (Corning Glass Works, Corning, N.Y.) under vacuo of 6-100 mm Hg at room temperature
for 2 hours, then at 10-100 milliTorr overnight (16 hours) to obtain a dry, thin
homogeneous lipid coating on the bottom of the dish.
B. Lipid Vesicle Formation/Enzyme Encapsulation
The dish containing the lipid film prepared in A was prewarmed in
an orbital shaker water bath at 38-39°C for 10-12 minutes. A solution of glucose-6-phosphate
dehydrogenase (37.5 kU) (Toyobo Biochemicals Co., Osaka, Japan) in 15 ml of 2%
glycerol/0.25 mM EDTA, pH7.3, was added to the warmed lipid film in the dish.
The dish was sealed with parafilm and shaken at 150-250 rpm at 38-39°C for 20 minutes
to form lipid vesicles.
The lipid vesicles prepared in B were allowed to cool to room temperature
in the dish, transferred to dialysis tubing using a Pasteur pipet and then dialyzed
against 1 liter of a cold Tris Buffer of 0.1M Tris /0.5 mM EDTA/0.03M NaCl/0.01%
sodium azide, pH 7.8, at 2-8°C for 2 days with one change of buffer after 24 hours.
D. Extrusion/Sizing of Lipid Vesicles
The dialyzed lipid vesicles of C were transferred to a beaker and
warmed at 30-32°C for 10 minutes in a water bath and then extruded through 25 mm
diameter polycarbonate membranes (Nucleopore Corp., Pleasanton, CA) of the following
pore sizes and at the respective pressures:
3.0 um - 18 psi
2.0 um - 20 psi
1.0 um - 24 psi
0.6 um - 30 psi
using the Liposome Extruder disclosed in copending U.S. Application Serial No.
940,519, filed December 10, 1986, the pertinent portions of which are herein incorporated
E. Washing of the Sized Vesicles
The extruded liposome vesicles from D were removed from the extruder
and centrifuged at 35K rpm (BECKMAN L5-75, with Type 65 rotor) at 5-10°C for 60
minutes. The clear supernatant was removed and the pellet was resuspended in the
Tris Buffer described in C and centrifuged. This step was repeated 3 more times.
The resultant washed pellet of liposome vesicles was suspended in 10 ml of the
Tris Buffer. The T&sub4;-liposomes so produced were then used in an Enzyme Membrane
1. A method for the preparation of a phosphodiester-ligand-analyte conjugate
characterised in that it comprises:
(a) reacting a polar phospholipid corresponding to the following general formula:
wherein R&sub1; and R&sub2; independently represent hydrogen, hydroxyl, R&sec;,
OR&sec; or O R&sec;
wherein R&sec; represents a saturated or unsaturated, branched- or straight-chain
alkyl or alkylene group having from 1 to 24 carbon atoms and wherein at least
one of R&sub1; and R&sub2; represents O R&sec;
or OR&sec;; and
wherein R&sub3; represents a branched- or straight-chain alkylamine having from
1 to 24 carbon atoms; with a ligand to produce a polar phospholipid-ligand composite;
(b) reacting the polar phospholipid-ligand composite, which may be previously
activated, with an analyte, to produce the phosphodiester-ligand-analyte conjugate;
the ligand being reactive with the R&sub3; moiety of the phospholipid and reactive
with the analyte; and optionally,
(c) purifying the phosphodiester-ligand-analyte conjugate.
2. A method as claimed in claim 1 wherein R&sub1; and R&sub2; independently
represent O R&sec;
or OR&sec;; and/or
R&sec; represents a saturated straight-chain alkyl having from 10 to 20 carbon
atoms; and/or R&sub3; represents a straight-chain alkylamino having from 2 to 4
3. A method as claimed in claim 1 or claim 2 wherein the ligand comprises alkyl
having from 2 to 20 carbon atoms, branched- or straight-chain, saturated or unsaturated,
containing from 0 to 20 heteroatoms and two distinct functional groups; and/or
is a hetero- or homo-bifunctional cross-linking reagent or a cyclic acid anhydride.
4. A method as claimed in any of claims 1 to 3 wherein the analyte is an amino-containing
analyte; and/or is thyroxine or triiodothyronine.
5. A method as claimed in any of claims 1 to 4 wherein the phosphodiester-ligand-analyte
conjugate is purified by preparative chromatography, preferably preparative thin
layer chromatography or flash column chromatography.
6. A method of forming an analyte-functionalized liposome for use in immunoassays
characterised in that it comprises:
(a) preparing a phosphodiester-ligand-analyte conjugate by a method as claimed
in any of claims 1 to 5: and
(b) forming therefrom an analyte-functionalized liposome which is functionalized
on its surface with the analyte and has a marker carried within the liposome.