 |
Description  |
|
|
BRIEF SUMMARY OF THE INVENTION
The invention relates to synthetic compounds which, when administered
orally to warm-blooded animals, inhibit the absorption of cholesterol.
Certain water/alcohol soluble extracts from plant sources have been found
to reduce cholesterolemia in chicks, pigs and rats (P. Griminger, et al.,
"Dietary Saponin and Plasma Cholesterol in the Chick." Proc. Soc. Exp.
Biol. Med. 99:424-426, 1958; H. A. I. Newman, et al. "Dietary Saponins, a
Factor Which May Reduce Liver and Serum Cholesterol Levels." Poultry Sci.
37:42-46, 1958; B. Morgan, et al., "The Interactions Between Dietary
Saponin, Cholesterol and Related Sterols in the Chick." Poultry Sci.
51:677-682, 1972; D. L. Topping, et al, "Effects of Dietary Saponins in
Fecal Bile Acids and Neutral Sterols, Plasma Lipids, and Lipoprotein
Turnover in the Pig." Am. J. Clin. Nutr. 33:783-786, 1980; D. G.
Oakenfull, et al., "Effects of Saponins on Bile-acids and Plasma Lipids in
the Rat." Br. J. Nutr. 42:209-216, 1979; and D. L. Topping, et al.,
"Prevention of Dietary Hypercholesterolemia in the Rat by Soy Flour High
and Low in Saponins." Nutr. Rep. Int. 22:513-519, 1980.)
More specifically, extracts from alfalfa hay are known to be active in
reducing the absorption of dietary cholesterol. Although such alfalfa
extracts are of unknown composition, they are found to contain saponins
identifiable by thin-layer chromatography. The alfalfa extracts contain,
in addition to saponins, unspecified amounts of carbohydrates, amino
acids, peptides, pigments, and free aglycones removed from alfalfa hay by
the water:alcohol solvent used during their preparation. Such crude
extracts are sometimes referred to herein as "alfalfa saponins" as an
operational definition. These alfalfa extracts reduce the intestinal
absorption of cholesterol in rats and monkeys (M. R. Malinow, et al.,
"Cholesterol and Bile Acid Balance in Macaca fascicularis: Effects of
Alfalfa Saponins." J. Clin. Invest. 67:156-162, 1981). The capacity of
such alfalfa extracts to interfere with cholesterol absorption is enhanced
by partial acid hydrolysis as reported in M. R. Malinow, et al., "Effect
of Alfalfa Saponins on Intestinal Cholesterol Absorption in Rats." Am. J.
Clin. Nutr. 30:2061-2067, 1977; and U.S. Pat. No. 4,242,502 (Malinow, et
al.)).
Digitonin binds cholesterol in vitro, inhibits the intestinal absorption of
cholesterol, and prevents the hypercholesterolemia expected in monkeys
ingesting high-fat, highcholesterol foods (M. R. Malinow, et al,
"Prevention of Hypercholesterolemia in Monkeys (Macaca fascicularis) by
Digitonin, Am. J. Clin. Nutr., 31:814-818, 1978). But, digitonin is
costly. Diosgenin has also been reported to inhibit the absorption of
cholesterol in rats when given in a massive dose (1000 mg/kg) (M. N.
Cayen, et al., "Effect of Diosgenin on Lipid Metabolism in Rats.", J.
Lipid Res. 20: 162-174, 1979).
It was previously reported that toxicity of plant saponins is decreased in
rats, mice, and birds by cholesterol in the diet (J. O. Anderson, "Effect
of Alfalfa Saponin on the Performance of Chicks and Laying Hens." Poult.
Sci. 36:873-876, 1957; I. Ishaaya, et al., "Soyabean Saponins. IX. Studies
of Their Effects on Birds, Mammals and Cold-blooded Organisms." J. Sci.
Food Agric., 20:433-436, 1969; G. Reshef, et al., "Effect of Alfalfa
Saponins on the Growth and Some Aspects of Lipid Metabolism of Mice and
Quails." J. Sci. Food Agric. 27:63-72, 1976; E. B. Wilcox, et al., "Serum
and Liver Cholesterol, Total Lipids and Lipid Phosphorus Levels of Rats
Under Various Dietary Regimes." Am. J. Clin. Nutr., 9:236-243, 1961).
Despite these encouraging results, it has remained a problem that plant
extracts, which are of variable composition, contain a volume of nonuseful
chemical substances. It is difficult, due to the variations in
composition, to set a standard dosage or predict the impurities present.
Thus, such extracts are not well suited for use by humans. Furthermore,
purification of plant extract substances and synthesis of saponins
suspected to exist in plants are likely to be very costly due to the
anticipated complexity of the required procedures.
It has now been discovered that certain synthetically produced, pure
"sapogenin-derived" compounds, e.g., substances compounded from
spirostane, spirostene, or sterol-derived" compounds are nontoxic. Such
compounds depress cholesterol absorption more effectively than alfalfa
extracts on a weight basis and thus can be administered in reasonably
sized doses. Because the chemical compositions of these substances are
known and because they can be synthesized at a high degree of purity, they
are suitable for use by any warm-blooded animal, including humans.
Precursor substances for use in the synthesis are available as by-products
of present industrial processes. For example, one spirostane compound
(tigogenin) is currently a wasted by-product of digitalis manufacture.
Unless administered in massive amounts, pure sapogenins do not
significantly bind cholesterol or inhibit its absorption. It is only when
compounded with another moiety that sapogenins have the desired effect.
Examples of such sapogenin compounds are compounds of tigogenin and
diosgenin, particularly glycosides having tigogenin or diosgenin as an
aglycone. Although it is not established that the size of the nonsapogenin
moiety has any effect on a compound's ability to inhibit cholesterol
absorption, at least one glycoside with a relatively longer sugar moiety
has an increased biological activity in regard to cholesterol absorption.
Specifically, cellobiose-sapogenins are more active than
glucose-sapogenins having the same aglycone. Of such substances,
cellobiose-tigogenin is particularly active to inhibit cholesterol
absorption. Somewhat less active is cellobiose-diosgenin, perhaps due to
the presence of a double bond in C.sub.5 of the spirostene aglycone of
diosgenin.
DETAILED DESCRIPTION
Certain synthetic compounds, when administered orally, inhibit the
absorption of cholesterol by warm-blooded animals, and consequently may
lower plasma cholesterol levels and induce regression of atherosclerosis.
These include compounds of sterols and of "spirostane substances" such as
diosgenin, tigogenin, smilagenin and the like. One group of such compounds
includes compounds of diosgenin
(5,20.alpha.,22.alpha.,25d-spirostan-3.beta.-ol) as represented by the
following formula:
##STR1##
Another group according to the invention include compounds of tigogenin
(5.alpha.,20.alpha.,22.alpha.,25D-spirostan-3.beta.-ol) represented by
formula:
##STR2##
A. Esters
Esters of the above formulas can be prepared by reacting an anhydride with
the "spirostane substance". Any of numerous organic anhydride substances
could be used in such molecules, although not all such molecules would be
effective or be sufficiently nontoxic for general use.
Esters have been formed from the following anhydrides which react with the
hydroxyl group of the sapogenin or sterol:
______________________________________
Anhydrides Used in the Synthesis of Sapogenin and Sterol
______________________________________
Esters
phtalyl-DL-glutamic cis-1,2-cyclobutane
dicarboxylic
1-octenyl-succinic citraronic
glutaric 3-nitrophtalic
nonenylsuccinic methylsuccinic
trans-1,2-cyclohexane
3-methylglutaric
dicarboxylic
cix-1,2-cyclohexanedicarboxylic
2,3-dimethyl maleic
3,3-dimethyl glutaric
1,2,3,4-cyclobutane
tetracarboxylic
trans-1,2-cyclohexanedi-
diphenyl
carboxylic
2-dodecen-1-ylsuccinic
maleic
dichloromaleic
______________________________________
Of the esters produced, only maleic esters proved to be effective in the
inhibition of intestinal absorption of cholesterol in laboratory animals.
Such maleic esters were synthesized by combining maleic anydride and the
desired sapogenin or sterol substance in chloroform at 50.degree. for a
period of four days. The result was a maleic ester of the formula:
##STR3##
wherein R is a sapogenin such as diosgenin or tigogenin or a sterol such
as cholesterol.
Tests were performed on laboratory animals according to the procedure
outlined in M. R. Malinow, et al., "Effect of Alfalfa Saponins on
Intestinal Cholesterol Absorption in Rats." Am. J. Clin. Nutr., 30,
December 1977, pps. 2061-2067. The tests were performed in groups of six
animals each (mean.+-.SE). The result of the tests are summarized in Table
I:
TABLE I
______________________________________
Effect of Sapogenin Esters on Intestinal Absorption of
Cholesterol in Rats. Tests performed in groups of
six animals each (mean .+-. SE)
Intestinal absorption of
cholesterol (% of I.D.)
Substance mg/rat Controls Experimental
-P
______________________________________
tigogenin maleate
15 80.5 .+-. 0.6
62.9 .+-. 1.0
0.001
diosgenin maleate
15 79.6 .+-. 1.4
56.0 .+-. 2.2
0.001
tigogenin 15 79.1 .+-. 1.5
81.2 .+-. 1.1
N.S.
diosgenin 15 73.5 .+-. 2.0
76.4 .+-. 2.6
N.S.
maleic acid 15 79.1 .+-. 1.5
78.1 .+-. 0.8
N.S.
maleic acid/tigogenin
5/10 77.0 .+-. 2.1
78.1 .+-. 0.8
N.S.
Maleic acid/diosgenin
5/10 77.0 .+-. 2.1
71.2 .+-. 2.8
N.S.
______________________________________
In vitro tests for micellar cholesterol binding were performed according to
the method of M. R. Malinow, et al., "Prevention of Hypercholesterolemia
in Monkeys (Macaca fascicularis) by Digitonin." Am. J. Clin. Nutr.
31:814-818, 1978. These tests confirmed that cholesterol was bound by the
sapogenin maleates. The results of these in vitro tests appear in Table
II:
TABLE II
______________________________________
In Vitro Micellar Cholesterol Binding
Bound cholesterol
Substance (mg/mg substance)
______________________________________
Tigogenin maleate
0.20
diosgenin maleate
0.24
digitonin 0.33
alfalfa saponins
0.24
tigogenin 0
diosgenin 0
______________________________________
When tested in primates, however, the sapongenin maleates induced vomiting
when administered orally. Thus, the maleates may be unacceptable for
administration to humans, or even be toxic.
B. Glycosides
Sapogenin and sterol compounds according to the invention, specifically
glycosides of tigogenin, diosgenin and cholesterol successfully bind
cholesterol and inhibit its absorption by the digestive system of
warm-blooded animals. Thus far, no toxic effect has been associated with
such substances. And, due to their structure, it is likely that they are
substantially nontoxic.
The glucosidic compounds were formed by reacting a carbohydrate-containing
molecule with the hydroxyl group of the aglycone substance. The
carbohydrate-containing molecule can, for example, be
.alpha.-D-(+)-glucose of the formula:
##STR4##
or .beta.-D-(+)-glucose of the formula:
##STR5##
or longer carbohydrate molecules such as (+)-cellobiose(.beta.-anomer)
otherwise known as 4-O-(.beta.-D-glucopyranosyl-D-glucopyranose):
##STR6##
The glycosidic bonds between the carbohydrate-containing molecule and the
aglycone could be either an .alpha. glycosidic bond:
##STR7##
or a .beta. glycosidic bond:
##STR8##
For compounds which are to bind cholesterol, it is anticipated that a
.beta. glycosidic bond will be preferred in most instances since compounds
having a .beta. glycosidic bond are less likely to be hydrolyzed in the
intestine of the subject animal.
Thus, although an .alpha.-cellobiose-tigogenin of the following formula
might be suitable,
##STR9##
it is anticipated that a .beta.-cellobiose-tigogenin, such as one of the
following formula, would be more effective:
##STR10##
Glucosides having tigogenin and diosgenin aglycones were synthesized and
tested according to the following procedures.
EXAMPLE 1
Synthesis
A series of saponins were synthesized by glycosylation of the hydroxyl
group of the two sapogenins, tigogenin
(5.alpha.,20.alpha.,22.alpha.,25D-spirostan-3.beta.-ol) and diosgenin
(5.alpha.,20.alpha.,22.alpha.,25D-spirosten-3.beta.-ol). Synthesis of the
saponins was accomplished with a modified version of the method described
by Rosevear at al., "Alkyl Glycoside Detergents" A Simpler Synthesis and
Their Effects on Kinetic and Physical Properties of Cytochrome C Oxidase."
Biochemistry, 19:4108-4115, 1980.
a. Weigh 1 mmol (678 mg) of cellobiose octoacetate (.beta.-anomer) or 1
mmol (390 mg) of glucose pentaacetate (.beta.-anomer) into a foil-covered
reaction vessel. While stirring magnetically, add 5 ml of glacial acetic
acid and stir for a few minutes. Quickly add 5 ml of 31% HBr in glacial
acetic acid. Immediately shut in the fumes with a stopper and stir
vigorously for 45 to 55 min (cellobiose) or 30 to 40 min (glucose).
b. Add 10 ml of choloform for cellobiose or dichloromethane for glucose.
Pour into a separating funnel containing 30 ml of ice and water. Shake for
2 min and extract the bottom nonaqueous layer into 30 ml of a cold
saturated sodium bicarbonate solution previously saturated with chloroform
or dichlormethane. Shake for 2 min. Repeat NaHCO.sub.3 extraction twice or
until the pH of the aqueous layer is >7.0. Wash the nonaqueous phase three
times with cold solvent-saturated distilled water.
c. Dry the extracted solvent layer over 500 mg. of magnesium sulfate while
stirring for 30 min. Centrifuge and wash the precipitate with dry solvent.
d. Pour the dried filtrate into a foil-covered, 50-ml screw-cap tube and
evaporate to about 10 ml with N.sub.2 at low heat.
e. Keeping the reaction vessel protected from light and air moisture as
much as possible, add: 1 mmol of tigogenin or diosgenin; 200 mg of dry
silver carbonate; one small iodine crystal; and 1 g of 4 .ANG. molecular
sieves. Stir in the dark for 12 to 24 h.
f. Centrifuge at a low speed for 10 min. and transfer the supernatant to a
foil-covered, 50-ml screw-cap tube. Wash the precipitate twice with 5 ml
of solvent and combine the solvents.
g. Evaporate the combined solvent with N.sub.2 to a cloudy syrup of about 1
to 2 ml. Add a stir bar and 10 ml of a solution of
triethylamine:methanol:water (1:2:1). Stir for 30 min. and let stand
overnight.
h. Transfer the solution quantitatively into dialyzing tubing with 40 ml of
water. Dialyze it against tap water for 48 h.
i. Transfer the solution to a container for freeze-drying. Evaporate the
water overnight.
j. Dissolve the resulting white powder in dichloromethane-methanol (10:1,
vol/vol) and transfer to a chromatography column filled with silica gel
type 60 (230-400 mesh, E. Merck Reagents). Separate 7 ml fractions with
the above solvent and after 150 ml, use methanol to elute the saponins.
k. Perform thin-layer chromatography (TLC) on a small amount (around 10
.sub..mu. g) with chloroform:methanol:water (65:38:10) as the solvent. Use
a Cu acetate solution for charring. Saponins typically have retardation
factor (R.sub.f) values around 0.6 to 0.8 and give a bluish or brownish
color.
This procedure probably synthesized saponins with .beta.-glycosidic bonds,
or, in the case of cellobiose, a mixture of .beta.- and
.alpha.-glycosides. It was possible to synthesize the .alpha.-isomer, by
using glucose pentaacetate (.alpha.-anomer) and ZnCl.sub.2 instead of
AgCO.sub.3 as catalyst.
Glucosides having tigogenin and diosgenin aglycones were tested for in
vitro binding of cholesterol according to the following procedure:
EXAMPLE 2
In Vitro Binding of Cholesterol
a. Prepare a micellar suspension of cholesterol by placing in a flask
caprylic acid (1.2.times.10.sup.-3 M), glyceryl monoleate
(0.6.times.10.sup.-3 M), and [4-.sup.14 C] cholesterol
(0.3.times.10.sup.-3 M; specific activity.about.25,000 dpm/mg), and
agitating these for 1 h at 38.degree. C. with sodium taurocholate
(10.times.10.sup.-3 M) in 0.1M phosphate buffer, pH 6.2 (20). The
suspension contains approximately 58 .mu.g of cholesterol/ml. Centrifuge
the suspension at 10,000 rpm for 30 min. before use.
b. Place 100 .mu.g of the synthetic saponin dissolved in methanol in a
50-ml screw-cap tube and evaporate the solvent under N.sub.2 in a
warm-water bath.
c. Add 4 ml of the micellar suspension and shake it for 2 h at room
temperature.
d. Transfer the medium to centrifuge tubes. Centrifuge at 3,000 rpm for 20
min.
e. Determine the radioactivity in 0.5 ml of the upper solvent phase with 10
ml of toluene-based scintillation fluid.
f. Treat blanks as above without adding the synthetic saponin.
Results of these tests showed that binding of cholesterol occurs with the
synthetic saponins. Representative data from the test appears in Table
III:
TABLE III
______________________________________
In Vitro Binding of Cholesterol by Synthetic Saponins.sup.a
Mass/flask
Cholesterol bound
Saponin (.mu.g) (.mu.g)
______________________________________
None.sup.b 0 0
Cellobiose-tigogenin.sup.b
250 21
Cellobiose-tigogenin.sup.b
500 37
Cellobiose-tigogenin.sup.b
750 46
Cellobiose-tigogenin.sup.b
1000 60
Cellobiose-diosgenin.sup.b
1000 26
Glucose-diosgenin.sup.c
1000 49
Glucose-tigogenin.sup.c
1000 40
______________________________________
.sup.a Results are average of three determinations.
.sup.b Mixture of and glycosides.
.sup.c Mainly glycoside.
Further experimentation was conducted to determine the effect of the
synthesized saponins on living animals.
EXAMPLE 3
Effects of Synthetic Glycosides in Vivo
Effects of the synthetic glycosides on the intestinal absorption of
cholesterol in rats were tested according to the procedure of M. R.
Malinow, et al., "Effect of Alfalfa Saponins on Intestinal Cholesterol
Absorption in Rats." Am. J. Clin. Nutr. 30:2061-2067, 1977. The animals
were fed semipurified cholesterol-free food from 8 a.m. to 10 a.m. for ten
days. On the day of the experiment, the rats were anesthetized at about
10:30 a.m. and the test substance, as well as a pulse dose of radioactive
cholesterol, were given per gastric tube. The excretion of .sup.14
C-neutral steroids was determined in feces collected for 72 h after
intragastric administration of the test substances and 2 mg of [4-.sup.14
C] cholesterol (specific activity.about.0.25 .mu.Ci/mg).
As shown in Table IV, these rate experiments demonstrated that the
synthetic glycosides inhibit the intestinal absorption of cholesterol. No
significant inhibition was observed with sapogenins. Better results were
obtained with a longer sugar moiety (cellobiose) than with a shorter
moiety (glucose).
TABLE IV
__________________________________________________________________________
Effects of Sapogenins and Synthetic Glycosides on Cholesterol Absorption
in Rats.sup.a
Intestinal
Student's
Number Absorption of
.sub.- t test,
Substance
of Weight
Dose cholesterol
.sub.-- P versus
Relative
Series
administered
rats (g) (mg/rat)
(I.D.) controls
Absorption
__________________________________________________________________________
I none 6 242 .+-. 3
0 74.6 .+-. 2.3
100
glucose-
6 246 .+-. 7
14 46.2 .+-. 1.8
<0.001
62
tigogenin
glucose-
6 245 .+-. 5
14 52.6 .+-. 3.7
<0.001
71
diosgenin
alfalfa
6 249 .+-. 9
14 60.2 .+-. 3.7
<0.01
81
extract
II none 6 290 .+-. 7
0 74.8 .+-. 1.6
100
cellobiose-
6 291 .+-. 12
14 39.6 .+-. 1.8
<0.001
53
tigogenin
cellobiose-
6 287 .+-. 4
14 53.7 .+-. 1.3
<0.001
72
diosgenin
alfalfa
6 275 .+-. 8
14 56.3 .+-. 1.1
<0.01
75
extract
III none 6 268 .+-. 5
0 78.0 .+-. 2.1
100.
tigogenin
6 259 .+-. 5
15 80.4 .+-. 1.4
N.S. 103
IV none 6 328 .+-. 4
0 73.5 .+-. 2.0
100
diosgenin
6 330 .+-. 4
15 76.4 .+-. 2.6
N.S. 104
V none 6 276 .+-. 7
0 77.7 .+-. 2.2
100
cellobiose-
6 282 .+-. 3
15 69.5 .+-. 1.3
0.01
89
cholesterol
glucose-
6 273 .+-. 8
15 72.5 .+-. 1.0
N.S. 93
cholesterol
alfalfa
6 277 .+-. 5
15 68.4 .+-. 2.3
0.02
88
extract
__________________________________________________________________________
Values are mean .+-. SE. Abbreviations: I.D., injected dose; N.S., not
significant.
.sup.a The excretion of .sup.14 Cneutral steroids was determined in feces
collected for 72 h after intragastric administration of glycosides or
sapogenins and 2 mg of [4.sup.14 Ccholesterol.
The mechanism whereby cholesterol absorption is inhibited by sapogenin and
sterol compounds is unknown. However, it is possible that the compounds
form an insoluble complex with cholesterol in the intestinal lumen and
thereby prevent the absorption of dietary cholesterol. Additionally, the
compounds may bind biliary cholesterol and cholesterol from desquamated
intestinal cells and, thus, may induce negative cholesterol balance
through the increased excretion of endogenous cholesterol.
In the above described experiments, synthetic sapogenin and sterol
compounds inhibited the absorption of exogenous cholesterol in laboratory
animals. It is thus anticipated that they will also be effective in
preventing atherosclerosis or inducing its regression without toxic
effects. It is an advantage that such synthetic compounds are pure
substances with known or determinable chemical structures and may be
synthesized in sufficient purity so as to be used to treat human beings.
As indicated by their ability to prevent the absorption of cholesterol,
synthetic sapogenin and sterol compounds may reduce deaths due to
atherosclerotic disease, a most significant cause of death in the adult
population of the Western world.
Having given examples of preferred embodiments of my invention, it will be
apparent to those skilled in the art that changes and modifications may be
made without departing from my invention in its broader aspects. I
therefore intend the appended claims to cover all such changes and
modifications as fall within the true spirit and scope of my invention.
* * * * *
|
|
 |
|
 |
Description |
|
