Use of cross-linked amylose as a matrix for the slow release of biologically active compounds

The present invention is concerned with a solid slow release oral pharmaceutical dosage unit which comprises a solid dosage unit made up of an admixture of a therapeutic dosage of an orally effective pharmaceutical product and a cross-linked polymer of amylose with a cross-linking agent selected from 2,3 dibromopropanol and epichlorohydrin, wherein the cross-linking of the polymer has been carried out with from about 0.1 to about 10 g of cross-linking agent per 100 g of amylose.

FIELD OF THE INVENTION 
The present invention relates to a slow release pharmaceutical tablet, and 
more particularly to a slow release pharmaceutical tablet incorporating a 
crosslinked polymer of amylose as the slow release matrix. 
PRIOR ART 
In the last two decades much interest has been paid to the development of 
monolithic devices for the controlled release of drugs by various routes 
of administration. 
There are several types of polymers which have already been used as matrix 
for the release of drug. Thus, polymeric materials such as polyvinyl 
chloride, polyethylene polyamides, ethylcellulose, silicone, 
poly(hydroxyethyl methacrylate)PHEMA, other acrylic copolymers, 
polyvinylacetate-polyvinylchloride copolymers and other polymers were 
described as adequate matrix for tablet preparation (see for example U.S. 
Pat. No. 3,087,860; U.S. Pat. No. 2,987,445; and Pharm. Acta Helv., 1980, 
55, 174-182, Salomon et al. 
The controlled or slow release of some drugs is of a high importance for 
biopharmaceutical applications. There are various systems of slow release, 
mostly based on diffusion-controlled release or swelling control release 
mechanisms. Diffusion-controlled polymeric systems allow some prolongation 
of drug release but provide no real means of control since the release 
rate is not constant. 
Recently, many efforts have been devoted to the development of systems able 
to release the drug at a constant rate, in other words, following 
zero-order kinetics, such as described in S.T.P. Pharma 1986, 2, 38-46 
(Peppas et al.). The approach called `swelling-controlled` systems 
consists in glassy polymers into which a water front penetrates at a 
constant rate. Behind this front, the polymer is in a rubbery state. 
Provided the drug diffusion coefficient in the rubbery polymer is much 
higher than in the glassy polymer, a zero order release can be achieved to 
a certain degree. However, the delivery rate is constant only for a 
limited fraction of the release, usually around 60% of the total amount of 
contained drug, and requires a low initial drug concentration. 
Accordingly, it would be highly desirable to provide a slow release system 
following a zero-order kinetics, and allowing a controlled release of a 
drug at a constant rate until all the drug is released, whatever the 
concentration of the drug in the system is. 
SUMMARY OF THE INVENTION 
In accordance with the present invention there is now provided a solid slow 
release oral pharmaceutical dosage unit which comprises a solid dosage 
unit made up of an admixture of a therapeutic dosage of an orally 
effective pharmaceutical product and a cross-linked polymer of amylose 
with a cross-linking agent selected from 2,3-dibromopropanol and 
epichlorohydrin, wherein the crosslinking of the polymer has been carried 
out with from about 0.1 to about 10 g of cross-linking agent per 100 g of 
amylose. 
In another aspect of the present invention, most of the particles of the 
cross-linked polymer of amylose with a cross-linking agent have a size 
that varies generally between about 25 and about 300 microns, but can be 
as high as 700 microns. 
In a further aspect of the present invention, the pharmaceutical product is 
present in the tablet in an amount of from about 10 to 60% w/w.

DETAILED DESCRIPTION 
Cross-linked amylose 
The cross-linking of amylose is well known in the literature. For example, 
the desired cross-linking can be carried out in the manner described in 
BIOCHIMIE 1978, 60, 535-537 (Mateescu) by reacting amylose with 
epichlorohydrin in an alkaline medium. In the same manner, amylose can 
also be cross-linked with 2,3-dibromopropanol. 
Essentially, the amylose is swollen in an alkaline medium such as sodium 
hydroxide and after homogenization, an appropriate amount of cross-linking 
agent is added. After complete homogenization, the reaction medium is 
transferred onto a water bath and heated for one hour at a temperature of 
from 40.degree. to 45.degree. C. and the temperature is then raised to 
from 60.degree. to 75.degree. C. for a further period of from 1 to 2 hours 
after which time the reaction is complete. The duration of heating can be 
varied as well as the amount of cross-linking agent used in the reaction. 
The resulting cross-linked gel is then sieved in wet form and the granules 
ranging from about 25 to about 700 .mu.m are collected for the preparation 
of the slow-release tablet of the present invention. The granules of 25 to 
about 300 .mu.m representing at least 50% of the granules are selected for 
use in accordance with the present invention. 
The preferred cross-linked polymers of amylose with epichlorohydrin (CLA) 
suitable for the purposes of the present invention are those where from 
about 0.1 to about 10 g of epichlorohydrin have been used to cross-linked 
100 g of amylose. More preferred cross-linked polymers were obtained when 
from about 0.5 to 7.5 g of epichlorohydrin per 100 g of amylose were used. 
In accordance with the present invention, it has been found that a polymer 
of amylose cross-linked with a cross-linking agent selected from 
2,3-dibromopropanol and epichlorohydrin, wherein from about 0.1 to about 
10 g of cross-linking agent have been used to cross-linked 100 g of 
amylose, are surprisingly and unexpectedly suitable for the slow-release 
of a large variety of drugs associated therewith. It has unexpectedly and 
surprisingly been found that the tablets prepared in accordance with the 
present invention are adapted to liberate the drug in a dose to linear 
release for a period of from 11 to 37 hours, or even more depending on the 
amount of epichlorohydrin used to cross-linked amylose. 
Preparation of tablets 
About 10 to 60% w/w of anhydrous theophylline was mixed with cross-linked 
amylose (CLA) in a shaking mixer for a few minutes. Tablets weighing about 
500 mg each were obtained by compression in a hydraulic press at more than 
0.15 T/cm.sup.2. Tablets of 1.26 cm diameter and thickness of about 2.9 to 
about 4.5 mm can be obtained depending on the applied pressure, but 
various geometry can also be realized. Hardness tests have also shown that 
it is not dependent on the crosslinking degree. 
In order to illustrate the advantages of the present invention, the release 
of theophylline from CLA tablets was selected as a model for kinetic 
studies of the release. Obviously other drugs could be incorporated in the 
CLA tablets of the present invention and provide similar slow release 
characteristics, as long as there is no interactions between the drug and 
the CLA. 
`In vitro` drug release from tablets 
Tablets were placed individually in 1L distilled water at 37.degree. C. in 
U.S.P. XX dissolution apparatus equipped with a rotating paddle (50 
r.p.m.). Theophylline release was followed spectrophotometrically at 254 
nm (Pharmacia single path monitor UV-1) and continuously recorded; a 
closed loop system and a peristaltic pump at a flow rate of 10.0 mL/min, 
were used. 
CLA is an interesting polymer for the preparation of controlled release 
drug tablets. Advantages of this material include the easy manufacturing 
of tablets, the possibility of maintaining controlled release even at a 
high drug concentration in the tablet, and the relative independence of 
release kinetics from drug loading in certain limits. Furthermore, the CLA 
slow release matrix of the present invention has high biocompatability, 
and total `in vivo` biodegradability. Also, tests have shown that the drug 
release kinetics are not influenced at pH values of from 1.5 to 11, which 
strongly suggest that the present release controlled system will be 
applicable in gastroenteric media. 
The present invention will be more readily understood by referring to the 
following examples which are given to illustrate the invention rather than 
limit its scope. 
EXAMPLE 1 
Cross-linking of amylose with epichlorohydrin 
10g of corn amylose are introduced, under agitation, in 35 mL 5N NaOH, at 
0.degree.-2.degree. C. and homogenized at the same temperature, on an ice 
bath, for about 30 min. Then, 0.1 g of epichlorohydrin are slowly added, 
and the homogenization continued, for another 30 min on the ice bath. 
After complete homogenization, the recipient containing the reaction 
medium is transferred onto a water bath and heated for one hour at 
40.degree.-45.degree. C., and then at 60.degree.-75 .degree. C. for 
another 100 minutes, for completion of the reticulation reaction. During 
heating, 1-2 mL of water is added from time to time in order to avoid an 
advanced dehydration of the reaction medium. After the reticulation is 
accomplished, the cross-linked amylose gel is washed several times with 
distilled water for elimination of sodium hydroxide excess, until a pH 
value of 6 of the distilled water is reached. The CLA gel is sieved in wet 
form, retaining grains ranging between 25-75 .mu.m, and then dried by 
treatment and subsequent decantation with increasing acetone 
concentration. The whole procedure is carried out over a period of several 
hours. The last step consists of washing the resulting solid with pure 
acetone, directly on a Buchner filter, followed by drying at air 
overnight. 
The product prepared according to this Example will be referred to 
hereinafter as CLA-1.0. 
A similar cross-linked polymer was obtained by substitution of 
2,3-dibromopropanol for epichlorohydrin. 
EXAMPLE 2 
By proceeding in the same manner as in Example 1 and replacing the 0.1 g of 
epichlorohydrin by 0.4; 0.75; 1.2 and 2.0 g, there is obtained 
corresponding cross linked products hereinafter identified as CLA-4.0, 
CLA-7.5, CLA-12, and CLA-20. 
EXAMPLE 3 
Preparation of Tablets. 
Anhydrous theophylline reagent grade (Baker) is mixed (10% w/w) with 
cross-linked amylose in Turbula shaking mixer for about 10 min. Tablets 
weighing about 500 mg each are obtained by compression in Carver hydraulic 
press at more than 2.4 T/cm.sup.2, with a 1.26 cm diameter and thickness 
of 2.9 mm. The same procedure can be applied for different amounts of 
theophylline in the tablets. For example, tablets containing 10, 20, 30, 
40, 50 and 60% w/w of theophylline were prepared. 
EXAMPLE 4 
`In vitro` drug release results. Equilibrium swelling 
Equilibrium swelling of various types of CLA polymeric powder, CLA drug 
free tables and CLA containing 10% theophylline tablets are measured in 
water and various aqueous solvents at room temperature. 
The tablets or the equivalent mass of 500 mg of CLA powder are placed in a 
50 mL graduate cylinder to which 50 mL water or aqueous solvents are 
added. After 96 hours, the equilibrium swelling volume was read directly. 
The swelling is expressed as swollen volume per weight unit of initial dry 
material (mL/g). The swelling volume of the powders in water varied from 
12 to 42 mL/g. 
Analysis of theophylline release from CLA tablets 
Theophylline release data are analyzed using the equation proposed by 
Peppas in Pharm. Acta Helv. 1985, 60, 110-111: 
##EQU1## 
TABLE I 
______________________________________ 
Analysis of diffusional release mechanism 
Diffusional Time 
release Overall solute release 
dependence of solute 
exponent (n) 
mechanism release rate (dMt/dt) 
______________________________________ 
n = 0.5 Fickian diffusion 
t.sup.-0.5 
0.5 &lt; n &lt; 1.0 
Anomalous diffusion 
t.sup.n-1 
n = 1 Case II transport 
zero-order (t.sup.0) 
n &gt; 1 Super case II transport 
t.sup.n-1 
______________________________________ 
In the above Table I, the Fickian diffusion is controlled by Fick's laws, 
and the release is in a hyperbolic function upon the time, and linear in 
function of t .sup.1/2. In the anomalous diffusion, the release curve upon 
the time is somewhat between the hyperbolic and linear dependency. In Case 
II transport, the release curve is linear in function of the time, whereas 
Supercase II transport is for an exponential function of the release upon 
the time. 
Table II shows examples of theophylline release from tablets initially 
containing 10 to 60% theophylline in CLA. Analysis of solute release data 
for CLA-1.0, CLA-4.0 and CLA-7.5 between t.sub.0 (t.sub.0 =initial time) 
and t 90 (corresponding to 90% of total theophylline release) are also 
presented in Table II. 
TABLE II 
______________________________________ 
Kinetics parameters of 90% release of the initial 
theophylline amount 
Drug Release Kinetic Kinetic 
content time(hours) parameter 
parameter 
Type of gel 
% M.sub.t /M.sub..infin. = 90% 
k* n* 
______________________________________ 
Amylose 10 1.2 0.785 0.876 
CLA-1.0 10 37.0 0.129 0.537 
CLA-1.0 50 32.0 0.096 0.634 
CLA-1.0 60 24.0 0.013 0.551 
CLA-4.0 10 25.0 0.126 0.634 
CLA-4.0 40 24.0 0.162 0.531 
CLA-4.0 50 13.0 0.283 0.423 
CLA-7.5 10 11.5 0.182 0.662 
CLA-7.5 20 11.5 0.161 0.702 
CLA-7.5 30 7.0 n.d. n.d. 
CLA-12.0 
10 1.3 0.760 0.890 
CLA-20.0 
10 0.7 1.330 1.210 
______________________________________ 
*Kinetic parameters from Peppas equation 
n.d. = No representative 
These data are obtained when release of the theophylline occurs from all 
faces of the tablet. Most values of n are ranging between 0.5 and 1, which 
is indicative of an anomalous solute release mechanism, as illustrated in 
Table I. Indeed, the theophylline release from the CLA tablets of the 
present invention follow a generally close to linear dependency upon time, 
which is a characteristic of anomalous release type. Since there is no 
glassy/rubbery transition in the polymer used in accordance with the 
present invention, the deviation from a fickian behaviour cannot be 
explained by the `swelling control` release mechanism as described in 
S.T.P. Pharma, (supra). 
The theophylline powder is mixed with polymeric granules prior to 
compression. Thus the tablet consists in an agglomerate of polymeric 
granules surrounded by theophylline. This is completely different from the 
`swelling control system` in which the drug is molecularly dispersed into 
a glassy polymer which turns into a rubbery one upon solvent penetration. 
When water penetrates into the tablet, the polymer hydrates and swells. 
CLA-1.0 has a low cross-linking degree and a small number of 
three-dimensional transversal glycerine bridges introduced by the 
crosslinking. As a consequence, it is assumed that an important number of 
hydrogen bounds can be created between neighbouring polymeric chains 
following the compression. The slow theophylline release from CLA-1.0 
tablets could therefore be attributed to a slow water penetration due to 
the presence of numerous intragranular hydrogens bonds. At higher 
cross-linking ratios, the higher density of glyceric bridges of a total 
length of 8.64 .ANG. (resulting from epichlorohydrin treatment) between 
adjacent amylose chains may prevent the network from coming near the 
distance necessary to form hydrogen bonds. Typically, this distance is 
about 5.6 .ANG.. 
On the other hand, following the introduction into water, the tablets made 
with CLA having a higher reticulation degree, e.g. CLA-12 and CLA-20, were 
totally disaggregated over a period of approximately 90 minutes. 
Consequently, theophylline release was faster and closer to linearity, 
with k=0.76 and n=0.890 for CLA-12 and with k=1.33 and n=1.21 for CLA-20 
respectively (Table II). 
It is interesting to note that small changes in the reticulation degree, 
for instance between CLA-7.5 and CLA-12, with swelling volumes of powders 
of 17.2 and 14.4 mL/g respectively, produce important differences in 
release kinetics, i.e. the release time decreased from 15 to 2 hrs. 
However, differences of the same order in the reticulation degree of 
CLA-12 and CLA-20, with swelling volumes of powders of 14.4 and 12.0 mL/g 
respectively, have no significant effect on the release kinetics, that is 
about 2 hours in both cases. These data confirm the importance of the 
hydrogen associations in the case of CLA-1.0, CLA-4.0 and CLA-7.5 tablets. 
Accordingly, one can assume that at higher cross-linking degree of amylose, 
fewer hydrogen bonds are formed, thus not allowing good cohesion of the 
tablet. Upon swelling, individual polymeric granules separate and the drug 
is released too fast. 
It is assumed that in the case of CLA-1.0, CLA-4.0 and CLA-7.5, hydrogen 
bonds are formed within polymeric granules as a result of the compression 
effort, ensuring good cohesion in the tablets, even under swollen state. 
This cohesion plays an important role in the control of the water 
penetration rate and in preventing the tablet from a rapid disintegration. 
It is therefore assumed that the drug release is controlled partly by the 
water penetration forming new water-amylose hydrogen associations, which 
can even replace the amylose-amylose interchains hydrogen bonds. This 
behaviour plays a role in the deviation from the direct dependency of 
fickian diffusion mechanism. The presence of strong intragranular hydrogen 
bonds in the CLA-1.0, CLA-4.0 and CLA-7.5 tablets are confirmed by 
measuring equilibrium swelling of CLA in tablets and powder, using water 
and 8 M urea respectively, to demonstrate the formation of interchain 
hydrogen bonds. Table III illustrates the equilibrium swelling of various 
cross-linked amylose. 
TABLE III 
______________________________________ 
Equilibrum swelling of cross-linked amylose 
(Swollen gel volume/initial dry polymer weight) 
Equilibrum Equilibrum Ratio 
swelling in 
swelling (8M 
swelling (8M 
Material water (mL/g) 
urea) mL/g urea/water) 
______________________________________ 
CLA-1.0 Powder 
42.0 n.d. n.d. 
CLA-1.0 Tablet 
2.0 n.d. n.d. 
Ratio* P/T 21.0 
CLA-4.0 Powder 
22.0 34.0 1.5 
CLA-4.0 Tablet 
5.0 26.0 5.2 
Ratio* P/T 4.4 
CLA-7.5 Powder 
17.2 23.0 1.3 
CLA-7.5 Tablet 
5.0 24.0 4.8 
Ratio* P/T 3.4 
CLA-12 Powder 
14.4 24.0 1.6 
CLA-12 Tablet 
12.4 24.0 1.9 
Ratio* P/T 1.16 
CLA-20 Powder 
12.0 24.0 2.0 
CLA-20 Tablet 
10.4 24.0 2.3 
Ratio* P/T 1.15 
______________________________________ 
n.d. = No deposition of the gel 
##STR1## 
As shown in Table III, the swelling volumes of all CLA tablets and powder 
were higher in 8M urea than in water. For CLA-1.0 and CLA-4.0, the ratio 
P/T was higher than for all other CLA types, supporting the hypothesis of 
interchain hydrogen association following the compression. For each 
cross-linking degree, equilibrium swelling in water was more important for 
powders than for tablets, probably because of the new intragranular 
hydrogen bonds created by compression. Thus, the ratio of swelling in 8M 
urea/water was significantly more pronounced in the case of lower 
reticulated CLA tablets which develops more intragranular hydrogen bonds 
than amylose having a higher reticulation degree. For instance, the values 
of the ratio swelling were 5.2 for CLA-4.0 tablets, and then decreasing to 
1.9 for CLA-12 and to 2.3 for CLA-20 tablets. Therefore, these results are 
consistent with the hypothesis that tablets of lower cross-linking of 
amylose show good cohesion because of strong intragranular hydrogen bonds. 
Since granullometrics and compression forces were also similar, tablets 
should present no significant difference in consistence. It is therefore 
unlikely that the observed difference in theophylline release kinetics 
could result from the effect of porosity on water penetration kinetics. 
Furthermore, the specific volume (42 mL/g) of dried CLA-1.0 powder was the 
highest, while the swelling of CLA-1.0 tablets was the lowest, and the 
theophylline release from this type of product was the slowest. Therefore, 
these results confirm that hydrogen associations are implied in the 
release control, rather than the porosity of CLA. 
The influence of theophylline content in tablets on the drug release 
kinetics was also studied. In all cases, the yield of release was more 
than 95% in various kinetic conditions, between t.sub.0 and t.sub.90 of 
total theophylline release, depending on the reticulation degree and the 
drug content of the tablet. The release kinetics were partially similar at 
about 10-20% of theophylline for CLA-7.5; 10 to 40% of theophylline for 
CLA-4.0 and 10-50% of theophylline for CLA-1.0. This observation is 
consistent with the fact that these tablets are neither behaving like 
conventional hydrophillic matrix nor like `swelling-controlled` systems. 
Indeed, in this last case, doubling the drug concentration would have 
modified the release kinetics more extensively. At the maximum capacities 
of drug content in the tablets, the release was much faster and showed 
irregular kinetics (Table II). Tablets of CLA-1.0, CLA-4.0 and CLA-7.5 
containing a drug concentration higher than the maximum capacity were also 
partially disaggregated. The high amount of drug between polymeric 
granules probably caused insufficient cohesion, therefore leading to 
partial desaggregation. 
Finally, the release of theophylline from pure amylose tablets was studied. 
The release is fast (1-2 hours when k=0.785, n=0.876), accompanied by 
erosion of the tablet. Furthermore, the release of theophylline from non 
crosslinked amylose tablets is quite similar to the release from CLA-12 
and CLA-20 tablets. This fast release in the case of non cross-linked 
amylose demonstrates the importance of the particular three-dimensional 
structure, in which the interchain hydrogen bonds play a major role in the 
drug release rate. 
From this dependence between the release time and the reticulation degree, 
evidence is given that for the low reticulation degrees, between 0 and 
10%, the interchain hydrogen bonding is maximal, and seems to participate 
in the control of the theophylline release.