Coagels from alkanoyl-6-O-ascorbic acid derivatives as drug carriers: Structure and rheology

Article abstract of DOI:10.1016/j.farmac.2003.07.010

6-O-Ascorbic acid alkanoates (ASCn, where n is the number of carbon atoms in the alkyl chain) behave as surfactants and form stable supramolecular assemblies in water, depending on chemical structure, concentration and temperature. In concentrated water dispersions, ASCn form liquid crystalline structures ('coagels'), below the critical micellar temperature (CMT), with a typical Krafft phenomenon. Such semisolid systems incorporate and stabilize drugs like anthralin, which is insoluble and unstable in aqueous media. The rheological behavior of coagels obtained from aqueous ASC8, ASC10, ASC11, ASC12, ASC14 and ASC16 was evaluated and related to the coagel structure. For ASC8, ASC12, ASC14 and ASC16 complex rheology was observed and spur values were determined. This behavior is indicative of a high three-dimensional structure. The spur value represents a sharp point of structural breakdown at low shear rate. At this point the semisolids acquire pseudoplastic flow with a very low viscosity. Instead, ASC10 and ASC11 coagels showed pseudoplastic flow and - in the case of ASC11 - thixotropy was observed. The ASCn coagel rheological behavior and their capability to load pharmacologically active compounds point to a potentially valuable capacity for such systems as drug carriers.

Full text of DOI:10.1016/j.farmac.2003.07.010

Il Farmaco 58 (2003) 1271ꢀ1276
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www.elsevier.com/locate/farmac
Coagels from alkanoylꢀ
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6-O-ascorbic acid derivatives as drug carriers:
structure and rheology
Santiago Palma a, Alvaro Jime´nez-Kairuz a, Laura Fratoni b, Pierandrea Lo Nostro b,
Ruben Manzo a, Daniel Allemandi a,*
a Departamento de Farmacia, Fac. de Ciencias Qu´ımicas, Universidad Nacional de Co´rdoba, Ciudad Universitaria, 5000 Co´rdoba, Argentina
b Dipartimento di Chimica, Universita` di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino, Florence, Italy
Received 20 January 2003; accepted 26 July 2003
Abstract
6-O-Ascorbic acid alkanoates (ASCn, where n is the number of carbon atoms in the alkyl chain) behave as surfactants and form
stable supramolecular assemblies in water, depending on chemical structure, concentration and temperature. In concentrated water
dispersions, ASCn form liquid crystalline structures (‘coagels’), below the critical micellar temperature (CMT), with a typical Krafft
phenomenon. Such semisolid systems incorporate and stabilize drugs like anthralin, which is insoluble and unstable in aqueous
media. The rheological behavior of coagels obtained from aqueous ASC8, ASC10, ASC11, ASC12, ASC14 and ASC16 was
evaluated and related to the coagel structure. For ASC8, ASC12, ASC14 and ASC16 complex rheology was observed and spur
values were determined. This behavior is indicative of a high three-dimensional structure. The spur value represents a sharp point of
structural breakdown at low shear rate. At this point the semisolids acquire pseudoplastic flow with a very low viscosity. Instead,
ASC10 and ASC11 coagels showed pseudoplastic flow and*
/
in the case of ASC11*thixotropy was observed. The ASCn coagel
/
rheological behavior and their capability to load pharmacologically active compounds point to a potentially valuable capacity for
such systems as drug carriers.
´
# 2003 Editions scientifiques et me´dicales Elsevier SAS. All rights reserved.
Keywords: Surfactants; Ascorbyl derivatives; Coagel; Rheology; Drug carrier
1. Introduction
micellar temperature (CMT) [4]. If the dispersions are
cooled down below the CMT, a semicrystalline meso-
phase (coagel) is obtained. Upon heating, coagels may
form either liquid and homogeneous micellar solutions
6-O-Ascorbic acid alkanoates (ASCn, Scheme 1)
behave as amphiphilic molecules in water. They possess
both a hydrophobic moeity (aliphatic chain) and a polar
group (ascorbic acid) [1,2].
These derivatives were synthesized in our laboratories
[3] with the main aim of preparing vitamin C-based
amphiphiles that combine the powerful antioxidant
properties of ascorbic with sufficiently good solubility
in lipophilic media.
The solubility of ASCn decreases with the alkyl chain
length, and increases with temperature. These com-
pounds form transparent dispersions in water, above
the critical micellar concentration (CMC) and critical
(n 5
/
10), or gels (n ]11), depending on chain length n,
/
temperature, and the addition of other solutes. In a
recent paper we investigated the phase behavior of
coagels as a function of the alkyl chain length, and in
the presence of sucrose and urea [5]. The effect of
different salts is now being tested on ASC12 coagels.
Further, these coagel systems are able to increase
substantially the apparent solubility [6], and the stability
of certain drugs [7]. They therefore present a promising
tool as drug carriers in pharmaceutical dosage formula-
tions.
In this paper we evaluate the rheological behavior of
coagels obtained from different ascorbylꢀ
(nꢁ8, 10, 11, 12, 14, 16). We interpret the results in
terms of their structural features.
/
alkanoates
/
* Corresponding author.
E-mail address: dalemand@dqo.fcq.unc.edu.ar (D. Allemandi).
´
0014-827X/03/$ - see front matter # 2003 Editions scientifiques et me´dicales Elsevier SAS. All rights reserved.
doi:10.1016/j.farmac.2003.07.010

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to a Shimadzu UV-160-A spectrophotometer. All mea-
surements were performed three times.
3. Results and discussion
ASCn coagels and gels can be obtained through a
sequence of heating-cooling cycles of the aqueous
suspensions around the CMT. In previous work, we
determined the CMT values for ASC8, ASC10, ASC11,
ASC12, ASC14 and ASC16 using differential scanning
calorimetry (DSC) and conductimetry. The results are
summarized in Table 1 [8].
Scheme 1.
2. Material and methods
2.1. Materials
Alkanoylꢀ
/
6-O-ascorbic acid esters (ASCn, with nꢁ8,
/
3.1. Preliminary examination of the coagels
10, 11, 12, 14 and 16) were made in our laboratories.
The procedure [3] uses condensation between the
corresponding carboxylic acid (H3C(CH2)nꢂ2COOH)
Qualitative information can be gained by observing
the macroscopic behaviour of the coagels. Coagels
obtained from ASC8, ASC12, ASC14 and ASC16
remain at the bottom of the container even when the
container is inverted upside-down and shaken. By
contrast coagels from ASC10 and ASC11 show an
apparent pseudoplastic flow and can be easily poured
from the vial after shaking. All coagels decreased in
apparent consistency with time on rubbing, suggesting
thixotropy or irreversible shear breakdown. Before
shear stress, coagels from ASC8, ASC12, ASC14 and
ASC16 appear homogeneous. No phase separation is
and the C6ꢀOH primary group of L-ascorbic acid, in
/
sulphuric acid at 40 8C. Purity was assessed through
TLC and elemental analysis. All reactants (analytical
grade) were purchased from Fluka (Milan, Italy), and
used without further purification. Bidistilled water was
purified with a MilliQ apparatus.
2.2. Rheology
Coagels were prepared by heating ASCn aqueous
suspension above the phase transition temperature, the
samples were then placed into the sensor cup and kept
for three hours at 8 8C and then assayed. Rheological
assays were performed at 20 8C in a Haake (Karlsrube,
Germany) viscometer VT500 equipped with a VT500/
VT 3.01 software, and an NV sensor. The curve fittings
and statistical analyses were performed using a Microcal
Origin† version 3.5.
observed, even after a long period (ꢀ
shear stress is applied, macroscopic structural changes
can be observed. After a brief pause (B30 minutes), free
/
7 days). When
/
water is released and phase separation occurs. Unlikely
as it seems, coagels from ASC10 and ASC11 form
semisolid systems where phase separation is observed
before shear stress and the formation of the coagel is
faster than for the other derivatives.
Structural differences can be tracked also at a
microscopic level. Fig. 1 shows the ESEM micrographs
of coagels from ASC8, ASC10, ASC12 and ASC16. For
ASC8 and ASC10, the coagel structures can be depicted
as a planar continuous lamellar phase, where bilayers
are ordered in sheets [9]. However, important size
differences in the plates are observed (see Fig. 1a and
1c).
2.3. ESEM
Environmental scanning electron microscopy experi-
ments were performed on coagels using an XL 30 ESEM
Philips apparatus at 10 kV, at about 10 8C, with a
relative humidity ranging between 45 and 85%.This
corresponds to a pressure of 2.9ꢀ6.0 mm Hg.
/
Table 1
CMT values for the phase transitions of coagels of ASCn in water
2.4. Drug incorporation into coagels
n
CMT (8C)
ASCn suspensions at different concentrations were
heated above the CMT, until the gel phase was formed.
Then an excess of anthralin was incorporated into the
samples.
After 2 h, the samples were filtered and system was
permitted to reach room temperature. Then, a weighted
amount of the loaded coagel was placed into a
volumetric flask, diluted with ethanol and transferred
DSC
Conductivity
8
18.5
34.5
40.0
47.3
56.0
63.8
19.3
36.8
38.5
44.5
54.2
61.3
10
11
12
14
16

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3.2. Continuous shear analysis
Newtonian flow. The differences are directly related to
their structure.
The rheological behavior of the coagels was evaluated
by recording the shear rate vs. shear stress flow curves.
The results are shown in Fig. 2. All coagels show a non-
Coagels from ASC10 and ASC11 (Fig. 2b and c)
showed pseudo-plastic flow, and in the case of ASC11
thixotropy was detected as well.
Fig. 1. ESEM for ASCn coagels.

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Fig. 2. Rheograms for ASCn coagels: (a) ASC8; (b) ASC10; (c) ASC11; (d) ASC12; (e) ASC14; and (f) ASC16.
Instead, coagels from ASC8, ASC12, ASC14 and
ASC16 exhibit a much more complex rheology (Fig.
2a,dꢀf). This behavior is characteristic of highly struc-
tured semisolids and several reports for a wide variety of
systems can be found in the literature [10ꢀ12]. The
rheograms form a hysteresis loop in which the ‘down’
curve is displaced to the left of the ‘up’ curve. A
characteristic feature of many such rheograms is the
presence of a ‘spur’ point on the ascending curve. The
rheology of these coagels displays a high yield or spur
value. This can be taken as a measure of the strength
necessary to break the ordered structure of the system
before a significant flow occur. The shear stress at spur
point is called the ‘static yield value’ (sS) [12].
/
Moreover, a ‘dynamic yield value’ (sD) can be
obtained by extrapolating the linear portion of the
down curve to the shear stress axis. sD measures the
energy input that is necessary to achieve a constant t/D
ratio at which the structure has been partially or totally
disrupted during the up curve stress [12]. Likewise, in
general terms, the loop area (LA) may indicate the
amount of structure breakdown that takes place during
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Table 2
Area under curve (LA, from Fig. 2), static yield value (sS) and
dynamic yield value (sD) for ASCn coagels
Coagel from
LA
sS (Pa)
sD (Pa)
ASC8
1253
1574
2977
643
6.24
8.71
0.42
1.06
1.02
0.18
ASC12
ASC14
ASC16
29.29
6.86
shearing. The values of sS and sD, for coagels from
ASC8, ASC12, ASC14 and ASC16 are reported in Table
2.
Fig. 3. ESEM for ASC 12 coagel after shear stress (t ]sS).
/
For such systems, an irreversible structure breakdown
is observed when the shear stress exceeds sS. The
original coagel structure is not restored on rest. The
values observed for sS are proportional to the alkyl
chain length for ASC8, ASC12 and ASC14. The
relatively low value for ASC16 coagel indicates a less
structured network. The semisolids exhibit a pseudo-
responsible for the formation of a three dimensional
structure (see Fig. 1a and c). Coagels from ASC12,
ASC14 and ASC16 behave similarly to ASC8 samples.
However, ESEM images (Fig. 1eꢀh) show a very
/
different arrangement of the lamellae that form a ‘house
of cards’ structure. Swelling and strengthening of the
semisolid network is owed to the presence of strongly
bound water regions between the amphiphilic bilayers.
Instead, for ASC10 and ASC11 coagels, this kind of
arrangement are apparently not possible. When coagels
of ASC12, ASC14 and ASC16 are subjected to shear
plastic flow at t ]sS.
/
3.3. Rheological behavior versus microstructure of
coagels
stress (t ꢀsS), the structure breaks irreversibly and the
/
The structural differences observed in ESEM images
(see Fig. 1) can be related to the different rheological
behavior of ASCn coagels. For ASC10, the coagel has a
typical lamellar structure, with flexible, parallel bimole-
cular sheets [13]. These account for a pseudoplastic flow
behavior.
A similar structure is observed for ASC8 as well.
However this system shows a more complex rheology
with a spur point. A reasonable explanation for this
finding is the significant size difference of ASC8 coagel
plates, with respect to those of ASC10 that are
viscosity decreases significantly. The sample acquires a
less ordered pattern, similar to that of ASC10. This
phenomenon can be clearly observed with the ESEM
micrograph of Fig. 3, where the structure of an ASC12
coagel after shear stress is depicted. This picture is
similar to Fig. 1c, indicating that the ASC12 coagel
acquires a pseudoplastic behavior after shear stress as in
the case of ASC10 and ASC11 coagels. Such rheology is
a characteristic feature of some coarse systems com-
monly used in pharmacy [14].
Fig. 4. Rheogram for anthralin loaded ASC14 coagel.

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S. Palma et al. / Il Farmaco 58 (2003) 1271ꢀ1276
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Finally, taking into account the potential utilization
carriers, especially for rather hydrophobic and easily
degradable products.
of these semisolids systems as drug carriers, the incor-
poration of an antipsoriatic drug (anthralin) [15] was
evaluated as well as its influence in the rheology of the
coagels.
The amount of anthralin loaded into ASCn coagels
increased linearly with n. However, the load capacity is
Acknowledgements
Partial financial support from Consejo Nacional de
Investigaciones Cient´ıficas y Te´cnicas (CONICET) and
the Consorzio Interuniversitario per lo Sviluppo dei
Sistemi a Grande Interfase (CSGI), Ministero dell’Is-
truzione, dell’Universita` e della Ricerca (MURST), is
greatly acknowledged.
higher for nꢀ12 [16]. This behavior could be attributed
/
to the gel configuration of ASC14 and ASC16 disper-
sions at temperature above TMC [8], where lamellar
configuration would permit higher drug solubilization
comparatively to micellar dispersions of ASCn with
lower molecular weight.
Regarding the rheological properties of anthralin
loaded coagels, the drug incorporation have no effect
on the semisolid structure. In Fig. 4 the rheogram of
anthralin loaded ASC14 coagel is depicted. As it occurs
with ASC14 unloaded coagel (Fig. 2e), a spur value is
observed and it is indicative of a high structured liquid
crystal. This fact permit to infer that the solubilization
of drug in the lipophilic portion of the coagel do not
affect the interaction between the solvent (water) and
the hydrophilic portion of ASCn.
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