Available online at www.sciencedirect.com
Microporous and Mesoporous Materials 107 (2008) 149–160
www.elsevier.com/locate/micromeso
Hydrotalcite-like compounds: Versatile layered hosts
of molecular anions with biological activity
Umberto Costantino
a
a,*
, Valeria Ambrogi b, Morena Nocchetti a, Luana Perioli
b
CEMIN ‘‘Centro di Eccellenza Materiali Innovativi Nanostrutturati’’, Dipartimento di Chimica Università di Perugia Via Elce di Sotto,
8-06123 Perugia, Italy
b
Dipartimento di Chimica e Tecnologia del Farmaco, Università degli Studi di Perugia, Via del Liceo, 1-06123 Perugia, Italy
Received 22 November 2006; received in revised form 22 December 2006; accepted 1 February 2007
Available online 9 February 2007
Abstract
This paper reviews a systematic work, mainly performed in our laboratories, on the intercalation into hydrotalcite-like compounds
(HTlc) of anti-inflammatory drugs, sunscreen and antimicrobials species. The paper is divided in three parts, the first part being
addressed to the synthesis, structural aspects and intercalation properties of HTlc, the second part to the properties of the intercalation
compounds with biologically active molecular anions, the third part to the preparation of polymeric nanocomposites for biomedical
applications. Ion exchange procedures have been used to obtain the intercalation compounds of MgAl-HTlc with ibuprofen, diclofenac,
indomethacin, ketoprofen and tiaprofenic acid and to study their release in solutions simulating biological fluids. Some of these intercalation compounds have been tested to improve the solubility of poorly water-soluble drugs such as indomethacin, ketoprofen, tiaprofenic acid and flurbiprofen. MgAl-HTlc and ZnAl-HTlc have been used as starting materials to prepare new sunscreen products, in which
organic filters are entrapped between HTlc lamellae, in order to improve ‘‘solar product’’ effectiveness and safety. The preparation of
polymeric composites constituted by HTlc exchanged with the anti-inflammatory diclofenac or the antimicrobial drugs and polycaprolactone are also described.
Ó 2007 Elsevier Inc. All rights reserved.
Keywords: Hydrotalcites; Intercalation chemistry; Sunscreen; Anti-inflammatory drugs; Polymeric nanocomposites
1. Introduction
The present paper deals with innovative applications of
layered materials in pharmaceutical, phytopharmaceutical
and cosmetic formulations. Essentially, the work done over
10 years and still in progress in our laboratories will be
reviewed. This research work started on the consideration
that the interlayer region of a lamellar solid capable of
intercalation may be considered a container of nanometric
dimensions, at least when the gallery height is considered,
where intercalated guest species are stored, often protected
from oxidation or photolysis. The guest species can be
withdrawn for use by a chemical signal, i.e., by a deinterca-
*
Corresponding author. Fax: +39 075 5855566.
E-mail address: [email protected] (U. Costantino).
1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.micromeso.2007.02.005
lation process. When the guest species are drugs, the above
considerations match the current trend of pharmaceutical
technology that requires formulations able to (i) modify
drug pharmacokinetics; (ii) maintain pharmacologically
active drug levels for long periods, avoiding repeated
administrations; (iii) vehicolate the drug release in its pharmaceutical target; (iv) increase the solubilization rate and
bioavailability of drug. Intercalation compounds of biocompatible layered hosts with biologically active species
could thus provide materials to design systems for modified
drug release. Hydrotalcite-like compounds (hereafter indicated as HTlc) based on Mg and Al hydroxycarbonates
are the most suitable layered hosts to test this idea, because
of their biocompatibility, and because they are already
used in medicine for their antiacid and antipepsin activity.
Early results of our researches appeared in proceedings of
thematic symposia before the end of last century and in a
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U. Costantino et al. / Microporous and Mesoporous Materials 107 (2008) 149–160
monograph [1,2]. However, the topic was mature in the scientific community and much work has been done by using
hydrotalcite as host for vitamins, such as ascorbic acid and
retinoic acid [3]; non-steroidal anti-inflammatory drugs
(NSAID), such as salicylate, naproxen [4] and diclofenac
[5]. Moreover, intercalation compounds with pesticides
and plant growth regulators [6] for agriculture application
and HTlc-DNA hybrid composites as non-viral inorganic
vectors [7] have been prepared.
Another field of application of HTlc in rapid expansion
resides in the preparation of novel nanocomposites based
on exfoliated inorganic layered materials and organic polymers. The composites obtained exhibit an interesting combination of chemical, physical and mechanical properties
when compared to those of the two separate components
[8]. The most part of literature concerns with montmorillonites and cationic clays, but recent papers suggest the use of
organically modified HTlc as suitable fillers of polymers
such as polycaprolactone [9], polyethylene [10], polystyrene
[11] and poly methylmetacrylate [12]. This use has allowed
new developments in the application of HTlc loaded with
bioactive species. In fact, it is possible to disperse them in
biodegradable polymers and obtain systems to be used as
films, membranes or fibres in biomedical field.
This review is divided in three parts. The first part recalls
preparation methods, structural aspects and reactivity of
hydrotalcites, the second part deals with the properties of
HTlc-intercalation compounds with anionic drugs and
UV-filters, the third part with the preparation of polymeric
nanocomposites.
2. Part 1
2.1. Synthesis and structural aspect of hydrotalcites
The synthesis, the structural characterisation, the intercalation properties and the applications of HTlc have been
the object of many recent monographs and reviews [13].
These compounds, also known as ‘‘anionic clays’’ or ‘‘layered double hydroxides’’ constitute a large family of
materials with general formula ½MðIIÞ1x MðIIIÞx ðOHÞ2 ðAn
x=n ÞmH2 O, where M(II) is a divalent cation such as
Mg, Ni, Zn, Cu or Co and M(III) is a trivalent cation such
as Al, Cr, Fe or Ga with An an anion of charge n such as
CO2
3 , Cl , NO3 or organic anion. The x value generally
range between 0.2 and 0.4 and determines the layer charge
density and the anion exchange capacity. The anions can be
exchanged by other inorganic, organic or metallo-organic
anions and even by biologically active molecules containing
ionisable acidic groups. Incorporation of anions containing
specific functionality into HTlc allows to prepare functional solids and this synthetic route has been applied to
prepare novel nanocomposite materials of interest in the
field of photo-physics and photo-chemistry [14], catalysis
[13b,15] and drug release [1,2].
Before to describe the different methods used to prepare
and modify the hydrotalcites, it seems useful to recall some
Fig. 1. Schematic illustration of the HTlc structure in carbonate form.
of their structural aspects. The structure of HTlc, shown in
Fig. 1, is similar to that of the mineral brucite (Mg(OH)2)
in which each layer being obtained by the edge concatenation of different Mg(OH)6 octahedra [16]. In HTlc structure
partial isomorphous replacement of the divalent cations by
trivalent cations occurs generating positive charges on the
sheet. The positive charges are balanced by anions located
in the interlayer region where also hydration water molecules are accommodated.
Different methods have been suggested to prepare
hydrotalcites, such as precipitation at constant pH (coprecipitation), precipitation at variable pH, hydrothermal synthesis, electrochemical methods, etc., each of them provides
different advantages [13b]. The systems reviewed in this
paper were prepared by urea method. Hydrotalcites having
a good crystallinity degree and micron-sized particles with
a narrow particle size distribution were obtained accomplishing a precipitation from ‘‘homogeneous’’ solution in
which urea is the precipitating reagent [17]. Lamellar solids
in carbonate form were obtained because of the high concentration of the carbonate anions in the solution, deriving
from the thermal decomposition of urea. In order to prepare intercalation compounds with guest anions different
from the carbonate one, various ways were adopted and
will be described in the following section.
2.2. Preparation of HTlc-intercalation compounds with
biological active anions
The first way to obtain hydrotalcite intercalation compounds is to perform an anion exchange reaction. In
U. Costantino et al. / Microporous and Mesoporous Materials 107 (2008) 149–160
151
Fig. 2. (A) Anion exchange isotherms of MgAl-HTlc-Cl toward DIK ( ), IBU (m) and of MgAl-HTlc-NO3 toward TIAP (j). Experimental conditions:
Concentration 0.1 M, temperature 25 °C, reaction time 3 days. (B) X-ray diffraction patterns of the MgAl-HTlc at different exchange percentages of TIAP:
(a) 24.2%, (b) 46.8%, (c) 64.9%, (d) 94.1%.
designing the reaction the nature of the counterion originally present in the HTlc should be considered. The diffusion of anionic species with high steric hindrance into the
interlamellar region will be facilitated if the counterion is
little held and determines a large gallery height. Considering the selectivity scale for the most common counterions
2
[18], CO2
3 > SO4 OH > F > Cl > Br > NO3 >
ClO4 , HTlc containing chloride, or rather, nitrate anions
are the most suitable precursors for the uptake of biologically active species. In particular, the hydrotalcite in carbonate form (HTlc-CO3) was first converted in chloride
form (HTlc-Cl) by titration with HCl 0.1 M at constant
pH of 5 and then, the HTlc-Cl was equilibrated with an
aqueous solution of NaNO3 0.5 M (molar ratio
NO
3 =Cl ¼ 10Þ to obtain the nitrate form of the hydrotal-
cite (HTlc-NO3). The intercalation mechanism and the relative selectivity coefficient was studied both by means of
anion exchange isotherm and following the structural
changes occurring at different degrees of exchange.
Fig. 2A shows the anion exchange isotherms of MgAlHTlc-Cl toward diclofenac (DIK), ibuprofen (IBU) and
of MgAl-HTlc-NO3 toward tiaprofenic acid (TIAP), while
Fig. 2B shows the X-ray diffraction patterns of the MgAlHTlc at different exchange percentages of TIAP. It may be
seen that the drugs are exchanged with high selectivity and
that the ion exchange process occurs with a first order
phase transition from the Cl- or NO3-phase to the drug
phase.
If high steric hindrance prevents direct exchange, two
other procedures may be used to obtain the intercalation
Ion Exchange
M(II)Al-HTlc-CO3
(7.65Å)
HCl 0.1M
M(II)Al-HTlc-Cl
(7.75Å)
pH=5
NaNO3 0.5M
M(II)Al-HTlc-NO3
(8.9Å)
NO3/Cl=10
Solution of the guest
in saline form
M(II)Al-HTlc-Guest
Coprecipitation
M(II) + M(III) + Guest
solution
Aging
NaOH
HTlc-Guest
pH=9-10
HTlc-Guest
(with improved cristallinity)
Reconstruction procedure (Memory effect)
MgAl-HTlc-CO3
MgAlOx
450˚C
MgAlOx
Solution of the guest in acid form
CO2-free water
MgAl-HTlc-Guest
MgAl-HTlc-OH
Fig. 3. Scheme of experimental routes followed to obtain HTlc-intercalation compounds.
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U. Costantino et al. / Microporous and Mesoporous Materials 107 (2008) 149–160
H2 C
Cl
COOH
H3 C
CHCH2
H3 C
H
N
Cl
Diclofenac (DIK)
H3CO
Ibuprofen (IBU)
H2C COOH
N
CO
CH3
HC
COOH
CH3
H 3C
O
C
COOH
CH
Cl
Ketoprofen (KET)
Indomethacin (IND)
HO
O
S
CH3
O
COOH
CH3
F
Thiaprofenic Acid (TIAP)
Flurbiprofen (FLUR)
Fig. 5. XRPD of the indicated intercalation compounds.
Fig. 4. Structural formulae of the anti-inflammatory drugs.
compounds: the direct synthesis by coprecipitation and the
reconstruction of the calcinated hydrotalcites in presence of
selected guests. The direct synthetic procedure requires the
precipitation of the HTlc in the presence of the anionic
form of the guests. The chloride or nitrate M(II) and
M(III) salts are often used and dissolved in a solution containing the selected guest. Coprecipitation is performed at
pH between 9 and 10 by addition of NaOH solution. Well
crystallised samples are formed when the guests have a high
self-assembly tendency. In other cases, a hydrothermal
treatment of intercalates formed may improve the crystallinity of the products [19].
The second procedure, typical of MgAl-HTlc-CO3,
takes in advantage of the ‘‘memory effect’’ of the hydrotalcite calcinated at 300–500 °C. The calcinated solid, constituted of a mixture of magnesium and aluminum oxides, is
able to reconstruct the lamellar structure in water or in
solution of given anions [20,21]. When the regeneration
occurs in CO2-free distilled water the positive charge of
the lamellae will be balanced by OH ions. The interlayer
OH groups can be replaced by other anions via an
acid–base reaction with the corresponding species in acid
form. Otherwise, the reconstruction should be carried out
in a solution containing the guest in acid form in order
to have the direct intercalation of the guest. The scheme
of experimental routes followed to obtain intercalation
compounds is shown in the scheme of Fig. 3.
Drugs, belonging to the anti-inflammatory class
(NSAID), such as indomethacin (IND), TIAP, ketoprofen
(KET) [22], DIK [23], IBU [24] and flurbiprofen (FLUR)
[25] were selected to prepare intercalation compounds with
the biocompatible MgAl-HTlc. The structural formulae of
the drugs are shown in Fig. 4, the –COOH group confers
the acid properties to the molecules. Two synthetic procedures were adopted in order to obtain the nanohybrids:
ion-exchange reactions starting from the HTlc-Cl and
reconstruction of the HTlc structure. The best results, in
terms of crystallinity and ion-exchange percentage of the
intercalation compounds, were obtained with the former
procedure. The compounds were characterized by thermogravimetric analysis (TGA), X ray diffraction patterns
(XRPD), DSC, FT-IR and SEM. The composition and
the interlayer distance of the obtained materials are
reported in Table 1. Fig. 5 shows the XRPD of the intercalation compounds with the anti-inflammatory compared
with the pristine chloride form. The interlayer distances
of the compounds containing DIK, KET and TIAP are
very close because of their structural analogy, while the
more complex IND gives the higher interlayer distance
increase. Furthermore, the interlayer distance is consistent
with the presence in the interlayer region of a monolayer of
NSAID anions, partially interdigitated, with their principal
axis almost perpendicular to the layer plane. It was also
found that hydrotalcites show a marked preference for
these species, probably because of their high tendency to
aggregate as a compact monofilm in the interlayer region
(see Fig. 2A). As an example the computer-generated disposition of the DIK anions into the interlamellar region
of MgAl-HTlc is shown in Fig. 6.
Further investigations were performed considering
several UV absorbents: p-aminobenzoic acid (PABA)
[26], 2-phenyl-1H-benzimidazole-5-sulfonic acid (Euso-
Table 1
Composition, interlayer distance and drug loading of intercalation
products HTlc and anti-inflammatory
NSAID
Intercalation product
d
(Å)
Drug loading
(%)
Ref.
IND
KET
TIAP
DIK
IBU
[MgAl]aIND0.20Cl0.13 Æ 0.3H2O
[MgAl]KET0.27Cl0.06 Æ 0.4H2O
[MgAl]TIAP0.27Cl0.06 Æ 0.4H2O
[MgAl]DIK0.33 Æ 1H2O
[MgAl]IBU0.33 Æ 0.5H2O
25.7
22.7
22.7
23.6
21.7
50.4
50.0
50.3
55.0
50.0
[22]
[22]
[22]
[23]
[24]
a
[MgAl] = [Mg0.67Al0.33(OH)2].
U. Costantino et al. / Microporous and Mesoporous Materials 107 (2008) 149–160
153
Table 2
Composition, interlayer distance and drug loading of intercalation
products HTlc and sunscreen
Sunscreen Intercalation product
PABA
PABA
EUS
EUS
FER
m4BHF
d4BHF
a
b
c
Fig. 6. Computer-generated representation of HTlc-DIK: DIK ions are
arranged in the interlayer region to form a bilayer plane and are partially
interdigitated, with their principal axis almost perpendicular to the layer
plane.
lexÒ) (EUS) [27], ferulic acid (FER) [28] and 5-benzoyl-4hydroxy-2-methoxy-benzenesulphonate acid (4BHF) [29].
In Fig. 7 the structural formulae of these species are shown.
All the intercalation products were prepared by ion
exchange procedure by contact between MgAl-HTlc or
ZnAl-HTlc, in chloride or nitrate form, and sunscreen in
saline form (sodium salt solution); the compounds
obtained were characterized by XRPD, TGA, DSC, FTIR, SEM techniques. HTlc-Cl showed great affinity toward
EUS and FER anions while, in the case of PABA and
4BHF, the HTlc-NO3 resulted a better starting material.
Aluminum and sunscreen content in HTlc was investigated
and the formula was calculated for each compounds (Table
2): the sunscreen loading resulted always very high, reaching almost the stoichiometric value. The interlayer distances of the intercalation compounds, evaluated by
[MgAl]0.34 aPABA0.21(NO3)0.13 Æ H2O
[ZnAl]0.34 bPABA0.26(CO3)0.04 Æ H2O
[MgAl]034EUS0.23Cl0.11 Æ 0.2H2O
[ZnAl]0.34EUS0.29Cl0.05 Æ 0.9H2O
[MgAl]0.39 cFER0.22Cl0.17 Æ 0.7H2O
[ZnAl]0.34m4BHF0.22(NO3)0.11 Æ 1 H2O
[ZnAl]0.34d4BHF0.12(CO3)0.038(NO3)0.02
Æ 0.9 H2O
d
Sunscreen Ref.
(Å) loading
(%)
15.5
15.9
21.1
21.9
17.1
19.4
15.3
24.8
24.7
48.5
42.7
35.5
37.7
25.6
[MgAl]0.34 = [Mg0.66Al0.34(OH)2].
[ZnAl]0.34 = [Zn0.66Al0.34(OH)2].
[MgAl]0.39 = [Mg0.61Al0.39(OH)2].
XRPD (Fig. 8), were dramatically increased with respect
the pristine chloride (7.75 Å) or nitrate (9 Å) form. Considering that, in general, the molecular anions are oriented
between the sheets with the C SO
3 and C–COO bond
orthogonal to the layer plane the interlayer distance
increases with the increasing of the anion complexity.
Moreover, taking advantage of the bi-protic nature of
4BHF, the mono-anions (m4BHF), the di-anions
(d4BHF) or a mixture of mono and divalent anions have
been intercalated into the hydrotalcite and two different
phases were obtained (19.4 and 15.3 Å for the ZnAlm4BHF and ZnAl-d4BHF, respectively) [29]. SEM micrographs showed that intercalation process did not modify
the crystal size and morphological characteristics, in fact
both starting HTlc (chloride or nitrate forms) and all the
intercalation products had regular and well-formed hexagonal plates with similar size showing that neither aggregation process or crystal breakdown occurred after sunscreen
immobilization. It is interesting to note that the crystal
edges resulted less regular and less sharp in the case of compounds containing zinc. Fig. 9 reports as examples the
SEM images of EUS and PABA intercalation products in
comparison to those of the starting HTlc.
COOH
H
N
N
SO3H
NH2
EUS
O
PABA
OH
O
SO3H
OMe
OH
HO
OMe
4-BHF
FER
Fig. 7. Structural formulae of the UV absorbents.
[26]
[26]
[27]
[27]
[28]
[29]
[29]
Fig. 8. XRPD of the indicated intercalation compounds.
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U. Costantino et al. / Microporous and Mesoporous Materials 107 (2008) 149–160
Fig. 9. Scanning electron micrographies of the indicated samples.
3. Part 2
3.1. HTlc-intercalation compounds to improve drug solubility
About 30–40% of drugs now available on the market are
poorly soluble in biological fluids and in this case the dissolution process is the limiting step for drug bioavailability.
In fact, before absorption the drug must be solved in biological fluids and, if it is well absorbed by gastrointestinal
membrane, the passive absorption rate is controlled by
Fick diffusion law and depends on drug concentration in
biological fluids in the absorption site [30].
On the basis of these considerations, solubility is an
important physicochemical property for a drug because it
plays a key role in drug liberation, absorption and, as a
final result, in its bioavailability.
Recently, FDA Guidance for Industry (USA) [31] and
the CPMP Note for Guidance on the Investigation of Bioavailability and Bioequivalence (Europe) [32] have introduced the biopharmaceutic classification system (BCS).
This classification is a scientific framework which classifies
drug substances on their aqueous solubility and permeability [33]. According to BCS, drug substances are classified in
four groups:
Class 1: high solubility and high permeability; Class 2:
low solubility and high permeability; Class 3: high solubility and low permeability; Class 4: low solubility and low
permeability.
For drugs owning to the class 2 of BCS, the improving
of their apparent solubility [34] is an important aim for the
development of a suitable formulation for oral administration. Intercalation in inorganic matrices such as hydrotalcites, which can host molecularly dispersed drugs into
their structures (liquid shell model), could induce an
improvement of apparent solubility. At acid pH (less than
4) hydrotalcite quickly dissolves, releasing the drug in
molecular form promptly suitable for absorption. IND,
TIAP and KET, were chosen as models of poorly soluble
drugs in acid environment [22] and were intercalated in
MgAl-HTlc. The intercalation compounds were all
obtained with a good drug loading (ca. 50% that is
1000 mg of the intercalated compound contains ca.
500 mg of drug) (Table 1).
The solubility measurements of the drug from the intercalate were determined in a gastric juice with pH 1.2 (USP
25 at 37 °C) in order to mimic gastric environment and for
comparison solubility measurement of the drug from the
physical mixture (HTlc-Cl/drug) and the drug powder were
performed too. The best results were obtained for IND. In
fact, the drug concentration from the physical mixture and
from the crystalline drug powder was less than 2 mg/ml,
whereas measurements from the intercalate (Fig. 10),
showed that after 1 min the IND concentration was
12 mg/ml and after 15 min reached 14 mg/ml. This value
was maintained during the experiment (3 h). Apparent solubility enhancement was observed also for the other two
tested drugs and it was 1.3 and 1.6 times higher than that
from the crystalline drug for KET and TIAP, respectively.
The drug solubility increase could presumably be explained
Fig. 10. Indomethacin concentrations from the intercalation compound in
pH 1.2 gastric fluid.
U. Costantino et al. / Microporous and Mesoporous Materials 107 (2008) 149–160
155
by the lack in crystallinity of intercalated drugs which are
directly released in ionic form by dissolution of HTlc in
acidic medium. Recently good results have been obtained
with FLUR too: improvement of drug dissolution rate in
gastric medium and permeability through gastric mucus
were observed [25].
3.2. HTlc to obtain a controlled drug release
When the intercalation compound between hydrotalcite
and a drug is surrounded by intestinal environment at pH
6.8–7.5, the interlayer region of this lamellar host may be
considered a microvessel from which an anionic drug, previously immobilized, is released as a consequence of a deintercalation process. Thus hydrotalcite could be used as
a matrix for a new controlled release formulation.
Controlled release dosage forms which permit a prolonged drug release find relevant applications for drugs
with low half life and chronic therapy. This strategy in fact
can permit reduction of administrations (once a day drug),
of drug dosage and side effects with patient compliance
improvement. In the intestinal tract the drug release from
intercalated product is due to exchange of drug ions with
the phosphates, hydroxides, carbonates present in the
intestinal medium. Interesting results were reported when
the non steroidal antiinflamatory drugs, DIK [23] and
IBU [24], were intercalated in hydrotalcite. These drugs
are characterized by a short biological half life and are used
for the relief of rheumatoid arthritis and osteoarthritis
symptoms, for the treatment of acute flares and in longterm management of these diseases. Their use is often limited by the frequent side effects affecting the gastrointestinal
tract. These problems could be reduced by a formulation
able to control the drug release.
The in vitro dissolution studies showed that the drug
was released by a de-intercalation process due to the
exchange of the drug with the ions present in the dissolution medium. The drug release (Fig. 11) from HTlc-DIK
was performed in simulated intestinal fluid at pH
7.5 ± 0.1 and in a solution designed to mimic the ionic conditions of the small intestine (pH 7.0) [35].
At pH 7.5 the dissolution rate of DIK from HTlc-DIK
was slower than that from the physical mixture. The drug
release from the physical mixture was already complete
after 15 min, whereas the DIK release from intercalation
product due to exchange of DIK ions with the phosphates
of the medium, was 38% after 15 min, 60% after 90 min
and 90% after 9 h. The first rapid drug release (burst effect)
can be explained with the release of DIK anions taken up
on the surface of the microcrystals and those intercalated
in the external part of the lamellar structure. Then, the
slower release is due to the exchange of drug ions which
are in the internal part of the lamellae and have to diffuse
through the HTlc particles. As small species (phosphates)
exchange bigger ions (DIK), a consequent decrease of the
interlayer distance occurs. Thus the initial exchange of
anions of the external part of the crystals causes the forma-
Fig. 11. Release of DIK from HTlc-DIK and the physical mixture at pH
7.5 and 7.0.
tion of an external phase with smaller distance (boundary
phase) [36] with a consequent DIK release rate decrease.
When the dissolution test was performed at pH 7.0, the
DIK release from physical mixture was almost complete
after 15 min, whereas the drug release from HTlc-DIK
was 20% after 15 min, 40% after 2 h, 50% after 4 h, up to
a maximum of 70% at the end of the experiment (24 h).
This means that in this fluid the DIK release was slower
than that at intestinal simulated pH 7.5 buffer and that
DIK ions were not completely exchanged by the medium
ions. The slower DIK release rate can be explained with
the lower concentration of phosphates in this medium,
whereas the release decreasing can be due to the occurrence
of the grafting reaction between acidic H2 PO
4 anions,
which prevail at pH 7 on HPO2
4 anions, and the hydroxyls
of the layer [37]. In this strongly bound form, phosphates
are no more movable and can obstruct the exit of the drug
anions.
In the case of IBU intercalation product (HTlc-IBU) the
drug release profile, even if was not as slow as in the case of
DIK, was not immediate and showed that 60% of the drug
was released after 20 min and the 100% after 100 min.
This different behaviour can be explained by both the
greater affinity of DIK for HTlc (Fig. 2A) and its bigger
molecular size in comparison to IBU. In fact, after the
exchange of the ions present in the external part of the particle, the formation of an external phase with smaller interlayer distance can slow down more the release of DIK than
that of IBU, as this latter ion has a smaller size.
On the basis of these results HTlc may be used to prepare modified release formulations. Since HTlc dissolves
in strong acidic media, thus releasing immediately the intercalated drug in the stomach, the intercalation compounds
need to be protected by a proper enteric coating. The combination of HTlc, able to provide a sustained drug release
in the distal intestinal tract, and a polymer, which avoids
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U. Costantino et al. / Microporous and Mesoporous Materials 107 (2008) 149–160
Fig. 12. Scanning Electron Micrographies of Eudragit S/HTlc-DIK: (a) microparticles and (b) relative cross-section.
drug delivery in the upper gastrointestinal tract is a strategy
which can be develop in order to obtain a system for DIK
colonic delivery. Thus, a new composite system for colonic
delivery of diclofenac has been designed by microencapsulating HTlc-DIK in EudragitÒ S or EudragitÒ L (Fig. 12).
This will be described in a forthcoming paper [38].
3.3. HTlc-intercalation compounds to improve sunscreen
effectiveness and safety
The exposure to solar radiations can cause not only
short-term responses, as eritema and sunburn, but longterm effects as cutaneous photoageing, immunosuppression
and skin cancers [39–41] which are increasing in all the
world. In particular, the ultraviolet components may damage DNA and, thus, lead to mutagenic and carcinogenic
events [42]. Therefore, protection against UV radiations
has become an increasingly important issue for human
health and induced cosmetic industries to improve sunscreen products for providing wide spectrum coverage
and to asses a method to determine the sun protecting factor (SPF). Also public institutes [43,44] and textile industries [45,46] took care of this problem and studied sunprotective clothing that may help to prevent illness or even
death in the case of cancer patients [44].
Moreover, at this time, sunscreen use represents probably the easiest and most available way for consumers to
protect their skin from excess of solar radiation and to prevent some of the damages. Protection of the skin by either
radiation reflection (e.g., micronized titanium dioxide, zinc
oxide) or absorption by organic (UV) filters are preventive
measures against such toxic events. The possibility, however, exists that the sunlight transforms the absorbing molecules into reactive intermediates, with loss of protective
and screen power [47–49], which may also promote phototoxic or photoallergic contact dermatitis [50,51], DNA
damage and photocross reaction with other molecules as
drugs [52–54]. Therefore, the photochemical stability is
the most important characteristic of an effective and safe
solar cosmetic products also in consideration that sunscreens and their derivatives can be absorbed trough the skin
and can have systemic effects, can accumulate in the skin
with repeated applications. The phodegradation can be
prevented by:
1. presence of more protective factors, in order to obtain a
synergic protection,
2. photo-stable sunscreens at fixed wavelengths and
3. molecular encapsulation or complexation.
As regard the last point, many strategies have been proposed and the use of hydrotalcites showed to be a good
approach. Recently, in fact, we demonstrated that sunscreen intercalation in these inorganic materials could represent a new strategy to improve sunscreen photostability
and, at same time, to avoid a direct contact with skin
[26–29]. The aim of these researches was to immobilize
and to store the sunscreens in the interlayer region of a
lamellar host and HTlc containing magnesium and aluminum (MgAl-HTlc) or zinc and aluminum (ZnAl-HTlc) cations were chosen because their oxides are good ultraviolet
light absorber. The used sunscreens were PABA, EUS,
FER and 4BHF.
PABA was a very interesting molecule and first it was
chosen as sunscreen model because of its high photoinstability and photosensitizing properties. It is a UV-B absorber (200–313 nm) used as sunscreen component as early
as the 1920s and then lasted when dermatologists became
aware that it was a fairly photosensitizer. In fact, it tends
to cross-sensitize with other para-substituted molecules
[55] and it is responsible for many cutaneous problems
[56,57] and carcinogenic effect, so that, it has not been
employed in cosmetic formulations and in many solar
products it is reported ‘‘PABA-free’’ [55].
EUS is a widely used UV absorber, approved by European Community (EC) cosmetic law. After sunlight exposition it can produce some free radicals and active oxygen
species responsible of photoallergic reactions [58].
U. Costantino et al. / Microporous and Mesoporous Materials 107 (2008) 149–160
157
FER is a very promising substance for solar formulations because its properties. In fact, it provides a high
degree of skin protection acting as UV-absorber screen
and also reducing the UVB-induced erythema, because of
its high effectiveness in scavenging nitric oxide [59]. FER
sunscreen is not approved by EU cosmetic law but it is largely used in several countries, such as Japan, as food preservative and as an active ingredient in many skin
lotions, sunscreens and pharmaceutical preparations; Food
and Drug Administration (FDA) did not establish its status yet. It was chosen because its use is limited because
its low stability after exposure to air and light.
4BHF, EU sunscreen approved, is a bi-protic molecule
and mono-anion and di-anion forms have been intercalated
in ZnAl-HTlc obtaining two compounds (HTlc-m4BHF
and HTlc-d4BHF, respectively) with different filter loading
(Table 2).
Free sunscreens and intercalation products (same
amount of filter) were formulated and sunscreen release
from formulations was studied. A silicon cream [60], not
oily and water proof, was chosen as vehicle for the following requirements:
1. the least presence of water to prevent the filter deintercalation from hydrotalcite and consequently to avoid its
contact with the skin;
2. absence of dissolved ions, whose presence could cause
filter deintercalation from hydrotalcite via ionic
exchange during the cream preparation and storage;
3. consistency, spreading, cosmetic pleasantness, water
resistance as requested from a good sunscreen
formulation.
The in vitro releases have been performed in different
media and in all the cases the entrapped sunscreen has been
released from the creams in very low amounts, sometimes
negligible amounts, in comparison to free filter. PABA
in vitro release profile was reported (Fig. 13) as an example.
When the test was performed in water (a), PABA was
released from cream in a quantity of 20% after 15 min,
43% after 5 h and the release increased up to 80% only after
8 h. Intercalated PABA was released in very lower percentage. In fact, the release is less than 5% for the first hour,
less than 10% in 5 h and less than 12% in 8 h.
The same release differentiations were observed in the
presence of a phosphate buffer (b). The release of PABA,
when in free solid state, was 25% in 15 min, around 50%
in 6 h and increased up to 67% in 7 h while PABA release
from both intercalated compounds is around 20% after 3 h
and less than 31% after 8 h.
The non-intercalated PABA release from the cream in
sea water (NaCl salt solution) (c) was more than 30% after
15 min and rises to 50% after 2 h, while from the intercalated formulation was less than 10% after 6 h and less than
12% in 8 h. These results showed that, even after a bath in
sea water, the sunscreen was barely released while most of
it remains entrapped inside the inorganic host practically
Fig. 13. PABA release from creams in water (a), in phosphate buffer pH
5.5 (b) and in sea water (c).
avoiding any its skin contact. Very good results were
obtained also for 6 months aged formulations in the case
of 4BHF, but these data will be published furthermore.
Successively the free sunscreens, the intercalation products and their formulations have been investigated by spectrophotometric analysis in order to study the matrix
influence on sunscreen sunlight protection effect and on
their photostability by means UV–Vis absorption
spectroscopy.
The absorption spectra were recorded before and after
different irradiation times. In all the case, HTlc showed
to have protective and stabilizing properties for the used
organic filters; FER behavior is reported as example.
158
U. Costantino et al. / Microporous and Mesoporous Materials 107 (2008) 149–160
Fig. 15. Absorption spectra of HTlc-FER, physical mixture of FER and
HTlc-Cl (PHYS-MIX) and pure FER, recorded before (full line) and after
(dashed line) irradiation.
Fig. 14. Absorption spectra of pure solid FER, intercalated product
HTlc-FER, inorganic matrix HTlc-Cl and physical mixture of FER and
HTlc-Cl (PHYS-MIX).
To achieve information on UV–Vis radiation protection, absorption spectra of different FER samples were
recorded (Fig. 14). Figure shows HTlc-FER spectrum to
those of pure solid FER, inorganic matrix HTlc-Cl and
physical mixture prepared by mixing FER with HTlc-Cl.
The spectra obtained from HTlc-FER and FER were
rather similar in the range 250–400 nm and a shifting of
the maximum from 390 to 300 nm was observed. However
the HTlc-FER spectrum was slightly broader above
390 nm signifying that in the intercalation product specific
interactions occurred between the chromophore and the
matrix. This observation was further confirmed by comparing the spectra of the matrix alone, that did not present any
remarkable absorption contribution, and physical mixture
where no broadening was observed. Therefore, this analysis showed that the intercalation process led to a new nanostructered material with a slightly broader protection
range.
In Fig. 15, the spectra recorded before (full line) and
after irradiation (dotted line) at 366 nm for 210 min of
HTlc-FER, physical mixture and pure FER have been
reported. It can be noted that the HTlc-FER sample spectra did not present any significant change after irradiation.
On the other hand, irradiation of the physical mixture led
to decreased absorbance all over the spectral range indicating a degradation of the chromophore material. These
observations suggested that the solar screen was protected
from photodegradation when intercalated in the matrix.
For comparison, FER spectrum (aqueous solution)
before (full line) and after irradiation (dotted line) at
313 nm for 90 min has been reported; in these conditions
the irradiation led to the modification expected for a
trans–cis isomerization.
In order to investigate the possible interactions with
cream components also the sunscreen loaded formulation
have been submitted to UV–Vis radiation exposure;
absorption spectra and fluorescence spectra (fluorescence
intensity vs. irradiation time) were recorded (data not
reported). The loaded cream absorption was similar to
not formulated intercalation compounds and in any case
it was 2–3 fold higher [26,27]. This effect is due to the contribution of cream ingredients to the screening properties
of the intercalation compounds, leading to a higher protection factor. All the creams showed a reasonable
photostability.
The use of HTlc in solar formulations therefore offers
many advantages such as:
1. sunscreen stabilization;
2. absorption of ultraviolet lights in both the UV-A region
and the UV-B region;
3. absence of a close contact between skin and filter with
the consequent elimination of allergy problems;
4. revaluation of old molecules, not expensive, currently
not used.
With this assumption, a new protection model has been
designed.
4. Part 3
4.1. HTlc as additives in biodegradable polymeric matrices
In recent years, polymer/layered crystals nanocomposites have been recognized as one of the most promising
research fields in material chemistry because of their
enhanced mechanical, thermal, gas barrier and flame retardant properties, when compared to those of the pristine
polymer. Another interesting aspect of nanocomposites,
not yet developed, regards the possibility to confer to the
polymer, through the dispersion of selected nanostructured
compounds, special functionalities. Polymeric matrices in
the form of films, membranes or fibers with bio-medical
U. Costantino et al. / Microporous and Mesoporous Materials 107 (2008) 149–160
properties can be obtained by dispersion into the polymer
of intercalation compounds containing anti-inflammatory,
anti-microbical or anti-oxidant species.
For this purpose, the hydrotalcite containing DIK was
used as filler of a biocompatible and biodegradable polyester, i.e., polycaprolactone [61]. The studies on of mechanical
properties of composites, containing different amounts of
MgAl-HTlc-DIK, showed that the presence of the inorganic
filler in the polymeric matrix caused an improvement of
mechanical parameters except for the fracture. Moreover,
the composites processed as films were submitted to
in vitro release tests in a physiological saline solution
(0.9% NaCl). The DIK release was very promising for tunable drug delivery systems and it consisted of two steps: (1) a
first stage, in which the DIK was quickly released, very
likely, from the surface of the lamellae; (2) a second stage,
much lower ascribable to the anti-inflammatory deintercalation from the interlayer region of HTlc inside the polymeric
film. The amount of DIK that released from composite
materials depends on both the nature of the counter anion,
that have to be exchanged, and on the anions diffusion
through the polymer. Similar results were obtained by using
the antimicrobial drug chloramphenicol [62].
5. Conclusion
The scientific results presented in this review indicate
that layered compounds belonging to the family of biocompatible hydrotalcites are versatile hosts of a variety of
molecular anions with biological activity. Applications of
Mg–Al and Fe–Al in medicinal chemistry has already
reached the commercial area and many patents have been
issued, however, the development of systems for drug
release, for immobilization of sunscreens and antimicrobial
agents still requires additional fundamental and applied
research. The systems of drug release from nanocomposites
constituted by biodegradable polymers and HTlc-intercalation compounds is surely an argument of research for the
interest in biomedical applications.
Acknowledgements
The Authors wish to thank Prof. L. Latterini for the
measurements of the photochemical stability and FIRB
(RBNE017MB5), PRISMA 2005 and COFIN 2003 projects for funding these researches.
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