and Microparticles as Controlled Drug Delivery Devices
10th, 2000, Revised May
11th, 2000; Accepted June 26th,
Majeti N. V. Ravi
of Chemistry, University
of Roorkee, India
Although, the drug delivery system (DDS) concept is not
new, great progress has recently been made in the treatment of a variety
of diseases. Targeting delivery of drugs to the diseased lesions is one of
the most important aspects of DDS. To convey a sufficient dose of drug to
the lesion, suitable carriers of drugs are needed. Nano and microparticle
carriers have important potential applications for the administration of
therapeutic molecules. The research in this area is being carried out all
over the world at a great pace. Research areas cover novel properties that
have been developed increased efficiency of drug delivery, improved
release profiles and drug targeting. The purpose of this review is to take
a closer look at nano/microparticles as drug delivery devices.
Controlled drug delivery
technology represents one of the frontier areas of science, which
involves multidisciplinary scientific approach, contributing to human
health care. These delivery systems offer numerous advantages compared
to conventional dosage forms, which include improved efficacy, reduced
toxicity, and improved patient compliance and convenience. Such systems
often use macromolecules as carriers for the drugs. By doing so,
treatments that would not otherwise be possible are now in conventional
use. This field of pharmaceutical technology has grown and diversified
rapidly in recent years. Understanding the derivation of the methods of
controlled release and the range of new polymers can be a barrier to
involvement from the nonspecialist. Of the different dosage forms
reported, nanoparticles and microparticles attained much importance, due
to a tendency to accumulate in inflamed areas of the body (1-3).
Nano and microparticles for their attractive properties occupy unique
position in drug delivery technology. Some of the current trends in this
area will be discussed.
Nanoparticles and Nanospheres
Nanoparticles were first
developed around 1970. They were initially devised as carriers for
vaccines and anticancer drugs (4). In order to enhance tumor uptake, the
strategy of drug targeting was employed, and as a first important step,
research focused on the development of methods to reduce the uptake of
the nanoparticles by the cells of the reticuloendothelial system (RES)
(5). Simultaneously, the use of nanoparticles for ophthalmic and oral
delivery was investigated (6).
Polymeric micelles often
self-assemble when block copolymers are used for their preparation (7).
Micelles, based on the biocompatible copolymers of poly(ethylene oxide)
PEO with poly(L-Lactic acid) PLA or with poly(b-benzyl-L-aspartate)
PBLA, have been described in literature (8,9). Synthesis of such
nanospheres with functional groups on their surface is shown in Figure
Aldehyde groups on the surface of
the PEO-PLA micelles may react with the lysine residues of cell’s
proteins. They may also be used for attachment of the amino-containing
ligands. The hydroxyl groups on the surface of the PEO-PBLA micelles can
be further derivatized and conjugated with molecules capable to pilot
the modified micelles to specific sites of living organism. Such
nanospheres have been tested as vehicles for delivery of
anti-inflammatory and anti-tumor drugs (10,11).
1 (a) poly(ethylene oxide)-co-b-benzyl-L-aspartate(PEO-PBLA)
and (b) poly(ethylene oxide) -co-L-lactide
(PEO-PLLa) micelles with aldehyde groups on their surface
Poly(lactide-co-glycolide)-[(propylene oxide)-poly(ethylene oxide)]
Nanoparticles (80-150 nm) of the
biocompatible and biodegradable polyester copolymer PLG [Poly(lactide-co-glycolide)]
Figure 2 have been reported (12,13) by
the nanoprecipitation method (they have been precipitated with acetone
from their oily colloidal nanodispersion in water). Thus formed
particles of PLG were coated with 5-10 nm thick layer of the
poly(propylene oxide) - poly(ethylene oxide) (PPO-PEO) block copolymer
or with tetrafunctional (PEO-PPO)2 -N-CH2-CH2-N-(PPO-PEO)2
(12,13). Such coats are bound to the core of the nanosphere by the
hydrophobic interactions of the PPO chains, while PEO chains protrude
into the surrounding medium and form a steric barrier, which hinders the
adsorption of certain plasma proteins onto the surface of such
particles. On the other hand, the PEO coat enhances adsorption of
certain other plasma compounds. In consequence, the PEO-coated
nanospheres are not recognized by macrophages as foreign bodies and are
not attacked by them (14).
Allock and coworkers developed
derivatives of the phosphazene polymers suitable for biomedical
applications (15-17). Long-circulating in the blood, 100-120 nm in
diameter, PEO-coated nanoparticles of the poly(organophospazenes)
containing amino acid, have been prepared. PEO-polyphosphazene
copolymer, or poloxamine 908 (a tetrafunctional PEO copolymer) has been
deposited on their surface (18,19). Chemical formulae of such
polyphosphazene derivatives are shown in Figure 3.
Polyphosphazenes for medical applications
2.4 Poly(ethylene glycol)
Poly(ethylene glycol) PEG-coated
nanospheres from PLA, PLG, or other biodegradable polymers viz., poly(e-caprolactone)
(PCL), may be used for the intravenous drug delivery. PEG and PEO denote
essentially identical polymers. The only difference between the
respective notations is that methoxy groups in PEO may replace the
terminal hydroxyls of PEG. It has been pointed out that PEG coating of
nanospheres provides protection against interaction with the blood
components, which induce removal of the foreign particles from the
blood. It prolongs, therefore, their circulation in the blood stream. In
consequence, thus coated nanospheres may function as circulation depots
of the administered drugs (20,21). Slowly releasing drugs into plasma,
and thus altering their concentration profiles can achieve obvious
therapeutical benefits. About 200 nm in diameter PEG-coated nanospheres,
in which PEG is chemically bound to the core have been prepared, in the
presence of monomethoxy PEG, by ring opening polymerization (with
stannous octoate as a catalyst) of such monomers as e-caprolactone,
lactide, and/or glycolide (21). Ring opening polymerization of these
monomers in the presence of such multifunctional hydroxy acids as citric
or tartaric, to which several molecules of the monomethoxy monoamine of
PEG (MPEG-NH2) have been attached, yields multiblock (PEG)n-(X)m
copolymers. PEG-PLA copolymer in which NH2 terminated methoxy
PEG molecules have been attached to tartaric acid is shown schematically
in Figure 4.
It has been demonstrated that
morphology, degradation, and drug encapsulation behavior of copolymers
containing PEG blocks strongly depends on their chemical composition and
structure. Studies of nanoparticles composed of the diblocks of the
with the methoxy terminated poly(ethylene glycol) [PLG-PEG] or of the
branched multi-blocks PLA-(PEG)3, in which three methoxy
terminated PEG chains are attached through a citric acid residue,
suggested that they have a corecorona structure in an aqueous medium.
The polyester blocks form the solid inner core. The nanoparticles,
prepared using equimolar amounts of the PLLA-PEG and PDLA-PEG
stereoisomers, are shaped as discs with PEG chains sticking out from
Figure 4 Multiblock
(PEG)n-(x)m copolymers. Amino terminated methoxy
polyethylene glycol molecules attached to tartaric acid with Pla side
content seems to be just right for applications in cancer and gene
therapies (7). Such nanospheres are prepared by dispersing the methylene
chloride solution of the copolymer in water and allowing the solvent to
Figure 5 PEG-dextran
By attaching biotin to its free
hydroxyl groups and complexing it with avidin, cell specific delivery
may be attained. NMR studies of such systems (22) revealed that the
flexibility and mobility of the thus attached PEG chains is similar to
that of the unattached PEG molecules dissolved in water. Recently, PLG
microspheres, with the PEG-dextran conjugates Figure
5, attached to their surface, have been investigated as another
variant of the above-described approach. Microspheres of diameter
400-600 nm have been prepared (23). To the glycopyranose hydroxyls of
the dextran units, targeting moieties can be attached (23).
Azidothymidin (AZT)/dideoxycytidine (ddc) nanoparticles
Recently, research took advantage
of the properties of nanoparticles, to be easily taken up by cells of
the RES (3). In the case of AIDS, for instance, the macrophages of the
RES represent one of the most important therapeutic targets (24). In
addition to the CD4+T lymphocyles, these cells
play a decisive role as a reservoir for the human immunodeficiency virus
(HIV). In tissues such as the lung and the brain, HIV is located
primarily in macrophage-like cells (i.e., alveolar macrophages and
microglia, respectively (25,26). In contrast to CD4+
T cells, in which HIV replication is proliferation dependent and finally
leads to cell death, macrophages in a mature nonproliferating but
immunologically active state can be productively infected with HIV type
1 (HIV-1) and HIV-2 (27-29). Altered cellular functions in the
macrophage population may contribute to the development and clinical
progression of AIDS. A large proportion of AIDS patients show HIV
encephalitis with diffuse neuronal damage which is thought to be
mediated by viral proteins and/or factors with neurotoxic activity (i.e.
cytokines). This can occur through proinflammatory cytokines or by
HIV-1-specific proteins secreted from cells of the mononuclear phagocyte
system, including brain macrophages, microglia and multinucleated giant
cells, which have been shown to be productively infected with HIV
It is evident that, apart from
having a function in the pathogenesis of the disease cells of the
macrophage lineage are vectors for the transmission of HIV. The
placental macrophage is likely to be the primary cell type responsible
for vertical (maternofetal) transmission of HIV (33). For mucosal
transmission it was found that an important property of the transmitted
HIV variant is its ability to infect macrophages (34). The phenotypic
characterization of virus populations which were transmitted sexually or
vertically a selection of variants with a predominate tropism for
macrophage in the recipient (35). Because of the important role of cells
of the monocyte/macrophage (Mo/Mac) lineage in the pathogenesis of HIV,
fully effective anti-HIV therapy must reach Mo/Mac in addition to other
without surfactant coating leading to stealth properties may represent
promising drug carriers for this disease. Therefore, the potential of
nanoparticles to deliver antiviral drugs to HIV-infected and uninfected
macrophages was investigated by Schafer et al (36,37). It was found in
in vitro cell cultures of human moncytes/macrophages (Mo/Mac) that the
uptake of nanoparticles by HIV-1-infected cells was about 30% higher
after 7 days of culture and about 65% after 21 days than by uninfected
cells. Similar results were observed with macrophages obtained from AIDS
patients, depending on the state of disease. The uptake by these cells
was also dependent on the surface coating with different surfactants
(37). Interestingly, despite their possible stealth properties, these
surfactants did not reduce the in vitro uptake of the nanoparticle:
ploxamer 338 had no influence and poloxamer 188 even increased the
The following investigation of
the in vitro inhibition of the development of infection of the above
human macrophage cultures by HIV-1 using the antiviral drugs
azidothymidin (AZT) and dideoxycytidine (ddc), however, revealed no
difference between free and nanoparticle-bound drugs (38):
nanoparticle-bound AZT or ddc retained their activity but were not
superior to free drug. The situation was totally different with the
antiviral protease inhibitor saquinavir (32). Here a 10-fold increase in
efficacy of nanoparticle-bound drug over free drug was observed. The
difference in these results probably is due to the fact that AZT and ddc
easily penetrate into cell uptake by the nanoparticles.
However, it has to be considered
that in tissue cultures a homogenous cell population exists, which is
totally different to the situation in the human or animal body: the body
consists of a huge number of different cell types. Macrophages represent
only a small percentage of the total number of cells. As a consequence,
it was conceivable that even with AZT or ddc nanoparticles still may
achieve a better targeting to these cells despite their inability to
show a better antiviral efficacy of these drugs in vitro in the
homologous cell cultures. Indeed that was observable: Lobenberg and
Kreuter (39) bound 14C-labeled AZT to nanoparticles. After
i.v. injection of nanoparticle-bound drug, the concentration of the
AZT-label in the organs that were rich in macrophages, such as the liver
(39), were up to 18 times higher than with AZT solution. Likewise, after
oral administration the nanoparticle formulation delivered the AZT-label
more efficiently to the RES than the aqueous solution. In addition, the
blood concentration was significantly higher after oral administration
using radioluminography supported the above results (40). The
radioactivity in the liver, lung, and spleen, organs with a high number
of macrophages was much higher than in other parts of the body with the
nanoparticles formulation. The above organs showed a spotted appearance
of the radioactivity that is typical for accumulation in macrophages
after i.v. as well as after oral administration of the drug bound to the
particles. In contrast, after administration of the solution, the
radioactivity in the above organs was homogeneously distributed and much
lower. These results show that the nanoparticles seemed to have reached
their target and may represent very promising delivery system for AIDS
Poly (isobutylcynoacrylate) nanocapsules
Intragastric administration of
insulin-loaded poly(isobutylcyanoacrylate) nanocapsules induced a
reduction of the glycemia to normal level in streptozotocin diabetic
rats (41-43) and is alloxan induced diabetic dogs (44). The
hypolglycemic effect was characterized by surprising events including a
lag time period of 2 days and a prolonged effect over 20 days. Insulin
is a very hydrosoluble peptide and should be inactivated by the enzymes
of the gastrointestinal tract. Thus, the reason why insulin could be
encapsulated with high efficiency in nanocapsules containing an oily
core and why these nanocapsules showed so unexpected biological effect
remained unexplained. Nanocapsules were prepared by interfacial
polymerization of isobutylcyanoacrylate (45). Any nucleophilic group
including those of some of the aminoacids of insulin (46) could initiate
the polymerization of such a monomer. In this case insulin could be
found covalently attached to the polymer forming the nanocapsule wall as
it was recently demonstrated with insulin-loaded nanosphers (47).
Aboubakar et al., (48) studied,
physico-chemical characterization of insulin-loaded poly(isobutyl
cyanoacrylate) nanocapsules obtained by interfacial polymerization. They
claimed that the large amount of ethanol used in the preparation of the
nanocapsules that initiated the polymerization of isobutylcyanoacrylate
preserving the peptide from a reaction with monomer resulting high
encapsulation rate of insulin. From their investigations, it appears
that insulin was located inside the core of the nanocapsules and not
simply adsorbed onto their surface.
Nanoparticles have been widely
investigated as the drug carriers (49-52). Biodegradable
poly(D,L-lactide) (53,54) polybutylcyanoacrylate (55) and poly(e-caprolactone) (56) are widely being used to
prepare nanoparticles. The advantages of the nanoparticles are the
reduced drug toxicity, the improvement of biodistribution, and the
increased therapeutic efficacy. Diblock copolymers have been studied in
the sustained release system as an alternative drug carrier (57,58),
since they are known to form a micelle structure.
Hydrophilic-hydrophobic diblock copolymers exhibit amphiphilic behavior
and form micelles with core-shell architecture. These polymeric carriers
have been used to solubilize hydrophobic drugs, to increase blood
circulation time, to obtain favorable biodistribution and to lower
interactions with reticuloendothelial system (59-61). In the same
direction, Oh et al (62) reported the preparation and characterization
of polymeric nanoparticles containing adriamycin as a model drug. The
nanoparticles are obtained from poly(g-benzyl-L-glutamate)/poly(ethylene
oxide) [PBLG/PEO] diblock copolymer, which form a hydrophobic inner core
and a hydrophilic outer shell of micellar structure (63,64), by adopting
dialysis procedure. Their results indicate that only 20% of the
entrapped drug was released in 24 h at 37 0C and the release
were dependent on the molecular weight of hydrophobic polymer.
Chitosan-poly(ethylene oxide) nanoparticles
Hydrophilic nanoparticle carriers
have important potential applications for the administration of
therapeutic molecules (7,24). Most of the recently developed
hydrophobic-hydrophilic carriers require the use of organic solvents for
their preparation and have a limited protein-loading capacity
(19,65-67). Calvo et al (68) reported a new approach for the preparation
of nanoparticles, made solely of hydrophilic polymer, to address these
limitations. The preparation technique, based on an ionic gelation
process, is extremely mild and involves the mixture of two aqueous
phases at room temperature.
Figure 6 The
preparation of CS nanoparticles- A schematic diagram
One phase contains the
polysaccharide chitosan (CS) and a diblock copolymer of ethylene oxide
and polyanion sodium tripolyphosphate (TPP) Figure 6. It was stated
that, the size (200-1000 n) and zeta potential (between + 20mv and
+60mv) of nanoparticles can be conventionally modulated by varying the
ratio CS/PEO-PPO. Furthermore, using bovine serum albumin (BSA) as a
model protein, it was shown that these new nanoparticles have great
protein loading capacity (entrapment efficiency up to 80% of the
protein) and provide a continuous release of the entrapped protein for
up to 1 week (68).
Nanoparticles of methotrexate
(MTX) were prepared using o-carboxymethylate
chitosan (o-CMC) as wall
forming materials, and an isoelectric-critical technique under ambient
condition (64). Drug controlled releases were studied in several media
including simulated gastric fluid, intestinal fluid and 1% fresh mice
serum. It was found that acidic media provide a fast release rate than
neutral media. The effect of MTX/o-CMC
ratio and amount of crosslinking agents of drug release in different
media were evaluated. The changes of size and effective diameter of o-CMC
nanoparticles were detected by SEM and laser light scattering system
before and after the drug release. The author claimed that, the o-CMC nanoparticles constitute an attractive alternative to other
anticancer drugs and enzyme carriers (69).
Solid lipid nanoparticles (SLNs)
Solif lipid nanoparticles (SLNs),
one of the colloidal carrier systems, has many advantages such as good
biocompatibility, low toxicity and stability (70). Schwarz and Mehnert
(71) studied the lipophilic model drugs tetracaine and etomidate. The
study highlights the maximum drug loading, entrapment efficiacy, effect
of drug incorporation on SLN size, zeta potential (charge) and long-term
physical stability. Drug loads of up to 10% were achieved, while
simultaneously maintaining a physical stable nanoparticle dispersion
(71). They claimed that the incorporation of drugs showed no or little
effect on particle size and zeta potential compared to drug free SLN
(71). In another study, Kim and Kim (72) studied the effect of drug
lipophilicity and surfactant on the drug loading capacity, particle size
and drug release profile. The prepared SLNs by homogenization of melted
lipid dispersed in an aqueous surfactant solution. Ketoprofen, ibuprofen
and pranoprofen were used as model drugs and tween and poloxamer
surfactants were tested (72). Mean particle size of prepared SLNs was
ranged from 100 to 150 nm. It was found that the drug loading capacity
was improved with the most lipophilic drug and low concentration of
Microcapsules and microSpheres
The term “microcapsule” is
defined, as a spherical particle with the size varying in between 50 nm
to 2 mm containing a core substance. Microspheres are in strict sense,
spherically empty particles. However, the terms microcapsules and
microspheres are often used synonymously. In addition, some related
terms are used as well. For example, “microbeads” and “beads”
are used alternatively. Sphere and spherical particles are also employed
for a large size and rigid morphology. Due to attractive properties and
wider applications of microcapsules and microspheres, a survey of the
applications in controlled drug release formulations is appropriate.
3.1 Multiporous beads of
Several researchers (73,74) have
studied simple coacervation of chitosan in the production of chitosan
beads. In general, chitosan is dissolved in aqueous acetic acid or
formic acid. Using a compressed air nozzle, this solution is blown into
NaOH, NaOH-methanol, or ethanediamine solution to form coacervate drops.
The drops are then filtered and washed wit hot and cold water
successively. Varying the exclusion rate of the chitosan solution or the
nozzle diameter can control the diameter of the droplets. The porosity
and strength of the beads correspond to the concentration of the
chitosan-acid solution, the degree of N-deacetylation of chitosan, and the type and concentration of
coacervation agents used.
The chitosan beads described
above have been applied in various fields viz., enzymatic
immobilization, chromatograph support, adsorbent of metal ions, or
lipoprotein, and cell cultures. It was confirmed that the porous
surfaces of chitosan beads make good cell culture carrier. Hayashi and
Ikada (75), immobilized protease onto the porous chitosan beads which
carry active groups with a spacer and found the immobilized protease had
higher pH, thermal storage stability, and gave rather higher activity
toward the small ester substrate, N-benzyl-L-arginine
ethyl ester. In addition, Nishimura et al (73) investigated the
possibilities of using chitosan beads as a cancer chemotherapeutic
carrier for adriamycin. Recently, Sharma et al (76-78) prepared chitosan
microbeads for oral sustained delivery of nefedipine, ampicillin and
various steroids by adding to chitosan and then going through a simple
coacervation process. These coacervate beads can be hardened by
crosslinking with glutaraldehyde or epoxychloropropane to produce
microcapsules containing rotundine (79). The release profiles of the
drugs from all these chitosan delivery systems were monitored and showed
in general the higher release rates at pH 1-2 than that at pH 7.2-7.4.
The effect of the amount of drug loaded, the molecular weight of
chitosan and the crosslinking agent on the drug delivery profiles have
been discussed (76-79).
3.2 Coated alginate
Many of the present controlled
release devices for in vivo delivery of macromolecular drugs involve
elaborate preparations, often employing either harsh chemicals, such as
organic solvents or extreme conditions, such as elevated temperature.
The conditions have the potential to destroy the activity of sensitive
macromolecular drugs, such as proteins or polypeptides. In addition,
many devices require surgical implantation and in some cases, the matrix
remains behind or must be surgically removed after the drug is exhausted
et al (81) studied a mild alginate/polycation microencapsulation
process, as applied to encapsulation of bioactive macromolecules such as
proteins. The protein drugs were suspended in sodium alginate solution
and sprayed into 1.3% buffered calcium chloride to form cross-linked
microcapsules, large (upto 90%) losses of encapsulation species were
encountered, and moderate to strong protein-alginate interactions caused
poor formation of capsules. As a result, a diffusion-filling technique
calcium alginate microcapsules were formed by spraying 10 ml of the
sodium alginate solution into 250 ml of buffered
2-[4-(2-hydroxyethyl)piperazin-1-yl]ethane sulfonic acid (HEPES) calcium
chloride (13 MM HEPES, 1.3% CaCl2 pH 7.4) from a 20 ml
plastipack syringe through a 22 G needle Figure 7.
Figure 7 Schematic of
spray device used in the preparation of calcium alginate microsphere
Protein was then loaded into
these capsules by stepwise diffusion from solutions of increasing drug
concentration. The drug loaded capsules were coated with a final layer
of polycation. In all, three polycation coatings were used, two prior
filling and one after filling. The first coating strongly influenced the
size, integrity, and loading capacity of the capsules. Low
concentrations of polycation resulted in poorly formed capsules with
very low retention of the drug in the final capsule, while very high
concentration prevented the drug from entering the capsule at the
filling stage. The first coat also affected the duration of drug release
from the capsule and the size of the burst effect. The second coat had
less effect on the capsule integrity, but it did influence the drug
payload and release profile. The final, sealing-coat had little effect
on drug payload and only limited effect on the release profile upto a
critical concentration, above which the release profile was not
affected. For all coats, increasing polycation concentration decreased
the burst effect, and caused the release profile to be more sustained.
Encapsulation of a series of dextrans with increasing molecular weight
revealed that the release profile was directly related to the molecular
weight of the diffusing species, which was more sustained as molecular
weight increased. More recently, Murata et al (82) investigated alginate
gel bead containing chitosan salt. When the bead was placed in bile acid
solution it rapidly took bile acid into itself. The uptake amount of
taurocholate was about 25 m mol/0.2 g dried gel beads. The phenomenon
was observed on the case of the beads incorporating colestyramine
instead of chitosan. From the studies reported, it appears that
ion-exchange reaction accompanying the insoluble complex-formation
between chitosan salt and bile acid occurs in alginate gel matrix (82).
3.3 N-(aminoalkyl) chitosan
The most promising encapsulation
system yet developed appears to be the encapsulation of calcium alginate
beads with poly-L-lysine. However, the use of this system on a large
scale, such as for oral vaccination of animals, is not feasible due to
the high cost ($200/g) of poly-L-lysine (PLL). It would therefore be
desirable to develop an economic and reliable microencapsulation system
based on chitosan and alginate. The better membrane-forming properties
of PLL over chitosan were for the following reasons: PLL contains a
number of long-chain alkylamino groups that extend from the polyamide
backbone. These chains may extend in a number of directions and interact
with various alginate molecules, through electrostatic interactions,
resulting in a highly crosslinked membrane. Chitosan, on the other hand,
has aminogroups that are very close to the polysaccharide backbone.
Interaction between the charged amino groups of chitosan and carboxylate
groups of alginate may be lessened due to steric repulsion between the
Goosen and coworkers (83)
attempted to mimic the properties of PPL by extending the length of the
cationic spacer arm on the chitosan main chain. In the chemical
modification, chitosan was first reacted with a-bromoacylbromide
followed by reaction with an amine. The major problem in this procedure
was the competing hydrolysis reaction of the bromoacylbromide.
Furthermore, the lack of characterization of the modified chitosan
caused ambiguity in the effectiveness of the chitosan modification. No
significant difference was found in membrane properties between modified
and unmodified chitosan. A two-step synthetic method for attaching long
alkylamine side chains to chitosan is represented in Figure
Figure 8 Modification
of chitosan with bromoalkylphthalimides and hydrazine
The approach outlined in Figure 8
is designed to attach flexible alkylamine side chains to the chitosan
polysaccharide backbone, possibly simulating the behavior of PPL. The
presence of two amino groups in this side chain may even enhance
membrane-forming properties. Chemical modification of poly(vinyl
alcohol) (PVA) by a similar procedure may also produce a-
polyamine with membrane-forming properties similar to that of PPL (83).
All alkylation products were characterized by solution 1H-
and 13C-NMR and by solid-state CP-MAS 13C-NMR. The
above synthetic polymer derivatives, as well as chitosan,
polyallylamine, and polyethyleneimine, were used to form membrane
coatings around the calcium alginate beads in which blue dextran of
molecular weight 7.08x104 or 26.6x104 was
entrapped. These microcapsules were prepared by extrusion of a solution
of blue dextran in sodium alginate into a solution containing calcuim
chloride and the membrane polymer. Measuring the elution of the blue
dextran from the capsules, spectrophotometrically (83), assessed
membrane integrity and permeability.
The encapsulation process of
chitosan and calcium alginate as applied to encapsulation of hemoglobin
was reported by Huguet et al .
In the first process, the mixture of hemoglobin and sodium alginate is
added dropwise to the solution of chitosan and the interior of capsules
thus formed in the presence of CaCl2 is hardened. In the
second method, the droplets were directly pulled off in a chitosan- CaCl2
mixture. Both procedures lead to beads containing a high concentration
of hemoglobin (more than 90% of the initial concentration (150 g/L) were
retained inside the beads) provided chitosan concentration is
The molecular weight of chitosan
(mol wt 245000 or 390000) and the pH (2, 4, or 5.4) had only a slight
effect on entrapment of hemoglobin, the best retention being obtained
with beads prepared at pH 5.4. The release of hemoglobin during the bead
storage in water was found to be dependent on the molecular weight of
chitosan. The best retention during storage in water was obtained with
beads prepared with the high molecular weight chitosan solution at pH
2.0. Considering the total loss in hemoglobin during the bead formation
and after 1 month of storage in water, the best results were obtained by
preparing the beads in an 8 g/L solution of a 390000 chitosan at pH 4
(less than 7% of loss with regard to the 150 g/L initial concentration).
Figure 9 Schematic
representation of the ionic interactions between alginate and chitosan
(a) pH 5.4; (b) pH 2.0
Similarly, the encapsulation of
various molecules [Hb, bovine serum albumin (BSA) and dextrans with
various molecular weights] in calcium alginate beads coated with
chitosan has been reported [85,86]. Their release has been compared and
the influence of the conformation, the chemical composition and the
molecular weight of the encapsulated materials have been analysed .
The ionic interaction between alginate and chitosan at different pH are
depicted in Figure 9.
3.5 Poly(adipic anhydride)
In ocular drug delivery, the high
rate of tear turnover, and the blinking action of the eyelids lead to
short precorneal residence times for applied eye drops. Typically, the
washout rate reduces the concentration of the drug in a tear film to
one-tenth of its starting value in 4-20 min. As a result, only the eye
absorbs a few percent of the administered drug and the duration of the
therapeutic action may be quite short. Early reports showed that the
formulation of the eye drops is decisive for the availability of an
ocular drug (87). The absorbed amount of the model substance, fluoro
metholone, and duration in the aqueous humor increased when a suspension
was used instead of a solution, and the best result was obtained when
the drug was formulated in an ointment. Similar results as with an
ointment were obtained when a suspension formulated in a hydrogel was
used (88). The combination of particles with a hydrogel thus increases
the bioavailability of an ocular drug. A hydrogel may increase the
precorneal residence time of a suspension, and the residence time of the
hydrogel, therefore, sets the maximal achievable residence time for a
given drug. Water-soluble drugs are generally not retained by hydrogels
because of their high diffusion coefficients. One way of solving this
problem is to incorporate these drugs into polymeric microparticles.
A novel microsphere-gel
formulation was investigated aiming to extend precorneal residence times
for ocular drugs (89). Poly(adipic anhydride) was used for
microencapsulation of timolol maleate. A nonaqueous method for the
microsphere preparation was employed due to the hydrolytical sensitivity
of the polymer. Microspheres were prepared with an average diameter of
The polymer and the microspheres were characterized before and during
degradation using size exclusion chromatography (SEC), differential
scanning calorimeter (DSC), X-ray diffraction, infrared spectroscopy
(IR), and scanning electron microscopy (SEM) (89). The microspheres had
a smooth external surface and a hallow center surrounded by a dense
outer shell. Degradation of the microspheres resulted in a constant
release of adipic acid, the degradation product, indicating a
surface-eroding degradation mechanism. The surface erosion of the
polymer controlled the release of incorporated substance, timolol
maleate. The drug release rate profile appeared to be suitable for
ocular drug delivery. However, the initial drug release rate was
decreased to some extent when the PAA-microspheres were incorporated
into an in situ gelling polysaccharide (89), GelriteÒ.
The authors claimed that the improved ocular bioavailability of these
novel microsphere-gel delivery formulations remains to be compared with
that of ordinary eye drops.
3.6 Gellan-gum beads
Gellan gum is a linear anionic
polysaccharide produced by the microorganism pseudomonas elodea (90,91). The natural form of the polysaccharide
consists of a linear structure repeating tetrasaccharide unit of
glucose, glucuronic acid and rhamnose (91-93) in a molar ratio of 2:1:1.
Native gellan is partially acylated with acetyl and L-glyceryl groups
located on the same glucose residue (94). X-ray diffraction analysis
shows that gellan gum exists as a half-staggered, parallel, double helix
which is stabilized by hydrogen bonds involving the hydroxymethyl groups
of one chain and both carboxylate and glyceryl groups of the other (95).
The presence of acetyl or glyceryl groups does not interfere with double
formation but does alter its ion-binding ability. The commercial gellan
gum is the deacetylated compound obtained by treatment with alkali (90),
yielding the gum in its low acyl form in which the acetate groups do not
interfere with helix aggregation during gel formation. Gellan forms gels
in the presence of mono and divalent ions, although its affinity for
divalent ions is much stronger (96). Milas et al (97), showed a
mechanism of gelation based on aggregation of the double helix
controlled by the thermodynamics of the solution in which the nature of
the counter-ion is of prime importance. The apparent viscosity of the
gellan gum dispersions can be markedly increased by increases in both pH
and cation concentration (98,99). Gellen gum is mainly used as a
stabilizer or thickening agent and it has a wide variety of
applications, particularly in food industry (100,101), as a bacterial
growth media (102,103) and in plant tissue culture (104). Its medical
and pharmaceutical uses are in the field of sustained release. Due to
its characteristic property of temperature-dependent and cation-induced
gelation, gellan has been used in the formulation of eye drops, which
gelify on interaction with the sodium ions naturally present in the eye
Microcapsules containing oil and
other core materials have been formed by complex coacervation of gellan
gum-gelatin mixture (108). Deacetylated gellan gum was used to produce a
bead formulation containing sulphamethizole by a hot extrusion process
into chilled ethylacetate (100). Recently, the ability of gellan gum to
form gels in the presence of calcium ions enabled capsules to be
prepared by gelation of the polysaccharide around a core containing
starch (109-111), or oil was investigated.
More recently, Kedzierewicz et al
(112) adopted a method rather simpler than the ones used so far, i.e.
the ionotropic gelation
method, to prepare gellan gum beads. Gellan gum beads of
propranolol-hydrochloride, a hydrophilic model drug were prepared by
solubilizing the drug in a dispersion of gellan gum and then dropping
the dispersion into calcium chloride solution. Major formulation and
process variables, which might influence the preparation of the beads
and drug release from gellan gum beads were studied. Very high
entrapment efficiencies were obtained (92%) after modifying the pH of
both the gellan gum dispersion and the calcium chloride solution. The
beads could be stored for 3 weeks in a wet or dried state without
modification of the drug release. Oven-dried beads released the drug
somewhat more slowly than the wet or freeze-dried beads. The drug
release from the oven-dried beads was slightly affected by the pH of the
dissolution medium (112). Gellan gum could be a useful carrier for the
encapsulation of fragile drugs and provide new opportunities in the
field of bioencapsulation.
The treatment of infiltrating
brain tumors, particularly oligodendrogliomas, requires radiotherapy,
which provides a median survival of 3.5-11 years (113). Since 5-iodo-21-deoxyuridine
(IdUrd) is a powerful radio sensitizer (114), the intracarnial
implantation of IdUrd loaded microparticles within the tumor might
increase the lethal effects of g-radiations
of malignant cells having incorporated IdUrd. The particles can be
administered by stereotactic injection, a precise surgical injection
technique (115). This approach requires microparticles of 40-50 mm
in size releasing in vivo their content over 6 weeks, the standard
period during which a radiotherapy course must be applied.
The solvent evaporation process
is commonly used to encapsulate drugs into poly(lactide-co-glycolide)
microparticles (PLGA) (116). It is well known that the candidate drugs
must be soluble in the organic phase. In the case, where the active
ingredient is not oil soluble, other alternative can be considered. The
W/O/W-multiple emulsion method is particularly suitable for the
encapsulation of highly hydrophilic drugs. For drugs which are slightly
water soluble, like IdUrd (2mg/ml), other approaches must be
investigated to achieve significant encapsulation: dissolution of the
drug in the organic phase through the use of a cosolvent or dispersion
of drug crystals in the dispersed phase. In the latter case, it is often
admitted that the suspension of crystals in the organic phase can lead
to an initial drug release, which is difficult control (117,118). To
reduce IdUrd particle size, two-grinding processes were used,
spray-drying and planetary ball milling (119-121). The optimal
conditions of grinding were studied through experimental design and the
impact on in vitro drug release from PLGA microspheres was then
examined. More recently, Geze et al (122), studied IdUrd loaded
(PLGA) microspheres with a reduced initial burst in the in vitro release
profile, by modifying the drug grinding conditions. IdUrd particle size
reduction has been performed using spray drying or ball milling. Spray
drying significantly reduced drug particle size with a change of the
initial crystalline form to an amorphous one and lead to a high initial
burst. Conversely, ball milling did not affect the initial Id Urd
crystallinity. Therefore, the grinding process was optimized to
emphasize the initial burst reduction. The first step was to set
qualitative parameters such as ball number, and cooling with liquid
nitrogen to obtain a mean size reduction and a narrow distribution. In
the second step, three parameters including milling speed, drug amount
and time were studied by a response surface analysis. The
interrelationship between drug amount and milling speed was the most
significant factor. To reduce particle size, moderate speed associated
with a sufficient amount of drug (400-500 mg) was used. Id Urd release
from microparticles prepared by the O/W emulsion/extraction solvent
evaporation process with the lowest crystalline particle size (15.3 mm)
was studied to overcome burst effect. In the first phase of drug
release, the burst was 8.7% for 15.3 mm compared to 19% for 19.5 mm
milled drug particles (122).
In the other procedure, Rojas et
al (123); optimized the encapsulation of b-lactoglobulin
(BLG) within PLGA microcaparticles prepared by the multiple emulsion
solvent evaporation method. The role of the pH of the external phase and
the introduction of the surfactant Tween 20, in the modulation of the
entrapment and release of BLG from microparticles, was studied. Better
encapsulation of BLG was noticed on decreasing the pH of external phase
to a value close to the PI of BLG, however, a larger burst release
effect. In contrast, the addition of Tween 20 increased the
encapsulation efficiency of BLG and considerably reduces in the burst
release effect. In addition, Tween 20 reduced the number of aqueous
channels between the internal aqueous droplets as well as those
communicating with the external medium. Inventors claimed that these
results constitute a step ahead in the improvement of an existing
technology in controlling protein encapsulation and delivery from
microspheres prepared by the multiple solvent evaporation method (123).
Blanco-Prieto et al (124) studied
the in vitro release kinetics of peptides from PLGA microspheres,
optimizing the test conditions for a given formulation, which is
customary to determine in vitro/in vivo correlation. The somatostatin
analogue vapreotide pamoate, an octapeptide, was microencapsulated into
PLGA 50:50 by spray drying. The solubility of this peptide and its in
vitro release kinetics from the microspheres were studied in various
test media. The solubility of vapreotide pamoate was approximately 20-40
in 67 mM phosphate buffer saline (PBS) at pH 7.4, but increased to
at a pH of 3.5. At low pH, the solubility increased with the buffer
concentration (1-66 mM). Very importantly, proteins (aqueous bovine
serum albumin (BSA) solution or human serum) appeared to solubilize the
peptide pamoate, resulting in solubilities ranging from 900 to 6100 mg/ml.
The release rate was also greatly affected by the medium composition.
The other results are, m PBS of pH 7.4 only 33+1% of the peptide
was released within 4 days, whereas, 53+2 and 61+0.95 were
released in 1% BSA solution and serum respectively. The type of medium
was found critical for the estimation of the in vivo release. From their
investigations, it was concluded that the in vivo release kinetics of
vapreotide pamoate form PLGA microspheres following administration to
rats were qualitatively in good agreement with those obtained in vitro
using serum as release medium and sterilization by g-irradiation
had only a minor effect on the in vivo pharmacokinetics (124).
Transplantation of islets of
Langerhans as a means of treating insulin-dependent diabetes mellitus
has become an important field of interest (125-127). However, tissue
rejection and relapse of the initial autoimmune process have limited the
success of this treatment. Immunoisolation of islets in semipermeable
microcapsules has been proposed to prevent their immune destruction
(128,129). Nevertheless, a pericapsular cellular reaction eventually
develops around microencapsulated islets, inducing graft failure (130).
Since empty microcapsule elicit a similar reaction (131), the reaction
is not related to the presence of islets within the capsule but is, at
least partially, caused by the capsule itself. Consequently,
microcapsule biocompatibility appears to constitute a major impediment
to successful microencapsulated islet transplantation.
Smaller microcapsules (<350 mm)
offer many advantages over standard microcapsules (700 to 1500 mm),
including reduced total implant volume, better insulin kinetics (132),
improved cell oxygenation (133), and potential access to diverse
implantation sites, such as the spleen and liver. A new electrostatic
pulse system produces microcapsules of <350 mm
in diameter (134,135) as compared with 700 to 1500 mm
produced with the usual air-jet system. Lum et al (134) have suggested
that these smaller microcapsules exhibit a higher degree of
biocompatibility compared to standard microcapsules. However, no
quantitative data or comparative studies have been published addressing
Previous investigations on
alginate-poly-L-lysine microcapsule biocompatibility have focused mainly
on intraperitoneal transplantation of microcapsulate islets into
diabetic rats. The use of peritoneal implantation for biocompatibility
studies is hindered by the fact that, in this site, microcapsules are
unevenly distributed. Free floating microcapsules, which are easily
recovered, are less likely to show pericapsular fibrosis than
irretrievable microcapsules. The selection bias hinders quantitative
evaluation of microcapsule biocompatibility. The low recovery of smaller
microcapsules increases the selection bias. To overcome these
methodological problems, Pariseau et al (136), developed and carefully
validated a method for the in vivo comparative evaluation of
microcapsule biocompatibility. This technique comprises implantation of
microcapsules into rat epididymal fat pads, retrieval of fat pads after
fixed time periods, and histological evaluations with the use of a
fibrosis scove. Microcapsule recovery rate from this site was 99.6+0.75%
(136). The pericapsular reaction is uniform within one fat pad and
between fat pads, allowing random sampling and comparative studies
Robitaille et al (137), reported
investigation comparing the biocompatibility of microcapsules <350 mm
in diameter made with an electrostatic pulse system to that of
microcapsules 124+120 mm
in diameter made with standard air-jet system, with the objectives;(1)
to compare the biocompatibility of smaller and standard microcapsules
while maintaining either equal implant volume or equal alginate content;
and (2) to analyze the biocompatibility of smaller versus
standard microcapsules with respect to the total implant surface exposed
to the surroundings. To evaluate the biocompatibility,
200,1000,1120,1340, or 3000 of smaller microcapsules (<350 mm)
or 20 standard microcapsules (1247+120 mm)
were implanted into rat epididymal fat pads, retrieved after 2 weeks,
and evaluated histologically. The average pericapsular reaction
increased with the number of small microcapsules implanted (p<0.05;
3000 Vs 200, 300 Vs 1000 and 1000 Vs 200 microcapsules). At equal volume
and alginate content, standard microcapsules caused a more intense
fibrosis reaction than smaller microcapsules (p<0.05). In addition,
20 standard microcapsules elicited a stronger pericapsules (p<0.05)
although the later represented a 3.4 fold larger total implant surface
exposed. Finally, from their investigation it appears that the
microcapsules of diameters <350 mm
made with an electrostatic pulse system are more biocompatible than
standard microcapsules (137).
3.9 Crosslinked chitosan
Procedure for the preparation of
crosslinked chitosan microspheres coated with polysaccharide or lipid
for intelligent drug delivery systems is reported Figure
Figure 10 Theoretical
structure of chitosan gel microsphere coated with polysaccharide or
The microspheres were prepared
with an inverse emulsion of 5-FU or its derivative solution of
hydrochloric acid of chitosan in toluene containing SPAN 80. Chitosan
was crosslinked with Schiff’s salt formation by adding glutaraldehyde
toluene solution. At the same time, the amino derivatives of 5-FU were
immobilized, obviously resulting in an increase in the amount of drug
within the microspheres. The microspheres were coated with anionic
polysaccharides (e.g. carboxymethylchitin, etc.) through a polyion
complex formation reaction. In the case of lipid coated microsphere, the
microspheres along with dipalmitoyl phosphalidyl choline (DPPC) were
dispersed in chloroform. After evaporation of the solvent, microspheres
were obtained coated with a DPPC lipid multilayer, which exhibited a
transition temperature of a liquid crystal phase at 41.4 oC.
The diameter range of microspheres was 250-300 nm with a narrow
distribution. The stability of the dispersion was improved by coating
the microsphere with anionic polysaccharide or a lipid multilayer Figure
Figure 11 Preparation
process of MS (CM), MS (CML) and MS(CML) and MC(CM) polysaccharide
A comparative study on the
release of 5-FU and its derivatives from a polysaccharide coated
microspheres MS (CM) was carried out in physiological saline at 37 oC.
Data indicated that the 5-FU-release rate decreased in the order:
free-5-FU > carboxymethyl type 5-FU > ester type 5-FU. The results
revealed that the coating layers on the microspheres were effective
barriers to 5-FU release.
The lipid mutilayers with a
homogeneous composition generally show a transition of gel-liquid
crystal. When the temperature is raised to 42 oC, which is
higher than the phase transition of 41.4 oC, the release
amount of 5-Fu increased, the amount of drug delivered decreased at 37 oC,
which is lower than the transition temperature. Due to the improved
recognition function of polysaccharide chains for animal cell membranes,
it is reasonable to develop targeting delivery systems from
polysaccharide coated microspheres, MS (CM).
In their studies on
pharmaceutical applications of chitin and chitosan, Yao and coworkers
(138) reported chitosan/gelatin network polymer microspheres for
controlled release of cimetidine. The drug loaded microspheres were
prepared by dissolving chitosan, gelatin (1:1 by weight) and cimetidine
in 5% acetic acid. A certain amount of Tween 80 and liquid paraffin at
water to oil a ratio of 1:10 was added to the chitosan/gelatin mixture
under agitation at 650 rev. per min at 30 oC. A suitable
amount of 25% aqueous glutaraldehyde solution was added to the inverse
emulsion and maintained for 2 h. Finally, the liquid paraffin was
vaporized under vacuum to obtain microspheres.
The drug release studies were
performed in hydrochloric acid solution (pH 1.0) and potassium
dihydrogen phosphate (pH 7.8) buffer at ionic strength 0.1 m/L. A pH
dependent pulsed-release behavior of the HPN matrix was observed (138).
Moreover, the release rate can be controlled via the composition of the
HPN and the degree of deacetylation of chitosan.
Crosslinked chitosan network beads with spacer groups
A novel technique for the
preparation of pH sensitive beads of chitosan is reported as a part of
the studies on controlled drug delivery applications of chitosan.
Diclofenac sodium, Thyamine hydrochloride, chlorphenramine maleate and
Isoniazid were used as model drugs (139-141). In these studies, widely
used products in medical and pharmaceutical areas viz., glycine and
polyethylene glycol were employed as spacer groups to enhance the
flexibility of the polymer networks and influence the swelling behavior
through macromolecular interactions. The procedure is based on adding
drugs to chitosan solution and beads were prepared by simple
coacervation (79). The swelling behavior, solubility, hydrolytic
degradation and drug loading capacity of the beads were investigated
(141,142). Effect of the crosslinker studied, by varying the amounts of
The beads exhibit high pH
sensitivity. The swelling ratio of the beads at pH 2.0 is obviously
higher than that at pH 7.4. This pH sensitive swelling is due to the
transition of bead network between the collapsed and the expanded rates,
which is related to ionization degree of amino groups on chitosan in
different pH solutions. In both these systems, a near zero-order release
is observed for about 4 days. The amounts and percent release in
chitosan-PEG system is a bit higher, when compared to the
chitosan-glycine system, due to the water diffusivity and pore forming
properties of PEG. The effects of the amount of drug loaded and the
crosslinking agent on the delivery profiles were reported as well
1,5-diozepan-2-one (DXO) and D,L-dilactide (D,L-LA) microspheres.
The most successful class of
degradable polymers so far have been aliphatic polyesters. The
degradation takes place via hydrolysis of the ester linkages in the
polymer backbone. These materials must be extensively tested and
characterized since many of them are new structures. This is however,
not sufficient. Equally important is the identification of degradation
mechanisms and degradation products. Since high molecular weight
polymers are seldom toxic, the toxicity and tissue response after the
initial post operative period is related to the compounds formed during
The most important polymer on the
market today is poly(lactic acid) (PLA) which upon degradation yields
acetic acid, a natural metabolite in the human body. The formation of
natural metabolites should be advantageous as the body has routes to
eliminate them. Other commercial degradable materials are
polyparadioxane, copolymer of glycolic acid and trimethylene carbonate
which do not give natural metabolites when degraded. Their most
important characteristic is probably the fact that the degradation
products are harmless in the concentration present.
Albertsson and coworkers (143)
carried out extensive research to develop polymers in which the polymer
properties are altered for different applications. The predominant
procedure is ring-opening polymerization which provides a way to achieve
pure and well defined structures, They have utilized cyclic monomers of
lactones, anhydrides, carbonates, ether-lactones, and specifically
oxepan-2,7-dione (AA), b-propiolactone,
Cl), 1,5-dioxepan-2-one (DXO), dilactide, and 1,3-dioxan-2-one (TMC).
The work involved the synthesis of monomers not commercially available,
studies of polymerization to form homopolymers, statistical and block
copolymers, development of crosslinked polymers and polymer blends,
surface modification in some cases, and characterization of the
materials formed. The characterization is carried out with respect to
the chemical composition and both chemical and physical structure, the
degradation behavior in vitro and in vivo, and in some cases the ability
to release drug components from microspheres prepared from the polymers.
Copolymers of 1,5-dioxepan-2-one
(DXO) and D, L-dilactide (D,L-LA) were synthesized in different molar
concentration, in their recent attempts (144). 1H-NMR was
used to determine the molar composition of the copolymers, and DSC was
used to determine the glass transition and melting temperatures. All the
polymers were amorphous with a glass transition varying between -36 oC
[poly(DXO)] to +55 oC
[poly(D, L-LA)]. In vitro hydrolysis studies on the copolymers showed
degradation times up to 250 days. Copolymers of 1,3-dioxan-2-one (TMC)
Cl) were also studied (144). Conversion studies were performed, and both
monomers separately showed almost the reactivity. Poly(e-caprolactone)
seemed to be more sensitive to transesterification at elevated
temperatures than poly(trimethylene carbonate). Using DSC, melting
endotherms were seen in molar compositions with a CL content as low as
65% which indicates a block structure. Microspheres intended for drug
delivery were prepared from poly(TMC-co-CL),
poly(adipic anhydride) and poly(lactide-co-glycolide)
(144). SEM studies showed that the microsphere morphology concentration
dependent at the time of preparation and on the choice of polymer. The
drug release profiles showed dependence on the polymer degradation
behavior and on the water penetration into the microsphere (144).
Liquid drug-delivery systems,
which do not require surgical implantation, may use vehicles made of
microparticulates or colloidal carriers composed of lipids,
carbohydrates or synthetic polymer matrices. Liposomes, the most widely
studied of these vesicles, can be formulated to include a variety of
compositions and structures that are potentially non-toxic, degradable
and nonimmunogenic. To produce a long acting local anesthetic effect,
vesicles have been used to entrap dibucaine (145), methoxyflurane (146),
tetracaine (147) and lidocaine (148) using formulations with polylactic
acid, lecithin, iophendylate and phosphatidylcholine and cholesterol,
respectively. With varying degrees of success, these treatments have
provided neural blokade for periods far outlasting, which is produced by
any drug given alone.
Masters and Domb (149) reported
on an injectable drug delivery system that uses liposomes (150) to
release the local anesthetic, bupivacaine, from a liposomal matrix that
is both biodegradable and biocompatible to produce sustained local
anesthetic blockade (SLAB). Bupivacaine due to its minimum vasodilating
properties was preferred to other local anesthetics (e.g., lidocaine)
allowing the released drug to remain at the site of injection longer
(151). Lipospheres are an aqueous microdispersion of water insoluble,
spherical microparticles (0.2 to 100 mm
in diameter), each consisting of a solid core of hydrophobic
triglycerides and drug particles that are embedded with phospholipids on
the surface. The in vivo studies with Liposheres have shown that a
single bolus injection can deliver antibiotics and anti-inflammatory
agents for 3 to 5 days (152) and also, control the delivery of vaccines
(153,154). Recent reports were of bupivacaine-liposphere formulation,
which produced 1-3 days reversible sensory and motor SLAB when applied
directly to the rat sciatic nerve (149). The particle size of the
liposheres was in between 5 and 15 mm, with over 90% surface phospholipid.
Lipospheres released bupivacaine over two days under ideal sink
conditions. Liposphere nerve application produced dose-dependent and
reversible block. Sustained local anesthetic block (SLAB) was observed
for 1-3 days in various in vivo tests: (a) Hind paw withdrawal latency
to noxious heat was increased over 50% for 96h period after application
of 3.6% or 5.6% bupivacaine-lipospheres. The 3.6% and 5.6% doses were
estimated to release bupivacaine at 200 and 311 mg
drug/h, respectively based on release spanning 72h. Application of 1.6%
bupivacaine-lipospheres increased withdraw latency 25-250% but for only
a 24h duration, (b) similarly, vocalization threshold to hind paw
stimulation was increased 25-50% for 72h following application of 3.6%
bupivacaine-lipospheres; (c) finally concluded that, sensory blockade
outlasted or equaled corresponding motor block duration for all
liposphere durg dosages (149).
3.14 Glutamate and TRH
L-glutamate is the principal
excitatory neurotransmitter in the mammalian central nervous system
(155) and has been shown to stimulate trigeminal motoneurons within the
trigeminal motor nucleus in acute, short-term physiologic studies (156).
Since trigeminal motoneurons innervate the muscles of mastication and
activity patterns (EMG) of those muscles directly affect the
growth/development of the carnifacial skeleton by biomechanical forces
produced (157), Byrd and coworkers (158,159), tried and successfully
used gultamate-impregnated microspheres to stimulate trigeminal
motoneurons in situ within the brainstem of young rats to produce
skeletal alterations. The underlying premise was that increased delivery
of glutamate in proximity to trigeminal motoneurons would increase
activity of both those motoneurons and the masticatory muscles they
supply. Indirect physiologic evidence for this premise was provided by
the presence of (1) more pronounced implantside wear facets on the
mandibular incisors in rats with gultamate-microsphere implants, and (2)
deviation of their facial skeletons toward the implant side (158,159).
The microspheres used were of biodegradable, polyanhydride construction
and released gultamate at a controlled rate within the intact rat. The
use of biodegradable polyanhydride microspheres as drug-carrier matrices
(160) is an effective method for delivery of long-term release of
neuroactive substances to a specific locus within the CNS with little
risk of infection (159).
The tripeptide TRH
(thyrotropin-releasing hormone) has been confirmed as an important
neurotransmitter/neuromodulator within the brain stem region of the CNS
(161-165). Trigeminal motoneurons are highly immunoreactive for TRH
(163) and also that TRH actually increases the excitability of other
brain stem motoneurons in vitro (165). TRH therefore proved to be useful
to increase activity levels of trigeminal motoneurons in vivo. Byrd et
al (158), investigated in the chronic, long-term effects of
administering TRH in proximity to trigeminal motoneurons in vivo, and
comparing any carniofacial sequelae with those effected by glutamate
Recently, Byrd et al (166)
investigated the sequelae of sustained, in vivo delivery of two
important neurotransmitter substances, glutamate and TRH, upon
carniofacial growth and development. In their studies, the relative
effects of glutamate and TRH microspheres stereotactically placed in
proximity to trigeminal motoneurons within the trigeminal motoneurons
were compared. Stereoactive neurosurgery at 35 days was conducted for 4
experimental groups comprising 10 male Sprague-Dawley rats a group. The
data was collected after killing 5 rats of each group at 14 and 21 days.
Histology revealed that implants were clustered in the pontine reticular
formation, close to ventrolateral tegmental nucleus. Both glutamate and
TRH rats had implant-side deviation of their facial skeleton and snout
regions 4x2 ANOVA and post hoc t-tests
revealed significant (p < 0.05,0.01) differences between
groups and sides for motoneuron count, muscle weight, and osteometric
Polyelectrolyte complexes of sodium alginate chitosan
Polyelectrolyte complexes (PECs)
are formed by the reaction of a polyelectrolyte with an oppositely
charged polyelectrolyte in an aqueous solution. Polysaccharides, which
have bulky pyranose rings and highly stereoregular configuration in
their rigid, linear backbone chains, have been frequently studied (167).
PECs have numerous applications such as membranes, antistatic coatings,
environmental sensors, and chemical detectors, medical prosthetic
materials etc (168). Among these, their wide use as membranes for
dialysis, ultrafiltration, and other solute separation processes are of
special interest and also made it possible for the use in microcapsule
membranes. Microcapsules can be used for mammalian cell culture and the
controlled release of drugs, vaccines, antibiotics, and hormones
(168-170), To prevent the loss of encapsulated material, the
microcapsules should be coated with another polymer that forms a
membrane at the bead surface. The most well known and promising system
is the encapsulation of alginate beads with poly-L-lysine (PLL). Because
this system has a limitation due to the high cost of PLL, other systems
such as alginate beads coated with chitosan or its derivatives have been
developed (83,84). Few results have been reported about the formation of
PECs of alginate with chitosan under acidic condition. Although
alginate/chitosan microcapsules have been studied a lot, the studies
have been limited in a narrow pH region due to the solubility of
Lee et al (171) described a new
procedure to overcome the solubility of chitosan. In this procedure,
chitosan was heterogeneously deacetylated with a 47% sodium hydroxide
solution and followed by a homogeneous reacetylation with acetic
anhydrides to control the N-acetyl
content of the chitosan having a similar molecular weight. The chitosan
having different degrees of N-deacetylation
were complexed with sodium alginate, and the formation behavior of
polyelectrolyte complexes (PECs) was examined by the viscometry in
various pH ranges. The maximum mixing ratio (Rmax) increased
with a decrease in the degree of N-acetylation
of the chitosan at the same pH, and with a decrease in pH at the same
degree of N-acetylation. Similarly, N-acylated
chitosans were also prepared. The N-acyl
chitosans scarcely affected the formation behavior of PECs with sodium
alginates. For the application of PECs produced, the microencapsulation
of a drug was performed and the release property of drug was tested. The
microcapsules were prepared in one step by the extrusion of a solution
of a guaifenesin and sodium alginate into a solution containing calcium
chloride and chitosan through inter-polymeric ionic interactions. The
drug release during the drug-loaded microcapsules storage in saline was
pH dependent, where the microcapsules were formed and the kind of N-acyl
groups introduced to the chitosan. The microcapsules prepared at pH 4.8
showed a minimum release rate and the release rate varied with the pH
due to the loop formation of backbone chains of polyelectrolytes. The N-acyl groups introduced to the chitosan enhanced the release rate
A number of synthetic
polypeptides have been investigated for medical applications such as
biodegradable suture, artificial skin substitutes and sustained release
devices. The rate of degradation of synthetic polypeptides can be
controlled by a proper selection of amino acid components. Recent
reports revealed, microcapsules prepared from [Glu(OMe)]m
(Sar)n (m=21, n=19) and [Lys(z)]m (Sar)n
(m=27, n=15), and were chemically modified to obtain a pH-responsive
releasing membranes. One membrane was prepared by partially deprotecting
the ester groups of [Glu(OMe)m (Sar)n. The other
membrane was prepared by connecting of poly(Glu) to side chain amino
groups that were generated by a partial deprotection of
[Lys(z)]m (Sar)n. In the later stages, two
types of polypeptide microcapsules were prepared; Glu residues in the
main chain, and Glu residues in the graft chains on the positively
charged main chain. Both microcapsules showed pH-responsive release of
FITC-dextran encapsulated in the microcapsules. The release rate was
observed to be slower in the medium at pH3.0 than pH 7.5. From optical
microscope observation, it appears that partially deblocked [Glu(OMe)]m
(Sar)n microcapsules swelled more at pH 7.5 than at pH 3.0;
might be due to enhanced permeation through the polypeptide membrane at
pH 7.5. However, the authors observed a little change in the shape of
poly(Glu)-grafted [Lys(z)]m (Sar)n microcapsules,
by changing pH of the medium. It is suggested that ion-pairing between
carboxylate groups of poly(Glu) and ammonium groups of Lys acts as
crosslinking to give the shape stability (172).
3.16 Albumin microspheres
Albumin is an attractive
macromolecular carrier and widely used to prepare microspheres and
microcapsules, due to its availability in pure form and its
biodegradability, nontoxicity and nonimmmunogenicity (173). A number of
studies have shown that albumin accumulates in solid tumors (174,175)
making it a potential macromolecular carrier for the site-directed
delivery of antitumor drugs. Recently, Katti and Krishnamurti (176)
prepared albumin microspheres by suspension crosslinking in the absence
of any surfactant using paraffin oil as the dispersion medium and
formaldehyde as the crosslinking agent. They characterized the
microspheres by SEM and found to be spherical having a particle size
distribution in the range of 50-400 mm.
A preliminary drug release study of chlorothiazide in vitro indicated a
diffusion controlled release of drug. Authors claimed that this method
is simple, cost effective and moreover, promising technique for the
large scale manufacturing (176).
The author is grateful to Council
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N.V. Ravi Kumar, Department of Chemistry, University of Roorkee,
Roorkee-247 667, India. email@example.com
Published by the Canadian Society for
Copyright © 1998 by the Canadian Society for Pharmaceutical Sciences.