Lectin-mediated drug targeting: history and applications
Christiane Biesa
, Claus-Michael Lehra
, John F. Woodleyb,*
a
Department of Biopharmaceutics and Pharmaceutical Technology, Saarland University, Saarbru¨cken, Germany
b
Laboratoire de Cine´tique des Xe´nobiotiques, Faculte´ des Sciences Pharmaceutiques, 35 chemin des Maraıˆchers, 31062 Toulouse, France
Received 8 October 2003; accepted 14 October 2003
Abstract
The purpose of this paper is to review the history of using lectins to target and deliver drugs to their site of action. The hour
of birth of ‘‘lectinology’’ may be defined as the description of the agglutinating properties of ricin, by Herrmann Stillmark in
1888, however, the modern era of lectinology began almost 100 years later in 1972 with the purification of different plant
lectins by Sharon and Lis. The idea to use lectins for drug delivery came in 1988 from Woodley and Naisbett, who proposed the
use of tomato lectin (TL) to target the luminal surface of the small intestine. Besides the targeting to specific cells, the lectin–
sugar interaction can also been used to trigger vesicular transport into or across epithelial cells. The concept of bioadhesion via
lectins may be applied not only for the GI tract but also for other biological barriers like the nasal mucosa, the lung, the buccal
cavity, the eye and the blood–brain barrier.
D 2003 Elsevier B.V. All rights reserved.
Keywords: Lectin; Protein–sugar interaction; Bioadhesion; Drug delivery; Gene delivery; Transcytosis; Vesicular transport; Endocytosis
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426
2. The rationale behind lectin-mediated drug targeting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426
3. Lectin-mediated targeting to the gastrointestinal (GI) tract: the early studies . . . . . . . . . . . . . . . . . . . . . . . 427
4. Bioadhesion studies with lectin-conjugated micro- and nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . 428
5. Lectin-mediated drug absorption enhancement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428
6. The reverse situation: targeting to endogenous lectins. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429
7. Other lectin targeting possibilies in the GI tract: specific cell types and diseased tissues . . . . . . . . . . . . . . . . . . 430
8. Lectin-mediated delivery to the nasal mucosa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430
9. Lectin-mediated delivery to the lungs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430
10. Lectin-mediated drug delivery to the buccal cavity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431
11. Lectin-mediated ocular drug delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431
12. Lectin-mediated delivery at the blood–brain barrier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432
13. Targeting the liver asialoglycoprotein receptor: drug and gene delivery. . . . . . . . . . . . . . . . . . . . . . . . . . 432
14. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433
0169-409X/$ - see front matter D 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.addr.2003.10.030
* Corresponding author. Tel./fax: +33-56-225-6885.
E-mail address: woodley@cict.fr (J.F. Woodley).
www.elsevier.com/locate/addr
Advanced Drug Delivery Reviews 56 (2004) 425–435
1. Introduction
Lectins are proteins that recognise and bind to
sugar complexes attached to proteins and lipids. They
do this with very high specificity for the chemical
structure of the glycan arrays. It is generally acknowledged
by the lectin scientific community that ‘‘lectinology’’
began in 1888 when a young doctor, Hermann
Stillmark, at the University of Dorpat (now
Tartu in Estonia), presented a thesis describing the
agglutinating properties of ricin, which had been
extracted and partially purified from castor seeds
[1]. It has been noted [2], however, that even earlier,
in the 1860s, the agglutinating activity of certain
snake venoms had been observed. The word ‘agglutinin’
was widely used to describe molecules and
extracts that caused the clumping together or agglutination
of erythrocytes and other cells. It was not until
the 1950s that the word ‘lectin’ was coined to describe
the substances from plants that recognised and distinguished
the blood group substances on the basis of the
different sugars expressed [3].
The modern era of lectinology might be considered
to have started with the seminal paper of Sharon and
Lis in 1972. Most lectins described at that time were
isolated from plants and were being widely used as
tools, particularly in histopathology; thanks to the
high levels of specificity that lectins demonstrated
for different cell types, both normal and pathological,
as well as for subcellular structures. Sharon and Lis
listed different lectins that had been purified and a
further seven plant extracts known to agglutinate red
cells and since then the number has increased dramatically
[4]. In that 1972 article, a number of potential
uses for lectins are mentioned, but ‘drug delivery’ is
not one of them. Since 1972, a considerable number
of lectins have also been identified from animal
sources (for a recent review, see [2]).
2. The rationale behind lectin-mediated drug
targeting
The rationale behind lectin-mediated drug targeting
is very simple. Most cell surface proteins and many
lipids in cell membranes are glycosylated and these
glycans are binding sites for lectins. The combination
of a small number of sugars can produce a vast range
of different chemical structures. Different cell types
express different glycan arrays and in particular,
diseased cells, such as transformed or cancerous cells,
often express different glycans compared with their
normal counterparts. Therefore, lectins could be used
as carrier molecules to target drugs specifically to
different cells and tissues.
Apart from the concept of using the specificity of
protein–sugar interactions for targeting to specific
cells only, this kind of receptor-mediated bioadhesion
Fig. 1. The targeting potential of bioadhesive lectins at epithelial barriers: binding, internalisation and intracellular transport (e.g. into or
bypassing acidic endosomal compartments) of lectins and lectin-conjugates depends on the protein–sugar interaction (from [79]).
C. Bies et al. / Advanced Drug Delivery Reviews 56 (2004) 425–435426
may also be used to convey signals to cells in order to
trigger vesicular transport processes into or across
polarised epithelial cells. Haltner et al. [5] compared
cellular binding and endocytosis of three N-acetylgalactosamine
specific mistletoe lectins (ML I–III) with
the N-acetylglucosamin specific lectins from stinging
nettle (UDA), tomato (TL/LEA), wheat germ (WGA)
and a succinylated derivative of the latter lectin
(WGAs) to monolayers of human intestinal epithelial
Caco-2 cells and in particular studied the temperature
dependence of this process on FITC-labelled lectins.
While the N-acetylgalactosamine-specific ML’s
showed identical binding curves at 4 and at 37 jC,
the four N-acetylglucosamine specific lectins showed
a marked difference in binding at the two temperatures.
Temperature independent lectin binding suggests
that those ligands only bind to the outer cell
surface and are not internalised. If binding is reduced
at 4 jC compared to 37 jC, which was observed for
LEA and WGA, the lectins are likely to be endocytosed.
Surprisingly binding of UDA and WGAs, was
significantly reduced at 37 jC compared to 4 jC,
which may be explained by cellular uptake and
subsequent transport to acidic intracellular compartments,
where the fluorescence of FITC was quenched.
Together, such data suggest that depending on the
lectin structure and sugar-specificity, lectins may or
may not be endocytosed, and if so, may or may not be
using an acidic endosomal pathway (Fig. 1). Thus,
when coupled to macromolecular drugs or particular
drug carriers, the selection of a suitable lectin may
perhaps allow the cellular uptake and subsequent
intracellular routing of such delivery systems to be
controlled.
3. Lectin-mediated targeting to the gastrointestinal
(GI) tract: the early studies
It was just 100 years after Stillmark’s thesis presentation
that Woodley suggested that lectins might be
used to target the GI tract. In a paper presented at the
15th Annual Conference of the Controlled Release
Society in Basle in 1988, he proposed the use of
tomato lectin (TL) to target and bind to the luminal
surface of the small intestine, that is the lectin would
demonstrate bioadhesion [6]. Bioadhesion has been
defined as the attachment of a drug carrier to a
specific biological location [7]. The objective was to
exploit the property of bioadhesion to slow down the
intestinal transit time of oral drugs and thus enhance
their bioavailability.
TL had been shown to have specificity for Nacetylglucosamine
and derivatives such as its tetramer
[8]. It was chosen because it was easy to purify, had
been shown to bind to the intestinal mucosa of rats
and was relatively resistant to degradation by intestinal
enzymes. In addition TL was non-toxic to rats
[9,10] and the fact that raw tomatoes were consumed
by millions of people world-wide suggested it was
not toxic to humans. Using radiolabelled TL, Naisbett
and Woodley showed that it bound strongly to rat
intestinal mucosa in vitro, targeting a number of the
major glycoproteins of the intestinal brush border
[11]. As a consequence of binding to mucosal cells,
it was also transported across the mucosa in vitro in
significantly higher amounts than other macromolecules
[12]. The binding was inhibited by a number
of sugars, which are specific binding targets of
the lectin, confirming the true lectin nature of the
interaction.
When the studies moved in vivo the results were
less encouraging. The intestinal transit of orally administered
radiolabelled TL was studied by administering
the lectin into the stomach of rats and
measuring the radioactivity in different sections of
the small intestine at 1, 5, 10 and 24 h. The distribution
was compared with the distribution of an inert
polymer, polyvinylpyrrolidone (PVP) and albumin as
a degradable protein of similar molecular weight. The
TL showed resistance to digestion compared with the
albumin, which was completely degraded during
passage down the gut. However, while there was
some retention of the TL in the upper parts of the
GI tract, there was not a major difference in the transit
between TL and the PVP [13]. The researchers
concluded that the lectin was probably binding to
the intestinal mucus, which is constantly being turned
over and carried down the gut by peristalsis in a
continuous flow. The mucus turnover time in rats has
been estimated to be between 47 and 270 min [14].
Due to the mucus layer covering the intestinal epithelium,
it is not evident that the cell surfaces are often
completely exposed. Subsequently, the interaction
between TL and mucin was confirmed by other
researchers [15–17]. The work described by Naisbett
C. Bies et al. / Advanced Drug Delivery Reviews 56 (2004) 425–435 427
and Woodley was carried out with the soluble lectin,
and it was clear that the next step was to investigate
lectin-modulated bioadhesion using realistic pharmaceutical
drug carriers, such as nanoparticles or liposomes,
as described in the next section.
4. Bioadhesion studies with lectin-conjugated
micro- and nanoparticles
Lehr et al. conjugated TL to the surface of
polystyrene microspheres and demonstrated that they
bound to isolated enterocytes. Inhibition of binding
by tetra-N-acetyl glucosamine confirmed that the
lectin was responsible for the interaction, and it
was also observed that mucin reduced the binding
[15]. As observed by Naisbett and Woodley, the
transit time of the TL-conjugated particles down
the gut was not significantly different from control
particles [18]. Similar results were obtained with
lectins specific for mannose (Galanthus nivalis agglutinin)
or mannose/glucose (concanavalin A). Other
lectins tested did, however, modify the intestinal
transit profile: for example, kidney bean lectin (Phasseous
vulgaris agglutinin) led to delayed transit of the
microspheres and a broad distribution down the small
intestine after administration. Irache and colleagues
conjugated lectins, including TL, to polystyrene latex
beads and demonstrated in vitro binding to intestinal
segments and mucus [19]. The same group has also
made lectin-conjugated nanoparticles. For example
they conjugated a fucose specific lectin from Ulex
europaeus, to nanoparticles made from gliadin, and
demonstrated binding of these particles to mucin [17].
Montisci et al. have used radiolabelled degradable
polylactide microspheres and conjugated to them, TL
or the lectin from Lotus tetragonolobus, which is
fucose specific [20]. Studies of the intestinal transit
of the labelled microspheres orally administered to
rats showed an overall delay in GI transit that was
almost entirely due to retention of the lectin-conjugated
microspheres in the stomach. Strangely this
retention did not appear to be lectin specific as it
was not reduced by preincubating the conjugates
with the lectin specific sugars. Thus the use of lectin
targeting to the GI tract to reduce the transit time of
pharmaceutical formulations has to date had limited
success.
5. Lectin-mediated drug absorption enhancement
The early studies of Woodley and Naisbett had
shown that TL could cross the intestinal mucosa in
vitro [12]. They used an improved everted rat gut sac
to demonstrate that the lectin was endocytosed by
small intestine into the enterocytes and that intact
lectin could be detected on the serosal side of the gut.
At a saturating concentration (15 Ag/ml) the uptake
into the cells was some 40-fold greater than that of the
control inert polymer, PVP, and transfer across the
mucosa some 8-fold greater. Using Caco-2 cells, Lehr
and Lee [21] showed that compared with BSA as
control, the cells took up/bound TL as well as phytohemagglutinin
L and E. However, the transcytosis
across the cell layer was low and the rate for lectins
no higher than the control. On the other hand, using
the same cell line, Russell-Jones et al. [22] were able
to demonstrate translocation across the cell layer of
nanoparticles which had been conjugated to the lectins
wheat germ agglutinin (WGA), concanavalin A (Con
A) and LTB, the binding subunit of heat labile toxin
from E. coli. These in vitro studies suggested that
conjugation of lectins to suitable drug formulations or
the conjugation of drugs to lectins as carrier molecules
might enhance drug delivery to the epithelial cells
and/or to the systemic circulation. Florence and his
colleagues at the London School of Pharmacy have
pioneered studies on the intestinal uptake of microparticles
in the face of prevailing dogma [23]. In 1997,
they reported the results of an in vivo study in rats
where they had conjugated TL to 500 nm polystyrene
nanoparticles and fed them to rats over a period of 5
days. There was a systemic uptake of 23% of the dose
compared with the controls, which were the same
nanoparticles but in which the lectin active sites had
been blocked with a competing sugar. If this scale of
absorption could be achieved in man, then there is
potential for an effective oral drug delivery system
[24] (Table 1).
Another type of lectin interaction that has been
used for similar purposes is that of bacterial adhesion
and invasion factors, proteins that bind to cell surfaces.
The 192 amino acid carboxy terminus of the
invasin of Yersinia pseudotuberculosis has been
shown to promote the binding and uptake of microparticles
in both cultured mammalian cells and the rat
intestine, causing them to enter the systemic circulaC.
Bies et al. / Advanced Drug Delivery Reviews 56 (2004) 425–435428
tion [25,26]. Additionally there are many bacterial and
plant toxins, which gain entry into intestinal epithelial
cells by receptor-mediated endocytosis. Amongst the
bacterial toxins are the A-B5 toxin of Vibrio cholerae,
the E. coli heat-labile toxin (LTB) and the toxin of
Shigella shigae. The B-subunit of this toxin binds to
glycolipids or glycoproteins on the luminal surface of
the epithelial cells and triggers endocytosis [27]. Even
if these adhesion/invasion factors are not lectins in the
strict sense, these mechanisms of bioinvasion have
definite potential for drug delivery.
6. The reverse situation: targeting to endogenous
lectins
While the majority of lectins used and studied are
from plant or microbial origin, it has become clear in
recent years that there exist numerous animal lectins
[2]. In the gut, it was known that certain bacteria
expressed glycan containing molecules in their cell
walls that bound to the epithelial surfaces via lectin
interactions, indicating that there were endogenous
lectins exposed on epithelial cell surfaces which could
be targeted by sugar bearing drug formulations
[28,29] In the 1980s, synthetic polymers bearing
pendant sugar moieties were synthesised as potential
drug carriers by Kopecek and his colleagues and these
were tested for interaction with the GI tract. Different
sugars gave different profiles of interaction with gut
tissue, with galactose bearing polymers showing
greater interaction in proximal regions of the gut,
while fucose bearing polymers consistently showed
the greatest interaction and were more specific for
distal gut regions [30]. This work has been advanced
in Kopecek’s laboratory and is discussed later in this
issue (Minko).
Another class of endogenous lectins which bind via
specific protein–sugar interactions are the selectins.
They have been recognised to play an important role
Table 1
Lectins described in this article and their use in drug delivery
Gal, galactose; GalNAc, N-acetylgalactosamine; GlcNAc, N-acetylglucosamine; NeuNAc, sialic acid; Man, mannose; Fuc, fucose.
C. Bies et al. / Advanced Drug Delivery Reviews 56 (2004) 425–435 429
in inflammatory processes [31]. For instances, they
are involved in the rolling and extravasation of
leukocytes on the endothelial surface of blood vessels.
As such phenomena are also relevant to transport
processes at biological barriers in the context of
controlled drug delivery, a particular chapter by
Bakowsky et al. has been dedicated to the selectins
in this issue.
7. Other lectin targeting possibilies in the GI tract:
specific cell types and diseased tissues
Given the fact that different cell types, both normal
and diseased, express different glycan arrays on their
surfaces as well as demonstrated over the years by the
use of lectins as histochemical tools; the idea of using
lectins as targeting molecules for cell specific drug
delivery is both attractive and feasible, and has
generated considerable interest. In particular the targeting
to the gut associated lymphoid tissue (GALT),
manifested as the Peyer’s patches, has clear possibilities
for the development of more effective oral
vaccines. Immunisation by the oral route has the
considerable advantage of generating secretory antibodies
in addition to systemic immunity. The Peyer’s
patches are covered by specialised cells called M-cells
which endocytose and process macromolecules to
stimulate a chain of immunological events. Lectin
targeting to M-cells has been the subject of intensive
studies as described in detail later in this issue in the
article by Jepson et al.
The other possibility for lectin targeting is to
diseased tissues, notably of the colon. Inflammatory
diseases of the colon and cancer of the colon are
both major medical problems. The possibility, therefore,
of targeting drugs specifically to the diseased
cells is very exciting and because of their specificity
lectins may well have the potential to achieve this
goal. For example, E. coli K99 fimbriae have been
used to target the corticosteroid 6-methyl-prednisolone
to the affected part of the GI tract of patients
with Crohn’s disease [32,33]. The group of Kopecek
has been concentrating on the development of
drug-bearing polymers with lectins to target to
diseased cells. Again, these issues will be covered
in more depth in the later articles by Minko and by
Gabor.
8. Lectin-mediated delivery to the nasal mucosa
While the surface area of the nasal mucosa is
relatively small (150 cm2
), it is highly vascularised
and has a relatively permeable membrane. In addition,
ease of access via the nasal cavity, the lack of first
pass metabolism and rapid onset of action make it an
interesting site for drug administration. The nasal
cavity also contains the equivalent of the GALT in
the form of the nasal-associated lymphoid tissue
(NALT) covered by an epithelial layer of M-cells.
Thus it is also a site of considerable interest for its
potential, for the administration of vaccines, particularly
as vaccination via the nasal cavity induces both
systemic and mucosal immunity. On the other hand
the main disadvantage of drug administration to the
nasal mucosa via the nasal cavity is the low retention
time due to rapid mucociliary clearance: the residence
half life is between 15 and 30 min. This makes it a
good candidate for the use of bioadhesive systems to
increase the contact time between drug or immunogen
and the sites of absorption. While there are many
studies on the nasal administration of bioadhesive
formulations, the emphasis has been on the use of
polymers such as chitosan and carbopols, and to date
there are few studies using lectins. Using the isolectin
B4 from Bandeiraea simplicifolia 1 (BSI-B4) Giannasca
et al. demonstrated lectin-mediated targeting of
antigen to hamster M-cells, resulting in the production
of specific serum IgG, and Kumar et al. have demonstrated
that equine nasopharyngeal tonsillar tissue
contains M-cells that react with a lectin from Bandeireae
simplicifolia. [34,35]. This latter lectin (GS I-B4)
has also been shown to be almost exclusively M-cell
specific for rat NALT, in contrast to other lectins
tested (UEA-1, DBA, WGA) and it suppressed the
uptake of yeast particles by the M-cells [36]. Thus
although studies on lectin targeting to the upper
respiratory tract are still very preliminary, the possibilities
for vaccine administration look interesting.
9. Lectin-mediated delivery to the lungs
Compared with the nasal cavity, the lungs have a
very large surface area (75 m2
) and the thinness of the
alveolar epithelium (0.1–0.5 Am) may facilitate rapid
drug absorption. As with the nasal cavity, first pass
C. Bies et al. / Advanced Drug Delivery Reviews 56 (2004) 425–435430
metabolism is avoided, and the relative lack of proteolytic
enzymes (compared with the gut, for example)
makes the pulmonary administration of peptides
and proteins an attractive proposition.
Early histological data and more recent studies
revealed the binding of lectins to tissues of the airway
epithelium [37–39]. In addition it has been shown
that several lectins that bound to the apical surfaces of
lung cells in culture were actively taken up by the
cells [40]. This binding and uptake of lectins by lung
cells including carcinoma cells has particularly attracted
the attention of gene therapists to enable them
to target and enhance the uptake of DNA. Thus, for
example, lectins have been shown to target a plasmid
selectively to carcinoma cells [41] and to enhance the
lipofection efficiency of different lung carcinoma cells
[42].
The disease that has attracted most attention for
gene delivery to lung cells is cystic fibrosis (CF), a
relatively common disease (one in 2000–2500 Caucasian
births) with several identified mutations in the
gene coding for a membrane chloride transporter. In
early studies, lectins were shown to enhance gene
transfer of polylysine and/or histone plasmid complex
to CFT1 cells, a human tracheal cell line derived from
a CF patient homozygous for the DF508 mutation.
More recently the reverse use of lectin targeting, i.e.
targeting sugar bearing carriers to endogenous lectins
has been exploited. Fajac et al. [43,44] have developed
glycosolated polycations (glycofectins) and
complexed them with DNA. These complexes were
more efficient at transfection than naked DNA. The
most successful were lactosylated complexes that
promoted both cellular uptake and intracellular trafficking
to the nucleus.
Further down the respiratory tract the alveolar
epithelium constitutes the major barrier to macromolecular
drug absorption into the pulmonary circulation
[45,46]. The alveolar epithelium consists of two
distinct epithelial cell types: the cuboidal type II
cells, that produce the lung surfactant and serve as
progenitor cells for the type I cells [47,48]. Type I
cells cover 93% of the surface of the alveolae and
appear as very thin cells with protruding nuclei
providing a short diffusion path for gas exchange.
Both cell types possess a different lectin binding
specificity. While Maclura pomifera agglutinin binds
specifically to type II cells, Ricinus communis agglutinin
and TL show specific binding to type I cells
[49,50]. Bru¨ck et al. (2001) showed the binding of
lectin-functionalised liposomes to human alveolar
cells in primary culture and the uptake of FITClabelled
dextrans encapsulated in these liposomes
[51]. Nebulisation of these functionalised liposomes
did not significantly influence their physical stability
and cell binding capacity and led to a deposition of
the liposomes in the lower parts of the lung. These
results make the functionalised liposomes potential
candidates as macromolecule-drug carriers for local
and systemic administration [52].
10. Lectin-mediated drug delivery to the buccal
cavity
The buccal cavity has a surface area of f 50 cm2
with a relatively poor permeable non-keratinised epithelium.
Like the nasal cavity, due to the large flow of
saliva, substances in the buccal cavity have a low
residence time ( < 5–10 min) and hence it is a prime
site for the use of bioadhesive formulations, some of
which are now marketed. Lectin targeting to this site
is being actively investigated and is dealt with in
detail in a later section of this volume (J. Smart).
11. Lectin-mediated ocular drug delivery
There are two major surface tissues of the eye
facing the outside world, the conjunctiva and the
cornea. The conjunctiva contains goblet cells secreting
mucin, but there are no goblet cells on the cornea.
Mucus is spread over both epithelia by the action of
blinking, and bioadhesive polymers will attach to the
conjunctival mucus. The turnover rate of the mucin is
f 15–20 h, whereas normal tear turnover time is
16%/min. It has been suggested that ocular drug
delivery may be prolonged by conjugation to lectins
that adhere to the corneal and conjunctival epithelia,
and this may enhance drug absorption across these
epithelia. The conjunctiva has associated lymphoid
tissues (CALT), although it is still unclear if the
associated epithelium contains equivalents to the Mcells
of the intestine [53]. The presence of lectin
binding sites has been demonstrated on the corneal
and conjunctival epithelia of humans and other speC.
Bies et al. / Advanced Drug Delivery Reviews 56 (2004) 425–435 431
cies [54,55]. Schaeffer et al. [56] showed that pretreatment
of rabbit corneas with wheat germ agglutinin
increased the binding of ganglioside-containing
liposomes, and carbachol entrapped in these liposomes
showed enhanced flux across the cornea. At
the present time, there does not appear to be the
development of major lectin-based systems for ocular
drug delivery. Because the systemic absorption of
ocularly applied drugs is relatively low, the risk of
systemic side effects of the lectins should be relatively
small.
12. Lectin-mediated delivery at the blood–brain
barrier
The endothelial cells of brain capillaries lack fenestrations,
have few pinocytic vesicles and form very
tight junctions, which are responsible for the formation
of the blood–brain barrier (BBM), which restricts the
movement of most molecules from the blood to the
brain [57]. The BBM is a formidable barrier to entry
of many drugs into the brain, notably anticancer
drugs, and a number of different approaches have
been proposed to overcome the limited access of
drugs to the brain. Fischer and Kissel [58] showed
the binding of some plant lectins to primary endothelial
cells isolated from porcine brain, especially
WGA, which seems to be a good candidate for drug
targeting to the blood–brain barrier due to its high
affinity for the cerebral capillary endothelium compared
with other lectins and its low cytotoxicity. In
addition WGA has been shown to enhance the uptake
of HIV-1 gp 120, usually slow at crossing the blood–
brain barrier, without disrupting the barrier function
[59].
13. Targeting the liver asialoglycoprotein receptor:
drug and gene delivery
As already described for gene targeting to lung
cells, genes can be targeted to their destination by
conjugation to sugar moieties specific for animal or
reverse lectins. The best-described example of such
an animal lectin is the asialoglycoprotein receptor
(ASGPr), a C-type animal lectin that is expressed on
the surface of hepatocytes [60]. It plays a role in the
clearance (endocytosis and lysosomal degradation) of
deasialylated proteins from the serum [61,62]. The
ASGPr recognises terminal ß-D-galactose or N-acetylgalactosamine-residues
[63]. This receptor has been
investigated as a site for drug targeting using carriers
bearing galactose residues. The most advanced example
of such systems is the N-(2-hydroxypropyl) methacrylamide
(HPMA) anticancer polymer conjugates
developed by Ruth Duncan and her colleagues [64].
These consist of a linear polymer backbone with
drugs attached to the backbone via short peptide
sequences that are hydrolysed by intracellular
enzymes to release the drug. Targeting moieties can
also be affixed to the polymer backbone. One of these
conjugates (PK2, FCE28069) consists of the HPMA
backbone with f 7.5 wt.% doxorubicin and f 1.5–
2.5 wt.% galactose to target to the ASPGr. This
compound has been studied in Phase I/II clinical trials
and gamma camera imaging confirmed 15–20% of
the dose targeted to the liver at 24 h following
administration [65,66]. Other carriers to target the
receptor have been proposed, notably liposomes. For
example, Yu and Lin showed that asialofetuin-labelled
liposomes enhanced the delivery of the hydrophilic
molecule inulin into hepatoma cells and they are
potential drug carriers for the intracellular delivery
of membrane-impermeable drugs to liver cells [67].
The ASPGr has also been investigated for its
potential in enhancing gene transfer into hepatic cells.
The first approaches to use this receptor for the
specific targeting of genes to liver cells have been
undertaken by Wu et al. [68,69]. They used PLLparticles
coupled to an asialoglycoprotein moiety to
complex DNA. They were able to successfully transform
rat and rabbit liver with this approach, but the
transformation was only transient and with low efficiency
[70,71]. To overcome these drawbacks, efforts
have been made to make more stable and soluble
particles with a well defined structure [72,73]. Lipoplexes
are a non-viral gene delivery system formed
when cationic liposomes are mixed with plasmid
DNA. This system has also been optimised for gene
targeting to the liver by the conjugation of modified
galactolipids [74,75]. Hara et al. developed asialofetuin-labelled
liposomes as a vector system that is
effective in gene expression in vivo after intraportal
injection in adult mice [76,77]. As far as the in vivo
efficiency has been documented the major limitation
C. Bies et al. / Advanced Drug Delivery Reviews 56 (2004) 425–435432
of most of these methods is the need for local
administration or a partial hepatectomy. Recently,
Arangoa et al. developed improved protamine-enhanced-asialofetuin-lipoplexes.
Addition of protamine
sulphate leads to smaller complexes that are better
suited for efficient endocytosis. A nuclear localisation
signal in the protamine sequence results in efficient
targeting to the nucleus and obviates the need for
partial hepatectomy [78].
14. Conclusions
From modest beginnings as potential tools for
specific drug targeting and bioadhesion applications
some 20 years ago, lectins are realising a number of
important applications in the field, as reflected by the
articles in this issue. Some of the problems associated
with any macromolecular targeting system still have
to be tackled, notably those of toxicity and immunogenicity.
It is hoped that some of these problems
might be overcome in the future by the application
of biotechnology techniques to produce quantities of
smaller fragments of lectins that will retain the high
target specificity that these fascinating molecules
possess, but will be easier to manipulate. The use of
lectins in drug targeting is a fledgling subject that will
surely grow in the years to come.
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