Drug delivery by an enzyme-mediated cyclization of a lipid prodrug with unique bilayer-formation properties

Article abstract of DOI:10.1002/anie.200805241

Special delivery: Liposomal drug-delivery systems in which prodrugs are activated specifically by disease-associated enzymes have great potential for the treatment of severe diseases, such as cancer. A new type of phospholipid-based prodrug has the ability to form stable small unilamellar vesicles (see picture). Activation of the prodrug vesicles by the enzyme sPLA₂ initiates a cyclization reaction, which leads to the release of the drug.

Full text of DOI:10.1002/anie.200805241

Angewandte
Chemie
DOI: 10.1002/anie.200805241
Medicinal Chemistry
Drug Delivery by an Enzyme-Mediated Cyclization of a Lipid Prodrug
with Unique Bilayer-Formation Properties**
Lars Linderoth, Gꢀnther H. Peters, Robert Madsen, and Thomas L. Andresen*
The development of advanced biomaterials and drug-delivery
systems has had a significant impact on our ability to treat
severe diseases.[1] In the design of nanoparticle-based drug-
delivery systems for intravenous administration, the objective
is to create a particle that is stable during blood circulation,
accumulates to a high degree in the diseased tissue, and is able
to release the drug after accumulation at a rate that matches
the pharmacodynamic profile of the drug.[2,3] Current strat-
egies include the use of lipid-based micelles and liposomes,[1,3]
hydrocolloids,[4] and more recently, polymersomes.[5] Herein,
we report the development of a novel drug-delivery system,
whereby lipid-based prodrugs are formulated as liposomes,
and drug release is affected by an enzymatically triggered
cyclization reaction. We illustrate the idea with secretory
phospholipase A2; however, the principle can equally well be
used with other enzymes, for example, matrix metallopro-
teases.
The use of liposomes for targeted drug delivery to tumor
tissue has attracted increasing attention in the last decade,
with a particular focus on the development of trigger
mechanisms for site-specific drug release.[2,6] We have been
particularly interested in liposomal drug-delivery systems
activated by the enzyme secretory phospholipase A2
(sPLA2).[7] This enzyme is overexpressed in cancerous and
inflammatory tissue and is present in the extracellular
matrix.[8] sPLA2 hydrolyzes the ester group in the sn-2
position of glycerophospholipids and shows dramatically
increased activity when absorbed onto a lipid membrane–
water interface, such as a liposome.[2] We have shown
previously that it is possible to construct long circulating
liposomes that can carry encapsulated drugs to cancerous
tissue and release the drugs upon activation by human
type IIA sPLA2 (sPLA2-IIA).[2] However, it would be very
useful if the activity and elevated levels of sPLA2-IIA in
cancerous tissue could be exploited to activate prodrugs
specifically at the diseased target site. To investigate this
possibility, we studied the versatility of sPLA2 for the
hydrolysis of different lipid structures.[9] This study showed
that sPLA2 tolerates a number of structural changes in the
sn-1 position of glycerophospholipids. On the basis of these
results, we envisioned that a drug could be attached cova-
lently to the sn-1 position of a glycerophospholipid derivative
in such a way that the hydroxy group liberated at the sn-2
position by sPLA2 hydrolysis of the lipid prodrug would react
subsequently with an ester group at the sn-1 position to form a
five-membered lactone and release the carried drug
(Scheme 1).
To investigate this new concept in liposomal drug delivery,
we used the anticancer drug capsaicin[10] as a model drug. The
capsaicin prodrug 8 was synthesized from the commercially
available lactone 1 (Scheme 2). Benzyl protection of lactone
1, followed by reduction with lithium borohydride, gave diol
3. The primary alcohol group of diol 3 was protected with a
silyl group, and the secondary alcohol was coupled with
octadecanoic acid. After removal of the benzyl group of the
resulting ester 4 by hydrogenolysis, the phosphate head group
was introduced by the phosphoramidite method[11] with 5-
phenyl-1H-tetrazole as the proton donor.[12] The silyl protect-
ing group in phosphate 5 was removed with TBAF, and the
alcohol generated was oxidized to the corresponding acid,
which was coupled directly to capsaicin without intermediate
purification. Treatment of the resulting phosphate 7 with
trimethylamine, followed by exposure to acidic conditions, led
to the removal of the protecting groups on the phosphate
moiety to afford the desired prodrug 8.
[*] Dr. T. L. Andresen
Department of Micro- and Nanotechnology
Technical University of Denmark, 4000 Roskilde (Denmark)
Fax: (+45)4677-4791
E-mail: thomas.andresen@nanotech.dtu.dk
Homepage: http://www.cbio.dk
Dr. L. Linderoth, Dr. G. H. Peters, Dr. R. Madsen
Department of Chemistry
Technical University of Denmark, 2800 Lyngby (Denmark)
Dr. G. H. Peters
MEMPHYS-Center for Biomembrane Physics (Denmark)
The capsaicin prodrug 8 was hydrated in a 4-(2-hydroxy-
ethyl)-1-piperazineethanesulfonic acid (HEPES) buffer
(pH 7.5). Its thermodynamic phase behavior was investigated
by differential scanning calorimetry (DSC); however, no
phase transitions were detected in the range 10–708C. The
lipid solution was investigated by dynamic light scattering
(DLS), which revealed a population of vesicles with an
average diameter of 66 nm and a low polydispersity. Fur-
thermore, cryo-TEM images revealed that prodrug 8 self-
aggregates into small unilamellar vesicles (SUVs; Figure 1).
The vesicles had a very uniform appearance, and no multi-
lamellar structures were observed, which was surprising, as it
Dr. R. Madsen
Center for Sustainable and Green Chemistry (Denmark)
[**] Gunnel Karlsson (Biomicroscopy Unit, Polymer and Materials
Chemistry, Lund, Sweden) is gratefully acknowledged for valuable
assistance with cryo-TEM, and Prof. Rolf H. Berg for valuable
discussions. Financial support from DTU, the Danish National
Cancer Agency, and LiPlasome Pharma is gratefully acknowledged.
The Center for Biomembrane Physics and the Center for Sustainable
and Green Chemistry are supported by the Danish National
Research Foundation.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200805241.
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speculate that this behavior
may be a consequence of the
phenyl group of capsaicin
lying in the interface region
in the lipsome bilayer, possi-
bly in combination with the
high net negative charge of
the liposomes.
The absence of a main
phase transition in the scan-
ned temperature range indi-
cates that the phospholipids
do not seem to be organized in
a highly ordered fashion in the
lipid bilayer, possibly as a
consequence of the reduced
hydrophobicity of capsaicin
relative to that of the fatty-
acid chain in naturally occur-
ring phospholipids. To further
Scheme 1. The lipid prodrug forms the liposome membrane and is hydrolyzed by sPLA2 to liberate the drug
after a cyclization reaction.
characterize the SUVs, we
carried out calcein-encapsula-
tion studies, which revealed
that the prodrug 8 does not form vesicles with
the capacity to encapsulate water-soluble com-
pounds. Finally, an attempt to determine the
critical aggregation concentration (CAC) of the
prodrug by isothermal titration calorimetry
(ITC) was not successful, as we found that the
CAC value was below the ITC detection limit
and thus below 10À8 m. From this result, we
conclude that the prodrug will be present
exclusively as aggregated structures.
The vesicles composed of the capsaicin
prodrug were investigated for their susceptibility
to sPLA2 activation and degradation. We used
purified snake-venom sPLA2 (Agkistrodon pis-
civorus piscivorus), which is known to be active
towards a large variety of substrates; we have
Scheme 2. Synthesis of the capsaicin prodrug 8. Bn=benzyl,
DCC=N,N’-dicyclohexylcarbodiimide, DMAP=4-dimethylaminopyri-
dine, DMF=N,N-dimethylformamide, PDC=pyridinium dichromate,
TBAF=tetrabutylammonium fluoride, TBDMS=tert-butyldimethylsilyl,
Tf =trifluoromethanesulfonyl, TMP=2,2,6,6-tetramethylpiperidine.
was not necessary to extrude or sonicate the lipid solution. We
have never previously encountered natural or synthetic lipids
that exclusively form uniform bilayer vesicles directly upon
dispersion in a buffer, as observed for prodrug 8, nor have we
been able to find reports of such lipid behavior in the
literature. An understanding of the basis of this behavior
would be of the highest interest in liposomal and drug-
delivery research, as it would dramatically change the
requisites for liposomal preparation for medical use. We
Figure 1. Cryo-TEM images (two representative images shown) clearly
revealed that the prodrug lipids formed small unilamellar vesicles
(SUVs); the bilayer thickness was measured to be approximately 4 nm.
The specimens were prepared as thin liquid films (<0.3 mm thick) on
lacey carbon films (black area) supported by a copper grid and
plunged into liquid ethane at À1808C. A few liposomes are lying on
top of one another owing to the thickness of the amorphous water
film.
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previously observed that the substrate specificity of this
enzyme is comparable to that of human sPLA2-IIA.[13a] We
used static light scattering at 908 as an indirect measurement
of enzymatic hydrolysis and used HPLC and MALDI-TOF to
quantify the degree of hydrolysis. After the addition of
sPLA2, there is a dramatic change in the static light scattering
(Figure 2a). This result strongly indicates a change in vesicle
Figure 2. a) Static light scattering at 908 as a measurement of the
action of sPLA2 on vesicles composed of the capsaicin prodrug.
b) HPLC chromatogram showing the amount of prodrug before the
addition of sPLA2 (0 h) and the amount of prodrug 24 h after the
addition of sPLA2 (I=intensity, tR =retention time).
morphology as a consequence of sPLA2 hydrolysis. HPLC
also indicated hydrolysis of the lipids by sPLA2 (Figure 2b).
The main focus of the present study was to show that
capsaicin can be released through sPLA2 activation of the
prodrug. The hydrolysis experiments were therefore analyzed
further by MALDI-TOF. For the measurements, we used a
DHB-KCl matrix, which did not interfere with the regions of
interest for the prodrug and free capsaicin (Figure 3a–c).
Figure 3c shows the MALDI-TOF measurements before the
addition of snake-venom sPLA2; it is evident that free
capsaicin is not present in the samples. The prodrug is
therefore stable in the vesicles and does not degrade in the
buffer solution for at least two weeks after the preparation of
the vesicles.
Snake-venom sPLA2 was added to the vesicles, and the
samples were analyzed at different time intervals. The
MALDI-TOF data show clearly that the prodrug is hydro-
lyzed after sPLA2 addition. The rate of hydrolysis shows that
the prodrugs are good substrates for sPLA2. Furthermore, it is
evident from the MALDI-TOF measurements in the region
with the molecular weight of capsaicin that capsaicin is
released from the phospholipids after hydrolysis. This result is
highly interesting, as it shows that the cyclization takes place
at a relatively fast rate. Moreover, it was not possible to detect
any lysophospholipids (lipids that had not cyclized after
hydrolysis by the enzyme), which indicates that the cycliza-
tion is highly favored.
Figure 3. MALDI-TOF measurements of the hydrolysis of capsaicin
prodrug vesicles by sPLA2. a) Matrix noise in the regions of interest.
b) Matrix and capsaicin, K+. c) The amount of prodrug and capsaicin
before (0 s) and 1000 s, 1500 s, and 5000 s after the addition of sPLA2.
d,e) MALDI-TOF measurement of the hydrolysis of prodrug vesicles by
human sPLA2-IIA before (d) and 24 h after (e) the addition of the
enzyme.
enzymatic activation. The amount of capsaicin released
through hydrolysis by human sPLA2-IIA was found to be
(90 Æ 11)% (n = 3) after 24 h, and no uncyclized lysophos-
pholipid was detected. We used sPLA2-IIA from human tear
fluid as a convenient source,[13a] but verified that the
hydrolysis of the prodrug in conditioned medium from Colo
205 colon cancer cells (Colo 205 cells secrete sPLA2-IIA),
with an enzyme concentration of (75 Æ 20) ngmLÀ1 [13b]
,
also
led to full conversion of the prodrug within 24 h.
In summary, we have developed a novel class of lipid-
based prodrugs with unique properties. The prodrugs sponta-
neously form SUVs in water and upon enzymatic activation
release the drug by a cyclization reaction. We illustrated the
concept by using secretory phospholipase A2 as the prodrug
activator; however, the cyclization principle can equally well
be used with other disease-associated enzymes. We expect
that the SUV-formation properties of prodrug 8 is specific to
this structure, and that prodrugs of drugs other than capsaicin
would not express this unique behavior. However, even if new
prodrugs do not form liposomes but other types of aggregates,
the hydrolysis of these aggregates by sPLA2 would still be
We also investigated the activity of human sPLA2-IIA
towards the prodrug vesicles by adding human sPLA2-IIA to
the vesicle solution and analyzing the mixture by MALDI-
TOF during the experiment (Figure 3d,e). The measurements
showed that the capsaicin prodrugs are indeed hydrolyzed by
human sPLA2-IIA and that capsaicin is released after
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and the attachment of other drug molecules are now in
progress.
Received: October 27, 2008
Revised: December 1, 2008
Published online: January 28, 2009
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Keywords: cyclization · drug delivery · liposomes ·
phospholipase · prodrugs
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