A new drug-release method using the Staudinger ligation

Article abstract of DOI:10.1016/j.bmcl.2006.03.073

Many drugs induce severe side-effects caused by their lack of selectivity. One way to overcome this problem is to design a specific system which releases a free drug in a controlled manner. Herein we describe a new way to liberate a drug from a prodrug using the Staudinger ligation as the trigger.

Full text of DOI:10.1016/j.bmcl.2006.03.073

Bioorganic & Medicinal Chemistry Letters 16 (2006) 3147–3149
A new drug-release method using the Staudinger ligation
*
´
Michel Azoulay, Gerald Tuffin, Wafa Sallem and Jean-Claude Florent
UMR 176 CNRS/Institut Curie, Centre de Recherche, 26 rue d’Ulm, F-75248 Paris Cedex 05, France
Received 27 February 2006; revised 16 March 2006; accepted 17 March 2006
Available online 18 April 2006
Abstract—Many drugs induce severe side-effects caused by their lack of selectivity. One way to overcome this problem is to design a
specific system which releases a free drug in a controlled manner. Herein we describe a new way to liberate a drug from a prodrug
using the Staudinger ligation as the trigger.
Ó 2006 Elsevier Ltd. All rights reserved.
The Staudinger reaction was developed in 1919 by Stau-
dinger and Meyer.1,2 This reaction consists in the nucle-
ophilic attack of a phosphine on an azide leading to a
phosphazide which, through the loss of nitrogen, gives
a reactive aza-ylide. In the presence of water, this inter-
mediate spontaneously hydrolyzes to yield a primary
amine and the corresponding phosphine oxide. Saxon
and Bertozzi reported a modification of the Staudinger
reaction for the first time in 2000, ‘the intramolecular
Staudinger ligation’.3,4 The elegant modification consists
in positioning an electrophilic trap, such as a methyl es-
ter, into the phosphine structure in order to capture the
aza-ylide intermediate by an intramolecular cyclization.
This reaction is compatible with a large number of func-
tional groups and therefore has various uses in organic
synthesis and biological chemistry. The Staudinger liga-
tion was applied to peptide synthesis5,6 or to the prepa-
ration of carbohydrate–protein conjugates.7 The
modified Staudinger reaction has already been used for
the immobilization of substrates to surfaces,8,9 in the
investigation of cellular metabolism of synthetic
azido-sugars,10 also for biological labeling11,12 and even
on living organisms such as mice.13
intramolecular cyclization to generate an hydroxy–ben-
zylic–carbonyl system, known to initiate a 1,6-elimina-
tion process.14,15 This kind of self-eliminating linker16–18
is widely used in drug targeting.
We designed a phosphine 1 as a potential prodrug that
would enable a new rearrangement of the aza-ylide a
(Scheme 1). We postulated that this aza-ylide a would
generate the phenol anion b which would undergo a
R
R
HN
O
HN
O
O
O
P
N3 R'
N2
R'
Ph
O
O
O
O
P N
NH
Ph
a
NH
Ph
Ph
1
Drug
Drug
CO2
O
P
O
Ph
Ph
Ph
O
O
P Ph
H
NH
Drug
N
R'
N
N
Here, we propose a new application of the Staudinger
ligation to trigger drug liberation from a prodrug. The
main goal of this project is to release a free drug in a
stringently controlled manner. For this purpose, the
Staudinger ligation was carried out with a modified
electrophilic trap, as a phenyl carbamate to capture
the aza-ylide intermediary formed. We expected the
NH2
Drug
R'
N R
H
O
R
O
c
b
H2O
OH
H
H
N
Ph
N
HO
P
R
R'
O
Ph
O
Keywords: Prodrug; Staudinger ligation; Doxorubicin; Drug releaser.
*
Scheme 1. Postulated mechanism of prodrug activation via the
Staudinger ligation.
Corresponding author. Tel.: +33 1 4234 6658; fax: +33 1 4234
6631; e-mail: jean-claude.florent@curie.fr
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.03.073

3148
M. Azoulay et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3147–3149
1,6-quinone methide rearrangement, followed by spon-
taneous decarboxylation to liberate the drug entity.
Then the quinone methide c would be hydrolyzed by
water to form urea and phosphine oxide compounds.
CHO
CHO
CHO
ii
iii
2
2
R
P
P
H
OH
OH
N O
O
3 R = H
4 R = I
To provide a proof of this concept, we synthesized the
two following compounds (Scheme 2): 1a, harboring a
4-nitroaniline (pNA) as a model compound representing
a potential amine-drug; 1b, carrying doxorubicin as a
drug. We then tested a model reaction between these
two phosphine compounds 1a or 1b and the commer-
cially available azido-compound 2 in aqueous THF
(1:1) at 37 °C.
i
6
5
iv
OH
O
O
O
P
NO2
vi
H
H
P
2
N O
O
N O
2
The synthesis of the prodrug analogue 1a is described in
Scheme 3. The commercially available compound 3 was
reacted with iodine monochloride19 to yield the mono-
iodoaldehyde 4. The tertiary phosphine 5 was obtained
by palladium-catalyzed cross-coupling between diphe-
nylphosphine and the iodo-compound 4 as described
by Herd et al.20 Condensation of compound 5 with the
commercially available n-propylisocyanate in the pres-
ence of a catalytic amount of pyridine21 led to the carba-
mate 6. Reduction of compound 6 by NaBH4 in a
mixture of chloroform and 2-propanol22 produced the
corresponding alcohol 7. The latter was mixed with
the 4-nitrophenylisocyanate to generate the 4-nitroanili-
necarbamate as the model compound 1a.
O
8
7
v
vii
1b
1a
Scheme 3. Reagents: (i) ICl, CH2Cl2, 60%; (ii) Ph2PH, CH3CN,
Pd(OAc)2, KOAc, 44%; (iii) n-propylisocyanate, CH2Cl2, 91%; (iv)
NaBH4, CHCl3/CH3(CH2)2OH (5:1), 85%; (v) 4-nitrophenylisocya-
nate, CH2Cl2, 60%; (vi) 4-nitrophenylchloroformate, pyridine,
CH2Cl2, 58%; (vii) doxorubicin, DMF, Et3N, 33%.
The doxorubicin prodrug 1b was obtained in two steps
from compound 7, as shown in Scheme 3. Acylation of
the alcohol 7 with 2 equivalents of 4-nitrophenylchloro-
formate23 produced the carbonate 8, which was then
reacted with doxorubicin to give the expected prodrug 1b.
With these two compounds in hand, we first examined
the reaction between the model compound 1a and the
water-soluble azido-compound 2 (Fig. 1).
When 1a was incubated with compound 2, the increase
in absorbance was immediately obvious. 4-Nitroaniline
formation was monitored by UV spectroscopy at
405 nm wavelength.24 The released 4-nitroaniline clearly
showed that compound 2 initiated an intracyclization
followed by a 1,6-quinone methide rearrangement,
Figure 1. (A) Incubation of compounds 1a (5 mM) and 2 (10 mM);
(B) pNA release was monitored by its absorbance, as a function of
time, with or without 2.
OH
O
O
OH
OH
NO2
NH
CH3O
O
OH
O
O
according to the proposed pathway presented in Scheme
1. Without 2, no absorbance of pNA was observed.
CH3
O
HO
O
In the second experiment, we tested the same reaction
with the doxorubicin prodrug 1b (Fig. 2).
NH
O
O
H
P
N
O
The release of the free doxorubicin was monitored by re-
verse-phase HPLC. As in the first experiment, no drug
liberation was observed in the absence of the azido-com-
pound. Doxorubicin liberation was the second evidence
that the reaction followed the mechanism illustrated in
Scheme 1. To evaluate the efficiency of the activation,
we quantified the drug release as described in
2
H
P
2
N
O
O
O
1b
1a
Scheme 2. Structures of the potential amine-prodrug 1a and the
doxorubin prodrug 1b.

M. Azoulay et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3147–3149
3149
References and notes
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Supplementary data. After 3 h at 37 °C in aqueous
THF, the reaction appeared to approach 90%
completion.
In summary, we have successfully demonstrated for the
first time, the utility of the Staudinger ligation to liberate
a drug selectively and in good yield. In this report, we
have used an experimental model. We now wish to
extend our results to the complex environment of a liv-
ing cell. The Staudinger ligation seems perfectly suitable
for such an application, since the two reaction partners
are biocompatible and orthogonal to most biological
functionalities. Further studies to take advantage of this
new drug-release method are currently in progress.
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Acknowledgment
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Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.bmcl.2006.
03.073.
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