An anticoagulant with light-triggered antidote activity

Article abstract of DOI:10.1002/anie.200602346

(Chemical Equation Presented) A light remedy to overdosing: Aptamers with an intramolecular "caged" antisense region remain active until they are irradiated. Upon irradiation, the antisense activity is released and the aptamer refolds into an inactive str

Full text of DOI:10.1002/anie.200602346

Communications
Antisense Agents
antisense-based antidote molecule that is able to anneal to
the aptamer and thus reverse its inhibitory function.
DOI: 10.1002/anie.200602346
Another aptamer with anticoagulant activity is the 15mer
single-stranded DNA molecule AW (Scheme 1a)that binds to
An Anticoagulant with Light-Triggered Antidote
Activity**
Alexander Heckel,* Maximilian C. R. Buff,
Marie-Sophie L. Raddatz, JensMüller, Bernd Pötzsch,
and Günter Mayer*
Drug-inhibiting antidotes are helpful tools for the prevention
and treatment of unwanted side effects caused by overdosing.
These are of particular importance in the case of anticoagu-
lants, the overdosing of which may induce life-threatening
bleeding. Among the available anticoagulants, unfractionated
heparin is the only one that can be effectively neutralized by a
specific antidote. The lack of a specific antidote is a serious
disadvantage of all novel anticoagulants (e.g. synthetic
heparin, factor Xa, and thrombin inhibitors)even though
they are more effective antithrombotics and possess several
other advantages over unfractionated heparin.[1] As a con-
sequence unfractionated heparin remains the anticoagulant
of choice for patients with a high risk of bleeding or in clinical
situations in which rapid reversal of the anticoagulant activity
is required, although its use is associated with potential
drawbacks including immune reactions.
An alternative approach to the development of antidotes
for anticoagulants was introduced recently by Sullenger and
co-workers.[2] They used aptamers that target factor IXa of
the blood-clotting cascade as effective anticoagulant. Apta-
mers are short, single-stranded nucleic acids that fold into
well-defined three-dimensional shapes.[3] They bind with high
affinity and specificity to their respective target molecules and
can be used, for example, as potent inhibitors. For the
development of an antidote, Sullenger took advantage of the
inherent properties of nucleic acids and developed an
Scheme 1. a) The aptamer AW binds thrombin selectively and can act
as anticoagulant. b) Our concept of an aptamer that contains its own
antidote (in an inactive form). The antidote activity is triggered by light
irradiation. c) The “caged” nucleic acid residue dCNPE, which acts as a
temporary mismatch, synthesized for this purpose. NPE=1-(2-nitro-
phenyl)ethyl.
and inactivates thrombin, which is a key protein in the blood-
clotting cascade.[4] The antithrombin aptamer folds under
physiological conditions into a stable G-quadruplex structure
and consists of six thymidine and nine guanosine nucleotides.
In our studies on the development of methods to control
biological processes spatiotemporally with light, we have
shown that aptamer activity can be triggered by light after the
active site has been temporarily blocked with a photolabile
group[5] or the formation of the active conformation of the
aptamer has been prevented by modification of a key
residue.[6]
Herein we describe the development of a single oligonu-
cleotide that contains both an aptamer region and a tempo-
rarily inactivated “caged” antisense region in the one
molecule and can hence act as both an anticoagulant and its
own antidote, the latter function being triggered by light. We
assumed that variants of the aptamer that bear an elongated
5’- or 3’-region that is complementary to the G-quadruplex
sequence are inactive because they are locked in a hairpin
conformation rather than folded into the active G-quadruplex
(Scheme 1b). However, if the antisense region contains
“temporary mismatches”,[7] the hairpin is not formed until
the molecule is irradiated and the photolabile group is
removed.
[*] Dr. A. Heckel, Dipl.-Chem. M. C. R. Buff,
Dipl.-Chem. M.-S. L. Raddatz, Dr. G. Mayer
Life and Medical Sciences (LIMES)
Program Unit Chemical Biology and Medicinal Chemistry
KekulØ Institute for Organic Chemistry and Biochemistry
University of Bonn
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
Fax: (+49)228-73-4809
E-mail: heckel@uni-bonn.de
gmayer@uni-bonn.de
Dr. J. Müller, Prof. Dr. B. Pötzsch
Institute for Experimental Hematology and Transfusion Medicine
University Hospital Bonn
Sigmund-Freud-Strasse 25, 53105 Bonn (Germany)
[**] This work was made possible by grants of the Fonds der
Chemischen Industrie (for A.H. and G.M.) and by an Emmy Noether
Fellowship (DFG) for A.H. The authors thank Prof. M. Famulok for
his continuingand encouragingsupport.
To ascertain the ideal length of the antisense region we
synthesized the aptamer variants A1–A3 (Figure 1a). The
respective antisense regions are linked to the 5’-end of AW by
Supportinginformation for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Angewandte
Chemie
On the basis of these data we decided to extend our
toolbox of “caged” deoxynucleotides that can be activated by
light to the residue dCNPE (see Scheme 1c).[9] A protected
phosphoramidite of dCNPE was used to synthesize the caged
aptamer–antidote chimeras A4 and A5 (Figure 2a; see the
Supporting Information for synthetic details).
Figure 1. a) The sequence of the 5’-end-elongated aptamers A1–A3.
b) Results of blood-clottingexperiments with the aptamers AW and
A1–A3 (relative blood-clottingtime versus aptamer concentration, see
the SupportingInformation for experimental details).
a GAAA sequence, which builds a stable GNRA tetraloop
motif (N = any nucleotide, R = purine). We then performed
filter-binding experiments with A1–A3 to determine which of
the variants are still able to bind to thrombin (Table 1). The
Table 1: Dissociation constants (in nm) of aptamer variants determined
by filter-bindingexperiments.
Without
irradiation
With
irradiation[a]
AW
A1
A2
A3
A4
A5
124Æ16
155Æ23
333Æ44
>5000
n.d.
n.d.
n.d.
Figure 2. a) The sequence of the caged aptamer–antidote chimeras A4
and A5. b) Results of the blood-clottingexperiments with the aptamers
A1, A3, and A4 (before and after irradiation, relative blood-clottingtime
versus aptamer concentration, see the SupportingInformation for
experimental details). c) For a better comparison, the values trel. in
(b) are shown at an aptamer concentration of 1500 nm.
n.d.
166Æ12
>5000
>500
297Æ44
[a] n.d.=not determined.
A4 and A5 were analyzed for their ability to bind to
thrombin and to inhibit thrombin-dependent blood clotting.
As demonstrated in Figure 2b and c, A4 is indeed able to
prolong the blood-clotting time in a concentration-dependent
manner and with only slightly less efficiency than A1. As
anticipated, A4 becomes inactive upon irradiation—except at
very high concentrations for which some remaining activity
could be detected. A5, however, remained inactive before and
after irradiation (data not shown). Circular dichroism (CD)
spectra of the aptamers in this study can be found in the
Supporting Information. From a photochemical standpoint, it
was interesting for us to see how well the dCNPE residue
performed in the photodeprotection step. Generally, one
might have anticipated from the literature that the removal of
elongation of the AW aptamer with the tetraloop-forming
stretch (A1)results in a slightly decreased affinity. The
addition of two further dC nucleotides (A2)decreases the
affinity of the aptamer even further, and almost no affinity
was detected after addition of two more dA nucleotides
(A3).[8] Thus, we conclude that already in the aptamer variant
A3, the hairpin rather than the G-quadruplex conformation is
predominant.
Furthermore, we analyzed which of the aptamer variants
inhibits thrombin-mediated blood clotting in human plasma.
As depicted in Figure 1b, aptamer A1 is still active. However,
in comparison with AW, the same effect is obtained only at
higher concentrations. This result has to be kept in mind
because even with a completely inactive antisense region the
resulting aptamer can only be as effective as A1. Aptamer A2
showed only very little remaining activity, and A3 was found
to be inactive within error limits. Thus, the data obtained from
the blood-clotting studies are in good accord with the results
observed in the interaction studies.
the NPE group would create problems because of the often-
[10]
À
encountered difficulty to photocleave C N bonds. How-
ever, with our experimental setup (360 nm from three UV
LEDs of ca. 100 mW each), 100 pmol of A4 could be
deprotected in 7 s, and the deprotection step was remarkably
clean.
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Communications
For the experiments discussed so far in the text, the
activity of nucleic acid based modulators of protein function
in a spatiotemporal fashion.
irradiation of the caged aptamers was performed before the
beginning of the experiment. Thus it was unclear whether a
preformed complex of A4 and thrombin could also be
destroyed by irradiation. Figure 3 shows the respective
blood-clotting and filter-binding assays, which show that the
photodeactivation also functions when A4 is already bound to
thrombin—a fact that will be important for applications
in vivo.
Received: June 12, 2006
Published online: September 19, 2006
Keywords: antisense agents · aptamers · blood clotting ·
.
caged compounds · DNA
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[8] The same was true for aptamers with two or four more
complementary nucleotides (data not shown).
[9] Literature on caged compounds in general and caged oligonu-
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Figure 3. a) Results of blood-clottingassays in which the complex of
thrombin and A4 was irradiated, as well as control experiments with A3
and without (w/o) aptamer (concentration of aptamer: 3 mm).
b) Results of filter-bindingassays in which the irradiation was per-
formed before and after formation of the complex with thrombin
(y axis: relative amount of A4 bound, concentration of thrombin
150 nm).
In conclusion, after having demonstrated that it is possible
to activate an aptamer with light,[5,6] we have now demon-
strated that deactivation with light is also possible by
incorporating a caged antisense region. Before photodeacti-
vation, the caged chimera showed only about half of the
activity of the unmodified wild-type aptamer, but this activity
can easily be compensated by increasing the concentration. It
is more important that the aptamer becomes virtually inactive
upon irradiation. As before,[6] the location of the caged
residue (in this case the caged dCNPE)is very important. The
new aptamer described herein combines the advantages of a
highly specific anticoagulant with rapid and effective control
of its function. These characteristics are of potential benefit to
patients who are at high risk of anticoagulant-associated
bleeding and in clinical situations in which rapid reversal of
the anticoagulant functions is required, such as in antico-
agulation of extracorporeal circulation devices. Furthermore,
this approach will be of general applicability to control the
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