Rapid noninvasive detection of experimental atherosclerotic lesions with novel 99mTc-labeled diadenosine tetraphosphates

Article (PDF Available)inProceedings of the National Academy of Sciences 95(2):691-5 · February 1998with10 Reads
DOI: 10.1073/pnas.95.2.691 · Source: PubMed
Abstract
The development of a noninvasive imaging procedure for identifying atherosclerotic lesions is extremely important for the clinical management of patients with coronary artery and peripheral vascular disease. Although numerous radiopharmaceuticals have been proposed for this purpose, none has demonstrated the diagnostic accuracy required to replace invasive angiography. In this report, we used the radiolabeled purine analog, 99mTc diadenosine tetraphosphate (Ap4A; AppppA, P1,P4-di(adenosine-5')-tetraphosphate) and its analogue 99mTc AppCHClppA for imaging experimental atherosclerotic lesions in New Zealand White rabbits. Serial gamma camera images were obtained after intravenous injection of the radiolabeled dinucleotides. After acquiring the final images, the animals were sacrificed, ex vivo images of the aortas were recorded, and biodistribution was measured. 99mTc-Ap4A and 99mTc AppCHClppA accumulated rapidly in atherosclerotic abdominal aorta, and lesions were clearly visible within 30 min after injection in all animals that were studied. Both radiopharmaceuticals were retained in the lesions for 3 hr, and the peak lesion to normal vessel ratio was 7.4 to 1. Neither of the purine analogs showed significant accumulation in the abdominal aorta of normal (control) rabbits. The excised aortas showed lesion patterns that were highly correlated with the in vivo and ex vivo imaging results. The present study demonstrates that purine receptors are up-regulated in experimental atherosclerotic lesions and 99mTc-labeled purine analogs have potential for rapid noninvasive detection of plaque formation.
Proc. Natl. Acad. Sci. USA
Vol. 95, pp. 691–695, January 1998
Medical Sciences
Rapid noninvasive detection of experimental atherosclerotic
lesions with novel
99m
Tc-labeled diadenosine tetraphosphates
DAVID R. ELMALEH*
,JAGAT NARULA*
,JOHN W. BABICH*, ARTIOM PETROV
,ALAN J. FISCHMAN*,
BAN-AN KHAW*
,ELIEZER RAPAPORT
§
, AND PAUL C. ZAMECNIK
§
*Division of Nuclear Medicine of the Department of Radiology, Massachusetts General Hospital and the Department of Radiology, Harvard Medical School,
Boston, MA 02114;
Center for Molecular Targeting, Northeastern University, Boston, MA 02210;
§
Hybridon, Inc., Cambridge, MA 02139; and
Imaging
Biopharmaceuticals, Cambridge, MA 02142
Contributed by Paul C. Zamecnik, October 31, 1997
ABSTRACT The development of a noninvasive imaging
procedure for identifying atherosclerotic lesions is extremely
important for the clinical management of patients with cor-
onary artery and peripheral vascular disease. Although nu-
merous radiopharmaceuticals have been proposed for this
purpose, none has demonstrated the diagnostic accuracy
required to replace invasive angiography. In this report, we
used the radiolabeled purine analog,
99m
Tc diadenosine tet-
raphosphate (Ap
4
A; AppppA, P
1
,P
4
-di(adenosine-5*)-tetra-
phosphate) and its analogue
99m
Tc AppCHClppA for imaging
experimental atherosclerotic lesions in New Zealand White
rabbits. Serial gamma camera images were obtained after
intravenous injection of the radiolabeled dinucleotides. After
acquiring the final images, the animals were sacrificed, ex vivo
images of the aortas were recorded, and biodistribution was
measured.
99m
Tc-Ap
4
A and
99m
Tc AppCHClppA accumulated
rapidly in atherosclerotic abdominal aorta, and lesions were
clearly visible within 30 min after injection in all animals that
were studied. Both radiopharmaceuticals were retained in the
lesions for 3 hr, and the peak lesion to normal vessel ratio was
7.4 to 1. Neither of the purine analogs showed significant
accumulation in the abdominal aorta of normal (control)
rabbits. The excised aortas showed lesion patterns that were
highly correlated with the in vivo and ex vivo imaging results.
The present study demonstrates that purine receptors are
up-regulated in experimental atherosclerotic lesions and
99m
Tc-labeled purine analogs have potential for rapid nonin-
vasive detection of plaque formation.
It is becoming increasingly clear that atherosclerosis is an
immuno-inflammatory process that involves complex interac-
tions between the vessel wall and blood components (1–5). The
atherogenetic process involves sequestration of partially oxi-
dized lipids in the vessel wall (6), which leads to endothelial
injury. Endothelial injury promotes adherence of mononuclear
cells and platelets that contribute to phenotypic transforma-
tion of medial smooth muscle cells (SMCs) from adult to
embryonic forms. The transformed muscle cells proliferate
and migrate to the intima in parallel with accumulation of
lipids by monocytes that leads to the formation of foam cells.
Clearly, platelets, macrophages, and proliferating SMCs of
atheroscolerotic plaque provide important targets for the
development of noninvasive diagnostic agents (4–10).
Extra cellular adenine nucleotides are released from a
variety of cells and regulate many physiological processes by
interaction with P2 purine receptors or adenosine (A1, A2, and
A3) receptors (11–14). These compounds can accumulate in
atherosclerotic plaque by two mechanisms: (i) binding to
platelets via interaction with platelet P2T receptors (15), which
inhibits platelet aggregation; and (ii) binding to P2x and P2y
purine receptors on macrophages, monocytes, and SMCs that
are present at high concentrations in atherosclerotic lesions.
Recently, it was shown that the adenine analog, Ap
4
A
(AppppA, P
1
,P
4
-di(adenosine-59)-tetraphosphate), is ubiqui-
tous in living cells and appears to play an important role in
extracellular signaling events in a variety of tissues (11, 12). In
particular, this compound was found to be a competitive
inhibitor of adenosine diphosphate-induced platelet aggrega-
tion (13). Because platelet aggregation plays a central role in
arterial thrombosis and plaque formation (10), Ap
4
A was
proposed as a therapeutic agent for inhibition and treatment
of plaque formation.
Proliferation of medial SMCs and their migration into vessel
intima is an important component of atherogenesis and also
occurs in postangioplastic restenosis, allograft-related vascu-
lopathy, and inflammatory vascular conditions such as Ka-
wasaki disease. Proliferation of the SMCs, which represents
conversion from adult (contractile) to the neonatal (synthetic)
phenotype, is initiated by endothelial damage and is followed
by platelet aggregation and release of growth factors. Recently,
we used a mouseyhuman chimeric Z2D3 F(ab)
2
specific for the
proliferating SMCs of human atheroma for imaging experi-
mental atherosclerotic lesions (16). Because ATP and its
analogs are potent inducers of rat aorta medial SMC prolif-
eration in culture, we hypothesized that radiolabeled diade-
nosine polyphosphates such as Ap
4
A could be useful reagents
for noninvasive imaging of atherosclerotic lesions.
In this report, Ap
4
A and AppCHClppA (Fig. 1) were labeled
with
99m
Tc by using glucoheptonate or mannitol as coligand,
purified by HPLC, and administered intravenously to New
Zealand White rabbits with experimental atherosclerotic le-
sions. Normal rabbits were used as controls. Serial gamma
camera images revealed clear focal areas of increased tracer
accumulation in the aortas of all the lesioned animals but none
of the controls. These results suggest that
99m
Tc-labeled Ap
4
A
and AppCHClppA are potentially useful reagents for rapid
noninvasive imaging of atherosclerotic plaque.
MATERIALS AND METHODS
Synthesis of
99m
Tc-Ap4A-Glucoheptonate (Mannitol). Fifty
to 100 millicuries of
99m
TcO
4
2
obtained from a
99
Moy
99m
Tc
generator (DuPont Merck, Billerica, MA) in 1.5 ml of 0.9%
NaCl was used for preparation of a standard glucoheptonate
kit (DuPont Merck). Twenty millicuries of
99m
Tc-glucohepto-
nate was added to a solution of 1.0 mg Ap
4
A (Sigma), and the
mixture was stirred for 30 min at room temperature. The
reaction mixture was purified by adsorption onto a C
8
-0DS
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Abbreviations: Ap
4
A, AppppA, P
1
,P
4
-di(adenosine-59)-tetraphos-
phate; SMC, smooth muscle cell; ID, injected dose.
691
reverse-phase column (5 3 250 mm, Waters) followed by
elution with CH
3
CN buffer (20:80). The buffer contained 3.1
ml concentrated H
3
P0
4
and 3.9 ml t-butylammonium hydrox-
ide, adjusted to pH 2.4 with NaOH. The radioactive peak
eluting at 16 min corresponded to
99m
Tc-Ap
4
A-glucoheptonate
and was injected into rabbits as described in the following
section. A second radioactive peak eluting at 1–3 min corre-
sponded with
99m
Tc-glucoheptonate. The radiochemical yield
was 10–30%. Similar results were obtained when Ap
4
A was
radiolabeled by using
99m
Tc-mannitol (17).
Synthesis of
99m
Tc-AppCHClppA-Glucoheptonate (or Man-
nitol). These compounds were prepared exactly as described
above by using AppCHClppA.
Experimental Atherosclerotic Lesions. Male New Zealand
White rabbits weighing 2.5–3.0 kg (Charles River Breeding
Laboratories) were maintained on a 2% cholesterol-6% pea-
nut oil diet (ICN) for 3 months. After 1 week of the hyper-
lipidemic diet, the abdominal aorta was denuded of endothe-
lium by a modified Baumgartener technique (16). Briefly, each
animal was anesthetized with a mixture of ketamine and
xylazine (100 mgyml, 10:1 volyvol; 1.5–2.5 ml sc), and the right
femoral artery was isolated. A 4F Fogarty embolectomy cath-
eter (12–040-4F; Edwards Laboratories, Santa Ana, CA) was
introduced through an arteriotomy and advanced under flu-
oroscopic guidance to the level of the diaphragm. The catheter
was inflated to a pressure of 3 psi above the balloon inflation
pressure with radiographic contrast media (Conray, Mallinck-
rodt), and three passes were made down the abdominal aorta
with the inflated catheter. The femoral artery then was ligated,
and the wound closed. The animals were allowed to recover
from anesthesia and then returned to their cages. This protocol
was approved by the animal care and use committees of
Northeastern University and Massachusetts General Hospital
and is in compliance with National Institutes of Health-
approved guidelines.
Gamma Camera Imaging of Atherosclerotic and Normal
Rabbits. Two to four millicuries of the
99m
Tc-Ap
4
A-
glucoheptonate,
99m
Tc-AppCHClppA-glucoheptonate, or
99m
Tc-glucoheptonate (control) was injected into marginal ear
veins of groups of three rabbits with experimental atheroscle-
rotic lesions. As a control, three unlesioned animals were
injected with 2–4 mCi of
99m
Tc-Ap
4
A-glucoheptonate. After
radiopharmaceutical administration, serial gamma images
were collected every minute for the first 5 min, every 2 min for
the next 25 min, and every 5 min for the next 2.5 hr. In all of
the rabbits, images were acquired in the anterior and lateral
decubitus projections (including the heart and aorta) by using
a standard field-of-view gamma camera (Series 100, Ohio
Nuclear, Solon, OH) equipped with a high-resolution parallel-
hole collimator and interfaced with a dedicated computer
system (Technicare 560, Solon, OH). The pulse height analyzer
was adjusted to record the 140 KeV photopeak of
99m
Tc, and
all images were recorded in a 256 3 256 matrix.
After acquiring the final images, the animals were sacrificed
with an overdose of sodium pentobarbital. The aortas were
removed, opened along the ventral surface, and mounted on
styrofoam blocks. The aortas then were placed on the face of
the gamma camera and ex vivo images were recorded (10 min).
Blood Clearance of
99m
Tc-Ap
4
A. Blood samples (0.1 ml) were
withdrawn in parallel with the imaging studies in atheroscle-
rotic and control rabbits and radioactivity was measured with
a well-type gamma counter (LKB model 1282, Wallac Oy,
Finland). To correct for decay and permit calculation of the
concentration of radioactivity as a fraction of the administered
dose, aliquots of the injected doses (IDs) were counted
simultaneously. Blood radioactivity [% IDyg] was plotted as a
function of time after injection, and the curves were fitted to
bi-exponential functions.
Biodistribution. After acquiring the ex vivo images, biodis-
tribution was measured. Samples of blood, heart, lung, liver,
spleen, kidney, skeletal muscle, normal aorta, and lesioned
aorta were washed with saline, weighed and radioactivity was
measured with a well type gamma counter (LKB model 1282).
To correct for radioactive decay and permit calculation of the
FIG. 1. Chemical structures of Ap
4
A and AppCHClppA.
FIG. 2. Blood clearance of
99m
Tc-labeled Ap
4
A and AppCHClppA
in atherosclerotic and control rabbits. The bi-exponential fits to the
data are also indicated:
99m
Tc-Ap
4
A in atherosclerotic rabbits (solid
line),
99m
Tc-Ap
4
A in control rabbits (dot-dashed line), and
99m
Tc-
AppCHClppA in atherosclerotic rabbits (dashed line). Each point is
the mean 6 SEM for three animals.
FIG. 3. Biodistribution of
99m
Tc-Ap
4
A-glucoheptonate in nontar-
get organs at 3 hr after administration. Each bar is the mean 6 SEM
for three animals.
692 Medical Sciences: Elmaleh et al. Proc. Natl. Acad. Sci. USA 95 (1998)
concentration of radioactivity in each organ as a fraction of the
administered dose, aliquots of the IDs were counted simulta-
neously. The results were expressed as % ID/g.
Statistical Methods. The results were evaluated by one-way
ANOVA followed by Duncan’s New Multiple Range Test. The
blood clearance curves were fitted to bi-exponential functions
by nonlinear least squares.
RESULTS
Radiochemistry. The method for radiolabeling Ap
4
A de-
scribed here resulted in a purer and more consistent product
than a previously reported procedure (18). This procedure
described above that used glucoheptonate as the coligand
yielded a mixture of the
99m
Tc-glucoheptonate (retention time
2–3 min, see above) and
99m
Tc-Ap
4
A-glucoheptonate at 16
min. With this procedure,
99m
Tc-Ap
4
A-glucoheptonate was
isolated with a radiochemical purity of .90%. Similar results
were obtained when
99m
Tc-mannitol was used as the coligand.
Blood Clearance of
99m
Tc-Ap
4
A. Blood clearance of the
99m
Tc-Ap
4
A-glucoheptonate was rapid. In the control rabbits,
the concentration of radioactivity (% IDyg) in the circulation
averaged 0.25% at 2 min, after injection, decreased to 0.08%
IDyg at 60 min and only slightly thereafter (up to 180 min). For
all groups of rabbits, blood clearance was well described by
bi-exponential functions with fast and slow components (t
1/2
s)
of '4 and ' 250 min, respectively (Fig. 2). Blood clearance was
not significantly different between rabbits with artheroscle-
rotic lesions and controls.
Biodistribution in Nontarget Organs. The biodistribution of
99m
Tc-Ap
4
A-glucoheptonate in nontarget organs at 3 hr after
injection is summarized in Fig. 3. Overall, these data clearly
demonstrate that accumulation (% IDyg) of the radiophar-
maceuticals in normal tissues was quite low: heart (0.02%),
liver (0.04%), lung (0.02%), kidney (0.06%), and spleen
(0.1%). The degree of tracer accumulation by the different
tissues was statistically significant (P , 0.01). However, con-
centrations were similar for all radiopharmaceuticals (P 5
NS). Of all the tissues that were sampled, kidney contained the
highest concentration of tracer (P , 0.01). Liver and lung had
higher concentrations than spleen and muscle (P , 0.05), and
spleen had higher concentration compared with muscle (P
, 0.05).
99m
Tc (Coligand)-Ap
4
A Accumulation in Atherosclerotic
Lesions. Fig. 4 summarizes the concentrations of radioactivity
in lesioned and normal aortic segments of rabbits at 3 hr after
i.v. administration of both
99m
Tc (coligand)-Ap
4
A analogs. The
mean % IDygof
99m
Tc (coligand)-Ap
4
A in the lesioned
segments was higher than the background activity in the
normal specimens: 0.074% vs. 0.01% (P , 0.01). Aortic arch
segments (which were not denuded) showed slightly greater
accumulation of
99m
Tc-Ap
4
A analogs comparable to thorac-
ic segments.
When the data were expressed at lesionedynormal rations,
accumulation of
99m
Tc (coligand)-Ap
4
A in atherosclerotic
lesions of balloon-denuded (abdominal) aorta was 7.4-fold
greater than in uninjured (abdominal) aorta of control rabbits
and '2-fold greater than the unlesioned thoracic segments of
aortas of rabbits with abdominal lesions. As the results in Table
1 indicate, focal regions of tracer accumulation were detected
in aortic segments of all lesioned rabbits imaged with either
Ap
4
A or AppCHClppA but in none of the controls. Although
accumulation in atherosclerotic lesions was slightly greater for
Ap
4
A compared with AppCHClppA, the difference was not
statistically significant.
Gamma Camera Imaging. All of the rabbits with experi-
mental atherosclerosis showed rapid accumulation of radio-
activity in the lesioned areas; representative images are shown
in Fig. 5. The lesions were clearly visible within 20 min after
injection, and radioactivity was retained in the lesions for the
full 3 hr of the imaging session. When the aortas were imaged
ex vivo, the pattern of radioactivity distribution closely paral-
leled the imaging results (Fig. 5). Inspection of the excised
aortas revealed lesion patterns that were virtually identical to
the results of in vivo and ex vivo imaging (Fig. 5). In contrast,
both in vivo and ex vivo gamma camera imaging failed to
demonstrate evidence of focal tracer accumulation in aortas of
unlesioned rabbits; representative images are shown in Fig. 6.
Inspection of the aortic specimens showed no evidence of
vessel damage. Imaging atherosclerotic rabbits with
99m
Tc-
labeled glucoheptonate showed no evidence of specific accu-
mulation in the aortic lesions, and the images were indistin-
guishable from those obtained in control rabbits imaged with
99m
Tc-labeled Ap
4
A or AppCHClppA (data not shown). With
this tracer, radioactivity cleared rapidly from all organs (t
1/2
s:
'5–10 min) and accumulated in the kidneys and bladder.
DISCUSSION
The development of a noninvasive test for identifying meta-
bolically active lesions should provide information on the
pathogenesis of atherosclerotic plaque formation. With such a
procedure it might be possible to detect ‘‘vulnerable’’ lesions
before they become unstable and lead to subintimal hemor-
rhage and coronary occlusion or clinical manifestations such
as ischemia. In addition, a noninvasive test of this type could
be important not only for diagnosis but also for development
FIG. 4. Accumulation of
99m
Tc-labeled Ap
4
A and AppCHClppA in
lesioned and normal aortic segments of rabbits at 3 hr after i.v.
administration. Each bar is the mean 6 SEM for three animals.
Table 1. Accumulation of
99m
Tc-labeled Ap
4
A and AppCHClppA in lesioned and control
aortic segments
Tracer No. Condition
Lesion
visualization
%IDygram
Abdominal
aorta
Thoracic
aorta
Ap
4
A 3 Atherosclerotic 3y3 0.074 6 0.01* 0.040 6 0.004
AppCHClppA 3 Atherosclerotic 3y3 0.060 6 0.007* 0.030 6 0.001
Ap
4
A 3 Control 0y3 0.010 6 0.004 0.016 6 0.006
*P 5 0.003.
Medical Sciences: Elmaleh et al. Proc. Natl. Acad. Sci. USA 95 (1998) 693
and monitoring of therapies directed at altering the natural
history of these lesions. There are a limited number of reports
describing noninvasive visualization of atherosclerotic lesions.
These studies have targeted the thrombotic component over-
lying the atherosclerotic lesion (with radiolabeled fibrinogen)
(19, 20) platelet aggregation at regions of turbulent flow (with
labeled platelets or platelet-specific antibodies) (21–23) or
proteins likely to be incorporated into atherosclerotic lesions
(with radiolabeled autologous lipoproteins) (24–29). Nonspe-
cific uptake of human IgG via Fc receptors of macrophages
also has been used as the basis for a targeting strategy (2, 30,
31). More recently, the Fab9 fragment of a monoclonal IgM
antibody, Z2D3, which is directed to an antigen associated with
the proliferating SMCs of human atherosclerotic lesions
showed rapid accumulation and good localization of lesions in
an experimental model (16).
Ap
4
A undergoes rapid degradation in the vascular bed by
the catalytic actions of ecto and phosphodiesterases and
soluble nucleotide pyrophosphates. We have demonstrated,
however, that the
99m
Tc-chelates of Ap
4
A and its analog are
much more stable in vivo compared with the corresponding
sodium salts (data not shown).
In the current study,
99m
Tc-Ap
4
A-glucoheptonate and its an-
alog accumulated rapidly in atherosclerotic lesions and were
retained for more than 3 hr. At 1 hr after injection, lesion to blood
radioactivity ratios were greater than 6 to 1. The
99m
Tc-Ap
4
A
analogs showed very low levels of accumulation in normal tissues
of the rabbit. When
99m
Tc-glucoheptonate (control) was injected
in lesioned animals, very low concentrations of radioactivity
accumulated in atherosclerotic lesions and normal tissues. Sim-
ilarly, very low levels of
99m
Tc-Ap
4
A and its analog accumulated
in the tissues of uninjured (control) animals. Thus, we have
demonstrated the feasibility of rapid noninvasive imaging of
experimentally induced atherosclerotic lesions with radiolabeled
99m
Tc-Ap
4
A analogs. In all cases, lesion visualization was possible
within 15–30 min after tracer administration.
In addition to demonstrating the feasibility of using this class
of reagents for scintigraphic localization of atherosclerotic
lesions, our results provide indirect evidence for up-regulation
of purinoreceptors in atherosclerotic plaques and could help to
unravel an important mechanism of atherogenesis. The role of
surface receptors coupled with intrinsic tyrosine kinase activ-
ity has been extensively studied in proliferating SMCs of
atheroma but the role of G protein-coupled receptors has not
been described. SMCs isolated from human and rat aorta,
which acquire synthetic phenotypic characteristics in tissue
culture, proliferate rapidly and show high intracellular Ca
21
content in response to extracellular ATP. The mitogenic and
Ca
21
mobilizing effects of diadenosine polyphosphates are as
potent as ATP (16). The present study provides an in vivo
characterization of purinoreceptors. Determination of the
precise cellular and subcellular site(s) of tracer binding within
the atherosclerotic lesions was beyond the scope of the present
investigations and awaits the results of future micro-
autoradiographic studies with
95m
Tc- or
1
H-labeled agents.
Although angiography is the ‘‘gold standard’’ for measuring
luminal narrowing and has been useful for monitoring therapeu-
tic interventions, it is of limited value for evaluating early lesions.
For optimal care of atherosclerotic disease, characterization of
plaque constituents and metabolism is imperative. With this
information it may be possible to differentiate stable plaques
from those in which rupture or encroachment on the lumen are
imminent. Thus, quantitation of concentration of foam cells,
SMC proliferation, and amount of overlying thrombotic plaque
could have significant impact in assessing prognosis.
Several clinical vasculopathies such as restenotic complica-
tions of angioplasty and allograft-related vasculopathy are
associated with rapid proliferation of SMCs; however, no
diagnostic or prognostic strategies have provided enough
information for early intervention in these patients. Recently
the Fab9 fragment of an antibody, Z2D3, which is specific for
a surface antigen of SMCs with the synthetic phenotype was
shown to be useful for rapid detection of experimental ath-
erosclerotic lesions (16). The results of the current study
indicate that
99m
Tc-Ap4A analogs accumulate in experimental
lesions even more rapidly than Z2D3.
CONCLUSIONS
The present study demonstrates that
99m
Tc-labeled adenine nu-
cleotide analogs (Ap
4
A and AppCHClppA) that are stable in vitro
and in vivo are easily prepared in high yield and purity. The
competitive inhibition of Ap
4
A and AppCHClppA at ADP
association sites on platelets makes them potentially highly se-
lective pharmacological agents for vascular lesions where plate-
lets show early localization and aggregation. These radiophar-
maceuticals have potential both as radiodiagnostic agents for the
rapid detection of atherosclerotic plaques and for probing the
fundamental pathophysiology of atherogenesis.
We thank Dr. Barry Zaret for his review and constructive comments
on this manuscript.
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Medical Sciences: Elmaleh et al. Proc. Natl. Acad. Sci. USA 95 (1998) 695
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    Full-text · Article · Apr 2009
    • "the most largely studied tracer of apoptosis, alternative peptidic and non-peptidic agents have been described109110111112113. The behavior of these new compounds warrant further investigation. "
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