Perfusion and Specific Radioprobes for Cardiac Imaging

Filipa Mendes, Lurdes Gano, Célia Fernandes, António Paulo and Isabel Santos

Radiopharmaceutical Sciences Group, IST/CTN, Instituto Superior Técnico,

Universidade Técnica de Lisboa, Estrada Nacional 10, km 139,7, 2695-066 Bobadela LRS, Portugal

Keywords: Nuclear Imaging, Molecular Imaging, Perfusion Agents, Targeted Radioprobes, Technetium, Fluorine.

Abstract: Major advancements have been made in treating cardiovascular disease. However, improving diagnosis is

crucial, because the detection of the early stages of disease would allow preventative approaches therapy.

Myocardial perfusion imaging is in clinical use for decades and is an effective tool for diagnosis, and long-

term follow-up of patients with suspected or known coronary artery disease. The technetium-based agents,

99mTc-sestamibi and 99mTc-tetrofosmin, are widely used myocardial blood flow tracers. However, since

both present drawbacks in their biodistribution properties, there is now resurgence in the study of both

neutral and cationic technetium agents to further improve the characteristics of perfusion

radiopharmaceuticals. Despite all the success of perfusion imaging, a unique strength of nuclear imaging is

its ability to provide tools for imaging processes at molecular and cellular levels in intact organisms under a

wide variety of physiologic conditions. Advances in new specific imaging agents that identify myocardium

injury and cellular dysfunction may contribute to the improvement of diagnosis and eventually better

therapeutic approaches. In this communication, we will review perfusion agents and their biological

mechanism of uptake. We will also discuss examples of target-specific radiopharmaceuticals for cardiac

imaging, including advances in pre-clinical imaging approaches.

1 MYOCARDIAL PERFUSION

IMAGING

In the past few decades, major improvements have

been made in treating some types of cardiovascular

disease. However, new treatment options are

urgently needed for all types of cardiovascular

disease. Moreover, improving diagnosis is crucial,

because by detecting the early stages of disease, the

focus of therapy could be shifted from treatment to

prevention.

Myocardial perfusion imaging has been in

clinical use for over 30 years, serving as an

effective, reliable, and relatively simple tool for

diagnosis, risk stratification, and long-term follow-

up of patients with suspected or known coronary

artery disease (Notghi and Low, 2011). Thallium-

201 chloride was the first pharmaceutical to be

widely used clinically for imaging myocardial

perfusion. Because of its relatively long half-life and

low energy X-ray emission, it is not the ideal agent

for imaging, giving a relatively large radiation dose

with lower image quality than technetium agents. It

enters the cells via the Na/K-ATPase, and is

redistributed fairly rapidly.

Technetium-based agents,

99m

Tc-sestamibi

(Cardiolite) and

99m

Tc-tetrofosmin (Myoview), are

now widely used myocardial blood flow tracers

(Figure 1). These perfusion agents have with

minimal redistribution, better imaging characteristics

and less radiation to the patient.

99m

Tc-sestamibi

enters the cell via a passive pathway due to its

lipophilicity and accumulates in the mitochondria in

response to the physiologically negative

mitochondrial and plasma membrane potentials. Due

to their elevated number of mitochondria, the heart,

muscles, liver and kidneys present a high uptake of

this radiopharmaceutical. Because the in vivo

behavior of

99m

Tc-tetrofosmin demonstrates

similarities with

99m

Tc-sestamibi it was initially

suggested that the mechanism determining cellular

distribution was also similar. The first studies

indicated that the uptake is through a metabolism-

dependent process, most likely by potential-driven

transport of the lipophilic cation. However,

subsequently it was shown that inhibition of the

Na+/K+ ATPase, partly inhibited the uptake of

Mendes F., Gano L., Fernandes C., Paulo A. and Santos I..

Perfusion and Speciﬁc Radioprobes for Cardiac Imaging.

DOI: 10.5220/0004664300790083

In Proceedings of the International Congress on Cardiovascular Technologies (VisualCardio-2013), pages 79-83

ISBN: 978-989-8565-78-5

 2013 SCITEPRESS (Science and Technology Publications, Lda.)

99m

Tc-tetrofosmin indicating that lipophilicity is not

the only factor involved in the cellular uptake.

Moreover, this agent appears to be more associated

with the cytosol than with mitochondria.

Nevertheless, it is consensual that

99m

Tc-tetrofosmin

uptake depends on both cell membrane and

mitochondrial potentials.

(1) (2)

(3)

Figure 1: Structures of

99m

Tc-sestamibi (1),

99m

Tc-

tetrofosmin (2) and

99m

Tc-TMEOP (3).

For the foreseeable future, myocardial perfusion

imaging will continue to be used for assessment of

ischaemia. However, both Cardiolite and Myoview

present biodistribution properties that suffer from

well-known drawbacks, the most important being

the high liver uptake, which can interfere in the

analysis of cardiac imaging, particularly of the

inferior left ventricular wall (

Germano, et al., 1994;

Kailasnath and Sinusas, 2001; Kapur, et al., 2002;

Llaurado, 2001; Parker, 2001)

. As a consequence, there

is now a resurgence and development of both neutral

and cationic technetium agents to further improve

the characteristics of perfusion radiopharmaceuticals

(Hatada et al., 2004); (Kim et al., 2008); (Liu, 2007);

(Liu et al., 2010).

Recently we were able to identify a new class of

organometallic complexes based on

tris(pyrazolyl)methane as lead structure (Garcia et

al., 2009); (Santos and Correia, 2005); (Maria et al.,

2007). This type of complexes has a cationic

character and they are stable both in vitro and in

vivo. In particular, we discovered that the

tricarbonyl complex

99m

Tc-TMEOP (Figure 1)

exhibited high heart uptake and biodistribution

properties suitable for myocardial imaging (Maria et

al., 2009); (Goethals et al., 2010) (Figure 2).

Data collected so far suggest that the

pharmacokinetic profile of

99m

Tc-TMEOP may

allow high quality imaging early after tracer

injection. Biodistribution and cardiac pinhole-gated

SPECT imaging studies in rats showed that

99m

Tc-

TMEOP has a cardiac uptake comparable to 99mTc-

sestamibi and

99m

Tc-tetrofosmin, but has a

significantly faster liver clearance (Goethals, et al.,

2010). At 40 min post injection, the heart/liver ratio

99m

Tc-TMEOP is twice that of

99m

Tc-sestamibi

and

99m

Tc-tetrofosmin (6.98±1.66, 2.48±0.30 and

2.66±0.40, respectively). Altogther, the data

collected so far suggest that the pharmacokinetic

profile of

99m

Tc-TMEOP may allow high quality

imaging early after tracer injection.

Figure 2: Planar image of a rat administered with

99m

Tc-

TMEOP, at 60 min pi. Image was performed in supine

position and acquired using a 128×128 matrix in a GE

gamma camera. Adapted from (Mendes et al., 2012).

Therefore, to get a better insight on the in vivo

behaviour of

99m

Tc-TMEOP, its mechanisms of

myocardial uptake and excretion have been

investigated. Our results indicate that the heart

uptake of

99m

Tc-TMEOP is related to its

accumulation in the mitochondria due to the

negative plasma and mitochondrial transmembrane

potentials (Mendes et al., 2012).

It is well know that cancer cells and tumours also

maintain a more negative potential owing to

increased metabolic requirements, and as a result,

there is an increased accumulation of

99m

Tc-

sestamibi,

99m

Tc-tetrofosmin and

99m

Tc-TMEOP in

malignant tumours. This feature permits the use of

these radiotracers for imaging cancers of the breast,

lung, brain and parathyroid adenomas (reviewed in

Mendes et al., 2011).

Despite all the success of perfusion imaging, a

unique strength of nuclear imaging is its ability to

provide tools for imaging biochemical and metabolic

CARDIOTECHNIX2013-InternationalCongressonCardiovascularTechnologies

processes and receptor and transporter functions at

molecular and cellular levels in intact organisms

under a wide variety of physiologic conditions.

osol

ragm

ent

tocho

ria

100

99m

Tc-TMEOP

99m

Tc-Sestamibi

% total recovered

Figure 3: Subcellular distribution of

99m

Tc-TMEOP and

99m

Tc-Sestamibi in isolated rat's heart tissue. Adapted

from (Mendes et al., 2012).

2 CARDIAC MOLECULAR

IMAGING

Molecular imaging studies are shedding important

light on the cellular and molecular biology

underlying important cardiovascular diseases

(Osborn and Jaffer, 2012). Therefore within the field

of cardiovascular medicine, potential applications of

molecular imaging include the analysis of vulnerable

plaques, heart failure, neurohormonal dysfunction,

myocardial metabolism, stem cell engraftment,

protein–protein interactions, and angiogenesis.

(Table 1).

Myocardial Pathology. Metabolic adaptation

probably represents one of the earliest responses to

myocardial ischemia. The application of a metabolic

radiotracer, as opposed to a perfusion tracer,

potentially extends the time window for noninvasive

imaging of an ischemic event beyond the resolution

of symptoms. Targeting intracellular metabolic

processes could expand our ability to diagnose and

treat subclinical or progressive cardiovascular

disorders that often remain elusive with traditional

imaging approaches. These therapeutic strategies in

turn create a demand for accurate, sensitive, and

physiological evaluation of therapeutic effects.

The autonomic nervous system plays an

important role in many cardiac functions, including

cardiac rhythm, conduction, and repolarization.

Several specific neurotransmitters interact with

receptors on pre- and post-synaptic binding sites

regulating the complex system of the heart.

Abnormalities in this interaction result in a variety

of cardioneuropathies.

Receptor imaging can be helpful for

prognosticating patients with heart failure, diabetes,

ischemic heart disease, heart transplantation, drug-

induced cardiotoxicity, and dysautonomias.

Table 1: Examples of targets for molecular imaging of

different cardiac pathologic events.

Pathological/Biological

Process

Cellular/Molecular Targets

Ischemia / myocardial

damage

Renin-angiotensin system

Chemokine receptor

VEGFR receptor

Metabolic imaging

Fatty acids metabolism

Glucose metabolism

Cardiac neuronal imaging

Pre-/Post-synaptic – sympathic

and parasympathic innervation

Receptors / Channels

Acute myocardial infarct -

Acute Necrosis

Disrupted myocytes

Calcium rich-areas

Myosin

Atherosclerosis - vulnerable

plaques

Apoptosis

Inflammation

Adhesion

Lipoproteins

Angiogenesis

Vascular Pathology. The diagnosis of vulnerable

plaques remains an elusive goal in clinical medicine.

The most widely accepted features of vulnerable

plaques, such as large lipid core, increased

inflammatory milieu and thin fibrous caps, have

been well characterized through pathological

studies. The ability to image a vulnerable plaque in

susceptible patients should theoretically result in

useful prognostic information that can be used to

either monitor or treat patients at risk more

aggressively.

The relatively poor correlation between risk of

plaque rupture and the degree of luminal obstruction

exposes the crucial need for in vivo detection of the

processes underlying progressive plaque

destabilization.

In addition to the morphologic characteristics,

apoptosis and inflammation are two other important

indicators of plaque instability. Apoptotic

macrophage death results in enlargement of the

plaque necrotic core and positive vascular

remodeling, whereas apoptosis of the smooth muscle

cells leads to attenuation of the fibrous cap.

Finally, angiogenesis is defined as the formation

of new capillaries by cellular outgrowth from

existing microvessels. It plays a crucial role in the

response to ischemia that is associated with

PerfusionandSpecificRadioprobesforCardiacImaging

peripheral arterial disease and myocardial infarction.

Imaging angiogenesis would therefore be valuable in

assessing risk stratification of patients with arterial

occlusive disease.

Selected Examples of Molecular Imaging Probes.

The biodistribution of molecular imaging probes is

determined by specific interactions between the

radioactive molecule and its target, which can be for

example antigen, enzymatic or receptor-binding.

Therefore the probe should present a high affinity to

its target, and also a high speciﬁcity, resulting in its

selective uptake and distribution at the target tissues.

Different types of biomolecules and

radionuclides, both metallic and non-metallic, have

been explored in nuclear imaging.

Within the field of cardiac molecular imaging

fluorine-18 fluorodeoxyglucose (

F-FDG) is the

most widely used agent (Figure 4).

F-FDG is a radiolabeled glucose analogue

transported into metabolically active cells, and

therefore it is an ideal agent for the assessment of

viable myocardium.

Moreover,

F-FDG presence is also correlated

with plaque macrophage content, and, therefore,

could be used as a surrogate reporter of this critical

cell involved in atherogenesis and plaque rupture

(Osborn, Jaffer, 2012).

Figure 4: Structure of

F-FDG.

The renin-angiotensin system (RAS) plays an

important role in regulating blood volume, arterial

pressure, cardiac and vascular function, and may

contribute to the pathogenesis of atherosclerosis.

The renin-angiotensin system is frequently activated

early in heart failure and is linked to left ventricular

remodeling and myocardial fibrosis.

A comprehensive in vivo approach to the study

of the RAS and its many components has been made

difficult by the complexity of the system. However,

this system has, at the same time, provided a number

of targets for nuclear imaging via radiolabeled

ligands, with special emphasis on the angiotensin-

converting enzyme (ACE).

The initial attempts at developing specific ACE-

binding radiotracers were made by use of

F–

labeled captopril, the first clinically available ACE

inhibitor. In normal rats, in vivo biodistribution at 30

minutes after injection revealed high uptake values

in the lungs, kidneys, and aorta, organs with known

high concentrations of ACE. This agent, however,

had a number of shortcomings that reduced its

potential as a suitable tracer for examining ACE

distribution, as it is believed to have a higher affinity

for vascular ACE than for tissue ACE and, thus, to

be less suited for examination of tissue-bound ACE

activity.

Another

F-labeled ACE inhibitor, lisinopril,

showed higher affinity for tissue ACE and allowed

higher resolution during in vitro autoradiography

when compared with

F-labeled captopril.

99m

Tc-

labeled lisinopril derivatives have been also

developed (Femia, et al., 2008) and recently it has

been shown that

99m

Tc-lisinopril localizes in the

heart of transgenic rats that over-express human

ACE-1 (Dilsizian, et al., 2012). The combination of

these studies has shown the feasibility of in vivo

imaging of ACE both by PET(

F) and SPECT

(

99m

Tc).

ACKNOWLEDGEMENTS

Fundação para a Ciência e a Tecnologia is

acknowledged for the Ciência 2007 grant to F

Mendes. The financial support of Covidien, Petten,

The Netherlands to IST/ITN is acknowledged.

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PerfusionandSpecificRadioprobesforCardiacImaging