Nuclear imaging of amyloidosis.

Summary Amyloidosis is a clinical condition caused by deposition of various protein fibrills in extracellular space. The presented symptoms depend on the type of deposits and the organ or organs involved. The correct diagnosis is often difficult, due to lack of nonivasive imaging techniques and insufficiency of morphological imaging procedures delievered by radiology. We presented a list of potential radiopharmaceuticals that can be used in detecting various types of amyloidoses. (123)I-SAP proved to have high sensitivity in imaging of AA and AL amyloidosis in visceral organs. (99m)Tc-Aprotinin was found to be useful in detecting cardiac amyloidosis. A couple of classical radiotracers, such as (201)Tl, (123)I-mIBG, together with (111)In-antimyosin were also tested for accuracy in cardiac imaging, however the main problem was low specificity. Potential applicability was also found in case of some bone-seeking agents and other radiotracers, e.g. (67)Ga-citrate and (99m)Tc-penta-DMSA. High sensitivity and specificity was achieved with β2-microglobulin labeled with (131)I or (111)In. Among PET tracers, (11)C-PIB deserves more attention, because it may have an important role in diagnosing of AD in the near future. Further clinical studies are expected to take place, because noninvasive diagnosing and monitoring of amyloidosis is still a challenge.

Background

Amyloidosis is a disease characterised by deposition of protein fibrils in the extracellular space. The fibril deposits (amyloid) are composed of low-molecular-weight subunits of various serum proteins, which are improperly metabolised [1]. The term ‘amyloid’ is misleading, because the deposits were initially thought to be related to starch components [2]. Excessive accumulation of amyloid in a given tissue or organ is initially asymptomatic, but later on leads to progressive organ failure or death. Amyloidosis can be local (limited to one organ or one side of the body) or systemic (spread throughout the body), which is usually fatal [3].

Amyloidosis can be congenital or acquired, but it is always related to some precursor plasma protein which forms the fibril deposits. These plasma proteins belong to a wide group of various biochemical substances, often unrelated to each other, but determining the type of amyloidosis. Different kinds of amyloid deposits usually present affinity to certain organs. However, the patterns of pathological accumulation in the body can be very diverse, resulting in various clinical manifestations of the disease. For example, the AL amyloidosis results from deposition of protein composed of monoclonal immunoglobulin light chains and in 26–50% is accompanied by cardiomyopathy. The AA type of amyloidosis, related to serum amyloid A protein (an acute-phase reactant) and associated with longstanding inflammatory disorders most often results in proteinuria and renal failure, and in less than 5% of patients it also leads to hepatomegaly, autonomic neuropathy or cardiomyopathy. Transthyretin is the precursor protein of ATTR amyloidosis, which can be hereditary or age-related, and frequently manifests with peripheral and autonomic neuropathy, renal failure or cardiomyopathy. Three types mentioned above comprise a group of major systemic amyloidoses. An example of localized amyloidosis may result from impaired clearance of β2-microglobulin in patients chronically haemodialysed, with pathological deposits in bones and jointes (Aβ2M-type amyloidosis).

Other types of amyloidoses are presented in the Table 1.

Table 1

presenting various types of amyloidoses [3,4].

Type of amyloidosis	Precursor protein	Generalized (G)/localized (L)	Clinical manifestations
AL	Ig light chain	G, L	Various symptoms, cardiomyopathy in 26–50%
AA	Serum amyloid A	G, L	Proteinuria, renal failure, hepatomegaly, neuropathy, cardiomyopathy
ATTR	Transthyretin	G	Hereditary or age-related; neuropathy, cardiomyopathy, renal failure
Aβ2M	β2-microglobulin	G	Associated with chronic hemodialysis, pathological deposits in bones and joints
AH	Ig heavy chain	G, L	All kinds of manifestations
AApoAI	Apolipoprotein AI	G, L	Hereditary; neuropathy, nephropathy, aorta (local)
AApoAII	Apolipoprotein AII	G	Hereditary
AApoAIV	Apolipoprotein AIV	G	Sporadic, senile
AGel	Gelsolin	G	Hereditary
ACys	Cystatin C	G	Hereditary, cerebral hemorrhage
ALys	Lysozyme	G	Hereditary
AFib	Fibrinogen	G	Hereditary, renal failure
Aβ	Aβ precursor protein (AβPP)	L	Alzheimer’s disease, ageing
ABri	ABriPP	G	British familial dementia
ADan	ADanPP	L	Danish familial dementia
ACal	Procalcitonin	L	Thyroid medullary cancer
AANF	Atrial natriuretic factor	L	Isolated atrial amyloid
AIAPP	Islet amyloid polypeptide	L	Islet of Langerhans, diabetes type II, insulinoma
APrP	Prion protein	L	Spongiform encephalopathies
AIns	Insulin	L	Iatrogenic
AMed	Lactadherin	L	Senile aortic (media)
AKer	Kerato-epithelin	L	Cornea, familial
ALac	Lactoferrin	L	Cornea
A	semenogelin	L	Seminal vesicle

Radiotracers Used for Imaging of Amyloid Deposits

Precise and early diagnosis of amyloidosis is necessary for efective treatment. Reliable diagnosis is provided only by histopathological examination, but it does not give information on the extent or progression of the disease. Nuclear imaging modalities, based on radiotracer accumulation within the sites of the disease play an important role in primary diagnosis and monitoring of illness [5]. There are two promising radiolabeled compounds, 123I serum amyloid protein (SAP) and 99mTc-Aprotinin, and several other potential indicators, such as, 99mTc-phosphates or phosphonates, 123I-mIBG, 201Tl, 67Ga-citrate, 111In-antimyosin, 99mTc-penta-DMSA, 131I-β2 microglobulin, which present various affinity to amyloid deposits. They are discussed in more detail below.

123I serum amyloid protein (SAP) – is a radiotracer composed of human serum amyloid P component labeled with radioactive 123I (half-life 13.2 h, energy 159 keV). It was first proposed by Hawkins et al. [6,7]. SAP is identical to glycoprotein amyloid P, which is contained in all amyloid deposits [8,9]. It binds in a calcium-dependent manner to amyloid fibrils of all types [10]. A standard dose of 123I-SAP is 100 μg of protein with 200 MBq of radioactivity. Imaging is performed 24 h after i.v. injection [11]. The 123I-SAP is used for detection of amyloid in systemic AA, AL and ATTR amyloidosis with high sensitivity and specificity. It allows to determine the extent and distribution of amyloid, especially in big visceral organs, such as liver, spleen or kidneys, but has lower potential in the assessment of cardiac involvement (probably due to the lack of fenestrated endothelium in the myocardium) [8]. A great value of SAP scintigraphy is the possibility to exclude systemic deposits in case of a localized disease. In a study of 219 patients with localized and systemic amyloidosis, Hazenberg reported a high 90% sensitivity of SAP scintigraphy in the detection of AA and AL amyloidosis and a lower, 48% sensitivity for detecting the ATTR type of the disease, while the overall specificity was 93% [8]. Hachulla [4] in a review article reported a 100% sensitivity when the scintigraphic analysis included the evaluation of 24h tracer retention in the body tissues.

Saïle [11], who compared the 24h-retention of SAP in 15 patients with various types of amyloidosis with control group proved higher whole-body retention (47–87% vs. 44–67% in controls).

Although amyloidosis is always diagnosed on the basis of positive biopsy results, the adventage of nuclear imaging is its ability to noninvasively assess the extent of involvement (whole-body scintigraphy) and the lack of potential complications (bleeding or perforation, infection). The drawback is that 123I is expensive and not readily available (Figure 1).

Figure 1

SAP scintigraphy showing pathologically increased uptake in spleen and adrenal glands ((A) – planar image, (B) – SPECT/CT). With permission of dr Andor Glaudemans from the Department of Nuclear Medicine & Molecular Imaging, University Medical Center Groningen, University of Groningen.

The 99mTc-Aprotinin – aprotinin is a proteinase inhibitor and is normally used as an anti-coagulant drug in open-heart surgeries. Various protein inhibitors were confirmed to be present in amyloid deposits [4], which gave the researchers an opportunity to investigate possible binding of aprotinin to amyloid [12]. When labeled with technetium-99m, aprotinin forms a stable complex which accumulates in kidneys and liver after intravenous injections. It does not cross the blood-brain barrier in normal conditions [13]. Hence, 99mTc-Aprotinin is not dedicated for assessment of visceral organs and brain. On the other hand, it occured to be valuable in imaging of myocardial amyloid. In a study by Han [5], all five patients (out of 35 examined) who had histologically confirmed heart amyloidosis showed positive uptake of 99mTc-Aprotinin in the myocardium (median heart-to-background ratio 2.0 vs. 1.1 in subjects without cardiac amyloid, p=00004).

Schaadt et al. [11] performed 99mTc-Aprotinin scintigraphy in 23 patients with confirmed or suspected amyloidosis, and found focal accumulations, mostly in organs such as lungs, pleura, liver, spleen, intestines, myocardium and tongue. A vast majority of lesions were histologically confirmed to contain amyloid deposits either during autopsy (3 patients) or in biopsy (the remaining 20 patients). The authors concluded that 99mTc-Aprotinin scintigraphy was fairly sensitive and specific diagnostic modality in patients with suspected amyloidosis.

The rest of the tracers present a wide range of specificity and sensitivity. Unfortunately, none of these tracers offer very high diagnostic accuracy, and thus tissue biopsies have to be performed to diagnose amyloidosis. The role of these tracers could be monitoring of disease progression/regression or assessment of disease spread in patients with established diagnosis and proven tracer accumulation especially where biopsy is problematic e.g. in the heart. Clinically relevant is to define amyloid chemical type, which can be performed with high accuracy with radioimmunochemical assays. Unfortunately, these radiotracers do not offer high accuracy in the determination of amyloid structural subtype and tissue sample is still indispensable.

Numerous papers have reported accumulation of bone-seeking tracers in tissues affected with amyloidosis, which was initially reported in 1977 [14]. The biochemical character of this affinity is not fully understood and most frequently it is explained with increased calcium content in amyloiditic deposits.

Several phosphonate- and phosphate-based radiotracers (e.g. 99mTc-PYP, 99mTc-MDP, 99mTc-HMDP, 99mTc-DPD) were tested and presented a broad spectrum of diagnostic performance. In the study by Falk et al., performed with the use of 99mTc-pyrophosphate, the result was positive in 9 of 11 patients with advanced amyloid cardiomyopathy, but only in 2 of 9 patients with biopsy-proven initial stage of the disease [15]. Lee et al. studied both 99mTc-PYP and 99mTc-MDP. The 99mTc-PYP scan was positive in all 7 patients, but only 4 patients presented 99mTc-MDP uptake [16]. Disappointing results were shown by Eriksson et al. [17] – only 4 of 12 patients with echocardiographic features of amyloidosis revealed increased 99mTc-PYP uptake. The results obtained by Tanaka et al. were equally discouraging – 99mTc-PYP uptake in only 4 of 12 patients with biopsy-proven cardiac amyloidosis [18]. More promising results were presented by Perugini et al. [19] – by scanning with the use of 99mTc-DPD and 99mTc-MDP they were able to differentiate ATTR vs. AL type of amyloidosis.

Many authors have reported a decreased uptake of Iodine-mIBG radiotracer in the amyloid-affected myocardium. This was initially described by Nakata et al. in 1995 [20]. Metaiodobenzylguanidine (mIBG) is structurally similar to noradrenalin and is taken up in presynaptic neural endings of the sympathetic system. Imaging of amyloidosis with mIBG radiotracer is indirect and based on sympathetic nerve destruction in amyloidosis.

Tanaka et al. performed mIBG scanning in 12 patients with familial amyloid polyneuropathy with biopsy-proven heart involvement. All patients demonstrated reduced radiotracer uptake (8 patients presented no heart uptake, 4 presented a deeply reduced uptake) [17]. Authors concluded that mIBG heart scintigraphy has potential to early detect amyloiditic changes in the myocardium. In the study by Dealahaye at al., all 17 patients with familial amyloid polyneuropathy demonstrated decreased heart uptake of 123I-mIBG and the level of uptake decrease correlated with clinical severity of polyneuropathy [21].

Perfusion in amyloid cardiomyopathy was studied with the use of 201Tl by Kobayashi et al. Perfusion remained almost unchanged, and deficits were seen only in bigger amyloid depositions [22]. Wechalekar et al. studied perfusion with 99mTc-Sestamibi and demonstrated no perfusion changes in amyloidosis [23]. Kodama et al. demonstrated increased 201Tl washout in amyloid cardiomyopathy group [24].

67Ga-citrate is a frequently utilized radiotracer having high affinity to inflammatory and neoplastic processes. The mechanism of gallium accumulation in target tissues is not fully understood. Its uptake is proportional to the density of transferrin receptors [25]. Reports on gallium citrate uptake in amyloid cardiomyopathy were published first by Brown et al. in 1979 [26], which was later confirmed by Montes et al. [27] and Li et al. [28]. It seems that gallium citrate does not play essential role in the discussed group of patients.

111In-antimyosin is a radiotracer with affinity to the injured myocardium. Its uptake is nonspecific and is present in many cardiological conditions such as infarction, inflammation, myocardiopathy, heart transplant complication, drug toxicity [29]. In a study by Lekakis et al. all seven patients with heart amyloidosis demonstrated increased uptake of 111In-antimyosin compared to control group [30].

99mTc-penta-DMSA is a radiotracer with affinity to several neoplastic diseases e.g. medullary thyroid cancer. Its uptake in AL amyloidosis was described by Ohta et al. [31] and Kobayashi et al. [22]. Disadvantage of 99mTc-penta-DMSA is its relatively high blood-pool concentration making the assessment of tissue uptake difficult, which was minimized by delayed image registration after 24h post injection and described by Arbab et al. [32].

β2-microglobulin (β2M) is a precursor protein of Aβ2M type of amyloidosis in patients chronically hemodialysed with involvement of joints. Radiolabeling of β2-microglobulin enabled specific scintigraphic detection of amyloid present in this particular type of amyloidosis. It was first performed by Floege in 1989 [33]. The isotope used was 131I and β2M was extracted from uremic hemofiltrate. The radiotracer is accumulated directly in amyloid deposits [32]. A disadvantage of 131I-labeled radiotracer was poor spatial resolution, which made imaging of small joints problematic and increased the radiation dose due to unnecessary β(−) radiation [34]. In 2000, Schaeffer combined β2M with gamma emitter 111In [35]. The registered images had a significantly better quality as compared with previously utilized radiotracers. To prevent a potential transmission of infection, “natural” blood-extracted β2M was substituted by a recombinant one with good results [35]. Sensitivity of 131I-β2M scan is better than the one achieved in combined radiological and clinical detection methods, specificity was high – the radiotracer was not present in inflammatory changes in joints and in short-term hemodialysis patients [36]. Disadvantage of β2M radionuclide imaging is inability to use it in patients with preserved residual renal function because of fast filtration of the radiotracer [37].

PET tracers

There is a question of incorporation of 18F-FDG-PET into the diagnostics of pulmonary amyloidosis. The role of 18F-FDG-PET in metabolic assessment of pulmonary nodules is well known [38]. Yadav et al. [39] described a case of a patient suffering from pulmonary amyloidosis who presented with intense uptake of 18F-fluorodeoxyglucose in one dominant pulmonary nodule, with a lack of uptake in the remaining nodules. The authors, however, explained that 18F-FDG is a nonspecific tracer for amyloid deposits. At the same time, positive focal accumulation of labeled glucose for a suspected malignancy should always be verified with microscopic examination if curative treatment is concerned.

Promising results were obtained by Klunk et al. [40] and Rabinovici et al. [41] in the imaging of beta-amyloid with 11C Pittsburgh compound-B (PIB) in Alzheimer’s disease. In the study by Klunk et al. who compared 16 patients with AD (Alzheimer’s disease) and 9 healthy patients, the retention of PIB was statistically higher in the areas which contained large amounts of amyloid deposits (frontal cortex: 1.94-fold, p=0.0001, parietal cortex: 1.71-fold, p=0.0002, temporal cortex: 1.52-fold, p=0.002, occipital cortex: 1.54-fold, p=0.002, striatum: 1.76-fold, p=0.0001). In the study by Rabinovici et al. [40] all the 7 patients with AD had a positive uptake of PIB. However, among patients with FTD (frontotemporal dementia) 4 out of 12 results were also positive, which means that 11C-PIB PET helps to discriminate AD from other types of neurodegeneration, but still a pathological correlation is needed.

Conclusions

Amyloidosis is a clinical condition which is difficult to diagnose and monitor, due to the lack of specific and non-invasive methods. Nuclear imaging brings hope to this field of medicine, since it allows to functionally detect amyloid deposits. Nuclear scans present many advantages, such as quick assessment of disease extent owing to whole-body scintigraphy, ability to monitor the progression or regression of illness, and very low invasiveness (apart from a small dose of ionising radiation). A few valuable radiotracers, such as 123I-SAP, 99mTc-Aprotinin or 11C-PIB dedicated for imaging of amyloidosis were already initially tested and the results are promising, which means that their role will probably be established in clinical practice in the near future. Certainly, some of the commonly available, “everyday” radiopharmaceuticals, for example bone-seeking tracers, could be more involved in the monitoring of the disease in cases with confirmed uptake. Nevertheless, further clinical studies are required to find new specific radiotracers which will allow to precisely detect, diagnose and monitor this troublesome and diverse clinical condition called amyloidosis.