PET Tracers in Cardiac Amyloidosis

Guest Bloggers: Pranav Chandrashekar, MD; Ahmad Masri, MD MS, The Amyloidosis Center, Knight Cardiovascular Institute, Oregon Health & Science University 
----- Molecular Basis of PET Imaging in Cardiac Amyloidosis------In the last 5 years, nuclear cardiology revolutionized our approach to cardiac amyloidosis (CA). Characterized by amyloid protein deposition in the extracellular space, CA is well suited to be imaged by molecular imaging techniques. While there are many types of amyloidosis, the two most common types affecting the heart are light chain cardiac amyloidosis (AL-CA) and transthyretin cardiac amyloidosis (ATTR-CA). The introduction of different molecular probes has expanded the potential applications of molecular imaging techniques in CA well beyond disease diagnosis extending to treatment monitoring and prognosis.
Amyloid is a filamentous protein which has a cross β-sheet structure where individual β-strands run perpendicular to the fiber axis. 99mTechnetium (99mTc) labeled radiotracers, which were previously used for their bone-avid properties such as 99mTc-pyrophosphate (99mTc-PYP), were recently found to bind avidly in the myocardium of patients with ATTR-CA in the absence of AL-CA (blood and urine immunofixation electrophoresis and serum free light chains), which has allowed the use of scintigraphy with single-photon emission computerized tomography (SPECT) to diagnose ATTR-CA in the correct clinical setting 1. It is thought that 99mTc-PYP selectively binds to calcium present in the in the amyloid fibrils which are more abundant in ATTR-CA compared to AL-CA2. However the exact mechanism for this is yet to be defined. Regardless, this is an important limitation whereby it can't reliably exclude AL-CA and we are still at early stages in using it quantitatively.  

Recently, multiple positron emission tomography (PET) tracers have been investigated in CA. Thioflavin-T is a histological dye similar to Congo Red dye that was found to display enhanced fluorescence when it binds to the β-pleated motif of the amyloid fibril (Figure A)3. Analogs of thioflavin-T were developed to create amyloid-binding PET tracers, originally for imaging of cerebral amyloid in Alzheimer's disease, but were later found to be taken up by the myocardium in CA (Figure B)4. There is also evidence that tracer uptake may be influenced by the type of amyloid fibril pattern based on studies of V30M ATTR, where uptake is influenced by the phenotype5.

A variety of semi-quantitative methods measuring PET tracer uptake [both dynamic: such as myocardial tracer retention index (RI) and static: such as standard uptake value ratio (SUVR) or target-to-blood pool ratio (TBR)] have been studied to not only better discriminate between the types of CA, but also identify CA earlier, quantify the burden of disease, and subsequently evaluate the response to therapy. We will review here the types of amyloid-binding PET tracers and their unique advantages over current imaging techniques.

11C-Labeled Pittsburg Compound-B
              11C-PIB was the one of the first PET tracers that was reported to have reliable myocardial uptake in biopsy proven CA4. Building on this, 11C-PIB PET scans were investigated as a single testing modality, with one small study identifying cut-off values for the semi-quantitative methods of SUVR and RI that had 100% accuracy in differentiating AL-CA from ATTR-CA6. Integrating the different patterns of 11C-PIB PET and 99mTc-PYP SPECT uptake can also further differentiate between AL-CA, certain hereditary ATTR-CA variants, and wild-type ATTR-CA7 since 11C-PIB PET showed uptake in all ATTR-CA types while 99mTc-PYP was negative in certain hereditary ATTR-CA variants. 11C-PIB could also be used as a prognostic tool as a higher SUVR was associated with an increased all-cause mortality in AL-CA8. An inherent limitation of 11C-PIB is its short half-life of 20 min that requires an onsite cyclotron for its production and clinical use, limiting its utilization outside of specialized settings.

18F-labeled PET tracers 
The 18F-labeled PET tracers offer an advantage over 11C-PIB as it has longer half-life of 110 minutes and can be utilized without the need for an on-site cyclotrons. The U.S. FDA also has already approved some of these agents for imaging cerebral amyloidosis.

              Using autoradiography, 18F-florbetapir uptake was seen in myocardial pathology sections, specifically with higher affinity to AL-CA compared to ATTR-CA9. Similar to 99mTc-PYP, the underlying mechanism for this selectivity is not yet delineated. 18F-florbetapir offers the unique ability to identify AL-CA earlier as uptake was demonstrated in patients with AL amyloidosis who don't meet traditional criteria for cardiac involvement, which potentially could be utilized to improve assessment of organ involvement in the disease10. It still does not appear to be able to assess response to therapy with no correlation between the change in RI or SUVR and changes in NTproBNP levels11,12.  

              In addition to correlation with results from 18F-florbetapir studies, 18F-florbetaben PET/CT was able to identify organs affected by AL amyloidosis among patients with multiple myeloma13. PET-MRI was also able to identify tracer uptake in biopsy proven CA, despite absence of classic cardiac MRI findings of CA14. The SUVR and RI correlated well with left ventricular global longitudinal strain on echocardiography15. Most promising however was its ability to accurately differentiate between non-CA, AL-CA, and ATTR-CA using the temporal change in SUV over an hour16. This makes it an attractive option as a single modality to diagnose CA, identify the type of CA, and assess cardiac function.

              The clinical utility of 18F-flutemetamol is not certain given conflicting evidence surrounding the diagnostic accuracy. Dietemann et al noted increased uptake with significantly higher left ventricular SUV and TBR in eight out of nine patients with CA (all ATTR, except one) compared to controls17. In contrast Papathanasiou et al noted that only two out of twelve CA patients (all ATTR, except two) showed visually increased uptake and no significant difference in SUV compared to their controls despite the presence of late gadolinium enhancement in the majority who underwent cardiac MRI18. Interestingly Möckelind et al found that 18F-flutemetamol uptake was significantly increased compared to controls in 17 patients with early onset V30M hereditary ATTR, the majority of whom had a negative 99mTc-DPD SPECT scan19. The reason for these conflicting results is not clear however the type of amyloid protein and amyloid fibril pattern may play a role. The type B fibril pattern seen in early-onset V30M ATTR that primarily causes neuropathy only contains full length TTR protein, as opposed to Type A fibril pattern that also contains N-terminal TTR fragments seen in late onset V30M ATTR disease that affects the heart. Four V30M ATTR patients had an SUV below the cut-off, one of whom had a positive 99mTc-DPD SPECT scan and Type A fibril pattern on biopsy.

 18F-Sodium Fluoride (18F-NaF)
              18F-NaF has been used previously for detecting micro-calcifications in prostate cancer and coronary plaques and is more readily available, however this tracer has been less extensively studied in CA. Its unique feature is that of all the 18F based tracers, 18F-NaF appears to have higher uptake with ATTR-CA as opposed to AL-CA based on small studies20-22.

Synthetically created tracers are being developed with the aim of specifically targeting amyloid protein. 124I-p5+14 (Figure C) is one such peptide radiolabeled by iodination that targets the hypersulfated heparan sulfate proteoglycan component of amyloid that has shown safety and uptake in AL, ATTR, ALECT2 patients in early phase trials23,24.

These numerous small studies should prompt larger multi-center studies that standardize CA specific PET imaging protocols, including which quantitative method should be adopted and defining optimal cut-off values. There are ongoing studies integrating PET tracers with cardiac MRI, which could help improve upon this already widely used modality and consolidate the number of studies patients will be required to undergo. The increasing recognition of CA has led to an exciting burgeoning field in nuclear cardiology with PET tracers showing promise to have unique applications that could improve the lives of those with cardiac amyloidosis, which is the ultimate goal of molecular imaging and nuclear cardiology.
Figure Legend:
A) Mechanism of PET tracer uptake in amyloidosis. Amyloid protein consists of parallel β sheets composed of β strands that run perpendicular to the long axis of the sheet. PET tracers are primarily thioflavin T analogues that bind along the long axis of the β sheets. These tracers emit a positron that subsequently produce a pair of gamma photons, which are detected by the PET scanner.
B) Chemical structure of 11C-Labeled Pittsburg Compound-B (11C-PIB). 11C-PIB is a thioflavin T analogue that is labeled with carbon-11, a radio-isotope. Other radiotracers have similar structures and are labeled with fluorine-18.
C) Schematic representation of p5+14. P5+14 is a 45 amino acid peptide with α-helical secondary structure. Lysine residues attached to the helix binds to amyloid protein

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Pranav Chandrashekar is supported by an educational grant from Pfizer paid to OHSU. Ahmad Masri received research grants from Pfizer, Akcea, and Ultromics (all paid to OHSU) and consulting fees from Eidos, Ionis and Cytokinetics.

Corresponding author:
Ahmad Masri, MD MS
Assistant Professor of Medicine
Phone: 503-494-8582
Fax: 503-494-8463
Mail code: UHN-62
3181 SW Sam Jackson Rd
Portland, OR 97239
Author Bios: 
Dr. Pranav Chandrashekar (@pranavc91) is a PGY-4 cardiac amyloidosis fellow at the Knight Cardiovascular Institute, Oregon Health and Science University in Portland, Oregon. Dr. Ahmad Masri (@masriahmadmd) is the Co-Director of the Amyloidosis Center, Oregon Health and Science University in Portland, OR and a member of the ASNC Social Media Task Force.