Systemic Amyloid Response in Patients with AL Amyloidosis Assessed By Serial Iodine ( ¹²⁴I) Evuzamitide PET/CT Imaging

Background: Immunoglobulin light chain (AL) amyloidosis is a systemic disorder. Fibrillar aggregates of monoclonal free light chains, or fragments thereof, can deposit in any organ or tissue. Invariably, more than one anatomic site is involved, including the heart, liver, spleen, kidneys, pancreas, nerves, gastrointestinal tract and vasculature. Cardiac and renal amyloidosis are clinically most ominous; however, amyloid in other sites significantly impacts quality of life and may cause other heretofore unappreciated clinical sequelae. This is also true of the other 17 types of systemic amyloidosis. The degree of amyloid distribution and the tissue-specific amyloid load can only be achieved by nuclear imaging. Moreover, monitoring changes in amyloid load in response to therapy would benefit from whole body quantitative molecular imaging techniques. Here we report a pilot study of eight ( n=8) patients with AL amyloidosis who underwent serial PET/CT imaging using a novel amyloid-binding, iodine-124-labelled peptide, designated iodine-124-evuzamitide. Data from a cohort of ten ( n=10) patients with transthyretin associated (ATTR) amyloidosis are also described for comparison (NCT05968846).

Methods: The study plans to enroll 20 ( n=20) patients with AL ( n=10) and ATTR ( n=10) amyloidosis who were previously imaged more than 12 months prior and who had positive uptake of ¹²⁴I-evuzamitide in the heart. To date, eight ( n=8) patients with AL and ten ( n=10) with ATTR amyloidosis have undergone repeat imaging since 2019. Between imaging sessions, patients with AL received immunotherapy ( n=2/8) or were untreated ( n=6/8). Patients were administered 37 or 74 MBq ¹²⁴I-evuzamitide, and PET/CT imaging was performed at ~5 h post injection. Clinical chemistry parameters were collected contemporaneously. Images were interpreted by a single reviewer, experienced in evaluating this tracer, for positive uptake. A fully manual 2D region of interest (ROI) analysis was performed on three representative image views of the heart in the transaxial plane as well as the liver, spleen, and kidneys. The blood pool was used as a reference tissue to determine standard uptake value ratios (SUVR _mean). Cardiac uptake of radiotracer was compared with serum NT-proBNP measurements. Pearson and Spearman correlations were used to test for associations.

Results: The mean time between imaging sessions for patients with AL amyloidosis was 3.21±0.95 years and 2.96±0.87 years for the entire cohort. In the AL amyloid cohort, 88% ( n=7/8) had a reduction in cardiac radiotracer uptake. In one ( n=1) patient, a 92.9% increase in cardiac uptake was seen, which was readily visible in scaled maximum intensity PET images. In this patient, the liver, spleen, and kidney uptake also increased. In contrast to the heart, renal uptake of the radiotracer was increased in 62.5% ( n=5/8) of patients with AL, with three ( n=3) patients showing a >20% increase in the SUVR _mean. A strongly positive, significant correlation between the change in the liver and spleen uptake was observed (Pearson r=0.88, p=0.004). The change in the cardiac radiotracer uptake correlated strongly with that of the spleen (Pearson r=0.74, p=0.04). For the entire cohort, AL and ATTR patients ( n=18), there was a significant moderate correlation between the change in cardiac SUVR _mean and change in single point serum NTproBNP concentration (Spearman rho=0.57, p=0.01).

Conclusion: AL amyloidosis, and indeed all types of systemic amyloid disease, is a complex systemic disorder. The amyloid response to treatment, or disease progression, remains enigmatic as compared to the hematologic and biomarker responses. Appreciating the changes in amyloid load may not only improve clinical management but may also illuminate aspects of pathogenesis. PET/CT imaging using ¹²⁴I-evuzamitide may, therefore, play a valuable role in understanding, diagnosing, and monitoring changes in organ-specific amyloid load and amyloid response. Additional longitudinal imaging studies are needed to investigate these possibilities further.