Longitudinal PET/CT Imaging with Iodine ( ¹²⁴I) Evuzamitide Reveals Organ Response to Plasma Cell Immunotherapy in a Patient with AL Amyloidosis

Background:Iodine-124-evuzamitide is a novel amyloidophilic peptide (p5+14), labeled with iodine-124, for detecting amyloid by positron emission/x-ray computed tomographic (PET/CT) imaging ( Pharmaceuticals 2023 Apr 21;16(4):629). PET is an inherently quantitative modality and thus ¹²⁴I-evuzamitide may be useful for monitoring disease progression and response to therapy. Amyloid deposits can impact any organ in patients with light chain-associated amyloidosis (AL); thus, tissue-specific changes in amyloid load, based on PET/CT imaging, may be informative. Currently, response to therapy is assessed by anatomic imaging and measurement of serum or urinary biomarkers. However, the correlation between these metrics and concomitant changes in amyloid load remains enigmatic. Here, we report a case of a patient with AL amyloidosis, on long-term daratumumab therapy, who underwent longitudinal ¹²⁴I-evuzamitide PET/CT imaging over 4 years (NCT03678259 and NCT05968846).

Patient and Methods: A 70-year-old male presented with abnormal liver function tests, notably an elevated serum alkaline phosphatase (ALK) of 465 IU/L. Biopsies of both the liver and celiac nodes revealed the characteristic apple-green birefringence of Congo red stain, indicating the presence of amyloid. The patient began daratumumab immunotherapy 14 months prior to his enrollment in the ¹²⁴I-evuzamitide imaging study and over the 4-year course of serial imaging. During this time, regular serial serum biomarkers were evaluated as standard of care. PET/CT images were acquired at 0, 23, and 45 months. For each imaging session, the patient received 2 mCi iodine-124-labeled tracer via intravenous injection and was imaged at 5 h post injection. Image data were assessed for visual uptake of radiotracer and quantified by manual 2D region-of-interest analysis where the semi-quantitative mean standard uptake value ratio (SUVR _mean) was calculated using blood pool as the reference tissue.

Results: Over the 45 months of evaluation, there was a 59% and 73% reduction in ¹²⁴I-evuzamitide SUVR in the liver and spleen. This was accompanied by decreases in the weight-scaled organ volume (cc/kg) for both the liver (15.2%) and spleen (30.0%) based on segmentation of the organ volume from CT images. This reduction in radiotracer uptake coincided with a sustained decrease in serum FLC, from 38.3 mg/L to 19.1 mg/L, and a positive liver response, where ALK levels dropped from ~200 IU/L to ~120 IU/L. In contrast, renal radiotracer uptake increased slightly in the context of stable creatinine levels, and a modest decline in eGFR. Cardiac uptake of ¹²⁴I-evuzamitide remained positive and relatively stable, concordant with serum NT-proBNP levels. These data suggest that organ-specific amyloid response, in the context of a sustained hematologic response, may occur independently and at different rates.

Conclusion: Previous reports have demonstrated the resolution of AL amyloid. However, this is the first report using an amyloid-binding radiotracer, capable of visualizing all major organs including the heart, demonstrating the complex pattern of organ response and regression of AL amyloid in the context of concurrent biomarker improvement and changes in organ volume. PET/CT imaging using ¹²⁴I-evuzamitide may, therefore, play a valuable role in both diagnosing amyloidosis and monitoring changes in organ-specific amyloid load. Additional longitudinal imaging studies are needed to support this clinical application.