Imaging the biodistribution of systemic AA amyloidosis in mice using ¹²⁵I-labeled protamine

Objectives Amyloid deposits in brain and peripheral organs contain in addition to protein fibrils a high concentration of heparan sulfate proteoglycans (HSPG). Data suggest that the chemical structure of amyloid-associated heparan sulfate differs from that found ubiquitously in the extra-cellular matrix of all normal tissues rendering it a potential new biomarker for these pathologic deposits. Therefore, we have studied the biodistribution of the heparin-binding peptide protamine as a novel agent for amyloid imaging.

Methods Synthetic protamine was purified by HPLC and the amino acid sequence confirmed by MS. ¹²⁵I-labeled protamine (¹²⁵I-protamine) was administered to mice with systemic AA amyloidosis or to amyloid-free animals and allowed to circulate for 1, 2, 4, 8 or 24 h. At each time point microSPECT/CT images were acquired and the tissue distribution of the radioactivity determined by gamma counting. Finally, 6 µm-thick tissue sections were prepared for autoradiography and histological evaluation.

Results In disease-free mice the ¹²⁵I-protamine was rapidly dehalogented and ¹²⁵I^- sequestered by the stomach which, with the thyroid was visible in SPECT images up to 8 h post-injection. In contrast, ¹²⁵I-protamine when injected into AA mice was observed by SPECT within the liver, kidney, pancreas and spleen (sites of AA amyloid), where tissue:muscle ratios were at least 2-fold greater in mice with amyloid as compared to the WT animals, indicating specific retention of the p48 peptide by AA deposits. Micro-autoradiography confirmed that ¹²⁵I-protamine preferentially associated with the AA amyloid deposits. In amyloid-free mice there was little accumulation of ¹²⁵I-protamine in any organ or tissue.

Conclusions These data indicate that even though HS is expressed in normal tissue, the heparin-reactive peptide protamine may be used for specific imaging of amyloid diseases.

Research Support Supported by USPHS grant 1R01DK079984