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A binding-site barrier affects imaging efficiency of high affinity amyloid-reactive peptide radiotracers in vivo.

Wall JS, Williams A, Richey T, Stuckey A, Huang Y, Wooliver C, Macy S, Heidel E, Gupta N, Lee A, Rader B, Martin EB, Kennel SJ - PLoS ONE (2013)

Bottom Line: Amyloid is a complex pathology associated with a growing number of diseases including Alzheimer's disease, type 2 diabetes, rheumatoid arthritis, and myeloma.The p5 peptide was posited to bind effectively to amyloid deposits, relative to similarly charged polybasic heparin-reactive peptides, because it adopted a polar α helix secondary structure.The p5R peptide had higher affinity for amyloid and visualized AA amyloid in mice by using SPECT/CT imaging; however, the microdistribution, as evidenced in micro-autoradiographs, was dramatically altered relative to the p5 peptide due to its increased affinity and a resultant "binding site barrier" effect.

View Article: PubMed Central - PubMed

Affiliation: Department of Medicine, University of Tennessee Graduate School of Medicine, Knoxville, Tennessee, USA. jwall@utmck.edu

ABSTRACT
Amyloid is a complex pathology associated with a growing number of diseases including Alzheimer's disease, type 2 diabetes, rheumatoid arthritis, and myeloma. The distribution and extent of amyloid deposition in body organs establishes the prognosis and can define treatment options; therefore, determining the amyloid load by using non-invasive molecular imaging is clinically important. We have identified a heparin-binding peptide designated p5 that, when radioiodinated, was capable of selectively imaging systemic visceral AA amyloidosis in a murine model of the disease. The p5 peptide was posited to bind effectively to amyloid deposits, relative to similarly charged polybasic heparin-reactive peptides, because it adopted a polar α helix secondary structure. We have now synthesized a variant, p5R, in which the 8 lysine amino acids of p5 have been replaced with arginine residues predisposing the peptide toward the α helical conformation in an effort to enhance the reactivity of the peptide with the amyloid substrate. The p5R peptide had higher affinity for amyloid and visualized AA amyloid in mice by using SPECT/CT imaging; however, the microdistribution, as evidenced in micro-autoradiographs, was dramatically altered relative to the p5 peptide due to its increased affinity and a resultant "binding site barrier" effect. These data suggest that radioiodinated peptide p5R may be optimal for the in vivo detection of discreet, perivascular amyloid, as found in the brain and pancreatic vasculature, by using molecular imaging techniques; however, peptide p5, due to its increased penetration, may yield more quantitative imaging of expansive tissue amyloid deposits.

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The distribution of peptide p5R differs from p5 in AA mice in vivo.Dual energy biodistribution studies of (A) 125I-p5R (black) and 123I-p5 (gray) and (B) 123I-p5R (black) and 125I-p5 (gray) in individual mice with moderate-to-severe AA amyloid (mean + SD, n = 5). (C) The microdistribution of 125I-p5 and 125I-p5R in separate mice with weak (1+) and strong (4+) AA assessed by autoradiography. Arrows indicate the presence of “large” AA splenic amyloid. (D) The difference between 125I-p5 and 125I-p5R uptake (i.e. %ID/g p5 – %ID/g p5R) in the spleen (○), liver (•) and pancreas (□) relative to amyloid load (%ID/g 125I-p5).
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pone-0066181-g005: The distribution of peptide p5R differs from p5 in AA mice in vivo.Dual energy biodistribution studies of (A) 125I-p5R (black) and 123I-p5 (gray) and (B) 123I-p5R (black) and 125I-p5 (gray) in individual mice with moderate-to-severe AA amyloid (mean + SD, n = 5). (C) The microdistribution of 125I-p5 and 125I-p5R in separate mice with weak (1+) and strong (4+) AA assessed by autoradiography. Arrows indicate the presence of “large” AA splenic amyloid. (D) The difference between 125I-p5 and 125I-p5R uptake (i.e. %ID/g p5 – %ID/g p5R) in the spleen (○), liver (•) and pancreas (□) relative to amyloid load (%ID/g 125I-p5).

Mentions: Faced with the counter-intuitive observation that peptide p5R had a greater affinity for heparin (Fig. 3A) and ex vivo AA amyloid (Fig. 3B), yet the reactivity with splenic AA amyloid in vivo appeared markedly reduced relative to studies with peptide p5, we examined the binding of both peptides in the same subject by using dual-energy biodistribution analyses (Fig. 5). Individual H2/IL-6 mice with moderate to severe AA amyloid (3 – 4+) received 125I-p5R and 123I-p5 or 125I-p5 and 123I-p5R. Biodistribution measurements at 2 h pi (Fig. 5A & B) showed significantly greater retention of p5 in splenic AA amyloid as compared to peptide p5R (p = 0.045, Fig. 5A; p = 0.032, Fig. 5B). There was no significant difference (p > 0.05) in the retention of peptides p5 and p5R in hepatic or pancreatic AA amyloid deposits.


A binding-site barrier affects imaging efficiency of high affinity amyloid-reactive peptide radiotracers in vivo.

Wall JS, Williams A, Richey T, Stuckey A, Huang Y, Wooliver C, Macy S, Heidel E, Gupta N, Lee A, Rader B, Martin EB, Kennel SJ - PLoS ONE (2013)

The distribution of peptide p5R differs from p5 in AA mice in vivo.Dual energy biodistribution studies of (A) 125I-p5R (black) and 123I-p5 (gray) and (B) 123I-p5R (black) and 125I-p5 (gray) in individual mice with moderate-to-severe AA amyloid (mean + SD, n = 5). (C) The microdistribution of 125I-p5 and 125I-p5R in separate mice with weak (1+) and strong (4+) AA assessed by autoradiography. Arrows indicate the presence of “large” AA splenic amyloid. (D) The difference between 125I-p5 and 125I-p5R uptake (i.e. %ID/g p5 – %ID/g p5R) in the spleen (○), liver (•) and pancreas (□) relative to amyloid load (%ID/g 125I-p5).
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3672142&req=5

pone-0066181-g005: The distribution of peptide p5R differs from p5 in AA mice in vivo.Dual energy biodistribution studies of (A) 125I-p5R (black) and 123I-p5 (gray) and (B) 123I-p5R (black) and 125I-p5 (gray) in individual mice with moderate-to-severe AA amyloid (mean + SD, n = 5). (C) The microdistribution of 125I-p5 and 125I-p5R in separate mice with weak (1+) and strong (4+) AA assessed by autoradiography. Arrows indicate the presence of “large” AA splenic amyloid. (D) The difference between 125I-p5 and 125I-p5R uptake (i.e. %ID/g p5 – %ID/g p5R) in the spleen (○), liver (•) and pancreas (□) relative to amyloid load (%ID/g 125I-p5).
Mentions: Faced with the counter-intuitive observation that peptide p5R had a greater affinity for heparin (Fig. 3A) and ex vivo AA amyloid (Fig. 3B), yet the reactivity with splenic AA amyloid in vivo appeared markedly reduced relative to studies with peptide p5, we examined the binding of both peptides in the same subject by using dual-energy biodistribution analyses (Fig. 5). Individual H2/IL-6 mice with moderate to severe AA amyloid (3 – 4+) received 125I-p5R and 123I-p5 or 125I-p5 and 123I-p5R. Biodistribution measurements at 2 h pi (Fig. 5A & B) showed significantly greater retention of p5 in splenic AA amyloid as compared to peptide p5R (p = 0.045, Fig. 5A; p = 0.032, Fig. 5B). There was no significant difference (p > 0.05) in the retention of peptides p5 and p5R in hepatic or pancreatic AA amyloid deposits.

Bottom Line: Amyloid is a complex pathology associated with a growing number of diseases including Alzheimer's disease, type 2 diabetes, rheumatoid arthritis, and myeloma.The p5 peptide was posited to bind effectively to amyloid deposits, relative to similarly charged polybasic heparin-reactive peptides, because it adopted a polar α helix secondary structure.The p5R peptide had higher affinity for amyloid and visualized AA amyloid in mice by using SPECT/CT imaging; however, the microdistribution, as evidenced in micro-autoradiographs, was dramatically altered relative to the p5 peptide due to its increased affinity and a resultant "binding site barrier" effect.

View Article: PubMed Central - PubMed

Affiliation: Department of Medicine, University of Tennessee Graduate School of Medicine, Knoxville, Tennessee, USA. jwall@utmck.edu

ABSTRACT
Amyloid is a complex pathology associated with a growing number of diseases including Alzheimer's disease, type 2 diabetes, rheumatoid arthritis, and myeloma. The distribution and extent of amyloid deposition in body organs establishes the prognosis and can define treatment options; therefore, determining the amyloid load by using non-invasive molecular imaging is clinically important. We have identified a heparin-binding peptide designated p5 that, when radioiodinated, was capable of selectively imaging systemic visceral AA amyloidosis in a murine model of the disease. The p5 peptide was posited to bind effectively to amyloid deposits, relative to similarly charged polybasic heparin-reactive peptides, because it adopted a polar α helix secondary structure. We have now synthesized a variant, p5R, in which the 8 lysine amino acids of p5 have been replaced with arginine residues predisposing the peptide toward the α helical conformation in an effort to enhance the reactivity of the peptide with the amyloid substrate. The p5R peptide had higher affinity for amyloid and visualized AA amyloid in mice by using SPECT/CT imaging; however, the microdistribution, as evidenced in micro-autoradiographs, was dramatically altered relative to the p5 peptide due to its increased affinity and a resultant "binding site barrier" effect. These data suggest that radioiodinated peptide p5R may be optimal for the in vivo detection of discreet, perivascular amyloid, as found in the brain and pancreatic vasculature, by using molecular imaging techniques; however, peptide p5, due to its increased penetration, may yield more quantitative imaging of expansive tissue amyloid deposits.

Show MeSH
Related in: MedlinePlus