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. 2016 Jul 22;2(8):e93.
doi: 10.1097/TXD.0000000000000598. eCollection 2016 Aug.

Quantification of β-Cell Mass in Intramuscular Islet Grafts Using Radiolabeled Exendin-4

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Free PMC article

Quantification of β-Cell Mass in Intramuscular Islet Grafts Using Radiolabeled Exendin-4

Daniel Espes et al. Transplant Direct. .
Free PMC article

Abstract

Background: There is an increasing interest in alternative implantation sites to the liver for islet transplantation. Intramuscular implantation has even been tested clinically. Possibilities to monitor β-cell mass would be of huge importance not only for the understanding of islet engraftment but also for the decision of changing the immunosuppressive regime. We have therefore evaluated the feasibility of quantifying intramuscular β-cell mass using the radiolabeled glucagon like peptide-1 receptor agonist DO3A-VS-Cys40-Exendin-4.

Methods: One hundred to 400 islets were transplanted to the abdominal muscle of nondiabetic mice. After 3 to 4 weeks, 0.2 to 0.5 MBq [177Lu]DO3A-VS-Cys40-Exendin-4 was administered intravenously. Sixty minutes postinjection abdominal organs and graft bearing muscle were retrieved, and the radioactive uptake measured in a well counter within 10 minutes. The specific uptake in native and transplanted islets was assessed by autoradiography. The total insulin-positive area of the islet grafts was determined by immunohistochemistry.

Results: Intramuscular islet grafts could easily be visualized by this tracer, and the background uptake was very low. There was a linear correlation between the radioactivity uptake and the number of transplanted islets, both for standardized uptake values and the total radiotracer uptake in each graft (percentage of injected dose). The quantified total insulin area of surviving β cells showed an even stronger correlation to both standardized uptake values (R = 0.96, P = 0.0002) and percentage of injected dose (R = 0.88, P = 0.0095). There was no correlation to estimated α cell mass.

Conclusions: [177Lu]DO3A-VS-Cys40-Exendin-4 could be used to quantify β-cell mass after experimental intramuscular islet transplantation. This technique may well be transferred to the clinical setting by exchanging Lutetium-177 radionuclide to a positron emitting Gallium-68.

Conflict of interest statement

The authors declare no conflicts of interest.

Figures

FIGURE 1
FIGURE 1
A, Biodistribution expressed as SUV (n = 10 animals). B, Competition with greater than 10 μg/kg DO3A-VS-Cys40-Exendin-4 decreased the tracer binding in pancreas (69%), in grafts with 400 islets (66%) and 200 islets (41%) but not in tissues not positive for GLP-1R such as liver (increase 19%). C, Linear correlation between concentration of radioactivity (SUV) measured in each muscle biopsy containing islet graft and the number of islets transplanted. D, Linear correlation between total radioactivity measured in each islet graft, corrected for the background muscle uptake, and the number of islets transplanted. There was a strong positive correlation between the surviving engrafted islets (assessed as insulin positive area in each graft containing muscle biopsy) and the radioactive uptake measured as concentration SUV (E) as well as the total uptake %ID (F).
FIGURE 2
FIGURE 2
Autoradiograms of a representative animal showing 4 consecutive sections of pancreas (A), intramuscular islet graft from an animal transplanted with 400 islets (B) and 200 islets (C). Arrows indicate background of islets. I, islet of Langerhans; X, exocrine pancreas; M, muscle. D, Immunohistochemical staining of paraffin embedded section of an intramuscular islet graft. Insulin stained in brown, counterstaining with hematoxylin. Scale bar represents 200 μm. [177Lu]DO3A-VS-Cys40-Exendin-4 binding was quantified as fmol tracer bound per mm3 of tissue (E). There was a linear correlation between total amount of fmol [177Lu]DO3A-VS-Cys40-Exendin-4 bound in each graft, as assessed by ex vivo autoradiography, and the number of islets transplanted (F) as well as the total insulin positive area in each graft (G).

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