Multimodal image coregistration and inducible selective cell ablation to evaluate imaging ligands

Abstract

We combined multimodal imaging (bioluminescence, X-ray computed tomography, and PET), tomographic reconstruction of bioluminescent sources, and two unique, complementary models to evaluate three previously synthesized PET radiotracers thought to target pancreatic beta cells. The three radiotracers {[(18)F]fluoropropyl-(+)-dihydrotetrabenazine ([(18)F]FP-DTBZ), [(18)F](+)-2-oxiranyl-3-isobutyl-9-(3-fluoropropoxy)-10-methoxy-2,3,4,6,7,11b-hexahydro-1H-pyrido[2,1-a]isoquinoline ((18)F-AV-266), and (2S,3R,11bR)-9-(3-fluoropropoxy)-2-(hydroxymethyl)-3-isobutyl-10-methoxy-2,3,4,6,7,11b-hexahydro-1H-pyrido[2,1-a]isoquinolin-2-ol ((18)F-AV-300)} bind vesicular monoamine transporter 2. Tomographic reconstruction of the bioluminescent signal in mice expressing luciferase only in pancreatic beta cells was used to delineate the pancreas and was coregistered with PET and X-ray computed tomography images. This strategy enabled unambiguous identification of the pancreas on PET images, permitting accurate quantification of the pancreatic PET signal. We show here that, after conditional, specific, and rapid mouse beta-cell ablation, beta-cell loss was detected by bioluminescence imaging but not by PET imaging, given that the pancreatic signal provided by three PET radiotracers was not altered. To determine whether these ligands bound human beta cells in vivo, we imaged mice transplanted with luciferase-expressing human islets. The human islets were imaged by bioluminescence but not with the PET ligands, indicating that these vesicular monoamine transporter 2-directed ligands did not specifically bind beta cells. These data demonstrate the utility of coregistered multimodal imaging as a platform for evaluation and validation of candidate ligands for imaging islets.

Fig. 1.

Coregistration of bioluminescence tomography and PET images permits evaluation of pancreatic [¹⁸F]FP-DTBZ uptake. (A) After luciferin injection, bioluminescence emanates from the pancreas of a MIP-Luc-VU/RIP-DTR mouse. (B) [¹⁸F]FP-DTBZ PET/CT image of the same mouse displays [¹⁸F]FP-DTBZ accumulation in several abdomen organs, including the liver, bladder, and intestines. (C) Reconstructed bioluminescence tomography of the same mouse displays the bioluminescence source location (red) within the CT mouse volume (blue pixels). (D) An axial slice through the tomographic bioluminescence image reveals the reconstructed bioluminescence source location in the expected anatomical location of the pancreas. The dashed red line defines the pancreatic ROI based on the bioluminescence reconstruction. (E) PET/CT of the same slice displays [¹⁸F]FP-DTBZ uptake in the kidneys, intestines, spleen, spine, and pancreas. The pancreatic ROI (dashed red line) from D is overlaid on the PET/CT image, demonstrating the pancreatic coregistration of bioluminescence and [¹⁸F]FP-DTBZ PET signal of the pancreas.

Fig. 2.

DT treatment ablates beta cells and bioluminescence signal but not pancreatic [¹⁸F]FP-DTBZ PET accumulation. (A) An islet from a control, untreated mouse (Left) displays normal insulin-expressing beta (green) and glucagon-expressing alpha (red) cells. After DT administration (Right), an islet displays almost complete loss of beta (green) and the persistence of alpha (red) cells. (B) MIP-Luc-VU/RIP-DTR mice treated with DT exhibited significantly increased (P < 0.0001) blood glucose at 4 d after DT administration (black bar) compared with normoglycemic untreated, control mice (white bar). (C) BLI of a control untreated mouse (Left) reveals photon emission from the pancreas. This signal was abolished at 4 d after DT treatment (Right). (D) Quantitative analysis showed a significant reduction (P < 0.01) of BLI after this treatment. (E) In contrast, the pancreatic uptake of [¹⁸F]FP-DTBZ, as evaluated from the PET signal, was similar in control and DT-treated mice. (F) Ex vivo biodistribution of [¹⁸F]FP-DTBZ was similar in organs dissected from DT-treated (black bars) and untreated MIP-Luc-VU/RIP-DTR mice (white bars). B and D–F display the mean + SE data of six mice per group.

Fig. 3.

Human islets are detected by BLI but not by [¹⁸F]FP-DTBZ PET imaging. (A) Mice with human islets transduced with luciferase and transplanted beneath the left renal capsule emit bioluminescence. (B) Coronal PET/CT slice of a mouse bearing transplanted human islets in the left renal capsule after [¹⁸F]FP-DTBZ administration. No difference is detectable between the left kidney, which was grafted with 300 islets, and the right kidney, which was not grafted. Both kidneys are delineated by a white dashed line. (C) Postmortem image of the left kidney reveals the persistence of the human islet graft (whitish area outlined with a black dashed line) underneath the kidney capsule at the end of the multimodal BLI/CT/PET imaging. (D) The renal uptake of [¹⁸F]FP-DTBZ, as evaluated from the PET signal, was similar in the left kidney containing transplanted human islets and the contralateral control right kidney. (E) Ex vivo biodistribution of [¹⁸F]FP-DTBZ was similar in the islet graft containing left kidney and the control right kidney. D and E display the mean + SE data of six mice per group.