Mutsumi Kosugi

What Can Be Seen by 18F-FDG PET in Atherosclerosis Imaging? The Effect of Foam Cell Formation on 18F-FDG Uptake to Macrophages In Vitro

18F-FDG PET is a promising tool for detecting vulnerable plaques, depending on the extent of macrophage infiltration; however, it is still not clear which stage of the lesion can be detected by 18F-FDG PET. Methods: In this study, we investigated the effect of foam cell formation on 18F-FDG uptake using cultured mouse peritoneal macrophages. Results: 18F-FDG accumulation was increased by foam cell formation, but the uptake was decreased to the control level after complete differentiation to foam cells. Changes in hexokinase activity tended to accompany changes in 18F-FDG uptake. In contrast, changes in glucose-6-phosphatase activity and glucose transporter 1 expression did not parallel 18F-FDG uptake. Conclusion: Our results suggest that 18F-FDG PET detects the early stage of foam cell formation in atherosclerosis.

Development of 111In-Labeled Liposomes for Vulnerable Atherosclerotic Plaque Imaging

Macrophage infiltration is a common characteristic feature of atherosclerotic-vulnerable plaques. Macrophages recognize phosphatidylserine (PS) exposed on the surface of apoptotic cells, which triggers the engulfment of the apoptotic cells by macrophages through phagocytosis. In this study, we prepared radiolabeled PS liposomes for detection of vulnerable plaques. Methods: PS liposomes were prepared by lipid film hydration. Phosphatidylcholine (PC) liposomes were prepared as controls. Liposomes (100 or 200 nm) were generated by an extruder to produce PS100, PS200, PC100, and PC200 liposomes. These were then radiolabeled by encapsulating 111In-nitrilotriacetic acid using an active-loading method. 111In liposomes were incubated with cultured macrophages for 2 h, and the uptake level was measured. For biodistribution studies, the 111In liposomes were injected intravenously into ddY mice. In addition, the 111In liposomes were injected into apolipoprotein E–deficient (apoE−/−) mice, and the aortas were harvested for autoradiography and oil red O staining. For SPECT imaging, 111In liposomes were injected intravenously into Watanabe heritable hyperlipidemic rabbits and scanned 48 h after injection. Results: The radiochemical yields were greater than 95% for all the prepared 111In liposomes. The level of in vitro uptake by macrophages was 60.5, 14.7, 32.0, and 14.4 percentage injected dose per milligram of protein for 111In-PS100, 111In-PC100, 111In-PS200, and 111In-PC200, respectively. In biodistribution studies, high spleen uptake was seen with PC liposomes. Liver uptake was high for all liposomes but was lowest with 111In-PS200. The blood half-lives were 3.2, 22.0, 3.6, and 7.4 min for 111In-PS100, 111In-PC100, 111In-PS200, and 111In-PC200, respectively. The distribution of 111In-labeled PS liposomes into atherosclerotic regions determined by autoradiography was well matched with the results of oil red O staining in apoE−/− mice. The target-to-nontarget ratios were 2.62, 2.23, 3.27, and 2.51 for 111In-PS100, 111In-PC100, 111In-PS200, and 111In-PC200, respectively. The aorta was successfully visualized by SPECT at 48 h after 111In-labeled PS liposome injection; however, high liver uptake was also observed. Discussion: From the in vitro uptake study, it has been demonstrated that macrophage targeting was accomplished by PS modification. Also, an atherosclerotic region was successfully detected by 111In-PS200 in apoE−/− mice and Watanabe heritable hyperlipidemic rabbits in vivo. Liposome modification to obtain slower blood clearance and lower liver uptake would be required to improve the SPECT images.

PEG modification on 111In-labeled phosphatidyl serine liposomes for imaging of atherosclerotic plaques

INTRODUCTION Previously, we reported a probe for imaging of atherosclerotic plaques: (111)In-labeled liposomes. Liposomes were modified with phosphatidylserine (PS) because macrophages recognize PS and phagocytize apoptotic cells in plaques. PS modification was successful and we could visualize atherosclerotic plaques by single-photon emission computed tomography (SPECT). However, too-rapid blood clearance reduced accumulation of PS-liposomes in plaques in vivo. Therefore, in the present study, PS-liposomes were modified with polyethylene glycol (PEG) to retard the rate of blood clearance. METHODS PS-liposomes (size, 100 nm or 200 nm) were PEGylated with PEG2000 or PEG5000 at 1 or 5 mol%, and radiolabeled with (111)In. For the study of uptake in vitro, liposomes were incubated with mouse peritoneal macrophages. Biodistribution studies in vivo were carried out in ddY mice. En face autoradiograms were obtained with apoE(-/-) mice upon intravenous injection of (111)In-liposomes. RESULTS Uptake was decreased significantly at 5 mol% PEGylation in 100-nm PS-liposomes (*P<0.05 vs. 0 mol%). All the PEGylated liposomes tested showed significantly lower uptake than the non-PEGylated control in 200-nm liposomes. In vivo results showed slower blood clearance in PEGylated liposomes. Autoradiograms in apoE(-/-) mice were well matched with Oil Red O staining. Additionally, 200-nm PS-liposomes modified with 5%PEG2000 ([(111)In]5%PEG2000PS200) showed the highest uptake to the region in vivo. CONCLUSIONS As expected, PEGylation retarded the rate of blood clearance. In addition, it affected liposome uptake by macrophages in vitro. These results suggest that the balance between the rate of blood clearance and macrophage recognition is important, and [(111)In]5%PEG2000PS200 showed the best results in our investigation.

Quantitation of rat cerebral blood flow using 99mTc-HMPAO

INTRODUCTION Technetium-99m-hexamethylpropyleneamine oxime (99mTc-HMPAO) is potentially useful for the assessment of cerebral blood flow (CBF) in small animals. In this paper, a procedure for quantitation of rat CBF using 99mTc-HMPAO was determined. METHODS Biodistribution of 99mTc-radioactivity in normal rats was determined after intravenous administration of 99mTc-HMPAO. Acetazolamide treated rats were intravenously administered with the mixture of 99mTc-HMPAO and N-isopropyl-[125I]iodoamphetamine ([125I]IMP), and arterial blood was then collected for 5min. After blood sampling, the brain radioactivity concentration was measured with the auto-well γ counter. RESULTS The brain radioactivity concentration after intravenous administration of 99mTc-HMPAO was steady from 14s to 60min post-injection. A double tracer experiment using 99mTc-HMPAO and [125I]IMP showed that 19s was the average of the optimal integration interval of arterial blood 99mTc-radioactivity concentration to obtain CBF values measured by 99mTc-HMPAO identical to those determined by [125I]IMP. The CBF value determined by 99mTc-HMPAO, calculated by dividing the brain radioactivity concentration at 5min post-injection by the integrated arterial blood radioactivity concentration until 19s post-injection, was well correlated with CBF as determined by [125I]IMP. CONCLUSION These results suggest that the CBF quantitation procedure described in this paper could be useful for rat CBF assessment.

Noninvasive quantitation of rat cerebral blood flow using 99m Tc-HMPAO—assessment of input function with dynamic chest planar imaging

BackgroundCerebral blood flow (CBF) quantitation using technetium-99m hexamethylpropyleneamine oxime (99mTc-HMPAO) generally requires assessment of input function by arterial blood sampling, which would be invasive for small animals. We therefore performed chest dynamic planar imaging, instead of arterial blood sampling, to estimate the input function and establish noninvasive quantitation method of rat CBF using the image-derived input function.ResultsIntegrated radioactivity concentration in the heart-blood pool on planar images (AUCBlood-planar) was identical to that in arterial blood samples (AUCBlood-sampling). Radioactivity concentration in the brain determined by SPECT imaging (CBrain-SPECT) was identical to that using brain sampling (CBrain-sampling). Noninvasively calculated CBF obtained by dividing CBrain-SPECT by AUCBlood-planar was well correlated with conventionally estimated CBF obtained by dividing CBrain-sampling by AUCBlood-sampling.ConclusionRat CBF could be noninvasively quantitated using 99mTc-HMPAO chest dynamic planar imaging and head SPECT imaging without arterial blood sampling.