Reproducibility of 3'-deoxy-3'-(18)F-fluorothymidine microPET studies in tumor xenografts in mice.

3'-Deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) has been used to image tumor proliferation in preclinical and clinical studies. Serial microPET studies may be useful for monitoring therapy response or for drug screening; however, the reproducibility of serial scans has not been determined. The purpose of this study was to determine the reproducibility of (18)F-FLT microPET studies. METHODS: C6 rat glioma xenografts were implanted into nude mice (n = 9) and grown to mean diameters of 5-17 mm for approximately 2 wk. A 10-min acquisition was performed on a microPET scanner approximately 1 h after (18)F-FLT (1.9-7.4 MBq [50-200 muCi]) was injected via the tail vein. A second microPET scan was performed approximately 6 h later on the same day after reinjection of (18)F-FLT to assess for reproducibility. Most of the mice were studied twice within the same week (for a total of 17 studies). Images were analyzed by drawing an ellipsoidal region of interest (ROI) around the tumor xenograft activity. Percentage injected dose per gram (%ID/g) values were calculated from the mean activity in the ROIs. Coefficients of variation and differences in %ID/g values between studies from the same day were calculated to determine the reproducibility after subtraction of the estimated residual tumor activity from the first (18)F-FLT injection. RESULTS: The coefficient of variation (mean +/- SD) for %ID/g values between (18)F-FLT microPET scans performed 6 h apart on the same day was 14% +/- 10%. The difference in %ID/g values between scans was -0.06% +/- 1.3%. Serum thymidine levels were mildly correlated with %ID/g values (R(2) = 0.40). Tumor size, mouse body weight, injected dose, and fasting state did not contribute to the variability of the scans; however, consistent scanning parameters were necessary to ensure accurate studies, in particular, controlling body temperature, the time of imaging after injection, and the ROI size. CONCLUSION: (18)F-FLT microPET mouse tumor xenograft studies are reproducible with moderately low variability. Serial studies may be performed to assess for significant changes in therapy response or for preclinical drug development.     

Preclinical efficacy of the c-Met inhibitor CE-355621 in a U87 MG mouse xenograft model evaluated by 18F-FDG small-animal PET.

The purpose of this study was to evaluate the efficacy of CE-355621, a novel antibody against c-Met, in a subcutaneous U87 MG xenograft mouse model using (18)F-FDG small-animal PET. METHODS: CE-355621 or control vehicle was administered intraperitoneally into nude mice (drug-treated group, n = 12; control group, n = 14) with U87 MG subcutaneous tumor xenografts. Drug efficacy was evaluated over 2 wk using (18)F-FDG small-animal PET and compared with tumor volume growth curves. RESULTS: The maximum %ID/g (percentage injected dose per gram of tissue) of (18)F-FDG accumulation in mice treated with CE-355621 remained essentially unchanged over 2 wk, whereas the %ID/g of the control tumors increased 66% compared with the baseline. Significant inhibition of (18)F-FDG accumulation was seen 3 d after drug treatment, which was earlier than the inhibition of tumor volume growth seen at 7 d after drug treatment. CONCLUSION: CE-355621 is an efficacious novel antineoplastic chemotherapeutic agent that inhibits (18)F-FDG accumulation earlier than tumor volume changes in a mouse xenograft model. These results support the use of (18)F-FDG PET to assess early tumor response for CE-355621.     

Characterization of osteolytic, osteoblastic, and mixed lesions in a prostate cancer mouse model using 18F-FDG and 18F-fluoride PET/CT.

The combination of small-animal PET/CT scans and conventional imaging methods may enhance the evaluation of in vivo biologic interactions of murine models in the study of prostate cancer metastasis to bone. METHODS: Small-animal PET/CT scans using (18)F-fluoride ion and (18)F-FDG coregistered with high-resolution small-animal CT scans were used to longitudinally assess the formation of osteoblastic, osteolytic, and mixed lesions formed by human prostate cancer cell lines in a severe combined immunodeficient (SCID) mouse tibial injection model. These scans were correlated with plain radiographs, histomorphometry, and soft-tissue measurements. RESULTS: Small-animal PET/CT scans were able to detect biologic activity of cells that induced an osteoblastic lesion 2 wk earlier than on plain radiographs. Furthermore, both the size and the activity of the lesions detected on PET/CT images significantly increased at each successive time point (P < 0.05). (18)F-FDG lesions strongly correlated with soft-tissue measurements, whereas (18)F-fluoride ion activity correlated with bone volume measured on histomorphometric analysis (P < 0.005). Osteolytic lesions were successfully quantified using small-animal CT, whereas lesion sizes measured on (18)F-FDG PET scans also strongly correlated with soft-tissue tumor burden (P < 0.05). In contrast, for mixed lesions, (18)F-fluoride ion and (18)F-FDG PET/CT scans detected only minimal activity. CONCLUSION: (18)F-FDG and (18)F-fluoride ion PET/CT scans can be useful tools in characterizing pure osteolytic and osteoblastic lesions induced by human prostate cancer cell lines. The value of this technology needs further evaluation to determine whether these studies can be used effectively to detect more subtle responses to different treatment regimens in animal models.     

In vivo VEGF imaging with radiolabeled bevacizumab in a human ovarian tumor xenograft.

Vascular endothelial growth factor (VEGF), released by tumor cells, is an important growth factor in tumor angiogenesis. The humanized monoclonal antibody bevacizumab blocks VEGF-induced tumor angiogenesis by binding, thereby neutralizing VEGF. Our aim was to develop radiolabeled bevacizumab for noninvasive in vivo VEGF visualization and quantification with the single gamma-emitting isotope 111In and the PET isotope 89Zr. METHODS: Labeling, stability, and binding studies were performed. Nude mice with a human SKOV-3 ovarian tumor xenograft were injected with 89Zr-bevacizumab, 111In-bevacizumab, or human 89Zr-IgG. Human 89Zr-IgG served as an aspecific control antibody. Small-animal PET and microCT studies were obtained at 24, 72, and 168 h after injection of 89Zr-bevacizumab and 89Zr-IgG (3.5 +/- 0.5 MBq, 100 +/- 6 microg, 0.2 mL [mean +/- SD]). Small-animal PET and microCT images were fused to calculate tumor uptake and compared with ex vivo biodistribution at 168 h after injection. 89Zr- and 111In-bevacizumab ex vivo biodistribution was compared at 24, 72, and 168 h after injection (2.0 +/- 0.5 MBq each, 100 +/- 4 microg in total, 0.2 mL). RESULTS: Labeling efficiencies, radiochemical purity, stability, and binding properties were optimal for the radioimmunoconjugates. Small-animal PET showed uptake in well-perfused organs at 24 h and clear tumor localization from 72 h onward. Tumor uptake determined by quantification of small-animal PET images was higher for 89Zr-bevacizumab-namely, 7.38 +/- 2.06 %ID/g compared with 3.39 +/- 1.16 %ID/g (percentage injected dose per gram) for human 89Zr-IgG (P = 0.011) at 168 h and equivalent to ex vivo biodistribution studies. Tracer uptake in other organs was seen primarily in liver and spleen. 89Zr- and 111In-bevacizumab biodistribution was comparable. CONCLUSION: Radiolabeled bevacizumab showed higher uptake compared with radiolabeled human IgG in a human SKOV-3 ovarian tumor xenograft. Noninvasive quantitative small-animal PET was similar to invasive ex vivo biodistribution. Radiolabeled bevacizumab is a new tracer for noninvasive in vivo imaging of VEGF in the tumor microenvironment.     

Effects of anesthetic agents and fasting duration on 18F-FDG biodistribution and insulin levels in tumor-bearing mice.

Small-animal PET has opened the way for imaging (18)F-FDG uptake in murine tumor models, but the need for anesthesia raises concern over its potential influence on (18)F-FDG kinetics. We thus investigated such effects on cultured cells and on tumor-bearing mice after short- and long-term fasting. METHODS: Lewis lung carcinoma (LLC) cells and cardiomyoblasts were treated for 2 h with a 100 micromol/L concentration of xylazine, ketamine, xylazine plus ketamine (Xy/Ke), or pentobarbital and were measured for (18)F-FDG uptake. LLC tumor-bearing C57BL6 mice that had been kept fasting for either 4 or 20 h were injected with Xy/Ke, pentobarbital, or saline and were administered 1.8 MBq of (18)F-FDG 15 min later. Biodistribution studies and plasma glucose and insulin assays were performed 45 min after injection. Separate anesthetized and control mice underwent (18)F-FDG PET. RESULTS: (18)F-FDG uptake in LLC cells was unaffected by anesthetic agents, whereas xylazine and ketamine caused a small increase of uptake in cardiomyoblasts. In mice kept fasting 4 h, Xy/Ke induced a marked elevation of (18)F-FDG activity (percentage injected dose [%ID]) in blood (6.8 +/- 0.9%ID/g vs. 1.1 +/- 0.6%ID/g) and kidneys while decreasing myocardial uptake (2.3 +/- 1.3%ID/g vs. 4.7 +/- 1.8%ID/g). Target-to-blood ratios were significantly reduced. Pentobarbital caused a moderate increase in blood activity (2.5 +/- 0.8%ID/g), decreased myocardial uptake (2.8 +/- 0.5%ID/g), and reduced target-to-blood ratios. PET images of mice kept fasting 4 h were consistent with the biodistribution data. Insulin levels were lower with Xy/Ke and higher with pentobarbital. In mice kept fasting 20 h, Xy/Ke and pentobarbital increased blood (18)F-FDG activity (5.5 +/- 2.2 and 4.9 +/- 0.9%ID/g vs. 2.4 +/- 0.3%ID/g) and reduced target-to-blood ratios, but these changes were substantially attenuated, compared with those in mice kept fasting 4 h. In addition, insulin levels were low and unaffected by anesthesia. CONCLUSION: Xy/Ke anesthesia markedly elevates blood (18)F-FDG activity and reduces tumor uptake ratios through inhibition of insulin release in mice kept fasting 4 h, whereas pentobarbital induces a similar but less severe response through insulin resistance. These metabolic effects, however, are substantially attenuated after 20 h of fasting. Hence both the choice of anesthetic and the duration of fasting have important effects on (18)F-FDG kinetics and PET images of tumor-bearing mice and should be considered when such studies are performed.     

Retro-orbital injection is an effective route for radiopharmaceutical administration in mice during small-animal PET studies

BACKGROUND AND AIM: Small-animal PET is acquiring importance for pre-clinical studies. In rodents, radiotracers are usually administrated via the tail vein. This procedure can be very difficult and time-consuming as soft tissue extravasations are very frequent and tail scars can prevent repeated injections after initial failure. The aim of our study was to compare the retro-orbital (RO) versus tail vein intravenous (i.v.) administration of (18)F-FDG and (11)C-choline in mice for small-animal PET studies. METHODS: We evaluated four healthy female ICR CD1 mice according to the following protocol. Day 1: each animal underwent an i.v. injection of 28 MBq of (11)C-choline. PET scan was performed after 10 min and 40 min. Day 2: each animal received an RO injection of 28 MBq of (11)C-choline. A PET scan was performed after 10 min and 40 min. Day 3: each animal received an i.v. injection of 28 MBq of (18)F-FDG. A PET scan was performed after 60 min and 120 min. Day 4: each animal received an RO injection of 28 MBq of (18)F-FDG. A PET scan was performed after 60 min and 120 min. Administration and image acquisition were performed under gas anaesthesia. For FDG studies the animals fasted for 2 h and were kept asleep for 20-30 min after injection, to avoid muscular uptake. Images were reconstructed with 2-D OSEM. For each scan ROIs were drawn on liver, kidneys, lung, brain, heart brown fat and muscles, and the SUV was calculated. We finally compared choline i.v. standard acquisition to choline RO standard acquisition; choline i.v. delayed acquisition to choline RO delayed acquisition; FDG i.v. standard acquisition to FDG RO standard acquisition; FDG i.v. delayed acquisition to FDG RO delayed acquisition. RESULTS: The RO injections for both (18)F-FDG and (11)C-choline were comparable to the intravenous injection of F-FDG for the standard and delayed acquisitions. CONCLUSION: The RO administration in mice represents a technical advantage over intravenous administration in being an easier and faster procedure. However, its use requires high specific activity while its value in peptides and other receptor-specific radiopharmaceuticals needs further assessment.     

A preliminary study of anti-1-amino-3-18F-fluorocyclobutyl-1-carboxylic acid for the detection of prostate cancer.

We evaluated the feasibility of anti-1-amino-3-(18)F-fluorocyclobutyl-1-carboxylic acid (anti-(18)F-FACBC) in diagnosing prostate cancer (PCa), using a rat orthotopic prostate cancer transplantation (OPCT) model. Furthermore, using in vivo experiments, we examined the potential of anti-(18)F-FACBC for differentiating between PCa and inflammation and between PCa and benign prostatic hyperplasia (BPH). METHODS: The OPCT model was developed by transplanting DU145, a human PCa cell line, into the ventral prostate of athymic F344 rats. To develop a dual PCa and inflammation (DPCI) model, MAT-Ly-Lu-B2--a rat PCa cell line--was transplanted subcutaneously into male Copenhagen rats. Streptozotocin was injected into the hind footpad of these rats for inducing popliteal lymphadenitis. For inducing the BPH, normal F344 rats were castrated and injected subcutaneously with testosterone propionate. In biodistribution studies, the rats were injected with anti-(18)F-FACBC or (18)F-FDG and sacrificed at 15 or 60 min after injection. We performed dynamic small-animal PET of the abdominal portion of the OPCT rats for 60 min after the injection of anti-(18)F-FACBC or (18)F-FDG. RESULTS: The biodistribution in the OPCT rats at 60 min after injection showed that the uptake of anti-(18)F-FACBC and (18)F-FDG into the PCa tissue was 1.58 +/- 0.40 %ID/cm(3) (percentage injected dose per cm(3)) and 1.48 +/- 0.90 %ID/cm(3), respectively (P > 0.05). The accumulation of anti-(18)F-FACBC in the urinary bladder at 60 min after injection was 3.09 +/- 1.43 %ID/cm(3), whereas that of (18)F-FDG was 69.31 +/- 16.55 %ID/cm(3) (P < 0.05). Consequently, small-animal imaging with anti-(18)F-FACBC facilitated the visualization of the PCa tissue of the OPCT rats with higher contrast than (18)F-FDG. Furthermore, in comparison with (18)F-FDG, apparently higher ratios of PCa to inflammation and PCa to BPH accumulation of anti-(18)F-FACBC were demonstrated in the animal models. CONCLUSION: FACBC PET is believed to be useful not only for the visualization of human PCa but also for differentiating between PCa and inflammation and between PCa and BHP.     

18F-Fluorodeoxyglucose positron emission tomography pulmonary imaging in idiopathic pulmonary fibrosis is reproducible: implications for future clinical trials

Purpose

Noninvasive markers of disease activity in patients with idiopathic pulmonary fibrosis (IPF) are lacking. We performed this study to investigate the reproducibility of pulmonary ¹⁸F-FDG PET/CT in patients with IPF.

Methods

The study group comprised 13 patients (11 men, 2 women; mean age 71.1?±?9.9?years) with IPF recruited for two thoracic ¹⁸F-FDG PET/CT studies performed within 2?weeks of each other. All patients were diagnosed with IPF in consensus at multidisciplinary meetings as a result of typical clinical, high-resolution CT and pulmonary function test features. Three methods for evaluating pulmonary ¹⁸F-FDG uptake were used. The maximal ¹⁸F-FDG pulmonary uptake (SUVmax) in the lungs was determined using manual region-of-interest placement. An ¹⁸F-FDG uptake intensity histogram was automatically constructed from segmented lungs to evaluate the distribution of SUVs. Finally, mean SUV was determined for volumes-of-interest in pulmonary regions with interstitial lung changes identified on CT scans. Processing included correction for tissue fraction effects. Bland-Altman analysis was performed and interclass correlation coefficients (ICC) were determined to assess the reproducibility between the first and second PET scans, as well as the level of intraobserver and interobserver agreement.

Results

The mean time between the two scans was 6.3?±?4.3?days. The interscan ICCs for pulmonary SUVmax analysis and mean SUV corrected for tissue fraction effects were 0.90 and 0.91, respectively. Intensity histograms were different in only 1 of the 13 paired studies. Intraobserver agreement was also excellent (0.80 and 0.85, respectively). Some bias was observed between observers, suggesting that serial studies would benefit from analysis by the same observer.

Conclusion

This study demonstrated that there is excellent short-term reproducibility in pulmonary ¹⁸F-FDG uptake in patients with IPF.     

Tumor blood flow measured by PET dynamic imaging of first-pass 18F-FDG uptake: a comparison with 15O-labeled water-measured blood flow.

PET molecular imaging of 15O-labeled water is the gold standard for measuring blood flow in humans. However, this requires an on-site cyclotron to produce the short-lived 15O tracer, which is cost-prohibitive for most clinical PET centers. The purpose of this study was to determine if the early uptake of 18F-FDG could be used to measure regional blood flow in tumors in the absence of 15O-water. METHODS: PET scans were obtained in patients being evaluated for tumor perfusion and glucose metabolism in a phase I dose-escalating protocol for endostatin, a novel antiangiogenic agent. A 2-min perfusion scan was performed with a bolus injection of 2,220 MBq (60 mCi) of 15O-water, which was followed by a 370-MBq (10 mCi) dose of 18F-FDG. Four sequential scans of 18F-FDG uptake were acquired, consisting of an early 2-min uptake scan-or first-pass scan-and 3 sequential 15-min late 18F-FDG uptake scans. Regions of interest (ROIs) were drawn on 2 or more tumor sites and on back muscle, as a control ROI, for each patient. Arterial blood concentration was derived from the PET scans by drawing an ROI over a large artery in the field of view. Blood flow was computed with a simple 1-compartment blood flow model using the first 2 min of data after injection. RESULTS: Blood flow estimated from the early uptake of 18F-FDG was linearly correlated with 15O-measured blood flow, with an intercept of 0.01, a slope of 0.86, and an R2 regression coefficient of 0.74 (r = 0.86). The 18F-FDG tumor extraction fraction relative to 15O-water averaged 0.86. A preliminary case study of a patient with prostate cancer confirms the utility of the first-pass 18F-FDG blood flow analysis in tumor diagnosis. CONCLUSION: These results suggest that the first-pass uptake of 18F-FDG may provide an estimate of perfusion in a tumor within the limitations of incomplete extraction of 18F-FDG compared with 15O-water.     

Comparison of F-FET and F-FDG PET in brain tumors

The purpose of this study was to compare the diagnostic value of positron emission tomography (PET) using [¹⁸F]-fluorodeoxyglucose (¹⁸F-FDG) and O-(2-[¹⁸F]fluoroethyl)-l-tyrosine (¹⁸F-FET) in patients with brain lesions suspicious of cerebral gliomas.

Methods

Fifty-two patients with suspicion of cerebral glioma were included in this study. From 30 to 50 min after injection of 180 MBq ¹⁸F-FET, a first PET scan (¹⁸F-FET scan) was performed. Thereafter, 240 MBq ¹⁸F-FDG was injected and a second PET scan was acquired from 30 to 60 min after the second injection (¹⁸F-FET/¹⁸F-FDG scan). The cerebral accumulation of ¹⁸F-FDG was calculated by decay corrected subtraction of the ¹⁸F-FET scan from the ¹⁸F-FET/¹⁸F-FDG scan. Tracer uptake was evaluated by visual scoring and by lesion-to-background (L/B) ratios. The imaging results were compared with the histological results and prognosis.

Results

Histology revealed 24 low-grade gliomas (LGG) of World Health Organization (WHO) Grade II and 19 high-grade gliomas (HGG) of WHO Grade III or IV, as well as nine others, mainly benign histologies. The gliomas showed increased ¹⁸F-FET uptake (>normal brain) in 86% and increased ¹⁸F-FDG uptake (>white matter) in 35%. ¹⁸F-FET PET provided diagnostically useful delineation of tumor extent while this was impractical with ¹⁸F-FDG due to high tracer uptake in the gray matter. A local maximum in the tumor area for biopsy guidance could be identified with ¹⁸F-FET in 76% and with ¹⁸F-FDG in 28%. The L/B ratios showed significant differences between LGG and HGG for both tracers but considerable overlap so that reliable preoperative grading was not possible. A significant correlation of tracer uptake with overall survival was found with ¹⁸F-FDG only. In some benign lesions like abscesses, increased uptake was observed for both tracers indicating a limited specificity of both techniques.

Conclusions

¹⁸F-FET PET is superior to ¹⁸F-FDG for biopsy guidance and treatment planning of cerebral gliomas. The uptake of ¹⁸F-FDG is associated with prognosis, but the predictive value is limited and a histological evaluation of tumor tissue remains necessary. Therefore, amino acids like ¹⁸F-FET are the preferred PET tracers for the clinical management of cerebral gliomas.     

Effects of pegfilgrastim on normal biodistribution of 18F-FDG: preclinical and clinical studies.

The purpose of this study was to evaluate the effects of pegfilgrastim, a long-acting granulocyte colony-stimulating factor, on the normal biodistribution of (18)F-FDG in an animal model and in humans. METHODS: Two groups of 12 rats received a single subcutaneous injection of either normal saline or pegfilgrastim. One, 7, 14, and 21 d after injection, biodistribution studies were performed 1 h after (18)F-FDG injection. Sixteen breast cancer patients underwent baseline (18)F-FDG PET/CT and, approximately 1 wk after receiving 1 dose of docetaxel and adjunctive pegfilgrastim, follow-up (18)F-FDG PET/CT (scan 2). Standardized uptake values corrected for lean body mass (SUL) were determined for several normal organs before and after therapy. RESULTS: In rats, bone marrow (18)F-FDG uptake (standardized uptake value) was higher in the pegfilgrastim group 1 d after injection (mean +/- SD, 8.3 +/- 4.1 vs. 2.5 +/- 0.2, P < 0.05), whereas (18)F-FDG uptake in blood was lower (0.41 +/- 0.06 vs. 0.49 +/- 0.01, P < 0.05). In patients, mean SUL was higher in bone marrow (4.49 +/- 1.50 vs. 1.33 +/- 0.22, P < 0.0001), spleen (3.29 +/- 0.83 vs. 1.23 +/- 0.23, P < 0.0001), and liver (1.45 +/- 0.25 vs. 1.31 +/- 0.23, P = 0.01) but lower in brain (4.18 +/- 0.76 vs. 5.14 +/- 1.44, P < 0.01) on scan 2 than on the baseline scan. CONCLUSION: In both the animal model and humans, pegfilgrastim markedly increased bone marrow uptake of (18)F-FDG and reduced (18)F-FDG uptake in some normal tissues. These profound alterations in (18)F-FDG biodistribution induced by pegfilgrastim must be considered when one is evaluating quantitative (18)F-FDG PET scans for tumor response to therapy.     

Multimodality imaging of tumor xenografts and metastases in mice with combined small-animal PET, small-animal CT, and bioluminescence imaging.

Recent developments have established molecular imaging of mouse models with small-animal PET and bioluminescence imaging (BLI) as an important tool in cancer research. One of the disadvantages of these imaging modalities is the lack of anatomic information. We combined small-animal PET and BLI technology with small-animal CT to obtain fusion images with both molecular and anatomic information. METHODS: We used small-animal PET/CT and BLI to detect xenografts of different cell lines and metastases of a melanoma cell line (A375M-3F) that had been transduced with a lentiviral vector containing a trimodality imaging reporter gene encoding a fusion protein with Renilla luciferase, monomeric red fluorescent protein, and a mutant herpes simplex virus type 1 thymidine kinase. RESULTS: Validation studies in mouse xenograft models showed a good coregistration of images from both PET and CT. Melanoma metastases were detected by 18F-FDG PET, 9-[4-(18)F-fluoro-3-(hydroxymethyl)butyl]guanine (18F-FHBG) PET, CT, and BLI and confirmed by ex vivo assays of Renilla luciferase and mutant thymidine kinase expression. 18F-FHBG PET/CT allowed detection and localization of lesions that were not seen on CT because of poor contrast resolution and were not seen on 18F-FDG PET because of higher background uptake relative to 18F-FHBG. CONCLUSION: The combination of 18F-FHBG PET, small-animal CT, and BLI allows a sensitive and improved quantification of tumor burden in mice. This technique is potentially useful for the study of the biologic determinants of metastasis and for the evaluation of novel cancer treatments.     

18F-fluoroacetate: a potential acetate analog for prostate tumor imaging--in vivo evaluation of 18F-fluoroacetate versus 11C-acetate.

PET with (11)C-acetate ((11)C-ACE) has a high sensitivity for detection of prostate cancer and several other cancers that are poorly detected with (18)F-FDG. However, the short half-life (20.4 min) of (11)C limits the general availability of (11)C-ACE. (18)F-Fluoroacetate ((18)F-FAC) is an analog of acetate with a longer radioactive half-life ((18)F = 110 min). This study was undertaken to assess the potential usefulness of (18)F-FAC as a prostate tumor imaging agent. METHODS: We developed an efficient radiosynthesis for (18)F-FAC, which has already been adapted to a commercial synthesizer. Biodistribution studies of (18)F-FAC were compared with (11)C-ACE in normal Sprague-Dawley male rats and CWR22 tumor-bearing nu/nu mice. We also performed a small-animal PET study of (18)F-FAC in CWR22 tumor-bearing nu/nu mice and a whole-body PET study in a baboon to examine defluorination. RESULTS: We obtained (18)F-FAC in a radiochemical yield of 55% +/- 5% (mean +/- SD) in approximately 35 min and with a radiochemical purity of >99%. Rat biodistribution showed extensive defluorination, which was not observed in the baboon PET, as indicated by the standardized uptake values (SUVs) (SUVs of iliac bones and femurs were 0.26 and 0.3 at 1 h and 0.22 and 0.4 at 2 h, respectively). CWR22 tumor-bearing nu/nu mice showed tumor uptake (mean +/- SD) of 0.78 +/- 0.06 %ID/g (injected dose per gram of tissue) for (11)C-ACE versus 4.01 +/- 0.32 %ID/g for (18)F-FAC. For most organs-except blood, muscle, and fat-the tumor-to-organ ratios at 30 min after injection were higher with (18)F-FAC, whereas the tumor-to-heart and tumor-to-prostate ratios were similar. CONCLUSION: All of these data indicate that (18)F-FAC may be a useful alternative to (11)C-ACE tracer for the detection of prostate tumors by PET.     

Optimizing experimental protocols for quantitative behavioral imaging with 18F-FDG in rodents.

Small-animal PET provides the opportunity to image brain activation during behavioral tasks in animal models of human conditions. The present studies aimed to simplify behavioral imaging procedures without a loss of quantitation by using an intraperitoneal route of administration (no cannulation, no anesthesia) and using a standardized uptake value (SUV) to reduce scan duration. METHODS: Sixteen animals with carotid artery cannulations were studied with 18F-FDG small-animal PET accompanied by serial arterial blood sampling. Ten of these animals were anesthetized and were inside the tomograph during 18F-FDG uptake, whereas 6 animals were awake in their home cages and scanned after 60 min of uptake. Of the 10 anesthetized animals, 6 received intraperitoneal 18F-FDG, whereas 4 received intravenous 18F-FDG, and all 6 awake animals received intraperitoneal 18F-FDG. Intravenously injected animals were positioned far enough inside the tomograph to obtain region-of-interest-based measures from the heart and the brain. In all animals, a full arterial input function and plasma glucose levels were obtained. To establish the optimal time during 18F-FDG uptake for blood sampling when using an SUV, a Patlak kinetic model was used to derive absolute rates of glucose metabolism and compared with SUVs calculated using different plasma points from the arterial input function. RESULTS: A single plasma point taken at 60 min after injection for intraperitoneal injections or 45 min after injection for intravenous injections provides a sensitive index of glucose metabolic rate with the highest correlation with data obtained from a fully quantitative input function. CONCLUSION: These studies support an experimental protocol in which animals can receive the 18F-FDG tracer injection intraperitoneally, away from the small-animal tomograph and with minimal impact on behavior. Further, animals can occupy the tomograph bed for a 10- to 30-min scan with a consequent increase in animal throughput.     

Increased metabolic activity of indolent liver metastases after resection of a primary colorectal tumor.

In murine models, resection of a primary tumor leads to increased vascularization and accelerated growth of metastases that previously had remained microscopic. To study such a potentially inhibitory effect of primary tumors on the outgrowth of distant metastases in humans, we assessed the metabolic activity of liver metastases by 18F-FDG PET before and after resection of primary colorectal tumors. METHODS: Group A consisted of 8 patients with synchronous colorectal liver metastases who were scheduled for resection of their primary tumor. These patients underwent an (18)F-FDG PET scan shortly before resection and 2-3 wk after resection of the primary tumor. The patients in a control group (group B, n = 9) underwent an 18F-FDG PET scan at the time of diagnosis of the liver metastases and a second scan several weeks later, before initiating treatment. There was no surgical intervention between the two 18F-FDG PET scans in this group. RESULTS: In group A, the maximum and mean standardized uptake values of the liver metastases clearly increased after resection of the primary tumor, by 38% +/- 55% and 42% +/- 52%, respectively, as compared with the first 18F-FDG PET scan. In group B, the maximum and mean standardized uptake values of the second 18F-FDG PET scan were not significantly higher than those of the first 18F-FDG PET scan; -11% +/- 23% and 1% +/- 29%, respectively. The difference in standardized uptake value increase between the 2 groups was statistically significant (P < 0.05). CONCLUSION: Our data cannot differentiate between the immunologic sequels caused by the surgical trauma itself and those caused by removal of the primary tumor. The observation itself, however, of increased metabolic activity after surgical resection of the primary tumor may have direct clinical applications and suggests the administration of antiangiogenic therapy after surgery of the primary tumor.     

PET imaging of colorectal cancer in xenograft-bearing mice by use of an 18F-labeled T84.66 anti-carcinoembryonic antigen diabody.

In this study, we investigated the 18F-labeled anti-carcinoembryonic antigen (CEA) T84.66 diabody, a genetically engineered noncovalent dimer of single-chain variable fragments, for small-animal PET imaging of CEA expression in xenograft-bearing mice. METHODS: 18F labeling of the anti-CEA T84.66 diabody (molecular mass, 55 kDa) was achieved with N-succinimidyl-4-18F-fluorobenzoate (18F-SFB). The biodistribution of the 18F-fluorobenzyl-T84.66 diabody (18F-FB-T84.66 diabody) was evaluated in athymic nude mice bearing subcutaneous LS 174T human colon carcinoma and C6 rat glioma tumors. Serial small-animal PET imaging studies were performed to further evaluate in vivo targeting efficacy and pharmacokinetics. RESULTS: Radiolabeling required 35 +/- 5 (mean +/- SD) min starting from 18F-SFB, and the tracer 18F-FB-T84.66 diabody was synthesized with a specific activity of 1.83 +/- 1.71 TBq/mmol. The decay-corrected radiochemical yield was 1.40% +/- 0.16% (n = 4), and the radiochemical purity was greater than 98%. The radioimmunoreactivity was 57.1% +/- 2.0%. The 18F-FB-T84.66 diabody showed rapid and high tumor uptake and fast clearance from the circulation in the LS 174T xenograft model, as evidenced by both small-animal PET imaging and biodistribution studies. High-contrast small-animal PET images were obtained as early as 1 h after injection of the 18F-FB-T84.66 diabody, and only a background level of activity accumulation was found in CEA-negative C6 tumors. The tracer exhibited predominantly renal clearance, with some activity in the liver and spleen at early time points. CONCLUSION: The 18F-labeled diabody represents a new class of tumor-specific probes for PET that are based on targeting cell surface antigen expression. The 18F-FB-T84.66 diabody can be used for high-contrast small-animal PET imaging of CEA-positive tumor xenografts. It may be translated to the clinic for PET of CEA-positive malignancies.     

Estimation of the 18F-FDG input function in mice by use of dynamic small-animal PET and minimal blood sample data.

Derivation of the plasma time-activity curve in murine small-animal PET studies is a challenging task when tracers that are sequestered by the myocardium are used, because plasma time-activity curve estimation usually involves drawing a region of interest within the area of the reconstructed image that corresponds to the left ventricle (LV) of the heart. The small size of the LV relative to the resolution of the small-animal PET system, coupled with spillover effects from adjacent myocardial pixels, makes this method reliable only for the earliest frames of the scan. We sought to develop a method for plasma time-activity curve estimation based on a model of tracer kinetics in blood, muscle, and liver. METHODS: Sixteen C57BL/6 mice were injected with (18)F-FDG, and approximately 15 serial blood samples were taken from the femoral artery via a surgically inserted catheter during 60-min small-animal PET scans. Image data were reconstructed by use of filtered backprojection with CT-based attenuation correction. We constructed a 5-compartment model designed to predict the plasma time-activity curve of (18)F-FDG by use of data from a minimum of 2 blood samples and the dynamic small-animal PET scan. The plasma time-activity curve (TACp) was assumed to have 4 exponential components (TAC(P)=A(1)e(lambda(1)t)+A(2)e(lambda(2)t)+A(3)e(lambda(3)t)-(A(1)+A(2)+A(3))e(lambda(4)t)) based on the serial blood samples. Using Bayesian constraints, we fitted 2-compartment submodels of muscle and liver to small-animal PET data for these organs and simultaneously fitted the input (forcing) function to early small-animal PET LV data and 2 blood samples (approximately 10 min and approximately 1 h). RESULTS: The area under the estimated plasma time-activity curve had an overall Spearman correlation of 0.99 when compared with the area under the gold standard plasma time-activity curve calculated from multiple blood samples. Calculated organ uptake rates (Patlak K(i)) based on the predicted plasma time-activity curve had a correlation of approximately 0.99 for liver, muscle, myocardium, and brain when compared with those based on the gold standard plasma time-activity curve. The model was also able to accurately predict the plasma time-activity curve under experimental conditions that resulted in different rates of clearance of the tracer from blood. CONCLUSION: We have developed a robust method for accurately estimating the plasma time-activity curve of (18)F-FDG by use of dynamic small-animal PET data and 2 blood samples.     

Evaluation of 3'-deoxy-3'-18F-fluorothymidine for monitoring tumor response to radiotherapy and photodynamic therapy in mice.

3'-Deoxy-3'-18F-fluorothymidine (18F-FLT) has been suggested as a new PET tracer for imaging tumor proliferation. We investigated the use of 18F-FLT to monitor the response of tumors to radiotherapy and photodynamic therapy (PDT) in mice. METHODS: C3H/He mice bearing an SCCVII tumor were treated with single-dose x-ray irradiation of 20 Gy. Tumor uptake was examined for 18F-FLT, 3H-thymidine (3H-Thd), 18F-FDG, and 14C-deoxyglucose (14C-DG) at 6 h, 12 h, 24 h, 3 d, and 7 d after radiotherapy. BALB/c nu/nu mice bearing a HeLa tumor were treated with PDT. Tumor uptake was examined for the 4 tracers at 24 h after PDT. Expression of proliferating cell nuclear antigen (PCNA) was determined in untreated and treated tumors. RESULTS: In the biodistribution study, considerable uptake of 18F-FLT was observed in both tumor types. Tumor volumes decreased to 39.3% +/- 22.4% at 7 d after radiotherapy. The PCNA labeling index was reduced in x-ray-irradiated tumors (control, 53.2% +/- 8.7%; 6 h, 38.5% +/- 5.3%; 24 h after radiotherapy, 36.8% +/- 5.3%). 18F-FLT uptake in tumor expressed as the percentage of the injected dose per gram of tumor (%ID/g) decreased significantly at 6 h and remained low until 3 d after radiotherapy (control, 9.7 +/- 1.2 %ID/g; 6 h, 5.9 +/- 0.4 %ID/g; 24 h, 6.1 +/- 1.3 %ID/g; 3 d after radiotherapy, 6.4 +/- 1.1 %ID/g). 18F-FDG uptake tended to gradually decrease but a significant decrease was found only at 3 d (control, 12.1 +/- 2.7 %ID/g; 6 h, 13.3 +/- 2.3 %ID/g; 24 h, 8.6 +/- 1.8 %ID/g; 3 d after radiotherapy, 6.9 +/- 1.2 %ID/g). PDT resulted in a reduction of the PCNA labeling index (control, 82.0% +/- 8.6%; 24 h after PDT, 13.5% +/- 12.7%). Tumor uptake of 18F-FLT decreased (control, 11.1 +/- 1.3 %ID/g; 24 h after PDT, 4.0 +/- 2.2 %ID/g), whereas 18F-FDG uptake did not decrease significantly after PDT (control, 3.5 +/- 0.6 %ID/g; 24 h after PDT, 2.3 +/- 1.1 %ID/g). Changes in the uptake of 18F-FLT and 18F-FDG were similar to those of 3H-Thd and 14C-DG, respectively. CONCLUSION: In our model system, changes in 18F-FLT uptake after radiotherapy and PDT were correlated with those of 3H-Thd and the PCNA labeling index. The decrease in 18F-FLT uptake after treatments was more rapid or pronounced than that of 18F-FDG. Therefore, 18F-FLT may be a feasible PET tracer for monitoring response to therapy in oncology.     

Whole-body tumor imaging using PET and 2-18F-fluoro-L-tyrosine: preliminary evaluation and comparison with 18F-FDG.

18F-FDG PET imaging is now established as a valuable tool for evaluating cancer patients. However, a limitation of (18)F-FDG is its absence of specificity for tumor. Both protein synthesis and amino acid transport are enhanced in most tumor cells, but their metabolism is less affected in inflammation. We therefore decided to evaluate the ability of PET with 2-(18)F-fluoro-L-tyrosine ((18)F-TYR) to visualize cancer lesions in patients compared with (18)F-FDG PET. METHODS: (18)F-FDG PET and (18)F-TYR PET were performed on 23 patients with histologically proven malignancies (11 non-small cell lung cancers (NSCLCs), 10 lymphomas, and 2 head and neck carcinomas). Fully corrected, whole-body PET studies were obtained on separate days. (18)F-FDG studies were performed after routine clinical fashion. (18)F-TYR studies were started 36 +/- 6 min after tracer injection and a second scan centered over a reference lesion was acquired after completion of the whole-body survey-on average, 87 min after injection. Standardized uptake values (SUVs) were calculated for all abnormal foci and for various normal structures. Results were compared with pathologic or correlative studies. RESULTS: (18)F-FDG PET correctly identified 54 malignant lesions, among which 36 were also visualized with (18)F-TYR (67%). (18)F-TYR did not detect any additional lesion. Tumor SUVs (SUV(bw), 5.2 vs. 2.5), tumor-to-muscle (7.4 vs. 2.7), and tumor-to-mediastinum activity ratios (3 vs. 1.4) were higher with (18)F-FDG than with (18)F-TYR. Two of 11 NSCLCs and 4 of 10 lymphomas were understaged with (18)F-TYR compared with (18)F-FDG. Although the NSCLC lesions missed by (18)F-TYR PET were small, several large lymphoma lesions did not accumulate the tracer. In 4 patients, (18)F-TYR-positive lesions coexisted with (18)F-TYR-negative lesions. There was a high physiologic (18)F-TYR uptake by the pancreas (average SUV(bw), 10.3) and the liver (average SUV(bw), 6.3). Muscle and bone marrow uptakes were also higher with (18)F-TYR than with (18)F-FDG: average SUV(bw), 1 versus 0.7 and 2.6 versus 1.8, respectively. There was no change over time in the (18)F-TYR uptake by the tumors or the normal structures. CONCLUSION: (18)F-TYR PET is not superior to (18)F-FDG PET for staging patients with NSCLC and lymphomas.     

Prevalence and patterns of physiologic muscle uptake detected with whole-body 18F-FDG PET

Recently, the use of 18F-FDG PET has progressed rapidly as a standard diagnostic imaging tool in many types of cancer. The purpose of this study was to evaluate the patterns and prevalence of muscle uptake as a result of muscle activity shortly before the 18F-FDG injection or during the uptake phase. METHODS: From October 2002 to October 2003, whole-body 18F-FDG PET scans (4-min emission and 3-min transmission per bed position) were performed on 1,164 patients with known or suspected malignancy. Images were acquired on a dedicated PET scanner 45-60 min after an intravenous injection of a weight-adjusted dose of 7.4 MBq/kg (0.2 mCi/kg) with a maximum of 925 MBq (25 mCi) 18F-FDG. A log of any nonphysiologic muscle activity during the uptake phase or reported excessive muscle activity the day before scanning was kept by the technologists. In addition, PET scans were reviewed retrospectively to evaluate any undesirably increased muscle uptake. RESULTS: A total of 146 of 1,164 patients (12.5%) had excessively increased muscle uptake detected on the PET scan that corresponded to the technologists' notes of muscle activity during the uptake phase or before 18F-FDG injection. Encountered patterns of muscle uptake due to muscle activity included uptake in neck, secondary to neck strain from being on a stretcher; masseter, secondary to chewing gum; vocal cords, secondary to speaking; chest wall, secondary to labored breathing; forearms and hands, secondary to reading; and lower extremities, secondary to nervous tapping of the feet. CONCLUSION: Undesirably increased physiologic muscle uptake is frequently encountered on 18F-FDG PET scans. In this study, 12.5% of patients were affected. It is prudent to instruct the patient to avoid any excessive physical activity at least 48 h before injection as well as to not exert muscle activity during the uptake phase. Furthermore, a record should be kept by the technologist of any observed excessive muscle activity during the uptake phase and reported to the reading physician-thus, eliminating a potential source of false-positive findings on interpreting PET scans.