Preclinical Evaluation of a Novel c-Met Inhibitor in a Gastric Cancer Xenograft Model Using Small Animal PET

Purpose

Here, we describe the efficacy of the novel small molecule c-Met inhibitor BAY 853474 in reducing tumor growth in the Hs746T gastric cancer xenograft model and tested the suitability of 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG) versus 3′-deoxy-3′-18F-fluorothymidine ([¹⁸F]FLT) for response monitoring in a gastric cancer xenograft mouse model using small animal PET.

Procedures

The c-Met inhibitor or vehicle control was administered orally at various doses in tumor-bearing mice. Glucose uptake and proliferation was measured using PET before, 48 and 96 h after the first treatment. The PET data were compared to data from tumor growth curves, autoradiography, Glut-1 and Ki-67 staining of tumor sections, and biochemical analysis of tissue probes, i.e., c-Met and ERK phosphorylation and cyclin D1 levels.

Results

BAY 853474 significantly reduces tumor growth. [¹⁸F]FDG uptake in Hs746T tumors was significantly reduced in the groups receiving the drug, compared with the control group. The [¹⁸F]FLT uptake in the tumor tissue was completely absent 96 h after treatment. Autoradiographic, immunohistochemical, and biochemical analyses confirmed the PET findings. Treatment with the c-Met inhibitor did not affect body weight or glucose levels, and no adverse effects were observed in the animals.

Conclusion

These preclinical findings suggest that clinical PET imaging is a useful tool for early response monitoring in clinical studies.     

Positron Emission Tomography with [18F]-3′-Deoxy-3′fluorothymidine (FLT) as a Predictor of Outcome in Patients with Locally Advanced Resectable Rectal Cancer: a Pilot Study

Purpose

This pilot study was performed to evaluate whether tumor uptake of ¹⁸F-labeled 3′-deoxy-3′fluorothymidine (FLT), a proliferative radiotracer, at baseline and early during therapy, is predictive of outcome in locally advanced rectal cancer.

Procedures

Fourteen patients underwent positron emission tomography (PET) with 2-deoxy-2-[¹⁸F]fluoro-d-glucose (FDG) and FLT before therapy and PET with FLT approximately 2 weeks after initiating neoadjuvant chemoradiotherapy. FLT and FDG uptake were evaluated qualitatively and by maximum standardized uptake value (SUV_max). Tumor FLT and FDG uptake were correlated with disease-free survival (DFS).

Results

Thirteen patients underwent surgery after therapy, one died before surgery with progressive disease. FDG-PET/computed tomography detected regional lymph node metastases in five and FLT-PET was positive in one. High pretherapy FDG uptake (SUV_max?≥?14.3), low during-therapy FLT uptake (SUV_max?<?2.2), and high percentage change in FLT uptake (≥60 %) were predictive of improved DFS (p?<?0.05 for all three values).

Conclusion

Pretherapy FDG uptake, during-therapy FLT uptake, and percentage change in FLT uptake were equally predictive of DFS.     

Examining Changes in [18 F]FDG and [18 F]FLT Uptake in U87-MG Glioma Xenografts as Early Response Biomarkers to Treatment with the Dual mTOR1/2 Inhibitor AZD8055

Purpose

The mTOR kinase inhibitor AZD8055 inhibits both mTORC1 and mTORC2 leading to disruption of glucose metabolism and proliferation pathways. This study assessed the impact of single and multiple doses of AZD8055 on the uptake of the glucose metabolism marker 2-deoxy-2-[¹⁸?F]fluoro-d-glucose ([¹⁸?F]FDG) and the proliferation marker 3′-deoxy-3′-[¹⁸?F]fluorothymidine ([¹⁸?F]FLT) in U87-MG glioma xenografts.

Procedures

Mice bearing U87-MG tumours received either vehicle or AZD8055 (20 mg/kg) once daily p.o. Mice were imaged with either [¹⁸?F]FDG or [¹⁸?F]FLT PET to assess treatment response. Comparisons were made between in vivo imaging and ex vivo histopathology data.

Results

Tumour uptake of [¹⁸?F]FDG was reduced by 33 % 1 h after a single dose of AZD8055 and by 49 % following 4 days of dosing. These changes coincided with suppression of the mTOR pathway biomarkers pS6 and pAKT. In contrast, the effect of AZD8055 on [¹⁸?F]FLT uptake was inconsistent.

Conclusions

The very rapid change in [¹⁸?F]FDG uptake following acute AZD8055 treatment suggests that this could be used as an early mechanistic biomarker of metabolic changes resulting from mTOR inhibition. The utility of [¹⁸?F]FLT for measuring the anti-proliferative effect of AZD8055 remains unclear.     

Evaluation of 2-Deoxy-2-[18F]Fluoro-D-glucose- and 3′-Deoxy-3′-[18F]Fluorothymidine–Positron Emission Tomography as Biomarkers of Therapy Response in Platinum-Resistant Ovarian Cancer

Purpose

We evaluated whether 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) and 3??-deoxy-3??-[¹⁸F]fluorothymidine ([¹⁸F]FLT) positron emission tomography (PET) could be used as imaging biomarkers of platinum resensitization in ovarian cancer.

Procedures

Paired platinum-sensitive and platinum-resistant ovarian cancer cells from the same patient, PEO1 and PEO4, grown as tumor xenografts in nude mice, were assessed by PET.

Results

The AKT inhibitor, API-2, resensitized platinum-resistant PEO4 tumors to cisplatin, leading to a markedly lower Ki67 labeling index (p????0.006, n?=?6 per group). [¹⁸F]FDG-PET and [¹⁸F]FLT-PET imaging variables were lower after combination treatment compared with vehicle treatment (p????0.006, n?=?6 per group). No changes were seen with either drug alone. PRAS40 phosphorylation status was a sensitive biochemical marker of pathway inhibition, whereas reductions thymidine kinase 1 expression defined the [¹⁸F]FLT response.

Conclusions

Therapeutic inhibition of AKT activation in acquired platinum-resistant disease can be imaged noninvasively by [¹⁸F]FDG-PET and [¹⁸F]FLT-PET warranting further assessment.     

Pharmacodynamic Evaluation of Irinotecan Therapy by FDG and FLT PET/CT Imaging in a Colorectal Cancer Xenograft Model

Purpose

Longitudinal changes of 3??-[¹⁸?F]fluoro-3??-deoxythymidine (FLT) and 2-deoxy-2-[¹⁸?F]fluoro-d-glucose (FDG) in response to irinotecan therapy in an animal model of colorectal cancer were compared.

Procedures

SCID/CB-17 mice with HCT116 tumors were treated with 50?mg/kg irinotecan by intraperitoneal injection weekly for 3?weeks. FLT and FDG-positron emission tomography (PET) were performed at baseline, the day after each treatment, and 5?days after the first treatment. Proliferation and apoptosis were evaluated by immunohistochemistry (IHC) after day 15 of imaging.

Results

Irinotecan treatment resulted in a suppression of tumor growth. Tumor FLT uptake was decreased the day after each treatment but to a lesser extent 5?days after the first treatment. FDG uptake increased the day after each treatment with a continuous increase throughout the experiment. IHC analysis of phospho-H3 and Ki67 confirmed FLT-PET results, indicating a decrease in proliferation the day after the final irinotecan treatment. Increased apoptosis monitored by caspase-3 was observed after day 15 with irinotecan treatment.

Conclusions

FLT-PET may be a better method than FDG-PET for assessing treatment response to irinotecan. Changes in imaging occur before changes in tumor volume.     

3′-Deoxy-3′-[<Superscript>18</Superscript>F]Fluorothymidine Uptake Is Related to Thymidine Phosphorylase Expression in Various Experimental Tumor Models

Purpose

We recently reported that high thymidine phosphorylase (TP) expression is accompanied by low tumor thymidine concentration and high 3′-deoxy-3′-[¹⁸F]fluorothymidine ([¹⁸F]FLT) uptake in four untreated lung cancer xenografts. Here, we investigated whether this relationship also holds true for a broader range of tumor models.

Procedures

Lysates from n = 15 different tumor models originating from n = 6 institutions were tested for TP and thymidylate synthase (TS) expression using western blots. Results were correlated to [¹⁸F]FLT accumulation in the tumors as determined by positron emission tomography (PET) measurements in the different institutions and to previously published thymidine concentrations.

Results

Expression of TP correlated positively with [¹⁸F]FLT SUV_max (ρ = 0.549, P < 0.05). Furthermore, tumors with high TP levels possessed lower levels of thymidine (ρ = ??0.939, P < 0.001).

Conclusions

In a broad range of tumors, [¹⁸F]FLT uptake as measured by PET is substantially influenced by TP expression and tumor thymidine concentrations. These data strengthen the role of TP as factor confounding [¹⁸F]FLT uptake.

     

Comparison of [18F]-Tracers in Various Experimental Tumor Models by PET Imaging and Identification of an Early Response Biomarker for the Novel Microtubule Stabilizer Patupilone

Purpose  The suitability of [¹⁸F]FDG, [¹⁸F]FLT, [¹⁸F]FET, and [¹⁸F]FCH as non-invasive positron emission tomography (PET) biomarkers for monitoring response to chemotherapy was analyzed in various experimental tumor models. Procedures  Tracer uptake into three syngeneic rodent tumor models and ten human xenograft models was evaluated using semiquantitative analysis of small-animal PET data. Murine RIF-1 fibrosarcomas and [¹⁸F]FLT were selected to monitor the effects of the novel cytotoxic patupilone. Results  Except [¹⁸F]FCH, all tracers provided good tumor visualization. Highest [¹⁸F]FDG uptake was identified in syngeneic tumors. Xenograft models, however, showed low [¹⁸F]FDG SUVs and were better visualized by [¹⁸F]FLT. Monitoring the effects of patupilone on [¹⁸F]FLT uptake in RIF-1 tumors revealed a significant decrease of tracer uptake after 24 h, which strongly negatively correlated with apoptosis. Conclusion  [¹⁸F]FLT PET of experimental tumors is a viable complement to [¹⁸F]FDG for preclinical drug development. [¹⁸F]FLT may be an excellent biomarker for patupilone-induced apoptosis. T. Ebenhan and M. Honer contributed equally to this work. An erratum to this article can be found at     

Multimodal <Emphasis Type="Italic">In Vivo</Emphasis> Imaging of Tumorigenesis and Response to Chemotherapy in a Transgenic Mouse Model of Mammary Cancer

Purpose

Transgenic mice expressing the polyoma middle T oncoprotein (PyMT) in the mammary epithelium were explored by multimodal imaging to monitor longitudinally spontaneous tumor growth and response to chemotherapy.

Procedures

Positron emission tomography (PET) with 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG) and 3'-deoxy-3'-[¹⁸F]fluorothymidine ([¹⁸F]FLT), single photon emission tomography (SPECT) with [^99mTc]TcO₄ ([^99mTc]TEC), X-ray computed tomography, and fluorescent confocal endomicroscopy (FCE) images were acquired during tumor progression in female PyMT mice. Imaging with [¹⁸F]FDG and [^99mTc]TEC was also performed in untreated, doxorubicin-treated, and docetaxel-treated PyMT mice. Total tumor volumes were quantified. Tumors were collected and macroscopic and histological examinations were performed.

Results

All PyMT mice developed multifocal tumors of the mammary epithelium that became palpable at 8 weeks of age (W8). Computed tomography (CT) detected tumors at W14, while a clear tumoral uptake of [^99mTc]TEC and [¹⁸F]FDG was present as early as W6 and W8, respectively. No contrast between mammary tumors and surrounding tissue was observed at any stage with [¹⁸F]FLT. FCE detected an angiogenic switch at W10. Lung metastases were not clearly evidenced by imaging. Doxorubicin and docetaxel treatments delayed tumor growth, as shown by [¹⁸F]FDG and [^99mTc]TEC, but tumor growth resumed upon treatment discontinuation. Tumor growth fitted an exponential model with time constant rates of 0.315, 0.145, and 0.212 week^?1 in untreated, doxorubicin, and docetaxel groups, respectively.

Conclusions

Molecular imaging of mammary tumors in PyMT is precocious, precise, and predictive. [¹⁸F]FDG-PET and [^99mTc]TEC SPECT monitor tumor response to chemotherapy.

     

2-[18F]Fludarabine, a Novel Positron Emission Tomography (PET) Tracer for Imaging Lymphoma: a Micro-PET Study in Murine Models

Purpose

Fludarabine has proven to be of considerable efficacy in the treatment of low-grade lymphomas. We have developed the labeling of this drug with fluorine-18 and evaluated 2-[¹⁸F]fludarabine as a novel positron emission tomography (PET) probe for in vivo imaging.

Procedures

Preclinical studies were conducted with 2-[¹⁸F]fludarabine, in parallel with 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG), in Swiss CD-1 and CB17 severely combined immunodeficient (SCID) mice, both as tumor-free control groups, and SCID mice bearing RL lymphomas.

Results

In Swiss mice, micro-PET studies with 2-[¹⁸F]fludarabine showed a distribution restricted to the organs of excretion and the spleen, the latter being less evident in SCID animals. In lymphoma-bearing SCID mice, 2-[¹⁸F]fludarabine demonstrated a rapid tumor uptake over the first 20 min which subsequently plateaued and provided an improved contrast than that of [¹⁸F]FDG.

Conclusion

This radiotracer merits further evaluation to establish its clinical usefulness to image low-grade lymphoma in humans in future clinical investigations.     

Pancreatic Beta Cell Mass PET Imaging and Quantification with [11C]DTBZ and [18F]FP-(+)-DTBZ in Rodent Models of Diabetes

Purpose

The aim of this study is to compare the utility of two positron emission tomography (PET) imaging ligands ((+)-[¹¹C]dihydrotetrabenazine ([¹¹C]DTBZ) and the fluoropropyl analog ([¹⁸F]FP-(+)-DTBZ)) that target islet ??-cell vesicular monoamine transporter type II to measure pancreatic ??-cell mass (BCM).

Procedures

[¹¹C]DTBZ or [¹⁸F]FP-(+)-DTBZ was injected, and serial PET images were acquired in rat models of diabetes (streptozotocin-treated and Zucker diabetic fatty) and ??-cell compensation (Zucker fatty). Radiotracer standardized uptake values (SUV) were correlated to pancreas insulin content measured biochemically and histomorphometrically.

Results

On a group level, a positive correlation of [¹¹C]DTBZ pancreatic SUV with pancreas insulin content and BCM was observed. In the STZ diabetic model, both [¹⁸F]FP-(+)-DTBZ and [¹¹C]DTBZ correlated positively with BCM, although only ??25% of uptake could be attributed to ??-cell uptake. [¹⁸F]FP-(+)-DTBZ displacement studies indicate that there is a substantial fraction of specific binding that is not to pancreatic islet ?? cells.

Conclusions

PET imaging with [¹⁸F]FP-(+)-DTBZ provides a noninvasive means to quantify insulin-positive BCM and may prove valuable as a diagnostic tool in assessing treatments to maintain or restore BCM.     

Validity of Simplified 3′-Deoxy-3′-[18F]Fluorothymidine Uptake Measures for Monitoring Response to Chemotherapy in Locally Advanced Breast Cancer

Purpose

Positron emission tomography using 3??-deoxy-3??-[¹⁸F]fluorothymidine ([¹⁸F]FLT) has been suggested as a means for monitoring response to chemotherapy. The aim of this study was to evaluate the validity of simplified uptake measures for assessing response to chemotherapy using [¹⁸F]FLT in locally advanced breast cancer (LABC).

Procedures

Fifteen LABC patients underwent dynamic [¹⁸F]FLT scans both prior to and after the first cycle of chemotherapy with fluorouracil, epirubicin or doxorubicin, and cyclophosphamide. The net uptake rate constant of [¹⁸F]FLT, K _i , determined by non-linear regression (NLR) of an irreversible two-tissue compartment model was used as the gold standard. In addition to Patlak graphical analysis, standardised uptake values (SUV) and tumour-to-whole blood ratio (TBR) were used for analysing [¹⁸F]FLT data. Correlations and relationships between simplified uptake measures and NLR before and after chemotherapy were assessed using regression analysis.

Results

No significant differences in both pre- and post-chemotherapy relationships between any of the simplified uptake measures and NLR were found. However, changes in SUV between baseline and post-therapy scans showed a significant negative bias and slope less than one, while TBR did not.

Conclusions

In LABC, TBR instead of SUV may be preferred for monitoring response to chemotherapy with [¹⁸F]FLT.     

Fusion of [18F]FDG PET with Fluorescence Diffuse Optical Tomography to Improve Validation of Probes and Tumor Imaging

Purpose

Given the progress of fluorescence diffuse optical tomography (fDOT) technology, here, we study the additional benefits provided by multimodal PET/fDOT imaging by comparing the biodistribution of 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG) in tumors with three fluorescent probes: a glucose analog, a protease activatable optical probe, and a ligand of αvβ3 integrin.

Procedures

Sequential fDOT/PET/computed tomography (CT) imaging of mice was performed with a custom multimodal mouse support that allows the subject to be transferred between the fDOT and the PET/CT scanners. Experiments were performed in xenografted tumor models derived from the human breast cancer line MDA-MB 231 and compared to ex vivo analysis.

Results

The three-dimensional signals showed that the fluorescent glucose analog is not colocalized with [¹⁸F]FDG, raising questions about its use as a surrogate probe of the PET tracer. Fusion of [¹⁸F]FDG with the other fluorescent probes showed evidence of high variability both for the protease activity and the αvβ3 integrin expression during tumor growth.

Conclusion

The added value of hybrid PET/fDOT over the two modalities was demonstrated for cross-validation of probes and for better characterization of tumor models.     

Preclinical Imaging Evaluation of Novel TSPO-PET Ligand 2-(5,7-Diethyl-2-(4-(2-[18F]fluoroethoxy)phenyl)pyrazolo[1,5-a]pyrimidin-3-yl)-N,N-diethylacetamide ([18F]VUIIS1008) in Glioma

Purpose

Translocator protein (TSPO) concentrations are elevated in glioma, suggesting a role for TSPO positron emission tomography (PET) imaging in this setting. In preclinical PET studies, we evaluated a novel, high-affinity TSPO PET ligand, [¹⁸F]VUIIS1008, in healthy mice and glioma-bearing rats.

Procedures

Dynamic PET data were acquired simultaneously with [¹⁸F]VUIIS1008 injection, with binding reversibility and specificity evaluated in vivo by non-radioactive ligand displacement or blocking. Compartmental analysis of PET data was performed using metabolite-corrected arterial input functions. Imaging was validated with histology and immunohistochemistry.

Results

[¹⁸F]VUIIS1008 exhibited rapid uptake in TSPO-rich organs. PET ligand uptake was displaceable with non-radioactive VUIIS1008 or PBR06 in mice. Tumor accumulation of [¹⁸F]VUIIS1008 was blocked by pretreatment with VUIIS1008 in rats. [¹⁸F]VUIIS1008 exhibited improved tumor-to-background ratio and higher binding potential in tumors compared to a structurally similar pyrazolopyrimidine TSPO ligand, [¹⁸F]DPA-714.

Conclusions

The PET ligand [¹⁸F]VUIIS1008 exhibits promising characteristics as a tracer for imaging glioma. Further translational studies appear warranted.     

Differences in Transport Mechanisms of trans-1-Amino-3-[18F]Fluorocyclobutanecarboxylic Acid in Inflammation,Prostate Cancer,and Glioma Cells: Comparison with l-[Methyl-11C]Methionine and 2-Deoxy-2-[18F]Fluoro-d-Glucose

Purpose

We aimed to elucidate trans-1-amino-3-[¹⁸F]fluorocyclobutanecarboxylic acid (anti-[¹⁸F]FACBC) uptake mechanisms in inflammatory and tumor cells, in comparison with those of l-[methyl-¹¹C]methionine ([¹¹C]Met) and 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG).

Procedures

Using carbon-14-labeled tracers, in vitro time-course, pH dependence, and competitive inhibition uptake experiments were performed in rat inflammatory (T cells, B cells, granulocytes, macrophages), prostate cancer (MLLB2), and glioma (C6) cells.

Results

Anti-[¹⁴C]FACBC uptake ratios of T/B cells to tumor cells were comparable, while those of granulocytes/macrophages to tumor cells were lower than those for [¹⁴C]FDG. Over half of anti-[¹⁴C]FACBC uptake by T/B and tumor cells was mediated by Na⁺-dependent amino acid transporters (system ASC), whereas most [¹⁴C]Met transport in all cells was mediated by Na⁺-independent carriers (system L).

Conclusions

The low anti-[¹⁸F]FACBC accumulation in granulocytes/macrophages may be advantageous in discriminating inflamed regions from tumors. The significant anti-[¹⁸F]FACBC uptake in T/B cells may cause false-positives in some cancer patients who undergo FACBC-positron emission tomography (PET).     

Comparison of [18F]-Tracers in Various Experimental Tumor Models by PET Imaging and Identification of an Early Response Biomarker for the Novel Microtubule Stabilizer Patupilone

Purpose  

The suitability of [¹⁸F]FDG, [¹⁸F]FLT, [¹⁸F]FET, and [¹⁸F]FCH as non-invasive positron emission tomography (PET) biomarkers for monitoring response to chemotherapy was analyzed in various experimental tumor models.     

Applying Amide Proton Transfer MR Imaging to Hybrid Brain PET/MR: Concordance with Gadolinium Enhancement and Added Value to [<Superscript>18</Superscript>F]FDG PET

Purpose

The purpose of this study is to evaluate the diagnostic concordance and metric correlations of amide proton transfer (APT) imaging with gadolinium-enhanced magnetic resonance imaging (MRI) and 2-deoxy-2-[¹⁸F-]fluoro-D-glucose ([¹⁸F]FDG) positron emission tomography (PET), using hybrid brain PET/MRI.

Procedures

Twenty-one subjects underwent brain gadolinium-enhanced [¹⁸F]FDG PET/MRI prospectively. Imaging accuracy was compared between unenhanced MRI, MRI with enhancement, APT-weighted (APTW) images, and PET based on six diagnostic criteria. Among tumors, the McNemar test was further used for concordance assessment between gadolinium-enhanced imaging, APT imaging, and [¹⁸F]FDG PET. As well, the relation of metrics between APT imaging and PET was analyzed by the Pearson correlation analysis.

Results

APT imaging and gadolinium-enhanced MRI showed superior and similar diagnostic accuracy. APTW signal intensity and gadolinium enhancement were concordant in 19 tumors (100 %), while high [¹⁸F]FDG avidity was shown in only 12 (63.2 %). For the metrics from APT imaging and PET, there was significant correlation for 13 hypermetabolic tumors (P < 0.05) and no correlation for the remaining six [¹⁸F]FDG-avid tumors.

Conclusions

APT imaging can be used to increase diagnostic accuracy with no need to administer gadolinium chelates. APT imaging may provide an added value to [¹⁸F]FDG PET in the evaluation of tumor metabolic activity during brain PET/MR studies.

     

Comparison of 18F-labeled CXCR4 antagonist peptides for PET imaging of CXCR4 expression

Purpose

CXCR4 is overexpressed on tumor cells from many types of human cancers. A high level of CXCR4 expression often correlates with poor prognosis, chemotherapy resistance, and metastasis. The development of CXCR4-specific radiotracers for positron emission tomography (PET) imaging will allow in vivo evaluation of receptor expression level for diagnosis or therapeutic evaluation.

Procedures

Two new ¹⁸F-labeled radiotracers based on an Ac-TC14012 peptide, [¹⁸F]FP-Ac-TC14012 and [¹⁸F]FB-Ac-TC14012, were synthesized and characterized. The affinities of the 2-fluoropropionate (FP)-conjugated or 4-fluorobenzoate (FB)-conjugated peptides to CXCR4-transfected Chinese hamster ovarian (CHO) cells were evaluated in a competitive binding assay with [¹²⁵I]CXCL12 radioligand. The cell uptake and retention of [¹⁸F]FP-labeled and [¹⁸F]FB-labeled peptides were measured. The tumor targetability and pharmacokinetics of these two tracers were also evaluated by microPET imaging and biodistribution studies.

Results

The labeled peptides retained high binding affinity to CXCR4 and showed much higher uptake in CXCR4-positive CHO cells than in CXCR4-negative cells in vitro. The smaller and more hydrophilic [¹⁸F]FP prosthetic group resulted in higher affinity and lower nonspecific cell uptake compared to the [¹⁸F]FB-labeled peptide. Both radiotracers showed much higher accumulation in CXCR4-positive than CXCR4-negative tumor xenografts in mice and allowed clear visualization of CXCR4 expression by PET. Among the two, [¹⁸F]FP-Ac-TC14012 showed higher tumor uptake and better tumor-to-background contrast. Unlike their N-terminal 4-F-benzoate analogs, these two tracers had minimal blood retention, likely due to reduced red blood cell binding. Metabolic organs, such as the liver and kidney, also showed high uptake. When blocked with low-dose cold peptide (10 μg), the tumor uptake was significantly increased, most likely due to the increased concentration in blood circulation, as evidenced by decreased liver uptake.

Conclusion

These results demonstrate that the [¹⁸F]FP-labeled Ac-TC14012 peptide with high tumor uptake, low nonspecific binding, and good tumor-to-background contrast promises [¹⁸F]FP-Ac-TC14012 as a PET tracer for in vivo PET imaging of CXCR4 expression.     

PET Imaging of Stroke-Induced Neuroinflammation in Mice Using [18F]PBR06

Purpose

The purpose of this study is to evaluate the 18 kDa translocator protein (TSPO) radioligand [¹⁸F]N-fluoroacetyl-N-(2,5-dimethoxybenzyl)-2-phenoxyaniline ([¹⁸F]PBR06) as a positron emission tomography (PET) imaging biomarker of stroke-induced neuroinflammation in a rodent model.

Procedures

Stroke was induced by transient middle cerebral artery occlusion in Balb/c mice. Dynamic PET/CT imaging with displacement and preblocking using PK111195 was performed 3 days later. PET data were correlated with immunohistochemistry (IHC) for the activated microglial markers TSPO and CD68 and with autoradiography.

Results

[¹⁸F]PBR06 accumulation peaked within the first 5 min postinjection, then decreased gradually, remaining significantly higher in infarct compared to noninfarct regions. Displacement or preblocking with PK11195 eliminated the difference in [¹⁸F]PBR06 uptake between infarct and noninfarct regions. Autoradiography and IHC correlated well spatially with uptake on PET.

Conclusions

[¹⁸F]PBR06 PET specifically images TSPO in microglial neuroinflammation in a mouse model of stroke and shows promise for imaging and monitoring microglial activation/neuroinflammation in other disease models.     

An Evaluation of 2-deoxy-2-[18F]Fluoro-D-Glucose and 3′-deoxy-3′-[18F]-Fluorothymidine Uptake in Human Tumor Xenograft Models

Purpose

The aim of this study is to assess the variability of 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]-FDG) and 3??-deoxy-3??-[¹⁸F]-fluorothymidine ([¹⁸F]-FLT) uptake in pre-clinical tumor models and examine the relationship between imaging data and related histological biomarkers.

Procedures

[¹⁸F]-FDG and [¹⁸F]-FLT studies were carried out in nine human tumor xenograft models in mice. A selection of the models underwent histological analysis for endpoints relevant to radiotracer uptake. Comparisons were made between in vitro uptake, in vivo imaging, and ex vivo histopathology data using quantitative and semi-quantitative analysis.

Results

In vitro data revealed that [1-¹⁴C]-2-deoxy-d-glucose ([¹⁴C]-2DG) uptake in the tumor cell lines was variable. In vivo, [¹⁸F]-FDG and [¹⁸F]-FLT uptake was highly variable across tumor types and uptake of one tracer was not predictive for the other. [¹⁴C]-2DG uptake in vitro did not predict for [¹⁸F]-FDG uptake in vivo. [¹⁸F]-FDG SUV was inversely proportional to Ki67 and necrosis levels and positively correlated with HKI. [¹⁸F]-FLT uptake positively correlated with Ki67 and TK1.

Conclusion

When evaluating imaging biomarkers in response to therapy, the choice of tumor model should take into account in vivo baseline radiotracer uptake, which can vary significantly between models.     

Improved Derivation of Input Function in Dynamic Mouse [18F]FDG PET Using Bladder Radioactivity Kinetics

Purpose

Accurate determination of the plasma input function (IF) is essential for absolute quantification of physiological parameters in positron emission tomography (PET). However, it requires an invasive and tedious procedure of arterial blood sampling that is challenging in mice because of the limited blood volume. In this study, a hybrid modeling approach is proposed to estimate the plasma IF of 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG) in mice using accumulated radioactivity in urinary bladder together with a single late-time blood sample measurement.

Methods

Dynamic PET scans were performed on nine isoflurane-anesthetized male C57BL/6 mice after a bolus injection of [¹⁸F]FDG at the lateral caudal vein. During a 60- or 90-min scan, serial blood samples were taken from the femoral artery. Image data were reconstructed using filtered backprojection with computed tomography-based attenuation correction. Total accumulated radioactivity in the urinary bladder at late times was fitted to a renal compartmental model with the last blood sample and a one-exponential function that described the [¹⁸F]FDG clearance in blood. Multiple late-time blood sample estimates were calculated by the blood [¹⁸F]FDG clearance equation. A sum of four-exponentials was assumed for the plasma IF that served as a forcing function to all tissues. The estimated plasma IF was obtained by simultaneously fitting the [¹⁸F]FDG model to the time–activity curves (TACs) of liver and muscle and the forcing function to early (0–1 min) left-ventricle data (corrected for delay, dispersion, partial-volume effects, and erythrocyte uptake) and the late-time blood estimates. Using only the blood sample collected at the end of the study to estimate the IF and the use of liver TAC as an alternative IF were also investigated.

Results

The area under the plasma IFs calculated for all studies using the hybrid approach was not significantly different from that using all blood samples. [¹⁸F]FDG uptake constants in brain, myocardium, skeletal muscle, and liver computed by the Patlak analysis using estimated and measured plasma IFs were in excellent agreement (slope?～?1; R ²?>?0.983). The IF estimated using only the last blood sample drawn at the end of the study and the use of liver TAC as the plasma IF provided less reliable results.

Conclusions

The estimated plasma IFs obtained with the hybrid method agreed well with those derived from arterial blood sampling. Importantly, the proposed method obviates the need of arterial catheterization, making it possible to perform repeated dynamic [¹⁸F]FDG PET studies on the same animal. Liver TAC is unsuitable as an input function for absolute quantification of [¹⁸F]FDG PET data.