PET image reconstruction using physical and mathematical modelling for time of flight PET-MR scanners in the STIR library.

This work demonstrates how computational and physical modelling of the positron emission tomography (PET) image acquisition process for a state-of-the-art integrated PET and magnetic resonance imaging (PET-MR) system can produce images comparable to the manufacturer. The GE SIGNA PET/MR scanner is manufactured by General Electric and has time-of-flight (TOF) capabilities of about 390 ps. All software development took place in the Software for Tomographic Image Reconstruction (STIR: http://stir.sf.net) library, which is a widely used open source software to reconstruct data as exported from emission tomography scanners. The new software developments will be integrated into STIR, providing the opportunity for researchers worldwide to establish and expand their image reconstruction methods. Furthermore, this work is of particular significance as it provides the first validation of TOF PET image reconstruction for real scanner datasets using the STIR library. This paper presents the methodology, analysis, and critical issues encountered in implementing an independent reconstruction software package. Acquired PET data were processed via several appropriate algorithms which are necessary to produce an accurate and precise quantitative image. This included mathematical, physical and anatomical modelling of the patient and simulation of various aspects of the acquisition. These included modelling of random coincidences using 'singles' rates per crystals, detector efficiencies and geometric effects. Attenuation effects were calculated by using the STIR's attenuation correction model. Modelling all these effects within the system matrix allowed the reconstruction of PET images which demonstrates the metabolic uptake of the administered radiopharmaceutical. These implementations were validated using measured phantom and clinical datasets. The developments are tested using the ordered subset expectation maximisation (OSEM) and the more recently proposed kernelised expectation maximisation (KEM) algorithm which incorporates anatomical information from MR images into PET reconstruction.

Iterative reconstruction of Fourier-rebinned PET data using sinogram blurring function estimated from point source scans.

PURPOSE: The accuracy of the system model that governs the transformation from the image space to the projection space in positron emission tomography (PET) greatly affects the quality of reconstructed images. For efficient computation in iterative reconstructions, the system model in PET can be factored into a product of geometric projection and sinogram blurring function. To further speed up reconstruction, fully 3D PET data can be rebinned into a stack of 2D sinograms and then be reconstructed using 2D iterative algorithms. The purpose of this work is to develop a method to estimate the sinogram blurring function to be used in reconstruction of Fourier-rebinned data. METHODS: In a previous work, the authors developed an approach to estimating the sinogram blurring function of nonrebinned PET data from experimental scans of point sources. In this study, the authors extend this method to the estimation of sinogram blurring function for Fourier-rebinned PET data. A point source was scanned at a set of sampled positions in the microPET II scanner. The sinogram blurring function is considered to be separable between the transaxial and axial directions. A radially and angularly variant 2D blurring function is estimated from Fourier-rebinned point source scans to model the transaxial blurring with consideration of the detector block structure of the scanner; a space-variant 1D blurring kernel along the axial direction is estimated separately to model the correlation between neighboring planes due to detector intrinsic blurring and Fourier rebinning. The estimated sinogram blurring function is incorporated in a 2D maximum a posteriori (MAP) reconstruction algorithm for image reconstruction. RESULTS: Physical phantom experiments were performed on the microPET II scanner to validate the proposed method. The authors compared the proposed method to 2D MAP reconstruction without sinogram blurring model and 2D MAP reconstruction with a Monte Carlo based blurring model. The results show that the proposed method produces images with improved contrast and spatial resolution. The reconstruction time is unaffected by the new method since the blurring component takes a relatively negligible part of the overall reconstruction time. CONCLUSIONS: The proposed method can estimate sinogram blurring matrix for Fourier-rebinned PET data and can be used to improve contrast and spatial resolution of reconstructed images. The method can be applied to other human and animal scanners.

Iterative image reconstruction for positron emission tomography based on a detector response function estimated from point source measurements.

The accuracy of the system model in an iterative reconstruction algorithm greatly affects the quality of reconstructed positron emission tomography (PET) images. For efficient computation in reconstruction, the system model in PET can be factored into a product of a geometric projection matrix and sinogram blurring matrix, where the former is often computed based on analytical calculation, and the latter is estimated using Monte Carlo simulations. Direct measurement of a sinogram blurring matrix is difficult in practice because of the requirement of a collimated source. In this work, we propose a method to estimate the 2D blurring kernels from uncollimated point source measurements. Since the resulting sinogram blurring matrix stems from actual measurements, it can take into account the physical effects in the photon detection process that are difficult or impossible to model in a Monte Carlo (MC) simulation, and hence provide a more accurate system model. Another advantage of the proposed method over MC simulation is that it can easily be applied to data that have undergone a transformation to reduce the data size (e.g., Fourier rebinning). Point source measurements were acquired with high count statistics in a relatively fine grid inside the microPET II scanner using a high-precision 2D motion stage. A monotonically convergent iterative algorithm has been derived to estimate the detector blurring matrix from the point source measurements. The algorithm takes advantage of the rotational symmetry of the PET scanner and explicitly models the detector block structure. The resulting sinogram blurring matrix is incorporated into a maximum a posteriori (MAP) image reconstruction algorithm. The proposed method has been validated using a 3 x 3 line phantom, an ultra-micro resolution phantom and a (22)Na point source superimposed on a warm background. The results of the proposed method show improvements in both resolution and contrast ratio when compared with the MAP reconstruction with a MC-based sinogram blurring matrix, and one without a detector response model. The reconstruction time is unaffected by the new method since the blurring component takes a relatively small part of the overall reconstruction time. The proposed method can be applied to other PET scanners for human and animal imaging.

Sinogram Blurring Matrix Estimation From Point Sources Measurements With Rank-One Approximation for Fully 3-D PET.

An accurate system matrix is essential in positron emission tomography (PET) for reconstructing high quality images. To reduce storage size and image reconstruction time, we factor the system matrix into a product of a geometry projection matrix and a sinogram blurring matrix. The geometric projection matrix is computed analytically and the sinogram blurring matrix is estimated from point source measurements. Previously, we have estimated a 2-D blurring matrix for a preclinical PET scanner. The 2-D blurring matrix only considers blurring effects within a transaxial sinogram and does not compensate for inter-sinogram blurring effects. For PET scanners with a long axial field of view, inter-sinogram blurring can be a major problem influencing the image quality in the axial direction. Hence, the estimation of a 4-D blurring matrix is desirable to further improve the image quality. The 4-D blurring matrix estimation is an ill-conditioned problem due to the large number of unknowns. Here, we propose a rank-one approximation for each blurring kernel image formed by a row vector of the sinogram blurring matrix to improve the stability of the 4-D blurring matrix estimation. The proposed method is applied to the simulated data as well as the real data obtained from an Inveon microPET scanner. The results show that the newly estimated 4-D blurring matrix can improve the image quality over those obtained with a 2-D blurring matrix and requires less point source scans to achieve similar image quality compared with an unconstrained 4-D blurring matrix estimation.

Longitudinal microPET imaging of brain tumor growth with F-18-labeled RGD peptide.

PURPOSE: EMD 121974, a potent cyclic RGD peptide inhibitor of alphav-integrins, demonstrated effectiveness in suppressing brain tumor growth in both preclinical models and phases I/II clinical trials. The ability to non-invasively evaluate alphav-integrin expression provides a novel and unique way to better understand brain tumor angiogenesis in relationship to alphav-integrin expression, and allow for direct assessment of anti-integrin treatment efficacy. PROCEDURES: We developed a F-18-labeled RGD peptide [F-18]FB-RGD and performed serial microPET imaging scans to follow brain tumor growth and angiogenesis as a function of time in an orthotopic U87MG glioblastoma xenograft model in athymic nude mice. RESULTS: The tumor was barely visible on microPET at the size of

Integrin alpha v beta 3-targeted imaging of lung cancer.

A series of radiolabeled cyclic arginine-glycine-aspartic acid (RGD) peptide ligands for cell adhesion molecule integrin alpha v beta 3-targeted tumor angiogenesis targeting are being developed in our laboratory. In this study, this effort continues by applying a positron emitter 64Cu-labeled PEGylated dimeric RGD peptide radiotracer 64Cu-DOTA-PEG-E[c(RGDyK)]2 for lung cancer imaging. The PEGylated RGD peptide indicated integrin alpha v beta 3 avidity, but the PEGylation reduced the receptor binding affinity of this ligand compared to the unmodified RGD dimer. The radiotracer revealed rapid blood clearance and predominant renal clearance route. The minimum nonspecific activity accumulation in normal lung tissue and heart rendered high-quality orthotopic lung cancer tumor images, enabling clear demarcation of both the primary tumor at the upper lobe of the left lung, as well as metastases in the mediastinum, contralateral lung, and diaphragm. As a comparison, fluorodeoxyglucose (FDG) scans on the same mice were only able to identify the primary tumor, with the metastatic lesions masked by intense cardiac uptake and high lung background. 64Cu-DOTA-PEG-E[c(RGDyK)]2 is an excellent position emission tomography (PET) tracer for integrin-positive tumor imaging. Further studies to improve the receptor binding affinity of the tracer and subsequently to increase the magnitude of tumor uptake without comprising the favorable in vivo kinetics are currently in progress.

Synthesis and evaluation of 2'-deoxy-2'-18F-fluoro-5-fluoro-1-beta-D-arabinofuranosyluracil as a potential PET imaging agent for suicide gene expression.

UNLABELLED: 2'-deoxy-2'-(18)F-fluoro-5-fluoro-1-beta-D-arabinofuranosyluracil ((18)F-FFAU) has been synthesized and evaluated in HT-29 cells as a potential PET agent for herpes simplex virus type 1 thymidine kinase (HSV1-tk) gene expression. METHODS: 2-Deoxy-2-(18)F-fluoro-1,3,5-tri-O-benzoyl-alpha-D-arabinofuranose was prepared by the reaction of the respective 2-triflate with tetrabutylammonium (18)F-fluoride. The fluorosugar was converted to its 1-bromo derivative and coupled with protected 5-fluorouracil. The crude product was hydrolyzed in base and purified by high-performance liquid chromatography to obtain the (18)F-FFAU. In vitro studies were performed in HT-29 cells by incubation at various time points. In vivo studies including biodistribution and microPET were performed in tumor-bearing nude mice. RESULTS: The radiochemical yield was 20%-30% decay corrected with an average of 25% in 4 runs. Radiochemical purity was >99% and average specific activity was 85 GBq/micromol (2,300 mCi/micromol) (end of synthesis). In vitro accumulation of (3)H-FFAU in HSV1-tk-expressing cells was approximately 180-fold (P < 0.001) higher than that in the wild-type cells between 30 and 120 min. In vivo uptake of (3)H-FFAU in HSV1-tk-positive tumors at 2 h was approximately 8-fold (P < 0.001) higher than that in the control tumors. Tumor uptake (percentage injected dose per gram of tissue) and the uptake ratio (tk-positive to wild type) of (3)H-FFAU in tk-positive cells was higher compared with those of our earlier studies using 2'-(14)C-deoxy-2'-fluoro-5-methyl-1-beta-D-arabinofuranosyluracil ((14)C-FMAU) and 9-(4-(18)F-fluoro-3-hydroxymethylbutyl)guanine ((18)F-FHBG) in the same cell lines. microPET on tumor-bearing nude mice also demonstrated a very high uptake of (18)F-FFAU in tk-positive tumors compared with that of the control tumor without significant accumulation in other organs. CONCLUSION: These results demonstrate that (18)F-FFAU has superior biodistribution characteristics and significantly higher in vivo uptake in HSV1-tk-expressing tumor compared with previously studied agents.

microPET and autoradiographic imaging of GRP receptor expression with 64Cu-DOTA-[Lys3]bombesin in human prostate adenocarcinoma xenografts.

UNLABELLED: Overexpression of gastrin-releasing peptide (GRP) receptor (GRPR) in both androgen-dependent (AD) and androgen-independent (AI) human neoplastic prostate tissues provides an attractive target for prostate cancer imaging and therapy. The goal of this study was to develop (64)Cu-radiolabeled GRP analogs for PET imaging of GRPR expression in prostate cancer xenografted mice. METHODS: [Lys(3)]bombesin ([Lys(3)]BBN) was conjugated with 1,4,7,10-tetraazadodecane-N,N',N",N"'-tetraacetic acid (DOTA) and labeled with the positron-emitting isotope (64)Cu (half-life = 12.8 h, 19% beta(+)). Receptor binding of DOTA-[Lys(3)]BBN and internalization of (64)Cu-DOTA-[Lys(3)]BBN by PC-3 prostate cancer cells were measured. Tissue biodistribution, microPET, and whole-body autoradiographic imaging of the radiotracer were also investigated in PC-3 (AI)/CRW22 (AD) prostate cancer tumor models. RESULTS: A competitive receptor- binding assay using (125)I-[Tyr(4)]BBN against PC-3 cells yielded a 50% inhibitory concentration value of 2.2 +/- 0.5 nmol/L for DOTA-[Lys(3)]BBN. Incubation of cells with the (64)Cu-labeled radiotracer showed temperature- and time-dependent internalization. At 37 degrees C, >60% of the tracer was internalized within the first 15 min and uptake remained constant for 2 h. Radiotracer uptake was higher in AI PC-3 tumor (5.62 +/- 0.08 %ID/g at 30 min after injection, where %ID/g is the percentage of injected dose per gram) than in AD CWR22 tumor (1.75 +/- 0.05 %ID/g at 30 min after injection). Significant accumulation of the activity in GRPR-positive pancreas was also observed (10.4 +/- 0.15 %ID/g at 30 min after injection). Coinjection of a blocking dose of [Lys(3)]BBN inhibited the activity accumulation in PC-3 tumor and pancreas but not in CWR22 tumor. microPET and autoradiographic imaging of (64)Cu-DOTA-[Lys(3)]BBN in athymic nude mice bearing subcutaneous PC-3 and CWR22 tumors showed strong tumor-to-background contrast. CONCLUSION: This study demonstrates that PET imaging of (64)Cu-DOTA-[Lys(3)]BBN is able to detect GRPR-positive prostate cancer.

Pegylated Arg-Gly-Asp peptide: 64Cu labeling and PET imaging of brain tumor alphavbeta3-integrin expression.

UNLABELLED: The alphav-integrins, cell adhesion molecules that are highly expressed on activated endothelial cells and tumor cells but not on dormant endothelial cells or normal cells, present an attractive target for tumor imaging and therapy. We previously coupled a cyclic Arg-Gly-Asp (RGD) peptide, c(RGDyK), with 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid (DOTA) and labeled the RGD-DOTA conjugate with 64Cu (half-life, 12.8 h; 19% beta+) for solid tumor targeting, with high tumor-to-background contrast. The rapid tumor washout rate and persistent liver and kidney retention of this tracer prompted us to optimize the tracer for improved pharmacokinetic behavior. In this study, we introduced a polyethylene glycol (PEG; molecular weight, 3,400) moiety between DOTA and RGD and evaluated the 64Cu-DOTA-PEG-RGD tracer for microPET imaging in brain tumor models. METHODS: DOTA was activated in situ and conjugated with RGD-PEG-NH2 under slightly basic conditions. alphavbeta3-Integrin-binding affinity was evaluated with a solid-phase receptor-binding assay in the presence of 125I-echistatin. Female nude mice bearing subcutaneous U87MG glioblastoma xenografts were administered 64Cu-DOTA-PEG-RGD, and the biodistributions of the radiotracer were evaluated from 30 min to 4 h after injection. microPET (20 min of static imaging at 1 h after injection) and then quantitative autoradiography were used for tumor visualization and quantification. The same tracer was also applied to an orthotopic U87MG model for tumor detection. RESULTS: The radiotracer was synthesized with a high specific activity (14,800-29,600 GBq/mmol [400-800 Ci/mmol]). The c(RGDyK)-PEG-DOTA ligand showed intermediate binding affinity for alphavbeta3-integrin (50% inhibitory concentration, 67.5 +/- 7.8 nmol/L [mean +/- SD]). The pegylated RGD peptide demonstrated rapid blood clearance (0.57 +/- 0.15 percentage injected dose [%ID]/g [mean +/- SD] at 30 min after injection and 0.03 +/- 0.02 %ID/g at 4 h after injection). Activity accumulation in the tumor was rapid and high at early time points (2.74 +/- 0.45 %ID/g at 30 min after injection), and some activity washout was seen over time (1.62 +/- 0.18 %ID/g at 4 h after injection). Compared with (64)Cu-DOTA-RGD, this tracer showed improved in vivo kinetics, with significantly reduced liver uptake (0.99 +/- 0.08 %ID/g vs. 1.73 +/- 0.39 %ID/g at 30 min after injection and 0.58 +/- 0.07 %ID/g vs. 2.57 +/- 0.49 %ID/g at 4 h after injection). The pegylated RGD peptide showed higher renal accumulation at early time points (3.51 +/- 0.24 %ID/g vs. 2.18 +/- 0.23 %ID/g at 30 min after infection) but more rapid clearance (1.82 +/- 0.29 %ID/g vs. 2.01 +/- 0.25 %ID/g at 1 h after injection) than 64Cu-DOTA-RGD. The integrin receptor specificity of this radiotracer was demonstrated by blocking of tumor uptake by coinjection with nonradiolabeled c(RGDyK). The high tumor-to-organ ratios for the pegylated RGD peptide tracer (at 1 h after injection: tumor-to-blood ratio, 20; tumor-to-muscle ratio, 12; tumor-to-liver ratio, 2.7; and tumor-to-kidney ratio, 1.2) were confirmed by microPET and autoradiographic imaging in a subcutaneous U87MG tumor model. This tracer was also able to detect an orthotopic brain tumor in a model in which U87MG cells were implanted into the mouse forebrain. Although the magnitude of tumor uptake in the orthotopic xenograft was lower than that in the subcutaneous xenograft, the orthotopic tumor was still visualized with clear contrast from normal brain tissue. CONCLUSION: This study demonstrated the suitability of a PEG moiety for improving the in vivo kinetics of a 64Cu-RGD peptide tracer without compromising the tumor-targeting ability and specificity of the peptide. Systematic investigations of the effects of the size and geometry of PEG on tumor targeting and in vivo kinetics will lead to the development of radiotracers suitable for clinical applications such as visualizing and quantifying alphav-integrin expression by PET. In addition, the same ligand labeled with therapeutic radionuclides may be applicable for integrin-targeted internal radiotherapy.

MicroPET imaging of breast cancer alphav-integrin expression with 64Cu-labeled dimeric RGD peptides.

PURPOSE: Alphavbeta3 and alphavbeta5 integrins are cell adhesion molecules that play a vital role in tumor angiogenesis and metastasis. The ability to visualize and quantify integrin expression in vivo will foster our understanding of the role of integrins alphavbeta3 and alphavbeta5 in tumor angiogenesis and allow for direct assessment of anti-angiogenic treatment efficacy based on integrin antagonists. This study compared the tumor targeting characteristics of two dimeric 64Cu-labeled RGD peptide agonists of alphavbeta3 integrin. PROCEDURES: Dimeric RGD peptides E[c(RGDyK)]2 and E[c(RGDfK)]2 were conjugated with 1,4,7,10-tetraazadodecane-N,N',N",N"'-tetraacetic acid (DOTA) and labeled with positron emitter 64Cu(t(1/2)=12.8 h, beta+=19%). Both 64Cu-DOTA-E[c(RGDyK)]2 and 64Cu-DOTA-E[c(RGDfK)]2 were used in biodistribution, microPET imaging and whole-body autoradiography studies in athymic female nude mice with orthotopically growing MDA-MB-435 breast carcinoma xenografts. RESULTS: At all time points, activity accumulation of 64)Cu-DOTA-E[c(RGDyK)]2 in tumors was significantly higher compared to the D-Phe analog. Liver uptake of the D-Tyr derivative was lower than the D-Phe derivative at early time points but the difference became marginal with time. Overall, 64Cu-DOTA-E[c(RGDyK)]2 yielded better position emission tomography (PET) images in orthotopic MDA-MB-435 bearing mice than did 64Cu-DOTA-E[c(RGDfK)]2. Both radiotracers had alphav-integrin specific tumor activity accumulation, as demonstrated by significant reduction of uptake with a coinjected blocking dose of c(RGDyK). CONCLUSIONS: The radiolabeled dimeric RGD peptides 64Cu-DOTA-E[c(RGDyK)]2 and 64Cu-DOTA-E[c(RGDfK)]2 have high and specific tumor uptake in a human breast cancer tumor xenograft, with the D-Tyr derivative showing better in vivo kinetics than the D-Phe derivative, most likely due to the increased hydrophilicity of the D-Tyr. Both dimeric peptides showed better tumor retention than the previously tested monomeric RGD counterparts, presumably because of bivalency and increase in apparent molecular size.

Synthesis of 2'-deoxy-2'-[18F]fluoro-5-bromo-1-beta-D-arabinofuranosyluracil ([18F]-FBAU) and 2'-deoxy-2'-[18F]fluoro-5-chloro-1-beta-D-arabinofuranosyl-uracil ([18F]-FCAU), and their biological evaluation as markers for gene expression.

[(18)F]-FBAU and [(18)F]-FCAU have been synthesized and evaluated in vivo as markers for HSV1-tk gene expression. At 2 hours, uptake of [(18)F]-FBAU and [(18)F]-FCAU in HSV1-tk-positive tumors was 7.9-fold and 6.0-fold higher than the control tumors, respectively. Micro-PET images also showed very high uptake in HSV-tk tumors. Compared to [(14)C]-FMAU, total uptake of [(18)F]-FBAU and [(18)F]-FCAU was similar in tk-positive cells, but the uptake ratio (tk+/wild) was higher. [(18)F]-FBAU and [(18)F]-FCAU appear to be potential PET imaging agents for gene expression.

18F-labeled RGD peptide: initial evaluation for imaging brain tumor angiogenesis.

Brain tumors are highly angiogenesis dependent. The cell adhesion receptor integrin alpha(v)beta(3) is overexpressed in glioma and activated endothelial cells and plays an important role in brain tumor growth, spread and angiogenesis. Suitably labeled alpha(v)beta(3)-integrin antagonists may therefore be useful for imaging brain tumor associated angiogenesis. Cyclic RGD peptide c(RGDyK) was labeled with (18)F via N-succinimidyl-4-[(18)F]fluorobenzoate through the side-chain epsilon-amino group of the lysine residue. The radiotracer was evaluated in vivo for its tumor targeting efficacy and pharmacokinetics in subcutaneously implanted U87MG and orthotopically implanted U251T glioblastoma nude mouse models by means of microPET, quantitative autoradiography and direct tissue sampling. The N-4-[(18)F]fluorobenzoyl-RGD ([(18)F]FB-RGD) was produced in less than 2 h with 20-25% decay-corrected yields and specific activity of 230 GBq/micromol at end of synthesis. The tracer showed very rapid blood clearance and both hepatobiliary and renal excretion. Tumor-to-muscle uptake ratio at 30 min was approximately 5 in the subcutaneous U87MG tumor model. MicroPET imaging with the orthotopic U251T brain tumor model revealed very high tumor-to-brain ratio, with virtually no uptake in the normal brain. Successful blocking of tumor uptake of [(18)F]FB-RGD in the presence of excess amount of c(RGDyK) revealed receptor specific activity accumulation. Hence, N-4-[(18)F]fluorobenzoyl labeled cyclic RGD peptide [(18)F]FB-RGD is a potential tracer for imaging alpha(v)beta(3)-integrin positive tumors in brain and other anatomic locations.

In vivo evaluation of 2'-deoxy-2'-[(18)F]fluoro-5-iodo-1-beta-D-arabinofuranosyluracil ([18F]FIAU) and 2'-deoxy-2'-[18F]fluoro-5-ethyl-1-beta-D-arabinofuranosyluracil ([18F]FEAU) as markers for suicide gene expression.

PURPOSE: FIAU and FEAU were evaluated in vitro and in vivo as markers for HSV1-tk gene expression. METHODS: In vitro and biodistribution studies were performed in wild type and transduced HT-29 cells using [14C]FIAU and [3H]FEAU. PET imaging was performed using [18F]FIAU and [18F]FEAU. RESULTS: In vitro uptake of [14C]FIAU in tk-positive cells was 39-fold, 49-fold, and 43-fold higher (p<0.001) than in wild type cells at 30, 60, and 120 min, respectively. Uptake of [3H]FEAU in transduced cells was 46-fold, 62-fold, and 121-fold higher (p<0.001) than in wild type cells at the same time points. In vivo uptake of [14C]FIAU at 2 h in HSV1-tk positive tumors was 15.48+/-3.94, 6.7-fold higher (p<0.001) than in wild type tumors. Uptake of [3H]FEAU in transduced tumors was 9.98+/-1.99, 5.0-fold higher (p<0.001) than in wild type tumors. Micro-PET images using [18F]FIAU and [18F]FEAU also showed very high uptake in HSV-tk tumors. CONCLUSION: [18F]FIAU and [18F]FEAU appear to be potential PET imaging agents for gene expression.

Glucose metabolism of human prostate cancer mouse xenografts.

We hypothesized that the glucose metabolism of prostate cancer is modulated by androgen. We performed in vivo biodistribution and imaging studies of [F-18] fluorodeoxyglucose (FDG) accumulation in androgen-sensitive (CWR-22) and androgen-independent (PC-3) human prostate cancer xenografts implanted in castrated and noncastrated male athymic mice. The growth pattern of the CWR-22 tumor was best approximated by an exponential function (tumor size in mm3 = 14.913 e(0.1086 x days), R2 = .96, n = 5). The growth pattern of the PC-3 tumor was best approximated by a quadratic function (tumor size in mm3 = 0.3511 x days2 + 49.418 x day - 753.33, R2 = .96, n = 3). The FDG accumulation in the CWR-22 tumor implanted in the castrated mice was significantly lower, by an average of 55%, in comparison to that implanted in the noncastrated host (1.27 vs. 2.83, respectively, p < .05). The 3-week maximal standardized uptake value (SUVmax) was 0.99 +/- 0.43 (mean +/- SD) for CWR-22 and 1.21 +/- 0.32 for PC-3, respectively. The 5-week SUVmax was 1.22 +/- 0.08 for CWR-22 and 1.35 +/- 0.17 for PC-3, respectively. The background muscle SUVmax was 0.53 +/- 0.11. Glucose metabolism was higher in the PC-3 tumor than in the CWR-22 tumor at both the 3-week (by 18%) and the 5-week (by 9.6%) micro-PET imaging sessions. Our results support the notions that FDG PET may be useful in the imaging evaluation of response to androgen ablation therapy and in the early prediction of hormone refractoriness in men with metastatic prostate cancer.

Micro-PET imaging of alphavbeta3-integrin expression with 18F-labeled dimeric RGD peptide.

The alphav integrins, which act as cell adhesion molecules, are closely involved with tumor invasion and angiogenesis. In particular, alphavbeta3 integrin, which is specifically expressed on proliferating endothelial cells and tumor cells, is a logical target for development of a radiotracer method to assess angiogenesis and anti-angiogenic therapy. In this study, a dimeric cyclic RGD peptide E[c(RGDyK)]2 was labeled with 18F (t(1/2) = 109.7 min) by using a prosthetic 4-[18F]fluorobenzoyl moiety to the amino group of the glutamate. The resulting [18F]FB-E[c(RGDyK)]2, with high specific activity (200-250 GBq/micromol at the end of synthesis), was administered to subcutaneous U87MG glioblastoma xenograft models for micro-PET and autoradiographic imaging as well as direct tissue sampling to assess tumor targeting efficacy and in vivo kinetics of this PET tracer. The dimeric RGD peptide demonstrated significantly higher tumor uptake and prolonged tumor retention in comparison with a monomeric RGD peptide analog [18F]FB-c(RGDyK). The dimeric RGD peptide had predominant renal excretion, whereas the monomeric analog was excreted primarily through the biliary route. Micro-PET imaging 1 hr after injection of the dimeric RGD peptide exhibited tumor to contralateral background ratio of 9.5 +/- 0.8. The synergistic effect of polyvalency and improved pharmacokinetics may be responsible for the superior imaging characteristics of [18F]FB-E[c(RGDyK)]2.

MicroPET and autoradiographic imaging of breast cancer alpha v-integrin expression using 18F- and 64Cu-labeled RGD peptide.

Cell adhesion molecules alphavbeta3 and alphavbeta5 play a pivotal role in tumor angiogenesis and metastasis. Antiangiogenic therapy by using small peptide antagonists of alphav-integrins slows tumor growth and prevents tumor spread. The ability to visualize and quantify integrin expression will enable selection of appropriate patients for clinical trials, following determination of treatment efficacy and development of new potent drugs. We have previously labeled cyclic RGD peptide c(RGDyK) with 125I and 18F and applied the radiotracers to both subcutaneous and orthotopic brain tumor models. Here we conjugated c(RGDyK) with 1,4,7,10-tetraaza-1,4,7,10-tetradodecane-N,N',N' ',N' "-tetraacetic acid (DOTA) and labeled the DOTA-RGD conjugate with 64Cu (t1/2) = 12.8 h, 19% beta+) in high radiochemical purity and specific activity. The tumor targeting ability and in vivo kinetics of 64Cu-DOTA-RGD was compared with [18F]FB-RGD and 125I-RGD in orthotopic MDA-MB-435 breast cancer model. All three radiotracers revealed fast blood clearance and high tumor-to-blood and tumor-to-muscle ratios. 125I-RGD had higher tumor uptake than the corresponding 18F and 64Cu analogues. [18F]FB-RGD indicated a fast tumor washout rate and an unfavorable hepatobiliary excretion pathway, resulting in significant activity accumulation in gallbladder and intestines. 64Cu-DOTA-RGD had prolonged tumor retention (1.44 +/- 0.09 %ID/g at 4 h postinjection) and persistent uptake in the liver. All three tracers revealed receptor specific tumor accumulation which were illustrated by effective blocking via coinjection with a blocking dose of c(RGDyK). Static microPET imaging and whole-body autoradiography showed strong contrast from the contralateral background. In conclusion, overall molecular charge and characteristics of radiolabels have profound effects on tumor accumulation and in vivo kinetics of radiolabeled RGD peptide. Further modification of the RGD peptide and optimization of the tracer for prolonged tumor uptake and improved in vivo kinetics are being explored.

MicroPET imaging of brain tumor angiogenesis with 18F-labeled PEGylated RGD peptide.

We have previously labeled cyclic RGD peptide c(RGDyK) with fluorine-18 through conjugation labeling via a prosthetic 4-[18F]fluorobenzoyl moiety and applied this [18F]FB-RGD radiotracer for alphav-integrin expression imaging in different preclinical tumor models with good tumor-to-background contrast. However, the unfavorable hepatobiliary excretion and rapid tumor washout rate of this tracer limit its potential clinical applications. The aims of this study were to modify the [18F]FB-RGD tracer by inserting a heterobifunctional poly(ethylene glycol) (PEG, M.W. =3,400) between the 18F radiolabel and the RGD moiety and to test this [18F]FB-PEG-RGD tracer for brain tumor targeting and in vivo kinetics. [18F]FB-PEG-RGD was prepared by coupling the RGD-PEG conjugate with N-succinimidyl 4-[18F]fluorobenzoate ([18F]SFB) under slightly basic conditions (pH=8.5). The radiochemical yield was about 20-30% based on the active ester [18F]SFB, and specific activity was over 100 GBq/micromol. This tracer had fast blood clearance, rapid and high tumor uptake in the subcutaneous U87MG glioblastoma model (5.2+/-0.5%ID/g at 30 min p.i.). Moderately rapid tumor washout was observed, with the activity accumulation decreased to 2.2+/-0.4%ID/g at 4 h p.i. MicroPET and autoradiography imaging showed a very high tumor-to-background ratio and limited activity accumulation in the liver, kidneys and intestinal tracts. U87MG tumor implanted into the mouse forebrain was well visualized with [18F]FB-PEG-RGD. Although uptake in the orthotopic tumor was significantly lower (P<0.01) than in the subcutaneous tumor, the maximum tumor-to-brain ratio still reached 5.0+/-0.6 due to low normal brain background. The results of H&E staining post mortem agreed with the anatomical information obtained from non-invasive microPET imaging. In conclusion, PEGylation suitably modifies the physiological behavior of the RGD peptide. [18F]FB-PEG-RGD gave improved tumor retention and in vivo kinetics compared with [18F]FB-RGD.