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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.

2′-Deoxy-2′-fluoro-5-methyl-1-β- d -arabinofuranosyluracil (FMAU), 9-(4-fluoro-3-hydroxy-methyl-butyl)guanine (FHBG) and 9-[(3-fluoro-1-hydroxy-2-propoxy)methyl]-guanine (FHPG) have been evaluated in a human breast cancer model as potential radiotracers for PET imaging of HSV1-tk gene expression. In vitro accumulation of [ 14 C]FMAU, [ 18 F]FHBG, and [ 18 F]FHPG in HSV1-tk-expressing cells was 14- to 16-fold ( p < .001), 9- to 13-fold ( p < .001), and 2- to 3-fold ( p < .05) higher than tk-negative control cells, respectively, between 30 and 240 min. Accumulation of FMAU and FHBG in vector-transduced cells was 10- to 14-fold and 6- to 10-fold higher than wild-type cells, respectively. At 2 hr, uptake of [ 14 C]FMAU in tk-positive cells was 6.3-fold and 60-fold higher than [ 18 F]FHBG and [ 18 F]FHPG, respectively. In vivo, tumor uptake of [ 14 C]FMAU in HSV1-tk-expressing cells was 3.7-fold and 5.5-fold ( p < .001) higher than tk-negative control cells at 1 and 2 hr, respectively. Tumor uptake of [ 18 F]FHBG was 4.2-fold and 12.6-fold higher ( p < .001) than tk-negative cells at the same time points. Incorporation of [ 14 C]FMAU in tk-positive tumor was 18-fold and 24-fold higher ( p < .001) than [ 18 F]FHBG at 1 and 2 hr, respectively. Micro-PET images support the biodistribution results and indicate that both [ 18 F]FMAU and [ 18 F]FHBG are useful for imaging HSV1-tk expression in breast cancer. Although FMAU demonstrates higher total incorporation (%dose/g) in tumor tissue compared with the other tracers, FHBG is superior in terms of specific accumulation in transfected cells at later time points.

2'-Deoxy-2'-flouro-5-methyl-1-beta-D-arabinofuranosyluracil (FMAU) has been evaluated in HT-29 cells as a potential positron emission tomography (PET) radiotracer for imaging HSV-tk gene expression in vivo. In vitro experiments demonstrate that the accumulation of [14C]-FMAU in HSV-tk-expressing cells is 2.4-fold (p < .02), 4.0-fold (p < .001), and 5.3-fold (p < .001) higher than the wild-type cells at 1, 3, and 5 hr, respectively. In vivo studies revealed that the tumor uptake in HSV-tk-expressing cells was 2.3-fold (p < .001), 3.0-fold (p < .001), and 5.5-fold (p < .001) higher than the control cells at 1, 2, and 5 hr, respectively. FMAU was found to be more sensitive compared to our earlier studies using 9-[(3-18F-fluoro-1-hydroxy-2-propoxy)methyl]-guanine ([18F]-FHPG) and 9-(4-[18F]-fluoro-3-hydroxy-methylbutyl)guanine ([18F]-FHBG) in the same cell lines, although, the specificity was less than FHBG. These results suggest that while FMAU labeled with PET isotopes may be useful for imaging HSV-tk-expressing tumors in vivo, multitracer studies across additional tumor models are necessary in order to identify an optimal PET radiotracer.

18 F positron emission tomography (PET) has a number of attributes that make it clinically attractive, including nearly 100% positron efficiency, very high specific radioactivity, and a short half-life of ≈ 110 minutes. However, the short half-life of 18 F and the poor nucleophilicity of fluoride introduce challenges for the incorporation of 18 F into complex molecules. Recently, the tetrazine- trans -cyclooctene ligation was introduced as a novel 18 F labeling method that proceeds with fast reaction rates without catalysis. Herein we report an efficient method for 18 F labeling of free cysteines of peptides and proteins based on sequential ligation with a bifunctional tetrazinyl-maleimide and an 18 F-labeled trans -cyclooctene. The newly developed method was tested for site-specific labeling of both c(RGDyC) peptide and vascular endothelial growth factor (VEGF)-SH protein. Starting with 4 mCi of 18 F- trans -cyclooctene and only 10 μg of tetrazine-RGD (80–100 μM) or 15 μg of tetrazine-VEGF (6.0 μM), 18 F-labeled RGD peptide and VEGF protein could be obtained within 5 minutes in 95% yield and 75% yield, respectively. The obtained tracers were then evaluated in mice. In conclusion, a highly efficient method has been developed for site-specific 18 F labeling of cysteine-containing peptides and proteins. The special characteristics of the tetrazine– trans -cyclooctene ligation provide unprecedented opportunities to synthesize 18 F-labeled probes with high specific activity for PET applications.

Accumulating experimental evidence indicates that overexpression of α 2 β 1 integrin may correlate with progression in human prostate cancer. The objective of this study was to design a novel imaging probe based on the Asp-Gly-Glu-Ala (DGEA) peptide for near-infrared-fluorescent (NIRF) imaging of α 2 β 1 , integrin expression in prostate cancer. The peptides were conjugated with appropriate fluorescent dyes, and the binding affinity of these probes was evaluated by flow cytometry in three human prostate cell lines (PC-3, CWR-22, and LNCaP). In vivo NIRF imaging of the α 2 β 1 -positive PC-3 xenograft model was performed to evaluate the α 2 β 1 targeted probe. In vitro immunofluorescence staining was carried out to confirm the α 2 β 1 integrin expression level. Flow cytometry analysis showed that PC-3 had the highest probe uptake, followed by CWR-22 and LNCaP tumor cells. In the subcutaneous PC-3 model, the tumor demonstrated prominent uptake with good tumor to background contrast. Immunohistochemistry staining also supported the in vivo optical imaging results. DGEA-based optical agents have been developed for specific imaging of α 2 β 1 , integrin expression. In vitro and in vivo localization demonstrated the potential of this agent to identify tumor subtypes amenable to anti-α 2 β 1 integrin treatment and potentially provide prognostic information regarding tumor progression.

Accumulating experimental evidence indicates that overexpression of α(2)β(1) integrin may correlate with progression in human prostate cancer. The objective of this study was to design a novel imaging probe based on the Asp-Gly-Glu-Ala (DGEA) peptide for near-infrared-fluorescent (NIRF) imaging of α(2)β(1) integrin expression in prostate cancer. The peptides were conjugated with appropriate fluorescent dyes, and the binding affinity of these probes was evaluated by flow cytometry in three human prostate cell lines (PC-3, CWR-22, and LNCaP). In vivo NIRF imaging of the α(2)β(1)-positive PC-3 xenograft model was performed to evaluate the α(2)β(1) targeted probe. In vitro immunofluorescence staining was carried out to confirm the α(2)β(1) integrin expression level. Flow cytometry analysis showed that PC-3 had the highest probe uptake, followed by CWR-22 and LNCaP tumor cells. In the subcutaneous PC-3 model, the tumor demonstrated prominent uptake with good tumor to background contrast. Immunohistochemistry staining also supported the in vivo optical imaging results. DGEA-based optical agents have been developed for specific imaging of α(2)β(1) integrin expression. In vitro and in vivo localization demonstrated the potential of this agent to identify tumor subtypes amenable to anti-α(2)β(1) integrin treatment and potentially provide prognostic information regarding tumor progression.

The purpose of this study was to investigate the effects of castration and tracer uptake time interval on the level of radiolabeled choline accumulation in murine-implanted human prostate tumor xenografts using quantitative autoradiography. We implanted androgen-dependent (CWR22) and androgen-independent (PC3) human prostate cancer cells in castrated ( n = 9) and noncastrated ( n = 9) athymic male mice and allowed tumors to grow to 1 cm 3 . The mice were euthanized at 5, 10, and 20 minutes after injection of 5 µCi [14C]-choline. Mice were prepared for quantitative autoradiography with density light units of viable tumor sections converted to units of radioactivity (nCi/mm 2 ) using calibration. Two-group comparisons were performed using a two-tailed Student t -test with unequal variance and with a significance probability level of less than .05. Two-group comparisons between the means of the tracer uptake level for each tumor type at each of three time points for each of two host types showed that (1) the level of tracer localization in the two tumor types was affected little in relation to the host type and (2) PC3 tumor uptake level tended to increase slowly with time only in the noncastrated host, whereas this was not observed in the castrated host or with CWR22 tumor in either host type. The uptake time interval and castration do not appear to significantly affect the level of radiolabeled choline uptake by the human prostate cancer xenograft.

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 e0.108 × 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 × days2 + 49.418 × 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.

The α v integrins, which act as cell adhesion molecules, are closely involved with tumor invasion and angiogenesis. In particular, α v β 3 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 18 F ( t 1 /2 = 109.7 min) by using a prosthetic 4-[ 18 F]fluorobenzoyl moiety to the amino group of the glutamate. The resulting [ 18 F]FB-E[c(RGDyK)] 2 , with high specific activity (200–250 GBq/μmol 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 [ 18 F]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 [ 18 F]FB-E[c(RGDyK)] 2 .

2′-Deoxy-2′-flouro-5-methyl-1-β-d-arabinofuranosyluracil (FMAU) has been evaluated in HT-29 cells as a potential positron emission tomography (PET) radiotracer for imaging HSV-tk gene expression in vivo. In vitro experiments demonstrate that the accumulation of [14C]-FMAU in HSV-tk-expressing cells is 2.4-fold ( p < .02), 4.0-fold ( p < .001), and 5.3-fold ( p < .001) higher than the wild-type cells at 1, 3, and 5 hr, respectively. In vivo studies revealed that the tumor uptake in HSV-tk-expressing cells was 2.3-fold ( p < .001), 3.0-fold ( p < .001), and 5.5-fold ( p < .001) higher than the control cells at 1, 2, and 5 hr, respectively. FMAU was found to be more sensitive compared to our earlier studies using 9-[(3-18F-fluoro-1-hydroxy-2-propoxy)methyl]-guanine ([18F]-FHPG) and 9-(4-[18F]-fluoro-3-hydroxy-methylbutyl)guanine ([18F]-FHBG) in the same cell lines, although, the specificity was less than FHBG. These results suggest that while FMAU labeled with PET isotopes may be useful for imaging HSV-tk-expressing tumors in vivo, multitracer studies across additional tumor models are necessary in order to identify an optimal PET radiotracer.

18F positron emission tomography (PET) has a number of attributes that make it clinically attractive, including nearly 100% positron efficiency, very high specific radioactivity, and a short half-life of ≈ 110 minutes. However, the short half-life of 18F and the poor nucleophilicity of fluoride introduce challenges for the incorporation of 18F into complex molecules. Recently, the tetrazine-trans-cyclooctene ligation was introduced as a novel 18F labeling method that proceeds with fast reaction rates without catalysis. Herein we report an efficient method for 18F labeling of free cysteines of peptides and proteins based on sequential ligation with a bifunctional tetrazinyl-maleimide and an 18F-labeled trans-cyclooctene. The newly developed method was tested for site-specific labeling of both c(RGDyC) peptide and vascular endothelial growth factor (VEGF)-SH protein. Starting with 4 mCi of 18F-trans-cyclooctene and only 10 μg of tetrazine-RGD (80-100 μM) or 15 μg of tetrazine-VEGF (6.0 μM), 18F-labeled RGD peptide and VEGF protein could be obtained within 5 minutes in 95% yield and 75% yield, respectively. The obtained tracers were then evaluated in mice. In conclusion, a highly efficient method has been developed for site-specific 18F labeling of cysteine-containing peptides and proteins. The special characteristics of the tetrazine-trans-cyclooctene ligation provide unprecedented opportunities to synthesize 18F-labeled probes with high specific activity for PET applications.

We hypothesized that imaging-based assessment of cellular proliferation in prostate cancer may improve tumor characterization. We therefore evaluated the biodistribution and effect of androgen on tumor uptake of the cellular proliferation imaging marker [ 18 F]-29fluoro-5-methyl-1-beta-D-arabinofuranosyluracil ( 18 F-FMAU) in xenograft mouse models of human prostate cancer. Castrated and noncastrated athymic male mice were implanted with androgen-independent PC3 and androgen-sensitive CWR22 human prostate cancer cells. Dynamic micro–positron emission tomography (PET)/computed tomography was performed for 1 hour followed by 10minute static scans at 2 and 3 hours. Animals were sacrificed after imaging for biodistribution studies and immunohistochemical staining of tumors for androgen receptor and Ki-67/MIB expression. 18 F-FMAU uptake was significantly higher in all major organs of the castrated animals in comparison with noncastrated mice, with the highest uptake in liver and the lowest uptake in muscle and bone. When compared to PC3 tumors, CWR22 xenografts showed significantly higher tumor to muscle (2.56 6 0.30 vs 1.99 6 0.30, p 5 .008) and tumor to liver (1.72 6 0.12 vs 1.26 6 0.17, p 5 .0003) uptake ratios in the noncastrated animal at the 3-hour time point. Androgen receptor and Ki-67/MIB expressions were higher in CWR22 than in PC3 xenografts. Our initial preclinical observations suggest that there may be an association between androgen signaling and thymidine metabolism and that 18 F-FMAU PET may be useful in prostate tumor characterization.