Reproducibility study of [(18)F]FPP(RGD)2 uptake in murine models of human tumor xenografts.

PURPOSE: An (18)F-labeled PEGylated arginine-glycine-aspartic acid (RGD) dimer {[(18)F]FPP(RGD)(2)} has been used to image tumor α(v)ß(3) integrin levels in preclinical and clinical studies. Serial positron emission tomography (PET) studies may be useful for monitoring antiangiogenic therapy response or for drug screening; however, the reproducibility of serial scans has not been determined for this PET probe. The purpose of this study was to determine the reproducibility of the integrin α(v)ß(3)-targeted PET probe, [(18)F]FPP(RGD)(2,) using small animal PET. METHODS: Human HCT116 colon cancer xenografts were implanted into nude mice (n = 12) in the breast and scapular region and grown to mean diameters of 5-15 mm for approximately 2.5 weeks. A 3-min acquisition was performed on a small animal PET scanner approximately 1 h after administration of [(18)F]FPP(RGD)(2) (1.9-3.8 MBq, 50-100 µCi) via the tail vein. A second small animal PET scan was performed approximately 6 h later after reinjection of the probe to assess for reproducibility. 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 or maximum 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. RESULTS: The coefficient of variation (mean±SD) for %ID(mean)/g and %ID(max)/g values between [(18)F]FPP(RGD)(2) small animal PET scans performed 6 h apart on the same day were 11.1 ± 7.6% and 10.4 ± 9.3%, respectively. The corresponding differences in %ID(mean)/g and %ID(max)/g values between scans were -0.025 ± 0.067 and -0.039 ± 0.426. Immunofluorescence studies revealed a direct relationship between extent of α(ν)ß(3) integrin expression in tumors and tumor vasculature with level of tracer uptake. 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, noting tumor volume, as well as making uniform: the time of imaging after injection and the ROI size. Reanalysis of ROI placement displayed variability for %ID(mean)/g of 6.6 ± 3.9% and 0.28 ± 0.12% for %ID(max)/g. CONCLUSION: [(18)F]FPP(RGD)(2) small animal PET mouse tumor xenograft studies are reproducible with relatively low variability.

microPET of tumor integrin alphavbeta3 expression using 18F-labeled PEGylated tetrameric RGD peptide (18F-FPRGD4).

UNLABELLED: In vivo imaging of alpha(v)beta(3) expression has important diagnostic and therapeutic applications. Multimeric cyclic RGD peptides are capable of improving the integrin alpha(v)beta(3)-binding affinity due to the polyvalency effect. Here we report an example of (18)F-labeled tetrameric RGD peptide for PET of alpha(v)beta(3) expression in both xenograft and spontaneous tumor models. METHODS: The tetrameric RGD peptide E{E[c(RGDyK)](2)}(2) was derived with amino-3,6,9-trioxaundecanoic acid (mini-PEG; PEG is poly(ethylene glycol)) linker through the glutamate alpha-amino group. NH(2)-mini-PEG-E{E[c(RGDyK)](2)}(2) (PRGD4) was labeled with (18)F via the N-succinimidyl-4-(18)F-fluorobenzoate ((18)F-SFB) prosthetic group. The receptor-binding characteristics of the tetrameric RGD peptide tracer (18)F-FPRGD4 were evaluated in vitro by a cell-binding assay and in vivo by quantitative microPET imaging studies. RESULTS: The decay-corrected radiochemical yield for (18)F-FPRGD4 was about 15%, with a total reaction time of 180 min starting from (18)F-F(-). The PEGylation had minimal effect on integrin-binding affinity of the RGD peptide. (18)F-FPRGD4 has significantly higher tumor uptake compared with monomeric and dimeric RGD peptide tracer analogs. The receptor specificity of (18)F-FPRGD4 in vivo was confirmed by effective blocking of the uptake in both tumors and normal organs or tissues with excess c(RGDyK). CONCLUSION: The tetrameric RGD peptide tracer (18)F-FPRGD4 possessing high integrin-binding affinity and favorable biokinetics is a promising tracer for PET of integrin alpha(v)beta(3) expression in cancer and other angiogenesis related diseases.

18F-labeled galacto and PEGylated RGD dimers for PET imaging of αvß3 integrin expression.

PURPOSE: In vivo imaging of α(v)ß(3) has important diagnostic and therapeutic applications. (18)F-Galacto-arginine-glycine-aspartic acid (RGD) has been developed for positron emission tomography (PET) imaging of integrin α(v)ß(3) expression and is now being tested on humans. Dimerization and multimerization of cyclic RGD peptides have been reported to improve the integrin α(v)ß(3)-binding affinity due to the polyvalency effect. Here, we compared a number of new dimeric RGD peptide tracers with the clinically used (18)F-galacto-RGD. PROCEDURES: RGD monomers and dimers were coupled with galacto or PEG(3) linkers, and labeled with (18)F using 4-nitrophenyl 2-(18)F-fluoropropionate ((18)F-NFP) or N-succinimidyl 4-(18)F-fluorobenzoate as a prosthetic group. The newly developed tracers were evaluated by cell-based receptor-binding assay, biodistribution, and small-animal PET studies in a subcutaneous U87MG glioblastoma xenograft model. RESULTS: Starting with (18)F-F(-), the total reaction time for (18)F-FP-SRGD2 and (18)F-FP-PRGD2 is about 120 min. The decay-corrected radiochemical yields for (18)F-FP-SRGD2 and (18)F-FP-PRGD2 are 52 ± 9% and 80 ± 7% calculated from (18)F-NFP. Noninvasive small-animal PET and direct tissue sampling experiments demonstrated that the dimeric RGD peptides had significantly higher tumor uptake as compared to (18)F-galacto-RGD. CONCLUSION: Dimeric RGD peptide tracers with relatively high tumor integrin-specific accumulation and favorable in vivo kinetics may have the potential to be translated into clinic for integrin α(v)ß(3) imaging.