Comparison of cyclic RGD peptides for αvβ3 integrin detection in a rat model of myocardial infarction

Laitinen I, Notni J, Pohle K, Rudelius M, Farrell E, Nekolla SG, Henriksen G, Neubauer S, Kessler H, Wester HJ, Schwaiger M

11.05.2013 [Original Artikel]

Expression of αvβ3 integrin is increased after myocardial infarction as part of the repair process. Increased expression of αvβ3 has been shown by molecular imaging with 18F-galacto-RGD in a rat model. The 68Ga-labelled RGD compounds 68Ga-NODAGA-RGD and 68Ga-TRAP(RGD)3 have high specificity and affinity, and may therefore serve as alternatives of 18F-galacto-RGD for integrin imaging. Left coronary artery ligation was performed in rats. After 1 week, rats were imaged with [13N]NH3, followed by 18F-galacto-RGD, 68Ga-NODAGA-RGD or 68Ga-TRAP(RGD)3 using a dedicated animal PET/CT device. Rats were killed, and the activity in tissues was measured by gamma counting. The heart was sectioned for autoradiography and histology. Immunohistochemistry was performed on consecutive sections using CD31 for the endothelial cells and CD61 for β3 expression (as part of the αvβ3 receptor). In vivo imaging showed focal RGD uptake in the hypoperfused area of infarcted myocardium as defined with [13N]NH3 scan. In autoradiography images, augmented uptake of all RGD tracers was observed within the infarct area as verified by the HE staining. The tracer uptake ratios (infarct vs. remote) were 4.7 ± 0.8 for 18F-galacto-RGD, 5.2 ± 0.8 for 68Ga-NODAGA-RGD, and 4.1 ± 0.7 for 68Ga-TRAP(RGD)3. The 68Ga-NODAGA-RGD ratio was higher compared to 68Ga-TRAP(RGD)3 (p = 0.04), but neither of the 68Ga tracers differed from 18F-galacto-RGD (p > 0.05). The area of augmented 68Ga-RGD uptake was associated with β3 integrin expression (CD61). 68Ga-NODAGA-RGD and 68Ga-TRAP(RGD)3 uptake was equally increased in the infarct area at 1 week post infarction as 18F-galacto-RGD. These results show the potential of 68Ga-labelled RGD peptides to monitor integrin expression as a part of myocardial repair and angiogenesis after ischaemic injury in vivo.

Be spoilt for choice with radiolabelled RGD peptides: Preclinical evaluation of 68 Ga-TRAP(RGD)3

Notni J, Pohle K, Wester HJ

01.01.2013 [Original Artikel]

Gallium-68 is rapidly gaining importance, as this generator-produced PET isotope is available independent of on-site cyclotrons, enabling radiopharmaceutical production with comparably simple techniques at low cost. The recently introduced TRAP chelator combines the advantage of straightforward design of multimeric 68 Ga-radiopharmaceuticals with very fast and efficient 68 Ga-labeling. We synthesized a series of five cyclo(RGDfK) peptide trimers and determined their αvβ3 integrin affinities in competition assays on αvβ3-expressing M21 human melanoma cells against 125I-echistatin. The compound with highest IC50, Ga-TRAP(RGD)3, showed more than 7-fold higher affinity compared to the monomers F-Galacto-RGD and Ga-NODAGA-c(RGDyK). TRAP(RGD)3 was radiolabeled with 68 Ga in a fully automated GMP compliant manner. CD-1 athymic nude mice bearing M21/M21L human melanoma xenografts were used for biodistribution studies, blockade experiments, metabolite studies and PET imaging. 68 Ga-TRAP(RGD)3 exhibited high M21 tumor uptake (6.08±0.63% ID/g, 60 min p.i.), was found to be fully stable in vivo, and showed a fast renal clearance. Blockade studies showed that uptake in the tumor, as well as in all other tissues, is highly integrin specific. A comparison of biodistribution and PET data of 68 Ga-TRAP(RGD)3 with those of 68 Ga-NODAGA-c(RGDyK) and 18 F-Galacto-RGD showed that the higher affinity of the trimer effects a larger dynamic response of tracer uptake to integrin expression, i.e., enhanced integrin-specific uptake in all tissues. We conclude that 68 Ga-TRAP(RGD)3could allow for imaging of low-level integrin expression in tissues which are not visible with the two competitors. Overall, the study constitutes proof of concept for the favourable in vivo properties of TRAP-based 68 Ga radiopharmaceuticals.

68Ga-NODAGA-RGD is a suitable substitute for 18F-Galacto-RGD and can be produced with high specific activity in a cGMP/GRP compliant automated process

Pohle K, Notni J, Bussemer J, Kessler H, Schwaiger M, Beer AJ

06.02.2012 [Original Artikel]

18F-Galacto-cyclo(RGDfK) is a well investigated tracer for imaging of ανβ3 expression in vivo, but suffers from the drawback of a time consuming multistep synthesis that can hardly be established under GMP conditions. In this study, we present a direct comparison of the pharmacokinetic properties of this tracer with 68Ga-NODAGA-cyclo(RGDyK), in order to assess its potential as an alternative for 18F-Galacto-cyclo(RGDfK). 68Ga labeling of NODAGA-cyclo(RGDyK) was done in full automation using HEPES-buffered eluate of an SnO2 based 68Ga-generator. Using M21 (human melanoma) xenografted BALB/c nude mice, biodistribution studies and micro-PET scans were performed for both 18F-Galacto-cyclo(RGDfK) and 68Ga-NODAGA-cyclo(RGDyK), and for the latter, in vivo stability was assessed. IC50 was determined in a displacement assay on M21 cells against 125I-echistatin. 68Ga-NODAGA-cyclo(RGDyK) was produced with high specific activity (routinely ca. 500 GBq/μmol) within 15 min. IC50 values are similar for both substances. Tracer uptake was similar in ανβ3 positive tumors (1.45%±0.11% ID/g and 1.35%±0.53% ID/g for 68Ga-NODAGA-RGD and 18F-Galacto-RGD, respectively) as well as for all other organs and tissues, with the exception of gall bladder and intestines, where 18F-Galacto-cyclo(RGDfK) uptake was significantly higher, which can be explained by the higher hydrophilicity of 68Ga-NODAGA-cyclo(RGDyK) (logP=−4.0 vs. −3.2 for 18F-Galacto-RGD). Only intact tracer was detected 30 min p.i. in organs and tumor; however, minor amounts of metabolites were found in the urine (6% of total urine activity). 68Ga-labeling of NODAGA-RGD can be performed rapidly and efficiently within 15 min in a GMP compliant process. Similar preclinical results were obtained in comparison with 18F-Galacto-RGD. Therefore, 68Ga-NODAGA-cyclo(RGDyK) is a suitable replacement for 18F-Galacto-cyclo(RGDfK).