99mTc-labeled SWL specific peptide for targeting EphA2 receptor

IntroductionEphA2, one member of the Eph receptor family, is widely expressed in multiple aggressive cancers. SWL, a small peptide identified by phage display, has high binding affinity to EphA2, suggesting that it could be exploited for targeted molecular imaging. Therefore, a novel peptide-based probe, 99mTc-HYNIC-SWL, was developed and its potential to specifically target EphA2-positive tumors was investigated.MethodsThe SWL peptide was labeled with hydrazinonicotinic acid (HYNIC), followed by 99mTc labeling. Immunofluorescence staining was carried out to detect the expression of EphA2 in A549 lung cancer cells and OCM-1 melanoma cells. Saturation binding experiments were performed by incubating A549 cells with increasing concentrations of radiolabeled peptide in vitro. To test the probe in vivo, nude mice bearing either A549 or OCM-1 derived tumors were established, injected with 99mTc-HYNIC-SWL, and subjected to SPECT imaging. Mice injected with excess unlabeled SWL were used as a specific control. Ex vivo γ-counting of dissected tissues from the mice was also performed to evaluate biodistribution.ResultsImmunofluorescence staining showed that A549 cells intensively expressed EphA2, while OCM-1 cells had little expression. 99mTc-HYNIC-SWL displayed high binding affinity with A549 cells (KD = 2.6 ± 0.7 nM). From the SPECT images and the results of the biodistribution study, significantly higher uptake of the tracer was seen in A549 tumors (1.44 ± 0.12 %ID/g) than in OCM-1 tumors (0.43 ± 0.20 %ID/g) at 1 h after injection. Pre-injection with excess unlabeled peptide in A549-bearing nude mice, significantly reduced tumor uptake of the radiolabeled probe (0.58 ± 0.20 %ID/g) was seen. These data suggest that 99mTc-HYNIC-SWL specifically targets EphA2 in tumors.ConclusionsThe expression of EphA2 can be noninvasively investigated using 99mTc-HYNIC-SWL by SPECT imaging. The in vitro and in vivo characteristics of 99mTc-HYNIC-SWL make it a promising probe for EphA2-positive tumor imaging.