FITC-Conjugated Cyclic RGD Peptides as Fluorescent Probes for Staining Integrin α_vβ₃/α_vβ₅ in Tumor Tissues

Abstract

This study sought to evaluate FITC-conjugated cyclic RGD peptides (FITC-RGD2, FITC-3P-RGD2, and FITC-Galacto-RGD2) as fluorescent probes for in vitro assays of integrin αvβ3/αvβ5 expression in tumor tissues. FITC-RGD2, FITC-3P-RGD2, and FITC-Galacto-RGD2 were prepared, and their integrin αvβ3/αvβ5 binding affinity was determined using the displacement assay against 125I-echistatin bound to U87MG glioma cells. IC50 values of FITC-Galacto-RGD2, FITC-3P-RGD2, and FITC-RGD2 were calculated to be 28 ± 8, 32 ± 7, and 89 ± 17 nM, respectively. The integrin αvβ3/αvβ5 binding affinity followed a general trend: FITC-Galacto-RGD2 ∼ FITC-3P-RGD2 > FITC-RGD2. The xenografted tumor-bearing models were established by subcutaneous injection of 5 × 106 tumor cells into shoulder flank (U87MG, A549, HT29, and PC-3) or mammary fat pad (MDA-MB-435) of each athymic nude mouse. Three to six weeks after inoculation, the tumor size was 0.1–0.3 g. Tumors were harvested for integrin αvβ3/αvβ5 staining, as well as hematoxylin and eosin (H&E) staining. Six human carcinoma tissues (colon cancer, pancreatic cancer, lung adenocarcinoma, squamous cell lung cancer, gastric cancer, and esophageal cancer) were obtained from recently diagnosed cancer patients. Human carcinoma slides were deparaffinized in xylene, rehydrated with ethanol, and then used for integrin αvβ3/αvβ5 staining, as well as H&E staining. It was found that the tumor staining procedures with FITC-conjugated cyclic RGD peptides were much simpler than those with the fluorescence-labeled integrin αvβ3 antibodies. Since FITC-RGD2, FITC-3P-RGD2, and FITC-Galacto-RGD2 were able to co-localize with the fluorescence-labeled integrin β3 antibody, their tumor localization and tumor cell binding are integrin αvβ3-specific. Quantification of the fluorescent intensity in five xenografted tumors (U87MG, MDA-MB-435, A549, HT29, and PC-3) and six human carcinoma tissues revealed an excellent linear relationship between the relative integrin αvβ3/αvβ5 expression levels determined with FITC-Galacto-RGD2 and those obtained with the fluorescence-labeled anti-human integrin β3 antibody. There was also an excellent linear relationship between the tumor uptake (%ID/g) of 99mTc-3P-RGD2 (an integrin αvβ3/αvβ5-targeted radiotracer) and the relative integrin αvβ3/αvβ5 expression levels from the quantification of fluorescent intensity in the tumor tissues stained with FITC-Galacto-RGD2. These results suggest that FITC-conjugated cyclic RGD peptides might be useful to correlate the in vitro findings with the in vivo imaging data from an integrin αvβ3/αvβ5-targeted radiotracer. The results from this study clearly showed that the FITC-conjugated cyclic RGD peptides (particularly FITC-3P-RGD2 and FITC-Galacto-RGD2) are useful fluorescent probes for assaying relative integrin αvβ3/αvβ5 expression levels in tumor tissues.

Radiolabeled Cyclic RGD Peptide Bioconjugates as Radiotracers Targeting Multiple Integrins

“…41,42 The U87MG glioma cells (Figure 13B) and tumor tissue (Figure 13C) could be stained with FITC-3P-RGD 2 , but not with FITC-3P-RGK 2 under the same conditions. 183 These results clearly show that the radiotracer uptake in tumors indeed is RGD–specific. It must be noted that using the “scrambled” peptides (e.g.…”

“…FITC-Galacto-RGD 2 ) for their potential as fluorescent probes to quantify the total α v β 3 expression levels in both the xenografted tumors and human carcinoma tissues. 183 It was found that the total α v β 3 levels followed a general order: colon cancer > pancreatic cancer > lung adenocarcinoma ≈ squamous cell lung cancer ≫ gastric cancer ≈ esophageal cancer. The same conclusion was also made using the fluorescence-labeled integrin β 3 antibody.…”

Evaluation of [89Zr]-Oxalate as a PET Tracer in Inflammation, Tumor, and Rheumatoid Arthritis Models

“…U87 MG tumors exhibit a high density of tumor neovasculature compared to HT29 cells. 27,28 Similarly, the high accumulation of 89 Zr-oxalate observed in tumor tissues was likely the result of nonspecific targeting.…”

Heterogeneous dimer peptide-conjugated polylysine dendrimer-Fe₃O₄composite as a novel nanoscale molecular probe for early diagnosis and therapy in hepatocellular carcinoma

“…It is known that cell adhesion molecules α v β 3 integrins and vascular endothelial growth factor receptors (VEGFRs) are highly expressed in many tumor tissues, [12][13][14] which are almost undetectable in normal tissues, 15,16 which provides a theoretical basis for synthesizing probes that can specifically bind to the tumor. Several peptide-based probes have been fabricated and have demonstrated promising applications in animal studies, 17,18 and some have even been successfully studied in human clinical trials.…”