Arginine-glycine-aspartic acid (RGD)-based imaging tracers allow specific imaging of integrin a v b 3 , a protein overexpressed during angiogenesis, leading to the possibility that it might serve as a tool to stratify patients for antiangiogenic treatment. However, these tracers have generally been characterized in xenograft models in which integrin a v b 3 was constitutively expressed by the tumor cells themselves. In the studies presented here, the use of 111 In-RGD 2 as a tracer to image only integrin a v b 3 expression on blood vessels in the tumor was determined using tumor xenografts in which tumor cells were integrin a v b 3 -negative. Methods: DOTA-E-[c(RGDfK)] 2 was radiolabeled with 111 In ( 111 In-RGD 2 ), and biodistribution studies were performed in squamous cell carcinoma of the head and neck (HNSCC) xenograft mouse models to determine the optimal peptide dose to image angiogenesis. Next, biodistribution and imaging studies were performed at the optimal peptide dose in 3 HNSCC mouse models, FaDu, SCCNij3, and SCCNij202. Immunohistochemical analysis of tumor vascular and cell surface expression of integrin a v b 3 and correlation analysis of vascular integrin a v b 3 and autoradiography were completed. Results: All 3 HNSCC xenografts expressed integrin a v b 3 on the vessels only. The optimal peptide dose of 111 In-RGD 2 was 1 mg or less for specific integrin a v b 3 -mediated uptake of the tracer. SPECT/CT imaging showed clear uptake of the tracer in the periphery of the tumors, corresponding with wellvascularized areas of the tumor. Within the tumor, 111 In-RGD 2 autoradiography coincided with vascular integrin a v b 3 expression, as determined immunohistochemically. Integrin a v b 3 -mediated uptake was also detected in nontumor tissues, which, through immunohistochemical analysis, proved positive for integrin a v b 3 . Conclusion: 111 In-RGD 2 allows the visualization of integrin a v b 3 in xenograft models in which integrin a v b 3 is expressed only on the neovasculature, such as in the HNSCC tumors. Thus, 111 In-RGD 2 allows specific visualization of angiogenesis in tumor models lacking constitutive tumoral integrin a v b 3 expression but may be less useful for this purpose in many tumors in which tumor cells express integrin a v b 3 .
a Overexpression of the epidermal growth factor receptor (EGFR) is linked to an adverse outcome in various solid tumors. Cetuximab is an EGFR inhibitor, which in combination with radiotherapy improves locoregional control and survival in a subgroup of patients with head and neck squamous cell carcinomas (HNSCCs). The aim of this study was to develop and characterize an EGFR-directed PET tracer, 64 Cu-cetuximab-F(ab′) 2 , to determine the systemic accessibility of EGFR. Mice with HNSCC xenografts, UT-SCC-8 (n = 6) or UT-SCC-45 (n = 6), were imaged 24 h post injection with 64 Cu-NODAGA-cetuximab-F(ab′) 2 using PET/CT. One mouse for each tumor model was co-injected with excess unlabeled cetuximab 3 days before radiotracer injection to determine non-EGFR-mediated uptake. Ex vivo biodistribution of the tracer was determined and tumors were analyzed by autoradiography and immunohistochemistry. The SUV max of UT-SCC-8 tumors was higher than that of UT-SCC-45: 1.5 ± 1.0 and 0.8 ± 0.2 (p < 0.05), respectively. SUV max after in vivo blocking of EGFR with cetuximab was 0.4. Immunohistochemistry showed that UT-SCC-8 had a significantly higher EGFR expression than UT-SCC-45: 0.50 ± 0.19 versus 0.12 ± 0.08 (p < 0.005), respectively. Autoradiography indicated that 64 Cu-cetuximab-F(ab′) 2 uptake correlated with EGFR expression in both tumors: r = 0.86 ± 0.06 (UT-SCC-8) and 0.90 ± 0.06 (UT-SCC-45).64 Cu-cetuxmab-F(ab′) 2 is a promising PET tracer to determine expression of EGFR in vivo. Clinically, this tracer has the potential to be used to determine cetuximab targeting of tumors and possibly to non-invasively monitor the response to EGFR-inhibitor treatment.
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