In vivo imaging of tumour angiogenesis in mice with the α_vβ₃ integrin-targeted tracer ^99mTc-RAFT-RGD

The molecular imaging of tumour neoangiogenesis currently represents a major field of research for the diagnostic and treatment strategy of solid tumours. Endothelial cells from tumour neovessels overexpress the α_vβ₃ integrin, which selectively binds to Arg-Gly-Asp (RGD)-containing peptides. We evaluated the potential of the novel radiotracer ^99mTc-RAFT-RGD for the non-invasive molecular imaging of α_vβ₃ integrin expression in mice models of tumour development.

^99mTc-RAFT-RGD, ^99mTc-cRGD (specific control) and ^99mTc-RAFT-RAD (non-specific control) were injected intravenously to mice bearing B16F0 or TS/A-pc tumours. In vivo whole-body tomographic imaging and post-mortem biodistribution studies were performed 60 min following tracer injection. Adjacent tumour slices were used to compare the localisation of neovessels from immunostaining and the pattern of ^99mTc-RAFT-RGD uptake from autoradiographic ex vivo imaging.

Biodistribution studies indicated that ^99mTc-RAFT-RGD tumour uptake was significantly higher than that of ^99mTc-RAFT-RAD in B16F0 (2.4±0.5 vs 1.0±0.1%ID/g, respectively) and in TS/A-pc tumours (2.7±0.8 vs 0.7±0.1%ID/g, respectively). Immunohistochemical and autoradiographic studies indicated that ^99mTc-RAFT-RGD intratumoural uptake preferentially occurred in angiogenic areas. Tomographic imaging allowed tumour visualisation following injection of ^99mTc-RAFT-RGD and ^99mTc-cRGD with similar tumour-to-contralateral muscle (T/CM) ratios in B16F0 and in TS/A-pc tumours whereas ^99mTc-RAFT-RAD T/CM ratios did not allow tumour imaging. In accordance with the higher level of α_vβ₃ integrin expression on TS/A-pc tumours than on B16F0 tumours as determined from western blot and immunoprecipitation analyses, the ^99mTc-RAFT-RGD T/CM ratio was significantly higher in TS/A-pc than in B16F0 tumours.

^99mTc-RAFT-RGD allowed the in vivo imaging of α_vβ₃ integrin tumour expression.