Molecular Imaging for the in Vivo Monitoring of Angiogenesis in a Hindlimb Ischemia Animal Model

ABSTRACT

Purpose: Integrin alph beta is a promising imaging target of angiogenic activity which is up-regulated on activated but not on quiescent endothelial cells. Molecular imaging of alph beta integrin expression with the aid of a dedicated high resolution gamma camera, is a very sensitive imaging approach for the evaluation of angiogenesis in the rabbit hindlimb ischemia model. Furthermore, in order to evaluate the whole spectrum of endogenous process of collateralization after occlusion of an rtery, Digital Subtraction Angiography [DSA] was also used for the visualization of larger collaterals. Methods: The study included seven New Zealand White rabbits that underwent unilateral percutaneous endovascular embolization of the femoral artery, for the establishment of hindlimb ischemia that triggers the endogenous process of collateralization. The contralateral limb was not embolized and served as a control. The radiotracer that was employed for the angiogenesis imaging, was a 99 mTc labeled cyclic RGD peptide [[c RGDfk-His]-99mTc] that binds specifically to alph beta integrin via a three amino acid sequence [Arginine-Glycine-Aspartic acid or RGD]. Image acquisition was performed with a high resolution gamma camera and all animals underwent molecular imaging on the 3rd day and the 9th day post-embolization. In all animals DSA was performed on the 9th day post-embolization. Results: The acquired images demonstrated the retention of the radiotracer at the ischemic tissue is remarkably increased compared to the non-ischemic hindlimb [normal limb] [mean value 16020 +/- 2309 vs. 13139 +/- 2493 on day 3; p=0.0014 and 21616 +/- 2528 vs. 13362 +/- 2529 on day 9; p<0.0001, respectively. In addition, radiotracer retention in normal limbs seemeds to be increased at day 9 in normal limbs compared to day 3 [p=0.0112]. DSA demonstrated the mean vessel length detected was significantly superior in the normal compared to the ischemic limb at day 9 [mean value 3680 +/- 369.8 vs. 2772 +/- 267.7; p< 0.0001, respectively]. Conclusion: Angiogenesis was successfully detected using a 99 mTc labeled cyclic RGD peptide molecular imaging technique and was significantly more pronounced in the ischemic compared to normal limbs, both at 3rd and 9th days after embolization. The peak of the phenomenon was detected at 9?th days. Finally increased retention of radiotracer in normal limbs at day 9 indicates presence and gradual accumulation of activated endothelium in normal tissues as well