Visualization of VEGRFR2 expression in salivary glands by a ⁶⁴Cu-labeled peptoid analog

Objectives Peptoids, oligomers of N-substituted glycines, are a specific subclass of peptidomimetics. A dimeric peptoid, GU40C4, was recently reported as a highly potent antagonist of Vascular Endothelial Growth Factor Receptor 2 (VEGFR2) activation, which inhibits angiogenesis and tumor growth in vivo. Herein we present our recent finding of VEGFR2 expression in salivary glands as revealed by PET imaging with ⁶⁴Cu-labeled GU40C4, which was designed to evaluate the prostate cancer growth in a mouse model.

Methods The VEGFR2 targeting peptoid, GU40C4, and its negative control peptoid were synthesized and conjugated with a bifunctional chelator (DOTA: 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). The in vivo evaluation was performed on a Siemens Inveon PET-CT system using a SCID mouse model bearing PC3 tumors. RNA was extracted from tumors and the submandibular glands and reverse transcribed into cDNA using a commercial kit. PCR was used to evaluate VEGFR2 mRNA expression. VEGFR2 protein expression was assessed by western blotting.

Results The specific activities of both ⁶⁴Cu-labeled peptoids were about 80 GBq/µmol. As expected, the PC3 tumors were clearly visualized by ⁶⁴Cu-DOTA-GU40C4 at 1, 4, and 20 h post injection (p.i.), whereas the negative control peptoid did not show appreciable tumor uptake. Interestingly, we observed a significant accumulation of ⁶⁴Cu-DOTA-GU40C4 in salivary glands (3.17 ± 0.25 %ID/g, 3.00 ± 0.36 %ID/g, 1.83 ± 0.21 %ID/g at 1, 4, and 20 h p.i., respectively), while the negative control uptake level was less than 0.16 ± 0.27 %ID/g. VEGFR2 expression was revealed at the mRNA and protein levels by PCR and western blotting, respectively.

Conclusions Given that both VEGFR1 and VEGFR2 are highly expressed in salivary glands, our imaging observation was not a surprise. However, it further substantiates the potential of developing GU40C4 for VEGFR2 targeted diagnostics and therapeutics.

Research Support NIH grants (P01 DK058398 and U24 CA126608