18F-Labeled heptapeptide for VEGF-R angiogenesis imaging

Objectives Over-expression of neuropilins (NRP) binding to the angiogenic VEGF receptor has been correlated with angiogenesis and tumor aggressiveness. Specific targeting of NRP could provide radiotracers with broad applications in monitoring of antiangiogenic treatments. The heptapeptide, ATWLPPR (A7R), has previously been shown to bind specifically to NRP-1 and to selectively inhibit VEGF binding to this receptor. We report here the first 18F-labeled derivative of A7R as a positron emission tomography (PET) imaging agent.

Methods [18F]FB-A7R was synthesized by coupling the N-terminal of A7R with N-succinimidyl-4-[18F]fluorobenzoate ([18F]SFB) and purified by HPLC. GS9L tumor cells underexpressing (GS9LVEGF-) or overexpressing (GS9LVEGF+) VEGF were incubated with [18F]FB-A7R for 10 and 40 minutes, washed three times, and measured in a scintillation counter. MicroPET imaging studies were performed with nude mice bearing ETM6 tumors at both sides of the thorax.

Results The total synthesis time was about 180 minutes with 20-30% radiochemical yields and great than 95% radiochemical purity. The results of [18F]FB-A7R binding to cells were shown in Table 1. Tracer retention in 9LVEGF+ cells was approximately 2 times higher compared to 9LVEGF- cells. MicroPET images showed high tumor uptake at early post injection period and then cleared out, but tumor to background (or muscle) ratios were still high.

Conclusions [18F]FB-A7R has been synthesized in moderate yield and high purity. Binding of the radiopeptide showed a clear preference to 9L tumor cells overexpressing VEGF. MicroPET imaging of mice bearing ETM6 tumor showed tumor uptake, but excretion afterwards. Blood metabolism studies indicated that [18F]FB-A7R was degradation in vivo. Structure modification of the radiotracer is under investigation. [18F]FB-A7R may have potential as a PET imaging agent for assessment of angiogenesis

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Table 1. Binding of 18F-A7R to 9L Tumor Cells