18F-labeled mini-PEG spacered RGD dimer (18F-FPRGD2): synthesis and microPET imaging of alphavbeta3 integrin expression

Abstract

Purpose: We have previously reported that (18)F-FB-E[c(RGDyK)](2) ((18)F-FRGD2) allows quantitative PET imaging of integrin alpha(v)beta(3) expression. However, the potential clinical translation was hampered by the relatively low radiochemical yield. The goal of this study was to improve the radiolabeling yield, without compromising the tumor targeting efficiency and in vivo kinetics, by incorporating a hydrophilic bifunctional mini-PEG spacer.

Methods: (18)F-FB-mini-PEG-E[c(RGDyK)](2) ((18)F-FPRGD2) was synthesized by coupling N-succinimidyl-4-(18)F-fluorobenzoate ((18)F-SFB) with NH(2)-mini-PEG-E[c(RGDyK)](2) (denoted as PRGD2). In vitro receptor binding affinity, metabolic stability, and integrin alpha(v)beta(3) specificity of the new tracer (18)F-FPRGD2 were assessed. The diagnostic value of (18)F-FPRGD2 was evaluated in subcutaneous U87MG glioblastoma xenografted mice and in c-neu transgenic mice by quantitative microPET imaging studies.

Results: The decay-corrected radiochemical yield based on (18)F-SFB was more than 60% with radiochemical purity of >99%. (18)F-FPRGD2 had high receptor binding affinity, metabolic stability, and integrin alpha(v)beta(3)-specific tumor uptake in the U87MG glioma xenograft model comparable to those of (18)F-FRGD2. The kidney uptake was appreciably lower for (18)F-FPRGD2 compared with (18)F-FRGD2 [2.0 +/- 0.2%ID/g for (18)F-FPRGD2 vs 3.0 +/- 0.2%ID/g for (18)F-FRGD2 at 1 h post injection (p.i.)]. The uptake in all the other organs except the urinary bladder was at background level. (18)F-FPRGD2 also exhibited excellent tumor uptake in c-neu oncomice (3.6 +/- 0.1%ID/g at 30 min p.i.).

Conclusion: Incorporation of a mini-PEG spacer significantly improved the overall radiolabeling yield of (18)F-FPRGD2. (18)F-FPRGD2 also had reduced renal uptake and similar tumor targeting efficacy as compared with (18)F-FRGD2. Further testing and clinical translation of (18)F-FPRGD2 are warranted.

Fig. 1

Chemical structures of ¹⁸F-FRGD2 (a) and ¹⁸F-FPRGD2 (b). The only difference between the two structures is the mini-PEG spacer

Fig. 2

a Serial microPET images of U87MG tumor-bearing mice after intravenous injection of ¹⁸F-FPRGD2. b For direct visual comparison, serial microPET images of U87MG tumor-bearing mice after intravenous injection of ¹⁸F-FRGD2 are also shown. c Coronal and sagittal microPET images of a U87MG tumor-bearing mouse 1 h after co-injection of ¹⁸F-FRGD2 and a blocking dose of c(RGDyK). Note that the scale (0–2.5%ID/g) is different from those in a and b (0–5%ID/g). d MicroPET images of a c-neu oncomouse after intravenous injection of ¹⁸F-FPRGD2. Arrows indicate tumors in all cases

Fig. 3

Time-activity curves of major organs after intravenous injection of ¹⁸F-FPRGD2

Fig. 4

Comparison between ¹⁸F-FRGD2 and ¹⁸F-FPRGD2 in U87MG tumor (a), kidney (b), liver (c), and muscle (d) over time

Fig. 5

Metabolic stability of ¹⁸F-FPRGD2 in mouse blood and urine samples and in liver, kidney, and U87MG tumor homogenates at 1 h after injection. The HPLC profile of pure ¹⁸F-FPRGD2 (Standard) is also shown