PET imaging of apoptosis in tumor-bearing mice and rabbits after paclitaxel treatment with (18)F(-)Labeled recombinant human His10-annexin V

Abstract

Monitoring response to chemo- or radiotherapy is of great importance in clinical practice. Apoptosis imaging serves as a very useful tool for the early evaluation of tumor response. The goal of this study was PET imaging of apoptosis with (18)F-labeled recombinant human annexin V linked with 10 histidine tag ((18)F-rh-His10-annexin V) in nude mice bearing an A549 tumor and rabbits bearing a VX2 lung cancer after paclitaxel therapy. (18)F-rh-His10-annexin V was prepared by conjugation of rh-His10-annexin V with N-succinimidyl 4-[(18)F]fluorobenzoate. Biodistribution was determined in mice by the dissection method and small-animal PET. Single-dose paclitaxel (175 mg/m(2)) was used to induce apoptosis in A549 and VX2 tumor models. (18)F-rh-His10-annexin V was injected into A549 mice and VX rabbits to acquire dynamic and static PET images 72 h after paclitaxel treatment. The uptake of (18)F-rh-His10-annexin V in apoptotic cells 4 h after induction was 6.45±0.52 fold higher than that in non-induced cells. High focal uptake of (18)F-rh-His10-annexin V was visualized in A549 (SUVmax: 0.35±0.13) and VX2 (0.41±0.23) tumor models after paclitaxel treatment, whereas lower uptake was found in the corresponding tumors before treatment (A549 SUVmax: 0.04±0.02; VX2: 0.009±0.002). The apoptotic index was 75.61±11.56% in the treated VX2 cancer, much higher than that in the untreated VX2 (8.03±2.81%). This study demonstrated the feasibility of (18)F-rh-His10-annexin V for the detection of apoptosis after chemotherapy in A549 and VX2 tumor models.

Keywords: Apoptosis; molecular imaging; recombinant human His10-annexin V; tumor response.

Figure 1

A: Synthesis scheme of ¹⁸F-rh-His₁₀-annexin V. Reaction conditions include a: 110°C, CH₃CN, 15 min; b: 95°C, 10 min; c: (CH₃)₄NOH, 90°C, CH₃CN, 5 min; d: 0.1 M borate buffer, room temperature, 20 min. B: Lane a, marker. Lane b, Wild type annexin V. Lane c, Recombinant His₁₀-annexin V. Lane d. Autoradiography.

Figure 2

In vitro binding assay indicated that ¹⁸F-rh-His₁₀-annexin V could bind to apoptotic cells specifically. Radiouptake in paclitaxel treatment cells was significantly higher than that in control, at 4 h, 6 h after treatment.

Figure 3

A: Biodistribution of ¹⁸F-rh-His₁₀-annexin V in mice at 15 min, 1 h and 2 h after injection (n=3). The radioactivity was expressed as ID %/g. B: Time-radioactivity curves in main organs were acquired with small-animal PET, the radioactivity was expressed as SUV.

Figure 4

Representative PET images of ¹⁸F-rh-His₁₀-annexin V in A549 tumor model (A). Left: Image in the tumor before treatment, Middle and Right: after paclitaxel treatment (Middle, same xenegraft with before treatment, Right, another mouse xenograft). Histological staining (B): Much more apoptotic cells were presented in the tumor after treatment in comparison of baseline image. The tumor was indicated by white arrow.

Figure 5

A: ¹⁸F-FDG image: Focal uptake was showed in the VX2 lung cancer before treatment. B: ¹⁸F-rh-His₁₀-annexin V in transverse, coronal and sagittal image showed intense uptake in the tumor 72 h after treatment. The tumor was indicated by yellow arrow head.

Figure 6

Histological staining in VX2 Lung cancer showed a small number of apoptotic cells were found before treatment, whereas a large number of apoptotic cells were observed after the treatment. TUNEL-positive nuclei confirmed the DNA fragmentation.