Introduction
Angiogenesis, the process by which new blood vessels are formed from the existing vasculature, is an important target and indicator in the therapy and prognosis of cancer, because it is a key feature of malignant solid tumors and plays a critical role in tumor growth, invasion, and metastasis [
1]. This process comprises several steps and is jointly regulated by the nature of the tumor and a complex network of several cell types and various growth factors/receptors and signaling pathways involved in endothelial proliferation, migration, and tube formation [
2,
3]. Several different angiogenesis inhibitors have been developed for blocking different steps of angiogenesis, whereby the formation of new blood vessels can be suppressed and the growth or spread of tumors can be stopped or slowed [
4‐
6]. Several agents have even been approved for therapeutic use in cancer patients and many are undergoing clinical trials. However, despite the rapid progress in drug development, the choice of an appropriate assay to evaluate the treatment response remains a challenge. Unlike cytotoxic chemotherapeutic drugs, angiogenesis inhibitors are typically cytostatic, that is, they slow or stop tumor growth rather than causing tumor shrinkage. Therefore, the routine methods for evaluating chemotherapeutic efficiency, including parameters such as changes in tumor volume or morphology detected by computed tomography (CT) and magnetic resonance imaging (MRI), may not be suitable for assessing the response to antiangiogenic treatment [
7]. Dynamic contrast-enhanced CT and MRI can be used to evaluate tumor blood flow, blood volume, and vascular permeability, but they cannot effectively quantify changes in tumor vascularity [
7]. The impact of molecular imaging on drug evaluation and development has been well reviewed [
8]. Positron emission tomography (PET) with the tracer fluorine-18(
18F)-labeled fluorodeoxyglucose (FDG), or
18F-FDG PET, has been widely used in clinical oncology for diagnosis, staging, and monitoring of treatment effects. This technique is based on the preferential uptake of the tracer by tumors having a high glucose metabolic activity [
9]. However, some studies that used
18F-FDG PET to monitor antiangiogenic efficiency showed no significant change in tumor tracer uptake [
10,
11].
α
Vβ
3 Integrin, which is strongly expressed on activated endothelial cells during angiogenesis, is one of the most extensively studied members of the integrin family, which comprises heterodimeric transmembrane glycoprotein receptors composed of 2 noncovalently associated α and β subunits [
12]. By specifically binding to extracellular matrix proteins via the tripeptide sequence Arg-Gly-Asp (RGD), α
Vβ
3 integrin plays an important role in endothelial cell survival and migration [
12]. Pentapeptides containing cyclic RGD (cRGDs) are optimized synthetic ligands that have a high affinity and selectivity for α
Vβ
3 integrin [
13], and a series of radiolabeled cRGD-containing peptides have been introduced for the noninvasive imaging of α
Vβ
3 integrin expression [
14,
15]. However, since tumor vessels represent about 5 % of the volume of a tumor and α
Vβ
3 integrin is expressed only on activated endothelial cells and not on quiescent endothelial cells, evaluation of angiogenesis by targeting α
Vβ
3 integrin is expected to be quite challenging; it would require a tracer with high specificity, strong affinity, and favorable pharmacokinetics [
16,
17]. Few reports have shown that an RGD-based PET probe can be effectively used to monitor the chemotherapeutic [
18,
19] or antiangiogenic treatment response [
10,
11,
20,
21]. We recently developed a multimeric cRGD-containing RAFT-c(-RGDfK-)
4-based PET probe, known as
64Cu-cyclam-RAFT-c(-RGDfK-)
4. It was synthesized by separately grafting 4 cyclo(-RGDfK-) peptide monomers onto the upper side of the regioselectively addressable functionalized template (RAFT) cyclic decapeptide platform [
22]. Further, it was labeled with
64Cu via the chelating agent cyclam conjugated to RAFT on the lower side [
23,
24]. In the present study, we evaluated the potential of this probe to enable the visualization and quantification of tumor angiogenesis and monitoring of the tumor response to the novel angiogenesis inhibitor TSU-68, a small-molecule oxindole compound.
Materials and methods
Generation of cyclam-RAFT-c(-RGDfK-)4 and radiolabeling with 64Cu
Cyclam-RAFT-c(-RGDfK-)
4 (molecular weight: 4,119.6) was designed and synthesized as reported previously [
23]. The peptide was radiolabeled with
64Cu in accordance with our previous report with minor modifications [
24]. In brief, the peptide was dissolved in 10 % HEPES/90 % MeOH (HEPES, 10 mM, pH 7.0) just before radiolabeling or in dimethyl sulfoxide (DMSO) (the aliquots can be kept at −20 °C for long-term storage).
64CuCl
2 powder was reconstituted in ammonium citrate buffer (100 mM, pH 5.5). The peptide (0.4 mM) and
64CuCl
2 (1.18 MBq/μl) were mixed in a 1/1 (vol/vol) ratio and incubated in a 37 °C water bath for 60 min. The radiolabeling efficiency was determined via reversed phase high-performance liquid chromatography (RP-HPLC) or thin-layer chromatography (TLC) with autoradiography using a bioimaging analyzer according to a previously described method [
24]. It was found to be >99 % for
64Cu-labeled cyclam-RAFT-c(-RGDfK-)
4, and the specific radioactivity was about 3 MBq/nmol. The maximum specific radioactivity that could be achieved for this peptide was ~37 MBq/nmol (unpublished data).
Cell lines and animal tumor models
Human hepatocellular carcinoma HuH-7 cells were purchased from the JCRB Cell Bank (Osaka, Japan) and cultured in RPMI 1640 medium (Wako Pure Chemical Industries, Ltd., Osaka, Japan) supplemented with 10 % fetal bovine serum (Nichirei Biosciences, Inc., Tokyo, Japan), 50 U/ml penicillin, and 50 μg/ml streptomycin (Wako Pure Chemical Industries, Ltd.). α
Vβ
3-Negative HEK293(β
1) and α
Vβ
3-overexpressing HEK293(β
3) cells (kindly provided by J.-F. Gourvest, Aventis, France), stable transfectants of the human embryonic kidney HEK293 cell line with human integrin β
1 and β
3 subunits, respectively, were cultured under conditions described previously [
25]. All the cells were cultured at 37 °C in a humidified 95 % air/5 % CO
2 atmosphere.
Animal procedures were approved by the Institutional Animal Care and Use Committee of the National Institute of Radiological Sciences. Cell suspensions containing 2 × 106 HuH-7, 1 × 107 HEK293(β1), or 2 × 107 HEK293(β3) cells were implanted by subcutaneous (s.c.) injection with a 25-gauge needle into the flank of female athymic nude mice (BALB/cAJcl-nu/nu; CLEA Japan, Inc., Tokyo, Japan). The suspensions also contained 50 vol % Matrigel (BD Biosciences, Bedford, MA) to facilitate tumor development. Mice with tumors of diameter approximately 5–10 mm were selected for subsequent experiments.
Flow cytometric analysis
The expression of α
Vβ
3 integrin on the surface of HuH-7 cells was analyzed by labeling the cells with R-phycoerythrin-conjugated anti-human α
Vβ
3 monoclonal antibody (clone LM609; Millipore Corporation, Temecula, CA), which was detected using the Guava EasyCytePlus Flow Cytometry System (Millipore Corporation) according to a previously described method [
24]. α
Vβ
3-Negative HEK293(β
1) and α
Vβ
3-overexpressing HEK293(β
3) cells were simultaneously analyzed as the negative and positive controls, respectively.
Immunohistochemical and histological analyses
Frozen tumor sections (7-μm thick) were fixed in acetone, stained with mouse anti-human αVβ3 monoclonal antibody (LM609; 1:100 dilution; Chemicon, Temecula, CA), and examined using the M.O.M. Immunodetection Kit (Vector Laboratories, Inc., Burlingame, CA). Purified rat anti-mouse CD31 monoclonal antibody (1:1,500 dilution; BD Biosciences) bound to biotinylated rabbit anti-rat secondary antibody (1:600 dilution; Dako, Glostrup, Denmark) was used for microvessel staining, and the Histostain-plus Bulk Immunostaining Kit (LAB-SA Detection System; Invitrogen, Camarillo, CA) was used for detection, with diaminobenzidine (DAB) as the chromogen. Nuclei were counter-stained with hematoxylin. Serial sections of the tumors were stained with hematoxylin and eosin (HE) for histological examination.
Antiangiogenesis therapy
TSU-68 (synonym: SU6668; molecular weight: 310.35) was purchased from Selleck Chemicals (Houston, TX). It was dissolved in DMSO at 30 mg/ml, and aliquots were stored at −20 °C until use. For each set of experiments, mice bearing HuH-7 tumors were divided into 2 groups (n = 4–6). The treatment group received intraperitoneal (i.p.) injections of TSU-68 (75 mg kg−1 d−1 in 50 μl of DMSO) for 14 days (days 1–14), and the control animals received i.p. injections of the vehicle alone (50 μl of DMSO). Every 2 days throughout the experiment, the body weight (g) was recorded, and the tumors were measured simultaneously by using a vernier caliper. Tumor volume (mm3) was determined using the formula 0.5 × length × width2, and the fold change in volume was calculated by dividing the obtained value by the tumor volume on the day before the treatment was started (day 0). The therapeutic response to TSU-68 was assessed on the day after the final drug injection (day 15).
Immunofluorescence staining and MVD measurement
Frozen tumor sections (7- or 30-μm thick) were fixed in acetone, stained with purified rat anti-mouse CD31 monoclonal antibody, and visualized using Alexa Fluor 488-conjugated goat anti-rat antibody (1:200 dilution; Invitrogen). Unlike the pan-endothelial cell marker CD31, CD105 is reported to be specifically an activated endothelial cell marker [
26]. To compare the pattern of staining between them, serial sections (7-μm thick) were stained with the CD31 antibody or rat anti-mouse CD105 monoclonal antibody (1:500 dilution; BD Biosciences) and visualized using Alexa Fluor 488-conjugated goat anti-rat secondary antibody. Double staining was conducted for CD31 and CD61, the mouse β
3 integrin subunit, wherein 7-μm-thick sections were simultaneously treated with the CD31 antibody and purified Armenian hamster anti-mouse CD61 antibody (1:50 dilution; BD Biosciences) and then coincubated with Alexa Fluor 594-conjugated goat anti-rat secondary antibody (1:200 dilution; Invitrogen) and Alexa Fluor 488-conjugated goat anti-Armenian hamster secondary antibody (1:100 dilution; Jackson Immunoresearch, West Grove, PA). The slides were then mounted with mounting agent (Dapi-Fluoromount-G™; SouthernBiotech, Birmingham, AL) containing 4′,6-diamidino-2-phenylindole (DAPI) for nucleus staining. Fluorescence images were acquired with an epifluorescence microscope (Olympus X61) equipped with an image tiling system (e-Tiling; Mitani Corporation, Fukui, Japan), which enabled the creation of a high-resolution image depicting whole tumor sections from separately captured photos.
The interwoven microvessel network observed in the HuH-7 tumor sections made it difficult to count the number of CD31-positive vessels. Therefore, the percentage of the CD31-stained area versus the area of the entire section, as assessed using the WinROOF image analysis software (version 6.5; Mitani Corporation), was used to express the relative microvessel density (MVD) (%) of the tumor.
Biodistribution study
HuH-7 tumor-bearing mice received tail vein (intravenous; i.v.) injections of 0.74 MBq 64Cu-cyclam-RAFT-c(-RGDfK-)4, and at the indicated times of 1 and/or 3 h postinjection (p.i.), the mice were sacrificed, and the tumor and normal organs of interest were collected, weighed, and processed for radioactivity counting using a γ-counter with decay correction. The radioactivity uptake in the tumor and normal organs was expressed as a percentage of the injected dose per gram of tissue (% ID/g) normalized to a mouse body weight of 20 g.
PET imaging
For PET imaging, the mice were i.v. injected with 11.1 MBq 64Cu-cyclam-RAFT-c(-RGDfK-)4, and at the indicated times (1 and/or 3 h p.i.,), static scans were acquired for 30 min using a small-animal PET system (Inveon; Siemens Medical Solutions USA, Inc., Malvern, PA) with the animals under 1 % isoflurane anesthesia. The acquired three-dimensional emission data were reconstructed using a maximum a posterior (MAP) reconstruction method with attenuation correction. Image display and analysis were performed using the ASIPro VM Micro PET Analysis software (Siemens Medical Solutions USA, Inc.). The radioactivity uptake in the tumor, expressed as standardized uptake value (SUV; corrected for body weight and injected radioactivity), was determined by selecting regions of interest (ROIs) encompassing the tumor on each of transverse frames and then using the 3D (VOI) dimensionality tool to link all the drawn ROIs to form a volume of interest. The maximum, mean, and minimum SUV (SUVmax, SUVmean, and SUVmin, respectively) were then acquired automatically.
Autoradiographic examination
After PET imaging, the mice were sacrificed, and the tumors and kidneys were excised, embedded in Tissue-Tek OCT compound (Sakura Finetek, Torrance, CA), and frozen by immersion in n-hexane pre-cooled at −80°. Next, 30-μm-thick tumor sections or 5-μm-thick kidney sections were air-dried and exposed to an imaging plate (BAS-MS 2040; Fujifilm Co. Ltd., Tokyo, Japan), and after overnight exposure at −80°, the plate was scanned using a bioimaging analyzer (FLA-7000; Fujifilm Co. Ltd.). The sections were then stored at −80 °C until the radioactivity decayed to negligible levels, after which immunofluorescence staining was conducted.
Statistical analysis
Quantitative data were given as mean ± standard deviation (SD). Statistical comparison between groups was performed using the unpaired t test, and P < 0.05 was considered significant. Correlation analysis was performed using Kaleida Graph (Synergy Software, Reading, PA).
Discussion
α
Vβ
3 Integrin is strongly expressed on activated endothelial cells during angiogenesis and on some types of tumor cells [
27]. Therefore, the α
Vβ
3 integrin expression levels in a tumor may predominantly be associated with its vasculature or with the vasculature and α
Vβ
3-positive tumor cells. Thus far, an increasing number of radiolabeled cRGD-containing peptides have been developed using various strategies for peptide modification and radiolabeling techniques for imaging α
Vβ
3 integrin [
14]. Since most studies used α
Vβ
3-positive tumor cells to establish the tumor model, and tumor vasculature represents about 5 % of the volume, the effectiveness of the RGD-based PET probe to detect angiogenesis requires greater validation. In the present study, we selected tumor xenografts derived from the α
Vβ
3-negative tumor cell line HuH-7 to eliminate interference from α
Vβ
3 integrin expressed on the tumor cells themselves. Another reason we selected this model is that it showed strong angiogenesis when subcutaneously transplanted into nude mice. RAFT-c(-RGDfK-)
4 with or without radio- or fluorescent dye labeling has been shown to have higher α
Vβ
3 integrin-binding specificity and enhanced binding activity than its monomeric analog in both in vitro and in vivo studies using different cell lines and murine tumor xenografts [
22‐
25,
28‐
30]. Here, we prove that
64Cu-cyclam-RAFT-c(-RGDfK-)
4 PET enabled visualization of tumor angiogenesis by targeting α
Vβ
3 integrin. The imaging quality was good, and this finding was supported by a biodistribution assay, which showed high tumor-to-blood and tumor-to-muscle ratios (31.6 and 6.7, respectively, at 3 h p.i.). Our results are in agreement with those of Haubner et al. [
31], showing that
18F-galacto-RGD PET using the α
Vβ
3-negative A431 tumor model can enable the visualization of α
Vβ
3 integrin expression resulting exclusively from tumor vasculature in mice.
As mentioned earlier, a few studies have reported the use of radiolabeled cRGD-containing peptides to monitor the response of tumors to angiogenesis inhibitors in animal models [
10,
11,
20,
21]. Battle et al. [
21] showed the inhibitory effect of sunitinib, a Food and Drug Administration (FDA) approved multi-targeted receptor tyrosine kinase (RTK) inhibitor, on the uptake of
18F-fluciclatide (formerly known as
18F-AH111585) by α
Vβ
3-positive U87MG tumors as revealed by repeated PET imaging during the 2-weeks treatment period; they also found a reduction in the tumor MVD at the end of the therapy. Compared with the monomeric peptide tracer of
18F-fluciclatide, the dimeric peptide FPPRGD2 labeled with
18F showed higher α
Vβ
3-specific accumulation and better pharmacokinetics [
32]. Chen’s group reported the use of
18F-FPPRGD2 for PET imaging of the tumor response to antiangiogenic therapy with ZD4190 [
11] or Abraxane [
10] in mice bearing α
Vβ
3-positive MDA-MB-435 breast tumors. ZD4190, an RTK inhibitor of vascular endothelial growth factor (VEGF) receptor, appeared to exert an acute effect on α
Vβ
3 integrin expression by downregulating the levels on both tumor cells and the endothelium as early as 2 h after the first drug administration, and this finding corresponded with reduced tumor
18F-FPPRGD2 uptake. Abraxane, an FDA-approved nanoparticle albumin-bound paclitaxel, showed antiangiogenic activity at a low dose (25 mg kg
−1). Significantly decreased tumor tracer uptake was found as early as 3 days after a single dose of Abraxane administration, correlating with a significant reduction in the β
3 integrin expression on the endothelium and no change in α
Vβ
3 integrin expression on the tumor cells themselves. Collectively, the results demonstrate that PET with an RGD peptide tracer is promising for the assessment of antiangiogenic effects. However, they also indicate that some antiangiogenic drugs such as ZD4190 may affect tumor cells expressing α
Vβ
3 integrin, making it difficult to directly correlate the reduction in tumor tracer uptake with the reduction in tumor MVD.
In our previous study, we found a strong and positive correlation between tumor uptake levels of
64Cu-cyclam-RAFT-c(-RGDfK-)
4 and the corresponding α
Vβ
3 integrin expression levels quantified by SDS-PAGE/autoradiography [
24]. Here, we used this tracer to monitor the tumor response to the antiangiogenic drug TSU-68, a novel and selective RTK inhibitor of the angiogenic receptors for VEGF, platelet-derived growth factor, and fibroblast growth factor. Preclinical studies have shown that oral or i.p. administration of TSU-68 resulted in significant growth inhibition of tumor xenografts of diverse origins [
33]. TSU-68 clinical trials are ongoing for patients with different advanced cancers such as hepatocellular carcinoma, gastrointestinal cancer, breast cancer, and non-small cell lung cancer. In an experimental study of C6 glioma xenografts in mice, daily i.p. administration of 75 mg kg
−1 d
−1 TSU-68 in 50 μl of DMSO resulted in a significant reduction in the tumor MVD after >10 days of therapy [
33]. A similar TSU-68 dosage was used in the present study, and it significantly delayed tumor growth and reduced tumor MVD in mice bearing HuH-7 xenografts after 14 days of therapy. The results obtained from the same set of experiments showed that the reduction in tumor MVD induced by TSU-68 treatment was accompanied by a reduction in the tumor SUV as quantified by PET imaging, and these results were further supported by those of the biodistribution assay. More importantly, a positive and significant correlation was found between the tumor MVD and the corresponding SUV (either the mean or maximum value) or % ID/g of tumor uptake evaluated using the biodistribution assay. The intratumoral colocalization of the tracer and vascular network distribution and the colocalization of CD31 and murine β
3 integrin support these results, which strongly demonstrate that the antiangiogenic effects of TSU-68 can be monitored by quantitative
64Cu-cyclam-RAFT-c(-RGDfK-)
4 PET imaging. In clinical studies, the treatment cycle for TSU-68 is usually daily administration for 28 days, and some patients even took the drug for >6 months [
34,
35]. In the present small animal imaging study,
64Cu-cyclam-RAFT-c(-RGDfK-)
4 PET showed significant changes in tumor tracer uptake after 2 weeks of drug administration. Although it is difficult to predict how early the antiangiogenic effect can be imaged, we believe that early detection is possible since the decrease in the tumor growth rate was observed as early as day 3 (after 2 injections of the drug) and because the decreasing tendency of tumor tracer uptake was observed after only 1 day of drug injection. In addition to its antiangiogenic effect on endothelial cells, the direct effect of TSU-68 on tumor cell proliferation must be recognized. Periodic imaging during the course of treatment would be valuable to determine the time point at which the response can be monitored, and we intend to conduct such a study in the future. The value of RGD PET for assessing and evaluating antiangiogenic treatment is that it is not only noninvasive and quantitative but also can evaluate the activation status of the tumor vasculature by targeting α
Vβ
3 integrin expressed on proliferating endothelial cells. Moreover, it helps determine whether an antiangiogenic drug is effective in individuals by enabling the assessment of alterations in tracer uptake not only in terms of the intensity but also the distribution.
Two weeks of TSU-68 administration did not cause a significant reduction in body weight, and no other side effects were noted. However, a biodistribution assay for 64Cu-cyclam-RAFT-c(-RGDfK-)4 showed that the radioactivity accumulated in most of the normal organs of the TSU-68-treated mice was significantly reduced. One possible explanation for this is that TSU-68 indirectly affects the expression pattern and/or activation state of integrins despite their low levels in normal tissues. Autoradiographic examination and immunostaining of endothelium/murine β3 integrin in kidney sections indicated that the reduced renal radioactivity was not due to the antiangiogenic effect of TSU-68, supporting this explanation.
The maximum radioactivity accumulation of
64Cu-cyclam-RAFT-c(-RGDfK-)
4 was found in the kidneys. The negligible levels of β
3 integrin expression in the kidney together with our previous finding [
24] that
64Cu-cyclam-RAFT-c(-RGDfK-)
4 renal radioactivity could not be blocked by excessive amounts of “cold” α
Vβ
3-specific RGD peptides strongly supported the conclusion that the kidney acts as the main excretory organ for
64Cu-cyclam-RAFT-c(-RGDfK-)
4. Renal radioactivity was observed predominantly in the cortex, the same site as that reported for other radiolabeled peptides [
36,
37], such as
111In-DOTA,Tyr
3-octreotate, whose radioactivity retention was significantly reduced by the coadministration of Gelofusine, a gelatin-based plasma expander [
36]. It would be worthwhile to determine whether Gelofusine is also effective in the case of
64Cu-cyclam-RAFT-c(-RGDfK-)
4, since the decrease in renal radioactivity may contribute to improving imaging quality, especially for disorders of the abdomen. Gelofusine may also reduce the risk of nephrotoxicity when
64Cu (
67Cu)-cyclam-RAFT-c(-RGDfK-)
4 is used for internal radiotherapy.
18F-labeled RGD peptides including galacto-RGD [
31], fluciclatide [
38], and FPPRGD2 [
39] have been used to treat cancer patients, and the FDA has approved the use of
18F-FPPRGD2 PET/CT imaging for evaluation of antiangiogenic therapy in solid tumors. However, the procedures for preparing such tracers are rather complex and time consuming, which limits their widespread clinical use [
40]. We previously reported
64Cu-radiolabeling of cyclam-RAFT-c(-RGDfK-)
4 [
24] and then made further improvements to achieve a specific radioactivity as high as ~37 MBq/nmol with a labeling efficiency >99 %, making purification unnecessary. Further, metabolic analysis of this agent in mice showed its high in vivo stability (unpublished data). Moreover, the labeling procedure is straightforward: the mixture of peptide and
64Cu only needs to be incubated at 37 °C for less than 60 min. The ease of preparation, high radiochemical yield, and high metabolic stability make
64Cu-cyclam-RAFT-c(-RGDfK-)
4 a promising PET probe for α
Vβ
3 integrin imaging.
In conclusion, we used murine xenografts from an αVβ3-negative tumor cell line and showed that 64Cu-cyclam-RAFT-c(-RGDfK-)4 PET enables the clear visualization of tumor angiogenesis and helps monitor the effectiveness of antiangiogenic therapy. In future studies, we intend to determine whether this strategy is effective for tumors in which αVβ3 is expressed on both tumor cells and the neovasculature by using longitudinal PET imaging to detect not only changes in tracer uptake but also changes in the tracer distribution pattern. With advances in technology, the spatial resolution of PET is expected to improve greatly to enable clear visualization of intratumoral tracer distribution. Because of the good imaging quality and easy preparation, 64Cu-cyclam-RAFT-c(-RGDfK-)4 is a promising PET tracer for tumor angiogenesis imaging. Further, it may also be applicable for monitoring angiogenic therapy in other angiogenesis-associated disorders such as ischemia, atherosclerosis, and myocardial infarction.