PET imaging of tumor vascular normalization in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a vascular tumor characterized by hypervascularity and marked vascular abnormalities. The hypervascular nature of the tumor highlights the importance of angiogenesis in the pathobiology of HCC [1]

severe vascular regression will increase tumor metastasis and relapse [8,9]. Indeed, restoring appropriate vascular normalization is crucial for increasing the benefits of anticancer therapy [10,11]. Pericyte coverage is necessary to stabilize immature endothelial tubes, contributing to vascular quiescence and integrity [11]. Normalization of vessels by improving pericyte coverage and restoring cell junctions leading to increased tumor perfusion would limit tumor hypoxia, prevent the selection of tumor cells with stronger invasive phenotypes, and improve drug delivery and efficacy [11,12].
We read with great interest the paper by Cai et al. recently published in the European Journal of Nuclear Medicine and Molecular Imaging investigating PDGFRβ-targeting PET imaging of HCC by using 68 Ga-labeled trimeric affibody-Z TRI [13]. Based on the successful development of monomeric and dimeric affibody probes against PDGFRβ [14,15], the diagnostic efficacy of [ 68 Ga]Ga-DOTA-Z TRI was further investigated in this work. This is a significant preclinical study for evaluating PDGFRβ-expressing pericytes in HCC models since HCC is a solid carcinoma with highly abnormal and dysfunctional vasculature. Transcriptomic data and protein expression profiles were analyzed first, which demonstrated that a series of typical angiogenesis-associated genes were strongly correlated to HCC and PDGFRβ was one of the most hyperactive genes suitable for vesseltargeted molecular imaging of HCC. Subsequently, through in vitro and in vivo experiments, the authors demonstrated the specific binding of [ 68 Ga]Ga-DOTA-Z TRI to PDGFRβexpressing pericytes. The selection of tumor models is crucial for studying tumor angiogenesis. There are differences in the vasculature and microenvironment of primary tumor grafts and subcutaneous tumor xenografts [16]. Diethylnitrosamine (DEN)-induced hepatocarcinoma in various animals presents histopathological similarities to that of human HCC, serving as standard models for studying the beneficial effects of many drugs and treatments on HCC [17]. Both DEN-induced primary HCC rats and idiopathical HCC rhesus monkeys used in this study are certainly strong points. Furthermore, the calculated effective dose based on the primate models demonstrated that [ 68 Ga]Ga-DOTA-Z TRI was ready for clinical translation.
Previous imaging researches support the view that restoring appropriate vascular normalization is crucial for alleviating hypoxia and increasing the benefits of anticancer therapy [10].
PET, a noninvasive technique for imaging of hypoxia, could determine the function of tumor blood vessels after anti-angiogenic therapy and help to determine the ideal regimen required for normalization [18,19]. In addition, radiolabeled arginine-glycine-aspartic acid (RGD)-based PET tracers can bind specifically to integrin α v β 3 highly expressed on vascular endothelial cells during active angiogenesis. Preclinical trials have shown that RGDbased PET tracers can help monitor the heterogeneity of α v β 3 as well as the normalization of blood vessels after anti-angiogenesis treatments [20][21][22] and help to determine the ideal regimen needed to improve the efficiency of radiotherapy [21]. 99m Tc-RGD immuno-single photon emission computed tomography/computed tomography (SPECT/CT) imaging showed a significant increase in tumor uptake 2 h after bevacizumab treatment compared to 24 h after bevacizumab treatment and control groups, which confirmed the vascular normalization window and showed the effect of combination therapy of bevacizumab when administered 2 h before radiotherapy [21]. Although the degree of pericyte coverage of ECs varied according to different tumor types, tumor blood vessels frequently lack adequate pericyte coverage, and recruitment of pericytes to newly formed blood vessels has been proposed as a therapeutic option to achieve vascular normalization [11]. PDGFRβ, a transmembrane receptor tyrosine kinase, is the typical biomarker on the surface of pericytes and is involved in pericyte recruitment and maturation [23,24]. The PDGFRβ-targeted tracer [ 68 Ga]Ga-DOTA-Z TRI developed by Cai and co-authors showed highly specific binding affinity to tumor-associated pericytes [13], providing a novel approach for visualizing pericytes and building up a toolbox for visualizing tumor vascular normalization.
Targeting tumor vascular normalization can overcome the shortcomings of anti-angiogenic therapy and further enhance the anticancer effect when combined with chemotherapy, radiation therapy, and immunotherapy [25,26]. A major challenge in inducing vascular normalization is optimizing the dose and schedule of combination therapy for individual patients [27]. Pharmacologically induced vascular normalization is transient. Histological staining, including microvascular density, vascular morphology, tumor perfusion, and permeability, is virtually nonreproducible in the same individual and cannot dynamically monitor the trend of the time window [28]. Target-specific molecular imaging techniques can noninvasively and dynamically visualize tumor vascular normalization (Fig. 1) and might refine therapeutic outcomes in combination therapy. With abundant preclinical data in hand, translational studies are needed to explore the clinical value of [ 68 Ga]Ga-DOTA-Z TRI PET imaging in terms of initial diagnosis and treatment optimization.

Declarations
Ethical approval Institutional Review Board approval was not required because the paper is an Editorial.
Informed consent Not applicable.

Conflict of interest
The authors declare no competing interests.