Molecular imaging can accelerate anti-angiogenic drug development and testing

Andrei Iagaru, Xiaoyuan Chen and Sanjiv Sam Gambhir*  About the authors

Several modalities, including anatomic, physiologic, and molecular imaging technologies, have a role in the process of drug development and validation. Anatomic imaging with CT, MRI and ultrasound has been used routinely in the process of oncology drug development, but these techniques primarily detect changes in size and structure, which can be delayed. Physiologic imaging detects blood flow within tumors. Many targeted cancer therapies are cytostatic rather than cytotoxic, and thus changes in tumor morphology might not be easily detectable. In contrast with conventional anatomical or physiological imaging, in vivo molecular imaging characterizes and monitors biological processes at the cellular and molecular levels. Molecular imaging techniques include PET, single photon emission computed tomography (SPECT), optical fluorescence, optical bioluminescence, magnetic resonance spectroscopy, molecular MRI (mMRI), and targeted ultrasound. Clinical molecular imaging provides functional information in patients following intravenous administration of tracers that are incorporated into various biochemical and cellular processes. ¹⁸F-fluoro-2-deoxyglucose (¹⁸F-FDG) PET/CT is a powerful imaging modality that is becoming widely available. This modality can detect active metabolic processes and the morphologic features associated with them in a single examination. The role of ¹⁸F-FDG PET has been proven in a variety of cancers, including lymphoma, colorectal carcinoma, lung cancer and melanoma, for which it significantly changes patient management.¹¹⁸F-FDG remains the most widely used PET radiotracer in clinical use; however, additional radiopharmaceuticals for molecular imaging with PET/CT could become available over the next few years.

Advances in cellular and molecular biology have facilitated the discovery and characterization of novel processes involved in disease progression, such as angiogenesis, which has led the pharmaceutical industry to target this process with novel angiogenesis inhibitors. Angiogenesis is one of the fundamental processes during tumor growth and disease progression, and is regulated by numerous molecular pathways, including, but not limited to, VEGF, matrix metalloproteinases, endoglin (CD105), integrin _v3 and E-selectin. All these pathways could serve as targets for molecular imaging. We have extensively reviewed the role of molecular imaging in anti-angiogenic drug development and concluded that PET and MRI are the two most important modalities that can be used to assess angiogenesis and the effect of anti-angiogenic treatment.²

PET is a major tool in the armamentarium of imaging angiogenesis. Several new probes have been developed and have been tested in the preclinical setting. Two of the most intensively studied angiogenesis-related molecular targets include integrin _v3 and the VEGF receptors. Cyclic Arg-Gly-Asp (RGD) peptides bind to integrin _v3 and can inhibit angiogenesis.³¹⁸F-labeling of such peptides was first reported by Haubner et al., and the tracer ¹⁸F-galacto-RGD exhibited integrin _v3-specific tumor uptake in the integrin-positive M21 melanoma xenograft model.⁴ Initial clinical trials in healthy volunteers and in a limited number of patients with cancer revealed that this tracer is safe and is able to delineate certain lesions that are integrin positive.^{5, 6} Despite the successful translation of ¹⁸F-galacto-RGD into clinical trials, several key issues remain to be resolved, such as tumor-targeting efficacy, pharmacokinetics, and the ability to quantify integrin _v3 density in vivo. We and others have, therefore, developed a series of dimeric and multimeric RGD peptides to improve the integrin _v3 targeting efficacy.² One tracer in particular, denoted as ¹⁸F-FRGD2, exhibited excellent integrin _v3-specific tumor imaging with favorable in vivo pharmacokinetics.⁷ The synergistic effect of polyvalency and improved pharmacokinetics may be responsible for the excellent imaging characteristics of ¹⁸F-FRGD2. We are currently in the process of translating ¹⁸F-FRGD2 and its derivatives into the clinic for the imaging of patients with cancer. Other PET isotopes such as ⁶⁴Cu-labeled RGD peptides and integrin-specific antibodies are also being developed.² It is noteworthy that RGD peptide tracers target not only the angiogenic blood vessels but also integrin-_v3 expressing tumor cells. Macromolecular and multimeric constructs are less likely to extravasate and might, thus, have advantages for specifically targeting the angiogenic vasculature. VEGF₁₆₅ was first labeled with ¹²³I and tested by SPECT imaging in both preclinical animal models and in patients with cancer. As with most other radioiodinated tracers, prominent activity accumulation in the stomach was observed owing to deiodination.⁸ We have labeled VEGF₁₂₁ with ⁶⁴Cu for PET imaging of tumor angiogenesis and VEGFR expression.⁹ MicroPET imaging revealed rapid, specific, and prominent uptake of ⁶⁴Cu-DOTA-VEGF₁₂₁ in highly vascularized small U87MG tumors with high VEGFR2 expression, but significantly lower and sporadic uptake in large U87MG tumors with low VEGFR2 expression. This difference in uptake might allow for the clinical translation of this radiopharmaceutical into imaging tumor angiogenesis. Furthermore, the difference in tracer uptake between tumors with high VEGFR2 expression and tumors with low VEGFR2 expression might guide anti-angiogenic treatment, and in particular could facilitate patient selection and the monitoring of VEGFR-targeted cancer therapy. The radioisotope ⁶⁴Cu was also used to site-specifically label VEGF₁₂₁ and it was found that pegylation considerably prolonged blood clearance time.¹⁰ Czernin et al. concluded that molecular imaging should be integrated into the drug development process.¹¹ However, even though PET imaging of angiogenesis markers is being developed, little has been done to use such tools for the assessment of anti-angiogenic treatment. This is due to the lack of FDA approval for general use of the new imaging probes.

mMRI is also emerging as a new investigative tool for angiogenesis. Mulder et al. used _v3-targeted bimodal liposomes with MRI to quantify angiogenesis in a tumor mouse-model and to evaluate the therapeutic efficacy of the angiogenesis inhibitors anginex and endostatin.¹² The results obtained by Mulder et al. suggest that mMRI can be used to non-invasively measure the efficacy of angiogenesis inhibitors during the course of therapy. Further efforts to use mMRI for angiogenesis imaging include Gd³ complexed nanoparticle T1 sequences and iron oxide nanoparticle T2 sequences. SPECT, optical imaging, and contrast-enhanced ultrasound may also be useful for imaging angiogenesis processes.

Although strategies for imaging angiogenesis are in the development phase, they have the potential to accelerate oncology drug development. Direct readouts of the levels of angiogenic-specific molecular targets before, during, and after therapy should help with preclinical drug testing and should eventually be useful within clinical trials evaluating combinations of targeted and imaging agents.

Competing interests

The authors declared no competing interests.

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