Noninvasive Visualization of Tumoral Fibrin Deposition Using a Peptidic Fibrin-Binding Single Photon Emission Computed Tomography Tracer

[18F]FDG-Labeled CGPRPPC Peptide Serving as a Small Thrombotic Lesions Probe, Including a Comparison with [99mTc]-Labeled Form

Purpose: Fibrin is a perfect target for specific imaging of all types of thrombotic lesions. Cyclic peptides were introduced as the best scaffolds out of the different types of probes for thrombi detection. This study was conducted to label previously synthesized peptide-targeting fibrin with [¹⁸F]FDG and its in vitro and in vivo assessments. Materials and Methods: CGPRPPC peptide functionalized with 6-hydrazinonicotinamide and Eei-NHS was synthesized and cyclized using air oxidation method. The cyclic sequences were labeled with [¹⁸F]FDG at 85°C within 30 min. The stability studies were performed in human plasma. Fibrin-binding and platelet aggregation tests were performed in vitro. Biodistribution and scintigraphy imaging in normal mice and carotid thrombotic rat model were considered as in vivo studies. Results: Radiolabeled peptides show a good stability in human plasma and also high-affinity binding for human fibrin. Platelet aggregation test confirmed specific binding of radiopeptides to fibrin. A key problem with the authors' previous research was inability to detect small-vessel thrombi. The results of positron emission tomography/computed tomography scanning show high specific uptake of [¹⁸F]FDG-labeled CGPRPPC in small-sized thrombosis. Conclusion: The experiment revealed that radiolabeling of cyclic peptide (CGPRPPC) with [¹⁸F]FDG enables us to detect small thrombotic lesions in small animal models with high resolution.

Molecular imaging of the tumor microenvironment

The tumor microenvironment plays a critical role in tumor initiation, progression, metastasis, and resistance to therapy. It is different from normal tissue in the extracellular matrix, vascular and lymphatic networks, as well as physiologic conditions. Molecular imaging of the tumor microenvironment provides a better understanding of its function in cancer biology, and thus allowing for the design of new diagnostics and therapeutics for early cancer diagnosis and treatment. The clinical translation of cancer molecular imaging is often hampered by the high cost of commercialization of targeted imaging agents as well as the limited clinical applications and small market size of some of the agents. Because many different cancer types share similar tumor microenvironment features, the ability to target these biomarkers has the potential to provide clinically translatable molecular imaging technologies for a spectrum of cancers and broad clinical applications. There has been significant progress in targeting the tumor microenvironment for cancer molecular imaging. In this review, we summarize the principles and strategies of recent advances made in molecular imaging of the tumor microenvironment, using various imaging modalities for early detection and diagnosis of cancer.

Fibrinolytic Enzyme Cotherapy Improves Tumor Perfusion and Therapeutic Efficacy of Anticancer Nanomedicine

Elevated interstitial fluid pressure and solid stress within tumors contribute to poor intratumoral distribution of nanomedicine. In this study, we hypothesized that the presence of fibrin in tumor extracellular matrix contributes to hindered intratumoral distribution of nanocarriers and that this can be overcome through the use of a fibrinolytic enzyme such as tissue plasminogen activator (tPA). Analysis of fibrin expression in human tumor biopsies showed significant fibrin staining in nearly all tumor types evaluated. However, staining was heterogeneous across and within tumor types. We determined the effect of fibrin on the diffusion, intratumoral distribution, and therapeutic efficacy of nanocarriers. Diffusivity of nanocarriers in fibrin matrices was limited and could be improved significantly by coincubation with tPA. In vivo, coadministration of tPA improved the anticancer efficacy of nanoparticle-encapsulated paclitaxel in subcutaneous syngeneic mouse melanoma and orthotopic xenograft lung cancer models. Furthermore, treatment with tPA led to decompression of blood vessels and improved tumor perfusion. Cotreatment with tPA resulted in greater intratumoral penetration of a model nanocarrier (Doxil), leading to enhanced availability of the drug in the tumor core. Fibrinolytics such as tPA are already approved for other indications. Fibrinolytic cotherapy is therefore a rapidly translatable strategy for improving therapeutic effectiveness of anticancer nanomedicine. Cancer Res; 77(6); 1465-75. ©2017 AACR.