Design of radiolabeled gelatinase inhibitor peptide ((99m)Tc-CLP) and evaluation in rats

Towards Optimized Bioavailability of 99mTc-Labeled Barbiturates for Non-invasive Imaging of Matrix Metalloproteinase Activity

Introduction

Dysregulated activity of matrix metalloproteinases (MMPs) drives a variety of pathophysiological conditions. Non-invasive imaging of MMP activity in vivo promises diagnostic and prognostic value. However, current targeting strategies by small molecules are typically limited with respect to the bioavailability of the labeled MMP binders in vivo. To this end, we here introduce and compare three chemical modifications of a recently developed barbiturate-based radiotracer with respect to bioavailability and potential to image MMP activity in vivo.

Methods

Barbiturate-based MMP inhibitors with an identical targeting unit but varying hydrophilicity were synthesized, labeled with technetium-99m, and evaluated in vitro and in vivo. Biodistribution and radiotracer elimination were determined in C57/BL6 mice by serial SPECT imaging. MMP activity was imaged in a MMP-positive subcutaneous xenograft model of human K1 papillary thyroid tumors. In vivo data were validated by scintillation counting, autoradiography, and MMP immunohistochemistry.

Results

We prepared three new ^99mTc‐labeled MMP inhibitors, bearing either a glycine ([^99mTc]MEA39), lysine ([^99mTc]MEA61), or the ligand HYNIC with the ionic co-ligand TPPTS ([^99mTc]MEA223) yielding gradually increasing hydrophilicity. [^99mTc]MEA39 and [^99mTc]MEA61 were rapidly eliminated via hepatobiliary pathways. In contrast, [^99mTc]MEA223 showed delayed in vivo clearance and primary renal elimination. In a thyroid tumor xenograft model, only [^99mTc]MEA223 exhibited a high tumor-to-blood ratio that could easily be delineated in SPECT images.

Conclusion

Introduction of HYNIC/TPPTS into the barbiturate lead structure ([^99mTc]MEA223) results in delayed renal elimination and allows non-invasive MMP imaging with high signal-to-noise ratios in a papillary thyroid tumor xenograft model.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11307-021-01668-z.

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Imaging Inflammation - From Whole Body Imaging to Cellular Resolution

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Molecular Imaging of Hydrolytic Enzymes Using PET and SPECT

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Imaging the Tumor Microenvironment

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Radiopharmaceuticals are localized in (malignant) tumor tissues by different mechanisms. One of these mechanisms, gelatinase enzyme activity, is associated with poor prognosis in cancer patients and potential targets for tumor imaging. There are some gelatinases to be associated with metastatic potential for tumor imaging to possibly predict metastases. In this study, a cyclic decapeptide conjugate, DTPA-CLP (DTPA-Cys-Leu-Pro-Gly-His-Trp-Gly-Phe-Pro-Ser-Cys), was selected as a peptide conjugate because of its selective inhibitory activity toward gelatinases. Peptide-conjugated DTPA-CLP was labeled with (99m) Tc with a radiolabeling efficiency of 97.0 ± 2.8%. After determining optimization conditions for radiolabeling, a biodistribution study of radiolabeled peptide in albino Wistar rats was performed. According to biodistribution data, (99m) Tc-DTPA-CLP showed high uptake in the lung, liver, uterus, and spleen. These results show that (99m) Tc-DTPA-CLP may be used for the imaging of gelatinase activity in metastatic tumors.