New 18F-Labelled Inhibitors of Matrix Metalloproteinases as Imaging Probes for PET Imaging

Introduction. Matrix Metalloproteinases (MMPs) are a family of zinc dependent endoproteases that degrade proteins in the extracellular matrix (ECM) and have a key role in tissue remodeling in normal (angiogenesis, tissue repair) as well as pathological conditions (cancer, stroke, neurodegeneration). There is a growing body of evidence indicating that different pathological states are characterized by a specific pattern of MMP activity, such that this pattern may be regarded to as a sort of signature of a given pathology. The level of MMPs expression and activity in malignant cancers is generally higher than in normal or premalignant tissues, with maximum activity occurring in areas of active invasion at the tumor-stroma interface. In oncology, the role of different members of the MMP family is under intense scrutiny. Gelatinases (MMP-2 and MMP-9) are well-established to sustain all stages of tumor progression, including proliferation, adhesion, migration, angiogenesis, and evasion of the immune system. On the other hand, other metalloproteinases (such as MMP-8 and MMP-26) seem to have anti-tumour properties. There is a growing consensus on the fact that the visualization in the body of the activity of a selected panel of MMPs would be a very valuable tool for the staging of tumors and the choice/calibration of the therapy. Amongst the available imaging techniques, nuclear medicine techniques offer the greatest sensitivity and the possibility to obtain true metabolic/molecular images, provided that suitable radiolabelled imaging probes are available.[1] Methods. We have developed a number of MMP inhibitors (MMP-I) that can be radiolabelled with 18F for Positron Emission Tomography (PET) imaging. These inhibitors are based upon biphenylsulfone scaffolds, with carboxylic acid as the ZBG. PET images of healthy and xenografted with U87MG glioblastoma mice (athymic nude) were obtained by using the YAP-(S)PET system. Anesthetized (isofluorane) mice were injected i.v. with [18F] MMP-I (250 ±50) uCi in 0.15 mL volume. Results. The fluorinated MMP-I showed a good affinity and selectivity against gelatinases, with best IC50 values against MMP-2 in the order of 10 nM. Rather surprisingly, the selectivity of fluorinated derivatives somewhat increased with respect to the precursors. Compounds could be radiolabelled with 18F with a RadioChemical Yield (RCY) suitable for preclinical studies and very good purity (> 90 %). The biodistribution of the most potent compounds was evaluated by PET imaging in mice. It was found to be largely dominated by the lipophilicity of the compound and by its quite strong interaction with blood proteins (especially albumin). Preliminary molecular PET images of MMP activity in a glioblastoma model subcoutaneously grafted in mice showed a weak accumulation of the tracer into the tumor. Conclusions. PET imaging of MMPs through 18F labelled inhibitors is feasible. A substantial improvement is expected if sequestration of the tracer by blood components is minimized. Acknowledgments. This work is supported by the Regione Piemonte (Italy) as part of the Converging Technologies BIO_THER project and part of the PIIMDMT project. References. [1] S. Wagner, H.J. Breyholz, A. Faust, C. Höltke, B. Levkau, O. Schober, M. Schafers, K. Kopka, Curr. Med. Chem., 2006, 13, 2819-2838.