A dual inhibitor of matrix metalloproteinases and a disintegrin and metalloproteinases, [¹8F]FB-ML5, as a molecular probe for non-invasive MMP/ADAM-targeted imaging.

BACKGROUND: Numerous clinical studies have shown a correlation between increased matrix metalloproteinase (MMP)/a disintegrin and metalloproteinase (ADAM) activity and poor outcome of cancer. Various MMP inhibitors (MMPIs) have been developed for therapeutic purposes in oncology. In addition, molecular imaging of MMP/ADAM levels in vivo would allow the diagnosis of tumors. We selected the dual inhibitor of MMPs and ADAMs, ML5, which is a hydroxamate-based inhibitor with affinities for many MMPs and ADAMs. ML5 was radiolabelled with (18)F and the newly obtained radiolabelled inhibitor was evaluated in vitro and in vivo. MATERIALS AND METHODS: ML5 was radiolabelled by direct acylation with N-succinimidyl-4-[(18)F]fluorobenzoate ([(18)F]SFB) for PET (positron emission tomography). The resulting radiotracer [(18)F]FB-ML5 was evaluated in vitro in human bronchial epithelium 16HBE cells and breast cancer MCF-7 cells. The non-radioactive probe FB-ML5 and native ML5 were tested in a fluorogenic inhibition assay against MMP-2, -9, -12 and ADAM-17. The in vivo kinetics of [(18)F]FB-ML5 were examined in a HT1080 tumor-bearing mouse model. Specificity of probe binding was examined by co-injection of 0 or 2.5mg/kg ML5. RESULTS: ML5 and FB-ML5 showed high affinity for MMP-2, -9, -12 and ADAM-17; indeed IC50 values were respectively 7.4 ± 2.0, 19.5 ± 2.8, 2.0 ± 0.2 and 5.7 ± 2.2 nM and 12.5 ± 3.1, 31.5 ± 13.7, 138.0 ± 10.9 and 24.7 ± 2.8 nM. Radiochemical yield of HPLC-purified [(18)F]FB-ML5 was 13-16% (corrected for decay). Cellular binding of [(18)F]FB-ML5 was reduced by 36.6% and 27.5% in MCF-7 and 16 HBE cells, respectively, after co-incubation with 10 µM of ML5. In microPET scans, HT1080 tumors exhibited a low and homogeneous uptake of the tracer. Tumors of mice injected with [(18)F]FB-ML5 showed a SUVmean of 0.145 ± 0.064 (n=6) which decreased to 0.041 ± 0.027 (n=6) after target blocking (p<0.05). Ex vivo biodistribution showed a rapid excretion through the kidneys and the liver. Metabolite assays indicated that the parent tracer represented 23.2 ± 7.3% (n=2) of total radioactivity in plasma, at 90 min post injection (p.i.). CONCLUSION: The nanomolar affinity MMP/ADAM inhibitor ML5 was successfully labelled with (18)F. [(18)F]FB-ML5 demonstrated rather low binding in ADAM-17 overexpressing cell lines. [(18)F]FB-ML5 uptake showed significant reduction in the HT1080 tumor in vivo after co-injection of ML5. [(18)F]FB-ML5 may be suitable for the visualization/quantification of diseases overexpressing simultaneously MMPs and ADAMs.

MicroPET Evaluation of a Hydroxamate-Based MMP Inhibitor, [(18)F]FB-ML5, in a Mouse Model of Cigarette Smoke-Induced Acute Airway Inflammation.

Matrix metalloproteinases (MMPs) are the main proteolytic enzymes involved in the pathogenesis of chronic obstructive pulmonary disease (COPD). A radiolabeled MMP inhibitor, [(18)F]FB-ML5, was prepared, and its in vivo kinetics were tested in a mouse model of pulmonary inflammation. BALB/c mice were exposed for 4 days to cigarette smoke (CS) or air. On the fifth day, a dynamic microPET scan was made with [(18)F]FB-ML5. Standardized uptake values (PET-SUVmean) were 0.19 ± 0.06 in the lungs of CS-exposed mice (n = 6) compared to 0.11 ± 0.03 (n = 5) in air-exposed controls (p < 0.05), 90 min post-injection MMP-9 levels in bronchoalveolar lavage fluid (BALF) were increased from undetectable level to 4615 ± 1963 pg/ml by CS exposure. Increased MMP expression in a COPD mouse model was shown to lead to increased retention of [(18)F]FB-ML5.

Probes for non-invasive matrix metalloproteinase-targeted imaging with PET and SPECT.

Dysregulation of matrix metalloproteinase (MMP) activity can lead to a wide range of disease states such as atherosclerosis, inflammation or cancer. The ability to image MMP activity non-invasively in vivo, by radiolabelled synthetic inhibitors, would allow the characterization of atherosclerotic plaques, inflammatory lesions or tumors. Here we present an overview of radiolabelled MMP inhibitors (MMPIs) and MMP peptides for positron emission tomography (PET) and single photon emission computed tomography (SPECT) for the detection of proteolytic activity of MMPs. So far, most studies are at a preliminary stage; however, some hydroxamate-based tracers such as the peptidomimetics [¹¹¹In]-DTPA-RP782, [99mTc]-(HYNIC-RP805)(tricine)(TPPTS), or Marimastat-ArB[¹8F]F3 and the picolyl- benzenesulfonamide [¹²³I]I-HO-CGS 27023A identified specifically the enzymatic action of MMPs in animal models of various pathologies. The development of new compounds that may lead to novel tracers (e.g. modification of zinc-binding group, variation of substituents attached to the S1', S2' and S3' pockets of the MMP inhibitors) and the use of antibodies and cell penetrating peptides are also discussed. In general, preclinical studies with atherosclerosis models proved to be more successful than those with oncological models.