Novel Gd(III)-based probes for MR molecular imaging of Matrix Metalloproteinases

Two novel Gd-based contrast agents (CAs) for the molecular imaging of matrix metalloproteinases (MMPs) were synthetized and characterized in vitro and in vivo. These probes were based on the PLG*LWAR peptide sequence, known to be hydrolyzed between Gly and Leu by a broad panel of MMPs. A Gd–DOTA chelate was conjugated to the N-terminal position through an amide bond, either directly to proline (compd Gd–K11) or through a hydrophilic spacer (compd Gd–K11N). Both CA were made strongly amphiphilic by conjugating an alkyl chain at the C-terminus of the peptide sequence. Gd–K11 and Gd–K11N have a good affinity for b-cyclodextrins (KD 310 and 670mM respectively) and for serum albumin (KD 350 and 90mM respectively), and can be efficiently cleaved in vitro at the expected site by MMP-2 and MMP-12. Upon MMP-dependent cleavage, the CAs lose the C-terminal tetrapeptide and the alkyl chain, thus undergoing to an amphiphilic-to-hydrophilic transformation that is expected to alter tissue pharmacokinetics. To prove this, Gd–K11 was systemically administered to mice bearing a subcutaneous B16.F10 melanoma, either pre-treated or not with the broad spectrum MMP inhibitor GM6001 (Ilomastat). The washout of the Gd-contrast enhancement in MR images was significantly faster for untreated subjects (displaying MMP activity) with respect to treated ones (MMP activity inhibited). The washout kinetics of Gd-contrast enhancement from the tumor microenvironment could be then interpreted in terms of the local activity of MMPs