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Abstract

Introduction Noninvasive mapping of enzyme activity by means of MRI holds a great potential for improved diagnostics of diseases and evaluation of cell therapies [1]. Controlled self-assembly of low molecular weight Gd(III) complexes into large aggregates is an efficient approach for delivering enzyme activatable MRI contrast agents. Herein, we report on peptide based self-assembling conjugates for the detection of carboxypeptidase B (CPB) activity by means of 1H MRI. Methods Gd(III)-peptide conjugate (Gd-PE) and unlabelled peptides (PEs) were obtained by utilizing Fmoc/Boc solid-phase synthetic scheme. Relaxation rates R1 and R2 of these conjugates in TRIS buffer (pH=7.6) were measured at a 3T MR system (~25°C) in the presence and absence of carboxypeptidase B. Corresponding T1-and T2-weighted MR images were acquired as well. In addition enzyme kinetics were assessed by ESI-MS. Results Gd-PE and PEs with a CPB cleavable sequence masking the self-assembling moiety were synthesized in a high yield. Their structure was verified by ESI-MS. Longitudinal and transverse relaxivity of Gd-PE were found to be 12.8±0.1 mM−1 s−1 and 27.7 ±0.1 mM−1 s−1, respectively, and thus were much higher compared to commercial agents (r1~ 4 mM−1 s−1). Upon enzymatic conversion of Gd-PE, the released molecules assembled into Gd-aggregates to result in marked changes of proton relaxation rates of ~17 for R1 and ~32 for R2. The observed changes were even larger when Gd-PE was mixed with PEs reaching ~36 in R1 and up to ~150 in R2. A strong contrast enhancement was observed for these samples in T1- and T2-weighted MR images. The conversion of Gd-PE by CPB in a time dependent manner was confirmed by ESI-MS.