An efficient MRI agent targeting extracellular markers in prostate adenocarcinoma

Purpose: Prostate cancer (PCa) is the most widespread tumor affecting males in Western countries. We propose a novel MRI molecular tetrameric probe based on the heptadentate gadolinium (Gd)-AAZTA (6-amino-6-methylperhydro-1,4-diazepinetetraacetic acid) that is able to in vivo detect PCa through the recognition of the fibrin–fibronectin (FB–FN) complex. Methods: The peptide CREKA (Cys-Arg-Glu-Lys-Ala), targeting the FB–FN complex in the reactive stroma of the tumor, was synthesized by solid phase peptide synthesis (SPPS) and conjugated to the tetramer dL-(Gd-AAZTA)₄. The resulting probe was characterized by ¹H relaxometry, tested in vitro on FB clots and in vivo on an orthotopic mouse model of PCa. Results: CREKA-dL-(Gd-AAZTA)₄ showed a remarkable relaxivity of 18.2 mM^-1s−1_Gd (0.47 T, 25°C) because of the presence of 2 water molecules (q = 2) in the inner coordination sphere of each Gd³⁺ ion, whose rotational motion (τ_R) is lengthened as the result of the relatively high molecular weight. The probe displayed a detectable affinity for plasma-derived FB clots. On intravenous injection of the probe in an orthotopic mouse model of PCa, a significant increase in the prostate T₁ contrast (~40%) was observed. The MRI signal appears statistically higher either with respect to the one observed for the control probes and to the one detected when CREKA-dL-(Gd-AAZTA)₄ was administered to healthy animals. Conclusions: This study demonstrated the ability of the CREKA-dL-(Gd-AAZTA)₄ probe to specifically localize in prostate tumor after injection. The high relaxivity of the probe allows the reduction of the injected dose to 20 µmol_Gd/kg, yielding a good in vivo contrast enhancement in the region of prostate tumor.