Optimizing the Biodistribution of Radiofluorinated Barbiturate Tracers for Matrix Metalloproteinase Imaging by...

Dysregulated expression or activation of matrix metalloproteinases (MMPs) is observed in many kinds of life-threatening diseases. Therefore, MMP imaging-for example, with radiolabeled MMP inhibitors (MMPIs)-potentially represents a valuable tool for clinical diagnostics using noninvasive single photon emission computed tomography (SPECT) or positron emission tomography (PET) imaging. Despite numerous preclinical imaging approaches, translation to a clinical setting has not yet been successful. We introduce and oppose three potential radiofluorinated MMP-targeted imaging probes, modified by the introduction of pentamethine cyanine (Cy5) dyes and therefore containing both radio- as well as fluorescent label with respect to their capability to assess MMP activity in vivo by means of scintigraphic (PET) and/or fluorescent (NIRF) imaging. New hybrid MMPI tracer candidates, structurally based on radiofluorinated pyrimidine-2,4,6-triones (barbiturates) from previous approaches, were synthesized by convenient two-step syntheses. In the first step, Cy5 dyes, varying in the number of sulfonate groups (n_SO = 1, 2, or 4) and bearing an additional "clickable" alkyne moiety, were coupled to the barbiturate MMPI by amide formation. In the second step, the [¹⁸F]fluoride radiolabel was introduced into the resulting Cy5 dye conjugates by "radio-click" chemistry. Biodistribution studies of these hybrid tracer candidates were assessed and compared in C57BL/6 mice by PET as well as fluorescence imaging. MMP activity was imaged in a MMP-positive mouse model of irritant contact dermatitis (ICD) by PET and sequential fluorescence reflectance imaging (FRI), respectively. In vivo data were validated by scintillation counting, gelatin zymography, and MMP-histology. Three new potential hybrid MMP imaging probes were prepared, differing essentially in the number of sulfonate groups, introduced by Cy5 dye components. Although the hydrophilicity of these compounds was substantially increased, 10a (n_SO = 1) and 10b (n_SO = 2) were still rapidly eliminated via unfavorable hepatobiliary pathways, as observed in earlier approaches. Only 11 (n_SO = 4) showed delayed in vivo clearance and a shift towards higher renal elimination. In the chosen mouse model of ICD, only 11 (n_SO = 4) significantly accumulated in the inflamed mouse ear, which could be precisely visualized by means of PET and FRI.