Design of targeting imaging probes
Active targeting is one of the most used approaches in pharmaceutical field in order to improve the concentration of a molecule (therapeutics or in vivo diagnostics) at the pathological site. Therefore, a targeting agent is a molecular system containing the “active principle” (a drug or an imaging probe) conjugated to a vector that has to exhibit a high affinity and specificity towards the biological target (cellular receptors, intracellular markers, enzymes, metabolite, extracellular components, ...). Moreover, the success of the approach is also based on the accessibility of the biological target by the targeting system. The design of a targeting imaging probe is affected by the sensitivity of the imaging modality. For instance, to overcome the relatively low sensitivity in the contrast detection of MRI, the targeting vector is conjugated to macromolecular or nanosized systems loaded with a high number of imaging units in order to significantly increase the contrast delivered at the biological target. On the contrary, in case of Optical or Nuclear imaging, sensibility is much better and a single fluorophore/radioisotope can be sufficient to generate enough signal from the disease marker. Considering the accessibility issue, MRI targeting agents are very suitable for endothelial markers that do not require the extravasation of the “large” probe, whereas optical probes can easily reach extracellular and intracellular targets.