Non-invasive molecular in vivo imaging, as an emerging technology, is a reality for several disease states but still requires refinement to progress into the clinical setting. In addition to the technological challenge for further increasing spatial resolution and sensitivity of the imaging hardware, a particular effort to improve or develop new imaging agents is crucial. For example, particle size and electrical charge are of great importance for the behavior of the agents in vivo, and therefore need to be carefully established. Moreover, future clinical applications for molecular imaging will require optimization of specificity, brightness (tissue penetration), and development of non-toxic agents.
Despite these current technological and cost-related limits, molecular imaging in general, and protease activity in particular, should contribute to biomedicine in the near future.
Detection of asymptomatic disease, and the prediction of severe complications, remains elusive to current diagnostic tools for many vascular disorders. Intravital imaging of protease activity promises to extend beyond anatomy and disclose biological aspects of the progress of atherosclerosis, myocardial infarct healing, heart transplant rejection, or aortic aneurysms. Moreover, novel therapeutic strategies in CVD should emerge from molecular imaging — including non-invasive imaging markers for prevention and personalized medicine, interventional imaging combined with treatment in the acute phase, and molecular assessment of therapeutic efficacy during treatment.
Preclinical studies in mice and rabbits have demonstrated how close we are to translating molecular imaging tools to the clinic. Imaging agents specifically targeting MMPs, cysteinyl cathepsins, or other enzymes implicated in CVD pathogenesis have demonstrated compatibility with established imaging platforms (PET, SPECT, MRI). The promising field of optical imaging will require special efforts in hardware to detect non-invasively small accumulations of active “smart probes” in inflamed or diseased tissues. Nonetheless, studies using MMPs and cathepsin-activatable probes have achieved proof of concept of the relevance and value of functional intravital imaging.
In addition to more reliable diagnosis and potential prediction of the outcome of a disease, molecular imaging for protease activity should also serve as a tremendous tool for development, evaluation, and dose ranging of novel therapeutics in vivo
. Moreover, the coupling of imaging and therapeutic interventions with multifunctional nanoparticles opens the possibility of “theranostic” approaches. This concept involves targeting therapeutic drugs in particles carrying imaging moieties, for selective drug delivery to diseased tissues and simultaneous monitoring of not only targeting, but also possibly efficacy. In an illustration of such an approach, agents targeting scavenger receptor 1A on macrophages can detect MMP-9 activity (84
). Multi-modality imaging — for example, tri-functional agents combining magnetic, optical, and radionuclide imaging functionality — promises to expand the gamut of molecular imaging (75
). Such multimodal imaging agents must contain an adequate ratio of the various contrast agents for matching the relative sensitivity of the applied imaging modalities. This particular requirement may lead to the development of chemical constructs to create multiple binding sites of contrast agents, from which overall charge and size remain compatible with penetration into the targeted tissues (88
Multimodal imaging strategies could also combine non-invasive and high-resolution technologies (e.g., confocal or multiphoton microscopy) to study protease activity in vivo, in interstitial as well as in intracellular milieu, to define their roles better and to characterize more precisely the behavior of imaging agents in the field of CVD. Current NIRF contrast agents, for example, may be applicable to combined FMT and (optical parametric oscillator-equipped) two-photon microscopy, where two-photon microscopy contributes both sub cellular resolution and imaging of intrinsic emission derived from extra cellular matrix components.
Protease imaging — and more broadly, molecular imaging — not only applies to CVD, but also to other diseases such as arthritis (89
), asthma (90
), and cancer (91
). Non-invasive intravital assessment of protease presence and activity will likely be feasible first by nuclear imaging in clinical trials, because of picomolar dosing and its widespread use in humans. The advantages of activatable probes over labeled inhibitors and the advent of multimodal imaging should increase efforts to introduce optical imaging into the clinic. The pace of progress in the development of novel molecular probes and imaging platforms during recent years highlights the promise of enabling advances in research and clinical applications through probing physiopathologic processes and assessment of specific molecular processes in intact subjects in vivo