Status and trends in the development of clinical diagnostic agents

Contrast agents (CA) are routinely used in clinical practice to improve the diagnosis of diseases and to monitor therapy response. The majority of CA comprises small molecules accumulating at pathological sites due to vascular abnormalities, such as changes in perfusion and permeability. For many diseases, high diagnostic accuracy can be achieved with contrast-enhanced imaging. This means that new CA will only succeed in translation if they either show superior performance with respect to diagnostic accuracy, safety and cost, support a new imaging modality, or are directly linked to the refinement of therapy, e.g., as a companion diagnostic. Unfortunately, these basic demands are often not carefully considered by the scientific community, leading to concepts with low chances of clinical translation. Thus, it is not surprising that, despite steadily increasing numbers of publications, there is quite the opposite trend when it comes to the clinical approval of new diagnostics. As a matter of fact, except for PET tracers, in the last decade, only a handful of CA received FDA or EMA approval. Furthermore, several approved products were discontinued by the manufacturers because of low market potential, a competitive own product, suboptimal clinical performance, or safety concerns. This review article discusses the current status of approved diagnostic probes for clinical imaging modalities, with a focus on new trends in CA development. In this context, molecularly targeted diagnostics or probes for emerging fields, such as image-guided surgery, nanomedicine, or theranostics, will be introduced and discussed with regard to their clinical translation.