Molecular imaging in drug discovery and development: potential and limitations of nonnuclear methods

Noninvasive conventional imaging methods are established technologies in modern drug discovery and development providing valuable morphological, physiological, and metabolic information to characterize disease phenotypes, to evaluate the efficacy of therapy and to identify and develop potential biomarkers for clinical drug evaluation. The development of target-specific or molecular imaging has added a new dimension: molecular events such as the target expression, the drug-target interaction, or the activation of signal transduction pathways can be studied in the intact organism with high spatial and temporal resolution. Molecular imaging is inherently a multimodality approach. In this article, we review the role of molecular imaging for drug discovery and development focusing on nonnuclear imaging methods, i.e., magnetic resonance imaging (MRI) and optical imaging techniques based on fluorescence and bioluminescence readouts. Examples discussed are direct visualization of target expression using target-specific ligands or reporter genes, pathway imaging, and cell-trafficking studies.