In vivo molecular imaging

The relatively young field of molecular imaging is focused on the visualization of molecular phenotypes in whole organisms. This is achieved using imaging systems based on radionuclides, nuclear magnetic resonance, ultrasound, or the visible-IR region of the optical spectrum. Molecularly defined contrast in these modalities is generated by exogenous probes of the endogenous proteome, or through transgenes. Examples of exogenous probes include those that are transported and trapped (glucose, nucleoside analogs), those directed against extracellular receptors (somatostatin, opioid, melanotropin), and those activated by extracellular proteases. Transgenes that have been used in molecular imaging include the above receptors, non-mammalian enzymes that trap pro-drugs (HSV-tk, yeast CD), and optical reporter proteins (luciferase, fluorescent proteins). Cutting edge technologies in this field include in vivo assays for protein-protein interactions, and in vivo assays for mRNA expression patterns. The number of degrees of freedom in designing new agents is daunting, and advancements in this field will require a significant participation from molecular and cellular biochemists.