Fig. 2.

Schematic shows two options for generating PET fluorescence (FL) probes on ideal ligand with three possible reaction sites.

A, Preforming reactive PET/FI probe and then isolating it before reaction with trisubstitutable ligand gives single agent with maximum possible ratio of PET to FI to ligand (3:3:1).

B, Simple and more common labeling strategy is to use commercial reagents. This scheme gives complicated mixture of products with lesser ratios of PET or FI to ligand. Approval for clinical application of complicated mixture of these products will be more challenging to gain.