Imaging Therapeutic PARP Inhibition In Vivo through Bioorthogonally Developed Companion Imaging Agents

A number of small-molecule poly (ADP-ribose) polymerase (PARP) inhibitors are currently undergoing advanced clinical trials. Determining the distribution and target inhibitory activity of these drugs in individual subjects, however, has proven problematic. Here, we used a PARP agent for positron emission tomography-computed tomography (PET-CT) imaging (¹⁸F-BO), which we developed based on the Olaparib scaffold using rapid bioorthogonal conjugation chemistries. We show that the bioorthogonal ¹⁸F modification of the parent molecule is simple, highly efficient, and well tolerated, resulting in a half maximal inhibitory concentration (IC₅₀) of 17.9 ± 1.1 nM. Intravital imaging showed ubiquitous distribution of the drug and uptake into cancer cells, with ultimate localization within the nucleus, all of which were inhibitable. Whole-body PET-CT imaging showed tumoral uptake of the drug, which decreased significantly, after a daily dose of Olaparib. Standard ¹⁸F-fludeoxyglucose imaging, however, failed to detect such therapy-induced changes. This research represents a step toward developing a more generic approach for the rapid codevelopment of companion imaging agents based on small-molecule therapeutic inhibitors.