Poly(ADP ribose) polymerase 1 (PARP1) inhibitors have had little clinical success. Thurber et al. used real-time in vivo single-cell pharmacokinetic imaging of a PARP1 inhibitor to show that the lack of clinical effect is probably not due to tumour cells receiving insufficient concentrations of a PARP1 inhibitor. They then used imaging data to model and predict the distribution of the PARP1 inhibitor in situations in which it would be difficult to experimentally determine pharmacokinetics, such as subcellular distribution in humans. The authors note that single-cell pharmacokinetic imaging could aid the understanding of other drugs in other diseases.