Nanoencapsulated drug delivery to solid tumors is a promising approach to overcome the pharmacokinetic limitations of therapeutic drugs. However, encapsulation leads to complex drug biodistribution and delivery making analysis of delivery efficacy challenging. As proxies, nanocarrier accumulation or total tumor drug uptake in the tumor are used to evaluate delivery. Yet these measurements fail to assess the delivery of active, released drug to the target, and thus it commonly remains unknown if drug-target occupancy is achieved. Here, an approach to evaluate the delivery of encapsulated drug to the target is developed, where residual drug target vacancy is measured using a fluorescent drug analog. In vitro measurements reveal that burst release governs drug delivery independent of nanoparticle uptake, and highlight limitations of evaluating nanoencapsulated drug delivery in these models. In vivo, however, the approach captures successful nanoencapsulated delivery, showing that tumor stromal cells drive nanoparticle accumulation and mediate drug delivery to adjacent cancer cells. These results, and generalizable approach, provide a critical advance to evaluate delivery of encapsulated drugs to the drug target-the central objective of nanotherapeutics.

Poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi) are a class of cancer drugs that enzymatically inhibit PARP activity at sites of DNA damage. In the context of BRCA mutations, PARPi can be synthetically lethal, presenting ideal genetic targeting. Yet, PARPi function primarily by trapping PARP1 onto sites of DNA damage. How PARPi trap and why some are better trappers remain unknown. Here, we show trapping occurs primarily through a kinetic phenomenon within biomolecular condensates that correlates with PARPi koff. Our results suggest PARP trapping is not the physical stalling of PARP1 on DNA, rather the high probability of PARP re-binding damaged DNA in the absence of other DNA binding protein recruitment. Furthermore, we found recruitment of the DNA binding protein RPA1 correlates to cell line PARPi sensitivity independent of RPA1 expression, demonstrating that condensate recruitment alone can impact efficacy. These results shed new light on how PARPi function, describe how PARPi properties correlate to trapping potency, and suggest previously unknown mechanisms of PARPi resistance.