Mechanistic insights into P-glycoprotein-driven transport of anti-cancer PARP inhibitors.

P-glycoprotein (Pgp), a membrane-bound transporter protein, plays significant roles in the development of multidrug resistance (MDR) and in drug disposition. Understanding the interactions between Pgp and its substrates is therefore crucial for optimal drug design and ultimately clinical success. In this study, we conducted a comparative analysis of the four FDA-approved anti-cancer PARPi and their interactions with Pgp to elucidate the mechanisms driving their Pgp-mediated transport. Through an assessment of PARPi-induced effect on Pgp-coupled ATP hydrolysis, we found that olaparib and talazoparib act as Pgp inducers, increasing Pgp's rate of ATP hydrolysis 3-fold from its basal activity. Rucaparib and niraparib slightly increased Pgp activity at lower concentrations but displayed biphasic kinetic trends and predominantly reduced Pgp ATPase activity below basal levels. Quenching of Pgp's intrinsic tryptophan residues revealed high binding affinities for the PARPi-Pgp complexes, apart from talazoparib, which displayed relatively lower affinity. Indirect conformational analyses via acrylamide quenching of tryptophan residues indicated that olaparib, rucaparib, and niraparib all induce Pgp to move more toward its "outward-open" position, while talazoparib induced a more intermediate conformation. Saturation transfer double difference (STDD) NMR revealed the PARPi structural basis for Pgp binding. Bi-directional transport assays suggested the rate of Pgp-mediated efflux to be olaparib > talazoparib > rucaparib > niraparib. Overall, our results provide insight for future potential development of PARPi that are less subject to Pgp-mediated transport.