Abstract

To date, the in vitro–in vivo correlation (IVIVC) of P-glycoprotein (P-gp)–mediated drug-drug interaction (DDI) at the blood-brain barrier (BBB) in rats indicated that the cutoff value to significantly affect the brain penetration of digoxin was [I_,unbound/K_i] of 1, where I_,unbound is the unbound plasma concentration of P-gp inhibitors. On the basis of the IVIVC in rats, we speculated that clinically used P-gp inhibitors do not cause DDI at the human BBB, because none of the compounds studied was [I_,unbound/K_i]>1 at therapeutic doses. Recently, positron emission tomography studies with P-gp substrates, such as [¹¹C]verapamil, [¹¹C]N-desmethyl loperamide, and [¹¹C]loperamide, together with potent P-gp inhibitors, have indicated that increases in the influx rate constant for brain entry were observed in humans. Therefore, we aimed to retrospectively analyze the results of P-gp–mediated DDIs with in vitro P-gp inhibition assays and to confirm the appropriate cutoff value. In vitro P-gp inhibition assays using verapamil, N-desmethyl loperamide, and loperamide as P-gp probe substrates were performed in human multidrug resistance protein 1-expressing LLC-PK1 cells. The efflux ratios decreased in the presence of P-gp inhibitors, and the K_i of tariquidar was 10 nmol/L, regardless of probe substrates. Taking the in vitro K_i and unbound plasma concentrations in clinical DDI studies together, the criterion [I_,unbound/K_i] of 1 was an appropriate cutoff limit to observe significant P-gp–mediated DDI at the BBB in humans. On the other hand, no significant DDI was observed in cases in which [I_,unbound/K_i] was less than 0.1. This criterion was comparable to the previous IVIVC result in rats.