Factors Governing P‑Glycoprotein-Mediated Drug–Drug Interactions at the Blood–Brain Barrier Measured with Positron Emission Tomography

The adenosine triphosphate-binding cassette transporter P-glycoprotein (ABCB1/Abcb1a) restricts at the blood–brain barrier (BBB) brain distribution of many drugs. ABCB1 may be involved in drug–drug interactions (DDIs) at the BBB, which may lead to changes in brain distribution and central nervous system side effects of drugs. Positron emission tomography (PET) with the ABCB1 substrates (<i>R</i>)-[¹¹C]­verapamil and [¹¹C]-<i>N</i>-desmethyl-loperamide and the ABCB1 inhibitor tariquidar has allowed direct comparison of ABCB1-mediated DDIs at the rodent and human BBB. In this work we evaluated different factors which could influence the magnitude of the interaction between tariquidar and (<i>R</i>)-[¹¹C]­verapamil or [¹¹C]-<i>N</i>-desmethyl-loperamide at the BBB and thereby contribute to previously observed species differences between rodents and humans. We performed <i>in vitro</i> transport experiments with [³H]­verapamil and [³H]-<i>N</i>-desmethyl-loperamide in ABCB1 and Abcb1a overexpressing cell lines. Moreover we conducted <i>in vivo</i> PET experiments and biodistribution studies with (<i>R</i>)-[¹¹C]­verapamil and [¹¹C]-<i>N</i>-desmethyl-loperamide in wild-type mice without and with tariquidar pretreatment and in homozygous <i>Abcb1a/1b^(−/−)</i> and heterozygous <i>Abcb1a/1b^(+/−)</i> mice. We found no differences for <i>in vitro</i> transport of [³H]­verapamil and [³H]-<i>N</i>-desmethyl-loperamide by ABCB1 and Abcb1a and its inhibition by tariquidar. [³H]-<i>N</i>-Desmethyl-loperamide was transported with a 5 to 9 times higher transport ratio than [³H]­verapamil in ABCB1- and Abcb1a-transfected cells. <i>In vivo</i>, brain radioactivity concentrations were lower for [¹¹C]-<i>N</i>-desmethyl-loperamide than for (<i>R</i>)-[¹¹C]­verapamil. Both radiotracers showed tariquidar dose dependent increases in brain distribution with tariquidar half-maximum inhibitory concentrations (IC₅₀) of 1052 nM (95% confidence interval CI: 930–1189) for (<i>R</i>)-[¹¹C]­verapamil and 1329 nM (95% CI: 980–1801) for [¹¹C]-<i>N</i>-desmethyl-loperamide. In homozygous <i>Abcb1a/1b^(−/−)</i> mice brain radioactivity distribution was increased by 3.9- and 2.8-fold and in heterozygous <i>Abcb1a/1b^(+/−)</i> mice by 1.5- and 1.1-fold, for (<i>R</i>)-[¹¹C]­verapamil and [¹¹C]-<i>N</i>-desmethyl-loperamide, respectively, as compared with wild-type mice. For both radiotracers radiolabeled metabolites were detected in plasma and brain. When brain and plasma radioactivity concentrations were corrected for radiolabeled metabolites, brain distribution of (<i>R</i>)-[¹¹C]­verapamil and [¹¹C]-<i>N</i>-desmethyl-loperamide was increased in tariquidar (15 mg/kg) treated animals by 14.1- and 18.3-fold, respectively, as compared with vehicle group. Isoflurane anesthesia altered [¹¹C]-<i>N</i>-desmethyl-loperamide but not (<i>R</i>)-[¹¹C]­verapamil metabolism, and this had a direct effect on the magnitude of the increase in brain distribution following ABCB1 inhibition. Our data furthermore suggest that in the absence of ABCB1 function brain distribution of [¹¹C]-<i>N</i>-desmethyl-loperamide but not (<i>R</i>)-[¹¹C]­verapamil may depend on cerebral blood flow. In conclusion, we have identified a number of important factors, i.e., substrate affinity to ABCB1, brain uptake of radiolabeled metabolites, anesthesia, and cerebral blood flow, which can directly influence the magnitude of ABCB1-mediated DDIs at the BBB and should therefore be taken into consideration when interpreting PET results