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>)-[<sup>11</sup>C]verapamil and [<sup>11</sup>C]-<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>)-[<sup>11</sup>C]verapamil or [<sup>11</sup>C]-<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 [<sup>3</sup>H]verapamil
and [<sup>3</sup>H]-<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>)-[<sup>11</sup>C]verapamil and [<sup>11</sup>C]-<i>N</i>-desmethyl-loperamide in wild-type mice without and with
tariquidar pretreatment and in homozygous <i>Abcb1a/1b<sup>(−/−)</sup></i> and heterozygous <i>Abcb1a/1b<sup>(+/−)</sup></i> mice. We found no differences for <i>in vitro</i> transport of [<sup>3</sup>H]verapamil and [<sup>3</sup>H]-<i>N</i>-desmethyl-loperamide by ABCB1 and Abcb1a and its inhibition
by tariquidar. [<sup>3</sup>H]-<i>N</i>-Desmethyl-loperamide
was transported with a 5 to 9 times higher transport ratio than [<sup>3</sup>H]verapamil in ABCB1- and Abcb1a-transfected cells. <i>In vivo</i>, brain radioactivity concentrations were lower for
[<sup>11</sup>C]-<i>N</i>-desmethyl-loperamide than for
(<i>R</i>)-[<sup>11</sup>C]verapamil. Both radiotracers
showed tariquidar dose dependent increases in brain distribution with
tariquidar half-maximum inhibitory concentrations (IC<sub>50</sub>) of 1052 nM (95% confidence interval CI: 930–1189) for (<i>R</i>)-[<sup>11</sup>C]verapamil and 1329 nM (95% CI: 980–1801)
for [<sup>11</sup>C]-<i>N</i>-desmethyl-loperamide. In homozygous <i>Abcb1a/1b<sup>(−/−)</sup></i> mice brain radioactivity
distribution was increased by 3.9- and 2.8-fold and in heterozygous <i>Abcb1a/1b<sup>(+/−)</sup></i> mice by 1.5- and 1.1-fold,
for (<i>R</i>)-[<sup>11</sup>C]verapamil and [<sup>11</sup>C]-<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>)-[<sup>11</sup>C]verapamil and
[<sup>11</sup>C]-<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 [<sup>11</sup>C]-<i>N</i>-desmethyl-loperamide but not (<i>R</i>)-[<sup>11</sup>C]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 [<sup>11</sup>C]-<i>N</i>-desmethyl-loperamide but not (<i>R</i>)-[<sup>11</sup>C]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