Imaging of P-glycoprotein-mediated pharmacoresistance in the hippocampus: proof-of-concept in a chronic rat model of temporal lobe epilepsy.

PURPOSE: Based on experimental findings, overexpression of P-glycoprotein at the blood-brain barrier has been suggested to be a contributor to pharmacoresistance of the epileptic brain. We test a technique for evaluation of interindividual differences of elevated transporter function, through microPET analysis of the impact of the P-glycoprotein modulator tariquidar. The preclinical study is intended for eventual translation to clinical research of patients with pharmacoresistant seizure disorders. METHODS: We made a microPET evaluation of the effects of tariquidar on the brain kinetics of the P-glycoprotein substrate [(18) F]MPPF in a rat model with spontaneous recurrent seizures, in which it has previously been demonstrated that phenobarbital nonresponders exhibit higher P-glycoprotein expression than do phenobarbital responders. RESULTS: Mean baseline parametric maps of the [(18) F]MPPF unidirectional blood-brain clearance (K(1) ; ml/g per min) and the efflux rate constant (k(2) ; per min) did not differ between the nonresponder and responder group. Tariquidar pretreatment increased the magnitude of [(18) F]MPPF K(1) in hippocampus by a mean of 142% in the nonresponders, which significantly exceeded the 92% increase observed in the responder group. The same treatment decreased the mean magnitude of [(18) F]MPPF k(2) in hippocampus by 27% in nonresponders, without comparable effects in the responder group. DISCUSSION: These results constitute a proof-of-concept for a novel imaging approach to evaluate blood-brain barrier P-glycoprotein function in animals. By extension, [(18) F]MPPF positron emission tomography (PET) with tariquidar pretreatment may be amenable for clinical applications exploring further the relevance of P-glycoprotein overexpression, and for enabling the rational design of pharmacotherapy according to individual differences in P-glycoprotein expression.

Uptake and binding of the serotonin 5-HT1A antagonist [18F]-MPPF in brain of rats: effects of the novel P-glycoprotein inhibitor tariquidar.

We used microPET to map the dose-response to the novel P-glycoprotein (P-gp) inhibitor tariquidar (TQD) of the initial influx of the P-gp substrate [(18)F]-MPPF in rat brain, and to test for effects of P-gp inhibition on the subsequent binding of [(18)F]-MPPF to serotonin 5-HT(1A) receptors. Summation maps of [(18)F]-MPPF uptake during the first 100 seconds after intravenous injection were calculated in groups of rats with vehicle (glucose 5%) pretreatment, or following pretreatment with TQD at doses of 5, 15, or 30 mg/kg. The early summation image (K(1)-weighted), were validated as a surrogate marker for the physiological blood-brain clearance (K(1); ml g(-)(1) min(-1)) by linear graphic analysis of the unidirectional blood-brain clearance relative to an image-based arterial input measured in the left ventricle of the heart. In the same animals, parametric maps of the [(18)F]-MPPF binding potential (BP(ND)) were calculated from the entire 60-minute emission recordings using conventional reference tissue methods. All [(18)F]-MPPF recordings were followed by an [(18)F]-FDG emission recording, the summation of which was used for spatial normalization to a rat brain atlas. Test-retest variability of K(1)-weighted uptake and BP(ND) was 25%. TQD treatment evoked a global dose-dependent increase in K(1)-weighted summation, which increased 2.5-fold with TQD (30 mg/kg), suggesting an IC(50) of 5 mg/kg TQD. All TQD doses increased the apparent [(18)F]-MPPF BP(ND) calculated by the Logan method by 30%-40%, a bias likely arising due to increased free [(18)F]-MPPF concentrations in brain. TQD (15 mg/kg) evoked a 45% global increase in [(18)F]-FDG uptake, suggesting perturbation of brain energy metabolism due to P-gp blockade.