Objectives

The main objective is to determine the contribution of P-gp activity to uptake and binding of existing PET tracer [18F]-MPPF, and [11C]-FMZ that are P-gp substrates.
Therefore the effect of P-gp modulators (such as tariquidar or CsA) for enhancing brain distribution of [18F]-MPPF or [11C]-FMZ will be investigated. The impact of epileptic seizures on radiotracer brain distribution will be evaluated. Considering these data further studies aim to test for a correlation between radiotracer binding and the pharmacosensitivity of epileptic seizures. 

 

Workpackage description

In view of the development of therapeutic strategies to overcome transporter-mediated resistance of epilepsy, it will be important to develop non-invasive tools to select patients with functionally relevant transporter overexpression. PET tracers that are P-gp substrates, given in combination with P-gp modulators, would help to image P-gp overexpression as a mechanism of pharmacoresistance in individuals with epilepsy, and would help to select patients that may benefit from specific therapeutic strategies to overcome transporter-mediated pharmacoresistance. Within this context, animal models of difficult-to-treat or pharmacoresistant epilepsy render a unique tool to validate radiotracers and P-gp modulators under standardized conditions. Respective human [11C]-FMZ and [18F]-MPPF PET data will be acquired.

Task 1:
Optimization of quantification procedures in naïve rats
The time activity course of [18F]-MPPF and [11C]-FMZ will be measured in different target areas following intravenous administration in naive rats. Different methods for the calculation of the binding potential of the radiotracers will be compared in order to choose the optimal method for further investigations. For more precise anatomical correlation additional MRI scans will be performed.

Task 2:
Impact of P-gp modulators on brain distribution of [18F]-MPPF and [11C]-FMZ in naïve rats
The impact of different Pgp modulators (e.g. ciclosporine A, tariquidar) on [18F]-MPPF and [11C]-FMZ brain distribution will be investigated following intravenous administration of the PET tracers in rats. An adequate dosing protocol of one selected Pgp modulator will be determined for further investigations.

Task 3:
Effect of epileptic seizures
The impact of epileptic seizures on [18F]-MPPF and [11C]-FMZ brain distribution and its modulation by Pgp modulators will be determined in post-status epilepticus models with development of spontaneous seizures. PET data are tested for a correlation with seizure data obtained by continuous EEG-video monitoring.

Task 4:
Correlation between the brain distribution of PET tracers and the therapeutic response in epileptic rats
Subgroups of pharmacoresistant and pharmacosensitive rats are identified during a phase of chronic phenobarbital treatment in a post-status epilepticus model with spontaneous recurrent seizures. The brain distribution of a selected PET tracer (FMZ or MPPF) and its modulation by a selected P-gp inhibitor is then compared between both groups.
 
Task 5:
PET tracer in drug-resistant and drug-responsive epilepsy – effect of P-gp inhibitors
PET scans for each tracer will be performed in 12 patients with pharmacoresistant TLE and 6-12 patients with pharmacosensitive TLE, one in the resting state and one after P-gp blockade. Effects will be correlated with response to medication as assessed by treatment history, seizure frequency, and time since last seizure. In addition, depressive symptoms will be valuated prior to each scan. Data from ten healthy controls provide measures of physiological intra-subject test-retest variability