Objectives

Our main objective is to validate radiotracers for P-gp activity as biomarkers for drug resistance using drug-resistant and drug-responsive epilepsy as a biological model, the required proof-of-concept of a non-invasive molecular imaging-based tool. Epilepsy is the ideal condition for testing suitability of these imaging biomarkers, as either surgical specimens or whole brains are available for direct in-vivo / ex-vivo comparative analysis (WP08).

Task 1:
To determine test-retest variability in drug-resistant and drug-responsive patients with epilepsy using PET and (R)-[11C]verapamil

Task 2, 4:
To determine the effects of chronic epilepsy and therapeutic anti-epileptic drugs (AED) on brain P-gp function

Task 3:
To determine the effects of P-gp inhibitors on the cerebral uptake of (R)-[11C]verapamil

 

Workpackage description

Background
Previous data from a pilot study in patients with pharmacoresistant unilateral temporal lobe epilepsy (TLE) showed a trend towards reduced (R)-[11C]verapamil uptake in epileptogenic as compared to non-epileptogenic brain areas, supporting the notion that regionally enhanced P-gp activity occurs in these patients (Langer et al, 2007). Abrahim et al (2008) recently found that metabolism of (R)-[11C]verapamil is significantly faster in patients with epilepsy than normal controls, which was attributed to hepatic CyP450 enzyme induction caused by AED.

(R)-[11C]verapamil-PET studies will be performed in two centres (P03-MUW, P08-UNIMAN). Drug-resistant and drug-responsive patients with well characterised epilepsy syndromes will be recruited from two large clinical bases (P01-UCL, P03-MUW). AED plasma levels will be measured for all patients to verify that differing plasma concentration does not confound the assessment of pharmacoresistance. The kinetic model developed in WP05 will be applied to test its robustness to changes in plasma pharmacokinetics of radiolabelled compounds resulting from chronic medication interaction with enzyme-inducing co-medication.

Task 1
Test-retest reproducibility in patients with epilepsy
16 patients with TLE, 8 drug-responsive and 8 drug-resistant, will undergo PET scanning with (R)-[11C]verapamil twice at baseline.

Since it is expected that seizure activity worsens variability, we hypothesise that drug-resistant patients have greater variability than drug-responsive patients and healthy controls (WP05).

Task 2
Effect of chronic epilepsy on brain P-gp function
Baseline (R)-[11C]verapamil uptake will be measured with PET in 8 healthy controls and two groups of patients:

• 8 patients with TLE who are drug-responsive and have not had a seizure for at least 12 months
• 16 patients with TLE who are refractory to at least 3 first-line AEDs and will undergo resective surgery at most 12 months after their PET scans.

Algorithms to correct for partial volume effects (WP05) will be crucial for quantification in the sclerotic hippocampus, the epileptogenic region.

We hypothesise that there will be reduced (R)-[11C]verapamil uptake from the epileptogenic region and areas of secondary seizure spread in drug-resistant TLE patients compared to the contralateral side and to same regions in drug-responsive patients. The difference is expected to be correlated with the level of transporter expression as measured independently using immunohistochemistry (WP08).

Similar experiments as those in tasks 1 and 2 will be performed at P02-VUmc using [11C]laniquidar, if the radiotracer emerges successfully from WP05.

Task 3
Effect of P-gp inhibitors on radiotracer uptake in patients with epilepsy
Blocking efflux pumps with cyclosporine-A (CsA) increased brain uptake of [11C]verapamil in a dose-dependent manner in rats (Hsiao et al, 2006) and by 80% at a tolerable dose in normal human volunteers (Sasongko et al, 2005). The 16 patients with refractory TLE and the 8 healthy controls who will have undergone baseline PET scanning with (R)-[11C]verapamil will be studied a second time under P-gp blockade. The two most suitable P-gp inhibitors and the optimal doses will be chosen based on the outcome of animal experiments using loperamide, CsA or tariquidar (WP02).

We hypothesise that after P-gp inhibitor administration, (R)-[11C]verapamil in refractory TLE patients will be increased in the epileptogenic region over and above the global increase in these patients and the increase in healthy controls.

Task 4
Influence of AEDs and acute seizures on brain P-gp function
Most AEDs are P-gp substrates and might thus compete with (R)-[11C]verapamil for transport by P-gp at the blood-brain barrier. 20 patients with refractory epilepsy due to malformations of cortical development or tumours will undergo paired (R)-[11C]verapamil PET scans, under treatment and during epilepsy monitoring. By comparing (R)-[11C]verapamil brain uptake in epileptogenic regions under full therapeutic medication with those under conditions of absent or reduced AED doses, we will assess the effects of medication and chronic epilepsy on P-gp function in vivo.