Targeting the Kappa Opioid System to Treat TLE

The high incidence of drug-resistant focal epilepsies poses a persistent challenge in medicine. Certain patients benefit from surgical removal of the epileptogenic focus. However, a large cohort of patients cannot be treated sufficiently at present. We and others have recently demonstrated the importance of dynorhin, an endogenous opioid peptide, in seizure control. In a pharmaco-resistant animal model of temporal lobe epilepsy (TLE) we showed antiepileptic effects upon exogenous application of agonists for the kappa opioid receptor (KOR), the primary receptor for dynorphin. However, KOR agonists typically induce dysphoric side effects, which hampers their use in clinics. These side effects appear dependent on one of two major signaling cascades (-arrestin) induced by KOR activation. In contrast, the other cascade induced through the G-protein most probably is important for seizure control. Targeted expression of modified dynorphins in the epileptogenic focus may overcome this problem. Preliminary experiments provided the proof of principle for such a therapy. The aim of tis project is to develop the basis for a gene therapy, which on the long run may help patients with drug-resistant epilepsies. Gene therapy offers a number of interesting aspects especially for epilepsies, which usually require life-time treatment with drugs causing severe side effects. On one hand, the treatment will be restricted to those brain areas causing the seizures, on the other the patient cannot forget to take the medication. Moreover, neuropeptides are expressed in neurons and released only upon strong stimulation (like onset of a seizure). This reduces the risk of tolerance, as it is frequently observed for permanently available drugs. To facilitate the options of gene therapy, it is important to optimize the peptides used. The optimization targets two aspects: 1) a specific activation of the target receptor with low or no activation of other receptors and 2) an as efficient as possible activation of the G-protein pathway combined with as little as possible activation of the -arrestin pathway, which would reduce the risk of side effects and internalization of the target receptor. The long-term goal of our studies is to develop novel therapeutic options for patients suffering from refractory mesial temporal lobe epilepsy, and potentially other types of intractable, focal epilepsies.

With a prevalence of 1-2% worldwide, epilepsy is one of the most frequent neurological diseases affecting people of all ages. Of the 870 million people living in the European Region, over 5 million suffer from epilepsy. Current pharmacotherapies are not sufficiently effective in over 30 % of patients and cause severe side effects. Therefore, research efforts are needed towards overcoming the limitations of present therapies, with the final goal to improve treatment efficacy and patient compliance. Recent evidence supports the idea of targeting the kappa opioid receptor (KOPr) with molecules activating the G-protein signaling pathway preferentially over the -arrestin2 signaling to achieve anti-ictal effects with low adverse effects. Within this project we characterized a number of dynorphin B variants in-vitro and in-vivo. Dynorphin B is one of the main endogenous ligands for the KOPr and a candidate for a gene therapy for temporal lobe epilepsy. This was shown by our group in an animal model over the recent years. The aim of the study was to identify dynorphin B variants with a higher selectivity for KOPr and a stronger activation of the G-protein pathway over the -arrestin2 pathway. This would reduce the risk of adverse effects. Indeed we identified a number of dynorphin B variants with a higher selectivity for KOPr over mu and delta opioid receptors. However, these variants did not show differences in the activation of G-protein and the -arrestin2. In parallel we investigated a variant of the KOPr, which is not activated by the endogenous ligands (the dyorphins) but by salvinorin B. Salvinorin B does not activate the wild-type KOPr. This concept is called Designer Receptor Exclusively Activated by Designer Drugs (DREADD). The Idea behind this strategy is to control the therapy in two ways: i) the DREADD is transferred by gene therapy only to the epileptogenic focus and ii) salvinorin B can be administered at the dose needed. Unfortunately this system proofed not useful, because the solvent needed for salvinorin B influenced seizures. Moreover, no effect of salvinorin B was observed after the effects of the solvent faded, suggesting a short duration of a potential effect. One important aspect of temporal lobe epilepsy is sex. Irrespective of this, at present almost exclusively male mice are tested in preclinical studies. To overcome this gap, we started to characterize the intrahippocampal kainic acid model in female mice. This opens the possibility not only to test the effect of novel treatments in both sexes, but also to test them during pregnancy and lactation and their impact on the offspring. First data were published and further investigations are planned for a follow-up project.