Abstracts

Rationale: Reductions of GABA-mediated inhibitory signaling have been associated with seizures and epilepsy. One mechanism to increase GABAergic neurotransmission is the inhibition of GABA-amino transferase (GABA-AT), the primary catabolic enzyme of GABA. Administration of a highly potent GABA-AT inhibitor, OV329, is proposed to increase levels of GABA within the CNS through a mechanism of action similar to that of vigabatrin (VGB). We previously reported that a single 10 mg/kg dose of OV329 significantly suppressed the number of focal seizures in the intrahippocampal kainate-Mesial Temporal Lobe Epilepsy (MTLE) mouse model. To further explore the potential efficacy of OV329, the NIH/NINDS Epilepsy Therapy Screening Program (ETSP) began a series of studies in 2020 in four distinct mouse seizure models.

Methods: Seizures and epilepsies were induced in mice through four distinct methods: 6 Hz electrical stimulation, maximal electroshock seizures (MES), corneal kindling model (CKM), and MTLE. Mice were then treated with OV329 at a range of doses (1–70 mg/kg PO) and assessed for seizure control at various timepoints (0.25 – 8 hours post-dose, in addition to baseline). Behavioral screening for tolerability and motor impairment was performed using the Rotarod test.

Results: No protection was demonstrated at any dose or timepoint in the 6 Hz or MES models. Unexpected deaths occurred in up to 50% of MES mice across doses. In the CKM, the OV329 ED₅₀ was found to be 27.9 and 46.7 mg/kg at two separate testing sites. OV329-treated CKM mice were significantly less likely to reestablish the kindled state at any point following a 55-day washout. Kindling status resumed immediately for control animals (100%), whereas it was delayed or absent in mice treated with OV329 (12.5% and 40% recovered fully kindled status in 40 and 50 mg/kg treatment groups, respectively). Finally, hippocampal paroxysmal discharges were completely abolished in all MTLE mice treated with a single 30 mg/kg dose, while sedation was noted in 25% of these animals at the 8-hour time point. No motor impairment on the Rotarod test was seen in mice with doses of 30 mg/kg or lower.

Conclusions: Seizures induced in acute models (6 Hz and MES) were not mitigated by OV329 pretreatment at the doses tested. However, OV329 demonstrated efficacy in the subacute models of epileptogenesis (CKM and MTLE). In these, a single, high dose of OV329 produced near complete protection from seizure activity. While OV329 administration in the CKM notably prevented mice from retaining the kindled state, further studies must be performed to determine whether this represents a disease-modifying effect or alteration of the kindling stimulus. As these were signal-finding studies, future studies will assess effective dosing strategies based on the pharmacokinetic and pharmacodynamic properties of OV329.

Funding: These data were generated in whole by the Epilepsy Therapy Screening Program (ETSP), NINDS, National Institutes of Health.