Abstracts

Rationale: One-third of people living with epilepsy (PWE) do not respond to their antiseizure medicine (ASM) therapy, putting them at greater risk for higher disease burden, lower quality of life and SUDEP compared to those individuals whose seizures are well controlled. Mechanisms of pharmacoresistance are likely multifactorial, yet few preclinical studies adhere to the ILAE definition of drug resistance (i.e., failure of 2 or more ASM therapy trials) when studying the pathophysiology of drug resistant epilepsy (DRE). We have previously developed a rat model of DRE where ~30% of rats are resistant to >2 ASM monotherapy trials when administered via a clinically relevant chronic oral dosing paradigm. Herein, we use this model to identify drug-, disease- and pathophysiological factors that contribute to DRE in an etiologically relevant rat model of temporal lobe epilepsy.

Methods: Male Sprague Dawley rats with confirmed epilepsy following post-kainic acid induced status epilepticus (KASE) were enrolled in a triple crossover study design to receive carbamazepine (300 mg/kg/day), levetiracetam (400 mg/kg/day), and lamotrigine (30 mg/kg/day) orally through food for 2-week monotherapy treatment arms. At the end of the study, rats received all three monotherapies but in a different order. Changes in seizure frequency and severity were recorded via 24/7 videoECoG monitoring. Plasma samples were collected at the end of each treatment period to confirm therapeutic drug levels. At the end of the study, rats were euthanized, and brains were collected for immunohistochemical labeling of markers of neuroinflammation (Iba-1, CD3) and neuronal network integrity (NeuN).

Results: Approximately 33% of rats developed DRE, regardless of treatment order and were independent of ASM concentrations in plasma. Seizure burden prior to initiation of treatment was not a predictor of DRE (mean seizures/day, sensitive: 1.7; resistant: 0.7), nor was it a predictor of the response to the first ASM delivered. Notably, we observed significant seizure aggravation with lamotrigine in most rats (median increase: 100%), however, this transient worsening of epilepsy severity did not influence subsequent ASM response to levetiracetam (median reduction 55%) or carbamazepine (median reduction 50%). Rats with DRE had increased numbers of IBA-1 reactive microglia in the hippocampus that was independent of treatment group or disease severity (e.g., seizure frequency). Moreover, this increase was independent of disruptions in NeuN labeling in the same brain regions.

Conclusions: Taken together, these data highlight that mechanisms of drug resistance are likely multifactorial, and an integrative approach should be considered when developing treatment options for DRE. Specifically, microglia reactivity and associated neuroinflammation may highlight novel, non-neuronal targets for the future of therapeutic development.

Funding: American Epilepsy Society Postdoctoral Fellowship and ITHS KL2 Career Development Award (5KL2TR002317-08) (MG). Work supported by Department of Pharmacy (HSW)