Novel test strategies for in vitro seizure liability assessment

Abstract

Introduction: The increasing incidence of mental illnesses and neurodegenerative diseases results in a high demand for drugs targeting the central nervous system (CNS). These drugs easily reach the CNS, have a high affinity for CNS targets, and are prone to cause seizures as an adverse drug reaction. Current seizure liability assessment heavily depends on in vivo or ex vivo animal models and is therefore ethically debated, labor intensive, expensive, and not always predictive for human risk.

Areas covered: The demand for CNS drugs urges the development of alternative safety assessment strategies. Yet, the complexity of the CNS hampers reliable detection of compound-induced seizures. This review provides an overview of the requirements of in vitro seizure liability assays and highlights recent advances, including micro-electrode array (MEA) recordings using rodent and human cell models.

Expert opinion: Successful and cost-effective replacement of in vivo and ex vivo models for seizure liability screening can reduce animal use for drug development, while increasing the predictive value of the assays, particularly if human cell models are used. However, these novel test strategies require further validation and standardization as well as additional refinements to better mimic the human in vivo situation and increase their predictive value.

Keywords: Alternatives to animal testing; GABAa receptor antagonists; drug safety assessment; human-induced pluripotent stem cell (hiPSC)-derived neuronal models; in vitro seizure liability assessment; ion channels; micro-electrode array (MEA) recordings; rodent primary cortical cultures; safety pharmacology.

Figure 1.

Schematic representation of neuronal signaling. Integration of dendritic input results in generation of an action potential that travels via the axon to the presynaptic terminal, where the neuron makes contact with the postsynaptic neuron (left side of picture). Arrows depict travel direction of action potential. When an action potential reaches the presynaptic terminal, Ca²⁺ influx via voltage-gated calcium channels (VGCC) triggers the fusion of vesicles loaded with neurotransmitter with the cell membrane, thereby releasing neurotransmitter in the synaptic cleft (right side of picture, top part). Neurotransmitters can bind to ionotropic receptors that undergo a confirmation change upon binding allowing for the passage of ions through the channel (right bottom half, left receptor). Neurotransmitters can also bind to metabotropic receptors (right receptor). Upon binding, this receptor activates a G-protein complex that then activates an enzyme. This enzyme either activates a second messenger system that triggers cellular responses or opens an ion channel. Most of these processes can be subject to modulation by drugs, potentially resulting in seizure induction

Figure 2.

MEA plates have an electrode grid on the bottom on top of which (neuronal) cells can be cultured (left) for noninvasive recordings of electrical activity. Recorded activity can be depicted in a raster plot (right) that illustrates the major MEA metric parameters. The example raster plot depicts the activity of a human iPSC-derived neuronal co-culture at 16 electrodes (horizontal lines) in a single well, where each tick mark (red circle) depicts one spike in a ~ 100 s recording window. An example of a burst is encircled in green and network burst in orange. Burst duration and network burst duration are depicted with a green and orange arrow, respectively, whereas an inter-burst-interval (IBI) is marked with a purple arrow. The cumulative trace above the raster plots indicates the synchronized activity between the different electrodes. The blue circle thus represents the level of synchronicity