While the cardiovascular system is a promising therapeutic target, it also presents significant safety hurdles. The primary advantages of human pluripotent stem cell (hPSC, which includes hES and iPS cells)-derived cardiomyocytes for use in drug discovery are: they are a human-based model; they display normal cardiac characteristics; and they survive under cell culture conditions for extended periods.
These factors are expected to enable more physiologically relevant efficacy studies as well as toxicity assessments for both electrical and traditional cytotoxic endpoints such as cell viability, ATP production, and mitochondrial toxicity.
Action potentials are the rhythmic electrical oscillations of cardiomyocyte membrane potential that underlie the heartbeat and basic cardiac function. The action potential waveform results from ions crossing the plasma membrane through a variety of ion channels. Drug interactions with cardiac ion channels resulting in action potential prolongation are of particular concern as they can lead to ventricular arrhythmias and sudden death.
The most common cause of drug-induced cardiac action potential prolongation is drug block of the hERG channel. Because the cardiac action potential waveform and ion-channel expression patterns are species specific, different toxicity assessments can be drawn from applying the same compound to cardiomyocytes from nonhuman species.
CDI’s hPSC-derived cardiomyocytes allow for an early indicator of drug-induced ion-channel dysfunction in a human model system (Figure 1). Single hPSC-derived cardiomyocytes were subjected to perforated-patch voltage clamp analysis. Action potentials were recorded from single cells under basal conditions and in the presence of terfenadine, a nonsedating antihistamine that was pulled from the market in 1998 for its potent hERG block and potential to cause sudden cardiac death.
Figure 1a shows representative patch clamp recordings of cardiac action potentials under basal conditions, during perfusion with 20 nM terfenadine, and following compound washout. Myocytes show action potential waveforms typical of ventricular hPSC-derived cardiomyocytes under basal conditions. Addition of 20 nM terfenadine causes action potential prolongation (2 min postdrug addition) that can lead to secondary premature depolarizations termed early-after depolarizations (EADs; 10 min post drug addition), which are considered a potential trigger for life-threatening cardiac arrhythmias.
Upon drug washout the action potential returns to its basal phenotype. The mean increase in action potential prolongation at 90% of repolarization (APD90) as a function of drug exposure duration is shown in Figure 1b, while the mean increase in APD90 at ten minutes of drug exposure is shown in Figure 1c.
As cardiomyocytes are aerobically poised, contractile, and metabolically active, they may present a predisposition to toxicity endpoints such as viability, apoptosis, ATP production/metabolism, and mitochondrial dysfunction. hPSC-derived cardiomyocytes are readily amenable to assessing these endpoints through commercially available test kits.