Cardiotoxicity testing in human 3D in vitro cardiac microtissues
A nine-compound study
SmartHeart® is a human 3D cardiac microtissue platform designed to investigate drug-induced changes in cardiac function. By combining human iPSC-derived cardiomyocytes and cardiac fibroblasts in self-assembled, ring-shaped microtissues, SmartHeart enables quantitative assessment of contractility, electrophysiology and calcium handling.
This white paper presents the experimental evaluation of nine cardioactive compounds, including seven CiPA reference drugs with different proarrhythmic risk profiles and two additional compounds selected to investigate inotropic responses. The results demonstrate how complementary functional readouts can help characterize drug-specific cardiac effects, including altered repolarization, changes in beating activity and positive or negative inotropy.
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KEY FINDINGS
Nine cardioactive compounds investigated: Seven CiPA reference compounds and two additional inotropic drugs were evaluated using selected functional readouts.
Electrophysiological responses: SmartHeart captured compound-specific alterations in action potential duration, amplitude, depolarization kinetics and beating activity.
Multichannel pharmacology: Responses to compounds such as ranolazine and terfenadine illustrate the value of integrated cardiac measurements for investigating complex ion-channel effects.
Cardiac contractility: Positive and negative inotropic responses were observed with isoprenaline and verapamil, respectively.
Calcium handling: Exploratory calcium transient measurements with metoprolol and nifedipine provided additional information on drug-induced functional changes.
Together, these findings support the use of human 3D cardiac microtissues for mechanistic cardiac safety investigations and the characterization of compound-specific functional responses.
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WHY HUMAN 3D CARDIAC MODELS MATTER FOR DRUG SAFETY
Drug-induced cardiotoxicity remains a significant challenge in pharmaceutical development. Cardiac adverse effects can arise through different mechanisms, including ion-channel inhibition, altered repolarization, disrupted calcium handling, impaired contractility and abnormal cardiac rhythm.
Conventional cardiac safety testing includes ion-channel assays, particularly hERG inhibition measurements, as well as complementary cellular, preclinical and clinical assessments. These approaches provide essential information, but individual endpoints do not fully characterize the integrated mechanical and electrical responses of human cardiac tissue.
The Comprehensive in vitro Proarrhythmia Assay (CiPA) initiative promotes a mechanistic approach to proarrhythmic risk assessment by integrating ion-channel pharmacology, computational modeling and human iPSC-derived cardiomyocyte measurements.
Human 3D cardiac microtissues provide a complementary experimental approach. Their multicellular organization enables researchers to investigate cardiac electrical activity, calcium dynamics and mechanical contraction in a tissue-level environment. By combining these functional endpoints, researchers can better characterize how compound-induced changes in cardiac signaling translate into measurable tissue responses.
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SMARTHEART®: A HUMAN IN VITRO 3D CARDIAC TISSUE PLATFORM
SmartHeart® uses a 96-well plate format in which human iPSC-derived cardiomyocytes and cardiac fibroblasts self-assemble into ring-shaped microtissues around deformable hydrogel pillars. The ring geometry provides mechanical resistance during contraction and enables quantitative assessment of tissue deformation. The glass-bottom configuration supports brightfield and fluorescence-based imaging directly in the culture plate.
Cardiac Contractility and Inotropic Effects
Brightfield video analysis tracks changes in cardiac microtissue geometry during contraction and relaxation. Measured or derived readouts include contraction strain, contractile stress, contraction and relaxation speeds, beating frequency and rhythm regularity. These measurements help identify positive or negative inotropic responses and characterize drug-induced changes in cardiac mechanical function.
Electrophysiology and Repolarization
Fluorescence-based optical electrophysiology enables the characterization of action potential dynamics in human cardiac microtissues. Readouts include action potential duration (APD), amplitude, depolarization kinetics, repolarization characteristics, beating frequency and irregularity. These measurements support investigation of ion-channel-related effects relevant to proarrhythmic risk.
Calcium Transients and Excitation–Contraction Coupling
Calcium-sensitive fluorescence imaging can be used to investigate intracellular calcium dynamics. Readouts include calcium transient amplitude, duration, rise and decay kinetics, and beating activity. These measurements provide complementary mechanistic information on calcium handling and excitation–contraction coupling.
Figure 1. Functional readouts available with SmartHeart®, including contractility, optical action potential measurements and calcium transient analysis.
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EXPERIMENTAL RESULTS: NINE CARDIOACTIVE COMPOUNDS
Nine cardioactive compounds with different pharmacological mechanisms were investigated using SmartHeart®. Experimental readouts were selected according to each compound and the study objectives; not every compound was assessed with every measurement modality. Seven compounds were investigated in the context of the CiPA reference panel, while isoprenaline and verapamil were included to evaluate positive and negative inotropic effects.
Table 1. Summary of cardioactive compounds investigated and their observed functional responses in SmartHeart®. Readouts and experimental sample sizes vary across compounds.
Dofetilide — IKr Inhibition and Delayed Repolarization
Dofetilide is a selective blocker of the rapid delayed rectifier potassium current (IKr) and a reference compound associated with high torsade de pointes (TdP) risk. In SmartHeart®, dofetilide induced substantial prolongation of late action potential repolarization, with a reported increase of more than twofold in the APD50–90 interval.
These electrophysiological changes were accompanied by reduced beating frequency and contractile stress. At the highest concentration tested, up to 38% of the cardiac rings became quiescent, while beating irregularity increased.
These observations illustrate how optical electrophysiology and tissue-level mechanical measurements can capture complementary functional consequences of IKr inhibition.
Figure 2. Dofetilide concentration-response in SmartHeart®. Electrophysiology: N = 3 independent experiments, n = 29 tissues. Contractility: N = 2 experiments, n = 27 tissues.
Disopyramide — Combined Sodium and Potassium Channel Effects
Disopyramide exhibits sodium- and potassium-channel blocking activities that can affect both cardiac depolarization and repolarization.
SmartHeart® measurements showed decreased action potential amplitude and depolarization speed, together with prolonged late repolarization. The electrophysiological response was accompanied by a reduction in contractile stress and changes in beating regularity across the tested concentrations.
These observations illustrate the value of assessing multiple cardiac functional endpoints when investigating compounds with more than one relevant ion-channel action.
Figure 3. Disopyramide concentration-response in SmartHeart®. Electrophysiology: N = 3 experiments, n = 25 tissues. Contractility: N = 2 experiments, n = 30 tissues.
Domperidone — Electrophysiological and Mechanical Alterations
Domperidone is a dopamine receptor antagonist with reported inhibitory effects on cardiac sodium and hERG potassium channels.
In SmartHeart®, increasing domperidone concentrations were associated with reduced action potential amplitude, slower depolarization and prolonged late repolarization.
The study also reported decreased beating frequency, reduced contractile stress and slower contraction and relaxation kinetics. At the highest tested concentration, up to 33% of tissues became quiescent.
These results demonstrate how electrophysiological alterations can be investigated alongside changes in cardiac mechanical activity.
Figure 4. Domperidone concentration-response in SmartHeart®. Electrophysiology: N = 3 experiments, n = 29 tissues. Contractility: N = 2 experiments, n = 16 tissues.
Metoprolol — Beta-Adrenergic Modulation and Calcium Handling
Metoprolol is a selective β1-adrenergic receptor antagonist that reduces adrenergic stimulation of cardiac activity.
In SmartHeart®, metoprolol decreased beating frequency and contractile stress. Changes in action potential characteristics and beating irregularity were also observed.
Exploratory calcium transient measurements showed reduced calcium transient amplitude and altered calcium release kinetics.
These calcium observations are preliminary and require further experimental replication before broader conclusions can be drawn.
Figure 5. Metoprolol concentration-response in SmartHeart®. Electrophysiology: N = 3 experiments, n = 35 tissues. Contractility: N = 2 experiments, n = 25 tissues. Calcium transient analysis under 1.5 Hz pacing: N = 1 experiment, n = 9 tissues.
Nifedipine — L-Type Calcium Channel Blockade
Nifedipine inhibits L-type calcium channels, which contribute to the cardiac action potential plateau and excitation–contraction coupling.
In SmartHeart®, nifedipine induced action potential shortening, accompanied by changes in action potential amplitude, depolarization kinetics and beating frequency.
An exploratory calcium transient experiment showed reductions in calcium transient amplitude, duration and rise rate, consistent with inhibition of calcium entry.
The calcium findings were obtained from a limited experimental dataset and should be considered preliminary.
Figure 6. Nifedipine concentration-response in SmartHeart®. Electrophysiology: N = 3 experiments, n = 25 tissues. Contractility: N = 2 experiments, n = 25 tissues. Calcium transient analysis: N = 1 experiment, n = 3 tissues.
Terfenadine — Multichannel Electrophysiological Effects
Terfenadine is an antihistamine associated with cardiac ion-channel blockade and established proarrhythmic liability.
SmartHeart® measurements showed decreased action potential amplitude and depolarization speed, together with prolonged late repolarization.
The compound also reduced beating frequency and increased beating irregularity across the tested concentrations.
These findings demonstrate the ability of the experimental platform to characterize several functional alterations associated with a compound exhibiting multichannel effects.
Figure 7. Terfenadine concentration-response in SmartHeart®. Electrophysiology: N = 3 experiments, n = 31 tissues. Contractility: N = 2 experiments, n = 25 tissues.
Ranolazine — Complex Ion-Channel Pharmacology
Ranolazine has a complex electrophysiological profile involving inhibition of late sodium current (INa,L) and IKr potassium current, among other concentration-dependent effects.
In SmartHeart®, ranolazine produced changes in action potential amplitude and depolarization kinetics, together with moderate prolongation of late repolarization.
The observed response illustrates how several ion-channel effects may contribute to the integrated electrical behavior of human cardiac tissue.
Because individual ion-channel inhibition does not necessarily predict the full electrophysiological response, these observations support the value of complementary tissue-level assessment.
Figure 8. Ranolazine concentration-response in SmartHeart®. Electrophysiology: N = 3 experiments, n = 34 tissues.
Isoprenaline — Positive Inotropic and Chronotropic Responses
Isoprenaline is a β-adrenergic receptor agonist that increases cardiac stimulation.
In SmartHeart®, increasing isoprenaline concentrations produced positive chronotropic and inotropic responses, reflected by increased beating frequency and derived contractile stress. Increased contraction and relaxation speeds were also observed.
These results demonstrate the response of the cardiac microtissues to β-adrenergic stimulation and the platform's ability to quantify drug-induced changes in cardiac mechanical activity.
Figure 9. Isoprenaline concentration-response in SmartHeart®. Contractility: N = 2 experiments, n = 15 tissues.
Verapamil — Negative Inotropic Effects
Verapamil is a calcium-channel blocker that can reduce calcium-dependent cardiac contractile activity.
In SmartHeart®, verapamil induced a reduction in derived contractile stress and contraction speed, consistent with a negative inotropic response.
These observations complement the positive inotropic response measured with isoprenaline and illustrate the ability of SmartHeart® to characterize changes in cardiac mechanical function in both directions.
Figure 10. Verapamil concentration-response in SmartHeart®. Contractility: N = 3 experiments, n = 32 tissues.
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DISCUSSION: WHAT DO THESE RESULTS DEMONSTRATE?
This nine-compound study illustrates how human 3D cardiac microtissues can be used to investigate drug-induced functional effects through complementary experimental readouts. The observed responses provide mechanistic information on electrophysiological disturbances, cardiac contractility and calcium handling.
Capturing Compound-Specific Electrophysiological Responses
The CiPA reference compounds produced distinct electrophysiological responses, including changes in action potential duration, amplitude and depolarization kinetics. The observations are consistent with different pharmacological mechanisms involving potassium, sodium and calcium currents.
For example, dofetilide produced substantial repolarization prolongation, whereas ranolazine exhibited a more complex electrophysiological profile involving several ion-channel effects.
These results support mechanistic investigation of drug-induced electrical disturbances. They do not establish a clinically validated classifier of torsade de pointes risk.
Combining Electrical and Mechanical Cardiac Readouts
Several compounds induced changes in both electrophysiology and contractility. The combined analysis of action potential characteristics, beating activity and tissue deformation provides a broader functional description than an individual endpoint alone.
For cardiac safety studies, this information can help researchers investigate whether an electrophysiological alteration is accompanied by measurable changes in tissue mechanical activity.
Detecting Positive and Negative Inotropic Effects
Isoprenaline and verapamil produced opposite effects on cardiac contractility. These responses demonstrate the ability of SmartHeart® to capture positive and negative inotropic activity through quantitative tissue-level measurements.
The characterization of inotropy is relevant to both cardiac safety assessment and pharmacological efficacy studies.
Complementary Calcium-Handling Measurements
Exploratory calcium transient experiments with metoprolol and nifedipine provided additional information on drug-induced changes in calcium dynamics.
However, because these measurements were obtained from a limited number of experiments and tissues, they should not be considered equivalent in evidential strength to the more extensively replicated electrophysiological and contractility measurements.
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MATERIALS & METHODS
Human Cardiac Microtissue Generation
SmartHeart® cardiac microtissues were generated using human iPSC-derived cardiomyocytes (iCell® Cardiomyocytes², FUJIFILM Cellular Dynamics) and human ventricular cardiac fibroblasts (Innoprot) at a 3:1 ratio.
Approximately 65,000 cells were seeded per well in SmartHeart® 96-well plates. The cell mixture was prepared in culture medium supplemented with Y-27632 ROCK inhibitor. Cells self-assembled into ring-shaped tissues around central hydrogel pillars.
Medium was added on the following day and subsequently changed every two days.
Cardiac Contractility Measurements
Contractile activity was assessed after approximately 14 days of culture. SmartHeart® cardiac rings were maintained at 37°C during acquisition using a temperature-controlled microscope stage.
Brightfield recordings were acquired using an inverted microscope with a 10× objective and a camera operating at a minimum of 35 frames per second. Baseline and drug-exposure recordings lasted 11 seconds.
Compounds were administered sequentially, with a 10-minute incubation period between acquisitions.
Videos were processed using SmartX 2.0 to extract contraction strain, derived contractile stress, beating frequency, contraction and relaxation speeds, and rhythm-related metrics.
Optical Electrophysiology
Optical action potential measurements were performed on tissues cultured for approximately 14–17 days.
Tissues were labeled with the voltage-sensitive dye FluoVolt™ and imaged using a Leica THUNDER fluorescence microscopy system at 37°C.
Fluorescence recordings were analyzed to quantify action potential amplitude, duration, depolarization kinetics and beating activity. These measurements were used to characterize compound-induced changes in electrophysiological function.
Calcium Transient Measurements
Calcium transient measurements were performed on tissues at approximately Day 25 using Cal-520 AM fluorescence imaging.
Fluorescence recordings were acquired at 37°C and processed to quantify calcium transient amplitude, frequency and kinetic parameters.
The calcium experiments with metoprolol and nifedipine were exploratory and included limited experimental replication.
Table 2. Tested cardioactive compounds and corresponding concentration ranges used in the SmartHeart® study. Concentrations are expressed as indicated for each compound.
Nine cardioactive compounds were evaluated across selected concentration ranges: dofetilide, disopyramide, domperidone, metoprolol, nifedipine, terfenadine, ranolazine, isoprenaline and verapamil.
Concentrations were selected according to the pharmacological properties of each compound and the experimental objectives. Compound solutions were pre-warmed to 37°C before administration. The concentration series used for each compound is detailed in Table 2.
Compound Exposure and Statistical Analysis
Nine cardioactive compounds were investigated over selected concentration ranges. Compound solutions were warmed to 37°C before administration.
Differences between experimental conditions and DMSO vehicle controls were analyzed using paired Wilcoxon tests. Values of p < 0.05 were considered statistically significant.
The original figures report significance thresholds as follows: * p < 0.05; ** p < 0.01; *** p < 0.001; **** p < 0.0001.
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CONCLUSION
This nine-compound study demonstrates the potential of SmartHeart® human 3D cardiac microtissues to characterize drug-induced changes in cardiac electrical and mechanical function.
Across compounds with different pharmacological mechanisms, the platform captured changes in repolarization, action potential dynamics, beating frequency, contractility and exploratory calcium-handling endpoints.
The combination of complementary functional measurements provides valuable mechanistic information for investigating cardiac liabilities during preclinical drug development.
These findings support further application and qualification of SmartHeart® for human-relevant cardiac safety assessment, with experimental design and performance requirements established according to the intended use.
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EXPLORE SMARTHEART® CARDIAC SAFETY TESTING
Investigate cardiac safety with human 3D in vitro functional data
SmartHeart® supports the characterization of drug-induced changes in cardiac contractility, electrophysiology and calcium handling.
4Dcell works with pharmaceutical and biotechnology teams to develop fit-for-purpose cardiac studies, combining quantitative functional measurements with scientific interpretation.