Nonclinical evaluation of chronic cardiac contractility modulation on 3D human engineered cardiac tissues

INTRODUCTION

Cardiac contractility modulation (CCM) is a medical device-based therapy delivering non-excitatory electrical stimulations to the heart to enhance cardiac function in heart failure (HF) patients. The lack of human in vitro tools to assess CCM hinders our understanding of CCM mechanisms of action. Here, we introduce a novel chronic (i.e., 2-day) in vitro CCM assay to evaluate the effects of CCM in a human 3D microphysiological system consisting of engineered cardiac tissues (ECTs).

METHODS

Cryopreserved human induced pluripotent stem cell-derived cardiomyocytes were used to generate 3D ECTs. The ECTs were cultured, incorporating human primary ventricular cardiac fibroblasts and a fibrin-based gel. Electrical stimulation was applied using two separate pulse generators for the CCM group and control group. Contractile properties and intracellular calcium were measured, and a cardiac gene quantitative PCR screen was conducted.

RESULTS

Chronic CCM increased contraction amplitude and duration, enhanced intracellular calcium transient amplitude, and altered gene expression related to HF (i.e., natriuretic peptide B, NPPB) and excitation-contraction coupling (i.e., sodium-calcium exchanger, SLC8).

CONCLUSION

These data represent the first study of chronic CCM in a 3D ECT model, providing a nonclinical tool to assess the effects of cardiac electrophysiology medical device signals complementing in vivo animal studies. The methodology established a standardized 3D ECT-based in vitro testbed for chronic CCM, allowing evaluation of physiological and molecular effects on human cardiac tissues.

Human in vitro assay for irreversible electroporation cardiac ablation

Introduction: Pulsed electric field (PEF) cardiac ablation has been recently proposed as a technique to treat drug resistant atrial fibrillation by inducing cell death through irreversible electroporation (IRE). Improper PEF dosing can result in thermal damage or reversible electroporation. The lack of comprehensive and systematic studies to select PEF parameters for safe and effective IRE cardiac treatments hinders device development and regulatory decision-making. Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have been proposed as an alternative to animal models in the evaluation of cardiac electrophysiology safety. Methods: We developed a novel high-throughput in vitro assay to quantify the electric field threshold (EFT) for electroporation (acute effect) and cell death (long-term effect) in hiPSC-CMs. Monolayers of hiPSC-CMs were cultured in high-throughput format and exposed to clinically relevant biphasic PEF treatments. Electroporation and cell death areas were identified using fluorescent probes and confocal microscopy; electroporation and cell death EFTs were quantified by comparison of fluorescent images with electric field numerical simulations. Results: Study results confirmed that PEF induces electroporation and cell death in hiPSC-CMs, dependent on the number of pulses and the amplitude, duration, and repetition frequency. In addition, PEF-induced temperature increase, absorbed dose, and total treatment time for each PEF parameter combination are reported. Discussion: Upon verification of the translatability of the in vitro results presented here to in vivo models, this novel hiPSC-CM-based assay could be used as an alternative to animal or human studies and can assist in early nonclinical device development, as well as inform regulatory decision-making for cardiac ablation medical devices.

Evaluation of Cardiac Contractility Modulation Therapy in 2D Human Stem Cell-Derived Cardiomyocytes

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are currently being explored for multiple in vitro applications and have been used in regulatory submissions. Here, we extend their use to cardiac medical device safety or performance assessments. We developed a novel method to evaluate cardiac medical device contractile properties in robustly contracting 2D hiPSC-CMs monolayers plated on a flexible extracellular matrix (ECM)-based hydrogel substrate. This tool enables the quantification of the effects of cardiac electrophysiology device signals on human cardiac function (e.g., contractile properties) with standard laboratory equipment. The 2D hiPSC-CM monolayers were cultured for 2-4 days on a flexible hydrogel substrate in a 48-well format. The hiPSC-CMs were exposed to standard cardiac contractility modulation (CCM) medical device electrical signals and compared to control (i.e., pacing only) hiPSC-CMs. The baseline contractile properties of the 2D hiPSC-CMs were quantified by video-based detection analysis based on pixel displacement. The CCM-stimulated 2D hiPSC-CMs plated on the flexible hydrogel substrate displayed significantly enhanced contractile properties relative to baseline (i.e., before CCM stimulation), including an increased peak contraction amplitude and accelerated contraction and relaxation kinetics. Furthermore, the utilization of the flexible hydrogel substrate enables the multiplexing of the video-based cardiac-excitation contraction coupling readouts (i.e., electrophysiology, calcium handling, and contraction) in healthy and diseased hiPSC-CMs. The accurate detection and quantification of the effects of cardiac electrophysiological signals on human cardiac contraction is vital for cardiac medical device development, optimization, and de-risking. This method enables the robust visualization and quantification of the contractile properties of the cardiac syncytium, which should be valuable for nonclinical cardiac medical device safety or effectiveness testing. This paper describes, in detail, the methodology to generate 2D hiPSC-CM hydrogel substrate monolayers.

Acute effects of cardiac contractility modulation stimulation in conventional 2D and 3D human induced pluripotent stem cell-derived cardiomyocyte models

Cardiac contractility modulation (CCM) is a medical device therapy whereby non-excitatory electrical stimulations are delivered to the myocardium during the absolute refractory period to enhance cardiac function. We previously evaluated the effects of the standard CCM pulse parameters in isolated rabbit ventricular cardiomyocytes and 2D human induced pluripotent stem cell-derived cardiomyocyte (hiPSC-CM) monolayers, on flexible substrate. In the present study, we sought to extend these results to human 3D microphysiological systems to develop a robust model to evaluate various clinical CCM pulse parameters in vitro. HiPSC-CMs were studied in conventional 2D monolayer format, on stiff substrate (i.e., glass), and as 3D human engineered cardiac tissues (ECTs). Cardiac contractile properties were evaluated by video (i.e., pixel) and force-based analysis. CCM pulses were assessed at varying electrical ‘doses’ using a commercial pulse generator. A robust CCM contractile response was observed for 3D ECTs. Under comparable conditions, conventional 2D monolayer hiPSC-CMs, on stiff substrate, displayed no contractile response. 3D ECTs displayed enhanced contractile properties including increased contraction amplitude (i.e., force), and accelerated contraction and relaxation slopes under standard acute CCM stimulation. Moreover, 3D ECTs displayed enhanced contractility in a CCM pulse parameter-dependent manner by adjustment of CCM pulse delay, duration, amplitude, and number relative to baseline. The observed acute effects subsided when the CCM stimulation was stopped and gradually returned to baseline. These data represent the first study of CCM in 3D hiPSC-CM models and provide a nonclinical tool to assess various CCM device signals in 3D human cardiac tissues prior to in vivo animal studies. Moreover, this work provides a foundation to evaluate the effects of additional cardiac medical devices in 3D ECTs.

Human in vitro neurocardiac coculture (ivNCC) assay development for evaluating cardiac contractility modulation

Two of the most prominent organ systems, the nervous and the cardiovascular systems, are intricately connected to maintain homeostasis in mammals. Recent years have shown tremendous efforts toward therapeutic modulation of cardiac contractility and electrophysiology by electrical stimulation. Neuronal innervation and cardiac ganglia regulation are often overlooked when developing in vitro models for cardiac devices, but it is likely that peripheral nervous system plays a role in the clinical effects. We developed an in vitro neurocardiac coculture (ivNCC) model system to study cardiac and neuronal interplay using human induced pluripotent stem cell (hiPSC) technology. We demonstrated significant expression and colocalization of cardiac markers including troponin, α-actinin, and neuronal marker peripherin in neurocardiac coculture. To assess functional coupling between the cardiomyocytes and neurons, we evaluated nicotine-induced β-adrenergic norepinephrine effect and found beat rate was significantly increased in ivNCC as compared to monoculture alone. The developed platform was used as a nonclinical model for the assessment of cardiac medical devices that deliver nonexcitatory electrical pulses to the heart during the absolute refractory period of the cardiac cycle, that is, cardiac contractility modulation (CCM) therapy. Robust coculture response was observed at 14 V/cm (5 V, 64 mA), monophasic, 2 ms pulse duration for pacing and 20 V/cm (7 V, 90 mA) phase amplitude, biphasic, 5.14 ms pulse duration for CCM. We observed that the CCM effect and kinetics were more pronounced in coculture as compared to cardiac monoculture, supporting a hypothesis that some part of CCM mechanism of action can be attributed to peripheral nervous system stimulation. This study provides novel characterization of CCM effects on hiPSC-derived neurocardiac cocultures. This innervated human heart model can be further extended to investigate arrhythmic mechanisms, neurocardiac safety, and toxicity post-chronic exposure to materials, drugs, and medical devices. We present data on acute CCM electrical stimulation effects on a functional and optimized coculture using commercially available hiPSC-derived cardiomyocytes and neurons. Moreover, this study provides an in vitro human heart model to evaluate neuronal innervation and cardiac ganglia regulation of contractility by applying CCM pulse parameters that closely resemble clinical setting. This ivNCC platform provides a potential tool for investigating aspects of cardiac and neurological device safety and performance.

Human cardiomyocytes are more susceptible to irreversible electroporation by pulsed electric field than human esophageal cells

Pulse electric field-based (PEF) ablation is a technique whereby short high-intensity electric fields inducing irreversible electroporation (IRE) are applied to various tissues. Here, we implemented a standardized in vitro model to compare the effects of biphasic symmetrical pulses (100 pulses, 1-10 μs phase duration (d), 10-1000 Hz pulse repetition rate (f)) using two different human cellular models: human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and human esophageal smooth muscle cells (hESMCs) cultured in monolayer format. We report the PEF-induced irreversibly electroporated cell monolayer areas and the corresponding electric field thresholds (EFTs) for both cardiac and esophageal cultures. Our results suggest marked cell type specificity with EFT estimated to be 2-2.5 times lower in hiPSC-CMs than in hESMCs when subjected to identical PEF treatments (e.g., 0.90 vs 1.85 kV/cm for the treatment of 100 pulses with d = 5 μs, f = 10 Hz, and 0.65 vs 1.67 kV/cm for the treatment of 100 pulses with d = 10 μs, f = 10 Hz). PEF treatment can result in increased temperature around the stimulating electrodes and lead to unanticipated thermal tissue damage that is proportional to the peak temperature rise and to the duration of the PEF-induced elevated temperatures. In our study, temperature increases ranged from less than 1°C to as high as 30°C, however, all temperature changes were transient and quickly returned to baseline and the highest observed ∆T returned to 50% of its maximum recorded temperature in tens of seconds.

Abstract P1123: Establishment Of An In Vitro Method To Evaluate Chronic Cardiac Contractility Modulation Signals In 3d Human Engineered Cardiac Tissues

Background: Cardiac contractility modulation (CCM) is a medical device-based therapy delivering non-excitatory electrical simulations to the heart during the absolute refractory period to enhance cardiac function. We previously evaluated the acute effects of CCM in isolated rabbit-CM and 2D hiPSC-CM monolayers, on flexible substrate, and found enhanced calcium and contractility. In the present study, we sought to extend these results to chronic studies in 3D human engineered cardiac tissues (ECTs) to develop a robust model to evaluate the long-term effects of CCM stimulation in vitro on intact human cardiac tissues.

Methods: HiPSC-CMs and cardiac fibroblasts in a fibrin-based gel were combined to form ECTs (Figure 1). Morphology and contractility were evaluated. We found ECTs displayed baseline peak contraction amplitude of 194.8±80.8 a.u., (n=5) and peak force and stress of 26.5±6.8 uN, (n=5) and 87.9±22.0.8 Pa, (n=5) respectively. ECT displayed robust baseline electrophysiological (AP) properties and calcium handling properties. Chronic CCM pulses (i.e., 30 minutes) were applied using a commercial pulse generator. Under these conditions ECTs displayed enhanced contractile kinetics 29.0±0.04 %, (P<0.01, n=4), relaxation kinetics 23.0±0.05 %, (P<0.05, n=4), and shorter duration 32.0±0.03 %, (P<0.01, n=4) relative to time matched control.

Conclusion: This study provides a comprehensive characterization of chronic CCM effects on 3D ECTs. Future studies will investigate prolonged time points. These data provide an in vitro model to assess physiologically relevant mechanisms and evaluate safety and efficacy of future cardiac electrophysiology medical devices.

Disclaimer: The mention of commercial products, their sources, or their use in connection with material reported herein is not to be construed as either an actual or implied endorsement of such products by the Department of Health and Human Services.

Chronic Cardiotoxicity Assays Using Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes (hiPSC-CMs)

Cardiomyocytes (CMs) differentiated from human induced pluripotent stem cells (hiPSCs) are increasingly used in cardiac safety assessment, disease modeling and regenerative medicine. A vast majority of cardiotoxicity studies in the past have tested acute effects of compounds and drugs; however, these studies lack information on the morphological or physiological responses that may occur after prolonged exposure to a cardiotoxic compound. In this review, we focus on recent advances in chronic cardiotoxicity assays using hiPSC-CMs. We summarize recently published literature on hiPSC-CMs assays applied to chronic cardiotoxicity induced by anticancer agents, as well as non-cancer classes of drugs, including antibiotics, anti-hepatitis C virus (HCV) and antidiabetic drugs. We then review publications on the implementation of hiPSC-CMs-based assays to investigate the effects of non-pharmaceutical cardiotoxicants, such as environmental chemicals or chronic alcohol consumption. We also highlight studies demonstrating the chronic effects of smoking and implementation of hiPSC-CMs to perform genomic screens and metabolomics-based biomarker assay development. The acceptance and wide implementation of hiPSC-CMs-based assays for chronic cardiotoxicity assessment will require multi-site standardization of assay protocols, chronic cardiac maturity marker reproducibility, time points optimization, minimal cellular variation (commercial vs. lab reprogrammed), stringent and matched controls and close clinical setting resemblance. A comprehensive investigation of long-term repeated exposure-induced effects on both the structure and function of cardiomyocytes can provide mechanistic insights and recapitulate drug and environmental cardiotoxicity.

Acute effects of cardiac contractility modulation on human induced pluripotent stem cell-derived cardiomyocytes

Cardiac contractility modulation (CCM) is an intracardiac therapy whereby nonexcitatory electrical simulations are delivered during the absolute refractory period of the cardiac cycle. We previously evaluated the effects of CCM in isolated adult rabbit ventricular cardiomyocytes and found a transient increase in calcium and contractility. In the present study, we sought to extend these results to human cardiomyocytes using human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) to develop a robust model to evaluate CCM in vitro. HiPSC-CMs (iCell Cardiomyocytes2 , Fujifilm Cellular Dynamic, Inc.) were studied in monolayer format plated on flexible substrate. Contractility, calcium handling, and electrophysiology were evaluated by fluorescence- and video-based analysis (CellOPTIQ, Clyde Biosciences). CCM pulses were applied using an A-M Systems 4100 pulse generator. Robust hiPSC-CMs response was observed at 14 V/cm (64 mA) for pacing and 28 V/cm (128 mA, phase amplitude) for CCM. Under these conditions, hiPSC-CMs displayed enhanced contractile properties including increased contraction amplitude and faster contraction kinetics. Likewise, calcium transient amplitude increased, and calcium kinetics were faster. Furthermore, electrophysiological properties were altered resulting in shortened action potential duration (APD). The observed effects subsided when the CCM stimulation was stopped. CCM-induced increase in hiPSC-CMs contractility was significantly more pronounced when extracellular calcium concentration was lowered from 2 mM to 0.5 mM. This study provides a comprehensive characterization of CCM effects on hiPSC-CMs. These data represent the first study of CCM in hiPSC-CMs and provide an in vitro model to assess physiologically relevant mechanisms and evaluate safety and effectiveness of future cardiac electrophysiology medical devices.

Mechanisms of QT prolongation by buprenorphine cannot be explained by direct hERG channel block

Buprenorphine is a μ-opioid receptor (MOR) partial agonist used to manage pain and addiction. QT_C prolongation that crosses the 10 msec threshold of regulatory concern was observed at a supratherapeutic dose in two thorough QT studies for the transdermal buprenorphine product BUTRANS^®. Because QT_C prolongation can be associated with Torsades de Pointes (TdP), a rare but potentially fatal ventricular arrhythmia, these results have led to further investigation of the electrophysiological effects of buprenorphine. Drug-induced QT_C prolongation and TdP are most commonly caused by acute inhibition of hERG current (I_hERG) that contribute to the repolarizing phase of the ventricular action potentials (APs). Concomitant inhibition of inward late Na⁺ (I_NaL) and/or L-type Ca²⁺ (I_CaL) current can offer some protection against proarrhythmia. Therefore, we characterized the effects of buprenorphine and its major metabolite norbuprenorphine on cardiac hERG, Ca²⁺, and Na⁺ ion channels, as well as cardiac APs. For comparison, methadone, a MOR agonist associated with QT_C prolongation and high TdP risk, and naltrexone and naloxone, two opioid receptor antagonists, were also studied. Whole cell recordings were performed at 37°C on cells stably expressing hERG, Ca_V1.2, and Na_V1.5 proteins. Microelectrode array (MEA) recordings were made on human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). The results showed that buprenorphine, norbuprenorphine, naltrexone, and naloxone had no effect on I_hERG, I_CaL, I_NaL, and peak Na⁺ current (I_NaP) at clinically relevant concentrations. In contrast, methadone inhibited I_hERG, I_CaL, and I_NaL. Experiments on iPSC-CMs showed a lack of effect for buprenorphine, norbuprenorphine, naltrexone, and naloxone, and delayed repolarization for methadone at clinically relevant concentrations. The mechanism of QT_C prolongation is opioid moiety-specific. This remains undefined for buprenorphine, while for methadone it involves direct hERG channel block. There is no evidence that buprenorphine use is associated with TdP. Whether this lack of TdP risk can be generalized to other drugs with QT_C prolongation not mediated by acute hERG channel block warrants further study.

Abstract 256: Human In Vitro Model for Preclinical Evaluation of Irreversible Electroporation Devices Used for Cardiac Ablation

Irreversible Electroporation (IRE) has gained significant interest as an alternative treatment for atrial fibrillation (AF). Typically, to terminate AF, thermal energy is delivered to the arrhythmic tissue causing rare but serious damage to surrounding areas. IRE is a non-thermal approach that has the potential to be faster yet more targeted. We are developing a novel, species relevant, standard laboratory test to evaluate IRE cardiac ablation using human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) as a model. hiPSC-CMs were cultured in monolayers. Pulses were delivered by a AVOZ D7B generator through contact electrodes. Electric field (E) distribution maps were obtained with numerical simulations, Sim4Life v3.2. Electroporation (EP) was quantified by florescence with YO-PRO-1 10 minutes after exposure. hiPSC-CMs were exposed to a range of electrical parameters including 120 unipolar pulses of 5 μs duration, at 1 kHz frequency (Fig. 1). We found that the EP threshold was ~1.5 kV/cm in line with previous reports on single adult ventricular mouse cardiomyocytes (Semenov et al. 2018). The treated area increased with the voltage (V) applied. These data suggest that this assay can delineate the extent of EP. In this work we will vary different pulse parameters (durations, amplitude, shape, repetition rate and number) to evaluate their effects on different human cell lines anatomically adjacent to the heart.

Fig.1: (A) Representation of the EP threshold determined by overlaying the E map (B) with a fluorescence image of pulse-treated hiPSC-CMs (C). To define the E threshold the distributions values obtained by modeling 1 V applied was scaled by the experimental V.

CRISPR/Cas9-mediated introduction of the sodium/iodide symporter gene enables noninvasive in vivo tracking of induced pluripotent stem cell-derived cardiomyocytes

Techniques that enable longitudinal tracking of cell fate after myocardial delivery are imperative for optimizing the efficacy of cell‐based cardiac therapies. However, these approaches have been underutilized in preclinical models and clinical trials, and there is considerable demand for site‐specific strategies achieving long‐term expression of reporter genes compatible with safe noninvasive imaging. In this study, the rhesus sodium/iodide symporter (NIS) gene was incorporated into rhesus macaque induced pluripotent stem cells (RhiPSCs) via CRISPR/Cas9. Cardiomyocytes derived from NIS‐RhiPSCs (NIS‐RhiPSC‐CMs) exhibited overall similar morphological and electrophysiological characteristics compared to parental control RhiPSC‐CMs at baseline and with exposure to physiological levels of sodium iodide. Mice were injected intramyocardially with 2 million NIS‐RhiPSC‐CMs immediately following myocardial infarction, and serial positron emission tomography/computed tomography was performed with ¹⁸F‐tetrafluoroborate to monitor transplanted cells in vivo. NIS‐RhiPSC‐CMs could be detected until study conclusion at 8 to 10 weeks postinjection. This NIS‐based molecular imaging platform, with optimal safety and sensitivity characteristics, is primed for translation into large‐animal preclinical models and clinical trials.

International Multisite Study of Human-Induced Pluripotent Stem Cell-Derived Cardiomyocytes for Drug Proarrhythmic Potential Assessment

To assess the utility of human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) as an in vitro proarrhythmia model, we evaluated the concentration dependence and sources of variability of electrophysiologic responses to 28 drugs linked to low, intermediate, and high torsades de pointes (TdP) risk categories using two commercial cell lines and standardized protocols in a blinded multisite study using multielectrode array or voltage-sensing optical approaches. Logistical and ordinal linear regression models were constructed using drug responses as predictors and TdP risk categories as outcomes. Three of seven predictors (drug-induced arrhythmia-like events and prolongation of repolarization at either maximum tested or maximal clinical exposures) categorized drugs with reasonable accuracy (area under the curve values of receiver operator curves ~0.8). hiPSC-CM line, test site, and platform had minimal influence on drug categorization. These results demonstrate the utility of hiPSCCMs to detect drug-induced proarrhythmic effects as part of the evolving Comprehensive In Vitro Proarrhythmia Assay paradigm.

Blinova et al. tested human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) for improving torsades de pointes arrhythmia risk prediction of drugs in the Comprehensive In Vitro Proarrhythmia Assay (CiPA) initiative. This validation study confirms their utility based on electrophysiologic responses to 28 blinded drugs, with minimal influence from cell lines, test sites, and electrophysiological platforms.

Clinical Trial in a Dish: Personalized Stem Cell-Derived Cardiomyocyte Assay Compared With Clinical Trial Results for Two QT-Prolonging Drugs

Induced pluripotent stem cells (iPSCs) have shown promise in investigating donor-specific phenotypes and pathologies. The iPSC-derived cardiomyocytes (iPSC-CMs) could potentially be utilized in personalized cardiotoxicity studies, assessing individual proarrhythmic risk. However, it is unclear how closely iPSC-CMs derived from healthy subjects can recapitulate a range of responses to drugs. It is well known that QT-prolonging drugs induce subject-specific clinical response and that all healthy subjects do not necessarily develop arrhythmias or exhibit similar amounts of QT prolongation. We previously reported this variability in a study of four human ether-a-go-go-related gene (hERG) potassium channel-blocking drugs in which each subject underwent intensive pharmacokinetic and pharmacodynamic sampling such that subjects had 15 time-matched plasma drug concentration and electrocardiogram measurements throughout 24 hours after dosing in a phase I clinical research unit. In this study, iPSC-CMs were generated from those subjects. Their drug-concentration-dependent QT prolongation response from the clinic was compared with in vitro drug-concentration-dependent action potential duration (APD) prolongation response to the same two hERG-blocking drugs, dofetilide and moxifloxacin. Comparative results showed no significant correlation between the subject-specific APD response slopes and clinical QT response slopes to either moxifloxacin (P = 0.75) or dofetilide (P = 0.69). Similarly, no significant correlation was found between baseline QT and baseline APD measurements (P = 0.93). This result advances our current understanding of subject-specific iPSC-CMs and facilitates discussion into factors obscuring correlation and considerations for future studies of subject-specific phenotypes in iPSC-CMs.

Comparative analysis of media effects on human induced pluripotent stem cell-derived cardiomyocytes in proarrhythmia risk assessment

INTRODUCTION

Cardiotoxicity assessment using human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) forms a key component of the Comprehensive in Vitro Proarrhythmia Assay (CiPA). A potentially impactful factor on iPSC-CM testing is the presence of serum in the experimental media. Generally, serum-free media is used to most accurately reproduce "free" drug concentration. However, caution is needed; drug solubility and cardiomyocyte electrophysiology could be affected by media formulation, potentially impacting interpretation of drug-induced effects.

METHODS

Effects of 25 drugs on properties of spontaneous field potentials in iPSC-CMs were assayed using a high-throughput microelectrode array (MEA) in two media formulations: serum-containing and serum-free. Comparative analysis was conducted on rate-corrected field potential duration (FPDc) and prevalence of arrhythmic events. Further MEA experiments were conducted, varying percentages of serum as well as carbon substrate components. Comparative LC-MS/MS analysis was done on two compounds to evaluate drug concentrations.

RESULTS

In serum-free media, 9 drugs prolonged FPDc. In serum-containing, 11 drugs prolonged FPDc. Eighteen drugs induced arrhythmias, 8 of these induced arrhythmias at lower concentrations in serum-containing media. At the highest non-arrhythmic concentrations, 13 of 25 drugs exhibited significant differences in FPDc prolongation/shortening between the media. Increasing fractions of serum in media yielded higher FPDc measurements. LC-MS/MS analysis of moxifloxacin and quinidine showed higher concentrations in serum-containing media.

DISCUSSION

The present study highlights media formulation as an important consideration for cardiac safety testing with iPSC-CMs. Results described here suggest that media formulation influences both compound availability and baseline electrophysiological properties. Special attention should be paid to media for future iPSC-CM assays.

Abstract 56: Individual Susceptibility to Drug-induced QTc Prolongation Did Not Correlate With Subject-specific Induced Pluripotent Stem Cells in a Small Sample of Healthy Subjects

Background: Induced pluripotent stem cells (iPSCs) carry patient-specific genetic information and have the potential to become an important tool for predicting individual clinical response to therapies in an in vitro assay. Here, we investigate if subject-specific iPSC cardiomyocytes (iPSC-CMs) derived from healthy volunteers could be used to predict individual response to 2 QT-prolonging drugs which they received in a clinical trial.

Methods and Results: Independent stem cell manufacturer (Stem Cell Theranostics) used commercially available Sendai virus encoding for Oct4 , Sox2 , Klf4 , and c-Myc to reprogram subject’s peripheral blood mononuclear cells (PBMCs) into iPSCs. They then applied chemically defined growth factors, and serum-free protocols to successfully differentiate iPSCs into iPSC-CMs for 17 out of 20 subjects (one batch per subject) enrolled in a randomized, double-blind, placebo-controlled, cross-over trial of dofetilide and moxifloxacin. Effects of the same 2 drugs on cellular action potential duration (APD) were recorded for 16 iPSC-CMs lines using a high-throughput optical imaging system (CellOPTIQ, Clyde Biosciences) using consistent in vitro protocols, including reproducible drug doses and exposure, cell culture and recording conditions. Both dofetilide (0.5-8 nM) and moxifloxacin (10-200 μM) induced dose-dependent prolongation of rate-corrected action potential duration (APD) in each iPSC-CM line. iPSC-CM drug responses were compared with individual clinical response represented by the slope of the linear regression fit through baseline and placebo-controlled drug-induced changes in QTc interval vs. plasma drug concentration. We found no statistical significant correlation between subject-specific iPSC-CMs drug-induced APD prolongation and individual QTc prolongation to either moxifloxacin [Spearman’s ρ = 0.191] or dofetilide [Spearman’s ρ = 0.068].

Conclusion: This study failed to find correlation between clinical and stem cell drug responses in a small sample of healthy subjects. More experiments with a larger sample size and iPSC-CMs batch-to-batch variability controls are required to further investigate the potential of patient-derived stem cells to predict individual clinical response.

Common Genetic Variant Risk Score Is Associated With Drug-Induced QT Prolongation and Torsade de Pointes Risk: A Pilot Study

BACKGROUND

Drug-induced QT interval prolongation, a risk factor for life-threatening ventricular arrhythmias, is a potential side effect of many marketed and withdrawn medications. The contribution of common genetic variants previously associated with baseline QT interval to drug-induced QT prolongation and arrhythmias is not known.

METHODS

We tested the hypothesis that a weighted combination of common genetic variants contributing to QT interval at baseline, identified through genome-wide association studies, can predict individual response to multiple QT-prolonging drugs. Genetic analysis of 22 subjects was performed in a secondary analysis of a randomized, double-blind, placebo-controlled, crossover trial of 3 QT-prolonging drugs with 15 time-matched QT and plasma drug concentration measurements. Subjects received single doses of dofetilide, quinidine, ranolazine, and placebo. The outcome was the correlation between a genetic QT score comprising 61 common genetic variants and the slope of an individual subject's drug-induced increase in heart rate-corrected QT (QTc) versus drug concentration.

RESULTS

The genetic QT score was correlated with drug-induced QTc prolongation. Among white subjects, genetic QT score explained 30% of the variability in response to dofetilide (r=0.55; 95% confidence interval, 0.09-0.81; P=0.02), 23% in response to quinidine (r=0.48; 95% confidence interval, -0.03 to 0.79; P=0.06), and 27% in response to ranolazine (r=0.52; 95% confidence interval, 0.05-0.80; P=0.03). Furthermore, the genetic QT score was a significant predictor of drug-induced torsade de pointes in an independent sample of 216 cases compared with 771 controls (r²=12%, P=1×10^-7).

CONCLUSIONS

We demonstrate that a genetic QT score comprising 61 common genetic variants explains a significant proportion of the variability in drug-induced QT prolongation and is a significant predictor of drug-induced torsade de pointes. These findings highlight an opportunity for recent genetic discoveries to improve individualized risk-benefit assessment for pharmacological therapies. Replication of these findings in larger samples is needed to more precisely estimate variance explained and to establish the individual variants that drive these effects.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrials.gov. Unique identifier: NCT01873950.

Clinical Trials in a Dish

Clinical trials 'in a dish' involve testing medical therapies for safety or effectiveness in the laboratory with human tissue. This has become possible owing to recent biotechnology advances including induced pluripotent stem cells, organs-on-a-chip, and whole-genome sequencing. We provide here an overview of the landscape and highlight steps the FDA is taking to advance the science of clinical trials in a dish and to support the development and validation of new regulatory paradigms to assess drug safety using these new technologies.

Comprehensive Translational Assessment of Human-Induced Pluripotent Stem Cell Derived Cardiomyocytes for Evaluating Drug-Induced Arrhythmias

Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CM) hold promise for assessment of drug-induced arrhythmias and are being considered for use under the comprehensive in vitro proarrhythmia assay (CiPA). We studied the effects of 26 drugs and 3 drug combinations on 2 commercially available iPSC-CM types using high-throughput voltage-sensitive dye and microelectrode-array assays being studied for the CiPA initiative and compared the results with clinical QT prolongation and torsade de pointes (TdP) risk. Concentration-dependent analysis comparing iPSC-CMs to clinical trial results demonstrated good correlation between drug-induced rate-corrected action potential duration and field potential duration (APDc and FPDc) prolongation and clinical trial QTc prolongation. Of 20 drugs studied that exhibit clinical QTc prolongation, 17 caused APDc prolongation (16 in Cor.4U and 13 in iCell cardiomyocytes) and 16 caused FPDc prolongation (16 in Cor.4U and 10 in iCell cardiomyocytes). Of 14 drugs that cause TdP, arrhythmias occurred with 10 drugs. Lack of arrhythmic beating in iPSC-CMs for the four remaining drugs could be due to differences in relative levels of expression of individual ion channels. iPSC-CMs responded consistently to human ether-a-go-go potassium channel blocking drugs (APD prolongation and arrhythmias) and calcium channel blocking drugs (APD shortening and prevention of arrhythmias), with a more variable response to late sodium current blocking drugs. Current results confirm the potential of iPSC-CMs for proarrhythmia prediction under CiPA, where iPSC-CM results would serve as a check to ion channel and in silico modeling prediction of proarrhythmic risk. A multi-site validation study is warranted.

Abstract 384: Evaluating Late Sodium Current in Human Induced Pluripotent Stem Cell Derived Cardiomyocytes

Background: Inhibiting late sodium current (I NaL ) reduced drug-induced QTc prolongation in a recent clinical trial. Induced pluripotent stem cell derived cardiomyocytes (iPSC-CMs) have emerged as a valuable tool in preclinical assessment of multichannel blocking drugs’ potential to prolong QT and induce arrhythmias. However, sodium channels in commercially available iPSC-CMs are known to be under expressed, necessitating investigation into the presence and effects of I NaL in this electrophysiological model.

Methods and Results: A platform combining simultaneous measurements of field potential and contraction (xCELLigence RTCA CardioECR, ACEA Biosciences) was used to assess the acute effects of three I NaL enhancing drugs, ATX-II, ibutilide, and alfuzosin, given alone or in combination with an I NaL blocker, lidocaine in iPSC-CMs (iCell Cardiomyocytes 2 , Cellular Dynamics). Additionally, dofetilide, diltiazem, and lidocaine alone were included as positive controls for hERG, L-type calcium, and sodium channel block.

ATX-II, a potent and specific I NaL enhancer, caused significant dose dependent rate-corrected field potential duration (FPDc) prolongation, which was then subsequently reduced in a dose dependent manner by the addition of lidocaine. At 100 nM ATX-II prolonged the FPDc by 1153.8 ± 135.8 ms from 360.5 ± 16.4 ms at the baseline, which was then reduced to 537 ± 37.4 ms with the addition of 30 μM lidocaine. Ibutilide (0.1-1 μM), a class III antiarrhythmic, caused beating rate decreases and early after depolarizations (EADs) that were not affected by lidocaine addition. Alfuzosin, which increases both peak and late sodium currents, caused dose-dependent reduction of beating rate, FPDc prolongation, and EADs at 5 μM and 10 μM. Alfuzosin-induced EADs were mitigated by addition of lidocaine (5-15 μM).

Conclusions: Late sodium current enhancers prolonged repolarization and induced arrhythmias in human iPSC-CMs. These effects were reversed by addition of lidocaine, a specific late sodium current blocker. These results are consistent with the late sodium current being present in iPSC-CMs in the presence of a late sodium current enhancer, which may have implications for drug safety testing.

Abstract 234: Physiological Toxicity Assessment of Tyrosine Kinase Inhibitors with Human Induced Pluripotent Stem Cell Cardiomyocytes

Tyrosine kinase inhibitors (TKIs) have improved targeted therapeutics for cancer management despite possible cardiac side effects, including decreased contractility. A combination microelectrode and impedance platform was used to assess field potential (FP) electrical activity and contractility, respectively, after acute and chronic exposure of six TKIs on human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs). Four non-TKI drugs were tested to confirm assay sensitivity.

hERG current inhibitors dofetilide and pentamidine increased beat rate (BR) corrected FP duration (FPDc). Dofetilide (6μM) induced transient early afterdepolarizations (EADs), characterized as ectopic beats/notching in the FP signal, and decreased impedance amplitude (BAmp) at 48hrs. Multi-channel blocker amiodarone (2μM) acutely decreased FPDc, with quiescence up to 1hr, and increased BAmp after 24hrs. Doxorubicin decreased BAmp, inducing quiescence starting at 48hrs (5μM and 10μM). Nilotinib, sorafenib and sunitinib, which have varying documented risks of clinical QT prolongation and/or cardiotoxicity, increased FPDc. With highest tested doses, nilotinib (6μM) generated persistent EADs and increased BR, sunitinib (2.5μM) produced tachyarrhythmias with minimal impedance-evident contractions that recovered by 24hrs, and sorafenib (10μM) induced quiescence that recovered by 3hrs. Imatinib, erlotinib and lapatinib, which have fewer clinical cardtiotoxic effects, did not cause rhythm disturbances. Erlotinib and lapatinib had no significant effect on FPDc, lapatinib (2.5μM and 5μM) decreased BR, while imatinib (15μM) acutely increased FPDc and increased BR after 48hrs.

Tandem electrical and impedance screening in hiPSC-CMs for drug cardiotoxicity was consistent with the clinical experience for TKIs, suggesting its potential utility for preclinical cardiotoxicity assessment for proarrhythmia and decreased contractility.

Late sodium current block for drug-induced long QT syndrome: Results from a prospective clinical trial

Drug-induced long QT syndrome has resulted in many drugs being withdrawn from the market. At the same time, the current regulatory paradigm for screening new drugs causing long QT syndrome is preventing drugs from reaching the market, sometimes inappropriately. In this study, we report the results of a first-of-a-kind clinical trial studying late sodium (mexiletine and lidocaine) and calcium (diltiazem) current blocking drugs to counteract the effects of hERG potassium channel blocking drugs (dofetilide and moxifloxacin). We demonstrate that both mexiletine and lidocaine substantially reduce heart-rate corrected QT (QTc) prolongation from dofetilide by 20 ms. Furthermore, all QTc shortening occurs in the heart-rate corrected J-Tpeak (J-Tpeak c) interval, the biomarker we identified as a sign of late sodium current block. This clinical trial demonstrates that late sodium blocking drugs can substantially reduce QTc prolongation from hERG potassium channel block and assessment of J-Tpeak c may add value beyond only assessing QTc.

Abstract 245: Assessing Proarrhythmic Potential of Drugs Using Human Induced Pluripotent Stem Cell Derived Cardiomyocytes and Optical Recordings With Voltage-sensitive Dyes

Background: Drug-induced arrhythmias have been a leading reason for withdrawing drugs from the market and abandoning the development of new drugs. For the past decade FDA has required new drugs to be tested for block of the hERG potassium channel and QT interval prolongation. While these requirements have prevented arrhythmia-related drug recalls, some new drugs affecting multiple ion channels are being dropped from development inappropriately. Human induced pluripotent stem cell derived cardiomyocytes (iPS-CMs) are a new technology for preclinical risk assessment; however they have not been fully characterized or validated.

Methods: We studied 26 drugs and 3 drug combinations that block multiple cardiac ion channels with two commercially available iPS-CM lines from Axiogenesis and Cellular Dynamics using optical recordings of action potentials with voltage-sensitive dyes. Drug-induced action potential duration (APD) prolongation was compared to clinical QT prolongation from two FDA-sponsored clinical trials. The effects of the drugs on multiple individual cardiac ion channels were assessed using manual patch clamp of overexpressed cell lines. Cardiac ion channel gene expression in the iPS-CMs was quantified and compared to primary human heart cell controls.

Results: Of 19 drugs with an FDA label of clinical QT prolongation, 15 exhibited iPS-CM APD prolongation after acute drug exposure. None of the 6 drugs without clinical QT prolongation caused iPS-CM APD prolongation. Of 14 drugs with arrhythmia risk on the FDA label, nine caused arrhythmias in iPS-CMs. iPS-CM response had good correlation with clinical QT data for drugs that block hERG and calcium channels, while drugs blocking the late sodium current had variable response. This was in line with gene expression data, which showed most robust expression of hERG and calcium channels.

Conclusion: Optical recordings from iPS-CMs with voltage sensitive dyes is a promising technology for high-throughput toxicity assessment for drug-induced arrhythmias. This study provides a comprehensive characterization of the cardiac ion channel properties of multiple commercially available iPS-CMs to support a potential new paradigm for assessing the arrhythmia risk of all new drugs.

Acute effects of nonexcitatory electrical stimulation during systole in isolated cardiac myocytes and perfused heart

Application of electrical field to the heart during the refractory period of the beat has been shown to increase the force of contraction both in animal models and in heart failure patients (cardiac contractility modulation, or CCM). A direct increase in intracellular calcium during CCM has been suggested to be the mechanism behind the positive inotropic effect of CCM. We studied the effect of CCM on isolated rabbit cardiomyocytes and perfused whole rat hearts. The effect of CCM was observed in single cells via fluorescent measurements of intracellular calcium concentration ([Ca²⁺]_i) and cell length (L). Cells were paced once per second throughout these recordings, and CCM stimulation was delivered via biphasic electric fields of 20 ms duration applied during the refractory period. CCM increased the peak amplitude of both [Ca²⁺]_i and L for the first beat during CCM compared to control, but then [Ca²⁺]_i and L decayed to levels lower than the control. During CCM, all contractions had a faster time to peak for both [Ca²⁺]_i and L; after stopping CCM the rise times returned to control levels. In the whole rat heart, the positive inotropic effect of CCM stimulation on left ventricular pressure was completely abolished in the presence of metoprolol, a beta‐1 adrenergic blocker. In summary, the CCM‐induced changes in intracellular calcium handling by cardiomyocytes did not explain the sustained positive inotropic effect in the whole heart and the β‐adrenergic pathway may be involved in the CCM mechanism of action.

Cardiac contractility modulation (CCM) is a heart failure therapy which delivers electrical pulses to the heart during refractory period. While there are some promising reports on the therapy's safety and effectiveness in humans, the underlining mechanism remains unknown. We studied the effect of CCM pulses in isolated rabbit cardiomyocytes and isolated rat heart in the presence of beta adrenergic blocker and recorded intracellular calcium transients and contractions. We concluded that the CCM‐induced changes in intracellular calcium handling by cardiomyocytes did not explain the sustained positive iotropic efect in the whole heart and beta‐adrenergic pathway may be involved in the CCM mechanism of action.

Differences in PGE2 production between primary human monocytes and differentiated macrophages: role of IL-1β and TRIF/IRF3

Prostaglandin E2 (PGE₂) is induced in vivo by bacterial products including TLR agonists. To determine whether PGE₂ is induced directly or via IL-1β, human monocytes and macrophages were cultured with LPS or with Pam3CSK4 in presence of caspase-1 inhibitor, ZVAD, or IL-1R antagonist, Kineret. TLR agonists induced PGE₂ in macrophages exclusively via IL-1β-independent mechanisms. In contrast, ZVAD and Kineret reduced PGE₂ production in LPS-treated (but not in Pam3CSK4-treated) monocytes, by 30–60%. Recombinant human IL-1β augmented COX-2 and mPGES-1 mRNA and PGE₂ production in LPS-pretreated monocytes but not in un-primed or Pam3CSK4-primed monocytes. This difference was explained by the finding that LPS but not Pam3CSK4 induced phosphorylation of IRF3 in monocytes suggesting activation of the TRIF signaling pathway. Knocking down TRIF, TRAM, or IRF3 genes by siRNA inhibited IL-1β-induced COX-2 and mPGES-1 mRNA. Blocking of TLR4 endocytosis during LPS priming prevented the increase in PGE₂ production by exogenous IL-1β. Our data showed that TLR2 agonists induce PGE₂ in monocytes independently from IL-1β. In the case of TLR4, IL-1β augments PGE₂ production in LPS-primed monocytes (but not in macrophages) through a mechanism that requires TLR4 internalization and activation of the TRIF/IRF3 pathway. These findings suggest a key role for blood monocytes in the rapid onset of fever in animals and humans exposed to bacterial products and some novel adjuvants.

Use of (32)P to study dynamics of the mitochondrial phosphoproteome

Protein phosphorylation is a well-characterized regulatory mechanism in the cytosol, but remains poorly defined in the mitochondrion. In this study, we characterized the use of (32)P-labeling to monitor the turnover of protein phosphorylation in the heart and liver mitochondria matrix. The (32)P labeling technique was compared and contrasted to Phos-tag protein phosphorylation fluorescent stain and 2D isoelectric focusing. Of the 64 proteins identified by MS spectroscopy in the Phos-Tag gels, over 20 proteins were correlated with (32)P labeling. The high sensitivity of (32)P incorporation detected proteins well below the mass spectrometry and even 2D gel protein detection limits. Phosphate-chase experiments revealed both turnover and phosphate associated protein pool size alterations dependent on initial incubation conditions. Extensive weak phosphate/phosphate metabolite interactions were observed using nondisruptive native gels, providing a novel approach to screen for potential allosteric interactions of phosphate metabolites with matrix proteins. We confirmed the phosphate associations in Complexes V and I due to their critical role in oxidative phosphorylation and to validate the 2D methods. These complexes were isolated by immunocapture, after (32)P labeling in the intact mitochondria, and revealed (32)P-incorporation for the alpha, beta, gamma, OSCP, and d subunits in Complex V and the 75, 51, 42, 23, and 13a kDa subunits in Complex I. These results demonstrate that a dynamic and extensive mitochondrial matrix phosphoproteome exists in heart and liver.

32P labeling of protein phosphorylation and metabolite association in the mitochondria matrix

Protein phosphorylations, as well as phosphate metabolite binding, are well characterized post-translational mechanisms that regulate enzyme activity in the cytosol, but remain poorly defined in mitochondria. Recently extensive matrix protein phosphorylation sites have been discovered but their functional significance is unclear. Herein we describe methods of using (32)P labeling of intact mitochondria to determine the dynamic pools of protein phosphorylation as well as phosphate metabolite association. This screening approach may be useful in not only characterizing the dynamics of these pools, but also provide insight into which phosphorylation sites have a functional significance. Using the mitochondrial ATP synthetic capacity under appropriate conditions, inorganic (32)P was added to energized mitochondria to generate high specific activity gamma-P(32)-ATP in the matrix. In general, SDS denaturing and gel electrophoresis was used to primarily follow protein phosphorylation, whereas native gel techniques were used to observe weaker metabolite associations since the structure of mitochondrial complexes was minimally affected. The protein phosphorylation and metabolite association within the matrix was found to be extensive using these approaches. (32)P labeling in 2D gels was detected in over 40 proteins, including most of the complexes of the cytochrome chain and proteins associated with intermediary metabolism, biosynthetic pathways, membrane transport, and reactive oxygen species metabolism. (32)P pulse-chase experiments further revealed the overall dynamics of these processes that included phosphorylation site turnover as well as phosphate-protein pool size alterations. The high sensitivity of (32)P resulted in many proteins being intensely labeled, but not identified due to the sensitivity limitations of mass spectrometry. These low concentration proteins may represent signaling proteins within the matrix. These results demonstrate that the mitochondrial matrix phosphoproteome is both extensive and dynamic. The use of this, in situ, labeling approach is extremely valuable in confirming protein phosphorylation sites as well as examining the dynamics of these processes under near physiological conditions.

Mitochondrial NADH fluorescence is enhanced by complex I binding

Mitochondrial NADH fluorescence has been a useful tool in evaluating mitochondrial energetics both in vitro and in vivo. Mitochondrial NADH fluorescence is enhanced several-fold in the matrix through extended fluorescence lifetimes (EFL). However, the actual binding sites responsible for NADH EFL are unknown. We tested the hypothesis that NADH binding to Complex I is a significant source of mitochondrial NADH fluorescence enhancement. To test this hypothesis, the effect of Complex I binding on NADH fluorescence efficiency was evaluated in purified protein, and in native gels of the entire porcine heart mitochondria proteome. To avoid the oxidation of NADH in these preparations, we conducted the binding experiments under anoxic conditions in a specially designed apparatus. Purified intact Complex I enhanced NADH fluorescence in native gels approximately 10-fold. However, no enhancement was detected in denatured individual Complex I subunit proteins. In the Clear and Ghost native gels of the entire mitochondrial proteome, NADH fluorescence enhancement was localized to regions where NADH oxidation occurred in the presence of oxygen. Inhibitor and mass spectroscopy studies revealed that the fluorescence enhancement was specific to Complex I proteins. No fluorescence enhancement was detected for MDH or other dehydrogenases in this assay system, at physiological mole fractions of the matrix proteins. These data suggest that NADH associated with Complex I significantly contributes to the overall mitochondrial NADH fluorescence signal and provides an explanation for the well established close correlation of mitochondrial NADH fluorescence and the metabolic state.

Distribution of mitochondrial NADH fluorescence lifetimes: steady-state kinetics of matrix NADH interactions

The lifetimes of fluorescent components of matrix NADH in isolated porcine heart mitochondria were investigated using time-resolved fluorescence spectroscopy. Three distinct lifetimes of fluorescence were resolved: 0.4 (63%), 1.8 (30%), and 5.7 (7%) ns (% total NADH). The 0.4 ns lifetime and the emission wavelength of the short component were consistent with free NADH. In addition to their longer lifetimes, the remaining pools also had a blue-shifted emission spectrum consistent with immobilized NADH. On the basis of emission frequency and lifetime data, the immobilized pools contributed >80% of NADH fluorescence. The steady-state kinetics of NADH entering the immobilized pools was measured in intact mitochondria and in isolated mitochondrial membranes. The apparent binding constants (K(D)s) for NADH in intact mitochondria, 2.8 mM (1.9 ns pool) and >3 mM (5.7 ns pool), were on the order of the estimated matrix [NADH] (approximately 3.5 mM). The affinities and fluorescence lifetimes resulted in an essentially linear relationship between matrix [NADH] and NADH fluorescence intensity. Mitochondrial membranes had shorter emission lifetimes in the immobilized poo1s [1 ns (34%) and 4.1 ns (8%)] with much higher apparent K(D)s of 100 microM and 20 microM, respectively. The source of the stronger NADH binding affinity in membranes is unknown but could be related to high order structure or other cofactors that are diluted out in the membrane preparation. In both preparations, the rate of NADH oxidation was proportional to the amount of NADH in the long lifetime pools, suggesting that a significant fraction of the bound NADH might be associated with oxidative phosphorylation, potentially in complex 1.

Fluctuation analysis of mitochondrial NADH fluorescence signals in confocal and two-photon microscopy images of living cardiac myocytes

A fluctuation analysis was performed on the reduced nicotine adenine dinucleotide (NADH) fluorescence signal from resting rabbit myocytes using confocal and two-photon microscopy. The purpose of this study was to establish whether any co-ordinated biochemical processes, such as binding, metabolism and inner mitochondrial membrane potential, were contributing to NADH signal fluctuations above background instrument noise. After a basic characterization of the instrument noise, time series of cellular NADH fluorescence images were collected and compared with an internal standard composed of NADH in the bathing medium. The coefficient of variation as a function of mean signal amplitude of cellular NADH fluorescence and bathing media NADH was identical even as a function of temperature. These data suggest that the fluctuations in cellular NADH fluorescence in resting myocytes are dominated by sampling noise of these instruments and not significantly modified by biological processes. Further analysis revealed no significant spatial correlations within the cell, and Fourier analysis revealed no coherent frequency information. These data suggest that the impact of biochemical processes, which might affect cellular NADH fluorescence emission, are either too small in magnitude, occurring in the wrong temporal scale or too highly spatially localized for detection using these standard optical microscopy approaches.