Cardiac contractility. A review

Considerations for an In Vitro, Cell-Based Testing Platform for Detection of Drug-Induced Inotropic Effects in Early Drug Development. Part 2: Designing and Fabricating Microsystems for Assaying Cardiac Contractility With Physiological Relevance Using Human iPSC-Cardiomyocytes

Contractility of the myocardium engines the pumping function of the heart and is enabled by the collective contractile activity of its muscle cells: cardiomyocytes. The effects of drugs on the contractility of human cardiomyocytes in vitro can provide mechanistic insight that can support the prediction of clinical cardiac drug effects early in drug development. Cardiomyocytes differentiated from human-induced pluripotent stem cells have high potential for overcoming the current limitations of contractility assays because they attach easily to extracellular materials and last long in culture, while having human- and patient-specific properties. Under these conditions, contractility measurements can be non-destructive and minimally invasive, which allow assaying sub-chronic effects of drugs. For this purpose, the function of cardiomyocytes in vitro must reflect physiological settings, which is not observed in cultured cardiomyocytes derived from induced pluripotent stem cells because of the fetal-like properties of their contractile machinery. Primary cardiomyocytes or tissues of human origin fully represent physiological cellular properties, but are not easily available, do not last long in culture, and do not attach easily to force sensors or mechanical actuators. Microengineered cellular systems with a more mature contractile function have been developed in the last 5 years to overcome this limitation of stem cell-derived cardiomyocytes, while simultaneously measuring contractile endpoints with integrated force sensors/actuators and image-based techniques. Known effects of engineered microenvironments on the maturity of cardiomyocyte contractility have also been discovered in the development of these systems. Based on these discoveries, we review here design criteria of microengineered platforms of cardiomyocytes derived from pluripotent stem cells for measuring contractility with higher physiological relevance. These criteria involve the use of electromechanical, chemical and morphological cues, co-culture of different cell types, and three-dimensional cellular microenvironments. We further discuss the use and the current challenges for developing and improving these novel technologies for predicting clinical effects of drugs based on contractility measurements with cardiomyocytes differentiated from induced pluripotent stem cells. Future research should establish contexts of use in drug development for novel contractility assays with stem cell-derived cardiomyocytes.

Clinical evaluation of cardiac effects of experimental doxycycline overdosing in healthy calves

Background

Cardiac morphologic and functional changes consistent with cardiomyopathy have been reported in field cases of calves with accidental doxycycline overdosing. The purpose of this study was to evaluate clinically the cardiac effects of an experimentally-induced doxycycline overdosing in healthy calves.

Twelve 2 months-old healthy Belgian Blue calves were studied. Six of them (group 1) received the normal dose (5 mg/kg, BID) and the six others (group 2) received five times the normal dose (25 mg/kg, BID) of oral doxycycline for five consecutive days (D1 to D5). Each calf was clinically examined daily. Measurement of serum AST, CK, Iso-CKs and LDH activities and an echocardiographic examination were performed before (D0) and one day after (D6) the last doxycycline administration. An ECG tracing was recorded at D0, D4, and D6.

Results

In both groups, no clinical, blood, echocardiographic or electrocardiographic changes suggestive of a cardiomyopathy were observed. Only a decreased appetite was observed in the calves of the group 2 between D3 and D6.

Conclusions

This trial failed to reproduce cardiac changes reported in accidental doxycycline-poisoning in calves, suggesting that high doses of doxycycline may not be the only etiologic factor of the cardiomyopathy reported in the field cases.

Preclinical cardiac safety assessment of drugs

The heart is a frequent site of toxicity of pharmaceutical compounds in humans, and when developing a new drug it is critical to conduct a thorough preclinical evaluation of its possible adverse effects on cardiac structure and function. Changes in cardiac morphology such as myocardial necrosis, hypertrophy or valvulopathy are assessed in regulatory toxicity studies in laboratory animals, although specific models may be needed for a more accurate detection of the risk. The potential proarrhythmic risk of new drugs is a major subject of concern and needs to be fully addressed before treatment of volunteers or patients takes place. In vitro assays are conducted to determine the effects on cardiac ion channels, in particular I(Kr) potassium channel antagonism. Prolongation of the QT interval is assessed in vivo, generally in telemetered dogs. Together, these two tests are considered to detect most arrhythmic drugs. The results of this core battery can be refined by additional studies, in particular assays on isolated cardiac tissues determining changes in cardiac action potential duration, shape and variability over time. Triggering of arrhythmia is assessed in hypokalaemic dogs with artificially created bradycardia, or in vitro in isolated whole hearts. The proarrhythmic risk of the new compound is then evaluated by integrating the results of these different tests. Drug adverse effects on cardiac electrophysiological function, in particular impulse formation and conduction, are evaluated through changes in ECG, generally recorded in dogs, pigs or monkeys. Changes in cardiac contractility occurring either as a primary effect of the drug on cardiac function or as a consequence of cardiac lesions should also be carefully assessed. In telemetered or anaesthetised animals, cardiac contractility is evaluated by measurement of left ventricular pressure and its first derivative over time. Echocardiography allows non-invasive measurement of drug-induced changes in ventricular wall movements and cardiac haemodynamics indicative of effects on contractility. In conclusion, a reliable and accurate evaluation of the cardiac safety of a new pharmaceutical agent is based on the results of in vitro tests, with overall moderate to high throughput, and in vivo experiments assessing the effects of the drug on the heart in its physiological environment. The specific sensitivities of the animals used in these assays to cardiac adverse effects should also be considered. The final evaluation of the cardiac risk is therefore based on an integrated analysis of the results from a battery of tests.

Results of the multicenter spaniel trial (MUST): taurine- and carnitine-responsive dilated cardiomyopathy in American cocker spaniels with decreased plasma taurine concentration

Fourteen American Cocker Spaniels (ACS) with dilated cardiomyopathy (DCM) were studied to determine if individuals of this breed with DCM are systemically taurine- or carnitine-deficient and to determine if they are responsive to taurine and carnitine supplementation. American Cocker Spaniels with DCM were identified using echocardiography, and plasma was analyzed for taurine and carnitine concentrations. Each dog was randomly assigned to receive either taurine and carnitine supplementation or placebos. Echocardiograms and clinical examinations were repeated monthly for 4 months. During this period, the investigators and owners were blinded with respect to the treatment being administered. Each dog was weaned off its cardiovascular drugs (furosemide, digoxin, and an angiotensin converting enzyme inhibitor) if an echocardiographic response was identified. At the 4-month time period, each investigator was asked to decide whether he or she thought his or her patient was receiving placebo or taurine/carnitine, based on presence or absence of clinical and echocardiographic improvement. Unblinding then occurred, and dogs receiving placebos were switched to taurine and carnitine supplementation and followed monthly for 4 additional months. All dogs were reexamined 6 months after starting supplementation; survival time and cause of death were recorded for each dog. Data from 3 dogs were not included because of multiple protocol violations. Each dog had a plasma taurine concentration < 50 nmol/mL (mean +/- SD for the group 15 +/- 17 nmol/ mL) at baseline; normal range, 50-180 nmol/mL. The plasma taurine concentration did not exceed 50 nmol/mL at any time in the dogs receiving placebos (n = 5), but increased to 357 +/- 157 nmol/mL (range 140-621 nmol/mL) during taurine and carnitine supplementation (n = 11). Plasma carnitine concentration was within, only slightly below, or slightly above reported limits of normality at baseline (29 +/- 15 mumol/L); did not change during placebo administration; and increased significantly during supplementation (349 +/- 119 mumol/L; n = 11). Echocardiographic variables did not change during placebo administration. During supplementation, left ventricular end-diastolic and end-systolic diameters, and mitral valve E point-to-septal separation decreased significantly in both groups. Shortening fraction increased significantly but not into the normal range. Echocardiographic variables remained improved at 6 months. All dogs were successfully weaned off furosemide, an angiotensin converting enzyme inhibitor, and digoxin once an echocardiographic response was identified. Nine of the dogs have died since the onset of the study in 1992. One dog died of recurrence of DCM and heart failure 31 months after starting supplementation; six dogs died of noncardiac causes. Two dogs developed degenerative mitral valve disease and died of complications of this disease. Dogs less than 10 years of age lived for 46 +/- 11 months, whereas dogs older than 10 years of age lived for 14 +/- 7 months. Two of the 11 dogs were alive at the time of publication, having survived for 3.5 and 4.5 years, respectively. We conclude that ACS with DCM are taurine-deficient and are responsive to taurine and carnitine supplementation. Whereas myocardial function did not return to normal in most dogs, it did improve enough to allow discontinuation of cardiovascular drug therapy and to maintain a normal quality of life for months to years.

Measurement of right ventricular ejection fraction and volume by the thermodilution technique in the unsedated calf

This study was designed to test the applicability and the reproducibility of the thermodilution method in the measurement of right ventricular ejection fraction (EF), end-diastolic volume (EDV) and end-systolic volume (ESV) in unsedated cattle. In the 16 healthy calves studied, it was possible, using a fast response thermodilution catheter, to obtain characteristic exponential steplike washout curves, allowing the calculation of right ventricular EF, EDV and ESV. Five to 10 successive thermodilution curves were recorded within a few minutes and two to five similar sets of measurements were performed throughout the same day to test the reproducibility of the technique. The same protocol was repeated the following day. Right ventricular EF, EDV and ESV were reproducible successively, throughout a given day and on following days. The mean intraset, intraday and interdays coefficients of variation ranged from a mean of 4.0 (SD 4.1) per cent to a mean of 18.2 (SD 7.9) per cent. Values of right ventricular volumes and EF agreed with those previously reported in various animal species. Thermodilution was, therefore, shown to be a useful method for measuring with a satisfactory reproducibility, right ventricular EF, EDV and ESV in the unsedated healthy calf.