{
  "abstract": "Introduction Drug-induced changes in cardiac function are a common cause of compound attrition. A decrease in cardiac contractility reduces perfusion to vital organs, while drug-induced increases may lead to increased work-load and oxygen demand, both of which can lead to increased mortality. Available in vitro contractility studies often rely on unloaded isometric twitches, which do not reflect the dynamic movements of the heart. In vitro assays are poorly predictive of the concentrations that may affect contractility in humans and furthermore, current in vivo contractility measurements are often invasive, resource/time consuming, low throughput and indirect. In vivo contractility is characterised by 4 distinct phases, as reflected in pressure-volume (PV) loops: isovolumic contraction, ejection, isovolumic relaxation and diastolic filling.There is therefore a need for more predictive pre-clinical cardiovascular in vitro assays that simulate in vivo contractility (cardiac cycle), have higher throughput than available test systems, but also able to detect negative and positive inotropic compounds (drugs that alter the force or energy of muscular contractions) in one single assay conducted using the same experimental conditions.Methods A robust highly sensitive cardiomyocyte contractility Work-Loop assay was developed and validated with the aim of assessing drug-induced inotropic responses at clinically relevant concentrations. Isolated primary cardiomyocytes were used to measure cardiac contractility (Ionoptix, Milton, USA). By clamping the diastolic and systolic forces (pre- and after-load) and mimicking the cardiac cycle. Mechanical work-loops were generated with contractility parameters included Emax (end-systolic force-length relation (marker of intrinsic contractility), Vmax (velocity of shortening), Fmax (maximum isometric force) to determine drug-induced inotropic effects of a panel of c.63 inotropes (19 positive, 34 negative & 10 neutral) covering over 20 mechanisms of action. Inotropes were tested within the clinical effective concentration ranges, at multiple concentrations (n=4-8).Results 19 positive and 35 negative inotropes exerted concentration-dependent increases and decreases in contractility parameters. For example, Milrinone (Phosphodiesterase-3 inhibitor) and Digoxin (Na +/K+ATPase inhibitor) increase the Emax (44; 49% respectively). Aprindine and Flecainide (Na+ channel blockers) decrease the Emax (95; 30% respectively). The Work-Loop assay provided multiple readouts for negative and positive inotropic responses (without system manipulation) and a detailed scoring system was compiled for all >60 compounds. The Work-Loop assay also detected concentration dependent electromechanical coupling alterations.Conclusion We have demonstrated the cardiomyocyte Work-Loop assay is robust and highly predictive, providing an improved and new approach to inotropy detection. Compared with published human data, the Work-Loop assay accurately predicts exposures in man that may cause a change in cardiac contractility with superior sensitivity (96.4%) and specificity (100%) compared to other inotropy assays. Assays that provide greater concordance with human findings are crucial in assessing cardiac safety and efficacy in drug development.",
  "authors": [
    {
      "affiliations": [
        "InoCardia Ltd, Coventry, United Kingdom"
      ],
      "name": "Adam Linekar"
    },
    {
      "affiliations": [
        "InoCardia Ltd, Coventry, United Kingdom"
      ],
      "name": "Jeremy Billson"
    },
    {
      "affiliations": [
        "InoCardia Ltd, Coventry, United Kingdom",
        "Coventry University, Coventry, United Kingdom",
        "Institute of Cardio-Metabolic Medicine, University Hospital Coventry, Coventry, United Kingdom",
        "Warwick Medical School, University of Warwick, Coventry, United Kingdom"
      ],
      "name": "Helen Maddock"
    }
  ],
  "title": "500 Measuring work in single isolated cardiomyocytes and replicating the cardiac cycle: an evaluation of over 60 clinical drugs.",
  "uid": "4e418943-93f3-5908-b784-8a67ecf0f8a4"
}