Rat Papillary Work Loop Assay: A Novel Method to Detect Drug Effects on Cardiac Contractility

Introduction: Cardiac contractility is an important physiological function of the heart and an assessment of drug effects on contractility is an important part of nonclinical drug safety testing. However, there are limited validated in-vitro tissue-based assays available that can be used to screen compounds in order to identify and eliminate inotropic liability during compound optimization (1).

Objectives: We investigated whether the rat myocardial work loop assay had the potential to provide a more predictive model of heart muscle dynamics. Validation included testing >15 inotropic agents (including beta-adrenergic agonists: dobutamine & isoproterenol; NaV1.5 blocker: flecainide; CaV1.2 blocker: diltiazem; calcium sensitizer: levosimendan; phosphodiesterase inhibitor: pimobendan & antifungal: itraconazole) at effective concentrations compared with those tested in the clinic.

Methods: Male rat papillary muscles were superfused with modified cardiac ringer. Muscle length and stimulation amplitude were optimised and active work loops were undertaken and net power outputs were calculated (2). Following stabilisation either a time response or a frequency response curve was undertaken in the absence or presence of inotropic drugs or vehicle controls (n=4-6). Data are presented as the mean±SEM. Data was analsyed by one-way ANOVA and Fischer’s LSD post test. Significance was considered P value ≤0.05.

Results: The Work Loop assay predicted >93% of the positive and negative inotropic effects at clinically relevant concentrations. It also predicted the relevant rank order of potency of compounds from the same class.

Table to show % Max Change from control of the Power Output by the Work Loop Assay and the ability to detect inotropy at clinically relevant concentrations. (Mean±SEM, n-4-6)

Conclusion: The myocardial Work Loop assay is a new approach to the early detection of drug effects on cardiac contractility, providing a superior predictivity of inotropy assessment and importantly identifying inotropy risk at clinically relevant concentrations and rank substances according to inotropy potency. This assay can be used for assessing inotropic risk and understanding the mechanisms of drug-induced effects on contractility.

References:

(1) Wallis R et al (2015) J Pharmacol Toxicol Methods. 75: 62-69

(2) Gharanei et al (2014) Toxicol In Vitro. 28(5): 722-3