Quantification and impact of circulating cardiotonic steroids in the RATE-AF randomised trial of patients with atrial fibrillation and heart failure

Background: The presence and role of endogenous digoxin-like cardiotonic steroids (CTS) in humans is controversial. This study utilises a novel pipeline to quantify CTS and examines their interaction with digoxin within a randomised trial.<br/><br/>Methods: The RAte control Therapy Evaluation in permanent Atrial Fibrillation (RATE-AF) trial randomised patients with permanent AF and symptoms of heart failure to low-dose digoxin or beta-blocker therapy; Clinicaltrials.gov NCT02391337. Circulating CTS were detected and quantified using a new ultra-high-performance liquid-chromatography tandem mass-spectrometry (LC-MS/MS) pipeline.<br/><br/>Results: All 160 participants of the RATE-AF trial were included, with mean age 76 years (SD 8) and 46% women. Endogenous CTS detected and quantified in baseline samples included digoxigenin and digitoxigenin, plus low or unquantifiable levels of ouabain, telocinobufagin, cinobufagin, marinobufagenin, bufalin, cinobufotalin, dihydroouabain and ouabagenin. Compared to beta-blockers, patients randomised to digoxin had better functional outcomes at 12 months for heart failure (-0.57 New York Heart Association class, 95% CI -0.82 to -0.32; p&lt;0.001) and atrial fibrillation (odds ratio 2.24 for a two-class improvement in modified European Heart Rhythm Association class, 95% CI 1.43-3.84; p&lt;0.001), with lower NT-pro B-type natriuretic peptide (geometric mean ratio 0.78, 95% CI 0.61 to 0.99; p=0.006). No interactions were observed for any baseline CTS with each outcome. Digoxin was associated with fewer adverse events (odds ratio 0.16, 95% CI 0.07-0.34; p&lt;0.001), again without any interaction from circulating CTS. Digoxin levels by LC-MS/MS were strongly correlated with measurement by a clinical immunoassay (r=0.87; p&lt;0.001), and treatment with digoxin did not affect CTS concentrations at 6-months follow-up. <br/><br/>Conclusions: A range of CTS are detected in the circulation of patients with atrial fibrillation and heart failure. Within this randomised trial but limited by low circulating levels, CTS do not appear to interact with the ability of digoxin to improve wellbeing compared to conventional first-line treatment with beta-blockers. <br/><br/>Keywords: Digoxin; Cardiotonic steroids; CTS; digoxigenin; digitoxigenin; ouabain; atrial fibrillation.<br/>