Clinical trials of endothelin antagonists in heart failure: a question of dose?

Circulating plasma endothelin (ET)-1 concentrations are substantially elevated, and correlate with the hemodynamic severity and New York Heart Association (NYHA) class, in patients with chronic heart failure (CHF). In early preclinical studies involving different models of experimental heart failure, ET antagonists reduced cardiac pressures, increased cardiac output, and prolonged survival. ET receptor antagonists also impressively improved systemic and pulmonary hemodynamics in patients with CHF, without causing neurohormonal activation. However, recent clinical trials, including the ENABLE (Endothelin Antagonist Bosentan for Lowering Cardiac Events in Heart Failure) and EARTH (Endothelin A Receptor Antagonist Trial in Heart Failure) studies, have shown neutral effects in terms of mortality and symptoms. This paper describes the possible reasons why benefit was not seen in these clinical studies, and suggests what lessons can be learnt from the way the studies were undertaken to apply to future studies.