There are currently very few proven pharmacologic options available for these patients. The
recent discovery of transient receptor potential vanilloid (TRPV) channels, particularly
TRPV2 channels, in the cardiovascular system is promising as a potential pathway for
pharmacologic intervention for Fontan patients. Probenecid, a drug best known as a treatment
for gout or as a penicillin adjunct, acts as a TRPV2 agonist and has recently become the
subject of study as a model therapy for the treatment of cardiomyopathy due to its positive
inotropic and lusitropic effects. The purpose of this pilot study is to determine if
probenecid will improve magnetic resonance (MRI) parameters of systolic and/or diastolic
dysfunction as well as associated symptoms in patients with a Fontan circulation. The
investigators will quantitatively assess functional improvement with pre- and post-treatment
cardiopulmonary exercise testing.